Solar Photovoltaics Overview 2005- J. El Spec Rel Ph 2006- p 105- Kazmerski

Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135Solar photovoltaics R&D at the tipping point: A 2005 technology overview Lawrence L. Kazmerski ∗ National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, CO 80401, USA Abstract The status of current and coming solar photovoltaic technologies and their future development are presented. The emphasis is on R&D advances and cell and module performances, with indications of the limitations and strengths of crystalline (Si and GaAs) and thin film (a-Si:H, Si, Cu(In,Ga)(Se,S)2 , CdTe). The contributions and technological pathways for now and near-term technologies (silicon, III–Vs, and thin films) and status and forecasts for next-next generation photovoltaics (organics, nanotechnologies, multi-multiple junctions) are evaluated. Recent advances in concentrators, new directions for thin films, and materials/device technology issues are discussed in terms of technology evolution and progress. Insights to technical and other investments needed to tip photovoltaics to its next level of contribution as a significant clean-energy partner in the world energy portfolio. © 2005 Published by Elsevier B.V. Keywords: Photovoltaics; Solar cells; Thin films 1. Introduction and progress Just over 50 years ago, this solar–electric technology marked a tipping point [1] at Bell Telephone Laboratories when Daryl Chapin, Gerald Pearson, and Calvin Fuller suddenly turned a research curiosity into a viable electricity producer [2]. Their research innovation brought the performances of these crystalline silicon devices from “laboratory interest” (conversion efficiencies hovering at 1%) to efficiencies five to eight times greater, earning consideration of these solar-powered devices as real electrical power sources. Although this threesome worked to develop a practical power supply for Bell’s remote telephone signal transmissions here on earth, they actually created the technology that first blossomed to power our early satellites—leading to a revolution in wireless communications that was not yet envisioned within their own forward-looking communications company. They were creative scientists and engineers ahead of their time. The revolution in the terrestrial markets they were addressing was delayed until at the end of their 20th century—much as Daryl Chapin himself had contemplated [3]. First, there would be a demonstration of technology readiness with remote power in the 1980s [4]. Then, in the late 1990s, this “PV” would emerge ∗ Tel.: +1 866 270 2962. E-mail address: larry [email protected]. to help electrify our homes and many aspects of our daily lives [4,5]. As this 21st century is starting, Chapin, Pearson, and Fuller would be pleased to finally see this PV market growth finally reaching respectable levels that are capturing the interests of investors and users [4–6]. The research progress over the past 25–30 years has been substantial and steady, as represented in Fig. 1. Photovoltaics is poised at what may be its most critical tipping point [1]; the one that will cause this technology to “spread like wildfire” as it finally becomes a major part of our world’s energy portfolio. PV as a technology and a business has just surpassed annual sales of 1 GW and U.S.$ 10B. As represented in Fig. 2, these worldwide shipments have been growing above 30% annually for the past decade; in fact, they have averaged above 35% for the past 5 years [7–9]. PV is a real business now—and should continue to exhibit such substantial annual increases for some time to come. Much of this growth has been the result of government incentives, mainly in Japan and in Germany [5,6,10,11]. Both these governments have shown that policies make a difference—using quite different approaches. The market stimulation in Japan has been based on cash subsidies, initially buying down the price to the consumer. Starting in 1994 with a 50% rebate, this program has followed its design to gradually phase down the government portion as the price for the PV system decreased. This coming year, the programs success is indicated by the more than 140,000 installations and reaching 0368-2048/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.elspec.2005.09.004 106 L.L. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 Fig. 1. Efficiency evolution of best research cells by technology type. This table identifies those cells that have been measured under standard conditions and confirmed at one of the world’s accepted centers for standard solar-cell measurements. the point that no subsidies are needed in their electricity price market. On the other hand, Germany introduced a “feed-in tariff” in 2000 that offers solar PV users a guaranteed “Euro per kWh” over a 20-year period, with each year the guaranteed price is reduced by 5%. With the availability of low-interest loans, the German markets have heated from less than 20 MW/year to above 150 MW/year currently. The cost is spread over the entire electricity user rate base so that utilities are not negatively impacted and the government does not have to appropriate the funds annually. The successes of these policies sometimes overshadow another important component—technology advancement. The 18–22 U.S. cents/kWh that has been reached also required a progression of substantial and creative R&D improvement in materials, devices, fabrication, characterization, and processing, leading better device performance and reliability, and lowered Fig. 2. Photovoltaic annual module shipments as reported by the industry. Shipments for U.S., Japan, Europe, and rest-of-world sectors have been identified [7]. L.L. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 107 systems costs that the “policies” have leveraged. This electricity price breaches into some electricity markets [11]. But it is still too high for the next wave of grid-tied applications (consumer side of the meter prices) and almost an order or magnitude too high for wholesale (central utility) generation. To tip this technology to its next level—building first multi-GW markets toward the terawatt levels, and manufacturing plants to hundreds of megawatts then perhaps the GW annual capacities, PV technology requires even more creativity, science, and engineering to meet the growing and diversified technical and consumer demands. This paper looks at the current PV technology status, the reasons underlying recent improvements, comparisons of approaches, with an emphasis on R&D needs and directions—and a bit of a look forward what future generation PV might encompass. There is a focus on the fundamental building blocks of the PV system—the cells and modules. The crystalline silicon, crystalline III–V, and polycrystalline and amorphous thin films that dominant now and near-term expectations will receive the primary attention. However, the next generations of PV are already on the laboratory research benches—organics, nanotechnologies, multi-multiple junctions, bandgap engineering, thermo-tuned concepts—all aimed either the ultra-high efficiency or the ultra-low cost regimes that the technologies of our quarter- to mid-century will likely require. Because even now our R&D is both undergoing and causing rapid and continuous change, this review can only present “snapshots” of today or tomorrow’s technologies. However, the attention and investment in this R&D is critical to realizing the new levels of performance and reduced-costs needs, both for the now and for the longerterm success of solar photovoltaics. 2. The technologies Photovoltaics technology includes a number of significant component performance “gaps” for various crystalline, polycrystalline, and amorphous; bulk, as well as thin-film technologies. The first is the difference between the theoretical limits, the attainable levels, and what has been demonstrated under the best conditions in the laboratory (the headline or record cells). These are shown in Fig. 1 for various crystalline, polycrystalline, bulk, and thin-film technologies [12]. Underlying these differences are losses that are inherent to the conversion process (theoretical to attainable), and the ability to fabricate the cell with the ensemble of optimal, interrelated properties, and parameters. The “gap between what can been attained and what has been reached” is a major focus for researchers (represented in Fig. 3) – a process of identifying, understanding, and minimizing losses – collecting every incident photon, allowing these to create the maximum number of electron–hole pairs, and then making these carriers live long enough to contribute to the current generation process. The second is the difference between the laboratory efficiency of cells and those produced in commercial lines. This has to do with scale up of the processing to larger areas, variations of materials (starting wafers, substrates, coatings, etc.), less controlled conditions, and higher required throughputs. The third gap is that between the cell efficiencies and those of the modules. This depends upon the ability to minimize the losses when wiring the cells into circuits, bringing the active area of the module to be closer to the cell area, and maximizing the optical transmission of the protective or support layers that are positioned between the cells and the incident sunlight. These “gaps” are ones that can and have to be addressed and minimized—and are Fig. 3. Performance gaps between best device efficiencies in the laboratory and attainable efficiencies for several solar cell technologies. there has been incredible progress in the understanding of the semiconductor and the device. It would appear that the single-crystal research phase plateaued some 10 years ago (Fig. Fig. maximizing electron–hole generation. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 active areas of research and development for all PV technologies [3–5]. (Table 1 presents a summary of the headline cells in various Si device technologies. (a) Bell cells and first solar module photograph and (b) device representation from Bell Laboratory (from page 68 of laboratory notebook of C. this over demand for the “semiconductor foundation” of the photovoltaics industry will likely lag behind the marketplace—as long as the incentives in Europe grow. Moreover. leaving an impression that other technologies can only gain on this frontrunner. passivated-emitter solar cells (PESC) [18].L. 1954 “Bell #1”. In fact. The evolution of these designs has included metal/insulator/n-type/p-type (MINP) [17]. and contacts that are made from the back of the cell—minimizing any obscuration at the front surface in order to accept every incident photon.20]. if those in the U. a dominant 94% were single-crystal. Fuller. Its design has a relatively shallow diffused junction (certainly the research tipping point to separate it from previous grown junctions). 2. multicrystalline. cells have reached laboratory performances converting nearly one-fourth of the incident photons into electrical power—reaching about 92% of its reported “theoretical limit”. 4. By comparison. Historical Bell Telephone Laboratories crystalline Si cell reported in 1954. Crystalline silicon Of the more than 1 GW of the PV commercial shipments in 2004. Although these capacities are expected to more than double in the next 3 years (beyond a 25% per year growth). 4b shows the cross-section of that device. witnessed by D. deployed for use in late 1954 [2]. .1.14]. singlesided and doubled-sided buried contacts (SSBC and DSBC) [19. and even more so. conventional p–n junctions of the early 1980s have evolved toward more complicated designs and structures—all aimed at capturing every incident phonon. ribbon. and prolonging the lifetime of those carriers to be collected for maximizing current generation [16]. After all. point contact [21]. (This “understanding” has been a major advantage for Si cells since it has leveraged the R&D accomplished for other more prevalent electro-optical technologies that have become parts of our everyday lives. This limited viewpoint has not clouded the research thinking and strategies of some parts of the world. 5a show the high-efficiency design of the SunPower cell [15]—strikingly similar in these two critical design features that brought about an early PV technology “tipping point”. but under which consumers and the environment suffer. are implemented. Some have had a misconception that there is little left in crystalline Si research.) The relatively simple.108 L. Chapin 1953). Fig.S.) It is also interesting to see how the fundamental understanding of that 1950s team endures today. Fig. where it has been recognized that there are significant improvements for both current commercial approaches—and especially for a potential next generation of Si solar technologies [23]. the “26% calculated limit” for crystalline Si technologies has recently been re-evaluated—with the result that the “bar” has now been raised. particularly in Europe and Japan. How does this Si cell differ in 2005 from that “invented” at Bell Labs a half century ago? Indeed. the markets would have absorbed even more except that the supplies were limited due to insufficient manufacturing capacities and supplies [13. 4a shows a photo of the April. measured at about 6% efficiency along with the first PV module. Fig. and bifacial cells [22]. The spectacle of “zero-inventory warehouses” is one that the industry relishes. 1). and sheet silicon [7]. “performance” is a marriage between the conversion parameters and the reliability criteria. The SunPower cell (Fig. It is engineered to contact from the rear side. The terrestrial commercial “20% club” represented by the cells in Fig.S. manufacturingfriendly approaches. Stability. 5a) has been discussed briefly above [27]. This has partially driven the development of the three major commercial cells having efficiencies in excess of 20%. 5. For example. 4. as represented in Fig. These devices must maintain their high power outputs for decades—meeting warranties that are now 20–25 years. Silicon is real business—and it is also a focus for real research excitement again. Silicon leads right now in these combined categories—and has potential for additional improvement. Swanson in the 1990s (first termed the “point contact” cell). It is also a partial basis of a new “Si Technology Initiative” proposed in the U. adding value to higher efficiency. This utilized interdigitated n+ and p+ diffusions and grid lines to collect the . 5 show three different approaches that are leading toward the 20% crystalline Si module. Additionally. This limit is now proposed to be near “29%” under standard conditions [24–26]. [23]—and such cell improvements may recast the thinking whether silicon can compete at the substantially sub-$1/W system price that other emerging technologies have taken as their exclusive real estate value in the longer term. with no light obscuration on the front surface—a design developed by R. It also establishes valuable targets and proofs of concept for the other PV approaches as systems gain from installation experience and use. and operating lifetime are equal partners in the economic viability (energy and cost recovery) and consumer acceptance (electrical power on demand) of our PV technology. It must also be recognized that there are other important progress metrics.L. there is a real and current economic benchmark for maximum “W/m2 ” for many rooftop applications. (Continued ). durability. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 109 Fig.L. Swanson and others have refined the calculations based on the greater understand and materials and device knowledge that has evolved over the last decade. This not only has reinvigorated a community of researchers to find how the crystalline Si device can enter into this expanded efficiency regime of 26–29%. with 30 years and beyond expected to be the industry standard in the not too distant future. 6 [30]. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 Fig. (b) BP Solar Saturn. The elimination of this cutting process affords potential materials and production savings—the logic behind the development sheet and ribbon approaches over the past 20 years. and the emerging Si can be to desired lengths without interrupting growth. The cell production and development is based upon sophisticated statistical process control and design of experiments—quality control leveraged from their parent Cypress Semiconductor to greatly tighten process specification limits. A third sheet method involves the growth of the Si layer by a high-speed process on a variety of substrates. Cells (125 mm × 125 mm) have been verified with efficiencies to 21. Module efficiencies exceeding 16% conversion efficiency are now commercially available using this innovative technology. photogenerated currents and low series resistance loss. The third cell is the Sanyo heterojunction with intrinsic thin layer (HIT) device (Fig. A major consideration with the cost of producing the Si cell is associated with producing the high-perfection wafer. This employs an n-type single-crystal wafer surrounded by ultra-thin a-Si:H films in a bifacial structure. which crystallizes between the stings. 5b shows the BP Solar buried contact cell [28]. SunPower has begun to market modules based on these cells that have high packing density for the cells. The former method involves shaping of the Si through a special die. forming a long (tens of meters) hollow octagon (although other shapes. Variations of this cell design are also being used in concentrating PV (100×–300×) with peak efficiencies of 25%.110 L. the strings are fed in from spools. The cell efficiencies are similar to these sheet processes to those made from the ingot-cut multicrystalline wafers—of the order of . This process has the positive feature of being continuous—the melt can be replenished. Commercial crystalline Si solar cell “20% club” showing cross-sections of: (a) SunPower A-300.5%.37]. Two major ones that have current commercial significance are edge film-fed growth (EFG) [34] and the string ribbon processes [35].L. The bulk Si methods have greatly benefited from the evolution of sawing techniques that utilize multiple thin wires that waste less of the precious semiconductor. including ceramics) [36. 5c) [29]. sacrificing efficiency points for the benefits of lower energy production and perhaps facilitating the use of lower-purity feedstock material. with some special attention to light trapping. even cylinders have been successfully produced) structures (metersize in diameter). This design has yielded production line cells with efficiencies to 20. Casting (and several sibling technologies) has become the manufacturing convention [31–33]. two high-temperature “strings” are pulled vertically through a shallow molten Si melt. In the latter technique. Headline cells at 20. and give the illusion of being uniformly black—designed for aesthetic enhancements for the built environment. A laser is used to separate the flat faces into individual cells. A typical breakdown of costs associated with the making of a commercial single-crystal Si cell is represented in Fig. and (c) Sanyo “HIT” solar cells.5%. One approach has been to employ less energy intensive processes to make the wafer.1% have been measured. The design incorporates a great deal of engineering. It uses advanced light trapping and minimizing the front contact “area” by “burying” the contacts in laser-produced grooves that lower the resistance through enhanced sidewall areas. 5. Fig. of the design by Martin Green cited above. 6 80.1 148. but present consider- able difficulties in handling and processing because of mechanical fragility for wafers of 100 m thickness and extremes in temperature cycling and thermal stresses they must endure during processing [44]. in addition conventional module use.5 36.9 100 147.98 602 10 Thin-film poly-. These have included processes involving hydrogen.0 17. developed by the Australian National University. (3/93) Silicon-film (700 m substrate) (2/97) Silicon-film (400 m substrate (5/05) 111 Monocrystalline silicon 706 42.4 4.0 661 Concentrator cells 0.5 714 37. The best bulk multicrystalline Si cell was verified in this past year by the Fraunhofer Institute at 20. long (50–100 mm).2 4.L. under standard measurement and reporting conditions [194] Voc (mV) Jsc (mA/cm2 ) FF (%) 82.017 589 35.7 Area (cm2 ) 4.26 68.0 18. .00 25000 3.6 8.6 76. The cells are then removed from the wafer support—providing narrow (1 mm). The development of plasma and other nitriding processes by the semiconductor electronics industry has led to improvements in single.00 Efficiency (%) 24.6 81.5 15. Additionally.8 80.4 76. aluminum arsenic. such as shown in Fig. passivation.7 80.60 1.9 77. The inventors argue for not only the materials savings these cells offer.1 4.4 77.3 634 36. A number of thin or thinned wafer technologies have emerged in this new century. including an interesting concept termed the Sliver® cell [45].002 144 1.3 20.7 632 35. “slivers”) bifacial cells.5 19. This has included understanding the nature of the defects resulting from the growth and processing process—and then devising surface and materials treatments to minimize the carrier losses at unwanted active sites. but also that the manufacturing process is greatly simplified and that the required manufacturing plant requires less capital investment.6 Multicrystalline silicon 664 37.1 36.1 80. lithium. and BP Solar UNSW/Euro-solare Georgia Tech Sharp AstroPower Sharp UNSW FhG-ISE ANU Kaneka University of Stuttgart Mitsubishi UNSW Pacific Solar Stanford U.8 1. and nano-crystalline silicon (<100 699 37.5 4. micro-.4 20.15 20.6 19.8 78.5 81. The cell processing (P and B diffusions.2 17. metallization) is completed while the Si strips are still supported by the Si substrate at the bottom edge.5 21.0 10. FZ Si.L.09 1.7 79.02 1. with several other cells in this materials category above 18% efficiency [39.0 80.4 4.6 81.6 15. Photowatt. 1–2 m Si (5/95) Point contact: 140 suns (5/87) Laser groove: 11 suns (9/90) Rear contact: 96 suns (10/95) Rear contact: 74 suns (7/93) 11–13% (although smaller grained sized sheets have somewhat lower performance). of Konstanz.3 600 31.1 77.12 645 32. The flexibility of these wafers and devices.04 668 37. The cells are being considered for transparent and concentrating modules.3 m) 21.2 16.6 625 36.3 81.3 95. Closing the differences between these less perfect materials and the single-crystal cell has involved considerable R&D.10 22. Silicon strips are produced by micro-machining narrow ∼100 m deep grooves into a Si wafer.1 16.6 16.00 22.8 661 32.6 26.8 81. This has fueled research on maintaining efficiencies for thinner layers. These have had efficiencies up to about 18%. UNSW SunPower SunPower Comments PERL (3/99) PERL (5/96) PERL (8/96) Rear contact.9 654 636 610 38.8 15.40].5 80.21 47 m Si (8/95) 46 m on SiOx (4/97) 20 m (thinned Si) (9/94) 2 m on glass (12/97) 45 m thin film transfer (7/01) 77 m VEST (2/97) CVD Si on Cz Si (5/95) PECVD on glass-submodule.2 26.6 17. thin (50–70 m) (thus.00 539 24.02 1.and polycrystalline Si cells. and phosphorus for front and back surfaces [41–43].4 Thin silicon (on thick low-cost Si substrate) 608 33.00 1.6 704 41. large area (9/02) Conv. p–n (10/85) Dendritic Web (5/85) [28 ◦ C] (5/04) Mechanically textured.4 23.9 81.3 78. have some very desirable application features.00 100. 7. with prototype minimodules (560 cm2 area) above 12% efficiency.7 Organization UNSW UNSW UNSW SunPower Sanyo BP Solar Spire Westinghouse FhG-ISE Univ.8 18.5 21.1% [38]. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 Table 1 Summary of confirmed.00 651 32. the economics of using less material has led the industry to investigate the production and use of thinner wafers—with objectives of trimming the industry standard of about 300 m thick wafers by a third or more. large area (2/03) (2/98) HEM Si (12/95) Textured mech.3 17. texturing the faces.7 21.5 678 39. large area (9/03) HIT large area (3/01) Laser Grooved. selected Si solar-cell efficiencies and related parameters.5 4.5 0.2 702 41.7 23. Ongoing research in crystalline Si aims to improve the throughput and yield of the growth processes.2. 6. So. There is some discussion over these feedstock issues.S. However. and better energy economy for production and product recovery. The first 25 years of thin films were dominated by that “cadmium sulfide” device—until its stability problems were judged to be insolvable. Incremental improvements will continue to help reduce these manufacturing costs to keep silicon as the major part of the PV business for the near future. cadmium telluride (CdTe).112 L. The surge in markets combined with a series of falls in the supplies needed for the semiconductor electronics manufacturers have elevated PV to be the major customer. and (c) energy required to make Si module components. In fact. Module finishing includes encapsulation materials. frame.$ 30–35/kg level that the solar industry can afford. (b) Si module. Low-cost processing and manufacturing for high-efficiency technologies is a major portion of efforts aimed at improving and simplifying all aspects of cell and module production. growing out of work by Reynolds et al. However. These first “5%” cells were cuprous sulfide/cadmium sulfide (Cu2 S/CdS)1 . and silicon.L. as the competing microelectronics and optoelectronics business again build – an industry that can pay far more for these supplies – the question of whether Si PV can continue to follow 90% learning curves1 that it has followed since the late 1990s 1 Aside: There were always termed “cadmium sulfide solar cells”. Finally. However. 2. this has caused the price for the feedstock to fall to the U. both the major PV activity and especially the problems were with the other heterojunction partner. large-scale manufacturing advantages. copper–indium–gallium–selenide (CIGS) and related compounds.13]. in 1955 [46]. glass. Now the major approaches include amorphous silicon:hydrogen (a-Si:H). contacting/wiring. This is particularly true of the developing ribbon and sheet processes). Thin films Thin-film photovoltaics is always looked at as the “younger cousin” of the silicon technology—poised to take over the energy production responsibilities of its older relative. for its product.to mid-1970s—although CdTe was of interest to earlier space PV work along with the Cu2 S/CdS . The arguments favoring thin-film PV have been based on materials utilization. Most of the work on these materials surged with the interests in terrestrial photovoltaics in the early. junction boxes. but never quite fulfilling its expectations or potential. The roles of defects and impurities – and their interactions and impacts on solar cell performance – remain important. some revolutionary developments in manufacturing or cell innovation could accelerate industry expectations significantly. the thinfilm “efficient” counterpart of the Bell Labs Si cell surfaced within a year of the Bell announcement. the CdS perhaps received some early bad press from being in the right place at the wrong time!. These range from evaluations that shortages are temporary to some dire arguments that additional investments are needed now to build plants to specifically supply this growing PV need. etc. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 Fig. But in fact. there remain questions about the resources for Si feedstock [4. Cost and energy breakdowns structures for crystalline Si technology: (a) PV system. and further stabilizing the semiconductor [52–55]. investments). This characteristic has been exploited for applications for tropical and desert areas. (named the Staebler-Wronki effect after its discoverers) was observed [49]. the instability has been minimized by engineering layer thicknesses (primarily the intrinsic layer) and by the use of multiple device structures [50–52]. its physics was completely different than the crystalline Si model. these technologies are now entering the world commercial markets. These thin-film technologies have more than doubled their sales over the previous year—and have a window of opportunity to further prove their merit. an inherent light instability. • Current research (thin wafer production. cost. . More importantly. competition from semiconductor electronics industry. Following the guidance provided by several research investigations. Research groups continue to give attention to this problem—with several recent new paths toward understanding. the amorphous Si was hydrogenated to reduce the bandgap states and to allow the development of open-circuit voltages. Having no long-range and perhaps only limited short-range order. the commercial a-Si:H module might actually have efficiencies better than crystalline Si counterparts under some specific conditions (e. quality assurance. yields. manufacturing. It also benefited technologically because it leveraged the R&D interests from other electronic technologies (transistors. Flexible cells: (a) thin-layer Si sliver cell and (b) thin-film polymer cell. diagnostics.2. 1 and 3). need for solar grade?). Its light optical characteristics make it 100 times more effective in absorbing the sun’s irradiance than crystalline Si. with a summary of champion devices in Table 2. in demand partially from the capacity problems with crystalline Si. robotics). complexities. this technical limitation still exists. Thin-film silicon: amorphous The introduction of this new class of semiconductors in the mid-1970s seemed to have posed the ideal photovoltaic candidate absorber [47]. and competitiveness. Early in the research investigation of this device. Thin films have advanced significantly over these past 30 years (Figs. • Manufacturing capacity (meeting market needs/growth.1. Its bandgap could be varied over tenth of eVs by changing the hydrogen content. The development and larger-scale adoption of this PV technology has been impaired primarily by single. at operating temperatures above 70 ◦ C). Devices could lose 50% or more of their power output over the first hundreds of hours of its operation exposed to light—not a desirable characteristic for a solar cell! Though there has been significant investment in both identifying the origin(s) of this problem and solving it. processing.L. (This “temperature benefit” has also been cited for the a-Si:H/Si Fig.g.L. processing. Because the performance tends to recover and improve at higher operating temperatures and the bandgap is higher. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 113 device owing to requirements for high power-to-weight ratios that still exist today in those important markets. plant scales toward 500 MW/year. Because of the defects associated with the “dangling Si bonds”. but cells and modules with less than 10% change in output characteristics are now attainable. novel approaches. and handling. depositing the material. 2. 1 and 8 show the evolution of the a-Si:H cell’s performance. process integration. There is a benefit to the a-Si:H technologies that is sometimes overlooked—a temperature performance characteristic that is better than its other semiconductor competitors [59]. important issue—stability. • R&D initiative (new efficiency regimes. materials optimization. 7. flat-panel displays). Technology concerns and issues • Manufacturing (costs. diagnostics).. new devices—next generations of Si technology). • Silicon feedstock (critical issue for the nearterm: availability. Figs. This includes combining or using nanocrystalline and/or microcrystalline Si in the device structure [56–58]. worth. The “cure” has not been found. and single-junction polycrystalline research cells are approaching 20%. 99 0.00 Efficiency (%) 12. The micromorph concept has progressed to commercial reality.27 1. they have reported cells in the 13% range and a module with an 11. selected a-SiH based solar-cell efficiencies and related parameters. many research groups started to look at the first stages of crystallization of their a-Si:H films into nanocrystalline (nc) and microcrystalline ( c) regimes. R&D at the edge In the mid-1990s.4 12.4 70 68. 9. 8. heterojunction in the last section.96 2289 7.08 0. showing progression of performances for small-area and large-area cells.72 2541 6.76 0. a combined a-Si:H/ c-Si:H into a stacked structure [61].1 70. showing “generic” and cells grown on glass and stainless steel).28 0. Recently.7% efficiency were reported for this arrangement—and a-SiH/ c-SiH tandem cell with 10% efficiency (both stabilized).9 .27 0.5 10. motivation for new “players” in PV industry.46 Dual-junction cells 1621 11. under standard measurement and reporting conditions [194] Voc (mV) Jsc (mA/cm2 ) FF (%) 74.7 11.1 74. Initial cells with 7. These have 20-year warranties—as well as aesthetic benefits of direct integration into the built environment.4 65.5 10. Thin-film amorphous-Si:H solar cells.5 12.L.03 Triple-junction cells 2375 7. It is the protective architectural roofing material—that also generates useful electricity. The use of these longer-range order films “at the edge” of the ordering process were deemed to provide the path toward more stable and higher performance devices. durability. • Perceptions of device lifetime issues. Technology concerns and issues • Stability.1 70.8 68.9 12.8% stabilized (aperture-area) efficiency (see Table 2) [70]. reliability.2. Some tagged this as the evolution of the amorphous technology toward thin-film crystalline silicon. Nanocrystalline and microcrystalline silicon-films. Kaneka offers a number of products.) To date. 2. The micromorph cell was further improved with the introduction of a ZnO layer as an intermediate reflector [62].3 13.114 L.4 897 18. Table 2 Summary of confirmed.8 886 17. the major application of this technology has been in successful integration into roofing products [60]—with these products in the 5–7% efficiency range typically.72 1685 9. ranging from rooftop to semitransparent designs for building integration [69].2.00 1.4 Organization Sanyo Solarex Glasstech USSC/Cannon Solarex USSC EDC Sharp Comments a-Si:H (not stabilized) (4/92) a-Si:H (not stabilized) (4/87) a-Si:H (not stabilized) (9/89) a-Si:H/a-SiGe:H/ss (not stabilized) (1/92) a-Si:H/a-SiGe:H (not stabilized) (10/87) a-Si:H/a-Si:H/a-SiGe:H (not stabilized) (10/96) a-Si:H/a-Si:H/a-SiGe:H (not stabilized) (12/88) a-Si:H/a-Si:H/a-SiGe:H (not stabilized) (10/96) SiGe:H (not stabilized) 12/92) Single-junction cells 887 19. A large research effort on the microcrystalline and nanocrystalline films exists today [63–68].5 Area (cm2 ) 1. The first progress was the introduction of the “micromorph” solar cell (Fig. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 Fig. The substrate is textured glass having a Si nitride deposited surface layer. Three layers of aSi films with appropriate doping and a silicon oxide capping film are deposited in a single deposition chamber. growing out of their analysis of the highest payoff paths to thin-film solar cell market penetration. deposition R&D). These layers are crystallized and hydrogenated using plasma techniques. which has now been taken under Q-cells.2. the goal being able to utilize inexpensive support structures in the latter case. Success with the prototype module technology led to the reformation the initial startup into CSG Solar. there has been some progress in polycrystalline thin-film Si on foreign substrates [76–78]—including some recent commercial ventures [79. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 115 Fig. These have ranged from thin expitaxial Si films on Si substrates to Si on glass or ceramics. using vacuum deposition and chemical vapor deposition—but always producing films with small grain sizes and high defect densities that limited carrier lifetimes. some creative “strip interconnection” circumvents potential shunts that can be issues with thin-film module performances. electrically insulating” (micron thickness) is applied and patterned in two stages: (1) the first producing the negative crater contact regions (where the Si is etched through to the n + layer) and (2) the positive dimple contacts to the player. Several demonstrations of thin Si cell performances have appeared in the literature over the past 10 years. 10 presents a crosssection of the CSG cell structure. and environmental advantages over some of the other PV thin films. These were mainly grown on foreign substrates. thinfilm Si was always the logical progression toward the ideal solar cell. For the purposes of learning more about the processes in thin-film Si. characterization. In addition. 2. substrates and packaging). Fig.3. Thin-film silicon: polycrystalline From consideration of improving materials utilization. volume. Prototype modules into the 8% efficiency regime have been reached. there has been some progress in both thinned and in epitaxial layers of Si on Si [72–75]. • Amorphous Si:H deposition rates (increase rates needed for manufacturing). Early work in this area was limited to cells having efficiencies in the 5% regime—much below expectations [71]. • Research (modeling. Inkjet or plasma methods form grooves (Fig. The cells offer integration with a small number of processing steps. such glass and graphite. A brief sum- mary is shown in Tables 2 and 3 for these various technology demonstrations.L. Cross-section of micromorph thin-film solar cell: (a) glass substrate configuration. 9.80]. while posing potential improvements over most current thin-film manufacturing approaches and offering some reliability. A low-cost “optically non-absorbing resin. analysis. materials availability. The banner for this technology is that is combines the proven strengths of wafer crystalline Si technology. Additionally.L. 10). yields. and the expectation is to reach 10% levels in bringing . with production facilities deployed in Germany. with ZnO intermediate reflector and (b) stainless-steel substrate configuration. • Manufacturing costs and capacities (vacuumbased production. The Si has sufficient conductivity to avoid the need for the usual transparent conducting oxide (TCO) film. most with the purpose of demonstrating viability of “thin-Si” technology from the performance and device engineering perspectives. Additionally. the relatively poor optical absorption characteristics of this indirect bandgap semiconductor required relatively thick layers to produce adequate electron–hole populations from the incident photons. A commercial entry that has attracted some attention and interest recently has been the “crystalline silicon on glass” (CSG) technology [79–83]. Aluminum (for example) is depositing over the entire surface and patterned appropriately by either injet or laser into thin strips joining the n+ to the p+ region of the adjacent cell. This approach had been under development by Pacific Solar in Australia since the mid-1990s. providing the active cell for the patterning process. but one that currently has significant technical and funding resources behind it.09 25.25 141.90 26.5 77.047 1. and reliability parameters are realized in its first-time manufacturing phase.47 15. quality assurance.0 15.418 0.3 Advance tandems 0.462 0. Thin-film copper indium selenide.8 14. This concept has progressed rapidly from demonstration to prototyping and certainly has advantages if the performance levels.7 17. diagnostics).4 21. yields.3 726 15.6 80. manufacturing cost.7 8. • Energy payback.1× (3/01) MgF2 /7059 glass/SnO2 /CdS/CdTe/C/Ag (6/92) MgF2 /7059 glass/SnO2 /CdS/CdTe/glass (4/99) CdSnO/CdS/CdTe/ glass (2/01) CdSnO/CdS/CdTe/ glass (9/01) 3–5 m CSS CdTe.or n-type conduction.0 11. • Manufacturing capacity (requires demonstration).102 1.2 71.8 1.0 0.5 16.5 6.3* 4. small area (11/00) CIGS on stainless steel (flexible) 2/00 ZnO/CdS/CIS (11/92) Concentrator: 14.768 0. and related chalcopyrites Interest in the Cu-ternary semiconductors began in the early 1970s for non-linear optics [84].90 68. • R&D initiative (part of next generation Si technology). under standard measurement and reporting conditions [194].2%.4 25. The bandgaps of several members (including CuInX2 .4 13.357 1. South Florida NREL NREL NREL Matsushita EPFL INAP ECN Toshiba Bell Labs/Lucent NREL Kopin/Boeing ARCO NREL Comments ZnO/CdS/CIGS (12/98).15 (Active area) 0.14 25. except (*). device fabrication). 18.7 510.1 25.1 cm2 cell (1/99) ZnO/CdS/CIGS (9/04) Large area (3/01) ZnO/CIGS(1/99) Cd-free cell ZnO/CdS/CIGS (electrodeposited) (2/99) ZnO/[Cd-doped CIGS] (2/01) Active area efficiency. industry involvement. It is an ambitious program. having high optical absorption.4.4 2. and bandgaps matched to the .4 16. 14.442 0. capable of either p.8 15.116 L.3 Organization NREL NREL NREL NREL NREL NREL Ritsumeikan University NREL Siemens Solar NREL Univ.8 522 22.6 75.8* 3.4 77. reproducibility.73 36.039 0. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 Table 3 Summary of confirmed.2. complexities. which are reported but not confirmed Voc (mV) Jsc (mA/cm2 ) FF (%) Area (cm2 ) 0.4 2.2 7.0 59.7 17. also.4 15. selected thin-film solar-cell efficiencies and related parameters.0 2.1 15. Se.5% direct.06 68.1 79.4 795 11.0 33. energy payback.192 0.5 75.410 1.5 18.7 835 6.7 35. 2. Technology concerns and issues • Manufacturing (cell and module performance.5 75. stable electro-optical properties. crystallinity.01 this into a viable commercial product.1 71. its alloys.9 4.00 Efficiency (%) 18. ZnS buffer.3× (21.86 25.51 34.1 68. with X = S.6 73.8 19. and Te) of this chalcopyrite family exhibited properties well suited for PV consideration. These were typically direct bandgap semiconductors. 1.8 16.131 1.2 70 63 Other advanced types 795 19.5 74.5 85.4 15.L.132 1.64 34.6 13.36 4. costs.449 0.0 4.5 73. • Cell and module efficiencies approaching the 15% levels (materials quality.19 30.6 71.3 693 669 605 666 636 671 539 736 CdTe 843 848 845 840 35. question QE-current (3/97) Nanocrystalline dye (Gr¨ tzel (12/96) a Nanocrystalline dye (Gr¨ tzel) submodule (2/98) a Nanocrystalline dye (Gr¨ tzel) (7/01) a GLE (polymer gel electrolyte) Photoe-electrochemical cell (5/00) “Plastic Cell” (ITA/Pentacene) (5/00) “Plastic Cell” (8/05) GaAs/CIS thin film (11/89) a-Si:H/CIGS (6/88) Transparent CdTe cell CIS cell Glass/Cd2 SnO4 /ZnSnOx /CdS:O/CdTe/Cux Te—Glass/ Mo/CIGS/CdS/ZnO CdTe/CIS 4-terminal mechanical stack (12/04) Cu-ternary and multinaries 678 32. passivation and other such schemes have been unnecessary. to the engineering of the cell for optimal performance and reliability. interfaces. physical vapor deposition and post-Se treatments. The most widely used back contact is a sputterdeposited molybdenum thin film.Ga)(Se. Several CIGS and CIGSS deposition methods have been successfully used to realize greater than 10% solar cells. . Two recent investigations are of interest to the device optimization from the materials point of view.04 eV for CIS) for better voltage output for this “heterojunction” solar cell [87].Ga)Se2 (or “CIGS”) and Cu(In. which have slightly higher bandgaps (to about 1. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 117 Fig. Cross-section of thin-film Si solar cell (crystalline silicon on glass or CSG technology). Studies by Sites et al. H2 Se) and Se-containing solids (including elemental Se).g. have identified the reasons underlying the optimal compositional profiles (Fig. including electron micrograph showing actual depositedlayer configurations. These include deposition from the elemental constituents (this having produced the highest efficiencies). Each of the layers. Cross-sectional representation of copper–indium–gallium–selenide thin-film solar cell. Cu(In.. The usual substrate is soda lime glass. chemical deposition. (In fact. thicknesses.L. sputtering of the metals and post-treatments in selenium-containing gas (e.S)2 . The CdS is usually deposited using immersion in liquid baths to provide a very thin (about ˚ 50 A) layer [93].88].2 eV for usual cell compositions compared to 1. Modeling and analysis by Zunger [90] and by Noufi [91] have shown that the grain boundaries in the p-type CIGS do not affect the carrier conduction—and the polycrystalline semiconductor behaves almost a singlecrystal device for the minority-carrier transport. Sputtering and chemical vapor deposition has been used for the ZnO [94]. with an SEM micrograph of the various layers. with the Na having some benefit to the CIGS properties and the electro-optical characteristics of the device [87.L. The cross-section of the device is represented in Fig. 11. The device evolved into a alloy cousin. especially optimizing concentrations and modeling its effects. and this phenomena resulting from normal deposition underlies the high light-generated current densities—a remarkable property for a polycrystalline semiconductor having micron-sized grains. the emphasis was on the heterostructure CdS/CuInSe2 (CIS). Thus. 11. 12) [89]. technology not using soda lime glass incorporated the Na during the deposition process. Additionally.) The issue of Na remains a research topic. and electro-chemical techniques [92]. Initially. solar spectrum. 10. The first relates to the compositional grading within the CIGS (the relative ratios of the Cu:In. and compositions are ascribed Fig. which was first demonstrated with efficiencies at 12% for single crystals [85] and at 6% for thin films in the 1974–1976 timeframe [86]. the effects of the grain boundaries have always been of concern in these polycrystalline devices. in particular). providing both lightweight and flexibility for the “power roofing” applications [100]. and commercial products for battery charging for military and for recreational applications has efficiencies in the 8–10% range [99]. Recently. 12. 13.S.1% under 14.5%). One objective is to reduce the amount of In used—a material that has issues with availability (abundancy). The best commercial modules have reached 13% with 4 ft2 areas (Fig. The requirement is obviously to . Research centers on the effects of alloying (with materials like Ga and S). and recent rising prices. current research has focused on reducing the Fig. and the use of non-glass substrates. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 Fig.4% conversion efficiency) and (b) shell solar (12. The most successful of the non-glass approaches has been the use of flexible stainless steel. 13).5 m regime to less than 1 m [101]. Analytical study of effects of Cu and In gradients in the CIGS active explaining the resulting performance and optimization of these thin-film solar cells [89]. and Europe report 10–11. Certainly.3× concentration [96]. Current–voltage characteristics of headline commercial CIGSeS modules: (a) showa shell (13. with the best such cell ZnO/CIGS at 16. competition from the large and growing flat-panel display industry. a commercial module of this technology was verified with 10. Looking toward the materials demands for multi-megawatts of this technology.118 L. replacing the CdS window layers with Cd-free layers (including ZnS and ZnO. The best research cells have been validated at a remarkable 19. thickness of the absorber layer from the 2. with several headline cells in this family summarized in Table 3.5% efficiency [95].1% efficiency.9% conversion efficiency).L.5% average efficiencies from their manufacturing lines [97. This device technology has also provided the first better than 20% efficiency for a polycrystalline cell—at 21. and manufacturers in the U. the positive and perhaps unique factors that favor this thin-film technology are stability and large-area production potential—with performance characteristics for smaller area cells similar to the module performances.98]. 2 16. Second is a current one—that of the cost of the In itself (tracked as a bi-product of the zinc refining process)—a price that has risen over an order of magnitude in the past year.0 (3-stage) 1.9 21. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 119 a lower Voc . and an issue that has also caused concern in the flat-panel industry which utilizes this element in its transparent conducting indium–tin-oxide [107]. the throughputs of devices in the manufacturing lines can be enhanced greatly by the ability to use higher deposition rates for the absorber.1 12. The results of higherrate deposition indicate problems with loss in Voc —likely due to the outdiffusion of Gas being limited by the reaction rate of the binary selenides during the second processing stage [101].L.9%.0 23.6 30.5 8.7 9.999% or better)—maximizing the performances by minimizing the introduction of possibly detrimental impurities.7 Efficiency (%) 16. and with the texturing of the back surface to enhance the path length of the longer wavelength light. for example. CuGaS2 . cell efficiencies near 13% have been reached (12. There have been some initial successes (see device cross-sections in Fig.6 26.585V. deposited by coevaporation (12. especially if the designs of the active layer reach the less than 1 m that they are targeting (or if this much thinner cell eventually is successful as part of a multiple approach) [108].0 60. In. Voc : 0.652 0. and CuInS2 [102–106].3 75. FF: .3 Fill factor (%) 78. 14. Finally. These have some additional importance for new polycrystalline device directions—multi-junction solar cells (discussed later in this paper).607 0.706 with no antireflection coating) [101].4 74.1% efficiency in this study.75 m thick absorber. Preliminary cells using 99. In order to improve energy payback and to enable the use of flexible polymer substrates.75 (codep) 0.5 (codep) 0.565 Jsc (mA/cm2 ) 31.L. Scanning electron micrographs of CIGS absorber layers for: (a) 1 m thick absorber deposited by three-stage process (16. Several single-junction device results are cited in Table 4. attributed to recombination in the back contact region. but device efficiencies decrease with absorber thicknesses <1 m (see Table 4).0 (codep) 0. and Ga have led to efficiencies about 2 percentage points less than the high-purity material control samples [101].4 m absorber thickness.7% efficiency).699 0. For completeness. with periodic reports of research progress on CuGaSe2 . These cells have .654 0.2% efficiency). Finally.4 (3-stage) Voc (V) 0. Central to discussions of this technology relate to materials. there is also work at producing these thinner layers at lower temperatures (nearing 400 ◦ C). these research and current commercial devices use high-purity materials (99. Initial results at 450 ◦ C with a 1. a 1 m absorber deposited by coevaporation yielded a 16.5 m thick absorber deposited by coevaporation (8. 14a–c). The current collection is expected to improve with the use of back reflectors.1 reduce thickness without adversely affecting the cell performance. if hundreds of megawatts are to be deployed. First is the availability of the In. (c) 0. The results of doubling the rates from the current ∼0. Most of those working in the technology are convinced that indium supply is sufficient.1mA/cm2 . Work on other Cu-ternaries continues. Table 4 Absorber thickness-dependent parameters for best CIGS thin-film solar cells [101] Thickness ( m) 1. Recent studies for use of lower-purity sources for possible cost reduction have been initiated because very little is know about the affects of various impurities on ternary semiconductor and device properties.9% elemental sources of Cu. (b) 0.06 m/min show that efficiencies ∼20% less than control samples are produced. Jsc : 31.0 75.5% efficiency). Fig. 2 eV for higher oxygen contents [117]. with off-the-shelf commercial products in the 7–9% range. when considering hundreds of megawatts of production. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 Technology concerns and issues • Research (chemical paths to materials realization. ability to control the CdTe conductivity with oxygen and other extrinsic dopants. contacts. safety. 15. Some alternatives to this process have been investigated. suitable large-scale deposition techniques/conditions for “20%” quality devices). alloy composition optimization). Thus. The champion commercial module has reached 11% efficiency (Table 5) [119]. to contacting. A representative cross-section of the device is shown in Fig. Having a nearly ideal bandgap for a single-junction solar cell. and health effects of the Cd in this device. • Complexity (manufacturing costs. the transparent conducting oxides at the top surface of the cell. Cadmium telluride Since the 1960s. and misconceptions about the environmental. cost. In fact. but also with the resolve and confidence this industry has for and in its product. . process development and integration. and Europe. control). and electrodeposition [111]. higher Isc than preferred). Cross-sectional representation of cadmium telluride thin-film solar cell.120 L. surface alteration/passivation. 2. recycling of the materials and modules. but attention to new packaging techniques and processes have been successful in overcoming most of the problems. operation in hot and humid climates). Although there are concerns. The commercial segment is growing. alteration of shallow. • Manufacturing base (currently embryonic commercial and product base). The best confirmed research-cell efficiency is 16. especially of the In. chemical and heat treatments. A major attribute of this technology (contrasting other thin films) is that its manufacturing is not limited by the deposition of the active semiconductor layer. including electron micrograph of this region. screen printing/sintering. yields. role of sodium. more than doubling capacities in the next couple of years both in the U. This is spurred partially by market opportunities (world demand coupled with the Si supply slowing). and now the leading terrestrial product (with sales expected to reach 20 MW this year) [110]. • Devices (lower Voc . improvement in morphology. with a resulting bandgap between that of the CdS and up to ∼3. and/or deep electronic levels.L. • Materials (availability. with a corresponding SEM micrograph showing the various layers. then as the “next-in-line” among the polycrystalline thin films [109]. environmental concerns). extensive studies indicate that all safety issues can be handled with modest investments in cost. throughputs. and the packaging critical for long-term life of the module [120]. this has initially caused some concern for the product operating in outdoor conditions. transient effects. The beneficial effects (increases in efficiency 10–25%) of this process have been attributed to enhanced grain size. this “front end” deposition step in the module fabrication is accomplished in several minutes. window. Areas of concern for devices relate Fig. CdTe has been a candidate PV material—first for space.5% [118].2. • Elimination of CdS window (environmental acceptance. spraying. including an oxygen-alloyed CdS. cells (efficiency increased with light exposure). perceptions. and the formation of an interfacial CdSTe layer. including physical deposition. 15. contact stability. evolution of a p–I–n or heterojunction. and tracking of deployed product [121]. Inherent to most cell processing is a CdCl2 chemical treatment [113]. the CdTe film produced using close vaportransport.S. CdTe thin films lead all other technologies.5. a solid solution with 2–15% sulfur [115–116]. efficient CdTe cells have been fabricated by a variety of potentially scalable and low-cost processes. less complex processing). either liquid (in CdCl2 :methanol solutions [113]) or a currently preferred dry vapor process [114] (because of better process control and ability to be incorporated in the manufacturing line)—both treatments done at temperatures of ∼400 ◦ C for periods of 8–20 min. In fact. • Stability of modules (encapsulations. • Scale-up (deposition techniques. Efficiency (%) 22. contact degradation.3 8670 4874 6728 5432 1376 905. The economics of these approaches have been argued for decades—but it has been the leveraging of the multiple-junction III–V cell technologies for space applications that have brought renewed interest and investment into the terrestrial concentrator system. .1 Organization UNSW/Gochermann Sandia Texas Instr.7 10.6 1.6 73. 2. One means for improving both the PV efficiency.285 12. Sandia/UNSW/ENTECH Siemens Solar Univ. Single-junction GaAs cells have been measured at 28% at 1000× concentration [124]. chemical treatments. 1–2 m Si (5/95) GaInP/GaInAsGe(10× conc.9 W (4/00) 61. efficiencies to 27%.334 6.3 63. • Stability (Cu diffusion and oxidation of contacts.46 0.7 W (12/99) (2/99) a-Si:H/a-Si:H/a-SiGe:H (stabilized) (10/98) a-Si:H/a-Si:H/a-SiGe:H (stabilized). • Manufacturing base. • Substrates (use of sodium-containing glasses.949 0.0 25. 44.7 68.4 7.8 W (9/97) a-Si:H/a-Si:H (stabilized) (9/93) a-Si:H/a-Si:H (unstabilized) (12/92) PECVD on glass-submodule. cost of other substrates).9 16.4 Aperture area is used unless indicated [194].181 2.3 W (6/96) 52. 70.643 1.) at rates compatible with the active layer rates.91 0.3 W (3/99) Submodule. recycling of modules. GaInAsP.3 73.9 12.6 8. • Modules (encapsulation).3 78. etc. • Environmental concerns and perceptions (Cd.6 3.5 10.353 3. interface optimization. with concentrations up to 400×.07 3.L.L. existing control of product currently in some major markets).5 10.3.3 12.1 9. packaging especially for hot and humid climates).7 9.21 3.4 10. selected PV module efficiencies.83 9. Sweden Siemens Solar Siemens Solar Shell Solar BP-Solarex BP-Solarex BP-Solarex First Solar Matsushita Matsushita USSC USSC Fuji Sanyo Pacific Solar ENTECH Sandia Comments 121 Crystalline silicon 5.9 69.0 8. and significantly reducing the systems cost is the use of concentrators—lenses. such as GaAs. contacts.205 64. modeling. • Materials availability.0 Polycrystalline Si 25000 3.7 13.9 73.6 14.93 14.0 18.7 12.5 68.45 26. 4 cells (3/00) submodule Submodule 12 cells series (7/99) CIGSS (4/94) CIGSS (8/02) 91.Stuttgart Uppsala. role of oxygen). and related parameters Voc (mV) Jsc (mA/cm2 ) FF (%) 80.0 68.0 Amorphous Si:H 4. hi-power 53. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 Table 5 Summary of confirmed.42 2. contacts.) (3/93) 67.2 66.1 3459 8760 46.1 13. GaAlAs.2 27.173 31.3 20.625 CdTe PERL (9/96) HEM multicrystalline (10/94) Spheral Si (9/92) Crystalline Si concentrator module (80 suns) 12 cells CIGSS.9 34.1 71.33 0. and larger-scale modules at 20% using 25% commercial cells.653 63. Very-high efficiency and concentrator devices Higher-cost semiconductors.2 0.7 15. or other optics that focus the sunlight onto the collection area of the solar cell.26 III–V Technology concerns and issues • Research (process development. • Ability to process other device layers (windows.5 W: highest power thin-film module (5/00) Large-area. potential of environmental regulation limiting deployment in some countries. reducing the high-value converter area.72 20.) (5/00) GaAs/GaSb (57× conc. Cost is the overriding consideration for terrestrial applications in conventional flat-plate technologies.1 9276 1200 100 661 34 41. 15 cells (4/96) CIGS.3 10.5 10.057 7.1 Cu-ternary based 23.36 20.7 75. Concentrators have been used successfully with crystalline silicon technology [123].6 Area (cm2 ) 778 1017 3931 1875 3651 90 16.1 2.5 1. InSb. and InP (Table 6) have been receiving attention as PV converters because they have exceptional performance demonstrations that have the potential to convert more than a third of the sun’s terrestrial power into electricity [122]. reflectors. 00 1402 1000 2622 2910 13. Two major device configurations have evolved for the highefficiency terrestrial concentrators. Ge/GaAs/GaInP (the same configuration developed for space).6 4034 6. The space markets of the late 1980s and 1990s demanded high power to weight ratio—and the multiple-junction approaches were ideal [129]. inverted structure (3/05) GaAs [255 suns] GaInAsP [171 suns] Entech cover InP [pp suns] Entech cover GaAs/GaSb 4-terminal.5 876 28. The first is the lattice-matched device.8 21.2428 3089 3377 88.6 16.108 4.011 4. one-sun cells 2488 85.5 24.8 30. shown in Fig.L. The latter configuration realized AM0 cell efficiencies approaching 30% for the non-concentrator space arrays. monolithic [236 suns] (5/05) Single-junction.AlGaAs window (3/90) GaAs Cleft (4/90) GaAs Cleft 5mm submodule (4/90) MBE GaAs (3/89) InP (4/90) ITO/InP (8/88) Poly-GaAs on Ge (11/95) GaInP/GaAs (4/96) 2-terminal.3 813 27.7% GaInP/GaAs/Ge 2-terminal. The parameters for the record devices are summarized in Table 6.3 35. The use of these in concentrators provided some benefits: lowering the system’s cost by minimizing the area of the expensive cell (by 400–1000 times for concentrations of 400×–1000×). such as GaInAsN that can be the bottom cell—or a fourth junction in this configuration.91 4.63 0.1 23.3 32. lattice-mismatch.1 Spectrolab NREL Organization Kopin Kopin Kopin Spire/Purdue Spire NREL RIT Japan Energy NREL Comments GaAs. the top cell tuned to the red and the bottom cell to the blue). one-sun cells 1022 28.310 0. each tuned to a different portion of the solar spectrum (e. Research centers on developing a “1 eV” cell.and 3-junction cell technologies were developed.24 0.56 878 29.7 37.04) GaInP/GaAs/GaInAs 2-terminal.9 18. monolithic [50 suns] GaInP/GaAs 2-terminal. monolithic InP/GaInAs (8/90) 3-terminal tandem Kopin/Boeing Varian GaAs/CISx (11/98) AlGaAs: 1.00 16. These cells attain “beyond the conventional” efficiencies because they use multiple devices in the same area. also. monolithic [100 suns] (9/99). monolithic. monolithic [175.9 85. Various 2-.1 suns] (4/05).6 27. but Ge is not the ideal third junction.075 0. have been investigated over this last 20 years (see also Fig.103 27.4 Area (cm2 ) 3. under standard measurement and reporting conditions [194] Voc (mV) Jsc (mA/cm2 ) FF (%) 87.78 14.7 85.1 83. 17a.075 0.2 30.126 0.00 Efficiency (%) 25. lattice-mismatch.3 20.4 88.0 85.0 87.54 3.0 Multi-junction. .25 4.3 32.6 84. 16.0 23. These notable results rekindled interest in the terrestrial use of these devices – and led to forecasts of cells with efficiencies of 40% and systems in the 30–35% range – the realization of “a third of the sun’s energy” converted to consumer electricity. same efficiency (5/00) at 560 suns GaInP/GaAs/Ge 2-terminal.0 Boeing/Spectrolab The first laboratory demonstrations of greater than 30% efficiency terrestrial multiple-junction devices were actually reported at the beginning of the 1990s [125–128]. 17).6 81.0 27.2 suns] (3. monolithic (1/03) GaInP/GaAs/GaInAs 2-terminal. 2-terminal monolithic structures. 18a). shown in Fig.2 82. and 4-terminal configurations. 3-.2 32. both 3-junction.4 82.8 79.92 13.2 0. also. The champion cell is 39. inverted structure [10.989 0.8 17. This cell captures the blue and green portions of the spectrum effectively.3 26.37 12.122 L.53 0.9 337 21. monolithic.9 Spire NREL NREL NREL NREL NREL Spectrolab/NREL Boeing/Spectrolab NREL 3039 3120 2435 1392 88.g.312 4.0 31. AM0 efficiency of 29. Both 2.2691 39.9 5. Thus.02 0.237 4. The best terrestrial triple-junction monolithic cell has been confirmed at 39% under 360× concentration (Spectrolab/Boeing)—the highest efficiency attained to date for any solar-cell technology [131].6 31. and making use of the increase in efficiency of these devices at higher concentration with increased current densities without significant lowering of the other cell parameters [130].0 0.55 1018 27.93 eV GaAs (3/89) 3-terminal tandem GaInP/GaAs/Ge 2-terminal.97 994 23. the total device is more effective in utilizing the total frequencies of the sunlight that intercepts its surface.8 83.0 0.0% under 400× concentration (see IV characteristics in Fig. monolithic [180 suns] GaInP/GaAs/Ge 2-terminal.94 72..2 25.3 21.2 1011 27. selected III–V solar-cell efficiencies and related parameters. mechanial stack [100 suns] InP/GaInAs 3-terminal.9 18.22 86.2639 0.1 Concentrator cells 0. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 Table 6 Summary of confirmed. long term reliability. • Utility market (must compete with electricity costs 1/3–1/4 those of rooftop PV ¢/kWh prices. tracking and electronics. and the modeling predicts greater than 40% can be attained without a fourth junction. and (c) 4-terminal mechanically stacked configurations. question of antireflection coatings. Cross-sectional representations of multiple-junctions solar cells. complex layers that may add to the cost and processing complexity of the device. This is the PV researchers’ field of dreams. In addition. • Systems (concentrator hardware suitable for PV. presents some interesting possibilities [134. These concentrator technologies are primarily aimed at large. cost-effective structures. utility scale applications (for high solar insolation regions such as the southwestern U. maintenance. materials availability). capacities). processing of latticemismatched structure. cooling of the cell. PV technologies: the race toward the next generations Some of the possible contenders for the next PV generations have started their journeys in the laboratory. Of course. but for most—even to demonstrate their abilities to generate voltage and current for the very first time. The benefit to this approach is that the bottom cell as a higher bandgap than for the latticematched configuration.L. other problems may provide difficulties—such as the stress that can cause bowing and making the final processing and handling of the cell difficult. markets harder to penetrate). the previous lattice-match device presents some challenges for the developing an anti-reflection coating that can effectively cover the large spectral range. Certainly. 18b). However. and processing. the 3rd generations are in marathon struggle that must not only bring them to commercialization. cell engineering. Fig. 17b.135]. materials. Fig.L. (b) 3-terminal monolithic. and wild ideas that have lined up in the race to meet the performance and cost goals needed to deliver those 15–30 terawatts by mid-century. Technology concerns and issues • Cost (cells.) [136]. the “concentrator” has developed a new life—thanks to the investment in space technology and to the persistence of this R&D community for terrestrial solar power service. a number of organizations have been pursuing “rooptop” potential systems—that can certainly be projected for commercial buildings and perhaps for some residences as well in the future [137.138]. however.S. then the next. system development). Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 123 case. In any 2. Whereas the “2nd generations” might be competing in the analogue of the 100 m dash to surpass Si in the now to near-term. • Research (development of 1 eV “fourth junction” for lattice-matched cell. the 4-junction Ge/GaInAsN/GaAs/GaInP structure has been modeled and is projected to exceed the 40% goal for the concentrator technology. 2nd generation as the thin films. and 3rd generations as a host of evolving devices.4. upstarts. hardware. speed bump up in computer microprocessors—and we . “Roadmaps” predict significant markets for such utility-scale PV in the 2020–2025 timeframe. daily energy output or two-access tracker compared to other approaches). 16. Problems. The lattice mismatch approach. (This is something many of us have experienced awaiting the next. with incorporating the nitrogen into the GaInAs without accompanying intraband defect levels has limited the progress of this approach [130–133]. This technology that has always been dubbed the “application of the future” may have made its first viable footprints in the nearer term markets with high efficiency and high electricity value. It has not yet been realized and potentially requires new materials development and engineering of multiple. • Industry (primarily directed to space applications. Martin Green [139] brought attention to this future when he classified 1st generation PV as crystalline Si. This 3-junction lattice-mismatched device avoids this problem. and “CPV” is positioning to serve those. manufacturing. It has already demonstrated one of the highest efficiencies confirmed for a solar cell (37.9% shown in the IV characteristic of Fig. It is also the parking lot of nightmares for the near-term real business of photovoltaics—delaying or inhibiting the adoption of real and working technologies that will serve for the next 20–30 years in order to wait for one that might not have even been demonstrated to generate electricity yet but in theoretically promise performance beyond Olympic levels. showing (a) 2-terminal monolithic. high-efficiency solar cells: (a) lattice-matched design. thin. but these materials should not be showstoppers from either cost or availability in the future. 19) [140]. 2. may never purchase a computer!) There must be an understanding and patience—knowing that the investment in these research areas is important for both future technology ownership and for readying the next generation(s) of solar electricity for many generations of consumers to come.L. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 Fig. a dual-junction dye cell (building on the efficiency of spectrum capture pioneered by several other technologies) has been reported with a 15% efficiency [146]. The dye cell has been in development for a relatively long period compared to a number of the other next-generation approaches. The materials for the dye are expensive today because it is a specialty molecule. Most in the PV community associate the problems and issues of the dye cell primarily with the dye. However. Recently. the interface is that between the titanium dioxide and the dye.1. 20. In the case of Gr¨ tzel’s dye a cell shown in Fig.124 L. Apparently. the electron is conducted in the TiO2 (the same material that is used in paint and toothpaste) and the hole returns to the counter electrode through the electrolyte. The organic dye region is very thin—and the exceiton is quenched very rapidly by injecting the electron into the TiO2 particle. Cross-sections of triple-junction. After separation. 17. Fooling mother nature Biomimetics (or mimicking nature’s use of chlorophyll) was underlying the first nanotechnology approaches to evolve ultralow-cost technologies (see Fig.4. This is an advantage of this cell in that the diffusion of the hole through the thin dye does not limit the performance as much as it does for organic cells reported in the next section. Michael Gr¨ tzel reported new success in PV convera sion [141–145]—in which the sunlight creates bound electrons and holes – excitons – that travel as a unit and separate only after reaching some material boundary. The manufacturing is extremely low cost because there is no vacuum equipment . and (c) lattice semimismatched. ARC is the antireflection coating. There is some question on whether it is a tandem using two dye-sensitized cells (mechanical stack equivalent) or one that uses a dye cell in conjunction with a conventional solar cell (perhaps Si). (b) lattice-mismatched (metamorphoric). not much has been released on this cell and its structure. inverted structure. Using dye molecules. intellectual property issue has held off the release of this important information at the time of this writing and the report [146]. This device has reached 11% conversion efficiency in its simple form [145]. thin lattice semi-mismatched triple-junction monolithic concentrator cell [37. 18.4. there have been some encouraging tests completed that indicate that this can achieve reliability level competitive with other PV approaches. and those still in the 3–5% range.2. 19. PV program and those for longer-term (“breakthrough”) approaches. indicating electricity cost targets and expectations of current U.152]. but organic semiconductors remained of interest to the electronics industry because of radiation resistance (the cold war paranoia) and lightweight. R&D was largely abandoned during the 1980s because of drops in available funding for PV. and Shiraway in the late 1970s for organic electronics led to some early investigations for PV in that same period—notably using polyacetylene. Device representation of dye-sensitized solar cell. 20. . Just one word—plastics Organic photovoltaics also operate through excitonic processes. It remains a serious member of the 2nd generation approaches—with some prototype manufacturing underway [150]. involved. but have not yet provided the same performance levels as the original designs [147–149]. In general.S. Gels and solid dyes are being investigated. perylene. MacDiarmid.L. But the Fig.155]. This research comes with what seems like an infinite number of possibilities—ranging from small molecules (<104 molecular weight) to polymers (>106 molecular weight) [154. Despite all the leveraging R&D in the organic optoelectronics areas. 1). organic solar cells have again come to the limelight (actually. However. and phthocyanine[151.L. Fig. Current–voltage characteristics of the champion cells for the: (a) latticematched triple-junction Ge/GaAs/GaInP monolithic concentrator cell [39%] and (b) inverted. Pioneering work by Heeger. Pathways toward next-generation photovoltaics technologies for ultralow cost and ultra-high efficiency. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 125 Fig. Only a few technologies have validated performances (see Fig.9%]. Now with the quest for new PV technologies and the growing interest in organic light-emitting diodes. the full solar spectrum!). the liquid dye and a solid–liquid interface raise some questions about 30-year lifetimes. many worry about the long-term viability of this technology. 2. these organic cells are certainly still in their infancy. However. Demonstrating a 50% 4 or 5 junction cell in the laboratory may well be come within a decade or less—but translating such a complex device from the laboratory bench to the commercial market will entail a massive effort covering a spectrum of complexities from the scale-up of the growth through the engineering of the concentrator systems. Of course. With each cell added. Of the 3rd generations approaches. with the prediction of exceeding 25% efficiency for two cells stacks and modules in the 20–25% regime [159].868 0. The choice of the high bandgap top cell semiconductor dictates the use of a mechanical or monolithic design. current matching. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 scientific research community is growing in this technology area—bolstered by the work with organic LEDs and the promise of ultralow-cost technologies for materials and manufacturing. and (b) GaAsbased monolithic designs (with very-high efficiencies to 39% already confirmed for two different. and a single ARC. An infinite number of stacked cells may not be practical.L.and 3-junction stacks have been previously cited—namely. The dream of unraveling our electricity from “Saran-Wrap-like” rolls of plastic may someday become reality. Polycrystalline tandems. Successes with 2. with some initial demonstrations of device feasibility. A summary of progress with these technologies. However. more materials are required (antireflection coatings.4. and testing. thin-film polycrystalline technologies are currently being developed. light absorption/transmission. many technical barriers need to be surmounted. Using more of the sun. With the time compressions experienced for technological advancement in the last five decades and the growing leveraging of technical creativity within the vast opto-electronics arena. current matching becomes critical. The monolithic approach has one grid or contact on each side. 3-junction approaches).126 L. etc.868 0. 2. interdiffusion. is provided in Table 8 [160].4. the complexity and number of problems increases with thermal expansion differences. (a) a-Si:H-based thin films (leading to moderate 13% laboratory cells for 3 junctions). one TCO. Carnot efficiency and calculations by Landsberg and Schockly and Queisser are included [132] Ideal Converter (at T = 300 K and Ts = 6000 K) with isotropic illumination) Carnot Landsberg Multiple-junction Impact ionization (highest Q factor) Solar thermal Hot electrons Thermophotovoltaic Thermophotonic Intermediate band (multi-quantum dot. inter-cell contacting and losses.632 0. Several polycrystalline thin-film tandem structures are being developed. 2.2. the champion cell to date with an efficiency of 16. Organic PV has a great deal to overcome before becoming part of the PV energy portfolio – but cost. still in their developmental stages. the organic materials can be chemically controlled to respond to different wavelengths—making multiple-junction approaches a potential pathway to increased efficiencies [156]. The mechanical stack (Fig.854 0. 21. Besides the tandem and triple-junction a-Si:H cells and modules. minimizing expensive real estate Multiple-junction solar cells are among the few very-high efficiency approaches that have been realized and commercialized. but concepts of 4–6 junctions are in the research stage. each tuned to a specific energy of the solar spectrum.950 0. substrates) in the structure. Multi-multiple junctions.3.632 0. close Fig.403 lattice-induced stresses.3. Not easy.854 0. Higher temperature operation may also provide limitations for some organics. But many questions need answers.3. Multiple-junctions are not the sole property of the “III–V” semiconductors.4. transparent conducting oxides. there are concerns and issues.854 0. and major R&D efforts need to be successful before these innovations make it to their major commercial phase. but also not beyond the ingenuity and persistence of the PV technical community. Although initial evidence indicates cells have better performances under concentrated light [156]—the combined effects of UV and thermal energy may limit this approach. 2. More positively. has a limiting efficiency for an infinite number of such junctions at about 87% (see Table 7) [158].1. defect generation. the 50% multiple-junction cell may not be that far off if sufficient resources are sustained for these approaches. 21. High-performance polycrystalline mechanically stacked tandem cell (CdTe–CIGS) with IV data for this concept that has reached 15. However. this one may really be a closer 2nd generation contender. alloy) Second photon pumped multiple-quantum well Shockley and Queisser Efficiency 0.3% conversion efficiency.854 0. The use of a number of junctions. . Table 7 Calculations of maximum efficiencies for various next-generation technologies.933 0. Organic materials are notoriously sensitive to ultraviolet radiation that can change bonding and lead to instabilities. manufacturability and performance potentials – and the leveraging from other electronic technologies – help maintain a serious interest in these approaches.5% [161]) has advantages of independent cell fabrication with minimization of cross contamination. 8 having corresponding bandaps 1. (5) MCuSnQ4 (with M = Ba. 22. organic solar cells. These cells have lower than expected Voc s.77 eV direct bandgap.1. This area is of immense technical interest—high-risk.L.5 eV [163]. These lie in the range of 15–25% currently. Because the active cells are tuned to a very specific wavelength (responding. leaving considerable room for improvement. and is considered a possible couple with CdTe. and characterization. Examples for this wider bandgap device include: (1) CuGaSe2 (CGSe). materials.. with the swell in R&D coming in the 1990s. and the semiconductor requires minimum sub-bandgap absorption.5–2. for example. Se) have been shown to have appropriate bandgaps [166].Ga)(Se. the surface of the CGS is modified. The technology awaited the development of suitable low-bandgap semiconductors—and the ability to process them. alternatives to the conventional nearing or at the outer fringes of science and engineering that might provide breakthroughs.668) have been realized. Of course. High-efficiency multiple-junctions. The first for “solar” is a technology that works.68 eV [162]. For higher 1. but feasible structures with efficiencies in excess of 20% are a challenge.707).5). and others were all part of the intellectual domain of the solar-cell research community in the 1950s though the 1970s. FF: 0. significant progress leaps. 2.5–1. a tunnel junction). cells with efficiencies to 11. TPV has made it to the niche consumer commercial markets [168. with a bandgap of 1. and the compound BaCuSnSe2 S2 has been measured with a 1.4% have been validated. Fig. (2) Cu(In. and the less-than-flat topography complicates the growth of sequential layers. Jsc : 17.33 has been 10.4. from the emission from a selective emitter or reflector). but has applications utilizing the infrared (primarily) portions of the spectrum and not currently a major consideration for terrestrial PV. these cells are in the bandgap range of 1. For compositions with x ∼ 0.9 eV.S)2 with a bandap of 1. Initial performance results are encouraging. Conceptual representations for: (a) thermophotovoltaic and (b) thermophotonic devices. the complexity of these semiconductors raises concerns about compositional control and stability—and they require considerable more investigation to determine their suitability. but potentially high payoff with the dual promise of high performance and low cost. Like other thin-film technologies. Typically.53 eV). 22a) has been under consideration for some time with origins back to the 1960s [167]. nanotechnologies. topographically. efficiencies to 8. Impatience (especially among funding sources!) sometimes shut the door too early on the ingenuity of scientists and inventors—but fortunately. stability and lifetime remain issues that have to be addressed as well. Q = S. and devices with efficiencies up to 10. system efficiencies are usually considered the benchmark of this technology. “Inventors” were usually far ahead of the ability of hardware and technology to prove the concepts—and remained part of the valued literature awaiting to be realized with the evolution of processing. (4) BaCuTeF.g.9 mA/cm2 . Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 127 tolerances for the connecting junction (e. The far side PV science and technology have always included higher risk approaches in its R&D portfolio.4. Jsc : 18.4. Thermophotovoltaics.2% (Voc : 0.3 eV [164]. FF: 0.1 eV [165].05 (Eg = 1. (3) Cd1−x Znx Te with 0 ≤ x ≤ 0. good ideas linger and wait their turn as that next possible “breakthrough”. but could exceed 40% for applications such as the generation of electricity from “power sources” aboard nuclear submarines or converting the waste heat from automobile exhausts or from the float-glass process that brings us mega .80.4.1% (Voc : 0.L.61 mA/cm2 . deposited via pulsed laser deposition has a bandgap of 2.8 eV bandgaps (x ∼ 0.51 and S/(Se + S) = 0. is used.169]—but has some wider interest in military ones. include concepts or relatives of those that many consider to be at least a bit risky if not approaching the radical fringe right now! 2. What we will be using in solar PV at the end of this century will be considerably different that what we have now—and will also likely Fig. Sr. The best bell with element ratios of Ga/(In + Ga) = 0. These structures are complex—physically. or even new technologies. Thermophotovoltaics (TPV—the conversion of “thermal” wavelengths into electricity. thin-films.823 V. For this application. Much of the R&D has focused on the top cell—because transmission through this structure and the performance of this device is critical [160]. and electro-optically.9% have been fabricated. Other concepts are just that. Nanotechnology approaches. The structure in Fig. no matter how much energy the photon has. Thus. which has the a potential thermodynamic conversion efficiency up to 66% by utilizing hot photogenerated carries to produce both higher photocurrents and voltages. Green et al. Then the rate of impact ionization could become competitive with the rate of carrier cooling. Related is thermophotonics (Fig. 24a uses the quantum dots sandwiched in an intrinsic portion between two PV cells [180]. but not realized yet in the laboratory. There have also been research and prototypes combining burning of bio-products (e. When the carriers in the semiconductor are confined by potential barriers to regions of space that are comparable or smaller than their deBroglie wavelength (or to the Bohr radius of excitons in the semiconductor bulk).g. R&D on the cells has probably kept in front of the critical needs for the emitters and reflectors in the thermophotovoltaic system. These are artificial semiconductors using one-dimensional quantum confinement to extend the bandgap. Several configurations have been proposed for these devices—including imbedding the quantum dots in polymers and other semiconductors. Related are intermediate bandgap solar cells [179]. and demonstration stages. This includes several nanotechnology and hot-electron approaches such as the quantum dot cell [171] and relatives using quantum rods and quantum pods [172]. with no full operation of cells demonstrated though they are theoretically possible. As a result. wood powder) to activate thermal emitters in the 1500 ◦ C region that would match the responses of several semiconductor technologies near 0. It is speculated that a 25%-efficient device could be ready by 2020. The high-energy carrier may be produced using an electric field. 2. concept. efficiencies are about 3/4 those for quantum-dot structures (Table 7).L. Photonic impact ionization using the photons available from sunlight is very inefficient in bulk semiconductor materials.. The two diodes are thermally isolated but optically coupled. The efficiency can approach the Carnot limit for conversion between the temperatures of the warmer and cooler device (Table 7). two or three excitons generated per incident photon). These thermophotonic devices have been modeled. • Industry (needs firm proof that these are significantly “better”—cost and performance. 24b) form the intermediate band of discrete states that allow sub-bandgap photon energies to be absorbed. Thermophotonics. 22b).4. . The problem of heat recovery from low temperature gradients (such as waste heat) is a limiting factor in system performance and economics. quantum-dot solar cells are still in their theoretical.4. The critical point is to avoid the thermalization of the carriers (excitons) [174]. Several groups have been investigating structures [183–186].3. transfers enough of its energy to bound carriers to free them. • Very high risk (though very high potential payoff). or from a high-energy photon. The electronic properties are controlled to a large extent on the size of the nano-particles themselves (Fig. for all intents and purposes a bulk semiconductor solar cell produces only one electron–hole pair per photon. It is the system performance (efficiency) that is critical. 23) [173].169]. as is done in many electronic devices. the hot-carrier collection rates may be significantly reduced. The extracted carriers are limited by the bandgap.2. Any excess energy carried by the photon is wasted as heat. However. The production of multiple carriers from a single high-energy carrier is a well-known process called impact ionization [175]. heat supplied to the warmer diode is converted to power in an electrical load that is connected between the two devices. government funding sources are typically shortsighted). The lower temperature one is maintained at ambient temperature. [183] have proposed some interesting approaches using Si and high temperature processing integrated with low-temperature quantum dot cells.e.5 eV [170]. These are significant research results—the first stages possibly leading to this new regime of ultra-high efficiency devices. Theoretically. by creating a single high-energy electron one may ultimately produce two or more electrons with low to moderate energy levels. A key to the efficiency increase is multiple-exciton generation (i. 2. These issues have been discussed in recent TPV conferences and in reviews of the technology [168. which uses a diode that emits a photon with energy above the bandgap (the radiator is an electroluminescent device such as a light-emitting diode) [172].4. so much so as to be virtually unnoticeable. These quantum dots (Fig. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 square miles of windows each year [169]. The control of the bandgap has been demonstrated. as happens in solar cells. Technology concerns and issues • Research concepts and technology (require research creativity. These structures can also used quantum dots of binary or ternary semiconductors to create and intermediate band that allows the harvesting of a much larger portion of the available solar spectrum. When both diodes operate at the radiative limit.4. patience and funding to prove or develop. but no devices using this approach have yet been validated. it has to be emphasized that the photovoltaic effect in any of these quantum-dot structures has still not been demonstrated. Multiple carriers are produced when an energetic carrier. The energy states of the quantum dots forming the intermediate bandgap can be controlled by the size of the dot. and it may be some time before the research community produces a true proof of concept. capacity or experience curve penetration needs to be accelerated). and it is anticipated that a device-proof of concept will be realized by the end of the decade. including the lifetime of the components outside the solid-state TPV converter itself.128 L. • Still small research efforts that need growth. Quantum dot cells take advantage of quantum size effects.. The heat is supplied to one diode to maintain it at a higher temperature than the other. such as an electron. This multiple-exciton production has now been demonstrated by several research groups (using quantum dots of Se. PbS) [176]. most expensive part of the PV system. as for crystalline Si. the module usually represents the first-level selling component of the PV system [187]. as for thin films) to deliver a desired voltage and current—and encapsulated into a supporting structure for environmental protection and strength.) It is the configuration of cells. (It is currently also the single. and cost of the PV technology. Micrographs of quantum-dot structures used in “quantum dot”. 3. or process-integrated.L. This package is the first line of defense in protecting the cells and the circuit . Module technologies In the business of photovoltaics. 23. lifetime. electrically connected into series and parallel strings (either separately. That is also the rea- son that worldwide R&D is centered on it because it represents the biggest and nearest potential in lowering the total system cost for the consumer [4–6]. A summary of the headline modules reported to date are presented in Table 5. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 129 Fig.L. • “Over-promising” inhibits adoption of current technologies (“waiting for solar godoh”). representing between 50 and 60% of the cost. “intermediate band” and other nanotechnology-based solar cells. The module construction determines not only its cost. Therefore. but also its reliability. The dimensions of these nanostructure provide the ability to tune the device to the incoming radiation. module issues are important because them directly impact the performance. The current standard for measuring the power of a module at a 25 ◦ C temperature provides a standard of comparison—but does not meet some requirements for installations which commonly operate at 40. materials science. ingress issues are considerably different—more critical to component lifetime and durability. it was only considered as a mechanical support for the expensive and high-tech converters that do the “real work” of photovoltaics. electromigration. contact openings. 50 ◦ C.as well as macro-area design to meet the demands of operation in varying ambient conditions. photovoltaics as a technology and a business is composed a complex network of co-dependent and intimately related tipping points [1]. However.188]. The module is more: it is a composite optical and electrical and mechanical structure that has required the collaborating and overlapping knowledge of physics.) are important for installation designs and use. Thin films have extremely high surface-to-volume ratios than the bulk technologies. These have shown technology worth. and the mistake was made at first just to use these for the developing thin films. varying electrical fields. 24. redesign. The most developed module concepts are for crystalline Si. First. Summary and future Currently. many of these are currently being re-evaluated to determine if these reporting criteria meet the needs of the installers and consumers to ensure operation in the real world. or even above 70 ◦ C. and reconfigure with the progress of the various cell approaches already discussed [189]. and engineering to ensure its viability. One that provides the integrity for ensuring performance for 30 years or more! R&D is as important here—especially for the emerging technologies which have more complexities and sensitivities than imposed by current crystalline Si approaches. Much was learned from the a-Si:H experience [190–192].130 L. Policy is important. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 mate interfaces among metals. and semiconductors. But. water vapor. Energy ratings (annual kWh. as well as economic and employment value. International standards for testing modules and reporting performance are in place [194]. Technologies that incorporate liquids (dye cells) and those that operate at higher temperatures (concentrators) have added orders of difficulty in packaging design and integrity. chemistry. and large surface areas that require micro. energy transfer locations. Several mis-steps have impeded module development in the past. Fig. First. growing the bonfires that have been lit in Japan and Germany.L. solar PV needs attention to government policy and consumer awareness and acceptance to take it to its next levels—those pushes that will make it “spread like wildfire” in markets around the world. but the wildfire needs additional and new fuels to make it endure. insulators. it is a real business that has reached $10B levels: it is clearly the fastest growing electricity source over the past year and past five years. solar PV has to tip to its next stages of technology development—this the need for R&D to improve now and near technologies in crystalline structures for the 25–30 year lifetimes that are now expected of this PV technology [11. Thus. Different technologies present different challenges. Intermediate band cell: (a) band diagram and (b) device cross-section. Second. These components have numerous inti- . other gases). This remains a concern for those working in the standards area—as well as for the installers and users of this technology. delamination. They also use materials that have different sensitivities to the ambient (such as water. and microdefects that affect macroscale electrical behavior [193]. which was a radical change to existing module approaches—presenting areas for shunting (bridges. It is important to understand that “packaging” is a major (perhaps monumental!) step to take following the delivery of an efficient solar device to the production of this first-line commercial product. Second. etc. one that has to adapt. More so are the ambient parameters—such as the available solar resource and temperature. it is an evolving area of photovoltaic technology and industry development. kWh/kW. interdiffusion. 4. Schmid. DC (www. Caesar. [23] Crystalline Silicon Initiative. California. G. J. Keller. It has the potential to grow as an energy resource 50 times more. 3.R.H. S. [16] M. Wenham. Brown and Company. Lautenschlager. P. Carlson. Warrenton.. SEIA. Bedford. Schubert. S. Department of Energy under Contract No. DC.A. in: V. www. pp. Wang. Acciarri.). A. Electrochem. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 131 Si and thin films—and to develop the next generations that will fuel the wildfire of business and deployment. 24. Phys. Strategies Unlimited. UK. Feerrazza. D. Proceedings of the European Photovoltaics Solar Energy Conference-Paris. Lett. Gluntz. A. Szpitalak. Yoshime.W. April 2005. Solar Cells: Operating Principles.edu/esd reports/ summer2005/solar power. 2004.M. New York.K. IEEE. McConnell. Proceedings of the 24th IEEE Photovoltaic Specialists Conference. [19] C.C. D. S. Report PM-57. UK. M. 1351–1355. H.S. [5] EPIA Roadmap: www. Swanson. Photovoltaic Five-Year Market Forecast 2005–2010. 1996. IEEE.L. 387–390. Swanson.L. 248–251. Taguchi. Wenham. This paper represents primarily the thoughts. Technology and System Applications. pp. [29] M. W. Rose. B. 44 (1983) 425. Jiqun.. NJ. G.html. A.W. Willison. Sidendsticker.M. CA. Sandstrom.B. 1993. See also. pp. K. pp. 604–607. Zhao. R. Reidel Publ. the resources. WIP. pp. Denver. New York. pp.do?categoryld=3050557&contentld =3060005. A. Kiyama. [12] R. F. Appl. for example T.L. Ciszek. S. 386–389. Appl. Mountain View. Proceedings of the DOE Solar Program Review. Mountain View. 1986. Photon International: The Photovoltaic Magazine. F. 2005. 1984 (present). 2005. no. Proceedings of the European Photovoltaic Solar Energy Conferences.R. M. Schneiderl¨ chner. 1988. [26] R. Cryst. pp. Munich. 2004–2005. [6] PV Roadmap 2030: www. NJ. Kluwer Publ. IEEE. 2004.P. Manufacturer Shipments 2004/2005. Green. Denver.org). 62 (1987) 243.S. Swanson. who help in reviewing this material. S. Rohatgi (Eds.go. CA. Chapin. DOE Solar Energy Technologies Review Meeting. Aachen. Kalejs. [18] M. [25] R. [38] O. Proceedings of the 18th IEEE Photovoltaic Specialists Conference. D. 1336–1340. C. Soc. pp. 25 (1954) 676. [11] Our Solar Power Future. Margadonna. E.. Special thanks goes to Keith Emery of NREL for his counsel and sharing his warehouse of knowledge on PV performance and characterization.R. New York. October 2004 (this is unpublished and not available in the proceedings from this meeting.J. Acknowledgements The author expresses sincere gratitude and appreciation to colleagues with the National Center for Photovoltaics at the National Renewable Energy Laboratory. New York.. Growth 66 (1984) 655. [31] T. Colorado. the creativity and innovation. L. IEEE. Rogol. IEEE. and the belief—and solar photovoltaics will be significant in our clean-energy future. Inc. New York. Goldschmidt. Khattak. 1995.P. Germany.M. Photovoltaics has advanced incredibly from its Bell Laboratories beginnings in 1954—the next decade will likely produce 50 times more technically than that first half century. [30] See.jp/englishy/archives/161027. Warabissako. Proceedings of the 25th IEEE Photovoltaic Specialists Conference. Photovoltaics Industry Roadmap Through 2030 and Beyond. 513–517. [24] R. Proceedings of the 15th IEEE Photovoltaic Specialists Conference. pp. SEIA. Growth 104 (1990) 14.H.bp. Hanoka. M. Green. 1995. This investment in R&D is essential to bringing down costs. IEEE. 2000.R. A. Tanhaka. Mulligan. see “Solar Power Through 2015” posted on http://esd. Willison. pp.S. Zhang. “Investing in Solar Power: Hot Spots and Shadows”. E. M. Proceedings of the 13th European Solar Photovoltaic Energy Conference. Aachen.S. [4] The U.R. June 2004. H. Narayanan. Pizzini. [37] R. Proceedings of the 23rd IEEE Photovoltaic Specialists Conference. Proceedings of the IEEE Photovoltaic Specialists Conferences. H. [13] M.P. Leimenstolt. Also I. E.com). 188. 1982. Altmatt. [22] H. 1977 (present). and WIP. Moutain View.mit. [32] M. Report PM-54. [21] R. March 2005. Blakers. CO. James and James. The U. Lett. Phys. [3] Strategies Unlimited. Presented at the U.htm. C. [15] W. The Tipping Point.K. James and James Ltd. G. H. However.D. [33] C. We have to provide the technical expertise.edu). J. Sun Screen: Investment Opportunities in Solar Power. Proceedings of the Photovoltaic Solar Energy Conferences. Luque. pp. Eguren. insights.com/genericarticle. 2005. [8] M. 39–42. Taouk. D. Swanson. Szpitalak. J. S. See. Proceedings of the 20th European PV Solar Energy Conference-Barcelona. D. Global Analysis of PV Markets and Application Forecast. 1343–1348.A.J. Dube. 529–531. and observations of the author. J. A. A. A. M. Swanson. Gladwell. New York. 1667–1770. in press. [17] M. Maycock. Honsberg.seia. Schmela. Cuevas. 1981. Proceedings of the o 19th European Photovoltaic Solar Energy Conference-Paris. Proceedings of the 17th IEEE Photovoltaic Specialists Conference. Wenham. [35] J. IEEE. Schultz.A.html.L. based upon his some 40 years in PV R&D. VA. Germany. See. Growth 39 (1977) 17. in press. Photon International: The Photovoltaic Magazine. 2005. S. R. Appl. Phys. Jester. .B.D. Blakers.P. 1078–1082. pp. Proceedings of the 19th European Photovoltaics Solar Energy Conference Paris. S. Fuller. Cryst. Kapur. PV Energy Systems. New York. Wenham. Stephens & Assoc. Solar Verlag GmbH. as well as to ensuring our next generations of consumers have technologies ready to meet the mounting demands for energy in this 21st century. Terakawa. [20] A. Solar Verlag GmbH. Washington. DE-AC36-98-GO10337.. References [1] M. S.P. pp. IEEE. See. A. May 2005. This paper was prepared partially through the support of the U. Englewood Cliffs.G. 24–35. Denver. Schmela. M. Photovoltaics for the 21st Century. Proceedings of the IEEE Photovoltaic Specialists Conference-Orlando. S. Savigni. R. [14] M.seia. Willeke.S. Green.. Cudzinovic.S.org. Y. Chong. Solar Energy Mater. [28] See. [9] Strategies Unlimited. Rogol. October 2004. Strategies Unlimited. Birkmire. but it is available directly from the presenter: michaelr@mit. Sakata.W. Also T. 1960 (present).nedo. New York. IEEE. 274–399. Proceedings of the 31st IEEE Photovoltaic Specialists Conference-Orlando. New York. Binetti. Lee. Luque. [10] Strategies Unlimited. Also. T. 1984. Washington. pp. McIntosh. Kazmerski.U. J. 866– 871. [27] R. New York. 2000. New York. Little. IEEE.W.R. Golden. WIP. Godfrey. Green.org/05Publications/EPIAPublications. UK. pp. Cryst. M. Sproul. 2004. Proceedings of the 31st IEEE PVSC-Orlando. pp. J. PV News. 2005.I. Also see Ref. Green. New York. March 2005. this is one of true “intelligent design”. [34] J. vol. Munich.epia. del Alamo. De Decuester. [7] P. Ebong. 2005. F. IEEE.R.M. James and James. Solar Cells 17 (1986) 85. C. Hanoka. (available at http://photon-magazine. [2] D. Prentice Hall. Pearson.A. Swanson. M. NREL. Maruyama. A. T. [36] C. Proceedings of the 20th IEEE Photovoltaic Specialists Conference. PV Industry Roadmap Through 2030 and Beyond: www. Strategies Unlimited. S. 66–82. Solar Cells 65 (2001) 231. Y. Sproul. Benagli. Fang. pp.com/downloads. IEEE. Gazuz. 1995. Evertt. Proceedings of the 28th IEEE Photovoltaic Specialists Conference-Anchorage. Proceedings of the 21st IEEE Photovoltaic Specialists Conference. F. 883–888. L. A. Ju.A. B. S.A. Doshi. Y. 2005. S. A. 2005. Y. J.B. G. Ganguly.R. Braunsschweig.A. J. Poortmans. IEEE.htm. Bennett. P. pp. New York. R. [80] www. Yang. R. Bailat. Meier. U. [55] V. 646–652. Wenham. M.earthlsci. pp. J. James and James.S. Sohlegemuth. [77] A.B. S. IEEE. in press.K. 2005. G. Anna Selvan. G. Philos. C. IEEE. Leies. 9967–9970. S. 2003.). [69] www. Focsa. Yang. Proceedings of the IEEE Photovoltaics Specialist Conference-Anchorage.J. Wronski. C. 1988. Narasimha. Goetz. u u B¨ chel. Barroux.S. 465–468. H. in press. pp. D. pp. Steinhauser. Branz. pp. Li. M. F. Widenborg. J. Fischer. CFreager. pp.S.W. M. 32 (2003) 1034. p. J. Narayanan. Bailat. Dubail. Munich. Ellert. Fay. H. [39] N. Munich. Weber.R. S. Basore. K. Leib. R. 12–16. Carlson. Vezzetti. Phys. IEEE.A. 1127–1130. 33 (1976) 935.). J. Reedy. A. New York. New York. [47] D. Rev. [81] P. A. 1993. G. R. Proceedings of the 19th European Photovoltaics Solar Energy Conference-Paris. F.E.J. Haas. Schillinger.I. Meier. A. Proceedings of the IEEE Photovoltaic Specialists Conference-Orlando. S. pp. G. C. Yashiki. M. Proceedings of the 20th European Photovoltaic Solar Energy Conference-Barcelona. M. NY. Lin. pp.E. [75] A. Chen. Aberle. K. Benett. I. Hamakawa. IEEE. Widenborg.E. C. S. T. IEEE.A. New York. 93 (2003) 121. Va. W.R. J. Field. D. D. [71] T. Verlinden. Pfanner. Neuendorf. [59] D. S. P. Michalski. R. Rajan. J. D. H. IEEE. Shah. Appl.go. E. 96 (1954) 533. Schropp. M. 1. 1996. vol.L. Wenham. 2005. L. P. I. vol. WIP. P. Phys. Solar Cells. Wronski. [50] H. [61] D. Proceedings of the 31st IEEE Photovoltaic Specialists ConferenceOrlando.I. Fisherman. V. Schindler. C. Electron. A. 1346–1351. Solar Energy 77 (2004) 857. Ch. Guha.V. pp. Hayashi. Munich. UK. Jones. [58] Y. Crandall. Kampas.oksolar. J. 9. Noack. Also A. S. Appl.html. Proceedings of the 31st IEEE Photovoltaic Specialists Conference-Orlando. New York.-M. Yoshimi. Proceedings of the 20th European Photovoltaics Solar Energy Conference-Barcelona. Stephens & Assoc. pp. M.E. 1990.I. N.-M. Kaleijs. S. A. Brendel. Nakata. C. Auer. Ghosh. R.L. L.S.aist. New York. Nakajima. 1448. 1194–1197.com. Rohatgi.W. pp. A.A. Zhang. Yah. Munich. H.kaneka. Vieweg. A. 2005. A. Non-Cryst. R.L. 1353–1358. Guillet. R. J. J. Eyer. 304–311. Deenapanray. K. 2005. [79] P. Dalal. 34. . K.H.pdf. Guha. Sproul. X. N. Thorp. Green. A. J. Proceedings of the IEEE Photovoltaics Specialists Conference. Everett. Device Lett. Feitknecht. [62] D. J. New York. Sawada. pp. UK. T. Y. Balberg. Martin-Brune. pp.B. T. Appl. Bachmann. See J. B59 (1999) 5498. Slaoui.P. IEEE. New York. 2005. IEEE. T. Carapella. Wyrsch. [78] A. Kunz. J. [49] D. Also. Straub. A. O.R. Rev. Chu. [40] A. in press. B. M. Sopori. pp. [73] M.F. K. Mag.I. [46] D. New York. G. [65] M. U. Yoo.G. S. Platz. [52] M. 935–938. D. Ovshinsky. Lett. 707–712. Adv.S. T. 2005. A. Campbell. Stocks. H¨ ghi. Shah. Green. J. 2005. [45] K. [44] See. New York. Willing. Bedford. R. [66] J.J. Li. Green. WIP. Begs. L. A. Amin. 28 (1976) 671. Proceedings of the 31st IEEE Photovoltaic Specialists ConferenceOrlando. S.com/solar panels/evergreen.L. P. [76] B.A. IEEE. Proceedings of the IEEE Photovoltaic Specialists Conference-Orlando. J. van Veen. IEEE. AIP. J. T. IEEE. B¨ chel. pp. A. Sproul.A. Helbig (Ed. IEEE. Shah. Miyamjima. Yue. A. A. Kroll. Emery. 1039–1042.E. J. Y. Q. Mahan. Weber. [41] B. [63] A. Cambridge.csgsolar. C. Xu. Zolper.html. B.L. Non-Cryst. Stieler.L. G. Malley. WIP. Ju. Tonon. Solff. Electron. P. K. 1541–1545. New York.R. Newton.H. Pilaton Vaucher.. pp. [56] J. Sopori. S. Arya. Morris. [42] C. IEEE.H. ED-24 (1977) 438. 1996. Proceedings of the 31st IEEE Photovoltaic Specialists Conference-Orlando. Chong.Y. 31 (1977) 292. Phys. r. Yan. K.E. Domin´ . A. Vallat-Sauvain. pp. pp. T. M. Campbell. Kroll. James and James.com/html/Technical%20Papers/pdf/PVSC00MCCellPaper.B. T. 69 (1991) 6728.html. Suzuki. A. Blakers. New York. S. Mueller. pp. pp. 473–480. S. [57] J. L. Electron Dev. Bergmann. [72] J. 1053–1056. M. Staebler. 2000. Reynolds.M. Roschek. Anna Selvan. M. 13–16. www. Trommer. 1019–1022.-F. G. 50 (1979) 3454. Terry. A. Proceedings of the 31st IEEE Photovoltaic Specialists Conference-Orlando. [54] A. Proceedings of the 31st IEEE Photovoltaic Specialists Conference-Orlando. 991–994.E. Geng. R. Proceedings of the 3rd World Conference on Photovoltaic Solar Energy Conversion-Osaka. WIP. Lett. Hurrle. Antes. Jansen. 1993. [64] J.-D. Ballif. Delahoy. Zheng. New York. 25 (1974) 283. New York. Aberle. Clark. Basore. 1995. 1328–1333. Al-Jassim. [70] K. L.org/energy/solar/page3html. Proceedings of the 31st IEEE Photovoltaic Specialists Conference-Orlando. Proceedings of the 19th Euorpean Photovoltaic Solar Energy Conference-Paris. Appl. Ch. Reinecke. A. Solar Energy 77 (2004) 865. pp. J. V. Proceedings of the 19th European Photovoltaics Solar Energy Conference-Paris. www. M. Johnson. Photovolaics Program Review. A. S. IEEE.. Phys. Marburger. K. T. 850–856. IEEE. pp.K. Shah. pp. Androutsopoulos. New York. B. Huang. Klein. Yamamoto.S. Phys. Inns. p. Proceedings of the Eighth European Photovoltaic Solar Energy Conference. pp. T. UK. Carlson. Munich. Beaucarne. in Solid State Phys. Watahiki.A.A. N. Koval. Solids 2 (1970) 99. Spitznagel. Carlson. [51] A. Kluwer Acad Publ. pp. Phys. Finger.J. Janz. R. New York. Keppner. P. Y. 115–125. Kritikson. Green. IEEE Trans. D. Phys. K. Weber. Proceedings of the 20th European Photovoltaics Solar Energy Conference-Barcelona. F. Lett. 2000. Bathey. Proe ceedings of the 20th European Photovoltaic Solar Energy ConferenceBarcelona. Also. Proceedings of the 19th European Photovoltaics Solar u Energy Conference-Paris.J. J. www. Proceedings of the 25th IEEE Photovoltaic Specialists Conference. Babaei. in press. Pola. Ballif. S. pp.132 L. Zhao. Proceedings of the 13th European Photovoltaics Solar Energy Conference. Ufert. 2005. Rech. Yun. New York. Wang. B. M. Reber.E. A. pp.R. New York. Proceedings of the 23rd IEEE Photovoltaic Specialists Conference PC IEEE. Spear. Proceedings of the IEEE Photovoltaic Specialists Conference-Orlando. Dube. Rowcin. Straub. pp. New York. Wang. Crandall. Proceedings of the 3rd World Conference on Photovoltaic Solar Energy ConversionOsaka. R. 2005. [74] G. Schulz. Duh. M. Pihan. Lett. Konagai. Cunningham. Mater. [83] www. Appl. A. Oswald. IEEE. Proceedings of the IEEE Photovoltaic Specialists Conference-Orlando. [82] M. 2004. pp. Garayanan.q-cells. Yamada. Hanoka. B. 1359–1364. 741–744. Phys. F. Vanecek. 2005. Willeke. Feitknecht. New York. Narayanan. 2004. IEEE.R. C.H. 1424–1427. Muthukrishanan. Y. K. Kamra. Proceedings of the 25th IEEE Photovoltaic Specialists Conference. D. New York. Wenham. Plesa. [67] A. www. Song. Solids 338–340 (2002) 639. J. K. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 [60] S. J. Blakers. Munich. K. J. N.G. V. R. Proceedings of the IEEE Photovoltaic Specialists Conference. R. Terrazzoni. Phys. Delahoy. S. AIP Conference Series 353. 678–680. M. 2005. Rajan. [53] D. Mai. Beilby. Chu. Reber. 1428– 1431. 877–882. J. Bennett. Eyer. [68] M. 1994. Harder. Rech. P. 1399–1402.jp/aist e/aist today/2004 13/feature2/feature 10. G.com/siliconpv. 28 (1976) 671. Lambertz. Griffin. Proceedings of the 13 NREL. Wenham. WIP. Hof. D.L. 2005. Werner. Y. N. S. Seifert. 1996. 2004. [43] B. Torres. IEEE. Guo. New York. WIP. Khattak. Also.C.sunpowercorp. E. McMahon (Eds. Meier. LeComber. Proceedings of the 31st IEEE Photovoltaic Specialists Conference. Walsh. M. 2003. Appl. Franklin. 1023–1027. J. The Netherlands.-P. 25 (2004) 27. Appl.R. E. in: R. T. Nelson. IEEE. Ternary Compounds 1977. Proceedings of the 31st IEEE Photovoltaic Specialists Conference-Orlando. 1996. Y. IEEE. Duda. [109] K. Berens. Wernick. Sci. Ramanathan. R. Wu. Sheldon. 1663–1667. Gay.R. B. U. M. Egaas. [89] J. Dimmler. A. Proceedings of the 20th European Photovoltaics Solar Energy Conference-Barcelona. Proceedings of the 19th European Photovoltaics Solar Energy Conference-Paris. D. V. IEEE. Prog. J. F. Sanborn. D. Solids. R. VanAlsburg. Wu. pp. Karpov. O. 1582–1585. R.R. Tarrant. Battachararya. James and James. Ramanathan. A. Zhang. pp. pp. S. [118] X. 323–326. [122] L. Also L. I. Enzenhofer.E. Luck. Powalla.M.H. Photovoltaics 11 (2003) 535. 319–322. Schock. Proceedings of the 31st IEEE Photovoltaic Specialists Conference-Orlando. 299–301. New York. Ullal. [119] B. J. J.R. S. R. 1062 (2003). pp. P. R. Proceedings of the 31st IEEE Photovoltaic Specialists Conference. R. To. [105] M. Book. DeHart. McMahon. R. 1998. Vac. R. Kazmerski. Electronic Properties and Applications. Wagner. Schaeffler. Rau. K. Hasoon. Kasper. 2005. Proceedings of the 15th NREL/SNL PV Program Review Meeting. IEEE. Friedimeier. Yan. [108] www. B. C. Proceedings of the 25th IEEE Photovoltaic Specialists Conference. 1623–1626. R. 363–366. pp. New York. IEEE. [107] www. WIP. New York. 1992. D. 835–838. Johnston. [102] K. Colorado. Zweibel. Munich. P. R. Proceedings of the 25th IEEE Photovoltaic Specialists Conference. Proceedings of the 26th IEEE Photovoltaic Specialists Conference. J.L. J. Asher. . Chem. 37–41. Hegedus. Proceedings of the 16th European Photovoltaic Solar Energy Conference. D. Noufi. Proceedings of the 31st IEEE Photovoltaic Specialists Conference-Orlando. Hariskos. Kazmerski. IEEE. Kazmerski. 2000. J. Scheer. Young. H.A. Sites. New York. Braunger. New York. Lee. Appl. 1010–2005. J. pp. R¨ hle. [100] D. [115] S.S. Cadmium Telluride. Dippo.-W. [116] X. 845–848. B. Munich.L. [92] See. Nadenau. Unold.A. H. S. New York. Denver. R.C. Berniard. Also B.D. 995–1000. Proceedings of the DOE Solar Energy Technologies Program Review. M. UK. J. pp. Demtsu. Proceedings of the 31st IEEE Photovoltaic Specialists Conference. 797–800.K. Yoon. 341–344. London.M. 1997.L. Albin. Contreras. Ruehle. M. Meyer. Noufi. [95] M. [87] Some recent review of CIGS. Munich. U. Y. James and James. pp. Sheldon. Contreras. J. in press. Shay. New York.J. IEEE. S. J.J. Scheer. Palm. K. 2005. Hariskos. Karg. Pergamon Press. Zunger. A. Prog. Schock. Noufi. R. Sites. T. in press. Denver. Keane. www. Britt. in press. T. Kluwer. 1991.W.nrel. Munich. pp. [103] N. King. R. S.-W. J. von Roedern. Proceedings of the 29th IEEE Photovoltaic Specialists Conference. M. Duda. [114] X. 2005. 2004. Coutts. Tuttle. W.C. Swanson. Romero. Willett. K. Scofield. The Netherlands. S. Gessert. G. IEEE.H. 967–972. J. B. Renewable Sustainable Energy Rev. pp. Roussilon. New York. C. J.J. B. Proceedings of the International Conference on Ternatry and Multinary Compounds (ICTMC-2004).H. Moutinho.compoundsemiconductor. 2005. [120] S.de. in press. M.S. K. X.com and www. Branata. Keane. New York. Phys. Wiedeman. J.V. Song. New York. R. A. Noufi.L. [91] R. New York. J. Kharchenko. K. Dimmler. Enzenrothy. Photovoltaics 13 (2005) 49. J. Sixth International Photovolaic Science and Engineering Conference. IEEE. Schock. Thin Solid Films 480–481 (2005) 499. R. Institute of Physics. 2003. 347–350. Asher. New York.N. Ramanathan. Wendt. in press.gov. Emery. Proceedings of the IEEE Photovoltaic Specialists ConferenceNew Orleans. 2001.L. R. Migliorato. Solids. New York. [94] M. R. Proceedings of the 31st IEEE Photovoltaic Specialists Conference-Orlando. J.H. R. AbuShama. H. C. 1996. DeHart.R. AbuShama.L. Solids. WIP. pp. New York. J. [99] M. Proceedings of the 13th European Photovoltaic Solar Energy Conference-Nice. Kim. UK. Demtsu. pp. pp. 189–194. Mason. G. for example L. CO. also J. Wu. A. K. Proceedings of the 17th European Photovoltaics Solar Energy Conference. 177–182. pp. Also X. B. P. J. Gould. T. H. IEEE. Seapan. J. pp. C. M.html. kKaiser. 217–228. R. Ch. New York. Perkins. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 [84] J.A. Ahn. IEEE. pp. Noufi. Also. [110] See. Proceedings of the 20th European Photovoltaic Solar Energy Conference-Barcelona. Proceedings of the 3rd World Conference on PV Energy Conversion. Levi. Jasenek. Proceedings of the 31st IEEE Photovoltaic Specialists Conference-Orlando. IEEE. Sheldon. Y. pp. IEEE. IEEE. Chernykh. R. Proceedings of the 20th European Photovoltaics Solar Energy ConferenceBarcelona.S. H.net/articles/magazine/11/5/5/1. New York.firstsolar. Proceedings of the 31st IEEE Photovoltaic Specialists Conference. Abusarma. F. Phys. Beck. A. 2003. Ellmer. New York. Bonnet.T. Solar Energy 77 (2004) 803. Powalla. 2005. R. R. IEEE. Crane. pp. Prog. S. F. Nguyen. 881–884. in press. New York. Klaer. NREL. H. IEEE. [98] M. J. 1977. Huntington. Sampath. Schock. Golden. Ramanathan. Proceedings of the International Conference on Ternary and Multinary Compounds (ICTMC-2004). J.N.S. K.E. 2005.C. [85] S. See also. Wanka. Keane. New York.A. H.). J. CO.J. Phys Lett. J. Mahathongdy. Holah (Ed.antecsolar. Igari. S. Dhere. WIP.G. 437–440. U. Keane. in: G. K. WIP.C. Appl. Wiesner. D. Also. Ruckh. Probst. Colorado. J. Golden. S.G. 13 (1976) 139. Shvydka. Verlinden. D. IEEE. Y. Proceedings of the 25th IEEE Photovoltaic Specialists Conference. U. Dhere. Kanto. Dhere.L. Perkins. Noufi. To. 211–214. 1975.H. pp. R. Wchock. Wieting. P. V. New York. IEEE.L. D. Photovoltaics 13 (2005) 209.H. 1 (1997) 71. 1996. E. D. Wickham.A.L. Albin. Proceedings of the 22nd IEEE Photovoltaic Specialists Conference. Schaetter.W-. D. [96] See M. IEEE. New York. NREL. 445–448. C. McCandless.A. Granata. James and James. 1975. Ribelin. Sol. D. [88] M. Webb.radiochemistry. Zhou. Proceedings of u the 25th IEEE Photovoltaic Specialists Conference. pp.W. Egaas.S. IEEE. Ramanathan. Wu.B.A. Phys. New York. H. Young. Tronson. Chalapathy. 1657–1662. McCandless. Voorwinden. Kessler. [93] T. 2005. 259–262. Stein. Proceedings of the 31st IEEE Photovoltaic Specialists Conference. H. R. Zanio.I. 2005. Ternary Chalcopyrite Semiconductors: Growth. [90] A.S. IEEE. Chem. D. Hariskos. Barth. J. pp. Mohring. Chem. Kazmerski. D. Technol. Desa. pp. T. Kessler. [113] B. J. Proceedings of the 31st IEEE Photovoltaic Specialists Conference. Warta. A.S. Butche. 2005. pp. Proceedings of the 19th European Photovoltaic Solar Energy Conference-Paris. 116. 2005. [106] V. [121] www. Denver. D. Cohen. Cebulla. Proceedings of the 31st IEEE Photovoltaic Specialists Conference-Orlando. G. Duyda.H. R. [101] K.S. Tarrant. Ryan. B.M. Also. [97] R. Denver. Wu. T. 1996. [111] D. IEEE. Green. Teeter. D.org/periodictable/elements/49.L. W. Contreras. IEEE. Phys. H. H. M. B.L. T. 25 (1974) 434. Yun. pp. Proceedings of the 31st IEEE Photovoltaic Specialists Conference.G. Proceedings of the 31st IEEE Photovoltaic Specialists Conference-Orlando. pp. [117] X. New York. pp. White. Gay. S. Stat. UK. Denver.L. H. J. Proceedings of the International Conference on Ternary and Multinary Compounds (ICTMC-2004).G. Noufi. R. 87 (2000) 584. D. 531–535. Ward. 183–188. Noufi. 2004. pp. R. Wu. 2004. Klenk. Lux-Steiner.A. Ayyagari. Swartzlander. 2005. [123] P. Hegedus. Academic Press. Rischmiller. 2004 R&D 100 Award (R&D Magazine). J. New York. 133 [104] T.E. pp.A. Shay. 219– 222. Ref. 1995. F. P. [86] L. Perkins. 2005. 2002. Keane.A. R. P. 2005. Colorado. Phys. Romero. J. WA. Doolittle. Heeger. C. S.html www. K. Geisz.M. Proceedings of the 20th European Photovoltaics Solar Energy Conference-Barcelona. Surapudi. Munich. A. New York.J. Golden. Palm.M. Graetzel. Stella.com/html/Resources/concbasic. [151] J. New York. Rev. T.J. Tandem solar Photovoltaic Energy Converters. B 66 (2002) R201403.html. 221–228.L. Krut. Sundaram. Proceedings of the 20th European Photovoltaics Solar Energy Conference-Barcelona. 163. Masters Thesis.htm. pp. [169] T. [136] www. Solar Cells 30 (1990) 363. Ullal. J. IEEE. Eng. F. Yerkes. Appl. [139] M. Special Report. [149] M. Graetzel.M. Yamaguchi. pp.pdf. C. A. Yasuda.californiasolarcenter. Park. Photovoltaics 11 (2003) 535. J. James and James. IEEE.S. 2005. R. D. B. Albin. S. pp. Davenport.wired.D. Mater. Freemantle.G. S. Noufi.R. Munich. Lett. Photovoltaics 9 (2002) 257. Rapp. Cohen. [138] www. 2004. D. M. R. Emery. J. [161] MECH STACK BEST.W. Jarefors. E. A. Komiya. 137–140.J. in press. [135] M.V. Mondt. James and James. Green. Proceedings of the 19th European Photovoltaics Solar Energy Conference-Paris. Lu. Ref. Moriary. Coutts.W.cchem. 530– 535. New York.html.org/articles/20040306/note13. Golden. T.M. [148] S. Romero. ref.berkeley. Monolithic.dyesol. (September) (1998) 90–95. (1978) 278. S. DC. Scottsdale. M. [145] N. T. www. Coutts. M. Niesen. M.J. [125] M. Proceedings of the SPIE Meeting. I. Photovoltaics 8 (2000) 171. [134] M.html.nasa. New York. S. Wanlass.J.M. King.A. R. K. . Coutts. Haller. UK. SPIE. Also. in press. 2004.co. Proceedings of the 19th European Photovoltaic Solar Energy Conference-Paris. pp. A. Wanlass. [132] O. J. [144] B. Schaff. 2005.J. UK. in press. Abushama. S.cfm?chanID=sa003&articleID=0004C09402CC-1CD0-B4A8809EC588EEDF. WIP. Curtis.J. Wanlass. [170] L. R. Publ. Appl. W. AZ. Yu. Ward. Alemu. IEEE. Romero. B.fi/Units/AES/studies/dis/halme abs. [174] M. R. Department of Energy. Tate. 2000.J. D. J.W. NREL IR#0505 (patent pending). J.M.php. J.H. [150] www.sunpowercorp. Neges. Tech. N. Dy-Sensitized Nanostructured and Organic Photoovoltaic Cells: Technical Review and Preliminary Tests. Solar Cell and Array Technology for Future Space Science Missions. Coutts. Ahrenkiel. Emery.ac. Mardesich. Ermer. McEvoy. Also.A. H. K. NREL. Carapella. M.07/20020709-6. Proceedings of the International Conference on Solar Concentrators for the Generation of Electricity or Hydrogen. Emery. Jani. A.tkk.A. Marvin.rdg. J. Duda. Proceedings of the 31st IEEE Photovoltaic Specialists Conference-Orlando. Park. Proceedings of the 31st IEEE Photovoltaic Specialists Conference-Orlando. IEEE. Fuke. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 [146] M.com/ and http://solar. C. R. pp.M. [173] See. Aalukiewicz. Wu.A.com/files/2005/04/the electron na. Also. Electron Dev. 1992. New York. Piszczor. J. [167] T.com/article.J. [131] R.J.blogspot. [143] M. R. Graetzel. Marks.R. in press. T. Freundlich. 1996. Broman. Honsberg. [160] M. Proceedings of the 22nd IEEE Photovoltaic Specialists Conference.sta.E. A. UK. J. [124] M. Gessert. http://solar-club. M. Avery.web. E. T. CO. S. Proceedings of the 31st IEEE Photovoltaic Specialists Conference-Orlando. AIP Conference Series 321.L.com/my community/Solar/Solar 20. K. Nesmith. Proceedings of the 31st IEEE Photovoltaic Specialists Conference-Orlando. CO. IEEE. [130] Summary of concent McConnell. [137] www. J.B. 579–582. Superfieies y Vacio 17 (2004) 22. 30–33. Kurtz.K.P. [164] T. Washington. [129] R. in press. and www.html.S. 38 (1981) 53. T. Asghar. Munich. M. [152] M. A. Phys 93 (2003) 3693. D. R. [172] A.J. M. 24 (2002) 2972. Denver. JPL D-24454. [158] See.S.edu/news/media/ releases/2000/03/03-03-2000. Proceedings of the 31st IEEE Photovoltaic Specialists Conference-Orlando. Chem. Noufi. V.pdf). Thin Solid Films (2005). Proceedings of the European Photovoltaics Solar Energy Conference-Barcelona. Phys.S.S. [140] www. in press. Sherif. Law. A.html. Fetzer. L. H.C. Proceedings of the DOE Solar Energy Technologies Program Review. 177–180.org/solareclips/2002. Lett. Karam. Munich.07/solar. energyinnovations. Ferguson. Albin. S. Toeben. www. Kluwer Acad. Waldrop.sciam. Cutts.com/1990/01/rooftop-solar-panels-couldfinally-see. J. IEEE. Wehrer. Emery. Keszler. C.-H. The Netherlands. Broman. V. J. Islam.J.cgi?entry id=2900 and http://lawnorder. Torchynska. A. [133] W. Fundlach. Wanlass. H. Renewable Sustainable Energy Rev. Moriarty. News 80 (December (51)) (2002) 6. [159] M. A. Duda. C. 190–195. Intl. Phys.R. pp. D. WIP. Tate.F. Cohen.C.G.9-7 (2004) 1 (www. Albrecht. Marks.sciencenews. New York.R. NREL/CD520-38172. Bailey. James and James. Graetzel. Rosselli. www. Peumans. WIP.F. [162] D. D¨ rr. U. V.A.S. K. Appl. J. Proceedings of the 21st IEEE Photovoltaic Specialists Conference-Kissimmee. Visbeck. pp.edu/pagrp/. J. Osterwald.html.P. K.W. O’Reagan. J. D.jp/en/review/17/review07. Y. [154] P. and www. C.treehugger. Nature 414 (2001) 338. 175–176. Shafarman.L. Basic Research Needs for Solar Energy Utilization. [156] S. 1994.com.asp. Symko-Davies. 3 (1999) 77. Proceedings of the 20th European Photovoltaic Solar Energy ConferenceConferenceBarcelona. 523. A. K.A. Keszler. [128] Other. James and James. M. Fukui. C. Kinh. Nature 353 (1991) 737. Perkins. 25 (1974) 664. www. Geisz. A. Halme. J. WIP. MacDiarmid.html. [141] M. 38–45. P. New York. Proceedings of the 17th European Photovoltaic Solar Energy Conference-Glasgow. Prog. 1st NREL Conference on Thermophotovoltaic Generation of Electricity. M. Harris Jr. Gossla. F. Zakeeruddin. Ager III. Bellingham. Symko-Davies.ac. [168] www. Prog. Semiconductors 33 (1999) 961.A.S. Ahrenkiel. Forest. J.T.uk/Biomim/about. AIP Conference Series 358. N. AIP.grc. See also. pp.R.K. M. Osterwa.A. in press. (2005). 4 (2005) 607. [175] www. D. Gee. Bunker. pp. Young. Approaches for Ultra-High Efficiency. Chiba. Sci. for example C. Graetzel. Proceedings of the Second NREL Conference on Thermophotovoltaic Generation of Electricity. Kazmerski. Wanlass. Thin Solid Films (2004).ch/solar-club/SolPV/autres/organics. Proceedings of the 12th European Photovoltaic Solar Energy Conference.E. Ward. Wernsman. Liliental-Weber.com/ThermoPV. Prog. Fitzgerald.com/mt/mt-comments. M. Hitachi Reviews. 2005. Bailey. G. James and James. Gessert.P. Yamanaka. u Nat.jxcrystals. C. M. New York. L. UK. Jones. Yamada. Park. IEEE. J. Symko-Davies. 2005. Commun. 159 and 160.134 L. Willig. Kurtz. Coutts. August 2005. G. Duda. [171] www. pp. D. Kurtz. [166] M. Takahashi. T. Dig.berkeley. L. NREL. [155] See. Obermaier.hitachi-cable. Am. Proceedings of the 20th European Photovoltaic Solar Energy Conference. Chem. 2005. Reucroft. K. Green. Mtg. and www. J. Unno. Denver. Honsberg.P.htm. Fraas. Phys Lett. J. Yanagi.J. KaRG. Yakimov. Koide. in press. Gaddy. Nature 254 (1975) 507. Noufi.com/wired/archive/13. 2003. [126] L. Jasinski.d.htm. Forrest. K. T. Carapella. W.J. L. [165] H. Proceedings of the European Photovoltaic Solar Energy Conference-Barcelona. Takahashi. Phys. T. Nicol. [163] M. A.J. Solid State Commun.com (www. Tang.sharpusa.cern. D. Probst. [142] M.W. A. Keane. J. T.S. 2004. December 2003. 80 (2002) 4741. T. J. AIP. Schwarzburg. S.com/files/sol dow 2005 products. Schmid. Ahrenkiel.A.. 1990.uk/∼en2jsr/examples1. Conibeer. [127] M. Nelles.html.gov/WWW/RT2001/5000/5410bailey1. H. J. Price.B. G.H.. S. N. Appl. Graetzel. Green.aps. UK.C.A. K.au). Hamilton. Z. New York 1994.. M. A. S.J. S. Hirano. Polupan. K. MultBandgap.bath. J. 410–413. [147] M.-H. Hosono. www. R. for example. Wanlass.futurepundit. Bailey. DOE Solar Energy Technologies Program Multiyear Technical Plan 2005-2010. A. [153] D. Cowen. Synth. P. Landi. C. Hamakawa. Proceedings of the 20th European Photovoltaics Solar Energy Conference-Barcelona. 2004.E. [157] J. Rev. Romero.asp?DocumentID=3209&CategoryID =1215. Rostalski. Pizzinato. Adv. Current Topics in Photovoltaics. in press. Connolly.nrel. Kozlowski. Phys. Belaidi. 135 [190] [191] [192] [193] [194] Also N. C. Bishop. M. Flat Plat Solar Array Project. 2002. M. . Cuadra. in press. Adv. D. E. New York. Nat. Akhane.W. Koch. P. Kondo. Further reading [48] Y. Konenkamp. May 6.P. New York. T. WIP. J. Mater. R.). pp. D’Aiello. Munich. Mater.M. vol. Alonso. S.W. 1057–1061. Cho. Palacios. http://www. D. Dhere. IEEE. pp. C. Shaller.). Washington. I–VIII. [179] A. Update (February) (2001) 9. A. Saito. Conibeer. Also. and www. in press. P.L. Guha. P.J. Ruiz.C. Science Daily. for example. [185] N. Williams (Ed. [183] M. Hof. Proceedings of the 25th IEEE Photovoltaic Specialists Conference. P. Golden. Lett. 1986 (JPL Publ.P. P. V. Proceedings of the Thin Film Module Reliability Project. Mater.gov/news/press/2005/1805 quantum dot. in press. Ball. Parisi. [177] R. 1996. Sci.). Castaner. J. Fairmaont Press. The Handbook of Photovoltaic Applications. A. in press. Hoppe. Luque. J. Oxford.oja-services. 2005). 2003–2005 (NREL. Harder. Marti.toolbase. WIP. CO. R. G. Proceedings of the 25th IEEE Photovoltaic Specialists Conference. C. I.S. B. L. Sinharoy. McMahon. Proceedings of the 25th IEEE Photovoltaic Specialists Conference.htm. in press. Sahnon. [188] U. Proceedings of the 3rd World Conference on Solar Energy Conversion-Osaka. WIP. Palacios. D. J. News 165 (2005) 259. Tablero. Proceedings of the 20th European Photovoltaics Solar Energy Conference-Barcelona. Hill. A. New York. Sariciftci. Calahan. S. R. Solar Cells 31 (1991) 527. Brabec.C. Mannig. M. in: C. J. S. V. Weiss. Proceedings of the 25th IEEE Photovoltaic Specialists Conference. 1986. I. L. Practical Handbook of Photovoltaics: Fundamentals and Applications. Munich. A Lin. A. Wilt. Takehara. Stanley. UK. Funct. Elsevier. E. Marti. N. Barnham.G. A. G. D. K. Birkmire. also. Munich. IEEE. Luque. T. F. Brabec. pp. J. Komiyama.nl/iea-pvps/pv/glossary. 1996. Ludi. Pink. NCPV Program Review Meeting. Shah. 2005. 940–943. WIP. Final Report. Luque. R. Proceedings of the 28th IEEE Photovoltaic Specialists Conference-Anchorage. R. Green.A. S. [181] M. K Sanchez. 1996. C.P. see PVWATTS performance calculator: http://rredc. JPL. Proceedings of the IEEE Photovoltaics Specialists Conference-Orlando. W. Kajita. Wahnon. Cuadra. Hammond. Sariciftci. 1227–1230. Proceedings of the DOE Solar Energy Technologies Program Review Meeting. M. [180] A. Denver. N. Werner. 2005. C. DC. A. Trupke. Proceedings of the 24th IEEE Photovoltaic Specialists Conference. Organic Photovoltaics: Concepts and Realization. Ernst. Fishcher.). Marti. Fukae. P. 3–8. in: T. Also. Drechesel. NREL. H. A. Wahnon. B.S. pp.L. 2002. pp. Proceedings of the 20th European Photovoltaics Solar Energy Conference-Barcelona. pp. Coutts. Markvart. Also J. Bessiere. D. U. WIP. S.gov/ solar/calculators/PVWATTS/. CO. Hegedus (Eds. D. (July/August) (2004) 35–40. N. C. Shiotsuka. Munich. A. K. Rev. [184] P. Lim. 1017–1022. 1191–1196. T. Witt.R. A. F. UK. Luque. [182] L. 2004. McCandless. Palacios.org/tertiaryT. 19 (2004) 1924.C. 2004. 2003. Sci. 1997. pp. 78 (1997) 5014. K. Corkish. Springer. Al-Jassim (Eds. 2003. Meissmer. Hummelen. Lopez.nrel. G. Goetz. 1995. Y Huang. New York.F. Y. Cho. WIP. Munich. IEEE.). J.S.C. N. Met 121 (2001) 1551. CA. Golden.W. Fairley. [112] B. R. N. Fernandez.S. Academic Press. Pfeiffer. 1190–1193. Lopez. D. Meakin (Eds.J. H. DOE. Proceedings of the a 20th European Photovoltaics Solar Energy Conference-Barcelona. M. Puzzer. McDonald (Eds. Bayon. N. J. J. Wiley and Sons. CO.J. [186] Y. McKee. New York. J. N. IEEE. 92 (2004) 186601. Proceedings of the 20th European Photovoltaics Solar Energy Conference-Barcelona. A. Proceedings of the 20th European Photovoltaics Solar Energy ConferenceBarcelona. New York.S. IEEE. [187] A. K. H. 86-31). Evans. Pasadena.E. Kazmerski / Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135 [176] A. New York. T. Dhere. Onesa. 885–888. S. [189] See.C. Zhou. Cuadra. Technol. New York. Handbook of Photovoltaic Science and Engineering. NREL. J. N. IEEE. S. Nozik. Masser.L. Roberts. Proceedings of the 20th European Photovoltaics Solar Energy Conference-Barcelona. Meissner. Munich. Klimov. C.J. Rev. AIP. K. pp. Leo. Phys. Sariciftci (Eds. 3 (1991) 129. Lett. E. Dyakonov. Ossenbrink. IEEE.D. Zerlaut. Fernˆ dez. Munich. WIP. Johnson. Marti. Atlanta. Osterwald.html?print. Gebeyehu. IEEE. Okada. Bailard. New York. 2004. R. J. 111–169. 1985. New York.S. J. Castro. Wohrle.). New York. Fangsuwannarak. Ohtani. Golden. Guha. in press. NREL/CP-590-31011. 2001. Luque. 2000. Proceedings of the 29th IEEE Photovoltaics Specialists Conference-New Orleans. IEEE. New York. M.J. Proceedings of the 14th NREL Photovoltaics Program Review. Tamechika. pp. J.J. Thin Solid Films 451–452 (2004) 515. 2003. C. Raffaelle. L. King. Res. 11 (2001) 15. www. N. M. R. [178] P. Also A. T. 1996.