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 Qy 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.