High-Efficiency Dye-Sensitized Solar Cells Based on the Composite Photoanodes of SnO2 Nanoparticles/ZnO Nanotetrapods

Abstract

We have devised dye-sensitized solar cells (DSSCs) with >6% efficiency by employing composite photoanodes of SnO(2) nanoparticles/ZnO nanotetrapods. Benefiting from material advantages of both constituents, the composite photoanodes exhibit extremely large roughness factors, good charge collection, and tunable light scattering properties. Among the three composite photoanodes with widely differing compositions tested, the best performance (efficiency = 6.31%) was obtained with a weight ratio of SnO(2)/ZnO 2:1 mainly due to the highest saturated J(sc) achieved at a thinnest film thickness. Charge collection losses in composite films with more ZnO nanotetrapods content and thus necessarily larger film thicknesses appear to be a main limiting factor on IPCE and therefore J(sc), which undermines the gain from their favorable light scattering ability. An ultrathin layer of ZnO spontaneously shelled on SnO(2) nanoparticles is found to enhance V(oc) primarily by lifting the band edges rather than by suppressing recombination. Finally, by intensity modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS), we have identified that recombination in SnO(2)/ZnO composite films is mainly determined by the ZnO shell condition on SnO(2), whereas electron transport is greatly influenced by the morphologies and sizes of the ZnO crystalline additives. In particular, ZnO nanotetrapods have proved to be superior in electron transport and therefore charge collection over ZnO particles additives in the SnO(2)/ZnO composite-based DSSCs.