Organic bulk heterojunction solar cells based on benzodithiophene and benzothiadiazole containing conjugated polymers

Organic photovoltaics (OPVs) or so-called organic solar cells particularly hold promise for manufacturing solar energy due to their advantages in low cost and production processes. In order to understand and improve the performance of OPVs, intense efforts have been dedicated around the world [1]. In particular, conjugated polymers are attractive for OPVs due to the π-conjugated systems in the polymer backbone which generates and transport the charge carriers [2]. Therefore, the design and synthesis of novel conjugated organic polymers play important role to obtain higher photovoltaic properties and improve the power conversion efficiencies (PCEs) of the OPVs. For this purpose, benzodithiophene and benzothiadiazole containing monomers were independently synthesized, then polymerized via Stille cross-coupling reaction to obtain P1 and P2 polymers. Oxidation and reduction behavior of the polymers were studied by cyclic voltammetry. Measurements indicated that the highest occupied molecular orbital (HOMO) levels were -5.25 eV for P1 and -5.38 eV for P2. The optical band gaps of P1 and P2 were calculated via UV-VIS-NIR spectroscopy as 1.54 eV and 1.64 eV, respectively. Bulk heterojunction solar cells were constructed with these polymers as the donor moieties together with PC71BM as the acceptor in the active layer. The current/voltage measurements showed that the highest PCEs of these photovoltaic devices were recorded as 2.52% for P1: PC71BM (1:4, w/w) in 2% DIO and 1.67% for P2: PC71BM (1:3, w/w) in 3% DIO solution