Flexible, Scalable and Reliable Fully Screen-Printed Thermoelectric Generators

Global energy demand is expected to increase by almost 50% in the next 20 years. Currently, traditional energy sources such as oil, gas, and coal have played a significant role in global energy production. However, they are also responsible for the increase in pollution and are harmful to human health. New energy sources are necessary to support this high demand, in a sustainable way. The thermal energy can be converted into electrical energy in a completely eco-friendly way thanks to thermoelectric generators (TEGs). Due to their flexibility, lightness, affordability, and natural abundance, organic thermoelectric materials are the perfect competitors for making flexible TEGs that can readily conform to complex surfaces. However, up to now, the research has primarily focused on making simple proof-of-concept devices not application-oriented, to highlight the thermoelectric performance of the organic materials. Moreover, demonstrator-devices typically show performance that is far below what one would expect based on the figures of merit of the constituent materials.<br/>Here, we present a new fabrication method to make vertical, fully printed, large-area, flexible, monolithic TEG with a high-density of thermocouples scalable up to m², compatible with all the screen-printable inks with performance that matched the expectations based on the figures of merit of the constituent materials.<br/>The TEGs were made screen printing five layers on top of each other on a 25 µm thick Kapton substrate, recreating the Π-shape structure, where device legs are oriented perpendicular to the heat flows. A silver bottom contact necessary for the electrical connection was screen printed, followed by an insulator layer with cavities designed to create a mask for the vertical thermocouples. Successively, the p-type layer and the silver layer were screen-printed to fill alternatively the cavities, thereby creating the vertical thermocouples. Finally, the top silver contact was screen printed to electrically complete the circuit.<br/>We demonstrated the scalability of the process employing as p-type the well-known and commercially available poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) to make vertical TEGs composed of 4 and 8 thermocouples, with a surface area of 0.5 cm²and 1 cm², respectively. The devices exhibited an ideal behaviour perfectly scalable in accordance with numerical simulations. After the validation of the fabrication method, we utilized a recently reported additive-free graphene, not previously assessed for thermoelectric applications, as p-type material to make a vertical TEG composed of 8 thermocouples. The power output at a ΔT = 25°C provided by the additive-free graphene device was over five times higher than that of the PEDOT:PSS TEG. Additionally, the measures were once again in accordance with numerical simulation. The additive-free graphene TEG provides a remarkable, among the organic thermoelectric TEG, power density of 15 nW/cm² at ΔT = 29,5 °C.<br/>Scaling this device to a 100 cm² large TEG will result in a power output of 1.5 µW, sufficient to power low-energy electronic devices such as sensors, wearables, and wireless communication systems.<br/>The fully screen-printable large-area TEGs presented are also extremely lightweight and flexible. Specifically, the additive-free graphene TEG provides an estimated power output of 1 µW/g, and its internal device resistance remained constant even when the device was rolled into a cylinder with a bending radius of 1 cm.<br/>Future work for this technology will involve making larger devices on thinner substrates to considerably increase the power output. Using a 5 µm substrate the power output of a 100 cm² additive-free graphene TEG will increase by an order of magnitude. Furthermore, due to the versatility and reliability of the process, it will also be possible to use inorganic materials as active inks with enhanced thermoelectric performance to further increase the power output.