Indolo[3,2-b]carbazole and benzofurazan based narrow band-gap polymers for photovoltaic cells

Abstract

Two indolo[3,2-b]carbazole and 5,6-bis(octyloxy)benzofurazan based narrow band-gap polymers PICzODTBF and PICzHDDTBF with linear and branched side chains respectively were synthesized, and their optical, electrochemical and photovoltaic properties were characterized. The number-averaged molecular weight (M_n) values of PICzODTBF and PICzHDDTBF were 8600 and 21 400 with polydispersity index (PDI) values of 1.97 and 2.43, respectively. The absorption spectra for the films showed that the peaks were located at 405 and 571 nm for PICzODTBF, and 396 and 531 nm for PICzHDDTBF, accompanying optical band gaps of 1.90 and 2.05 eV, respectively. Compared with PICzHDDTBF, the absorption of PICzODTBF was obviously red shifted resulting from its linear side chains leading to better π–π stacking between polymer chains. Through the electrochemical characterization, the HOMO energy levels were found to be −5.02 and −5.28 eV, with LUMO energy levels of −3.09 and −3.08 eV for PICzODTBF and PICzHDDTBF, respectively. The photovoltaic devices were fabricated with the structure ITO/PEDOT:PSS/polymer:PC₇₁BM/PFN/Al. For PICzODTBF, the short circuit current density (J_sc), open circuit voltage (V_oc), fill factor (FF) and power conversion efficiency (PCE) were 4.81 mA cm⁻², 0.80 V, 57% and 3.23%, respectively. PICzHDDTBF had J_sc, V_oc, FF and PCE values of 4.61 mA cm⁻², 0.95 V, 48% and 3.13%, respectively.