From 1999 - The 1999 Australia Prize
Dean's Awards 2004
Awarded for Excellence in the Field of Energy Science and Technology*
* Please note: This article was written in 1999, and some of the information contained therein may be outdated.
The Race For Solar Cell Efficiency
The winners of the 1999 Australia Prize offered in the field of Energy Science and Technology, Professor Martin Green and Professor Stuart Wenham, say a switch from fossil fuel-generated power to solar power is inevitable because of changes to the world's climate caused by the Greenhouse Effect.
Professor Green and Professor Wenham, of the University of New South Wales, have held the world record for solar cell efficiency for more than a decade and worldwide sales of products using their innovative technology are expected to total billions of dollars over coming decades.
Green and Wenham point to the fact that forty four per cent of Australia's Greenhouse gas production comes from burning fossil fuels for electricity production and say that photovoltaics offers clean electricity to a world grasping for ways to counter Greenhouse.
Photovoltaics are ideally suited to meet the power needs of the two billion people on Earth who presently have no access to electricity.
Before Green and Wenham's ground-breaking work on solar cells in the 1980s, the technology had been in stagnation for more than 20 years with the world's best solar cells converting only 15 per cent of sunlight into electricity. This was thought to be the highest efficiency that practical cells could achieve.
Last year, cells produced by the Green and Wenham team at University of New South Wales' Photovoltaics Special Research Centre achieved 24.5 per cent efficiency, the current world record by a large margin.
Professor Green is the Director of the Centre, and Professor Wenham is the Director of the University's Key Centre of Teaching and Research in Photovoltaics.
The awarding of the 1999 Australia Prize to Martin Green and Stuart Wenham represents only the second time in the ten year history of the Prize that it's been won by an all-Australian team. This is an indication of the pair's dominance in the world of photovoltaic research.
Green and Wenham's research into photovoltaics has not followed trends set overseas. "We've been exploring our own ideas," says Green. "The work that led us to first break the world efficiency record for solar cells involved an unconventional cell design: the buried contact cell. We got ahead of the rest of the world by doing something completely different."
Martin Green established a research program in photovoltaics in 1974 at the University of New South Wales, and soon attracted international attention for his theoretical and laboratory work.
However it wasn't until the company, Tideland, decided to set up a solar cell manufacturing facility in Sydney in 1980 that the results of this work on solar power moved out of the laboratory and into the energy marketplace.
Tideland employed two of Green's former students at its manufacturing facility. One of them was Stuart Wenham.
Wenham had been interested in solar energy since his high school days. Tideland employed him to establish its production line for solar cells and, in 1982, the company's cells achieved a conversion efficiency of 14.3 per cent, a world record at the time for a commercially-produced solar cell.
Meanwhile, Martin Green had become focused on what was termed the "four minute mile" of his field - to produce the first silicon cell capable of converting 20 per cent of incident sunlight energy into electricity, an efficiency then thought to be the absolute limit for solar cells.
Through fundamental research, Green determined that this limit arose from a specific energy loss mechanism in existing cells. To appreciate the significance of this insight, it is necessary to understand the internal workings of a solar cell.
A solar cell is made of a semiconductor, usually silicon, which absorbs sunlight. During absorption, the energy from the sunlight rips free electrons from atoms within the silicon. These energised electrons - electricity - then flow to the negative contact in the cell and from there move through a connected wire to which an electrical appliance can be attached. Electrons passing through an appliance give up their power and then flow back to the positive metal contact in the solar cell, returning to the atoms from which they were released. This process is endlessly repeated.
In his fundamental work in the early 1980s, Green found that the energy loss that limited the performance of silicon solar cells occurred when a photo-excited electron gave up its energy to a neighbour, which was unable to use it in the photovoltaic process to create electricity. Based on this insight, Green predicted that solar cells could eventually achieve an efficiency of 30 per cent. The problem was at that stage solar cells were still less that 20 per cent efficient.
Enter Stuart Wenham, who joined Martin Green in 1983, to begin what has become one of the most productive collaborations in Australian science.
Interestingly, some of the pair's best work came in the hours following their gruelling, twice weekly games of squash. "We'd grind each other into near exhaustion on the squash court," says Wenham. "And after the game, we'd flake out in our chairs and have these great brain storming sessions. They were extremely productive and led to a lot of good ideas. We both find squash a bit demanding these days so we have lunch together instead. That's where we brainstorm now."
Squash may not be a required subject towards a science degree, but perhaps it should be because, in 1983, Green, Wenham and their team boosted the highest independently confirmed solar cell efficiency from 16.4 per cent to 18 per cent.
Soon after, Stuart Wenham began exploring the use of lasers to etch patterns on the surface of a solar cell to reduce light reflection. The work resulted in cells with sophisticated surface geometries, which, in 1985, saw the UNSW team report the production of the world's first 20 per cent efficient solar cell. Green and Wenham had achieved their "four minute mile" and, in doing so, had focused world attention on the Photovoltaics Special Research Centre at the University of New South Wales.
The 20 per cent cell incorporated geometric surface structures based on Wenham's earlier work, a double layer of anti-reflection coating, a thin layer of oxide along the top surface and a smaller contact area to improve the output voltage. The device proved relatively easy to manufacture and is now extensively used on spacecraft and in solar car racing.
Prior to this breakthrough, in 1983, Wenham and Green stumbled on a process which would result in the most successfully commercialised photovoltaic device: the buried contact solar cell.
Like so many advances in science, the buried contact solar cell was something of an accident. One afternoon, Wenham was screen printing metal lines to form contacts on a new type of solar cell that had laser scribed groves running perpendicular to the metal lines. During this process, he found that the viscosity of the metal paste he was using was lower than usual, causing it to ooze down into the grooves in the wafer, rather than bridge across them. "That led us to the idea that relocating the metal contact to within the grooves could solve the limitations imposed by the conventional screen printing approach," says Wenham.
The buried contact solar cell soon achieved efficiency close to 20 per cent - compared with the 14 per cent efficient screen printed cell - and it was no more difficult to make.
Tideland bought licensing rights to the buried contact technology in 1985 and, in the same year, BP Solar purchased Tideland and with it the rights to commercialise the buried contact technology.
Buried contact solar cells, which have dominated some of the major solar car races across the world over the past decade, produce up to 30 per cent more energy than competing technologies. They are 20 per cent cheaper to produce and, last year, they became the largest manufactured solar cell technology in Europe.
Evidence of the superiority of the cells came during the 1993 Sunrayce, a solar car race across the United States in which all leading commercial cell technologies were used. Approximately half of the teams chose to use buried contact solar cells fabricated by BP Solar under licence to the University of New South Wales. Nine of the top ten place getters, including the top five went to teams using buried contact solar cells.
The cells have seen BP Solar emerge from relative market obscurity to international leadership in solar energy sales and the technology has been licensed to many of the world's largest solar manufacturers.
The early 1980s were a very productive time for the UNSW team. In 1984, Green suggested changes to the backs of solar cells which he said might increase their internal light reflectance and thus increase their "light trapping" ability.
The University of New South Wales team spent years improving the contacts at the rear of cells and, in 1995, reported a record 21.5 per cent efficiency for a solar cell of a thickness of only 47 microns - about the diameter of a human hair. The cell had demonstrated a light trapping ability equivalent to a device 50 times its thickness.
Green and Wenham have invented, or co-invented, seven distinct cell technologies over the past 15 years. In 1990, their group once again broke their own world efficiency record with a cell that achieved efficiencies of 23.2 per cent. This cell featured a surface of inverted pyramids to reduce reflection, improve internal light trapping, and was almost completely covered by oxide, with small area contacts on both the front and rear surfaces.
In 1996 the Honda Dream car, powered by these cells, established a new solar race record averaging speeds of 90 kph. The experience gained through building the "Dream" cells enabled the University of New South Wales team last year to increase silicon cell efficiency to 24.5 per cent, the current world record for cell efficiency.
With the commercialisation of their buried contact solar cell well advanced, 1987 saw Green and Wenham start work on a long term project to develop the next generation of low cost solar cells.
This was a technology which incorporated a thin film of multilayered cells set on a solid base. "The holy grail of photovoltaic research has long been to successfully deposit a thin film of solar cells onto a cheap substrate," says Martin Green.
"Other researchers had tried a variety of exotic and expensive materials for use in this thin film technology, but these efforts have problems with the availability and toxicity of these materials," he says. "We decided to simply choose the material we wanted to use and we then looked at how to make a high performance cell using that material."
Green and Wenham chose silicon, the material used in most photovoltaic cells. "We knew a lot about silicon, it's a benign material, and it's the second most abundant element in the Earth's crust after oxygen," says Green.
The resulting technology incorporates buried contact technology with the use of several parallel electricity collection junctions, which allow the whole cell to be active regardless of the material quality. The different layers of silicon are deposited uniformly over a large glass sheet, which is partitioned into individual cells.
The University of New South Wales and Pacific Power formed a company - Pacific Solar - in 1995 to commercialise this multilayer technology, and the establishment of the company's production line is well advanced.
Pacific Solar believes its multilayer cells will be priced to allow a dramatic increase in the number of solar powered residences.
The current cost of powering a house with solar panels is about $30,000, the major expense being the silicon wafers in the solar cells. "Our success in depositing thin layers of silicon cells onto glass changes the economics of solar power," says Green. "There's no longer a massive material cost. No longer do you make the cell on individual wafers. These large sheets of glass become your production unit and that radically reduces manufacturing costs."
Each of the homes in the Athletes' Village for the Sydney 2000 Olympic Games will generate its own electricity using solar cells developed by Green and Wenham. In 2002, Pacific Solar will release a home solar power package which will bring a substantial reduction in price for those buying into the technology.
Stuart Wenham says the economics of solar technology, rather than its efficiency, will be the big issue over the next few years.
The cost of producing one watt of solar electricity is presently about $4. Stuart Wenham points to a number of international studies which show that the photovoltaic industry will grow by between one hundred and one thousand fold bringing the cost of solar power down to $1 per watt.
"I expect that within a few years of our new thin film technology coming onto the market, the cost of solar electricity could drop to $1 per watt," says Wenham. "It's then that I'd expect the photovoltaic industry to reach a critical mass which will see an enormous growth in consumer uptake of the technology.
"The next couple of years will see a tripling of the manufacturing capacity for photovoltaic cells," says Wenham, "And as the market grows, the economies of scale will lead to further price reductions which will further stimulate the market. There is positive feedback in the system. Ultimately, most houses will have photovoltaic cells on their roof tops, perhaps imbedded in their roof tiles, generating most of the electricity they require."
Green and Wenham have invented a roof tile with an imbedded solar cell. The University of New South Wales is collaborating with a Japanese company to commercialise the tile. Stuart Wenham says the uptake of photovoltaics will not be universal. "It will always involve a substantial outlay to put the cells on your roof," he says.
Martin Green predicts that, within 30 years, every new house could be photovoltaically active, and the electricity houses produce will be cheaper than that supplied by coal fired power stations "This technology will become even cheaper over time and, of course, it will be even further refined," he says. "We're now working on photovoltaic technology with an efficiency of 30 per cent, but it's hard to see it getting above that and remaining low in cost."
Martin Green says that as the consequences of the Greenhouse Effect become apparent, they will work to speed up the dawning of the photovoltaics era. "I don't see governments' role in photovoltaics as directly investing in the technology," he says, "but government must encourage commercial interests to invest in the technology through tax breaks, appropriate regulations and other incentives."
Professor Wenham says solar cells' big advantage over conventional forms of power generation is that they produce electricity at times when the demand for electricity is highest - in summer, when air conditioners are in high use, and during the day when industry requires a lot of electricity. "Households equipped with photovoltaics will be able to export their excess electricity onto the grid during the peak demand daylight hours," says Wenham, "And draw electricity from the grid at night."
Stuart Wenham pays tribute to Professor Martin Green: "Martin is the world's most successful photovoltaic researcher," he says. "We work with a team of extremely talented researchers at the University of New South Wales, and there are similar teams around the world working on the same problems we're trying to solve. Martin's leadership has given us the edge over all of them."
Martin Green says his 20 year partnership with Stuart Wenham has been extremely harmonious and productive. "We complement each other well in the way we think about things and provide sounding boards for each other's ideas," he says. "It's very hard to see how any of this would have come about if we hadn't been able to work so well together."
Stuart Wenham says winning the Australia Prize is an absolute thrill. "I was flattered that I was considered worthy to be nominated alongside Martin for the Award," he says. "I'm particularly pleased that the award focuses on those areas of science that promote human welfare."
"The researchers I've worked with over the years are all highly motivated by the opportunity to use science and technology in a way that makes a difference, not only to the environment, but also to those less well off around the world. Many of our research group have been attracted to this work because they believe that it can make a difference."
Martin Green says the winning of the Australia Prize will help advance the commercial credibility of the technology he and Wenham invented.
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Professor Martin Green and Professor Stuart Wenham
Before Professor Martin Green and Professor Stuart Wenham's ground-breaking work on solar cells in the 1980s, photovoltaic technology had been in stagnation for more than 20 years with the world's best solar cells converting only 15 per cent of sunlight into electricity. This was thought to be highest efficiency that practical cells could achieve.
Martin Green and Stuart Wenham, from the University of New South Wales, have invented or co-invented seven distinct cell technologies over the past 15 years.
These solar cells have held the world efficiency record for converting sunlight into electricity for more than a decade and last year achieved an efficiency of 24.5 per cent, the current world record by a large margin.
Worldwide sales of products using Wenham and Green's innovative technology are expected to total billions of dollars over coming decades.
Professor Green is the Director of the Photovoltaics Special Research Centre and Professor Wenham is Director of the University's Key Centre of Teaching and Research in Photovoltaics.
The awarding of the 1999 Australia Prize in Energy Science and Technology to Professors Green and Wenham represents only the second time in the ten year history of the Prize that it's been won by an all-Australian team. This is an indication of the pair's dominance in the world of photovoltaic research.
Their Buried Contact Solar Cells have dominated some of the major solar car races across the world over the past decade. The cells produce up to 30 per cent more energy than competing technologies, they are 20 per cent cheaper to produce, and last year they became the largest manufactured solar cell technology in Europe.
Each of the homes in the Athletes' Village for the Sydney 2000 Olympic Games will generate its own electricity using solar cells developed by Green and Wenham.
In 2002, Pacific Solar, which is partly owned by the University of New South Wales, will release a home solar power package using thin film multilayered solar cells developed by Green and Wenham. This power package will bring a substantial price reduction for those buying into solar power technology.
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Web site addresses - http://www.abc.net.au/science/slab/ozprize99/default.htm
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