Concentrated PV Cells
October 27, 2011, International Solar Energy Technology Conference, Santa Clara, CA—Homan Yuen from Solar Junction described efforts in materials processing to engineer band gaps and achieve higher efficiencies. The technology of choice for this type of material is flat plate thin film.
Concentrated solar comes in two flavors; thermal and PV. Concentrators work at light intensity levels between 500 and 1,500 sun equivalents through lenses or shaped mirrors. The dropping costs for the semiconductor materials and the engineered band gaps leads to greater efficiencies and lower costs. Concentrator PV is moving into the mainstream as changing technologies, costs, and other factors reduce costs and improve performance.
A multi-junction device can spread out the power peak, increase peak power production, and increase efficiency. Other technologies are hitting plateaus and thus have to be de-rated at higher temperatures. The areas where concentrators have seen the most activity are in increasing light concentration, volume and scale, and efficiency in the cells.
For highest conversion efficiency, manufacturers have to engineer the band gaps to balance current production across the light spectrum. One compound in research is InGaP/GaAs/Ge. The band-gap tuning has to be in a lattice-matched material to minimize lattice dislocations, which reduce efficiency. By adding InAs, they decrease the lattice constant and have an easier time engineering the bandgap.
Production is in a standard LPCVD process with MOCVD and molecular electron beam epitaxy. Other technologies are too immature to consider at this time. The advantages of the engineered thin layers are high efficiency, higher output voltage, and reduced IR loss. They could add more junctions to gather other wavelengths of light or to balance current.
Because the cells operate in concentrated light, they have to work at high temperature. The bandgap goes down at higher temperatures which reduces the output voltage. They have to consider the optics to optimize the cell area. They could tune the junctions for local conditions like spectrum, time of day, seasons, and prevailing atmospheric conditions. At the same time, they have to work to get everything working together, since the tracking mechanisms are complex systems in their own right.
