Kelvin probe force microscopy of nanocrystalline TiO₂ photoelectrodes

Abstract

SummaryDye-sensitized solar cells (DSCs) provide a promising third-generation photovoltaic concept based on the spectral sensitization of a wide-bandgap metal oxide. Although the nanocrystalline TiO2 photoelectrode of a DSC consists of sintered nanoparticles, there are few studies on the nanoscale properties. We focus on the microscopic work function and surface photovoltage (SPV) determination of TiO2 photoelectrodes using Kelvin probe force microscopy in combination with a tunable illumination system. A comparison of the surface potentials for TiO2 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 N719 photoelectrode. The surface potential increase due to the surface dipole correlates with a higher DSC performance. Concluding from this, microscopic surface potential variations, attributed to the complex nanostructure of the photoelectrode, influence the DSC performance. For both bare and sensitized TiO2 photoelectrodes, the measurements reveal microscopic inhomogeneities of more 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 TiO2 layer. The effect of the built-in potential on the DSC performance at the TiO2/SnO2:F interface, investigated on a nanometer scale by KPFM measurements under visible light illumination, has not been resolved so far.

Use of Kelvin probe force microscopy to achieve a locally and time-resolved analysis of the photovoltage generated in dye-sensitized ZnO electrodes

“…Stable trapping of electrons in trap states of the porous ZnO electrode, which are subject to only quite slow recombination or transfer to electron acceptors like O 2 or water from air governed by diffusion of the electrons within the porous semiconductor is, thereby, indicated. A similar time‐scale was reported for the decay of the photovoltage derived from KPFM measurements under comparable conditions at porous TiO 2 electrodes sensitized by the Ruthenium complex N719 . The signal was reported to have relaxed to the dark value within (43 ± 4) s. If we assume that the signal had relaxed to about 1% of its original value within that time, a lifetime of (9.3 ± 0.9) s would result, a value of very similar magnitude to the value of τ 3 determined in the present study.…”

“…It was related to electron diffusion within the porous semiconductor network among individual grains. A faster relaxation component not quantified in that study was assigned to electron transport within individual grains . Despite a different character of the electrodeposited ZnO as opposed to granular TiO 2 such assignment of τ 2 = 73 ms in the present study to electron transport within crystalline domains appears a reasonable assignment.…”

Robust nanogenerators based on graft copolymers via control of dielectrics for remarkable output power enhancement

“…Thus, the increase in net dipole moments may be attributed to the formation of the polar molecules, that is, the polar β phase. In general, the change in the surface potential (Δ V ) is defined through the Helmholtz equation ( 40 )$\mathrm{normal\Delta }\mathrm{italicV}=4\mathrm{normal\pi }\frac{\mathrm{italicN}{\mathit{P}}_0\text{cos}false(\mathrm{normal\theta }false)}{\mathrm{italicA}\mathrm{normal\epsilon }}$where A is the surface area, N is the number of molecules, P is the dipole moment, P 0 is the dipole moment of the free molecule in a vacuum, θ is the molecular tilt angle, and ε = ( P 0 / P ) is the effective dielectric constant of a molecular monolayer. The surface potential changes of Δ V = −214 mV for pristine PVDF and Δ V = +416 mV for PVDF-G18 result in the dipole moments of 1.26 and 3.41 D with opposite directions, respectively.…”

Characterization of individual molecular adsorption geometries by atomic force microscopy: Cu-TCPP on rutile TiO2 (110)

“…6,7 The electron injection might also be influenced by the resulting dipole moment normal to the surface. 8 Density functional theory (DFT) calculations have further shown that the interfacial dipole can change the position of the TiO 2 conduction band, which has direct influence on the open circuit voltage of DSCs. 9,10 In a real device, several anchoring geometries are possible, due to the fact that they are often based on mesoporous substrates with different crystallographic surfaces and defects.…”