Stability of silicon based homojunction and heterojunction solar cells under space conditions

Two different types of n-a-Si:H/p-c-Si heterostructure solar cells with intrinsic buffer layer, one type with and one type without back surface field (BSF), and one purely crystalline silicon solar cell have been irradiated with protons at 1.7 MeV. For both types of heterojunctions and for the c-Si homojunction cells a similar dose been found for which degradation starts and also a similar decrease of the effective minority carrier diffusion length with increasing irradiation dose. For one type of heterojunction cells we observed a small increase of the collection efficiency in the blue part of the spectrum after irradiation what can be explained only by modifications of the interface between the crystalline silicon and the amorphous silicon or of the thin intrinsic a-Si:H layer. As a new tool for the evaluation of the solar cell degradation due to proton irradiation, electroluminescence measurements under forward bias have been used. The emission spectra confirms that for doses up to 5 × 1012 protons/cm2, silicon band-to-band recombination is the most important contribution to the light emission and the emission efficiency decreases monotonically with increasing proton dose.