Improving the nanoscale morphology and processibility for PCDTBT-based polymer solar cells via solvent mixtures

Abstract

The effect of solvent mixtures on the morphologies of poly[N-9′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) blend films is investigated. 1,2,4-Trichlorobenzene (TCB) which is a good solvent for PCDTBT is selected to mix with chloroform (CF), chlorobenzene (CB) and o-dichlorobenzene (oDCB) for tuning the morphology of the PCDTBT:PC71BM blend. It is found that formation of nanoscale phase separation with a fibrillar PCDTBT nanostructure of PCDTBT:PC71BM blend which is favorable for exciton separation and charge carrier transport is strongly dependent on the solubility parameters of the solvent mixtures. A clearly defined nanoscale phase separation of the PCDTBT:PC71BM blend can be obtained with TCB:CF mixture. The resulted morphology is similar to that produced with sole DCB solvent that is currently the best solvent for PCDTBT:PC71BM blend solar cells. Moreover, the TCB:CF mixture demonstrates better solubility and processibility for PCDTBT:PC71BM blend and allows us to prepare thick active layer that is required in large-area roll-to-roll process. The polymer solar cells with 250nm- thick active layer are fabricated by using TCB:CF solvent mixture and the power conversion efficiency of the devices reaches 6.45%. A highest short-circuit current of 13.6mA/cm2 is achieved due to enhanced optical absorption of thick active layer.