Photovoltaic research has much focused on maximizing the electricity yield obtained from solar panels: nowadays, commercial panels have a maximum conversion efficiency from sunlight into electricity of around 22%. To reach such high efficiency, silicon solar cells have been equipped with a textured surface with an antireflection layer to absorb as much light as possible. This creates a dark blue or black appearance of the solar panels.
Green solar panels
To create the colored solar panels the researchers have used the effect of Mie scattering, the resonant backscattering of light with a particular color by nanoparticles. They integrated dense arrays of silicon nanocylinders with a diameter of 100 nm in the top module cover slide of a high-efficiency silicon heterojunction solar cell. Due to the resonant nature of the light scattering effect, only the green part of the spectrum is reflected; the other colors are fully coupled into the solar cell. The current generated by the mini solar panel (0,7 x 0,7 cm2) is only reduced by 10%. The solar panel appears green over a broad range of angles up to 75 degrees. The nanoparticles are fabricated using soft-imprint lithography, a technique that can readily be scaled up to large-area fabrication.
Building materials
The light scattering effect due to Mie resonances is easily controllable: by changing the size of the nanoparticles the wavelength of the resonant light scattering can be tuned. Following this principle the researchers are now working to realize solar cells in other colors, and on a combination of different colors to create solar panels with a white appearance. For the large-scale application of solar panels, it is essential that their color can be tailored. In this way, solar panels become building materials that can be used in many different ways: red panels can serve as roof tiles, white ones can serve as walls in buildings. And solar panels placed in nature are ideally green, so they are invisible.
Green solar cell (a) Schematic of green solar cell design (b) AM1.5G solar spectrum (dark blue) and solar spectrum after transmission of the nanoparticle array (green). Because green light is reflected from the nanoparticle array, the transmitted spectrum has a dip at 550-580 nm. Photographs under diffused sunlight (0.5 x 0.5 cm²) of (c) untreated solar cell and (d) green silicon heterojunction solar minimodule
Reference:
Verena Neder, Stefan L. Luxembourg, and Albert Polman, Efficient colored silicon solar modules using integrated resonant dielectric nanoscatterers, Appl. Phys. Lett. 111, in press (2017) DOI:10.1063/1.4986796
