Method to determine the collection length in field-driven a-Si _1-xGe _x:H solar cells

Abstract

Amorphous silicon germanium alloys are widely used as a p-i-n bottom cell for amorphous silicon based tandem solar cells due to its small and tailored band gap. Decreasing the band gap is achieved by increasing the germanium atomic content. Increasing the germanium content reduces the material electronic quality and hence decreases the collection probability of the generated photocarriers. The determination of the collection distance in p-n solar cells is determined from the internal quantum efficiency because the minority carrier diffusion dominates. This method is not valid for p-i-n structures because the carrier drift dominates. This contribution presents a simple method to determine the collection distance for drift-driven solar cells from the measured quantum efficiency. The method is self checked by reproducing i-layer thickness beside the collection distance. We show that increasing the hydrogen dilution from 20 % to 25 % increases the collection length from 146 nm to 194 nm. The method estimates the original absorber thickness with error < 3%. © 2011 Published by Elsevier Ltd.