Room-Temperature Photodeposited Amorphous VO _x Hole-Transport Layers for Organic Devices

Hole-transport layers (HTLs) and hole-injection layers (HIL) are an integral part of optoelectronic devices such as organic photovoltaic cells (OPVs) and organic light-emitting diodes (OLEDs). A class of materials commonly used as HTLs are metal oxides because they have high transparency and stability. These metal oxides are, however, often made using techniques that are not conducive to large-scale fabrication, a challenge that must be resolved for the widespread adoption of these devices. In this work, we demonstrate the use of a room-temperature, ambient photochemical deposition route to form vanadium oxide films. We show, using a combination of X-ray absorption and X-ray photoemission spectroscopies, that the VO x film consists of V 2 O 5 but with a significant amount of V 4+ present. These films are initially created amorphous and become nanocrystalline after annealing in air at a temperature of 250 °C. After incorporating these VO x thin films as HTLs in both OPV and OLED devices, we surprisingly find this increase in crystallinity does not translate to improvement in device performance. All devices perform similarly to─or better than─control devices using PEDOT:PSS as an HTL. We furthermore demonstrate that these films are not affected by the operation of these devices and that the technique can be employed in combination with slot-die coating printing techniques. This work provides an easily upscaled, low-temperature method for depositing metal-oxide HTL layers.