DSSCs with ZnO@TiO2 core-shell photoanodes 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 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 degrees C and lower dye adsorption as a result larger particle sizes caused by Cd doping. (C) 2016 Elsevier B.V. All rights reserved.