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.