Spin coating of Silver Nanoparticles and Silicon quantum dots for enhanced down conversion efficiency

The commercial efficiency of a c-Si solar cell is ~18% although the thermodynamic limit is ~95%. This indicates a good scope for improvements. The major loss mechanism in a solar cell is spectral mismatch which is contributed to by non-absorption of low energy photons and thermalization of high energy photons. In this thesis, it is sought to reduce the loss that occurs through thermalization of high energy photons. For this purpose, the concept of down conversion is used. Silicon being relevant to the semiconductor industry, being abundant in nature and having been proved to exhibit down conversion in the form of spherical particles in the nm size range through space separated quantum cutting is opted. However, the down conversion efficiency of these is low owing to their indirect bandgap which leads to higher absorption within the material than enhancement in the number of photons through down conversion. In order to reduce the absorption within the material, it is sought to enhance the rate of radiative decay through the use of plasmonics exhibited by metal nanoparticles. Silver nanoparticles are used for our purpose as they exhibit resonance in the visible region of the spectrum and have the lowest absorption among different plasmonic materials. The silicon nanoparticles (quantum dots) powders are fabricated through the expanding thermal plasma chemical vapor deposition route while the silver nanoparticles fabricated through wet chemical synthesis are purchased from the market as powders. The silver nanoparticles are also deposited as metal island films in-house. Optimal deposition parameters for the deposition of quantum dots are arrived upon by depositing samples using different parameters and analyzing the results. A configuration for the down conversion layer is arrived upon based on simulations and analysis of design. The powders are then dispersed in ethanol for the purpose of spin coating. The dispersions are then spin coated on glass substrates. The quantum dot dispersion is also spin coated onto the metal island film substrate as per the chosen configuration. The silver nanoparticle samples are analyzed for plasmonic behavior, quantum dots for their absorption characteristics and the combination for enhancement in transmission through down conversion from their reflection and transmission spectra. To confirm the interaction between the silver nanoparticles and quantum dots, the enhancement in the photoluminescence spectra is checked. From the results, it is observed that there is agglomeration in the spin coated silver nanoparticles resulting in a loss of plasmonic behavior. The quantum dots are also agglomerated due to which an enhancement in the transmission spectra was not observed. However, the interaction between the quantum dots and the silver nanoparticles could be observed through the enhanced photoluminescence of the quantum dots. The enhancement is found to vary from 9 folds to about 50 folds which are high compared to similar results in the literature. Keeping in mind that this down conversion layer has not yet been optimized, even higher enhancements may be possible. This indicates the potential for the combination of silicon quantum dots and silver nanoparticles for application in solar cell down conversion layers.

Subject

solar cell
down conversion
silicon quantum dots
plasmonics
silver nanoparticles

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Embargo date

2013-12-20

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Student theses

Document type

master thesis

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