Impact of Molecular Charge-Transfer States on Photocurrent Generation in Solid State Dye-Sensitized Solar Cells Employing Low-Band-Gap Dyes

"Push-pull" structures have been considered a winning strategy for the design of fully organic molecules as sensitizers in dye-sensitized solar cells (DSSC). In this work we show that the presence of a molecular excited state with a strong charge-transfer character may be critical for charge generation when the total energy of the photoexcitation is too low to intercept accepting states in the TiO2 photoanode. Though hole transfer to the 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene can be very fast, an electron hole pair is likely to form at the organic interface, resulting in a possible traplike excitation. This leads to poor photocurrent generation in the solid state DSSC (ss-DSSC) device. We demonstrate that it is possible to overcome this issue by fabricating SnO2-based ss-DSSC. The resulting solar cell shows, for the first time, that a SnO2-based ss-DSSC can outperform a TiO2-based one when a perylene-based, low-band-gap, push-pull dye is used as sensitizer.