Sulphur-Doped Copper(I) Iodide as Record Figure of Merit P-Type Transparent Conductor

CuI is the best-known p-type transparent conductor (TC) to date, yet its conductivity still lags behind n-type TCs like ITO. Herein, we demonstrate S-doped CuI films via pulsed laser deposition and provide an in-depth defect analysis to describe its enhanced conductivity. Combining compositional and optoelectronic characterization of the films, we show that 3 at.% S incorporation in CuI leads to an increase in hole carrier density from ˜8x1019 cm-3 to ˜9x1020 cm-3, resulting in a conductivity boost from 78 S∙cm-1 to 435 S∙cm-1 while maintaining >75% transparency in the visible spectrum. Defect calculations suggest an interplay between S and I incorporation as responsible for the increased concentration of Cu vacancies, the latter being the main source of free holes in CuI. The high conductivities of the S:CuI films validate its record figure of merit and motivate further exploration of dopants and allow strategies with CuI to achieve high-performing p-type TCs.