Anticorrelated Photoluminescence and Free Charge Generation Proves Field-Assisted Exciton Dissociation in Low-Offset PM6:Y5 Organic Solar Cells

TL;DR

This study demonstrates that in low-offset PM6:Y5 organic solar cells, photocurrent generation is governed by field-dependent exciton dissociation, revealed through anticorrelated photoluminescence and free charge generation measurements.

Contribution

The paper introduces a new TDCF method (m-TDCF) and establishes the direct link between photoluminescence and free charge generation in low-offset organic solar cells.

Findings

Photocurrent is limited by field-dependent free charge generation.

Anticorrelation between photoluminescence and free charge generation.

Field-assisted exciton dissociation controls photocurrent in low-offset systems.

Abstract

Understanding the origin of inefficient photocurrent generation in organic solar cells with low energy offset remains key to realizing high performance donor-acceptor systems. Here, we probe the origin of field-dependent free charge generation and photoluminescence in non-fullerene acceptor (NFA) based organic solar cells using the polymer PM6 and NFA Y5 - a non-halogenated sibling to Y6, with a smaller energetic offset to PM6. By performing time-delayed collection field (TDCF) measurements on a variety of samples with different electron transport layers and active layer thickness, we show that the fill factor and photocurrent are limited by field-dependent free charge generation in the bulk of the blend. We also introduce a new method of TDCF called m-TDCF to prove the absence of artefacts from non-geminate recombination of photogenerated- and dark charge carriers near the electrodes. We then correlate free charge generation with steady state photoluminescence intensity, and find perfect anticorrelation between these two properties. Through this, we conclude that photocurrent generation in this low offset system is entirely controlled by the field dependent exciton dissociation into charge transfer states.