Charge transfer dynamics in chlorophyll-based biosolar cells.

We fabricated a chlorophyll (Chl)-based biosolar cell with HChl-sensitized TiO as an acceptor and (ZnChl) 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 TiO-HChl and (ZnChl) was observed at 640 nm after excitation at the Q peaks, 680 nm and 720 nm. This CT state is entirely different from the CT states observed for either TiO-HChl (TiO-HChl/spiro-OMeTAD) or TiO-(ZnChl) systems. Due to the slower charge transfer process from HChl to TiO as compared to that from (ZnChl) to HChl, the CT lifetimes of HChl-(ZnChl) (τ = 0.1 ps, τ = 1.4 ps) excited at 720 nm are slightly shorter than that excited at 680 nm (τ = 0.2 ps, τ = 5.6 ps). The TAS results suggest that the interface of TiO-HChl and (ZnChl) not only transfers holes as spiro-OMeTAD does, but also provides a built-in field for charge dissociation between the two Chl species.