Ultrafast Charge Carrier Dynamics in Extremely Thin Absorber (ETA) Solar Cells Consisting of CdSe-Coated ZnO Nanowires

The extremely thin absorber (ETA) solar cell architecture can enable higher efficiencies than planar cells for absorbers that have low carrier lifetimes or mobilities. Efficient charge separation requires that interfacial electron and hole transfer proceed much faster than the recombination lifetime of photoexcited carriers. In this work, transient absorption spectroscopy was employed to measure these ultrafast photophysical processes in CdSe-coated ZnO nanowire ETA cells and model planar films. At low pump fluences, carrier lifetime was controlled by Shockley Read Hall and surface recombination. Annealing the electrodeposited CdSe films increased the lifetime 50 -fold, to >1 ns as measured by transient absorption spectroscopy, which correlated to improved ETA cell performance. Interfacial electron transfer from the CdSe coating into the ZnO nanowires occurred 3 orders of magnitude faster, within 1 ps, independent of the presence or absence of an interfacial CdS buffer layer. Interfacial hole transfer to the ferri(o)cyanide redox couple could not be directly measured, but photodegradation of the semiconductor surface in the presence of electrolyte under prolonged light exposure resulted in faster dynamic response due to higher densities of surface electron trap states. This study provides a framework for understanding photophysics and improving performance of nanostructured, semiconductor -sensitized solar cells through a combination of device measurements and ultrafast probes.