Abstract
Three new small molecules based on the benzo [1,2-b:4,5-b’]dithiophene (BDT) fused central core with different side-chains, namely DPBDT-4Cl, POBDT-4Cl and COBDT-4Cl, are designed and synthesized to investigate the side-chain effect on the properties of nonfullerene acceptors. DPBDT-4Cl has symmetrical phenylalkyl side-chains on the central BDT unit. In order to narrow the bandgap and reduce the steric hindrance, the phenylalkyl chains are systematically replaced with the flexible electron-donating alkoxy side-chain (POBDT-4Cl) and alkyl side-chain (COBDT-4Cl). As a result, POBDT-4Cl and COBDT-4Cl are characterized with asymmetry-featured side-chains. From DPBDT-4Cl to POBDT-4Cl to COBDT-4Cl, their light absorption abilities, molecular packing behaviors and crystallinity are gradually enhanced. The devices based on these three acceptors all show power conversion efficiencies (PCEs) over 11% with energy loss below 0.55 eV. Compared to DPBDT-4Cl, POBDT-4Cl and COBDT-4Cl obviously exhibit enhanced device performance with improved short-circuit current densities (Jsc) and fill factors (FFs), which mainly ascribe to their reduced charge recombination and enhanced charge transport. In addition, the COBDT-4Cl achieved a high efficiency of 13.5% with a Jsc of 21.8 mA cm−2 and an FF of 0.71. This result is among the best performance obtained from asymmetry-featured small molecules.
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