Synthesis and characterization of an isoindigo-dithienocarbazole-isoindigo oligomer for organic solar cells

An isoindigo-dithienocarbazole-isoindigo oligomer (II-DTC-II) was synthesized by a Stille coupling reaction between N-hexadecyl-2,8- bis(trimethylstannyl)dithieno[3,2-b:6,7-b]carbazole and 6-bromo-N,N′- dioctylisoindigo. The oligomer exhibited a broad absorption with an optical band gap of 1.75 eV and a highest occupied molecular orbital energy level of -5.46 eV. Photovoltaic devices were fabricated using the II-DTC-II oligomer and [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM), to obtain the configuration ITO/PEDOT:PSS/II-DTC-II:PC₇₁BM/LiF/Al. The best power conversion efficiency of the II-DTC-II-based devices was 1.13% when 0.8 wt% diiodooctane was mixed into the active layer of II-DTC-II/PC ₇₁BM (1:1). The low conversion efficiency was attributed to the oligomer's poor solubility and miscibility with PC₇₁BM.