Technical Insight
Solar cell manufacturers come back down to Earth
For III-V solar cell manufacturers, it is Earth rather than space that represents the final frontier. However, recent advances in the performance of cells and high-concentration optical modules suggest that multi-junction photovoltaics can infiltrate the silicon-dominated terrestrial energy market. Michael Hatcher reports on the latest developments.
While multi-junction III-V cells now dominate the solar energy market for space applications, their cheaper silicon rivals have maintained a stranglehold on the terrestrial photovoltaics industry.
Large-scale terrestrial solar power plants that are currently being built in Arizona and Australia rely on silicon cells, despite recent advances that have seen multi-junction cell efficiencies reach almost double that of their silicon counterparts.
Late last year the US company Amonix installed the first 35 kW unit of the 1 MW solar energy plant that it is building in Arizona. By June this year, the plant s installed capacity had reached almost 600 kW. Meanwhile, the Japanese solar systems builder Kyocera is stepping up its manufacturing output to 120 MW per year to meet increased demand for residential solar energy.
Given this dominance of silicon technology in Earth-bound photovoltaics, the opportunities for III-V technology might seem limited. However, the latest III-V technology from Emcore, Spectrolab and Sharp looks likely to change this scenario in the medium term.
This year, all three companies have pushed the efficiency of their multi-junction cells towards the 40% mark. In May, Japan-based electronics giant Sharp announced the development of its 36.5% efficiency InGaP/InGaAs/Ge cell at the 3rd World Conference on Photovoltaic Energy Conversion in Osaka. At the 11th Biannual Workshop on OMVPE in July, Boeing subsidiary Spectrolab of Sylmar, CA, announced that it had developed a 36.9% terrestrial concentrator cell (figure 1). Emcore Photovoltaics has produced its own terrestrial triple-junction cells with an efficiency of 30% at 200 suns concentration, and recently won a contract to supply the solar panels for the two STEREO satellites being built by Johns Hopkins University.
In terrestrial applications, direct sunlight must be concentrated onto the multi-junction cells using focusing optics. A significant advantage of these concentrator systems is that fewer solar cells are required to achieve a specific power output, thus replacing large areas of semiconductor materials with relatively inexpensive optics that provide optical concentration. The higher cost of multi-junction cells is offset by the use of fewer cells. Their higher efficiency means that only a small fraction of the total cell area is required to generate the equivalent power output, compared with crystalline silicon or thin-film flat-plate modules.
Nasser Karam, Spectrolab s vice-president for Advanced Technology, told Compound Semiconductor that recent improvements in space cells should result in better efficiency for terrestrial cells. "During the last few years, multi-junction solar cells have doubled the power output of large commercial satellites, and substantially improved their revenue-generating capability. We believe that further optimization of terrestrial concentrator cells will yield the potential to surpass 40% conversion efficiency," he said.
One of the major stumbling blocks in adapting multi-junction cells for terrestrial use has been the tunnel junction used to combine the currents produced from each of the three sub-cells. Compared with space applications, terrestrial solar modules with a high sunlight concentration ratio generate a much higher current density than the tunnel junctions had originally been designed to handle. Further complications arise from the need to extract current and heat efficiently from the cells without causing hot spots that can degrade cells.
Spectrolab has been tackling the tunnel junction issue on two fronts, by developing both lattice-matched and metamorphic solar cells. In the first approach, the idea is to match the lattice spacing in all three junctions to address the terrestrial spectrum. However, in both dual- and triple-junction cells, the bandgap combination of 1.8-1.9 eV for the GaInP top cell and 1.424 eV for the GaAs junction is not ideal for either the space or terrestrial spectrum. According to Spectrolab, the conversion efficiency can be improved by grading the composition of a GaInAs buffer on a germanium substrate. In this lattice-mismatched or "metamorphic" approach (figure 2), the top two sub-cells can then be grown with a higher indium content and larger lattice constant than the lattice-matched cells. This lowers the bandgap of both the GaInP and GaInAs cells, providing a better match to the terrestrial solar spectrum.
Karam said that the metamorphic and lattice-matched structures were "neck and neck" in terms of the progress made so far. The lattice-matched structure has shown a 32.5% conversion efficiency, with the metamorphic cell reaching 31.3%. "We believe that both can be improved further to reach the 40% efficiency goal under concentration in the future," he said.
Crucially, Spectrolab s new cell and receiver designs are said to be capable of supporting concentrator modules that increase the sun s irradiation by up to a factor of 1000. In Spectrolab s tests on cells optimized to operate at 300 suns concentration, the cells successfully withstood prolonged illumination under 997 suns concentration, although conversion efficiency dropped to 27.9% from 34.2% at 161 suns.
Mark O Neill is president of Entech Solar, a company based in Keller, TX, that builds concentrator solar energy systems. Entech made the 720 Fresnel lenses that were used to concentrate sunlight onto 3600 dual-junction cells used to power NASA s Deep Space 1 probe launched in 1998, which included the first solar array to use dual-junction cells as a spacecraft s major power source.
Large-scale terrestrial solar power plants that are currently being built in Arizona and Australia rely on silicon cells, despite recent advances that have seen multi-junction cell efficiencies reach almost double that of their silicon counterparts.
Late last year the US company Amonix installed the first 35 kW unit of the 1 MW solar energy plant that it is building in Arizona. By June this year, the plant s installed capacity had reached almost 600 kW. Meanwhile, the Japanese solar systems builder Kyocera is stepping up its manufacturing output to 120 MW per year to meet increased demand for residential solar energy.
Given this dominance of silicon technology in Earth-bound photovoltaics, the opportunities for III-V technology might seem limited. However, the latest III-V technology from Emcore, Spectrolab and Sharp looks likely to change this scenario in the medium term.
This year, all three companies have pushed the efficiency of their multi-junction cells towards the 40% mark. In May, Japan-based electronics giant Sharp announced the development of its 36.5% efficiency InGaP/InGaAs/Ge cell at the 3rd World Conference on Photovoltaic Energy Conversion in Osaka. At the 11th Biannual Workshop on OMVPE in July, Boeing subsidiary Spectrolab of Sylmar, CA, announced that it had developed a 36.9% terrestrial concentrator cell (figure 1). Emcore Photovoltaics has produced its own terrestrial triple-junction cells with an efficiency of 30% at 200 suns concentration, and recently won a contract to supply the solar panels for the two STEREO satellites being built by Johns Hopkins University.
In terrestrial applications, direct sunlight must be concentrated onto the multi-junction cells using focusing optics. A significant advantage of these concentrator systems is that fewer solar cells are required to achieve a specific power output, thus replacing large areas of semiconductor materials with relatively inexpensive optics that provide optical concentration. The higher cost of multi-junction cells is offset by the use of fewer cells. Their higher efficiency means that only a small fraction of the total cell area is required to generate the equivalent power output, compared with crystalline silicon or thin-film flat-plate modules.
Nasser Karam, Spectrolab s vice-president for Advanced Technology, told Compound Semiconductor that recent improvements in space cells should result in better efficiency for terrestrial cells. "During the last few years, multi-junction solar cells have doubled the power output of large commercial satellites, and substantially improved their revenue-generating capability. We believe that further optimization of terrestrial concentrator cells will yield the potential to surpass 40% conversion efficiency," he said.
One of the major stumbling blocks in adapting multi-junction cells for terrestrial use has been the tunnel junction used to combine the currents produced from each of the three sub-cells. Compared with space applications, terrestrial solar modules with a high sunlight concentration ratio generate a much higher current density than the tunnel junctions had originally been designed to handle. Further complications arise from the need to extract current and heat efficiently from the cells without causing hot spots that can degrade cells.
Spectrolab has been tackling the tunnel junction issue on two fronts, by developing both lattice-matched and metamorphic solar cells. In the first approach, the idea is to match the lattice spacing in all three junctions to address the terrestrial spectrum. However, in both dual- and triple-junction cells, the bandgap combination of 1.8-1.9 eV for the GaInP top cell and 1.424 eV for the GaAs junction is not ideal for either the space or terrestrial spectrum. According to Spectrolab, the conversion efficiency can be improved by grading the composition of a GaInAs buffer on a germanium substrate. In this lattice-mismatched or "metamorphic" approach (figure 2), the top two sub-cells can then be grown with a higher indium content and larger lattice constant than the lattice-matched cells. This lowers the bandgap of both the GaInP and GaInAs cells, providing a better match to the terrestrial solar spectrum.
Karam said that the metamorphic and lattice-matched structures were "neck and neck" in terms of the progress made so far. The lattice-matched structure has shown a 32.5% conversion efficiency, with the metamorphic cell reaching 31.3%. "We believe that both can be improved further to reach the 40% efficiency goal under concentration in the future," he said.
Crucially, Spectrolab s new cell and receiver designs are said to be capable of supporting concentrator modules that increase the sun s irradiation by up to a factor of 1000. In Spectrolab s tests on cells optimized to operate at 300 suns concentration, the cells successfully withstood prolonged illumination under 997 suns concentration, although conversion efficiency dropped to 27.9% from 34.2% at 161 suns.
Mark O Neill is president of Entech Solar, a company based in Keller, TX, that builds concentrator solar energy systems. Entech made the 720 Fresnel lenses that were used to concentrate sunlight onto 3600 dual-junction cells used to power NASA s Deep Space 1 probe launched in 1998, which included the first solar array to use dual-junction cells as a spacecraft s major power source.
