  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.
