Impact of a doping-induced space-charge region on the collection of photo-generated charge carriers in thin-film solar cells based on low-mobility semiconductors

TL;DR

This paper investigates how doping-induced space-charge regions affect charge collection in low-mobility thin-film solar cells, revealing voltage-dependent photocurrent behavior and providing analytical and numerical tools to improve device performance.

Contribution

It introduces an analytical model for photocurrent behavior in doped low-mobility solar cells, validated by simulations and experiments, aiding in understanding and overcoming charge collection losses.

Findings

Photocurrent becomes voltage-dependent due to doping-induced space-charge regions.

Analytical expression accurately describes photocurrent behavior.

Experimental validation confirms the model's applicability to organic solar cells.

Abstract

Unintentional doping of the active layer is a source for lowered device performance in organic solar cells. The effect of doping is to induce a space-charge region within the active layer, generally resulting in increased recombination losses. In this work, the impact of a doping-induced space-charge region on the current-voltage characteristics of low-mobility solar cell devices has been clarified by means of analytical derivations and numerical device simulations. It is found that, in case of a doped active layer, the collection efficiency of photo-generated charge carriers is independent of the light intensity and exhibits a distinct voltage dependence, resulting in an apparent electric-field dependence of the photocurrent. Furthermore, an analytical expression describing the behavior of the photocurrent is derived. The validity of the analytical model is verified by numerical drift-diffusion simulations and demonstrated experimentally on solution-processed organic solar cells. Based on the theoretical results, conditions of how to overcome charge collection losses caused by doping are discussed. Furthermore, the presented analytical framework provides tools to distinguish between different mechanisms leading to voltage dependent photocurrents.