Determining Ultra-low Absorption Coefficients of Organic Semiconductors from the Sub-bandgap Photovoltaic External Quantum Efficiency

TL;DR

This paper introduces a method to accurately measure ultra-low absorption coefficients of organic semiconductors by fitting EQE spectra across different thicknesses, accounting for interference effects, thus improving understanding of sub-gap states.

Contribution

It presents an iterative transfer matrix method to determine sub-bandgap absorption coefficients from EQE spectra, addressing limitations of traditional spectrophotometry.

Findings

Accurate sub-gap absorption coefficients were obtained for two organic systems.

Interference effects significantly influence EQE spectral interpretation.

The method enhances understanding of sub-gap states in organic semiconductors.

Abstract

Energy states below the bandgap of a semiconductor, such as trap states or charge transfer states in organic donor acceptor blends, can contribute to light absorption. Due to their low number density or ultrasmall absorption cross-section, the absorption coefficient of these states is challenging to measure using conventional transmission reflection spectrophotometry. As an alternative, the external quantum efficiency (EQE) of photovoltaic devices is often used as a representative of the absorption coefficient, where the spectral line shape of the EQE is considered to follow the absorption coefficient of the active layer material. In this work, it is shown that the subbandgap EQE is subject to thickness dependent low finesse cavity interference effects within the device, making this assumption questionable. A better estimate for the absorption coefficient is obtained when EQE spectra corresponding to different active layer thicknesses are fitted simultaneously for one attenuation coefficient using an iterative transfer matrix method. The principle is demonstrated for two model acceptor-donor systems (PCE12ITIC and PBTTTPC71BM) and accurate subgap absorption coefficients are determined. This approach has particular relevance for both understanding sub-gap states and their utilization in organic optoelectronic devices.