Organic photosensitizers containing fused indole-imidazole ancillary acceptor with triphenylamine donor moieties for efficient dye-sensitized solar cells

Controlling the morphology of sensitizer on a TiO2 nanocrystalline surface is beneficial to facilitating electron injection and suppressing charge recombination. Given that the N,N-dimethylaniline-substituted imidazole-fused-indole on the middle segment for preventing π aggregation can deteriorate its intrinsic photostability, we incorporate a promising building block of fused-indole-imidazole [(1,4-dihydroimidazo[4,5-b]indole) DHII)] ring as the additional acceptor to construct a novel TD2, TD3 and YD3 with D–(A)π-D, D–(DA2)π-D, D–π-D(A)-π-D architecture, which exhibits several characteristics: (i) possible chelation of imidazole ring (through N) to the titanium ions on the TiO2 surface which can assist in increasing the electron injection into the conduction band of photoanode, (ii) showing a moderate electron-withdrawing capability for an ideal push-pull balance in both promising photocurrent and photovoltage; (iii) endowing an ideal morphology control with strong capability of restraining the intermolecular aggregation and facilitating the formation of a compact sensitizer layer via N,N-dimethylaniline groups grafted onto the fused-indole-imidazole unit. The co-adsorbent-free dye-sensitized solar cell (DSSC) based on dye TD3 exhibits very promising conversion efficiency as high as 6.04 ± 0.01%, with a short-circuit current density (Jsc) of 13.57 mA cm−2, an open-circuit voltage (Voc) of 0.80 V, and a fill factor (FF) of 0.774 under AM 1.5 illumination (100 mW cm−2). TD3-based device showed better performance because of the two anchoring groups, which play a significant role for better adsorption on the TiO2 surface along with the enhanced kinetics of photoexcited electron injection.