Effects of side chains on physical and photovoltaic properties of methyl thiophene-3-carboxylate containing dithienopyrrole-based copolymers

To enhance the open circuit voltage (V_oc) of bulk heterojunction (BHJ) polymer solar cells (PSCs) fabricated using dithieno[3,2-b:2′, 3′-d]pyrrole (DTP)-based copolymers, methyl thiophene-3-carboxylate was introduced as a weak acceptor into the polymer backbone. Two types of copolymers showed different optical and electrochemical properties even though both had an identical polymer backbone, with one of them having alkyl side chains tethered to the DTP unit. The energy level of the highest occupied molecular orbital (HOMO) of PDTPMT-C11 with a Y-branched alkyl chain was -5.22 eV, which was lower by 0.53 eV than that of PDTPMT-C12 with linear alkyl chains. The PSC, fabricated using PDTPMT-C11 and [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM), showed a power conversion efficiency (PCE) of 1.8% with a V_oc of 0.73 V, while the PSC comprising PDTPMT-C12:PC₇₁BM BHJ-PSC had PCE of 0.23%, which was limited by a relatively lower V_oc (0.26 V) corresponding to the HOMO level (-4.69 eV) of PDTPMT-C12.