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

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 250 nm- 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 13.6 mA/cm2 is achieved due to enhanced optical absorption of thick active layer.

The PCDTBT:PC71BM blend-based polymer solar cells with thick active layer were fabricated by using 1,2,4-trichlorobenzene (TCB)-based solvent mixtures, resulting in a power conversion efficiency of 6.45% and a highest short-circuit current of 13.6 mA/cm2 due to enhanced optical absorption of thick active layer. The TCB-based solvent mixtures not only favour constructing nanoscale phase separation with fibrillar PCDTBT nanostructure of the PCDTBT:PC71BM blend, but also demonstrate good solubility to the blend allowing us to prepare thick active layer of the polymer solar cells.Figure optionsDownload as PowerPoint slideHighlights
► The PCDTBT:PC71BM blend morphology correlates with solubility parameter of solvent.
► Nanoscale PCDTBT:PC71BM phase separation is constructed via solvent mixtures.
► Solvent mixtures show better solubility for PCDTBT:PC71BM blend.
► Polymer solar cell with thick active layer was fabricated via solvent mixtures.
► The polymer solar cell demonstrates a highest short-circuit current of 13.6 mA/cm2.