Synthesis and characterization of two carbazole-based alternating copolymers with 4-nitrophenylcyanovinylene pendant groups and their use as electron donors for bulk heterojunction solar cells

Abstract

Two new carbazole-based copolymers PT and PP carrying thienyl or phenyl, respectively, alternating segments and 4-nitrophenylcyanovinylene pendant groups attached to the carbazole nitrogen were synthesized and characterized. Their optical and electrochemical properties were investigated. Both copolymers showed broad absorption bands in the longer wavelength region, which is complementary to those of PC₆₁BM and PC₇₁BM acceptors, and showed optical band gaps about 1.64 eV and 1.69 eV for PT and PP, respectively. The deep lying HOMO energy level benefited a high open circuit voltage (V_oc), leading to an overall power conversion efficiency (PCE) of 2.19%, 2.98% 2.02% and 2.54% for bulk heterojunction (BHJ) solar cells based on PT:PC₆₁BM, PT:PC₇₁BM, PP:PC₆₁BM and PP:PC₇₁BM blends, respectively, processed from tetrahydrofuran (THF) solvent. The BHJ solar cells based on PT:PC₇₁BM and PP:PC₇₁BM blends processed from THF and 1-chloronaphthalene (CN) mixed solvents showed overall PCE of about 5.39% and 4.20%, respectively. The increase in the overall PCE for the BHJ devices based on solvent additives was attributed to the balanced charge transport in the devices.