Solution Synthesized <i>p</i>‑Type Copper Gallium Oxide Nanoplates as Hole Transport Layer for Organic Photovoltaic Devices

<i>p</i>-Type metal-oxide hole transport layer (HTL) suppresses recombination at the anode and hence improves the organic photovoltaic (OPV) device performance. While NiO_<i>x</i> has been shown to exhibit good HTL performance, very thin films (<10 nm) are needed due to its poor conductivity and high absorption. To overcome these limitations, we utilize CuGaO₂, a <i>p</i>-type transparent conducting oxide, as HTL for OPV devices. Pure delafossite phase CuGaO₂ nanoplates are synthesized via microwave-assisted hydrothermal reaction in a significantly shorter reaction time compared to via conventional heating. A thick CuGaO₂ HTL (∼280 nm) in poly­(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) devices achieves 3.2% power conversion efficiency, on par with devices made with standard HTL materials. Such a thick CuGaO₂ HTL is more compatible with large-area and high-volume printing process

Impurities and Electronic Property Variations of Natural MoS₂ Crystal Surfaces

Room temperature X-ray photoelectron spectroscopy (XPS), inductively coupled plasma mass spectrometry (ICPMS), high resolution Rutherford backscattering spectrometry (HR-RBS), Kelvin probe method, and scanning tunneling microscopy (STM) are employed to study the properties of a freshly exfoliated surface of geological MoS₂ crystals. Our findings reveal that the semiconductor 2H-MoS₂ exhibits both n- and p-type behavior, and the work function as measured by the Kelvin probe is found to vary from 4.4 to 5.3 eV. The presence of impurities in parts-per-million (ppm) and a surface defect density of up to 8% of the total area could explain the variation of the Fermi level position. High resolution RBS data also show a large variation in the MoS_<i>x</i> composition (1.8 < <i>x</i> < 2.05) at the surface. Thus, the variation in the conductivity, the work function, and stoichiometry across small areas of MoS₂ will have to be controlled during crystal growth in order to provide high quality uniform materials for future device fabrication