ZnO solar cells with an indoline sensitizer: a comparison between nanoparticulate films and electrodeposited nanowire arrays

The photovoltaic properties of nanostructured ZnO films sensitized with the indoline derivative dye D149 were studied. The performance of dye-sensitized solar cells built from ZnO building blocks with different morphology, (a) randomly oriented nanoparticle network and (b) nanowire arrays, was compared. The nanoparticle networks were prepared by the standard doctor blade technique from commercial ZnO powders and the nanowire arrays were electrodeposited in aqueous media. Two different lengths for the nanowire arrays (2.5 and 5 mu m) were considered. The characterization included electron microscopy, adsorption measurements, optical spectroscopy, current-voltage characteristics, open-circuit voltage versus light intensity, incident-photon-to-current efficiency, open circuit voltage decay and impedance spectroscopy under illumination. In spite of the smaller dye loadings of the nanowires with respect to the nanoparticles, the former showed a remarkably effective integrated optical absorption (in the range from 370 to 700 nm, 57% versus 69% for the latter). However the photocurrents for nanowires were lower than expected from this good absorption, which suggests that recombination rather than solar light harvesting can be the limiting factor in these nanowire-based solar cells. The impedance analysis and the open-circuit voltage decays showed smaller recombination resistances and shorter lifetimes for the nanowire-based solar cells. However, the interpretation of the recombination resistances, capacitances and lifetimes in the case of the nanowires is likely affected by space-charge effects and back-reaction through the substrate. An understanding of the effects discovered in this study is very valuable for the development of strategies to enhance the energy conversion efficiency for the ZnO nanowire array based solar cells.