3D-Printed Bulk Organic Thermoelectrics with High Power Output for Stretchable Electronics

Distributed electronics are gaining traction for application in wearables and the Internet of Things (IoT). The increasing complexity of these systems comes with an acute need for energy that batteries struggle to satisfy, either because they are too stiff to be seamlessly integrated into soft wearable electronics, or because their replacement is unfeasible for the numerous and highly distributed nodes of the IoT. In those scenarios, thermoelectric (TE) materials, which can convert directly waste heat into electrical power, hold great potential. Conveniently, it has been suggested that TEs are more efficient than other thermal engines for low-power applications[1] like wearables and the IoT. However, the widespread of TE generators (TEG) is hampered by their expensive fabrication process; the use of critical elements such as Bi and Sb; and their limited form factor (rigid, flat, and small).[2] The ubiquity of the IoT nodes and the particular form factor of wearable devices (stretchable and large-area) call for a new technology that strongly relies on clean and abundant materials like polymers, and on scalable and versatile fabrication techniques, like printing. While the development of conducting polymers for organic thermoelectrics (OTEs) has recently witnessed tremendous progress, the reality is that the integration of such materials in actual devices has been seldom attempted, and when attempted, the high performance of the materials did not translate into performing devices. The reason for such lackluster outcome resides in the fact that most OTE materials are developed as thin films using technologies borrowed from organic FETs and photovoltaics. In sharp contrast, performing TEGs (such as commercial Peltier elements) are inherently bulk devices, presenting low internal resistance and capable of sustaining large temperature gradients in the through-plane direction, none of which is possible for thin films.<br/> <br/>In my research group, we are working towards the development of high-performing 3D-printed bulk OTEs, which can be directly printed over large areas on flexible/stretchable substrates. In particular, we are developing direct ink writing (DIW) of conjugated polymers, which requires the careful formulation of pastes with very particular rheology, namely shear thinning and high yield strength. Moreover, specific strategies for drying and post-processing are necessary to ensure high shape fidelity. In return, DIW is not only potentially cheap and suitable for soft substrates, but offers also enough processing versatility to tune material morphology while enabling original device architectures that allow on-skin integration (to power wearables using the heat emitted by the human body) and conformability to hot curved surfaces, like hot pipes or engines (to power IoT nodes).<br/> <br/><i>Acknowledgments</i><br/>This work was supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme: Grant Agreement No. 948922 – 3DALIGN.<br/> <br/> <br/>[1] C. B. Vining, Nat. Mater. 2009, 8, 83.<br/>[2] F. Molina-Lopez, In 2020 IEEE Sensors, IEEE, Rotterdam, 2020, pp. 1–4.