3,6-Carbazole Incorporated into Poly[9,9-dioctylfluorene-<i>alt</i>-(bisthienyl)benzothiadiazole]s Improving the Power Conversion Efficiency

A novel concept of D–A-type copolymers based on fluorene polymer incorporated with 3,6-carbazole unit enhances the device performance for organic photovoltaic cells. <b>P­(F</b>_<b>45</b><b>C</b>_<b>5</b><b>-DTBT)</b>, incorporating 5 mol % 3,6-carbazole into <b>P­(2,7F-DTBT)</b>, shows an almost 2-fold improvement (5.1%) in power conversion efficiency relative to <b>P­(2,7F-DTBT)</b> (2.6%). This results is ascribed to the good balance between electron and hole mobilities in the devices (μ_e/μ_h ∼ 1.8 for <b>P­(F</b>_<b>45</b><b>C</b>_<b>5</b><b>-DTBT)</b> vs 152 for <b>P­(2,7F-DTBT)</b>), and the formation of a nanoscale morphology in the blend of the copolymer and [6,6]-phenyl C71-butyric acid methyl ester (PC₇₁BM)