\textbf{Nanostructured SnO}$_{\mathrm{\mathbf{2}}}$\textbf{ current collectors for solar energy conversion devices: relating morphology and conductivity}

Large bandgap (3.8 eV), high bulk conductivity, and a low-lying valence band make nanostructured SnO2 a promising candidate material for extracting photoexcited electrons from absorbers in solar energy conversion devices. Efficient charge collection requires high surface to volume ratio of a nanostructured SnO2 network, which comes at a cost of reduced conductivity due to incorporation of defects and grain boundaries, and reduction of electrical connectivity. We use terahertz time-domain spectroscopy (THz-TDS) to measure conductivity in nanoporous SnO2 films and nanowire arrays with different average lengths and packing densities. THz-TDS allows a non-contact measurement of frequency-resolved conductivity over nanoscale distances. Modeling the THz-TDS data using the the Drude--Smith model, we extract intrinsic properties of SnO2 as well as the effects of morphology on nanoscale conductivity. We find that the intrinsic carrier mobility of SnO2 making up a nano-porous film is 100 cm$^{\mathrm{2}}$/Vs, while the nanoscale mobility is 25 cm$^{\mathrm{2}}$/Vs. Correlating THz conductivity of nanostructured SnO2 with morphology allows us to establish optimal morphology and growth conditions for achieving highest conductivity while maintaining high surface to volume ratio.