H₂ diluted low temperature deposited a-Si:H, initial degradation and modeling

In a previous study of the hydrogen diluted low temperature deposited (HDLTD) a-Si:H materials no difference was found between the phototransport properties of these materials and those of the standard 'undiluted' materials deposited at a substrate temperature of 260°C. However, solar cells made of HDLTD materials exhibited higher open circuit voltage, initially faster light-induced degradation, followed by saturation of this degradation. Here we report a study of the initial degradation of the standard and the HDLTD materials using the steady state photocarrier grating (PCG) technique. The results reveal differences in the degradation of the minority carrier, mobility-lifetime product, (μτ)_h, that are consistent with the degradation of the cells. However, the degradation of the majority carrier product, (μτ)_e, of the two materials is the same. Our model suggests that the probable cause for the higher V_oc and saturated degradation in HDLTD material is a lower density of valence band tail states.