Synthesis and applications of novel low bandgap star-burst molecules containing a triphenylamine core and dialkylated diketopyrrolopyrrole arms for organic photovoltaics

In this study, we used facile synthetic routes to construct two well-defined starburst donor/acceptor conjugated small molecules with broad absorption features; in TPAKP-2 and TPAKP-3, triphenylamine (TPA) moieties served as electron donor core units and dialkylated diketopyrrolopyrrole (DKP) moieties with symmetrical thiophene units served as electron acceptors, in 1:2 and 1:3 ratios, respectively. Our investigation of the photophysical properties indicated that the absorption bands of TPAKP-2, and TPAKP-3 extended up to 793 nm, with low optical band gaps of 1.56 and 1.65 eV respectively. Under illumination with AM 1.5 white light (100 mW cm−2), we investigated the performance of bulk heterojunction (BHJ) photovoltaic devices incorporating an active layer of an electron-donor small molecule (TPAKP-2 or TPAKP-3) blended with an electron acceptor: [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) at various weight ratios. The photovoltaic device containing the donor TPAKP-3 and the acceptor PC71BM at a 1:3 weight ratio exhibited the best power conversion efficiency (1.81%), with an open circuit voltage of 0.66 V, a short circuit current density of 7.93 mA cm−2, and a fill factor of 34.7%.