Sulfur-Alloyed CuI for Highly Conducting and Stable p-Type Transparent Conductor via Scalable Iodination of Cu₂S

High-performance p-type transparent conductors are crucial for next-generation optoelectronics but currently lag behind their n-type counterparts. Copper iodide (CuI), despite promising hole mobility, suffers from limited conductivity and stability. We address these limitations by reporting highly conducting and stable Cu–I–S thin films, fabricated via scalable solid iodination of sputtered Cu2S. Comprehensive characterization reveals that these S-alloyed CuI films primarily consist of polycrystalline zincblende S-incorporated CuI as the dominant phase, along with a minor amorphous CuxS phase, exhibiting outstanding electrical properties: a remarkable hole concentration of ∼3 × 1021 cm–3, a hole mobility of ∼1 cm2 V–1 s–1, and a low resistivity of ∼2 × 10–3 Ω·cm, surpassing most p-type transparent conductors. These films demonstrate 50–70% visible transparency (with an optical bandgap of ∼3.1 eV) and robust environmental stability. This enhanced conductivity and stability arise from S-alloying-induced copper vacancies within the S-incorporated CuI matrix, and notably within the CuxS phase, which also significantly contributes to the improved stability. Post-thermal annealing of longer-iodinated Cu–I–S films increases copper sulfide content due to iodine out-diffusion at elevated temperatures, promoting copper and sulfur segregation. Our results also suggest a low concentration of sulfur substitution at iodine sites (SI) in the zincblende CuI phase under equilibrium growth conditions, consistent with the high formation energy of SI predicted by recent density functional theory calculations. These findings provide valuable insights into sulfur’s role in modifying CuI properties to achieve superior electrical and optical performance along with excellent durability, making this scalable approach promising for advanced transparent electronic devices.