Effect of blend composition in BisEH-PFDTBT:PC70BM solar cells

The photovoltaic properties of bulk-heterojunction solar cells with an active layer made of an alternating fluorene copolymer, poly{[2,7-(9,9-bis-(2-ethylhexyl)-fluorene)]-alt-[5,5-(4,7-di-2′-thienyl-2,1,3-benzothiadiazole)]}, and [6,6]-phenyl-C71-butyric acid methylester are investigated by varying the blend composition and the incident light power intensity. An efficient donor–acceptor intermixing is reached with donor to acceptor weight ratios of at least 1:3, while at lower acceptor content charge trapping effects are evidenced by the light intensity dependent study. Reverse bias analysis demonstrates that a major limiting factor for the performance of the investigated solar cells is represented by the poor ability of the generation of free charge carriers, attributed to the low hole mobility in the polymer phase.

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► Effect of blend composition in bulk-heterojunction BisEH-PFDTBT:PC70BM solar cells is reported.
► Efficient donor–acceptor intermixing is reached for at least 75% content of PC70BM.
► Charge trapping effects are evidenced at lower acceptor content.
► 70% of the photogenerated bound hole–electron pairs dissociates into free carriers at short-circuit condition in best cells.
► Poor generation of free charge carriers represents a major limiting factor.