Flexible and Transmissive All-Polymeric Heater Based on n-Doped Poly(Benzodifurandione)

Transparent heaters (THs) are pivotal for a range of applications, from defogging systems and electrochromic devices to next-generation AR/VR technologies. Traditionally, transparent conductive materials such as Indium Tin Oxide (ITO) have been employed due to their high electrical conductivity and optical transparency. However, ITO is mechanically fragile, energy-intensive to process, and poses scalability challenges for large-area and flexible applications. These limitations have motivated the urgent need for alternative materials.
Our group recently reported a one-pot oxidative polymerization and reductive doping reaction, performed in open air under ambient conditions, that yields n-PBDF—the first thermodynamically and kinetically stable n-doped conducting polymer. However, because this method affords n-PBDF as a solution in DMSO, the high boiling point and viscosity of DMSO present significant challenges for scalable thin-film coating methods such as ultrasonic spray coating.
In this work, we address these challenges by formulating n-PBDF into an ethanol-based ink, offering an environmentally sustainable and industrially viable alternative for large-area coatings. This ethanol formulation allows for precise deposition using scalable methods like ultrasonic spray coating. This study represents the first major demonstration of high-performance ultrasonic spray-coated n-PBDF films, exhibiting excellent optical transmittance (T550 ≈ 80%+) and remarkably high electrical conductivity (> 3,000 S/cm+), rivalling ITO and surpassing other organic materials. Furthermore, we demonstrate Joule heating in thin films of n-PBDF using this newly developed ethanol-based ink, alongside in-depth thermal characterization.
The n-PBDF films exhibit a remarkable Joule heating effect, with highly controllable temperature profiles tuned by film thickness and applied voltage (1−24V DC). These films achieve temperatures above 150°C and support linear areal power densities up to 600 mW/cm2, while maintaining a low haze value (< 0.30%) for high optical clarity. We also present a comprehensive optoelectronic characterization, including transmittance, electrical conductivity, sheet resistance, and the thickness dependence of these properties, as well as color and absorption metrics.
Thermal and Surface Properties
For the first time, we report the thermal transport properties of n-PBDF, revealing anisotropic thermal conductivity with an in-plane value of 3.4 W/mK. This positions n-PBDF as an ideal material for applications requiring efficient heat dissipation, such as transparent heaters and flexible electronics. The films exhibit excellent surface smoothness, with RMS roughness < 2.0 nm, as confirmed by AFM imaging, demonstrating uniform thickness and precise control over morphology.
Sustainability and Applications
The ethanol-based formulation not only improves the processability of n-PBDF but also supports a greener production method aligned with industrial standards. The scalability of this process allows for a wide range of applications, including flexible substrates like PET, which can be used in wearable technologies (e.g., ski goggles, motorcycle visors) and smart windows for defogging and deicing systems.
Durability and Outlook
Preliminary durability tests show minimal degradation in performance after multiple heating cycles, indicating the robustness of n-PBDF for long-term use. Ongoing work focuses on further optimizing the thermal and optoelectronic performance of n-PBDF films, with the aim of integrating these materials into advanced applications in AR/VR technologies, solar cells, and electrochromic devices.
In conclusion, this work demonstrates the first thorough device characterization and scalable application of n-PBDF in transmissive heaters and showcases n-PBDF as a scalable, sustainable, and high-performance material for widespread adoption in next-generation optoelectronic devices.