Tunable bandgap of copper sulfide via cation exchange as a p-type transparent electrode

Owing to the inherent challenges of oxide-based p-type TCOs such as the localized O 2p-derived valence band, deep acceptor levels, and the requirement for thermally demanding crystallization, recent research has increasingly focused on non-oxide halide and chalcogenide systems as alternative wide-bandgap semiconductors for transparent electronics. Here, we report a low-temperature cation-exchange approach for tuning CuS transparent electrodes by incorporating Zn into preformed CuS thin films. Unlike conventional non-oxide transparent electrodes such as carbon-based films, metal nanowires, or conducting polymers, which lack tunable optoelectronic properties and often suffer from limited mechanical durability, the present Zn:CuS film offers a continuously tunable optical bandgap (1.81-2.67 eV) together with a transmittance of >90% at 550 nm, a sheet resistance of ∼150 Ω sq^-1, and a negligible sheet-resistance change over 10⁴ bending cycles. Integration of 9% Zn:CuS as a transparent electrode with a CVD-grown MoS₂ channel yielded reliable photodetection under 450 nm illumination at DrainV = 1.0 V, with a photoresponsivity of up to ∼3 A W^-1 and rise/decay times of τ_r = 4.3s and τ_f = 16.1s. These results establish Zn:CuS as a solution-processable, bandgap-tunable alternative to conventional non-oxide transparent electrodes for next-generation 2D optoelectronics.