Development of hybrid photonic and plasmonic light management interfaces for thin film semiconductor devices

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2015
Nasser, Hisham
Hydrogenated amorphous silicon (a-Si:H) is a versatile and an attractive material of photovoltaics whose manufacturing has reached an immense and advanced level of maturity. Owing to its moderate conversion efficiency compared to its crystalline counterparts, it should target either power plants with miniature installation cost or applications with added value like building-integration. Since this photovoltaic technology is based on very thin films of a weakly light-absorbing material, light-management is, and always has been, a vital and indispensable aspect of the a-Si:H thin film solar cells technology. The highest conversion efficiencies of a-Si:H solar cells produced today basically involve light trapping approaches which employ randomly textured transparent substrate and a highly reflective rear contact. Obtaining new alternative approaches for light management in a-Si:H thin film solar cells is a great challenge. In this thesis, I propose to use plasmonic metal nanoparticles to enhance the light absorption in a-Si:H thin film solar cells. In the first part of this thesis, I demonstrate fabrication of plasmonically active interface consisting of silver nanoparticles (AgNPs) embedded in aluminum doped zinc oxide (Al:ZnO) that has the potential to be used at the front surface and at the back reflector of a thin film solar cell to enhance light-trapping and increase conversion efficiency. Then several single and double plasmonically active interfaces embedded in dielectric spacer thin films of different dielectric constant were successfully constructed and integrated to the front and at the rear device-quality a-Si:H thin films to investigate their light management potentials in terms of enhanced spectral dependence of photocurrent driven by a constant bias in the a-Si:H thin films use as indicators for an effective plasmonic effect. Single plasmonic interfaces exhibit plasmonic resonances whose frequency is redshifted with increasing particle size and with the thickness of a dielectric spacer layer. Double plasmonic interfaces consisting of two different particle sizes exhibit resonances consisting of double minima in the transmittance spectra. I investigate the enhancement of photocurrent in a-Si:H as a function of nanoparticle size and spacer layer thickness placed to the front and at the rear of the a-Si:H absorber. By comparing the photocurrent enhancement due to plasmonic interfaces integrated to the front and at the rear of a-Si:H thin films, we were able to judge that the true position of plasmonic AgNPs is at the rear of a-Si:H and with an optimum spacer layer of at most 30 nm thick film. A new advanced light trapping concept is constructed for the first time. In this concept, I merge the scattering potentials of Al:ZnO surface texturing and AgNP plasmonics in a single light trapping interface. The results show that surface texturing by wet etching of Al:ZnO combined with AgNPs produces the highest optical extinction of a-Si:H thin film at the band edge and the measured photocurrent shows a clear increase not only at AgNPs resonance wavelength but over the entire wavelength range. In parallel to the study related to the integration of plasmonic structures in a-SiH: thin films; the effect of SiO2 spacer layer thickness on the optical response of AgNPs of potential integration in crystalline silicon solar cells has been investigated. By carefully studying the thickness of the spacer layer, I have identified the critical thickness that defines the border between plasmonic and photonic regimes.

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Citation Formats
H. Nasser, “Development of hybrid photonic and plasmonic light management interfaces for thin film semiconductor devices,” Ph.D. - Doctoral Program, Middle East Technical University, 2015.