Solar cells; silicon solar cells; photovoltaics.
Martin Green is Scientia Professor at the University of New South Wales, Sydney and Director of the Australian Centre for Advanced Photovoltaics, involving several other Australian Universities and research groups. His group's contributions to photovoltaics are well known and include holding the record for silicon solar cell efficiency for 30 of the last 39 years, described as one of the “Top Ten” Milestones in the history of solar...view more
Martin Green is Scientia Professor at the University of New South Wales, Sydney and Director of the Australian Centre for Advanced Photovoltaics, involving several other Australian Universities and research groups. His group's contributions to photovoltaics are well known and include holding the record for silicon solar cell efficiency for 30 of the last 39 years, described as one of the “Top Ten” Milestones in the history of solar photovoltaics. The PERC solar cell that he invented in 1983 and his team developed to its full potential accounted for 91.2% of worldwide silicon solar module production in 2021 (CPIA).
Major international awards include the 1999 Australia Prize, the 2002 Right Livelihood Award, also known as the Alternative Nobel Prize, the 2007 SolarWorld Einstein Award, the 2016 Ian Wark Medal from the Australian Academy of Science, the prestigious Global Energy Prize in 2018, the 2021 Japan Prize, the 2022 Millenium Technology Prize and, with three former students, the 2023 Queen Elizabeth Prize for Engineering.
- Semiconductor Device physics
- Novel Semiconductor devices
- Photovoltaic Solar Energy Conversion
- Silicon Solar Cells
- Photovoltaic module design
- Photovoltaic device fabrication and characterisation
- Semiconductor device modelling
- Electrical energy storage
- Thin film crustalline silicon photovoltaic devices
Summary of Prof. Martin Green's Research Achievements
Professor Martin Green’s contributions to photovoltaics are unique internationally. His work has resulted in a massive, over 50%, relative improvement in the energy conversion efficiency of the commercially dominant silicon solar cells from 1983 to 2008, with these improvements now captured commercially. Such improved silicon cell efficiency has been identified as the key factor in recent photovoltaic cost reductions (Kavlak et al., 2018), now making solar “one of the lowest cost sources of electricity in history” (IEA, 2020) and the key weapon in recent climate change mitigation strategies (IEA, 2021; IPCC, 2022).
Green was first to describe, and his team the first to develop and experimentally demonstrate, two fundamentally different approaches to improving silicon cell performance that now underpin all modern silicon cell designs. Silicon cells dominate an annually increasing 96% of the global solar market, valued at US$308 billion in 2022 (Bloomberg, 2023).
These approaches were used by his team to demonstrate the world’s first 18%, 19%, 20%, 21%, 23%, 24% and 25% efficient silicon cells, amongst the 14 successive world records in this area that his team established. With his 3rd PhD student, the late Stuart Wenham, he also pioneered the now widespread use of lasers in cell processing and the development and first large-scale commercialisation of Cu plating to replace the ultimately unsustainable use of Ag for solar cell metallisation (BP Solar’s “Saturn” cell and Suntech’s “Pluto” cells).
Green has also made several notable theoretical contributions to the field, including the first identification of Auger recombination as placing the most severe bounds on silicon cell performance, developing the methodology to calculate these bounds and discussing cell designs capable of limiting performance (Green, 1984a). He was also first to explore “non-ergodic” light trapping schemes based on the crystallographically-defined pyramidal structures used in all modern silicon cells (Campbell and Green, 1987), showing these schemes had advantages over the ergodic approaches earlier identified (Yablonovitch, 1981), by more fully exploiting angular response. He also introduced the term “third generation photovoltaics” into the photovoltaic vernacular in his research monograph of this title (Green, 2003) discussing, self-consistently, all known approaches for improving cell efficiency above classic limits (Shockley and Queisser, 1961).
The first of the approaches he pioneered that underpin modern cell design is now described under the heading of “passivated contacts”, where Green was first to identify the potential of this approach and investigate theoretically and experimentally for solar, as tunnelling metal/thin-oxide/silicon contacts (Green et al., 1974). This resulted in the first silicon cells with output voltage above 650mV in 1978 (Godfrey and Green, 1979), increasing to 694mV in 1982, by additionally controlling silicon surface doping to give the “MINP” cell (Green et al., 1982). This MINP approach produced his team’s first world record cell, with the world’s first 18% efficient silicon cell confirmed in 1983 (Green et al., 1984). In 1981, he was also first to suggest and his group first to demonstrate record voltages with doped-polysilicon/thin-oxide/silicon versions of these contacts (Green and Blakers, 1983), now known as TOPCon (“tunnel oxide passivated contacts”), a high-efficiency cell technology now starting to compete for market share with the market dominant PERC cell (“passivated emitter and rear cell”), another device also pioneered by Green and his team.
This first polysilicon based TOPCon cell was intended to demonstrate consistency of Green’s tunnel oxide “passivated contacts” with the high temperatures involved in screen-printing contacts to silicon solar cells, then gaining market dominance. He was also first to suggest that these tunnel oxide devices could be used either as “majority carrier blocking” (junction replacement) or “minority carrier blocking” (“back surface field” replacement) contacts (Green et al., 1976), also likely the first to introduce this blocking concept and terminology into cell conceptualisation (Green, 1982; Green and Blakers, 1983).
The second of the currently important approaches was using small area contacts as an alternative, more simply implemented, approach to reducing detrimental recombination at metal-semiconductor interfaces (Green, 1975). This approach is most clearly demonstrated in the PERC cells briefly introduced above. When annealing the above 18% efficient MINP cells, Green noticed they could withstand much higher temperatures than expected, suggesting filamentary metal penetration of the oxide, along with the resulting small-area contact, could be tolerated. Using the small area contact test structure described in his earlier work (Green, 1975), NASA confirmed 687mV voltage output for his team’s devices in Sept. 1983, close to his group’s earlier record using tunnelling. The simpler processing allowed rapid improvement in efficiency with 19% efficiency confirmed by end-1983, using small-area top contacts combined with thin-oxide surface passivation, an approach Green dubbed PESC (the “passivated emitter solar cell”).
His contemporaneous conception of the more advanced PERC cell also in 1983 arose from the success of the small area contact approach, augmented by his then recently completed theoretical work on Auger-limited efficiencies, specifically showing that the then standard Al-alloyed “back surface field” (Al-BSF) would not allow these limits to be reached. Instead, he suggested “tunnelling or reduced contact area” approaches for the cell rear (Green, 1984a). His first PERC drawing was published in two reports being prepared end-1983 (Green, 1984b; Green, 1984c).
With texturing added to reduce reflection, PESC produced the first 20% and 21% efficient silicon cells in 1985 and 1988, using standard Al-BSF rear contacting. The team extended these results by fabricating the first efficient PERC cell in 1988, the first 23% cell in 1989, 24% cell in 1994 and 25% cell in 1999, all using PERC. The latter record stood for the next 15 years, encouraging PERC’s commercialisation. This began in earnest in 2012, with PERC becoming the globally dominant production technology in 2018. Over the years since, PERC has accounted for approximately 90% of global solar module production, with more PERC modules now installed than all other solar generation in human history.
The TOPCon approach using polycrystalline silicon contacts (Green and Blakers, 1983), was developed commercially by SunPower in the 1990s (Swanson and Gan, 1991) and subsequently by Tetrasun (Schultz-Wittmann and DeCeuster, 2015), before being “publicly rediscovered” by German groups in 2013 (Peibst et al., 2023). It is now the silicon cell produced in the second highest volume commercially, behind PERC, with “heterojuction” cells (HJT) third.
Japanese involvement in HJT arose from the late Professor Hamakawa’s interest in “honeymoon cells”, mating amorphous silicon/hydrogen alloy (a-Si:H) and crystalline Si cells. Citing Green’s work, Mitsubishi found that inserting a thin oxide between the two materials improved performance (Kawabata et al., 1990) with Sanyo, citing Mitsubishi’s work (Taguchi et al., 1990) finding a thin intrinsic a-Si:H layer worked better, with this subsequently standard. However, oxide followed by intrinsic a-Si:H is used in the most recent 26.8% efficient devices, with the overlying doped amorphous layers replaced by nanocrystalline, as used in the early Mitsubishi work. Sanyo marketed HJT product from 1997, with many more companies more recently involved.
The final cell of commercial interest is the “interdigitated back contact” (IBC) cell, with both polarity contacts on the cell rear. These cells are now linked to Green’s work since HJT, TOPCon or PERC contacts, or a combination of these, are now used for these rear contacts.
As well as the contributions to the above results made by his students, leading to substantial performance increase and cost reduction, Green has additionally trained the students who have had another particularly significant impact upon the industry over recent years. This has been by establishing manufacturing in low-cost Asian regions previously lacking the required expertise and infrastructure; largely via multiple Australian – Chinese joint ventures able to attract significant funding via listings on US stock-markets over the 2005-2010 period.
This manufacturing transformation was triggered by Dr Zhengrong Shi (Green’s 12th PhD student) founding Suntech, China’s first commercial cell manufacturer, in 2001. Despite limited funding (US$6 million), Suntech grew rapidly assisted by Green and other current and former team members, notably Stuart Wenham and Ted Szpitalak. Green was Suntech’s Chief Scientist – involved in its seminal IPO, prompted by US investment bank interest. Suntech was the first private Chinese company listing on the New York Stock Exchange in 2005’s largest technology float, raising US$400 million, boosting Suntech to global manufacturing leadership by 2007.
Jianhua Zhao (6th PhD student) and Aihua Wang (15th) founded CSUN, the second into volume cell manufacturing in China. Ximing Dai (18th) founded JA Solar, whilst Fei Yun (20th) and Guangfu Zheng (26th) were ‘Key Personnel’ for Solarfun (now Hanwha Q Cells), the 4th and 5th into such production. Strong competition between these pioneers – and those Additional to these technical contributions, Professor Green’s text “Solar cells: Operating principles, technology and system applications” (Prentice Hall, 1982) is the field’s most highly cited – reprinted many times, translated to Chinese, Portuguese, Arabic, Thai, Italian and Spanish. With former student, the late Stuart Wenham, he introduced the world’s first photovoltaic undergraduate engineering degree program in 2000, leading to formation of the UNSW School of Photovoltaic and Renewable Energy Engineering (SPREE) with circa 500 students now enrolled.
Green is also one of the world’s most highly cited researchers, selected as a Clarivate “Highly Cited Researcher” (top 0.1% in field) each year since the scheme’s instigation in 2014 and named “one of the most influential minds in engineering” (Google h-index >140, Cites >100,000). Based on publication impact in 2021, a Stanford University study identified him as “Top-5” worldwide in Applied Physics. He is a Fellow of the Australian Academy of Science (1991) – one of the youngest then elected – the Australian Academy of Technology and Engineering (1992) and the Royal Society, London (2013); also, an international member of the Spanish (2000) and US National (2023) Academies of Engineering.
Professional awards include the 1991 IEEE Cherry Award – the most prestigious in photovoltaics, for the first 20% silicon cell – and the 1995 IEEE Ebers Award for "sustained technical leadership in silicon photovoltaics”. In 2018, he became 10th inductee as an IEEE Celebrated Member, joining Nobel Prize winners Smith, Kroemer and Esaki, also Moore of “Moore’s Law” fame. In 2002 Green was awarded the Right Livelihood Award (aka “The Alternative Nobel Prize”) in the Swedish parliament.
Other awards include five rated by the International Rankings Expert Group (IREG) as amongst the “Top-99” academic awards globally: the 2009 ENI Renewable and Non-Conventional Energy Award (IREG 0.55, with a Nobel Prize rated 1.00); the 2018 Global Energy Prize for having “revolutionised the efficiency and costs of solar photovoltaics, making this now the lowest cost option for bulk electricity supply” (IREG 0.48); the 2021 Japan Prize for “development of high-efficiency silicon photovoltaic devices” (IREG 0.66); the 2022 Millennium Technology Prize for “innovation that has transformed the production of solar energy” (IREG 0.50); and, with former students, the 2023 Queen Elizabeth Prize for Engineering (IREG 0.51).
. Bloomberg (2023) “Energy transition investment trends”, January.
. Campbell P and Green MA (1987) “Light trapping properties of pyramidally textured surfaces”, J. Appl. Physics, Vol. 62, pp. 243-249 (Angular aspects discussed by same authors in IEEE Trans., Vol. ED-33, pp. 234-239, 1986).
. Godfrey RB and Green MA (1979) "655 mV Open Circuit Voltage, 17.6% Efficient Silicon MIS Solar Cells", Appl. Phys. Letters, Vol. 34, pp. 790-793.
. Green MA (1975) "Enhancement of Schottky Solar Cell Efficiency Above Its Semiempirical Limit", Appl. Phys. Letters, Vol. 27, pp. 287-288.
. Green MA (1982) "Solar cells: Operating principles, technology and system applications" (Prentice-Hall, NJ).
. Green MA (1984a) "Limits on the Open Circuit Voltage and Efficiency of Silicon Solar Cells Imposed by Intrinsic Auger Processes", IEEE Trans. Electron Devices, Vol. ED-31, pp. 671-678 (A paper published by Tiedje et al. later in the same Special Issue, covering some of the same material, was submitted 6-weeks after Green’s paper, after this had been reviewed by three unidentified specialists and well after the issue’s published deadline).
. Green MA (1984b) "Ultimate Performance Silicon Solar Cells", NERDDP 81/1264, Final Report, Feb. (76 pp.).
. Green MA (1984c) "High Efficiency Silicon Solar Cells", Proposal in response to RFP RB-4-04033, SERI (now NREL), March (126pp.).
. Green MA (2003) “Third Generation Photovoltaics: Advanced Solar Energy Conversion” (Springer-Verlag).
. Green MA (2019) “How did solar cells get so cheap?”, Joule, Vol.3, pp.631-633.
. Green MA and Blakers, AW (1983) “Advantages of metal-insulator-semiconductor structures for silicon solar cells”, Solar Cells, Vol. 8, pp. 3-16 (First mention and demonstration of polysilicon TOPCon for solar conversion including record voltages, outside UNSW, of 660-665mV - obtained in 1981. Earlier mentions of polysilicon for solar, without demonstration and without mention of tunnel oxide, are Fossum and Shibib, IEEE Int’l Electron Devices Meeting, Dec.1980, p.280 and Van Overstraeten, 15th IEEE PVSC, May 1981, p.372).
. Green MA et al. (1974) "Minority Carrier MIS Tunnel Diodes and their Application to Electron- and Photo-voltaic Energy Conversion - Part I: Theory" and “Part II: Experiment”, Solid-State Electronics, Vol 17, pp. 551-572 (First journal paper describing silicon MIS solar cells. A conference paper with same title and authors was presented in Toronto in 1972 reporting advantages over p-n junctions, with report AEC 5421-8 submitted to AEC Canada in December 1972, roughly contemporaneous with a late-news presentation by Charlson et al. at the Dec. 1972 IEDM, later published in 1975, although with no such advantages reported).
. Green MA et al. (1976) “Large open-circuit photovoltages in silicon minority carrier MIS solar cells”, 12th IEEE PVSC Conf., pp. 896 - 899.
. Green MA et al. (1982) “Towards a 700mV silicon solar cell”, Conf. Record, 15th IEEE PVSC, pp. 1219 – 1222.
. Green MA et al. (1984) "High Efficiency Silicon Solar Cells", IEEE Trans. Electron Dev., Vol. ED-31, pp. 679-683.
. IEA (2020) “World energy outlook”.
. IEA (2021) “Net zero by 2050: A roadmap for the global energy sector”.
. IPCC (2022) “Mitigation of climate change”, April.
. Kavlak G et al. (2018) “Evaluating the causes of cost reduction in photovoltaic modules”, Energy Policy, Vol. 123, pp. 700-710.
. Kawabata K et al. (1990) “Formation of PN junction with hydrogenated microcrystalline silicon”, 20th IEEE PVSC Conf., May 1990, p.659 - 663.
. Peibst R et al. (2023) “On the chances and challenges of combining electron-collecting nPOLO and hole-collecting Al-p+ contacts in highly efficient p-type c-Si solar cells”, Prog. PV, Vol. 31, pp. 327-340.
. Schultz-Wittmann O and DeCeuster D (2015) “Method of manufacturing high-efficiency solar cell”, US Patent 9130074B2.
. Shockley W and Queisser H (1961) "Detailed balance limit of efficiency of p-n junction solar cells", J. Appl. Physics, Vol. 32, pp. 510–519.
. Swanson RM and Gan JY (1991) “Method of fabricating polysilicon emitters for solar cells”, US Pat. 5057439.
. Taguchi M et al. (1990) “Improvement of the conversion efficiency of polycrystalline silicon thin film solar cell”, PVSEC-5, Nov. 1990, pp. 689-692.
. Yablonovitch E (1981) “Statistical ray optics”, J. Opt. Soc. Am., Vol. 72, pp. 899-907.