Indolocarbazoles end-capped with diketopyrrolopyrroles: impact of regioisomerism on the solid-state properties and the performance of solution-processed bulk heterojunction solar cells

Abstract

Two regioisomeric indolo[3,2-b]carbazoles end-substituted with diketopyrrolopyrrole dyes (3,9-TDPPIC and 2,8-TDPPIC) were synthesized, and their solid-state properties and performance as donor materials for bulk heterojunction solar cells using [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as an acceptor material were evaluated. Depending on the regiochemistry of backbone linkages (3,9- or 2,8-linkage), these two compounds showed quite different solid-state aggregation behaviors (crystalline versus amorphous), leading to a significant difference in the degree of phase separation upon blending with PCBM. As a result, a distinct effect of thermal annealing on the device parameters of solar cells (V_oc and J_sc) was observed for these compounds although both the devices showed almost the same power conversion efficiency (PCE) of ca. 0.7% with thermal annealing at 130 °C. For 3,9-TDPPIC, the highest PCE of 1.8% was achieved by further optimization of the device.