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In organic semiconductor based bulk heterojunction solar cells, the presenceof acceptor increases the formation of charge transfer (CT) excitons, therebyleading to higher exciton dissociation probabilities. In this work we usedsteady state EA measurements to probe the change in the nature of excitons asthe blend composition of the solar cell active layer material is varied. Weinvestigated blends ofpoly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]thiadiazole)](PPDT2FBT) and (6,6)-Phenyl C71 butyric acid methyl ester (PCBM). Analysis ofthe EA spectra showed that in presence of fullerene based acceptor, like PCBM,CT characteristics of the excitons were modified, though, no new CT signaturewas observed in the blend. Enhancement in the CT characteristic in the blendwas reflected in the photoluminescence (PL) measurements of the blends, where,PL quenching of $\sim$ 63\% was observed for 1\% PCBM. The quenching reachessaturation at about 20\% PCBM. However, the maximum efficiency of the deviceswas obtained for the blend having 50\% PCBM. Comparing experimental resultswith simulations, the variation of the device efficiency with PCBM percentagewas shown to be arising from multiple factors like increase in polarizabilityand dipole moment of excitons, and the efficiency of the carrier collectionfrom the bulk of the active layer.

Nature of excitons in PPDT2FBT: PCBM solar cell: Role played by PCBM