D–A–π–A featured sensitizers by modification of auxiliary acceptor for preventing “trade-off” effect

Abstract

Four D–A–π–A motif organic sensitizers (IQ9, IQ10, IQ11 and IQ12) in the absence or presence of thiophene substituents grafted onto the auxiliary acceptor of a quinoxaline unit have been developed for dye sensitized solar cells (DSSCs). Upon changing the π-linker from benzene to thiophene, the photocurrent (J_sc) of IQ10 increases around 2-fold, and the photovoltage (V_oc) decreases by 52 mV compared to that of IQ9. It is attributed to the fact that compared with the benzene linker, the thiophene conjugated bridge in dye IQ10 induces a small twist in the molecular planarity, thus resulting in the high light-harvesting capability (beneficial to J_sc) and high charge recombination (unbeneficial to V_oc). To prevent this “trade-off” effect between photocurrent and photovoltage, the building block of 2,3-dithienylquinoxaline as an auxiliary unit is specifically incorporated, which brings forth several advantages such as distinctly extending the light-harvesting region, increasing molar absorption coefficients, and blocking the dye self-aggregation to reduce charge recombination. Remarkably, dye IQ12 exhibits a beneficial balance between J_sc (17.97 mA cm⁻²) and V_oc (715 mV), along with a promising photovoltaic efficiency of 8.76%, much better than the corresponding dyes IQ9 (2.91%), IQ10 (7.75%) and IQ11 (6.56%). As demonstrated, the two twisted thiophene groups grafted onto the quinoxaline unit facilitate the resulting compact sensitizer layer to effectively overcome the charge recombination drawbacks in V_oc arising from the thiophene π-bridge linker, providing a rational molecular engineering to pursue the synergistic enhancement in the photocurrent and photovoltage for highly efficient organic sensitizers.