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Young Investigator Group Nanoscale Solid-Liquid Interfaces

Surface Photovoltage Spectroscopy

Surface photovoltage for the characterization of materials

The contact potential difference between a sample and a reference electrode can change under illumination of the sample. The negative light induced change of the contact potential difference is called surface photovoltage (SPV). SPV signals arise whenever photogenerated charge carriers are separated in space. This opens a huge potential for SPV analysis of materials in photovoltaics, photocatalysis, sensing and semiconductor technology….

Measurements of SPV signals do not need contact preparation. This dramatically expands the opportunities for applications of SPV techniques. Furthermore, samples can be studied by SPV even after intermediate steps of technology. SPV measurements can be performed on crystals, thin films, molecular monolayers and powders. This strongly widens the classes of photoactive materials which can be analyzed by SPV techniques.

SPV signals depend on light absorption, separation of photogenerated charge carriers, charge transport and recombination. Charge separation can be caused not only by drift in a space charge region but also by preferential diffusion, selective charge transfer at interfaces, intra-molecular polarization, dissociation of excitons at interfaces, carrier injection, photo-induced adsorption and desorption of molecules. Therefore, related phenomena can be studied by SPV techniques. However, the complexity of involved phenomena can make interpretation of SPV signals often challenging.

SPV - enlarged view

Surface Photovoltage Spectroscopy on wide bandgap semiconductors

Surface photovoltage techniques at HZB

SPV signals can be measured by the light induced change of the contact potential difference (Kelvin-probe techniques, shift of peaks in photoelectron emission spectroscopy techniques) or by the light induced charge at a fixed measurement capacitor (capacitive outcoupling). In the SPV laboratory of the CE-NSLI, SPV measurements are performed with macroscopic Kelvin probes and with fixed capacitors formed by the sample and the reference electrodes.

In our group, SPV transients can be measured continously from ns to ms…s by capacitive outcoupling of SPV signals with a high-impedance buffer (450 MHz, in-house development). Such transient SPV measurements close the time gap between Kelvin-probe (dc) and pump-probe photoelectron emission spectroscopy (<ps…ns) techniques. In addition, charge amplifiers were developed in collaboration with EMM and enable SPV measurements from ns to dc with the same electrode.

Tunable pulsed and continuous light sources are used for transient SPV spectroscopy and for modulated and dc SPV spectroscopy from the near infrared to the deep ultraviolet. For advanced  studying defect transitions in materials with ultra-wide bandgap, a mirrorless double-prism monochromator based on fused silica has been developed in cooperation with Freiberg Instruments.

References

  • Th. Dittrich, S. Fengler „Surface photovoltage spectroscopy of ultra-wide bandgap materials”, physica status solidi (RRL) (2025) 2400384.

  • I. Levine, D. Menzel, A, Musiienko, R. Macqueen, N. Romano, M. Vasquez-Montoya, E. Unger, C. Mora Perez, A. Forde, A. Neukirch, L. Korte, Th. Dittrich, “Revisiting sub-bandgap emission mechanism in 2D halide perovskites: The role of defect states”, JACS 146 (2024) 23437.

  • A. Chemin, I. Levine, M. Rusu, R. Vaujour, P. Knittel, P. Reinke, K. Hinrichs, T. Unold, Th. Dittrich, T. Petit “Surface-mediated charge transfer of photogenerated carriers in diamond”, Small Methods 7 (2023) 2300423.

  • R. Chen, Z. Ren, Y. Liang, G. Zhang, T. Dittrich, R. Liu, Y. Liu, Y. Zhao, S. Pang, H. An, C. Ni, P. Zhonu, K. Han, F. Fan, C. Li “Spatiotemporal imaging of charge transfer in photocatalyst particles”, Nature 610 (2022) 296.

  • Th. Dittrich, S. Fengler „Surface Photovoltage Analysis of Photoactive Materials“, World Scientific (2020).