Harvesting solar energy using self-assembled nanoparticles

Solar-thermal technology is a promising energy harvesting method that could play a role in mitigating the fossil fuel energy crisis. The technology transforms sunlight into thermal energy, but it is challenging to suppress energy dissipation while maintaining high absorption. Existing solar energy harvesters that rely on micro or nanoengineering don’t have sufficient scalability and flexibility, and will require a novel strategy for high-performance solar light capture while simultaneously simplifying fabrication and reducing costs.

Researchers from Harbin University, Zhejiang University, the Changchun Institute of Optics and the National University of Singapore have now designed a solar harvester with enhanced energy conversion capabilities. Their findings were published in APL Photonics.

The device employs a quasiperiodic nanoscale pattern — meaning most of it is an alternating and consistent pattern, while the remaining portion contains random defects (unlike a nanofabricated structure) that do not affect its performance. In fact, loosening the strict requirements on the periodicity of the structure significantly increases the device’s scalability. The fabrication process uses self-assembling nanoparticles, which form an organised material structure based on their interactions with nearby particles without any external instructions. Thermal energy harvested by the device can be transformed into electricity using thermoelectric materials.

Author Ying Li of Zhejiang University said solar energy is transferred as an electromagnetic wave within a broad frequency range. “A good solar-thermal harvester should be able to absorb the wave and get hot, thereby converting solar energy into thermal energy. The process requires a high absorbance (100% is perfect), and a solar harvester should also suppress its thermal radiation to preserve the thermal energy, which requires a low thermal emissivity (zero means no radiation),” Li said.

To achieve these goals, a harvester is usually a system with a periodic nanophotonic structure. But the flexibility and scalability of these modules can be limited due to the rigidity of the pattern and high fabrication costs. “Unlike previous strategies, our quasiperiodic nanophotonic structure is self-assembled by iron oxide (Fe₃O₄) nanoparticles, rather than cumbersome and costly nanofabrication,” Li said.

These images show the device’s solar-thermal conversion (left) and solar thermoelectric harvesting (right). Image credit: Zifu Xu.

The quasiperiodic nanophotonic structure achieves high absorbance (greater than 94%) with suppressed thermal emissivity (less than 0.2) and, under natural solar illumination, the absorber features a fast and significant temperature rise (greater than 80°C). Based on the absorber, the team built a flexible planar solar thermoelectric harvester, which reached a sustaining voltage of over 20 mV/cm². It is expected to power 20 light-emitting diodes per square metre of solar irradiation. This strategy can serve low-power-density applications for more flexible and scalable engineering of solar energy harvesting.

“We hope our quasiperiodic nanophotonic structure will inspire other work. This highly versatile structure and our fundamental research can be used to explore the upper limit of solar energy harvesting, such as flexible scalable solar thermoelectric generators, which can serve as an assistant solar harvesting component to increase the total efficiency of photovoltaic architectures,” Li said.

Top image credit: iStock.com/pixelfit