Abstract
Metal nanoparticles and diffractive nanostructures are widely studied for enhancing light trapping efficiency in thin-film solar cells. Both have achieved high performance enhancements, but there are very few direct comparisons between the two. Also, it is difficult to accurately determine the parasitic absorption of metal nanoparticles. Here, we assess the light trapping efficiencies of both approaches in an identical absorber configuration. We use a 240 nm thick amorphous silicon slab as the absorber layer and either a quasi-random supercell diffractive nanostructure or a layer of self-assembled metal nanoparticles for light trapping. Both the plasmonic and diffractive structures strongly enhance the absorption in the red/near-infrared regime. At longer wavelengths, however, parasitic absorption becomes evident in the metal nanoparticles, which reduces the overall performance of the plasmonic approach. We have formulated a simple analytical model to assess the parasitic absorption and effective reflectivity of a plasmonic reflector and to demonstrate good agreement with the experimental data.
Highlights
Thin films are a promising approach for further reducing the cost of solar cells, and for increasing their efficiency via the open-circuit voltage [1]
In order to minimize the number of parameters, we conducted the comparison on a simple slab of amorphous silicon (a-Si) on a glass substrate, i.e., without the usual rear mirror implemented in plasmonic backreflectors [6,7,8]
The key question is whether the additional absorption is due to stronger scattering of the plasmonic nanoparticles, or whether it is due to undesired parasitic absorption
Summary
Thin films are a promising approach for further reducing the cost of solar cells, and for increasing their efficiency via the open-circuit voltage [1]. Thin-film technologies strongly benefit from light-trapping approaches because the required absorption length may exceed the cell thickness, especially at longer wavelengths. Metal nanoparticle scatterers and diffractive nanostructures have been widely studied for light trapping, and both concepts have been shown to enhance the absorption efficiency [2,3,4,5]. A direct comparison on identical absorber layers has not yet been carried out, . We used the quasi-random supercell design [9,10], which we demonstrated previously as one of the most effective diffractive nanostructures [11]; for the plasmonic approach, we used a layer of self-assembled silver nanoparticles, which has shown some of the strongest light trapping performance among metallic nanostructures [6,12]. The absorber material and its processing were identical in both cases, whereas the absorption measurements were performed in two laboratories in to cross-check the experimental results
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