On the identification of deeper defect levels in organic photovoltaic devices

Abstract

Defect levels play a significant role in altering organic photovoltaic (OPV) performance, affecting device aspects such as recombination, carrier transport, and Fermi-level pinning. In the ongoing effort to optimize the promising OPV technology, the identification, characterization, and potential mitigation or enhancement of such defect states remain important regions of interest. Herein, low frequency admittance spectroscopy is coupled with a high frequency, point-by-point capacitance versus voltage measurement to reveal a previously unknown deep-defect distribution in poly(3-hexylthiophene) based OPVs. The capacitance models of Cohen and Lang, Walter et al. and Kimmerling are employed alongside a trap-free dark current model to give good characterization and substantiation to the discovered band. Repetitions of the measurements on devices with and without a fullerene acceptor show the measured distribution to contain acceptor-like traps spatially located in the polymer bulk. The findings presented here are important for the understanding and optimization of organic solar cells and we expect the presented methods to be generally applicable to other OPV material sets.