Hard x-ray photoelectron spectroscopy (HAXPES) is a powerful technique to characterize the chemical and electronic structures of materials. In energy conversion devices, often composed of a stack of thin layers and thus containing multiple buried interfaces, the increased probing depth of HAXPES, compared to conventional x-ray photoelectron spectroscopy, makes it a technique of choice to ultimately reveal a more comprehensive device-relevant picture. In this contribution, we provide a brief review of recent HAXPES experiments conducted at the High Kinetic Energy Photoelectron Spectrometer endstation located at the BESSY II KMC-1 beamline at Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, focusing on three different prominent sample material systems widely used in energy conversion devices: (1) Cu-chalcogenides, (2) metal oxides, and (3) halide perovskites. The findings revealed by these studies highlight the advantage of knowledge-based heterointerface design in energy conversion devices, building interface models based on direct measurements targeting the entire structure as only possible by HAXPES. We conclude by giving an update on the new and enhanced HAXPES experimental capabilities starting to be offered by the Energy Materials In-situ Laboratory Berlin facility.