Effects of HAT-CN Layer Thickness on Molecular Orientation and Energy-Level Alignment with ZnPc

Abstract

Efficient energy-level alignment is crucial for achieving high performance in organic electronic devices. Because the electronic structure of an organic semiconductor is significantly influenced by its molecular orientation, comprehensively understanding the molecular orientation and electronic structure of the organic layer is essential. In this study, we investigated the interface between a 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) hole injection layer and a zinc-phthalocyanine (ZnPc) p-type organic semiconductor. To determine the energy-level alignment and molecular orientation, we conducted in situ ultraviolet and X-ray photoelectron spectroscopies, as well as angle-resolved X-ray absorption spectroscopy. We found that the HAT-CN molecules were oriented relatively face-on (40°) in the thin (5 nm) layer, whereas they were oriented relatively edge-on (62°) in the thick (100 nm) layer. By contrast, ZnPc orientation was not significantly altered by the underlying HAT-CN orientation. The highest occupied molecular orbital (HOMO) level of ZnPc was closer to the Fermi level on the 100 nm thick HAT-CN layer than on the 5 nm thick HAT-CN layer because of the higher work function. Consequently, a considerably low energy gap between the lowest unoccupied molecular orbital level of HAT-CN and the HOMO level of ZnPc was formed in the 100 nm thick HAT-CN case. This may improve the hole injection ability of the anode system, which can be utilized in various electronic devices.

Thickness Dependence of MoO3 Hole Injection Layer on Energy-Level Alignment with NPB Hole Transport Layers in OLEDs

“…The HOMO level of the HTL can be further decreased by increasing the thickness of the MoO 3 HIL. A recent study reported that a sufficiently thick layer is necessary to decrease the HOMO level of an HTL with a 1,4,5,8,9,11hexaazatriphenylene hexacarbonitrile (HAT-CN) HIL, which has an electronic structure similar to that of the MoO 3 HIL [14]. This highlights the importance of understanding the energy-level alignment with different thicknesses of the MoO 3 HIL in formulating a strategy to further enhance OLED performance.…”