Temperature-dependent thermoelectric and multiband photoresponse of undoped, Co–doped and Co–B co-doped ZnO thin films

Abstract This study investigated the thermoelectric and multiband photoresponse characteristics under temperature variation for undoped, Co-doped and Co–B co-doped zinc oxide (ZnO) thin films. The films were successfully synthesized via the sol–gel spray method. The electrical resistance of the samples was measured in the temperature range of 300–500 K under dark, ultraviolet (UV), visible (Vis), and infrared (IR) illumination conditions to determine their photoresponse behavior. The thermoelectric performances of the samples were evaluated in the temperature range of 300–550 K by measuring the Seebeck coefficient and electrical conductivity, followed by calculating the power factor ( PF ). All films exhibited n-type conductivity with thermally activated transport dominated by defect-assisted hopping. Co doping enhanced carrier activation and introduced shallow donor states, yielding Seebeck coefficients up to −2250 μV/K (1% Co-doped ZnO). Co–B co-doping further improved electrical conductivity and reduced activation energy to ~0.16 eV, resulting in a power factor of 610 μW·m −1 ·K −2 for 2% Co-1% B co-doped ZnO at 500 K. This value is approximately 6000 times greater than that of undoped ZnO. 1% Co-1% B co-doped ZnO sample demonstrated broadband photoresponse with Rp > 90% across UV–Vis–IR spectra, confirming synergistic defect modulation. These findings highlight the potential of Co–B co-doped ZnO films for multifunctional energy harvesting and photodetection applications under variable thermal and optical conditions.