Triplet Excitons and associated Efficiency-Limiting Pathways in Organic Solar Cell Blends based on (Non-) Halogenated PBDB-T and Y-Series

TL;DR

This study investigates how triplet excitons form in organic solar cell blends, revealing pathways influenced by halogenation and suggesting strategies to reduce losses and enhance efficiency.

Contribution

It uncovers triplet formation mechanisms in OPV blends using advanced spin-sensitive techniques, highlighting the impact of halogenation on exciton dynamics.

Findings

Triplet excitons form via non-geminate hole back transfer in all blends.

Halogenated donors facilitate intersystem crossing to triplet states.

Understanding these pathways offers routes to improve OPV efficiency.

Abstract

The great progress in organic photovoltaics (OPV) over the past few years has been largely achieved by the development of non-fullerene acceptors (NFAs), with power conversion efficiencies now approaching 20%. To further improve device performance, loss mechanisms must be identified and minimized. Triplet states are known to adversely affect device performance, since they can form energetically trapped excitons on low-lying states that are responsible for non-radiative losses or even device degradation. Halogenation of OPV materials has long been employed to tailor energy levels and to enhance open circuit voltage. Yet, the influence on recombination to triplet excitons has been largely unexplored. Using the complementary spin-sensitive methods of photoluminescence detected magnetic resonance (PLDMR) and transient electron paramagnetic resonance (trEPR) corroborated by transient absorption and quantum-chemical calculations, we unravel exciton pathways in OPV blends employing the polymer donors PBDB-T, PM6 and PM7 together with NFAs Y6 and Y7. All blends reveal triplet excitons on the NFA populated via non-geminate hole back transfer and, in blends with halogenated donors, also by spin-orbit coupling driven intersystem crossing. Identifying these triplet formation pathways in all tested solar cell absorber films highlights the untapped potential for improved charge generation to further increase plateauing OPV efficiencies.