Novel 4,4′-bis(alkylphenyl/alkyloxyphenyl)-2,2′-bithiophene bridged cyclic thiourea functionalized triphenylamine sensitizers for efficient dye-sensitized solar cells

Two novel 4,4'-bis(allcylphenyl/alkyloxyphenyl)-2,2'-bithiophene bridged cyclic thiourea functionalized triphenylamine sensitizers (08 and 09), comprising 2-cyanoacrylic acid as the acceptor, have been designed and synthesized for dye-sensitized solar cells (DSSCs). For comparison, an analogue (D7) with 4,4'-dihexyl-2,2'-bithiophene spacer was synthesized as well. Their photophysical, electrochemical and photovoltaic properties have been systematically investigated. Compared with D7, D8 and D9 exhibit red-shifted and expanded absorption bands because aromatic alkylphenyl/alkyloxyphenyl was incorporated into the st-conjugated system, which resulted in an increased short-circuit photocurrent density (J(sc)). In addition, the relatively bulky alkylphenyl/alkyloxyphenyl suppresses intermolecular pi-pi aggregation and undesired charge recombination more effectively than alkyl chains, thus enhanced open-circuit voltages (V-oc) were obtained for D8 and D9. Due to the higher J(sc) together with the enhanced V-oc, D8 and D9 sensitized DSSC devices demonstrate improved power conversion efficiencies (PCEs) up to 7.88% under AM 1.5G irradiation, which surpassed that of D7 (7.41%).