Multi Junction Cells for Standard and Spectrum Splitting Concentrated Photovoltaics- Indoor and Outdoor Performances

In this paper we report the experimental results obtained from the manufacturing and characterization of III-V solar cells based on single junction, dual junction and triple junction structures. The main solar cell materials are GaAs and (Al)InGaP compounds. A test plan for qualification of “bare” solar cells for concentration systems is presented with its results. The test plan was derived from the space experience and it is based on the European Standards ECSS-E ST20-08, modified keeping into account the terrestrial standard for CPV modules (IEC 62108). The solar cells performances and their reliability is discussed with emphasis to the particular operation conditions induced by the CPV approach. These solar cells were then used and tested in two different CPV approaches: one based on standard concentrators and the other based on spectrum splitting concentrators. The systems, designed for standard concentration, need very high efficiency solar cells, therefore triple junction solar cells InGaP/GaAs/Ge are preferable. HCPV (High Concentration Photovoltaic) systems, based on this approach, have demonstrated efficiencies as high as 29%. The limit of this approach is that the multi junction solar cell structure is optimised for a specified incident spectrum (generally AM1.5D). During the day the spectrum is changing and the efficiency cannot be maintained, especially in the early morning and in the late afternoon. Spectrum splitting systems divide the light in several wavelength regions and use single or dual junction solar cells, optimised on each wavelength region. In this case a very high efficient system can be obtained that is less affected by spectrum variation. The use of standard concentrator and spectrum splitting systems is compared from a theoretical, experimental and economical point of view. The first results of outdoor performances are presented and discussed.