Tailoring Oxidation State of Solution-Processed MoO3-x Layer Using Laser-Irradiation and Its Application in Organic Solar Cells

Abstract

Solution-processed molybdenum oxide (MoO3-x) is a promising material for charge transport layer in optoelectronic devices. However, no clear relationship between oxidation state and electrical properties has been experimentally derived to guide the optimization of MoO3-x. Here, oxidation state of solution-processed MoO3-x is controlled by both a photoassisted reduction and a spontaneous reduction. The photoassisted reduction proceeds by absorbing the incoming high flux photons, resulting in oxygen vacancies. Moreover, the spontaneous reduction occurs when MoO3-x layer is in contact with indium-tin oxide (ITO) electrode and affects for several nanometers away from the interface. Thus, both the photon flux and the thickness of MoO3-x layer play a key role in determining the oxidation state. Synchrotron radiation photoelectron spectroscopy confirms a relationship that the oxidation state gradually decreases with increasing the photon flux and thinning the MoO3-x layer. As a result, at an optimized photon flux and thickness of MoO3-x layer, both high work function (>4.7 eV) and high electrical conductivity (29.1 mu S cm(-1)) could be simultaneously obtained. These beneficial electronic properties make the solution-processed MoO3-x thin films to be an efficient hole transport layers in organic solar cells.