Origin of Hole-Trapping States in Solution-Processed Copper(I) Thiocyanate and Defect-Healing by I2 Doping

Solution-processed copper(I) thiocyanate (CuSCN) typically exhibits low crystallinity with short-range order; the defects result in a high density of trap states that limit the device's performance. Despite the extensive electronic applications of CuSCN, its defect properties are not understood in detail. Through X-ray absorption spectroscopy, pristine CuSCN prepared from the standard diethyl sulfide-based recipe is found to contain under-coordinated Cu atoms, pointing to the presence of SCN⁻ vacancies. A defect passivation strategy is introduced by adding solid I₂ to the processing solution. At small concentrations, the iodine is found to exist as I⁻ which can substitute for the missing SCN⁻ ligand, effectively healing the defective sites and restoring the coordination around Cu. Computational study results also verify this point. Applying I₂-doped CuSCN as a p-channel in thin-film transistors shows that the hole mobility increases by more than five times at the optimal doping concentration of 0.5 mol.%. Importantly, the on/off current ratio and the subthreshold characteristics also improve as the I₂ doping method leads to the defect-healing effect while avoiding the creation of detrimental impurity states. An analysis of the capacitance-voltage characteristics corroborates that the trap state density is reduced upon I₂ addition.