Unifying Charge Generation, Recombination, and Extraction in Low‐Offset Non‐Fullerene Acceptor Organic Solar Cells

A significant leap in record-breaking power conversion efficiencies (PCEs) of single-junction bulk-heterojunction (BHJ) organic solar cells (OSCs) to over 18%[1] has recently been achieved. This can be credited to the rapid developments of new non-fullerene acceptors (NFAs) paired with suitable high performing polymer donors. While these breakthroughs are encouraging, it remains crucial to attain a deeper and more comprehensive understanding of the underlying mechanisms governing these novel and high performing polymer:NFA systems. Several recent studies have attributed the high performances of NFA-based solar cells to an improvement in the open-circuit voltage (VOC) without significantly diminishing the charge generation efficiency.[2–4] Particularly, in polymer:NFA systems, high VOC values have been achieved with efficient charge generation regardless of a very small energetic driving force for Even though significant breakthroughs with over 18% power conversion efficiencies (PCEs) in polymer:non-fullerene acceptor (NFA) bulk heterojunction organic solar cells (OSCs) have been achieved, not many studies have focused on acquiring a comprehensive understanding of the underlying mechanisms governing these systems. This is because it can be challenging to delineate device photophysics in polymer:NFA blends comprehensively, and even more complicated to trace the origins of the differences in device photophysics to the subtle differences in energetics and morphology. Here, a systematic study of a series of polymer:NFA blends is conducted to unify and correlate the cumulative effects of i) voltage losses, ii) charge generation efficiencies, iii) non-geminate recombination and extraction dynamics, and iv) nuanced morphological differences with device performances. Most importantly, a deconvolution of the major loss processes in polymer:NFA blends and their connections to the complex BHJ morphology and energetics are established. An extension to advanced morphological techniques, such as solid-state NMR (for atomic level insights on the local ordering and donor:acceptor ππ interactions) and resonant soft X-ray scattering (for donor and acceptor interfacial area and domain spacings), provide detailed insights on how efficient charge generation, transport, and extraction processes can outweigh increased voltage losses to yield high PCEs.

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