Grain Boundary-Induced Work Function Heterogeneity in Yb₂O₃ Thin Films on ITO

Ytterbium oxide offers innovations in photovoltaics and organic light emitting as it enhances electron injection between the cathode and the electron injection layer. That said, the tuning as well as the heterogeneity of its work function remain poorly understood. Here, we report a systematic study of the thickness-dependent work function of thermally evaporated Yb₂O₃ on indium tin oxide using pulsed force Kelvin probe force microscopy (PF-KPFM). The results demonstrate the reduction of the work function as a function of thickness, which has its origin in the formation of the interfacial dipole at the interface of both materials. High-resolution PF-KPFM mapping reveals the heterogeneity of the work function, especially at the grain boundaries. Based on these observations, we postulate a cascade mechanism for the electron transport in which the lateral variations in the work function in the electron injection layer facilitate the transport of electrons from the cathode to the electron transport layer. These results suggest that charge transport in electron injection layers may proceed through multistep rather than few-step pathways.