Kelvin Probe Force Microscopy of Nanocrystalline TiO2 Photoelectrodes

Dye-sensitized solar cells (DSCs) provide a promising third-generation photovoltaic concept based on the spectral sensitization of awide-bandgap metal oxide. Although the nanocrystalline TiO 2 photoelectrode of a DSC consists of sintered nanoparticles, there arefew studies on the nanoscale properties. We focus on the microscopic work function and surface photovoltage (SPV) determinationof TiO 2 photoelectrodes using Kelvin probe force microscopy in combination with a tunable illumination system. A comparison ofthe surface potentials for TiO 2 photoelectrodes sensitized with two different dyes, i.e., the standard dye N719 and a copper(I)bis(imine) complex, reveals an inverse orientation of the surface dipole. A higher surface potential was determined for an N719photoelectrode. The surface potential increase due to the surface dipole correlates with a higher DSC performance. Concludingfrom this, microscopic surface potential variations, attributed to the complex nanostructure of the photoelectrode, influence theDSC performance. For both bare and sensitized TiO 2 photoelectrodes, the measurements reveal microscopic inhomogeneities ofmore than 100 mV in the work function and show recombination time differences at different locations. The bandgap of 3.2 eV,determined by SPV spectroscopy, remained constant throughout the TiO 2 layer. The effect of the built-in potential on the DSCperformance at the TiO 2 /SnO 2 :F interface, investigated on a nanometer scale by KPFM measurements under visible light illumina-tion, has not been resolved so far.