Interfacial electronic structure of molybdenum oxide on the fullerene layer, a potential hole-injecting layer in inverted top-emitting organic light-emitting diodes

摘要

The interfacial electronic structures of molybdenum oxide (MoO_x) deposited on fullerene (C_(60)) which could be used as a hole-injecting layer in inverted top-emitting organic light-emitting diodes (TE-OLEDs) were investigated by photoemission spectroscopy. The hole-injecting barrier height (Φ_B ~h) at each interface investigated by an ultraviolet photoemission spectroscopy was reduced to from 1.4 to 0.1 eV as the thickness of MoO_x (ΘM_(oOx)) was increased from 0.1 to 5.0 nm on C_(60). In these interface system, the sign of vacuum-level shift, highest occupied molecular orbital (HOMO)-level shift, and core-level shifts were all positive indicating that the interface mechanism is attributed to the work-function differences due to a band bending at these interfaces. Moreover, the near-edge X-ray absorption fine structure spectra at carbon K-edge did not show any structural modification as well as any chemical reaction at the MoO_x-on-C_(60) interfaces when Θ_(MoOx) was changed on C_(60). From these results, the inverted TE-OLED with C _(60) (5.0 nm)/MoO_x (5.0 nm) showed the power efficiency of 1.7 lm/W at a luminance of about 1000 cd/m~2 and the maximum luminance of about 76.000 cd/m~2 at the bias voltage of 11.0 V. It exhibited the highest performance among the inverted TE-OLEDs fabricated as a function of MoO_x thickness from 0 to 5.0 nm.