Porous Magnesium Zinc Oxide Layers for Photoanodes in Dye-Sensitized Solar Cells

Abstract

Dye-sensitized solar cells (DSSCs) open new possibilities for photovoltaic applications that are inaccessible to established solar cell concepts, such as silicon-based photovoltaics. Due to its superior electron transport characteristics, zinc oxide (ZnO) has long been studied as a promising alternative to the traditionally used titanium dioxide (TiO2)-based photoanodes in DSSCs. However, photo conversion efficiencies (PCEs) of ZnO-based DSSCs generally lack behind the ones reached by titania-based cells, specifically due to lower open-circuit voltages VOC. The present study investigated Mg doping of ZnO as a promising way to significantly increase the VOC in ZnO-based DSSCs through an upward shift of the conduction band energy. Mesoporous layers of Mg-doped ZnO (MZO) have been prepared in the form of homogenously doped nanoparticles and core-shell particles with an MZO shell and a pure ZnO core. Optical and structural analysis was performed to confirm the integration of Mg into the ZnO crystal lattice, which increased the optical band gap depending on the respective Mg concentration. DSSCs prepared from homogeneously doped MZO nanoparticles exhibited significantly increased VOC but suffered from strongly reduced short-circuit currents JSC at higher Mg concentrations. Detailed photoelectrochemical analysis of these cells revealed a trap-related increase in recombination rate and transport resistance. Optimized core-shell structures were fabricated by atomic layer deposition (ALD) of conformal MZO layers of controlled composition and thickness on the internal surface of porous ZnO layers. DSSCs built from this hybrid architecture showed much higher conservation of JSC at high Mg concentrations than homogenously doped nanoparticles due to the prevention of increased transport resistance. Coupled with significantly increased VOC, this resulted in an overall increased PCE for cells containing MZO.