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Correlating Local Morphology and Charge Dynamics via Kelvin Probe Force Microscopy to Explain Photoelectrode Performance

Maryam Pourmahdavi1,2, Mauricio Schieda1, Ragle Raudsepp1, Steffen Fengler1, Jiri Kollmann1, Yvonne Pieper1, Thomas Dittrich3, Thomas Klassen2,4, and Francesca M. Toma1,2,5,*

  • *Contact author: francesca.toma@hereon.de
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PRX Energy 4, 023010 – Published 9 June, 2025

DOI: https://doi.org/10.1103/PRXEnergy.4.023010

Abstract

Photoelectrochemical (PEC) cells have emerged as a promising and sustainable route for the production of valuable chemicals and have attracted significant attention over the last few decades. However, photoelectrodes, essential components of PEC devices, are limited by solar-to-hydrogen conversion efficiencies significantly below the theoretical values, as well as by low stability under working conditions. An important hurdle toward achieving high efficiency is the limited number of available tools that can directly probe the interplay between the local morphology and charge carrier transport dynamics within the photoelectrode top layer. This layer has different functions: providing photocatalytic active sites and protecting underlying layers. Therefore, the development of a powerful technique to reveal the influence of morphology on transport dynamics is crucial. In this work, we show how to precisely correlate local morphology with optoelectronic properties via the investigation of photoinduced charge transport processes, by time-dependent Kelvin probe force microscopy (KPFM) measurements. Our approach allows the extraction of time-dependent photovoltage for each pixel of a KPFM scan. We used crystalline-amorphous mixed phase TiO2 as a well-established model system to apply and validate our method. Through local correlations between structure-morphology and optoelectronic properties, our results reveal more efficient photoinduced charge separation on the regions corresponding to the crystalline TiO2 phase, which exhibit faster transport, and generate a significantly higher surface photovoltage signal (approximately 440 mV) upon UV illumination (super-band-gap illumination). This finding supports the considerable role of microstructure in charge carrier dynamics, and the ability of our analytical method to resolve it. The analysis technique outlined here should find broad applicability, enabling in-depth insight into the effect of local microstructure, and contributing to the design and development of more efficient photoelectrodes by tailoring of the microstructure at the nanoscale.

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Popular Summary

Photoelectrochemical (PEC) cells are promising for sustainably producing valuable chemicals, but are limited by efficiency and stability under working conditions. A major challenge is understanding how microscopic structures within photoelectrodes affect their ability to convert solar energy. Here, the authors introduce a method using time-resolved Kelvin Probe Force Microscopy (KPFM) to correlate local morphology with optoelectronic properties. The technique is applied to a crystalline-amorphous titanium dioxide system, revealing that crystalline regions promote faster charge transport and higher surface voltages than amorphous ones. These insights demonstrate the considerable role of microstructure on performance. The analysis technique presented here can guide the design of more efficient, stable photoelectrodes by optimizing material structure.

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PRX Energy 4, 023010– Published 9 June, 2025
  • Received 16 August 2024
  • Accepted 15 April 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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