Enabling Efficient Creation of Long-Lived Charge-Separation on Dye-Sensitized NiO Photocathodes

The hole-injection and recombination photophysics for NiO sensitized with RuP ([Ru-II(bpy)(2)(4,4'-(PO3H2)(2)-bpy)](2+)) are explored. Ultrafast transient absorption (TA) measurements performed with an external electrochemical bias reveal the efficiency for productive hole-injection, that is, quenching of the dye excited state that results in a detectable charge-separated electron hole pair, is linearly dependent on the electronic occupation of intragap states in the NiO film. Population of these states via a negative applied potential increases the efficiency from 0% to 100%. The results indicate the primary loss mechanism for dye-sensitized NiO is rapid nongeminate recombination enabled by the presence of latent holes in the surface of the NiO film. Our findings suggest a new design paradigm for NiO photocathodes and devices centered on the avoidance of this recombination pathway.