Thermodynamic and kinetic engineering of bi-doped InSe for ultrafast, broadband, and highly stable optoelectronics

Two-dimensional (2D) indium selenide (InSe) holds great promise for optoelectronic applications, yet its practical application remains severely limited by high contact resistance, narrow spectral absorption range, and pronounced susceptibility to environmental degradation. Here, a bismuth (Bi) doping strategy is studied to engineer the thermodynamic and kinetic transport properties of InSe field-effect transistors. An optimal nominal Bi content of 25% leads to a pronounced increase in electron density, which effectively narrows the Schottky barrier at the metal–semiconductor interface, yielding a minimized contact resistance of 0.9 kΩ μm and a high field-effect mobility of 412 cm2 V−1 s−1. Concurrently, dopant-induced bandgap renormalization extends the optical response up to 980 nm. This synergistic modulation of interfacial tunneling probability and the joint density of states culminates in a high photoresponsivity and detectivity of 580 A W−1 and 2.52 × 1011 Jones, respectively, while elegantly preserving sub-millisecond response kinetics. Moreover, Bi incorporation suppresses the adsorption of oxygen and moisture on the InSe surface, substantially improving the stability of the unencapsulated devices, which retain 90.2% of their initial photocurrent after 90 days of exposure. This work comprehensively elucidates the interplay between defect dynamics and bandgap engineering, presenting a viable paradigm for robust and broadband 2D optoelectronics.