Effects of proton irradiation on upright metamorphic GaInP/GaInAs/Ge triple junction solar cells

The electrical parameters and external quantum efficiency (EQE) of 3 MeV and 8 MeV proton beam irradiated upright metamorphic (UMM) GaInP/GaInAs/Ge triple-junction solar cells, grown by metal-organic chemical vapor deposition, have been investigated and compared to lattice-matched (LM) GaInP/GaInAs/Ge solar cells. High-resolution X-ray diffraction was used to study the relaxation of strain by analysing reciprocal space maps. Threading dislocation density was estimated from cathodoluminescence image. SRIM simulation results have been applied for analysing the irradiation induced displacement damage and its effects on cell performance. The results show that the electrical parameters of both UMM and LM cells degraded more by 3 MeV proton compared to 8 MeV proton irradiation. The degradation of Voc and Isc depends on each other in both UMM and LM cells but have different features under proton irradiation due to different cell configurations, materials, and parameters etc. EQE spectra of UMM and LM cells mainly degrades in longer wavelength region due to the reduction of minority carrier diffusion length. Top GaInP subcell in UMM cell shows better radiation resistance than LM cell due to the higher In-P composition, and middle GaInAs middle subcell in UMM structure shows weaker radiation resistance because of the relatively higher indium composition.