Spatially resolved potential measurements for the characterization of contact properties
The device performance of (hybrid-)organic optoelectronic devices, e.g., field-effect transistors and solar cells is often limited by electrical contact resistances between active semiconductors and charge injecting/extracting layers. In perovskite absorber layers, ion migration plays a relevant role. To investigate such phenomena, Kelvin Probe Force Microscopy (KPFM) is an appropriate technique for analyzing potential distributions on the microscale (experimental setup in Fig.(a)). Microstructured electrode arrays (Fig. (b)) served as substrates for organic field-effect transistors and, further, provided symmetrical contacts for in-situ studies on perovskite absorber materials. In Fig. (c) an in-operando potential landscape of an organic field-effect transistor based on perfluorinated copper phthalocyanine (F16PcCu) under stepwise variation of the source-drain voltage is shown. Significant voltage drops at the metal/F16PcCu interfaces evolve due to contact resistances hindering injection (ΔVd) and extraction (ΔVs) of electrons. Fig. (d) depicts the response of the perovskite (PEA)4AgBiBr8 to polarization by a constant voltage. During external polarization at -4V, ions migrate along the film due to a contact resistance near the source on the left-hand side of Fig. (d), which decreases during polarization leading to a widely linear potential drop. Upon short circuiting (arrow), this local separation of ions is resolved by the high surface voltage (red potential peak). Subsequently, ions diffuse along their concentration gradient towards the original even distribution (data hidden behind the peak).
