Morphology control in polycarbazole based bulk heterojunction solar cells and its impact on device performance

Incremental increase in dimethyl sulfoxide (or dimethyl formamide) in ortho-dichlorobenzene solution of poly[$N$ -heptadecanyl-2,7-carbazole-alt-5,5-($4^{\prime },7^{\prime }$ -di-2-thienyl-$2^{\prime },1^{\prime },3^{\prime }$ -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 $\text{PCDTBT}/{\text{PC}}_{71}\text{BM}$ 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 $1.0{\text{cm}}^2$ .