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%, V_oc = 692 mV, J_sc = 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.