Key factors behind the superior performance of polymer-based NFA blends.

Abstract

All-small molecule (ASMs) solar cells have great potential to actualize the commercialization of organic photovoltaics owing to their higher solubility, lesser batch-to-batch variety and simpler synthesis routes compared to the blend systems that utilize conjugated polymers. However, the efficiencies of the ASMs are slightly lacking behind the polymer: small molecule bulk-heterojunctions. To address this discrepancy, we compare an ASM blend ZR1:Y6 with a polymer:small molecule blend PM7:Y6, sharing the same non-fullerene acceptor (NFA). Our analyses reveal similar energetic offset between the exciton singlet state and charge transfer state (ΔES1-CT) in ZR1:Y6 and PM7:Y6. In comparison to the latter, surprisingly, the ZR1:Y6 has noticeably a stronger field-dependency of charge generation. Low charge carrier mobilities of ZR1:Y6 measured, using space charge limited current measurements, entail a viable explanation for suppressed charge dissociation. Less crystalline and more intermixed domains as observed in the ZR1:Y6 system compared to polymer:Y6 blends, makes it difficult for NFA to form a continuous pathway for electron transport, which reduces the charge carrier mobility.