Incorporating Benzotriazole Moiety to Construct D–A−π–A Organic Sensitizers for Solar Cells: Significant Enhancement of Open-Circuit Photovoltage with Long Alkyl Group

Two novel benzotriazole-containing organic dyes based on D–A−π–A configuration, WS-5 with octyl group and WS-8 with methyl group, have been designed and synthesized for use in dye-sensitized solar cells (DSSCs). Compared with the traditional D−π–A sensitizers, the benzotriazole unit as an additional acceptor has several merits: (i) essentially facilitating the electron transfer from the donor to the acceptor/anchor; (ii) conveniently tailoring the solar cell performance with a facile structural modification on 2-position in the benzotriazole unit; and (iii) the nitrogen-containing heterocyclic group of benzotriazole being expected to improve the open-circuit photovoltage. The analysis of controlled intensity modulated photovoltage spectroscopy reveals that the replacement of methyl with octyl group enhances electron lifetime by 4-fold and retards charge recombination rate constant by 4-fold. The two dye-loaded TiO2 films possess almost the same conduction band position under the same condition, as revealed by the charge densities at open-circuit against open-circuit photovoltage. Therefore, the significant enhancement of open-circuit photovoltage from methyl to octyl group is attributed to the suppressed charge recombination. Under simulated AM1.5G solar light (100 mW cm–2), the DSSC based on WS-5 produces a short-circuit photocurrent of 13.18 mA cm–2, an open-circuit photovoltage of 0.78 V, a fill factor of 0.78, corresponding to a power conversion efficiency of 8.02%.