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

摘要

Two novel electron-deficient pi-conjugated moieties, quinoxalinedithienothiophene (QBTT) and phenazine-dithieno-thiophene (PBTT) were designed and synthesized as the pi-bridge for the construction of D-pi-A and D-A(1)pi-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 pi-conjugation expansion of PBTT pi-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 pi-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 pi-conjugated expansion is found more useful than linear pi-conjugated extension in enhancing absorption and device performance, and electron-deficient pi-conjugated moieties are a promising pi-bridge for efficient charge transport for photosensitizing applications.