Carbazole and thienyl benzo[1,2,5]thiadiazole based polymers with improved open circuit voltages and processability for application in solar cells

Abstract

The preparation of low energy gap carbazole based main-chain polymers having improved open circuit voltages (V_oc) and solubility is described. Poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt-(5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)-5,5-diyl] P1 and poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt-(5,6-bis(octyloxy)-4,7-di(2,2′-bithiophen-5-yl)benzo[c][1,2,5]thiadiazole)-5,5-diyl] P2 were prepared in good yields using Suzuki coupling methodologies. The polymers were characterized by NMR spectroscopy; UV-Vis absorption spectroscopy, cyclic voltammetry and their molecular weights were estimated using gel permeation chromatography. Introduction of octyloxy substituents on the benzothiadiazole acceptor repeat units on polymers P1 and P2 enhances their solubility compared to the polymer PCDTBT and alters their electronic and photophysical properties. Investigation of the photovoltaic properties of the two new polymers side by side with those of PCDTBT in bulk heterojunction cells using PC₇₀BM as a molecular acceptor indicated that both polymers provide higher V_oc values than that of PCDTBT (0.96 and 0.90 V for P1 and P2 respectively vs. 0.82 V for PCDTBT) and good power conversion efficiencies with PCE values of 4.22 and 4.12% respectively for P1 and P2 and a PCE of 4.30% for PCDTBT.