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Geminate and Nongeminate Pathways for Triplet Exciton Formation in Organic Solar Cells
Author(s) -
Privitera Alberto,
Grüne Jeannine,
Karki Akchheta,
Myers William K.,
Dyakonov Vladimir,
Nguyen ThucQuyen,
Riede Moritz K.,
Friend Richard H.,
Sperlich Andreas,
Gillett Alexander J.
Publication year - 2022
Publication title -
advanced energy materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 10.08
H-Index - 220
eISSN - 1614-6840
pISSN - 1614-6832
DOI - 10.1002/aenm.202103944
Subject(s) - intersystem crossing , fullerene , organic solar cell , exciton , materials science , acceptor , electron acceptor , chemical physics , photochemistry , polymer solar cell , energy conversion efficiency , optoelectronics , polymer , chemistry , atomic physics , physics , excited state , condensed matter physics , singlet state , organic chemistry , composite material
Organic solar cells (OSCs) have recently shown a rapid improvement in their performance, bringing power conversion efficiencies to above 18%. However, the open‐circuit voltage of OSCs remains low relative to their optical gap and this currently limits efficiency. Recombination to spin‐triplet excitons is a key contributing factor, and is widely, but not universally, observed in donor–acceptor blends using both fullerene and nonfullerenes as electron acceptors. Here, an experimental framework that combines time‐resolved optical and magnetic resonance spectroscopies to detect triplet excitons and identify their formation mechanisms, is reported. The methodology is applied to two well‐studied polymer:fullerene systems, PM6:PC 60 BM and PTB7‐Th:PC 60 BM. In contrast to the more efficient nonfullerene acceptor systems that show only triplet states formed via nongeminate recombination, the fullerene systems also show significant triplet formation via geminate processes. This requires that geminate electron–hole pairs be trapped long enough to allow intersystem crossing. It is proposed that this is a general feature of fullerene acceptor systems, where isolated fullerenes are known to intercalate within the alkyl sidechains of the donor polymers. Thus, the study demonstrates that engineering good donor and acceptor domain purity is key for suppressing losses via triplet excitons in OSCs.