Triplet Excitons Reconcile Charge Generation and Recombination in Low-Offset Organic Solar Cells: Efficiency Limits from a 5-State Model

TL;DR

This paper introduces a comprehensive 5-state rate model for organic solar cells that captures key photophysical processes, explaining efficiency limits and guiding material selection based on energy offsets and triplet properties.

Contribution

The novel 5-state model integrates singlet and triplet states to accurately predict device performance and elucidate the role of triplet states in charge recombination and efficiency.

Findings

Triplet states significantly influence charge recombination pathways.

Optimal energy offset for maximum efficiency is around 150 meV.

Model accurately predicts efficiency limits and device properties.

Abstract

The power conversion efficiency of organic solar cells has recently improved beyond 20%. The active layers of these devices comprise of at least two organic semiconductors, forming a type II heterojunction. Hereby, the device performance is determined by the kinetic interplay of various species, including localized excitons, charge transfer states as well as charge-separated states. However, a model which describes all relevant photovoltaic measures has yet to be developed. Herein, we present a comprehensive 5-state rate model which includes both singlet and triplet charge transfer states and takes into account the formation, re-splitting and decay of the local triplet state, parametric in the respective energy offset. We show that this model not only describes key device properties such as charge generation efficiency, photoluminescence, electroluminescence and Langevin reduction factor simultaneously but also elucidate how these vary across material combinations based on the D:A interfacial energy offset alone. We find that the electroluminescence and Langevin reduction factor depend strongly on the triplet properties and that the triplet decay becomes the dominant charge recombination pathway for systems with moderate offset, in full agreement to previous experimental results. Validation against literature data demonstrates the model's ability to predict the device efficiency accurately. Subsequently, we identify material combinations with singlet exciton to charge transfer state energetic offset of roughly 150meV as particularly promising. Our model explains further why recent certified efficiency records for binary blends remain at ca. 20% if no further means to improve photon and charge carrier harvesting are taken.