Bulk and Interface Degradation of Amorphous Silicon / Crystalline Silicon Heterojunction Solar Cells Under Proton Irradiation

Two different types of n-type amorphous silicon/p-type crystalline silicon heterojunction solar cells-with and without insertion of a thin intrinsic a-Si:H layer-have been irradiated with proton doses between 5.10¹⁰ and 5.10¹² protons/cm₂ at 1.7 MeV. They have been investigated as well by classical measurement techniques like spectral response and current-voltage characteristics under illumination as well as by electroluminescence measurements of the forward biased solar cell. As another interface sensitive technique, admittance spectroscopy has been applied before and after irradiation. Recently we have shown under which sample preparation conditions this latter technique can be applied to large area solar cells without the need to prepare special test structures [1]. Regarding the insertion of a thin intrinsic silicon layer at the interface between the n-type a-Si:H top layer and the p-type c-Si substrate (HIT structure), we find that this layer does not change the degradation behavior of the effective minority carrier diffusion length (obtained from spectral response measurements) in the crystalline silicon. The resulting damage constant, k_L, is 1.2 10^-6. Solar cell efficiencies dropped to slightly less than 50% of the original values for irradiation doses of 5 10¹² protons/cm². These results are comparable to the degradation found for crystalline silicon homojunction solar cells [2]. Comparing admittance spectroscopy and electroluminescence efficiency measurements, we found that the latter technique is more sensitive to proton irradiation induced interface modifications. In particular we observed a stronger degradation after irradiation for the heterostructure with the insertion of the intrinsic a-Si:H layer. The electroluminescence is dominated by the crystalline silicon band-to-band recombination and decreases monotonically for increasing irradiation doses