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Transient extreme ultraviolet (XUV) spectroscopy is becoming a valuable tool for characterizing solar energy materials because it can separate photoexcited electron and hole dynamics with element specificity. Here, we use surface-sensitive femtosecond XUV reflection spectroscopy to separately measure photoexcited electron, hole, and band gap dynamics of ZnTe, a promising photocathode for CO₂ reduction. We develop an ab initio theoretical framework based on density functional theory and the Bethe–Salpeter equation to robustly assign the complex transient XUV spectra to the material’s electronic states. Applying this framework, we identify the relaxation pathways and quantify their time scales in photoexcited ZnTe, including subpicosecond hot electron and hole thermalization, surface carrier diffusion, ultrafast band gap renormalization, and evidence of acoustic phonon oscillations.

Summary Transient extreme ultraviolet (XUV) and soft X-ray absorption techniques can measure element-specific ultrafast carrier and structural dynamics in materials or multilayer junctions. However, interpretation of the excited-state spectra is not straightforward. The core-hole couples with valence states, distorting the measured spectra. In this perspective, we describe the implementation and interpretation of XUV experiments. This description includes a guide for how to design a transient XUV/soft X-ray spectroscopy experiment by choosing a suitable edge and corresponding X-ray sources. We particularly focus on the rising use of XUV spectroscopy for understanding solar energy materials, such as measurements of polaron formation, electron and hole kinetics, and layer-resolved charge transport in junctions. The ability to measure photoexcited carriers in each layer of a multilayer junction, as well as associated reaction products, could prove particularly impactful in the study of molecules, materials, and their combinations that lead to functional devices in photochemistry and photoelectrochemistry.

Element-specific charge transfer dynamics are measured in solar fuel materials using ultrafast extreme ultraviolet pulses. This includes hole transport in a Si-TiO2-Ni photoelectrode and the role of polarons in metal oxide photocatalysts.

The absorption of solid state materials in complex photonic and optoelectronic devices overlap in the visible spectrum. Due to the overlap of spectral features, ultrafast measurements of charge carrier dynamics and transport is obscured. Here, the element specificity of transient extreme ultraviolet (XUV) spectroscopy is advanced as a probe for studying photoexcited charge transport in multiple-material junctions. The core-hole excited by the XUV transitions also imparts structural information on to the probed electronic transition. Transient XUV can therefore measure electron and averaged phonon dynamics for each elemental species in a junction. Application to polaron measurement in α-Fe_2O_3, valley-specific scattering in Si, and charge transfer in a nanoscale Ni-TiO_2-Si junction will be discussed.