Enhanced Sensitivity in Low‐Energy Inverse Photoemission Spectroscopy with an Off‐Axis Parabolic Mirror for Efficient Light Collection

Abstract Inverse photoemission spectroscopy (IPES) is a powerful tool for investigating unoccupied electronic states where the electron beam is incident upon the sample and the emitted photons are detected. However, its broader application is hindered by inherently low sensitivity, caused by the low probability of photon emission during the transition of free electrons to the unoccupied states. To enhance sensitivity through improved photon collection, an off‐axis parabolic (OAP) mirror is designed and fabricated for the low‐energy version of the IPES (LEIPS). Optical simulations showed that the OAP mirror increased photon collection efficiency from 3.06% to 63.3%, which is experimentally validated in the lowest unoccupied molecular orbital (LUMO) spectra of C₆₀ thin films. The OAP mirror‐LEIPS system is applied to study the energy level alignment (ELA) of pentacene films on substrates with different work functions (WF). By measuring both the highest occupied molecular orbital (HOMO) and LUMO levels, the evolution of transport gaps and the ELA of HOMO and LUMO with the Fermi level are analyzed. Pentacene exhibited n‐type behavior on a low WF substrate (Cs₂CO₃) and switched to p‐type on a high WF substrate (ITO). The OAP mirror‐enhanced LEIPS system significantly reduced the time required for such experiments, enabling efficient and reliable measurements.