The role of sulfur in sulfur-doped copper(I) iodide p-type transparent conductors

CuI has been the best-known p-type transparent conductor (TC) for years, yet its conductivity still lags behind n-type TCs. 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 characterizations of the films, we show that 3 atom % S incorporation in CuI leads to an increase in hole carrier density from ∼8 × 10¹⁹ to ∼9 × 10²⁰ cm⁻³, resulting in a conductivity boost from 78 to 435 S cm⁻¹ while maintaining >75% transparency in the visible spectrum. The increase in carrier density is linked to the formation of a Cu_xS amorphous phase at grain boundaries and copper-vacancy-rich phases intragrain, the latter suggested by defect calculations. The high conductivities of the S:CuI films validate the recently reported high figure of merit and motivate further exploration of dopants and alloy strategies with CuI to achieve high-performing p-type TCs.