The paper in Nature Communications is here: https://go.nature.com/2Q9CvUu

Thermoelectric (TE) materials allow conversion between heat and electricity via the Seebeck and Peltier effects to function as power generators and solid-cooling elements. Organic materials are emerging TE candidates not only because of their prominent performance at room temperature, but also due to unique features of flexibility and potential low-cost application. Recent extensive research into the Seebeck effect of organic TE (OTE) materials has led to improved ZT values greater than 0.2, which can meet the requirement of low power wearable devices. Unlike the Seebeck effect, the Peltier effect in OTE films, however, has not been explored, despite its critical role in enabling lightweight solid-cooling elements without moving parts that exist in typical refrigerators.

The Peltier effect occurs in a TE device concurrently with Joule heating, heat conduction, and various heat dissipation processes. In addition, the low electrical and thermal conductivity, which are intrinsic features of most OTE materials, lead to significant contributions from these processes. This means that precise identification of the cooling ability among the complex combination of thermal processes is a bottleneck challenge for the study of Peltier effect in OTE films.

To address this open issue, we utilized a thermally suspended device and an infrared (IR) imaging technique to probe the Peltier effect in an organic film. The Parylene-based substrate we used is extremely thin—less than 500 nm in thickness—to ensure minimized heat dissipation to the substrate. Moreover, we strictly controlled the amount of vacuum to higher than 10^-3 Pa to guarantee ultralow heat convection. Thereafter, spatially resolved temperature distribution was measured using IR microscopy. It is worth noting that the Peltier effect and Joule heating can be distinguished in a single measurement scan, since the former effect is a reversible thermodynamic phenomenon that depends linearly on the current (∝I) and matches the bias frequency, whereas Joule heating is fundamentally different and exhibits irreversible quadratic response (∝I²). These combined techniques allow us to evaluate the Peltier effect in OTE films.

While investigating the Peltier effect of poly(Ni-ett) film with the aforementioned method, we discovered that the internal heat conduction is involved even after a well-designed separation process. One way to remove this contribution is transient mapping of temperature distribution at a timescale of few milliseconds when the temperature difference induced by the Peltier effect occurs near the contact, whereas heat diffusion can be neglected. In our case, we observed that the heating/cooling energy diffuses into the central part of the film at 0.17 s and reaches a thermal balance within 3 s, indicating the rapid heating/cooling feature of TE devices.

Measurement of the Peltier coefficient (Π) provides an exclusive way to describe the Peltier effect quantitatively. However, the experimental determination of Π in an OTE film is extremely difficult because the overall heat flux and thermal diffusion flux have to be measured simultaneously. Benefiting from the utilization of a lock-in IR approach, we obtained a Π value of −21.6 mV at room temperature. This value is consistent with the results (−23.5 mV) predicted by the Thomson relation (Π = ST, where S is the Seebeck coefficient). In other words, our result offers the first evidence that the well-known Thomson relation is applicable to OTE materials.

From the point of view of cooling applications, large temperature differences and realistic cooling ability are of vital importance. In the manuscript, we show that a device based on the poly(Ni-ett) film can exhibit a large temperature difference of up to 41 K at a current density of 5 A/mm². Interestingly, realistic cooling of one contact by 0.2 K is observed at a current density of 0.1 A/mm² even if the device operates in a heat-insulated condition. More importantly, much superior cooling ability of 25 K is feasible for future ultrathin OTE devices, according to the theoretical prediction of Prof. Jia Zhu, Prof. Shuzhou Li, and Prof. Lifeng Chi.

As the first step in a study of the Peltier effect in OTE films, these findings will stimulate investigation of underlying physical processes in OTE materials and ultimately the construction of solid-cooling devices using organic materials. There are still many challenges to overcome, especially in the development of materials with superior performance and reasonable devices with ideal contacts. Our goal in the near future is to understand how to engineer this behavior in other state-of-the-art materials and to build p-n-based cooling modules to demonstrate their ability in wearable cooling elements.