Two channels of charge generation in perylene monoimide solid-state dye-sensitized solar cells

The mechanism of charge generation in solid-state dye-sensitized solar cells using triarylamine-substituted perylene monoimide dyes is studied by vis-NIR broadband pump-probe transient absorption spectroscopy. The experiments demonstrate that photoinduced electron injection into the TiO₂ can only occur in regions where Li⁺, from the commonly used Li-TFSI additive salt, is present on the TiO₂ surface. Incomplete surface coverage by Li⁺ means that some dye excitons cannot inject their electron into the TiO₂. However it is observed in the solar cell structure that some of the dye excitons that cannot directly inject an electron still contribute to free charge generation by the previously hypothesized reductive quenching mechanism (hole transfer to the solid-state hole transporter followed by electron injection from the dye anion into the TiO₂). The contribution of reductive quenching to the quantum efficiency of charge generation is significant, raising it from 68% to over 80%. Optimization of this reductive quenching pathway could be exploited to maintain high quantum efficiency in dyes with greater NIR absorption to achieve overall enhancements in device performance. It is demonstrated that broadband NIR transient spectroscopy is necessary to obtain population kinetics in these systems, as strong Stark effects distort the population kinetics in the visible region. Broadband ultrafast spectroscopy shows that electron injection into TiO ₂ from a red absorbing organic dye can be enhanced by a process in which a hole is transferred to the hole transport material to allow injection from the anion state of the dye. Quantitative analysis suggests this mechanism accounts for ≈10% of the charges generated and could be further enhanced. The reductive quenching mechanism of charge generation contributes to photocurrent generation.