Origin of Hole-Trapping States in Solution-Processed Copper(I) Thiocyanate (CuSCN) and Defect-Healing by I$_2$ Doping

TL;DR

This study reveals that I$_2$ doping heals defect states in solution-processed CuSCN by substituting missing ligands, significantly enhancing hole mobility and device stability in thin-film transistors.

Contribution

It introduces a simple I$_2$ doping method to passivate defects in CuSCN, improving its electronic properties and device performance.

Findings

Hole mobility increased by over five times.

Trap state density was reduced.

Device stability and performance improved.

Abstract

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 performance. Despite the extensive electronic applications of CuSCN, its defect properties have not been studied 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 I$_2$ 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. Applying I$_2$-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 I$_2$ 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 I$_2$ addition. The contact resistance and bias-stress stability of the devices also improve. This work shows a simple and effective route to improve hole transport properties of CuSCN which is applicable to wide-ranging electronic and optoelectronic applications.