Proceedings of Materials for Sustainable Development Conference (MAT-SUS) (NFM22)
Publication date: 11th July 2022
Long diffusion lengths of photo-excited charge carriers are crucial for high power conversion efficiencies of photoelectrochemical and photovoltaic devices. Metal oxides are considered as stable and low-cost photoelectrode materials for hydrogen production by photoelectrochemical solar water splitting. However, their power conversion efficiencies are usually limited by poor transport of photo-generated charge carriers. Time-resolved photoconductance measurements are often used to determine the carrier mobilities and diffusion lengths of metal oxide photoelectrodes, but effects such as polaron formation, trapping, and carrier localization can lead to time-varying mobilities and lifetimes that are not accounted for in the conventional analysis. Here, a generalized analysis is presented that is valid for time-dependent mobilities and time-dependent lifetimes. It determines the diffusion length directly from the integral of a photoconductivity transient, regardless of the nature of carrier relaxation. To this end, photoconductivity transients are measured from 100 fs to 100 µs by the combination of time-resolved terahertz and microwave spectroscopy. For BiVO4, one of the most studied photoanode materials, the time resolved THz spectroscopy (TRTS)- and time resolved microwave conductivity (TRMC) transient signals overlap in time and amplitude. This allows to monitor the temporal evolution of the charge carrier displacement which converges after ~100 ns to a diffusion length of ~15 nm, which rationalizes the photocurrent loss in the corresponding photoelectrochemical device. The presented method is further validated on a-Si: H, c-Si and a lead halide perovskite and could be applied to determine the diffusion length in a wide range of semiconductors, including disordered materials.
Key words: diffusion length, photoconductivity, TRMC, terahertz, TRTS, Metal Oxide, Water splitting, charge transport, mobility, lifetime, disorder, multiple trapping
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