The role of charge recombination to spin-triplet excitons in non-fullerene acceptor organic solar cells

TL;DR

This paper investigates charge recombination in non-fullerene acceptor organic solar cells, revealing that triplet excitons significantly reduce efficiency and proposing a new design strategy to suppress this recombination for higher power conversion efficiencies.

Contribution

It introduces a novel approach to prevent triplet exciton formation, reducing non-radiative recombination and improving OSC efficiency beyond 20%.

Findings

Majority of charge recombination proceeds via non-emissive triplet excitons.

Recombination via triplet excitons reduces open-circuit voltage by 60 mV.

Designing hybridisation between T1 and 3CTE suppresses triplet formation.

Abstract

The power conversion efficiencies (PCEs) of organic solar cells (OSCs) using non-fullerene acceptors (NFAs) have now reached 18%. However, this is still lower than inorganic solar cells, for which PCEs >20% are commonplace. A key reason is that OSCs still show low open-circuit voltages (Voc) relative to their optical band gaps, attributed to non-radiative recombination. For OSCs to compete with inorganics in efficiency, all non-radiative loss pathways must be identified and where possible, removed. Here, we show that in most NFA OSCs, the majority of charge recombination at open-circuit proceeds via formation of non-emissive NFA triplet excitons (T1); in the benchmark PM6:Y6 blend, this fraction reaches 90%, contributing 60 mV to the reduction of Voc. We develop a new design to prevent recombination via this non-radiative channel through the engineering of significant hybridisation between the NFA T1 and the spin-triplet charge transfer exciton (3CTE). We model that the rate of the back charge transfer from 3CTE to T1 can be reduced by an order of magnitude, allowing re-dissociation of the 3CTE. We then demonstrate NFA systems where T1 formation is suppressed. This work therefore provides a clear design pathway for improved OSC performance to 20% PCE and beyond.