Incremental increase in dimethyl sulfoxide (or dimethyl formamide) in ortho-dichlorobenzene solution of poly[N-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)] (PCDTBT) gradually reduces the polymer-solvent interaction, the attraction forces between polymer chains become more dominant, and the polymer chains adopt a tight and contracted conformation with more interchain interactions, resulting in a progressive aggregation in both solutions and films. This was used to fine tune the morphology of PCDTBT/PC71BM based solar cells, leading to improved domain structure and hole mobility in the active layer, and significantly improved photovoltaic performance. The power conversion efficiency increased from 6.0% to 7.1% on devices with an active area of 1.0cm2.

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