Morphological dependance of charge transport in nanostructured ZnO-based dye sensitized solar cells

Zinc oxide (ZnO) photoanodes of two different morphologies — nanorods and nanosheets are used to fabricate dye sensitised solar cells in order to study the influence of morphology on device performance. Characteristics such as dye loading capacity and light scattering ability of the photoanodes depend on the morphology to a large extent. This leads to change in the electron transport properties, which in turn affects power conversion efficiency. Here we report the difference in the charge transport properties of these morphologies evaluated using the time resolved photocurrent measurements. The photocurrent transients for ZnO nano-rod film exhibited dual peak behaviour, whereas transients for ZnO nano-sheet film exhibited only one peak as the time delayed second peak corresponding to slow electron diffusion was totally suppressed. The transient photocurrent decay also indicates that the charge transport rate is much faster in the ZnO nano-sheets, which allows the efficient charge collection over much larger thickness compared to other ZnO nano-rod film. The power conversion efficiency (PCE) of the devices was found to be 1.6% and 1.5 % for the ZnO-nano-rod and nano-sheet based dye sensitized solar cells respectively. A comparative study is performed and the photovoltaic parameters obtained in each case are correlated with the photocurrent transients enabling better understanding of the impact of morphology.