image: Design and syntheses of organic radical photothermal cocrystals for solar thermoelectric generators
Credit: By Sheng Zhuo, Yu Dong Zhao et al.
Recent advancements in solar thermoelectric generators (STEGs) have brought us closer to efficiently converting solar heat into electricity, with promising applications in wearable electronics, the Internet of Things, and personal thermal management. Researchers have been working tirelessly to enhance the performance of STEGs by developing high-performance thermoelectric materials, optimizing device configurations, and regulating thermal dissipation. Among the various materials explored, photothermal materials such as carbon-based materials, metal oxides, polymers, and phase-change materials have shown great potential for creating the desired temperature gradients for efficient energy conversion.
In a groundbreaking study, the open-shell radical 2,6-dibromonaphthalene-1,4,5,8-tetracarboxylic dianhydride (Br2NDA) was purposefully applied as an electron acceptor to rationally design and synthesize an organic radical photothermal charge-transfer (CT) cocrystal, coronene (COR)-Br2NDA (CBC). The prepared CBC cocrystal has demonstrated an exceptional photothermal conversion efficiency (PCE) of 67.2% under 808 nm irradiation with the intensity of 0.367 W cm-2, marking a significant leap forward in the field.
The CBC cocrystal was successfully synthesized through a simple solution self-assembly strategy, resulting in needle-like microrods with high crystallinity. A series of sophisticated analyses, including X-ray diffraction (XRD), selected area electron diffraction (SAED), UV-Vis absorption, photoluminescence (PL), Fourier transform infrared (FT-IR), and solid-state nuclear magnetic resonance (NMR), provided compelling evidence of strong charge transfer (CT) interaction between COR and Br2NDA. The cocrystal exhibited a broad absorption spectrum spanning 350-1100 nm, with a pronounced redshift compared to the individual components, indicative of robust CT interaction. Remarkably, the cocrystal showed nearly complete photoluminescence quenching, suggesting the enhanced nonradiative transitions. Electron spin resonance (ESR) measurements further confirmed the presence of unpaired electrons resulting from CT process, supporting the CT interaction in the ground state. Thermogravimetric analysis (TGA) also confirmed the cocrystal's thermal stability. The photothermal performance of CBC cocrystal was truly remarkable. Under 808 nm laser irradiation at 0.367 W cm-2, the prepared cocrystal reached an equilibrium temperature of 86°C within seconds. The PCE of 67.2% was calculated using a reliable method, surpassing many previously reported organic photothermal materials. The cocrystal also exhibited excellent cyclic stability, maintaining consistent performance across multiple irradiation cycles. When exposed to simulated solar irradiation, the CBC cocrystal achieved a high temperature of 64°C under 2 sun intensity, with a linear relationship observed between temperature rise and irradiation intensity.
To harness the CBC cocrystal's outstanding light-harvesting capacity and photothermal conversion performance, researchers incorporated it into a transparent resin to create a photothermal ink, which was then coated onto a thermoelectric generator (TEG) to develop a high-performance STEG. Under 2 sun simulated solar irradiation, the CBC-coated TEG achieved a photothermal conversion temperature of 70.3°C and an impressive output voltage of 209 mV, a 375% improvement over the bare TEG. The device demonstrated excellent stability, maintaining consistent voltage and current outputs across multiple heating and cooling cycles under varying solar intensities. Moreover, the CBC-TEG device exhibited responsive behavior to near-infrared (NIR) light, enabling non-contact information conversion. By modulating laser power intensity and duration, the device could transmit encoded signals, such as Morse code, demonstrating its potential for remote communication and encryption in wearable electronics and long-distance signal transmission.
This pioneering work not only provides a straightforward approach to designing high-efficiency organic radical photothermal cocrystals but also opens new avenues for solar-thermoelectric energy harvesting and information conversion technologies. The successful integration of CBC cocrystals into TEGs heralds a promising future for efficient solar-thermoelectric conversion and real-time information processing, with potential applications across a wide range of industries and research fields.