Terminal π-group engineering of organic co-sensitizers for thiourea dye based dye-sensitized solar cells

Abstract To enhance the conversion efficiency of organic dye sensitized solar cells (DSSCs) via the co-sensitization strategy, a series of organic co-sensitizers (CS1–3) for thiourea dye D8 were designed and synthesized. They take carbazole as the donor unit and 2-cyanoacrylic acid as the anchoring group. Their π-bridges contain a thiophene primary spacer and a different terminal aromatic linker, i.e., thiophene, furan, and benzene for CS1–3, respectively. The effects of different terminal spacer group on the optical, electrochemical, and photovoltaic properties of carbazole-based co-sensitizers were investigated comprehensively. Moreover, the roles of CS1–3 as co-sensitizers for D8 are further examined. The co-sensitized cells based on D8 and CS3 exhibited the highest power conversion efficiency (PCE) of 8.53%, which was superior to that of devices with D8 (PCE = 7.92%). Theoretical calculation investigation revealed that CS3 incorporating a benzene terminal group possessed the most twisty configuration, which may be beneficial to the prevention of aggregation and electron recombination. Furthermore, incident photon to current efficiency (IPCE) spectra and electrochemical impedance spectroscopy (EIS) investigations were performed to clarify the mechanism of the co-sensitization behavior.