Novel electron-deficient quinoxalinedithienothiophene- and phenazinedithienothiophene-based photosensitizers: The effect of conjugation expansion on DSSC performance

Two novel electron-deficient π-conjugated moieties, quinoxalinedithienothiophene (QBTT) and phenazinedithieno-thiophene (PBTT) were designed and synthesized as the π-bridge for the construction of D-π-A and D-A1-π-A configured photosensitizers for DSSC applications. Five new metal free photosensitizers, namely QC5-m and PC5-n where m = 1-2 and n = 1-3 were designed and synthesized using carbazole as an electron-donor, benzothiadiazole (BTZ) as an auxiliary group and cyanoacylic acid as an electron-acceptor. Despite only lateral π-conjugation expansion of PBTT π-bridge, PC5-n based dyes show broader spectral absorption than those of QC5-m counterparts. Both electron-deficient QBTT and PBTT moieties are found to be a useful π-conjugated bridge to achieve broad spectral absorption and strong charge transfer of a photosensitizer. DSSCs based on these photosensitizers were fabricated and investigated which exhibited power conversion efficiency (PCE) in the range of 5.23-7.77% with a simple sandwich structure under AM 1.5 irradiation, 100 mW cm−1. Among DSSCs fabricated, PC5-1 based device afforded the best photovoltaic performance with PCE up to 7.77%, Voc = 692 mV, Jsc = 15.6 mA cm−2 and FF = 72%. Consistently, the EIS and IMVS studies showed that PC5-1 based device exhibited the most efficient electron transfer at the interface between the electrolyte and the sensitized TiO2 and the slowest charge recombination rate among the devices studied. Our results demonstrated that lateral π-conjugated expansion is found more useful than linear π-conjugated extension in enhancing absorption and device performance, and electron-deficient π-conjugated moieties are a promising π-bridge for efficient charge-transport for photosensitizing applications.