Effect of Chromatic Aberrations on Two-Junction, Two-Terminal Devices in a Concentrator System

Of the modules monitored in PVUSA (Photovoltaics for Utility-Scale Applications), the one with the highest efficiency has been the ENTECH concentrator system. If the 18%-efficient Si cells in this module could be replaced by 26%-28% efficient two-terminal, two-junction (Ga_0.5In_0.5P/GaAs) devices, the efficiency of the system could be increased. However, two-junction, two-terminal devices present a special problem in that they must be current matched to retain their high efficiencies. This implies that variations in the spectrum caused by the focusing optics could be a potential problem. In this study we undertake to assess the severity of this problem and find that, when properly aligned, ENTECH's current lens introduces spectral nonuniformities which cause about 4% loss in power output for two-terminal Ga_0.5In_0.5P/GaAs devices compared with similar four-terminal devices. The power loss increases significantly if the lens is not correctly aligned, and can be diminished somewhat by reducing the maximum refraction angle of the lens from 40° to 30°. While the spectral nonuniformities reduce the power output, they also reduce the sensitivity of the system to variations in the spectrum. Thus, we conclude that, while care should be taken to minimize losses from chromatic aberrations, because of the variable spectrum, chromatic aberrations are unlikely to be a serious problem in any well designed concentrator system.