Revealing the Excited-State Dynamics in a Dye-Sensitized Solar Cell Based on Natural Chlorophylls

Chlorophylls (Chls) used as the sensitizers in the fabrication of natural dye-sensitized solar cells (NDSSCs) are usually extracted directly using a soaking method. Consequently, they are actually multipigment mixtures. In this study, the Chl mixture-based NDSSC exhibited a low photoconversion efficiency of ∼0.09% and a fill-factor (FF) of 0.55. We, thereby, explored the photophysical mechanisms underlying the various energy transfer (ET) and relaxation processes using a range of spectroscopic techniques, including steady UV–vis absorption, steady and time-resolved fluorescence, and transient absorption (TA) spectra. The spectral bands of steady absorption indicated that the Chl solution extracted directly from spinach contained three groups of pigments: flavonoids, carotenoids, and Chls (Chl a and Chl b). Upon adsorption to mesoporous TiO2, the Chls formed dimers and aggregates, resulting in blue-shifted fluorescence and reduced fluorescence lifetimes. Through comparative analysis of time-correlated single-photon counting (TCSPC) spectra, we identified the ET modes, i.e., carotenoids-Chl a at 375 nm excitation, carotenoids-Chl b at 470 nm excitation in solution, and carotenoids-Chl a aggregate at 375 nm excitation, carotenoids-Chl a/Chl b aggregate at 470 nm excitation in the TiO2 film. Additionally, a long fluorescence lifetime of 9.2 ns indicated a minor Chl b-Chl a ET (4%) in a solution of 470 nm excitation. Global fitting analysis of time-resolved TA spectra further supported these ET modes and additionally clarified the reliable excited-state relaxation channels along with the various components. It was determined that the primary ET route of carotenoids to Chl a occurs via the S2, not S* or S1 (the first excited dark state of carotenoids), to Qy state of chlorophylls, competing with relaxation to the S1 state within a few hundreds of femtoseconds. In conclusion, we outlined the kinetic schemes of two excited-state ET processes between pigments, where both the moderately long-time component (S* state of carotenoids) and the long-time component of 124 ps (Qy of Chls) play crucial roles in an effective excited-state depopulation mechanism, facilitating charge separation and electron transfer in NDSSCs.