Role of the Fermi level in the formation of electronic band-tails and mid-gap states of hydrogenated amorphous silicon in thin-film solar cells

Abstract

Hydrogenated amorphous silicon solar cells in p-i-n and n-i-p configurations were made with the intrinsic absorber layer deposited at different temperatures, between 200 and 350 °C. Using Fourier-transform photocurrent spectroscopy, the sub-gap absorption was measured, allowing the evaluation of the band-tail width and mid-gap defect quantity of the intrinsic absorber layer of the working device. When deposited at high temperature (>200 °C), p-i-n cells showed a larger performance decrease than n-i-p cells, along with broader band-tails as well as a larger number of defects created in the absorber layer. Hydrogen content measurements showed that for high temperature deposition (>200 °C), the Si-H bond becomes markedly less stable if the Fermi level of the intrinsic layer is shifted toward the valence band by an adjacent p-layer. Furthermore, by annealing samples at different stages of their layer stack deposition, the impact of the band-tail and mid-gap defect states on the open-circuit voltage and on the fill factor was evaluated. Based on these insights, we propose a model to predict the losses of solar cell parameters.