In 1999, prompted by the expanding research on n-type semiconductor based dye sensitized solar cells (DSSCs), the first observation of photocurrent from a sensitized p-type nanocrystalline material was reported by He et al.[1] Research on n-type semiconductors as photoelectrodes has seen a steady increase for the past 16 years, while the field of p-type semiconductors has been comparatively unexplored. The feasibility of NiO-based photocathodes has been demonstrated by only a few groups and remains a promising area of DSSC research. Ultrafast spectroscopic studies of sensitized NiO suggest that the strong electronic coupling between the NiO and the sensitizer is detrimental to the efficiency of NiO-based cells, due to the rapid back electron transfer observed for both sensitizers.[2,3]
The results we present focus on the hole-injection (< 1 ps) and subsequent recombination processes (~100 ps) associated with sensitized NiO nanocrystalline thin films. By employing femtosecond time-resolved visible transient absorption spectroscopy we are able to examine how the intrinsic early time dynamics of NiO-based dye sensitized solar cells vary between different organic sensitizers. Ultrafast measurements on complete cells are also reported in an effort to illustrate the influence of electrolyte on the observed dynamics of these novel p type devices.

[1] He, J. et.al. J. Phys. Chem. B. 1999, 103, 8940-8943.
[2] Borgström, M. et.al. J. Phys. Chem. B 2005, 109, 22928.
[3] Moranderia, A. et.al. J. Phys. Chem. B 2005, 109,19403.