Tailoring Oxidation State of Solution‐Processed MoO_3–x Layer Using Laser‐Irradiation and Its Application in Organic Solar Cells

Abstract Solution‐processed molybdenum oxide (MoO_3–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 MoO_3–x. Here, oxidation state of solution‐processed MoO_3–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 MoO_3–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 MoO_3–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 MoO_3–x layer. As a result, at an optimized photon flux and thickness of MoO_3–x layer, both high work function (>4.7 eV) and high electrical conductivity (29.1 µS cm^‐1) could be simultaneously obtained. These beneficial electronic properties make the solution‐processed MoO_3–x thin films to be an efficient hole transport layers in organic solar cells.