Polymer solar cells fabricated with 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4, 5-b′]dithiophene and alkyl-substituted thiophene-3-carboxylate-containing conjugated polymers: Effect of alkyl side-chain in thiophene-3-carboxylate monomer on the device performance

Four alkyl-substituted thiophene-3-carboxylate containing donor-acceptor (D-A) copolymers were designed, synthesized, and characterized. Thiophene-3-carboxylate was used as a weak electron acceptor unit in the copolymers to provide a deeper highest occupied molecular orbital (HOMO) level for obtaining a higher open-circuit voltage in polymer solar cells (PSCs). The resulting bulk heterojunction PSCs, made of the copolymers and [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM), exhibited different short circuit currents (J_SCs) and open-circuit voltages (V_OCs), depending on the length of alkyl side-chain in the thiophene-3-carboxylate unit. Among all fabricated photovoltaic (PV) devices, PC2:PC₇₁BM (1:1 wt. ratio) showed the highest efficiency with the highest J_SC of 10.5 mA/cm². Although PC5:PC₇₁BM (1:1) displayed the highest V_OC of 0.93 V, the device efficiency was observed to be poor, which is due to poor nanophase segregation. This comparison shows that the side-chain of thiophene carboxylate in these copolymers plays a very important role in the device efficiency.