DSSCs with ZnO@TiO₂ core-shell photoanodes showing improved V_oc: Modification of energy gradients and potential barriers with Cd and Mg ion dopants

ZnO@TiO₂ core-shell nanostructures have been manipulated with metal ion dopants to create DSSC photoanodes with reduced charge transfer recombination. ZnO and TiO₂ were modified by doping with Cd and Mg, respectively, resulting in a reduction in the band-gap for Cd:ZnO and an increase in the band-gap for Mg:TiO₂. A series of core-shell nanostructures based on a ZnO or a Cd:ZnO core and TiO₂ or a Mg:TiO₂ shell have been investigated as DSSC photoanodes. In another series, the core-shell materials were reversed for complete evaluation of our hypothesis. The structures of the core-shell nanoparticles, optoelectronic and interfacial charge transfer of the DSSCs were analyzed by XRD, SEM, TEM, XPS, Mott-Schottky experiment, UV-vis DRS, J-V and EIS measurements. The power conversion efficiencies (PCEs) of DSSCs based on these core-shell structures were observed to depend on the flat band of the shell oxide relative to that of the core oxide. DSSCs with Cd:ZnO@Mg:TiO₂ and Cd:ZnO@TiO₂ photoanodes achieved V_oc and FF values comparable to those of DSSCs with TiO₂ anodes. A moderate energy gradient and high potential barrier in these core-shell structures resulted in longer effective electron lifetimes and lower electron recombination in the corresponding DSSCs. The low PCE registered for DSSCs with anodes of Cd:ZnO@Mg:TiO₂ and Cd:ZnO@TiO₂ were attributed to the presence of a high proportion of rutile phase, caused by sintering at 650 °C and lower dye adsorption as a result larger particle sizes caused by Cd doping.