Synthesis and Photovoltaic Properties of Conjugated Copolymers Based on Benzimidazole and Various Thiophene

Abstract

A new accepter unit, dimethyl-2H-benzimidazole, was prepared and used for the synthesis of the conjugated polymers containing electron donor-acceptor pair for organic photovoltaics (OPVs). Dimethyl-2H-benzimidazole unit was designed to substitute the BT unit of poly(N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)) (PCDTBT). A series of new semiconducting polymers with 2,2-dimethyl-2H-benzimidazole, 9-heptadecanyl-9H-carbazole, and thiophene (or bithiophene) units was synthesized using Stille polymerization to generate PCDTMBIs (or PCBBTMBIs). In dimethyl-2H-benzimidazole, the sulfur at 2-position of BT unit was replaced with dialkyl substituted carbon, while keeping the 1,2-quinoid form, to improve the solubility of the polymers. The absorption spectra of PCDTMBIs with thiophene units exhibit two maximum peaks at about 430 and 613-645 nm in solution. The solutions of PCBBTMBIs show two absorption peaks at about 445-456 and 630-645 nm which is red-shifted about 20 nm when compared with PCDTMBIs caused by the introduction of bithiophene units. In most efficient polymer PCBBTMBI3, the device annealed at 100 degrees C for 10 min demonstrated a V-OC value of 0.60 V, a J(SC) value of 4.31 mA/cm(2), and a FF of 0.35, leading to the power conversion efficiency (PCE) of 0.91%, under white light illumination (AM 1.5 G and 100 mW/cm(2)).