Fully spray-printed flexible thermoelectric generators with PEDOT:PSS legs and silver electrodes

: Flexible thermoelectric generators (TEGs) offer a promising solution for self-powered wearable electronics. In this study, a custom-built spray-printing system was designed and fabricated by modifying a commercial filament 3D printer to enable precise, programmable thin-film deposition. Using this system, a fully spray-printed flexible TEG was fabricated using poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as thermoelectric (TE) legs and silver as electrodes. The custom-built printing system enabled precise control of key parameters to deposit uniform, low-resistance thin films. After optimising the printing parameters, a flow rate of 2.9 mL h -1 spray volume of 20 mL, and nozzle movement speed of 1000 mm min -1 were used to deposit PEDOT:PSS legs. For the silver electrodes, a 0.15:1 paste-to-solvent ratio, a nozzle–substrate distance of 7 cm, and an increased flow rate provided smooth, conductive films while mitigating ink sedimentation. Using these optimised conditions, a flexible TEG comprising 19 PEDOT:PSS legs interconnected by silver electrodes was fabricated on a Kapton substrate. The device exhibited excellent flexibility and stable output, generating an open-circuit voltage of 20 mV at ΔT = 68 °C. At this temperature difference, the device delivered a maximum output power of 0.25 nW, corresponding to a power density of 0.12 nW cm -2 . When applied to human skin under a natural temperature difference of ∼10 °C, the TEG produced 3.36 mV, demonstrating its feasibility for wearable TE energy harvesting. Moreover, after H 2 SO 4 post-treatment, the device delivered a maximum output power of 0.52 nW at ΔT = 46 °C, corresponding to a power density of ∼0.242 nW cm -2 . Compared with the untreated device (0.11 nW at ΔT ≈ 45 °C), this represents an approximately 4.8-fold enhancement in output power, demonstrating the effectiveness of post-treatment in improving the performance of spray-printed flexible TEGs.