In this research, we have characterized the La_0.7Ca_0.3MnO₃ (LCMO) or La_0.7Sr_0.3MnO₃ (LSMO)/N,N’-bis(1-naphthyl)-N,N’-diphentl-4,4’-diamine (NPB) interface by Ultraviolet Photoemission Spectrum (UPS) and Current-Voltage (I-V) measurements. UPS is used to get the energy level alignment and hole injection barrier height at the interface. I-V measurements at different temperatures are made on single-layer devices with LCMO or LSMO/NPB (70 nm or 140 nm)/Aluminum structure. From the UPS spectra, the injection barrier at the LCMO/NPB interface is less than 0.19 eV, indicating the bulk-limited conduction property of our devices. The experimental I-V curves show typical diode characteristics. In comparison with various theoretical models the devices I-V characteristics are further confirmed to...[ Read more ]

In this research, we have characterized the La_0.7Ca_0.3MnO₃ (LCMO) or La_0.7Sr_0.3MnO₃ (LSMO)/N,N’-bis(1-naphthyl)-N,N’-diphentl-4,4’-diamine (NPB) interface by Ultraviolet Photoemission Spectrum (UPS) and Current-Voltage (I-V) measurements. UPS is used to get the energy level alignment and hole injection barrier height at the interface. I-V measurements at different temperatures are made on single-layer devices with LCMO or LSMO/NPB (70 nm or 140 nm)/Aluminum structure. From the UPS spectra, the injection barrier at the LCMO/NPB interface is less than 0.19 eV, indicating the bulk-limited conduction property of our devices. The experimental I-V curves show typical diode characteristics. In comparison with various theoretical models the devices I-V characteristics are further confirmed to be bulk-limited and controlled by the trap-limited conduction (TLC) which assumes an exponential distribution of traps in the organic material. Several such distributions of traps have been found in the NPB film with different characteristic trap energies (E_c) ranging from 0.021 eV to 0.267 eV. For ‘deep’ traps with relatively large E_c the trap density is ~ 10²³ m^-3, while for ‘shallow’ traps close to the HOMO level the trap density is several orders of magnitude larger. The higher trap density for ‘shallow’ traps is explained by the short lifetime of trapped carriers in ‘shallow’ traps. Hysteresis in current densities between the voltage sweep-up and sweep-down measurements in the forward bias has also been found at different temperatures. The origin of the hysteresis and its dependence with integration time and temperature are ascribed to the temperature-related trapping/de-trapping time in the TLC process.