The space charge region (SCR) is a majority carrier-depleted region in a semiconductor. The width of this region and the size of the potential drop across it control photochemical charge separation under illumination, as relevant to solar energy conversion with photocatalyst particles, for example. For photocatalysts, the space charge region is often difficult to evaluate due to the small dimensions of the particles. Here, we show that surface photovoltage spectroscopy (SPS) can be used to observe the SCR in nano- and microparticles of strontium titanate (SrTiO3), aluminum-doped strontium titanate, and gallium arsenide (GaAs) on gold substrates. Depending on particle film thickness and thermal annealing conditions, we observe negative or positive photovoltages corresponding to majority or minority carrier diffusion toward the gold substrate. The direction of charge transport is controlled by the potential barrier across the depletion layer at the gold-particle interface. From the inverted photovoltage signal, potential barriers are estimated between 0.005 and 0.189 V and SCR widths between 1.3 and 2.8 μm. The observed barriers are 10–100 times smaller than the theory for an ideal Schottky junction, and the observed SCR widths are up to 100 times larger. The difference can be attributed to trapping of majority carriers in surface states arising from broken bonds and disorder, in the case of GaAs, or from chemisorbed oxygen, in the case of SrTiO3. Trapping of majority carriers in surface states (Fermi level pinning) also diminishes the effective carrier concentrations in the particles, explaining the larger SCR width in the films. These findings showcase the importance of surface states for charge separation in photocatalyst particles and their films.

Surface photovoltage spectroscopy discovers abnormally large photovoltage signals possibly associated with a previously unknown ferroelectric effect in Cr-doped SrTiO3 nanocrystals.

The space charge region (SCR) is a majority-carrier-depleted region near the interface of a semiconductor. The width of this region and the size of the potential drop across it influence the charge separation at the semiconductor interface. The space charge region is difficult to evaluate for photocatalyst particles due to their small dimensions and poor electric contact. Surface Photovoltage Spectroscopy (SPS) is a highly sensitive spectroscopic technique which can be used to monitor charge transfer and polarization in particle films. Here we apply SPS to observe the SCR in thin films of strontium titanate (SrTiO3), aluminum-doped strontium titanate, and gallium arsenide (GaAs) particles on gold substrates. Negative photovoltages correspond to light-induced majority carrier injection into the gold substrate while positive photovoltages correspond to majority carrier repulsion away from the particle-gold interface. From the inverted photovoltage size, potential barriers are estimated between 0.005 V and 0.189 V and SCR widths between 2.4 μm and 2.8 μm. The small size of the observed barriers and the large SCR widths are attributed to charge trapping in particle surface states (Fermi Level pinning), likely from chemisorbed oxygen. This suggests that depletion layers in particles are mainly controlled by surface states. Figure 1