Defect healing and improved hole transport in CuSCN by copper(i) halides†

Copper(I) thiocyanate (CuSCN) is a unique wide band gap, p-type inorganic semiconductor with extensive opto/electronic applications. Being a coordination polymer, CuSCN requires processing by coordinating solvents, such as diethyl sulfide (DES). The strong interactions between CuSCN and DES lead to the formation of SCN⁻ vacancies (V_SCN), which are detrimental to hole transport. In this work, we rationally modify copper(I) thiocyanate (CuSCN) through the use of chemically compatible copper(I) halides (CuX, where X = Cl, Br, or I). On assessing the device characteristics of thin-film transistors employing CuX-modified CuSCN as the p-channel layer, adding 5% of CuBr is found to be the most optimal condition. The hole mobility is increased by 5-fold to 0.05 cm² V⁻¹ s⁻¹ while the on/off current ratio is also enhanced up to 4 × 10⁴. The drain current in the off-state does not increase whereas the trap state density is reduced, and the performance improvement can be attributed to the defect healing effect. Detailed characterization by synchrotron-based X-ray absorption spectroscopy reveals the recovery of the coordination environment around Cu, confirming that Cl⁻ and Br⁻ can effectively passivate V_SCN defects. In particular, CuBr further improves film uniformity and smoothness. The simple protocol based on common chemicals reported herein is applicable to the standard CuSCN processing recipe, which is currently applied across a wide range of electronic and optoelectronic devices.