Analysis of bulk and interface defects in hydrogenated amorphous silicon solar cells by Fourier transform photocurrent spectroscopy

A series of hydrogenated amorphous silicon (a-Si: H) p-i-n solar cells with varying absorber thicknesses (t(i) = 10-500 nm) were investigated by Fourier transform photocurrent spectroscopy to determine the sub-andgap absorption originating from the native and metastable defects. Differences between annealed and degraded states of a-Si: H solar cells, as well as between diode and triode plasma-enhanced chemical vapor deposition methods for the absorber layer depositions were analyzed. Despite having a comparable annealed state performance to diode-deposited cells, the triode-deposited a-Si:H solar cells exhibit notably less light-induced metastable defects over the whole range of absorber thickness. Furthermore, the analysis of thin solar cells (t(i) < 100 nm) suggests that defects are not preferentially present at the interfaces of the intrinsic layer, but that there are regions with a higher absorption coefficient than the bulk of the absorber near the doped layers. Finally, we show that the solar cell performance, particularly the fill factor, is directly governed by the total amount of sub-gap defects, regardless of the thickness, state (annealed or degraded), and deposition method. (C) 2015 AIP Publishing LLC.