Hard X-ray Photoelectron Spectroscopy for Depth-Resolved Analysis of Optoelectronic and Photoelectrochemical Devices.

Laboratory-based hard X-ray photoelectron spectroscopy (HAXPES) extends conventional XPS into the multi-keV regime, enabling 20-30 nm probing depths while preserving high chemical-state sensitivity and energy resolution. This Perspective highlights recent advances that establish HAXPES as a practical and quantitative tool for characterizing buried interfaces in optoelectronic and photoelectrochemical devices. We discuss analytical strategies (multienergy excitation, angle-resolved detection, inelastic-background modeling, and Auger-parameter analysis) that permit depth-resolved quantification of band offsets, band bending, interface dipoles, and chemical gradients while distinguishing initial-state effects from final-state screening. Representative case studies span perovskite, organic, quantum-dot, and chalcogenide photovoltaics; oxide-based optoelectronic systems; catalyst-semiconductor and protection-semiconductor junctions; and operando solid-liquid measurements that track oxidation-state evolution and ion redistribution. We also outline key challenges including cross-instrument calibration, limited laboratory photon flux, and the development of stable operando cells. Finally, we identify opportunities in standardized measurement protocols, data-driven spectral analysis, and correlative in situ and operando methods that directly link the buried electronic structure to device performance and long-term stability.