LAUSR

Few‐layer indium selenide (InSe) holds promise for next‐generation optoelectronics but suffers from defect‐related limitations. While thermal annealing is a common post‐synthesis technique for tuning the properties of 2D materials, its application in InSe is hindered by the complex interplay between defect evolution and phase transitions. In this work, a low‐temperature, high‐vacuum annealing strategy is introduced that allows for fine‐tuned regulation of selenium vacancies, enabling significant optoelectronic improvements while suppressing unwanted phase transitions. By performing measurements on InSe optoelectronic devices throughout the vacuum annealing process, it is demonstrated that vacuum annealing can serve as an n‐type doping, coupled with the enhancement of mobility (from 1.2 to 276 cm2 V−1 s−1) and photoresponsivity (from 1000 to 2.3 × 104 A W−1). Through thermodynamic hypothesis and semi‐quantitative X‐ray photoelectron spectroscopy (XPS) results, a direct correlation is established between these performance improvements and the controlled increase of Se vacancy concentration (from ≈2% to ≈7%). Based on this, a band diagram model is proposed to explain the change of charge transport and photocurrent generation processes in InSe devices. This work establishes defect engineering as a critical standalone parameter for optimizing InSe device performance, offering both theoretical insights and practical guidelines for the fabrication of high‐performance InSe functional devices.