Nanoparticles provide efficiency gains for thinner solar cells
Prof. Schmid’s research demonstrated that nanoparticles interact with one another via their electromagnetic near-fields, so that local ‘hot spots’ arise where light is concentrated especially strongly.
To save on fabrication costs it is desirable with thin-film solar cells to utilize less material. As an example, chalcopyrite cells (copper-indium-gallium-diselenide, or ‘CIGS’ cells) in part consist of rare-earth elements like indium and gallium. If the active layer is made too thin it absorbs too little light and the efficiency level drops. Nanostructures on top of the active material might be able to capture the light and increase the efficiency.
The concept is being investigated by Prof. Martina Schmid, who heads the NanooptiX group of junior scientists at HZB and holds a junior professorship at Freie University Berlin. “Our objective is to optimize nanostructures so they selectively direct certain wavelengths of the solar spectrum into the cells.”
The topography of the sample surface can be seen here (white lines around the nano-particles) as well as the local optical excitations. The image displays several “hot spots” (yellow) that arise through interactions of the nanoparticles with the light and also with one another. Image: HZB/CalTech
One option to achieve this is to construct simple nanostructures from metallic particles that self-organise by heat-treatment of a thin metallic film. Prof. Schmid initially coated a glass substrate with a thin film of silver (20 nm), which was subsequently subjected to heat treatment. Irregular silver particles are formed in this way having diameters of around 100 nm.
The silver nanoparticles are irregularly shaped and randomly distributed over the surface, as shown by the scanning electron microscope image.
Image: HZB
In collaboration with colleagues at the California Institute of Technology (CalTech), Prof. Schmid investigated how these types of randomly distributed nanoparticles influence the incidence of light on a cell below. Using a scanning near-field optical microscopy (SNOM) technique, a tiny point scans the sample, determining the topography as with atomic force microscopy. The sample is simultaneously illuminated through an even smaller aperture in the probe point to create optical excitations (plasmons) in the nanoparticles. The optical excitations can either couple the light into the solar cell as desired – or instead transform the light into heat, whereby it is lost to the solar cell.
Measurements showed that there can be strong interactions between densely situated, irregularly distributed nanoparticles leading to local ‘hot spots’.
"Whereas the darker regions tend to absorb light and transform it into heat, the hot spots show where nanoparticles strongly interact via their electromagnetic near-fields. In these regions of enhanced fields, energy transformation in the solar cell could potentially be enhanced," Martina Schmid explains.
In the end, areas of stronger fields but also of comparatively weaker ones arise. However, it is difficult to establish a clear relationship between the occurrence of these hot spots and specific nanoparticles. “The particles mutually affect one another through their electromagnetic near-fields, which are notably more complex than suspected until now. We need to ascertain how we can intentionally create the desired field distributions," explained Schmid.
Reference
M. Schmid, J. Grandidier and H. A. Atwater, “Scanning near-field optical microscopy on dense random assemblies of metal nanoparticles“, J. Opt., 15, 125001 (2013)
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