Urea-assisted N-type conversion of laser-induced graphene for thermoelectric applications

Abstract Thermoelectric materials enable direct heat-to-electricity conversion. They have gained increasing interest in sustainable and wearable energy-harvesting systems. However, progress in flexible thermoelectric devices remains limited by the shortage of high-performance n-type organic materials, hindering the development of efficient p–n complementary modules. Laser-induced graphene (LIG) has attracted considerable attention for thermoelectric applications due to its high electrical conductivity, scalable fabrication, and compatibility with flexible substrates. However, LIG produced from polyimide typically exhibits p-type behavior, which limits its applicability in thermoelectric modules requiring complementary n-type materials. In this study, we address this limitation by introducing a simple and scalable urea-assisted strategy to convert p-type LIG into n-type LIG through nitrogen doping during the laser-induced graphitization process. First, we have fabricated LIG by direct laser writing on polyimide and subsequently treated with 5 and 10 wt% urea solutions, followed by mild annealing. Subsequent structural and chemical characterizations confirmed effective nitrogen incorporation, dominated by graphitic-N species, while preserving the porous 3D LIG network. Thermoelectric measurements revealed enhanced electrical conductivity (up to 1120 S/m) and a clear p-to-n transition, as evidenced by negative Seebeck coefficients in urea-treated films. The optimized LIG-10 N sample delivered the highest thermoelectric performance, reaching a power factor of 0.136 µW m⁻¹ K⁻² at 60 °C. Overall, this work provides a practical route for engineering n-type LIG, supporting the development of flexible thermoelectric modules for wearable and low-power energy-harvesting applications. Graphical Abstract