DSSCs with ZnO@TiO₂ core-shell photoanodes and I^-/I₃^- electrolyte showing improved Voc: 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 structures with reduced interfacial 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 a complete evaluation of our hypothesis. The conduction band minima for the core and the shell structures were both extended. The structure of the nanoparticles, optoelectrical and interfacial charge recombination of of DSSC based on the nanoparticle films were characterized by XRD, SEM, TEM, UV-Vis DRS, Mott-Schottky, current density-voltage and EIS methods. The power conversion efficiencies (PCEs) of DSSCs based on these core - shell structures were observed to depend on the elevation of the conduction band minimum 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.