Australian Photovoltaics Research and Development - ACS Energy

School of Photovoltaic and Renewable Energy Engineering, University of New South Wales (UNSW) Australia, Sydney 2052, Australia. ACS Energy Lett. , 20...
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Australian Photovoltaics Research and Development produced the first 22% efficient silicon cell using what now generally is described as an IBC (interdigitated back contact) cell,4 UNSW held the record for silicon cell efficiency from 1983 until 2014. In early 2014, a new record of 25.6% efficiency was measured for an IBC cell from Panasonic.5 The Prime Minister’s statement regarding market share stems from a mid2015 Bloomberg projection6 for PERC cells (passivated emitter and rear cells), invented and developed in Australia and expected to dominate the solar market over the coming decade. Australian PV R&D arguably has had an even larger impact than that mentioned by the Prime Minister because it underpins the recent game-changing PV cost reductions contributing to the buoyancy of the mood in Paris. These cost reductions stem directly from a shift of cell manufacturing over the 2005−2010 period to low-cost regions of Asia, spearheaded by researchers and engineers trained in Australian R&D programs.7 This shift was triggered by Suntech Power, founded in 2001 by UNSW Ph.D. graduate, Dr. Zhengrong Shi and a largely Australian team. After Suntech, other team members went on to found, over a short period, CSUN, JA Solar and Global Sunrise, later pioneering the commercialization of PERCs. These companies were able to grow quickly through capital raisings largely through U.S. stock exchanges.7 The UNSW research achievement documented in Figure 1 has been recognized in the Elsevier Handbook of Energy by selection as Number 8 in a timeline of the “Top Ten” milestones in the history of PVs.8 Number 5 on this list was the announcement of the first useable silicon solar cells in 1954, associated with the initial steep rise in efficiency in Figure 1. Number 6 is the first use as a power source on a satellite in 1958 that stimulated further efficiency improvements and establishment of a manufacturing industry. Number 7 is the derivation of thermodynamic limits on solar cell efficiency by Shockley and Queisser, with the UNSW contribution in taking silicon cell efficiency from ∼17 to 25% coming in as Number 8. Prior to UNSW contributions from the early 1980s, the highest-efficiency silicon cells were “black” cells of about 17% efficiency developed by COMSAT laboratories in the mid1970s. The basic black cell design (similar to the back surface field (BSF) cell of Figure 2A), in combination with the screenprinting process for forming contacts developed at Spectrolab at about same time,9 has formed the basis for most solar cell manufacturing up to the present. UNSW research led to substantial improvements, initially by minimizing front surface recombination in what wa termed PESC cells (passivated emitter solar cells, where “emitter” refers to the top region of the cell). In 1983, the more advanced PERC cell was first suggested,10 with experimental demon-

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ustralia has a long and illustrious research history in solar photovoltaic (PV) research and development (R&D), achieving several significant milestones in the history of PV R&D. These include the world’s first silicon cells with 20% and then 25% energy conversion efficiency,1 with the most recent achievement being the first conversion of sunlight to electricity with efficiency above 40% (ref 2). At the recent COP21 conference on climate change in Paris in late 2015, the Australian Prime Minister, Malcolm Turnbull, in his speech arguing how Australia was pulling its weight in climate change mitigation efforts, proudly stated: “The University of New South Wales (UNSW) has held the world record for solar cell eff iciency for 30 of the last 32 years. And by 2018 over 60 % of the world’s solar cells are to use technology developed by Australian researchers” (ref 3). He further added that Australia supported Mission Innovation that “aims to double investment in clean energy innovation over the next 5 years”, which might reasonably be expected to result in ongoing research support to build on these past achievements. Figure 1 shows the research outcomes underpinning the Prime Minister’s statement. Shown is the evolution of silicon solar cell efficiency from early implementations at Bell Laboratories to the UNSW demonstration of the first cells with 20% and then 25% energy conversion efficiency.1 Apart from a brief period around 1988 when Stanford University

Figure 1. Evolution of silicon solar cell efficiency from early implementations at Bell laboratories to the demonstration of the first cell with 25% energy conversion efficiency. © XXXX American Chemical Society

Received: July 15, 2016 Accepted: August 5, 2016

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Figure 2. (A) Four silicon cell technologies presently used in commercial production: BSF, back surface field; HJ: heterojunction; IBC: interdigitated back contact; PERC: passivated emitter and rear cell. (B) Likely market share of the different cell technologies, suggesting that PERCs will overtake BSF cells as the cell produced in highest volume in ∼2023 (ref 33). Other studies have suggested different dates for this transition, ranging from 2018 to 2020, with a smaller role for HJ and IBC devices.6

Research Council funding schemes. The Institute’s programs were taken over by the Australian Renewable Energy Agency (ARENA) in 2013. One such program inherited by ARENA was a strategic research initiative in PVs resulting in the formation of the Australian Centre for Advanced Photovoltaics (ACAP), which has links to many key strands of Australian PV research.12 Commencing in 2013, ACAP involves five of Australia’s “Group of Eight” leading research universities plus the Commonwealth Scientific and Industrial Research Organisation (CSIRO). ACAP’s research program focuses on a wide range of PV device research, ranging from silicon, where ACAP partners UNSW and ANU (Australian National University) have particular strengths, to organic and organic−inorganic hybrid devices where remaining partners, UoM (University of Melbourne), Monash University, and CSIRO, all closely located in Melbourne, as well as the University of Queensland (UQ), all have strong programs. Full details on present programs are contained in ACAP’s annual reports, including research highlights over recent years.12 One such highlight has been the UNSW work on hydrogenation of silicon with supporting work at ANU, particularly relevant to the commercialization of the PERC cell.13−15 Because this cell is able to take full advantage of improvements in the quality of a low-cost silicon wafer and its interfaces, improving that quality and sustaining this improvement over the 30 year operational life of the module are key to the success of this technology and to it achieving efficiencies close to 25% in production. Understanding the charge state of hydrogen and how this affects transport through the wafer and hydrogen’s effectiveness in passivation of the defects with which it interacts has been key to the development of practical sequences for enhancing and stabilizing PERC performance.13−15 This work complements work at ANU on fabricating cells on potentially low-cost UMG (upgraded metallurgical grade) silicon.16 An efficiency of 20.9% has been independently confirmed for a PERC made on such material,12 the highest to date by a large margin. Taking full advantage of the UNSW

stration of improved efficiency in 1989 ultimately leading to 25% efficiency in 1999 (under present standards). This remains the highest efficiency for a PERC today, although the record for silicon cell efficiency of 25.6% is now held by an IBC cell, as previously mentioned. The IBC cell was suggested at Purdue University, with efficient devices first demonstrated at Stanford University4 and IBC cells first commercialized by SunPower. Commercial modules over 22% efficiency are now available, although at a premium, compared to the present commercial norm of 16− 17% (ref 11). By combining the PERC approach with hydrogenation to boost the performance of low-cost substrates, such performance levels are expected to become the industry standard over the coming decade. Most commercial cells until recently have not taken full advantage of these post-1980 efficiency improvements, but this is quickly changing. Figure 2A shows the four silicon cell types presently used in commercial manufacturing. The first of these, the BSF cell, incorporates the BSF feature of “black” cells, where BSF refers to the heavily doped region underlying the rear contact, formed by alloying Al in the rear contact with the silicon. The PERC cell incorporates a much more refined treatment of the rear contact region, combining reduced contact area with an efficient rear surface mirror and, in the PERC implementation shown,10 localized substrate doping in the contact region. The two remaining structures in Figure 2A have also recently achieved the 25% efficiency milestone, some 15−16 years after the initial UNSW PERC result, both using Pdoped, n-type silicon rather than the commercially dominant Bdoped, p-type silicon wafers. The IBC cell has been previously mentioned, featured in Figure 1 as the first silicon cell to achieve 22% efficiency. The other is the heterojunction (HJ) cell, pioneered by Panasonic, that uses thin layers of a highband-gap hydrogenated amorphous silicon material (a-Si) to form both p-type and n-type contacts. PV research in Australia received a boost toward the end of the period shown in Figure 1 with the formation of the Australian Solar Institute in 2009, providing a targeted source of PV funding. Most previous Australian research had been funded on its academic merits under the general Australian 517

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Figure 3. (A) Dr. Xiaojing Hao with record efficiency CZTS solar cells. (B) Dr. Mark Keevers with a 34.5% prism minimodule designed for 1 Sun operation. (C) Flexible thin-film cells continuously printed by CSIRO, Melbourne using technology inspired by Australian production of plastic banknotes.

Another strand of Australian PV research is linked to the Australian development of the first plastic banknotes.22 Such notes were brought into circulation in 1988, with manufacturing of notes for several other countries occurring in Melbourne. This stimulated the founding of the Victorian Organic Solar Cell Consortium (VICOSC), based in Melbourne, formed by UoM, Monash, and CSIRO. VICOSC initially investigated organic and related dye-sensitized cells, focusing on developing high-throughput fabrication processes and improving the efficiency and longevity of these cells. The work has recently been extended to perovskites in which all ACAP nodes have an interest due to their wide potential range of application, including silicon-based tandem cells. Examples of recent high-impact papers from VICOSC and UQ, also interested in similar cell approaches, include one, with lead author Kuan Sun, affiliated with the UoM ACAP node, reporting a new class of nematic liquid crystal material capable of providing improved organic solar cell performance while also enabling easier roll-to-roll printing of environmentally friendly, mechanically flexible, and cost-effective PV devices.23 This work has formed the basis of new collaborations with the UQ node, as well as with U.S. partners NIST and GeorgiaTech. This paper has been identified as a “Hot Paper”, within the top 0.1% in its field. A second paper, also on organic PVs, also earned the “Hot Paper” label. The lead author was Jegadesan Subbiah, also from the UoM node, with coauthors from the CSIRO node. The paper reported the synthesis of a high molecular weight donor−acceptor conjugated polymer and its use in fabricating a 9.4% efficient solar cell.24 Another “Hot Paper”, with lead author Kyeongil Hwang of CSIRO also involving coauthors from the UoM node and the South Korean Gwangju Institute of Science and Technology, reported progress in the scale-up and printing of perovskite solar cells on flexible substrates.25 A fourth “Hot Paper”, with lead author Qianqian Lin from the UQ node, addressed the electro-optics of perovskite solar cells.26 A “Highly Cited Paper” with lead author Yu Han of the Monash node, but also involving coauthors from both UoM and CSIRO, reported on the degradation of encapsulated perovskite solar cells,27 one of the key issues determining the future prospects for this technology. Another “Highly Cited Paper” with lead author Yasmina Dkhissi of the UoM, but also involving coauthors from Monash and CSIRO, reported on low-temperature deposition of perovskites onto flexible polymer substrates,28 something that is difficult to achieve with other cell technologies, giving rise to potentially unique applications. Yet another “Highly Cited Paper” with lead author

hydrogenation work is expected to lead to even higher efficiency. Other work at ANU jointly with ACAP commercial partner Trina Solar led to what was briefly a world record efficiency of 24.4% for an IBC cell using n-type silicon wafers17 with ACAP, for a period holding world records for both IBC and PERC cells. Trina has since announced building on this work to produce a 23.5% efficient IBC cell on a full-sized 156 mm square wafer, the highest ever for a cell of this size.18 Another world record was recently achieved at UNSW in the ACAP program for pure sulfide CZTS (Cu2ZnSnS4) cells. These cells are of interest in their own right both as a thin-film alternative to silicon and also as a prototype material for use in silicon-based tandem cells. Efficiency approaching 8% was confirmed for the first time for a cell over 1 cm2 in area.19 Other work within ACAP targets the development of siliconbased tandem cell stacks, building on the realization that silicon has almost the perfect band gap for tandem stacks involving up to three or four cells.11,20 Although multiple projects span the integration of III−V chalcogenide and perovskite cells as monolithic thin-film layers on silicon, best results to date have been obtained with split-spectrum approaches. Efficiencies over 23% (based on internal measurements) have been reported at ANU by directing light reflected from a perovskite cell to a silicon cell21 and at UNSW using a dichroic filter to direct the light to the appropriate cell.12 Perhaps the most striking results with multiple junction cells have been obtained through work with the local Australian company, RayGen (www.raygen.com). RayGen markets a “power tower” concentrating PV (CPV) system, with a “dense array” of high-performance group III−V triple-junction cells mounted on a central tower onto which a field of heliostats (suntracking mirrors) reflects light. Using a small prototype of this system involving a single 20 cm diameter mirror, UNSW researchers, with assistance from RayGen, designed a receiver that steers near-infrared light, unable to be used effectively by the normal commercial III−V triple-junction cell, to a silicon cell. This gives an approximately 10% relative boost in performance. Independent measurements at the National Renewable Energy Laboratory (NREL) outdoor test facility in Colorado confirmed that this system demonstrated the first ever conversion of sunlight to electricity with efficiency above 40% (ref 2). More recently, 34.5% efficiency has been confirmed, again at NREL, for a small 30 cm2 minimodule that does not use concentrated sunlight, the highest ever for a device of this size (Figure 3B). 518

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(9) Ralph, E. L. Recent Advancements In Low Cost Solar Cell Processing. Conference Record, 11th IEEE Photovoltaic Specialists Conference, Scottsdale, May 6−8, 1975; 315−316. (10) Green, M. A. The Passivated Emitter and Rear Cell (PERC): From Conception To Mass Production. Sol. Energy Mater. Sol. Cells 2015, 143, 190−197. (11) Green, M. A. Commercial Progress and Challenges for Photovoltaics. Nat. Energy 2016, 1, 15015. (12) Australian Centre for Advanced Photovoltaics Annual Report 2015. http://www.acap.net.au/annual-reports (2016). (13) Hallam, B. J.; Hamer, P. G.; Wenham, S. R.; Abbott, M. D.; Sugianto, A.; Wenham, A. M.; Chan, C. E.; Xu, G. Q.; Kraiem, J.; Degoulange, J.; Einhaus, R. Advanced Bulk Defect Passivation for Silicon Solar Cells. IEEE J. Photovolt. 2014, 4, 88−95. (14) Nampalli, N.; Hallam, B. J.; Chan, C. E.; Abbott, M. D.; Wenham, S. R. Influence of Hydrogen on the Mechanism of Permanent Passivation of Boron-Oxygen Defects in p-Type Czochralski Silicon. IEEE J. Photovolt. 2015, 5, 1580−1585. (15) Payne, D. N. R.; Chan, C. E.; Hallam, B. J.; Hoex, B.; Abbott, M. D.; Wenham, S. R.; Bagnall, D. M. Acceleration and Mitigation of Carrier-Induced Degradation in p-Type Multi-Crystalline Silicon. Phys. Status Solidi RRL 2016, 10, 237−241. (16) Rougieux, F.; Samundsett, C.; Fong, K. C.; Fell, A.; Zheng, P.; Macdonald, D.; Degoulange, J.; Einhaus, R.; Forster, M. High Efficiency UMG Silicon Solar Cells: Impact of Compensation on Cell Parameters. Prog. Photovoltaics 2016, 24, 725−734. (17) Franklin, E.; Fong, K.; McIntosh, K.; Fell, A.; Blakers, A.; Kho, T.; Walter, D.; Wang, D.; Zin, N.; Stocks, M.; et al. Design, Fabrication and Characterisation of a 24.4% Efficient Interdigitated Back Contact Solar Cell. Prog. Photovoltaics 2016, 24, 411−427. (18) Trina Solar Announces New Efficiency Record of 23.5% for LargeArea Interdigitated Back Contact Silicon Solar Cell. http://www. prnewswire.com/news-releases/trina-solar-announces-new-efficiencyrecord-of-235-for-large-area-interdigitated-back-contact-silicon-solarcell-300257457.html (2016). (19) Sun, K.; Yan, C.; Liu, F.; Huang, J.; Zhou, F.; Stride, J. A.; Green, M.; Hao, X. Over 9% Efficient Kesterite Cu2ZnSnS4 Solar Cell Fabricated by Using Zn1‑xCdxS Buffer Layer. Adv. Energy Mater. 2016, 6, 1600046. (20) Green, M. A. Third Generation Photovoltaics: Theoretical and Experimental Progress In 19th European Photovoltaic Solar Energy Conference Paris, France; Davitti, V., Kallunki, S., Pollini, S., Rachlik, I., Munoz, E. U., Eds.; June 7−11, 2004; pp 3−8. (21) Duong, T.; Grant, D.; Rahman, S.; Blakers, A.; Weber, K.; White, T.; Catchpole, K. High Efficiency Perovskite/Silicon Tandem Cells with Low Parasitic Absorption. IEEE 43rd Photovoltaic Specialist Conference (PVSC), Portland, OR, 2016. (22) Solomon, D.; Spurling, T. The Plastic Banknote From Concept to Reality; CSIRO Publishing: Melbourne, 2014. (23) Sun, K.; Xiao, Z.; Lu, S.; et al. A Molecular Nematic Liquid Crystalline Material for High-Performance Organic Photovoltaics. Nat. Commun. 2015, 6, 6013. (24) Subbiah, J.; Purushothaman, B.; Chen, M.; Qin, T.; Gao, M.; Vak, D.; Scholes, F. H.; Chen, X.; Watkins, S. E.; Wilson, G. J.; Holmes, A. B.; et al. Organic Solar Cells Using a High-MolecularWeight Benzodithiophene-Benzothiadiazole Copolymer with an Efficiency of 9.4%. Adv. Mater. 2015, 27, 702−705. (25) Hwang, K.; Jung, Y.-S.; Heo, Y.-J.; Scholes, F. H.; Watkins, S. E.; Subbiah, J.; Jones, D. J.; Kim, D. Y.; Vak, D. Toward Large Scale Rollto-Roll Production of Fully Printed Perovskite Solar Cells. Adv. Mater. 2015, 27, 1241−1247. (26) Lin, Q.; Armin, A.; Nagiri, R. C. R.; Burn, P. L.; Meredith, P. Electro-Optics of Perovskite Solar Cells. Nat. Photonics 2014, 9, 106− 112. (27) Han, Y.; Meyer, S.; Dkhissi, Y.; Weber, K.; Pringle, J. M.; Bach, U.; Spiccia, L.; Cheng, Y. B. Degradation Observations of Encapsulated Planar CH3NH3PbI3 Perovskite Solar Cells at High Temperatures and Humidity. J. Mater. Chem. A 2015, 3, 8139−8147.

Rui Sheng of UNSW involved vapor-assisted deposition of lead bromide perovskite, of interest as a high-band-gap cell in a double-junction bromide/iodide/silicon device.29 One more “Highly Cited Paper” in the ACAP thin-film strand of activities, with lead author Jae Sun Yun from UNSW, but also involving coauthors from Monash, describes the experimental role of grain boundaries in improving perovskite cell performance.30 Other “Hot Papers” arose from collaboration between UNSW and NREL, documenting recent efficiency improvements in PVs across a range of technologies, including the recent ACAP efficiency records previously mentioned and the record 21.3% result for multicrystalline silicon obtained by ACAP industrial partner Trina Solar using PERC technology.31 Australia is a participant in the International Energy Agency (IEA) Co-operative Programme on PV Power Systems and prepares an annual national survey report on PV power applications in Australia. This includes extensive additional information on current Australian PV R&D.32 R&D activities largely independent of the ACAP program include a program at CSIRO (a division based at Newcastle rather than Melbourne) investigating measurement procedures for perovskite solar cells because these can pose severe measurement challenges due to hysteresis effects. Charles Darwin University is focusing on remote area hybrid PV systems, network integration, and performance of various PV module technologies (CIGS, CdTe, and c-Si) under the cyclonic tropical locale of the Northern Territory where the university is located. PV activities at Murdoch University in Western Australia cover a range of PV research activities focusing on the field performance of modules including long-term degradation and soiling impacts.32

Martin A. Green

School of Photovoltaic and Renewable Energy Engineering, University of New South Wales (UNSW) Australia, Sydney 2052, Australia

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AUTHOR INFORMATION

Notes

The author declares no competing financial interest.

REFERENCES

(1) Green, M. A. The Path to 25% Silicon Solar Cell Efficiency: History of Silicon Cell Evolution. Prog. Photovoltaics 2009, 17, 183− 189. (2) Green, M. A.; Keevers, M.; Thomas, I.; Lasich, J.B.; Emery, K.; King, R. R. 40% Efficient Sunlight to Electricity Conversion. Prog. Photovoltaics 2015, 23, 685−691. (3) 2015 United Nations Climate Change Conference (COP21), Paris, Nov. 30, 2015. http://unfccc.int/files/meetings/paris_nov_2015/ application/pdf/cop21cmp11_leaders_event_australia.pdf (2016). (4) King, R. R.; Sinton, R. A.; Swanson, R. M. Front and Back Surface Fields for Point-Contact Solar Cells. Conference Record of the Twentieth IEEE Photovoltaic Specialists Conference, Las Vegas, NV, 1988; pp 538− 544. (5) Masuko, K.; Shigematsu, M.; Hashiguchi, T.; Fujishima, D.; Kai, M.; Yoshimura, N.; Yamaguchi, T.; Ichihashi, Y.; Yamanishi, T.; Takahama, T.; et al. Achievement of More Than 25% Conversion Efficiency with Crystalline Silicon Heterojunction Solar Cell. IEEE J. Photovolt. 2014, 4, 1433−1435. (6) Wang, X. 2015 PV Market Outlook; Asia Solar Energy Forum: Manila, June 15, 2015. (7) Green, M. A. Revisiting the History Books. PV Magazine, June 2016; pp. 96−101. (8) Cleveland, C. J.; Morris, C. Handbook of Energy, Volume II: Chronologies, Top Ten Lists and Word Clouds; Elsevier Science, December 2013; p vii, 10.1016/B978-0-08-046405-3.05001-4. 519

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ACS Energy Letters (28) Dkhissi, Y.; Huang, F.; Rubanov, S.; Xiao, M.; Bach, U.; Spiccia, L.; Caruso, R. A.; Cheng, Y. B. Low Temperature Processing of Flexible Planar Perovskite Solar Cells with Efficiency Over 10%. J. Power Sources 2015, 278, 325−331. (29) Sheng, R.; Ho-Baillie, A.; Huang, S.; Chen, S.; Wen, X.; Hao, X.; Green, M. A. Methylammonium Lead Bromide Perovskite-Based Solar Cells by Vapor-Assisted Deposition. J. Phys. Chem. C 2015, 119, 3545−3549. (30) Yun, J.; Ho-Baillie, A.; Huang, S.; Woo, S. H.; Heo, Y.; Seidel, J.; Huang, F.; Cheng, Y. B.; Green, M. A. Benefit of Grain Boundaries in Organic-Inorgainc Halide Planar Perovskite Solar Cells. J. Phys. Chem. Lett. 2015, 6, 875−880. (31) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar Cell Efficiency Tables (Version 48). Prog. Photovoltaics 2016, 24, 905−913. (32) International Energy Agency Photovoltaic Power Systems Programme National Survey Reports. http://www.iea-pvps.org/index. php?id=93 (2016). (33) International Technology Roadmap for Photovoltaic (ITRPV) 2015. http://www.itrpv.net/Reports/Downloads/2016/ (2016).

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