Synthesis of indolo[3,2-b]carbazole-based random copolymers for polymer solar cell applications

• We report two indolocarbazole-based copolymers for photovoltaic applications.
• Two copolymers exhibited excellent thermal stability.
• Energy levels of copolymers can be modulated by varying the monomers ratios.
• Increasing of planar monomer content leads to a relatively smooth morphology.
• The optimal device performance reached a power conversion efficiency of 1.63%.

In addition to preparing two indolocarbazole-based random copolymers (named as r-PICTBT1 and r-PICTBT2), this work investigated their feasibility for bulk heterojunction polymer solar cells (PSCs). These copolymers consisted of commercially available 3,9-dibromo-5,11-dioctyl-5,11-dihydroindole[3,2-b]carbazole, 2,5-bis(trimethylstannyl) thiophene and dibromobenzo[c][1,2,5]thiadiazole by varying the feed in ratios via Stille cross-coupling reactions. The photophysical and electrochemical properties of the resulting copolymers could be fine-modulated easily by adjusting the feed ratios of monomers. Both copolymers in the thin film state exhibited two obvious peaks and a vibronic shoulder in the absorption spectra. Electrochemical experiments indicated that the highest occupied molecular orbital energy levels were − 4.95, − 5.00 eV; meanwhile, the lowest unoccupied molecular orbital energy levels were − 3.38, − 3.54 eV for r-PICTBT1 and r-PICTBT2, respectively. Bulk heterojunction PSCs composed of an electron-donor copolymer blended with an electron acceptor [6] and [6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6] and [6]-phenyl-C71-butyric acid methyl ester (PC71BM) at a weight ratio of 1:1 or 1:3 were investigated. Moreover, the r-PICTBT2/PC71BM-based (w/w = 1:1) PSC performed the best with an open-circuit voltage of 0.54 V, short-circuit current of 6.83 mA/cm2, fill factor of 0.44, and power conversion efficiency of 1.63%.