Watching Space Charge Build up in an Organic Solar Cell

TL;DR

This paper introduces a new optical method to monitor space charge buildup in organic solar cells caused by imbalanced transport, aiding material screening and understanding charge collection efficiency.

Contribution

A novel optical reconstruction technique is developed to detect space charge effects due to imbalanced transport in operating organic solar cells.

Findings

The method effectively distinguishes space charge effects in real devices.

Balanced transport is not essential for efficient thick-film OPVs when recombination is low.

Numerical and analytical models predict the impact of transport imbalance on charge collection.

Abstract

Space charge effects can significantly degrade charge collection in organic photovoltaics (OPVs), especially in thick-film devices. The two main causes of space charge are doping and imbalanced transport. Although these are completely different phenomena, they lead to the same voltage dependence of the photocurrent, making them difficult to distinguish. In this work, a method is introduced how the build-up of space charge due to imbalanced transport can be monitored in a real operating organic solar cell. The method is based on the reconstruction of quantum efficiency spectra and requires only optical input parameters that are straightforward to measure. This makes it suitable for the screening of new OPV materials. Furthermore, numerical and analytical means are derived to predict the impact of imbalanced transport on the charge collection. It is shown that when charge recombination is sufficiently reduced, balanced transport is not a necessary condition for efficient thick-film OPVs.