Comparative Study of Ni Doping (0–0.3 wt %) in Bi ₂ Se ₃ Polycrystalline Materials Synthesized via a Solvothermal Method: Structural, Optical, and Thermoelectric Analysis

Nickel-doped bismuth selenide (Bi₂Se₃) polycrystalline material, synthesized via a solvothermal method, emerged as a dual-functional material for mid-temperature optoelectronic and thermoelectric applications. Structural analysis revealed a rhombohedral phase with a new (0 1 8) peak and a 39% reduced crystallite size alongside morphological transitions to spherical and rod-like grains. Nickel doping narrowed the optical bandgap by 50%, enabling broadband absorption (317-678 nm) and inducing a 500 nm photoluminescence redshift for near-infrared emission. Thermoelectrically, nickel doping enhanced the electrical conductivity by 220% at 425 K while maintaining a high Seebeck coefficient and reducing the lattice thermal conductivity by 19%. These coordinated improvements yielded a record thermoelectric figure of merit (ZT = 1.15 at 525 K) for Ni₀.₃Bi₂Se₃a 259% enhancement over undoped Bi₂Se₃. The work establishes nickel-doped Bi₂Se₃ as a scalable and cost-effective material for high-efficiency photonic devices and waste-heat recovery systems, addressing critical challenges in sustainable energy technologies in the mid-temperature range.