First-principles study on the cosensitization effects of Ru and squaraine dyes on a TiO₂ surface

Cosensitization is widely used to increase the incident photon-to-current conversion efficiency (IPCE) of dye-sensitized solar cells (DSCs). One of the authors recently showed that cosensitization with squaraine HSQ1 dye improves the IPCE of a DSC based on the Ru dye N3, whereas little improvement was observed with the similar squaraine dye SQ1 [Adv. Funct. Mater. 23 (2013) 3782-3789]. Here we investigated the mechanism of the cosensitization effect by comparing the N3/HSQ1 and N3/SQ1 systems by means of first-principles calculations. The calculations were performed for the single-dye adsorption structures of N3, HSQ1, and SQ1 and for the coadsorption structures of N3/HSQ1 and N3/SQ1. We found that the orbital energy alignment and the distribution of the molecular orbitals, which affect the electron transfer kinetics, did not change upon coadsorption. More importantly, there was steric repulsion between HSQ1 and N3, whereas SQ1 formed a hydrogen bond with N3. The SQ1 structure tightly covering the N3 dye may have prevented N3 regeneration. The electronic states suggest that intermolecular electron transfer could occur between N3 and SQ1. This may also lower the IPCE of the N3/SQ1-cosensitized DSC due to the retardation of regeneration via SQ1. Therefore, the binding interaction between N3 and SQ1 suppressed the cosensitization effect, whereas HSQ1 improved the IPCE owing to the steric repulsion.