Origin of Hole-Trapping States in Solution-Processed Copper(I) Thiocyanate (CuSCN) and Defect-Healing by I2_2 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 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

Mixed-Metal Cu-Zn Thiocyanate Coordination Polymers with Melting Behavior, Glass Transition, and Tunable Electronic Properties

The solid-state mechanochemical reactions under ambient conditions of CuSCN and Zn(SCN)2 resulted in two novel materials: partially Zn-substituted α-CuSCN and a new phase CuxZny(SCN)x+2y. The reactions take place at the labile S-terminal, and both products show melting and glass transition behaviors. The optical band gap and solid-state ionization potential can be adjusted systematically by adjusting the Cu:Zn ratio. Density functional theory calculations also reveal that the Zn-substituted CuSCN structure features a complementary electronic structure of Cu 3d states at the valence band maximum (VBM) and Zn 4s states at the conduction band minimum (CBM). This work shows a new route to develop semiconductors based on coordination polymers which are becoming technologically relevant for electronic and optoelectronic applications.</p

Engineering High‑<i>k</i> Oxide/CuSCN Interface for p‑Channel Thin-Film Transistors

Copper(I) thiocyanate (CuSCN) is a unique coordination polymer semiconductor with excellent hole-transport properties and a wide band gap. CuSCN enables the construction of thin-film transistors (TFTs) based on a transparent p-type inorganic channel layera rare component. Despite the tremendous progress of TFTs based on transparent n-type oxides, the development of the p-type counterparts has been limited, especially for TFTs based on CuSCN. In this work, we explored three aspects in bottom-gate top-contact TFTs: the use of high-k metal oxides (AlOx, GaOx, and HfOx) as the dielectric layer, organic molecules (methacrylic acid or MAA and tetraethyl orthosilicate or TEOS) as the dielectric passivation layer, and an antisolvent (tetrahydrofuran or THF) to improve the hole-transport properties. The best condition was found to be AlOx annealed at 200 °C with the TEOS layer as the dielectric combined with THF-treated CuSCN as the semiconducting channel. The resulting TFTs operated under low voltages and showed a field-effect hole mobility in the range of 7–8 × 10–3 cm2 V–1 s–1, representing an increase of 4- to 5-fold from the previous report. In particular, the TEOS passivation layer and THF treatment increased the mobility as a result of the reduced trap state density