Study of the Hole Transport Processes in Solution-Processed Layers of the Wide Bandgap Semiconductor Copper(I) Thiocyanate (CuSCN)

Abstract

Wide bandgap hole-transporting semiconductor copper(I) thiocyanate (CuSCN) has recently shown promise both as a transparent p-type channel material for thin-film transistors and as a hole-transporting layer in organic light-emitting diodes and organic photovoltaics. Herein, the hole-transport properties of solution-processed CuSCN layers are investigated. Metal-insulator-semiconductor capacitors are employed to determine key material parameters including: dielectric constant [5.1 (±1.0)], flat-band voltage [-0.7 (±0.1) V], and unintentional hole doping concentration [7.2 (±1.4) × 1017 cm-3]. The density of localized hole states in the mobility gap is analyzed using electrical field-effect measurements; the distribution can be approximated invoking an exponential function with a characteristic energy of 42.4 (±0.1) meV. Further investigation using temperature-dependent mobility measurements in the range 78-318 K reveals the existence of three transport regimes. The first two regimes observed at high (303-228 K) and intermediate (228-123 K) temperatures are described with multiple trapping and release and variable range hopping processes, respectively. The third regime observed at low temperatures (123-78 K) exhibits weak temperature dependence and is attributed to a field-assisted hopping process. The transitions between the mechanisms are discussed based on the temperature dependence of the transport energy. The wide bandgap p-type semiconductor copper(I) thiocyanate (CuSCN) has the potential to replace conventional hole-transport materials in numerous opto/electronics applications. This work provides a comprehensive analysis of the charge transport properties of solution-processed CuSCN layers. Various techniques are employed to evaluate the dielectric constant, flat-band voltage, unintentional doping concentration, density of states in the mobility gap, and hole-transport mechanisms.