Interface Energy Alignment of Atomic-Layer-Deposited VO on Pentacene: an in Situ Photoelectron Spectroscopy Investigation.

Ultrathin atomic-layer-deposited (ALD) vanadium oxide (VO) interlayer has recently been demonstrated for remarkably reducing the contact resistance in organic electronic devices (Adv. Funct. Mater. 2016, 26, 4456). Herein, we present an in situ photoelectron spectroscopy investigation (including X-ray and ultraviolet photoelectron spectroscopies) of ALD VO grown on pentacene to understand the role of the ALD VO interlayer for the improved contact resistance. The in situ photoelectron spectroscopy characterizations allow us to monitor the ALD growth process of VO and trace the evolutions of the work function, pentacene HOMO level, and VO defect states during the growth. The initial VO growth is found to be partially delayed on pentacene in the first ∼20 ALD cycles. The underneath pentacene layer is largely intact after ALD. The ALD VO is found to contain a high density of defect states starting from 0.67 eV below the Fermi level, and the energy level of these defect states is in excellent alignment with the HOMO level of pentacene, which therefore allows these VO defect states to provide an efficient hole-injection pathway at the contact interface.