Charge transfer dynamics in chlorophyll-based biosolar cells

We fabricated a chlorophyll (Chl)-based biosolar cell with H₂Chl-sensitized TiO₂ as an acceptor and (ZnChl)_n as a donor. This solar cell gives a relatively high quantum yield from the absorption spectral contribution from both the donor and acceptor species. We employed subpicosecond time-resolved absorption spectroscopy (TAS) to study the excited state dynamics at the Chl interface. A charge transfer (CT) state between TiO₂-H₂Chl and (ZnChl)_n was observed at 640 nm after excitation at the Q_y peaks, 680 nm and 720 nm. This CT state is entirely different from the CT states observed for either TiO₂-H₂Chl (TiO₂-H₂Chl/spiro-OMeTAD) or TiO₂-(ZnChl)_n systems. Due to the slower charge transfer process from H₂Chl⁺ to TiO₂ as compared to that from (ZnChl)_n⁺ to H₂Chl, the CT lifetimes of H₂Chl^--(ZnChl)_n⁺ (τ₁ = 0.1 ps, τ₂ = 1.4 ps) excited at 720 nm are slightly shorter than that excited at 680 nm (τ₁ = 0.2 ps, τ₂ = 5.6 ps). The TAS results suggest that the interface of TiO₂-H₂Chl and (ZnChl)_n not only transfers holes as spiro-OMeTAD does, but also provides a built-in field for charge dissociation between the two Chl species.