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Wavelength-scale dielectric diffraction gratings for light trapping in ingaas/gaas quantum well solar cells




Turner, Samuel

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A light trapping structure can improve the efficiency of a photovoltaic device by increasing the path length for weakly absorbed light, and hence reduce the cost of solar electricity. With a broad scope for their application to photovoltaic devices of varied design and composition, the study of light trapping structures holds promise for a photovoltaic industry undergoing rapid growth. Wavelength-scale dielectric diffraction gratings have recently received significant attention as light trapping structures whose theoretical upper limits for path length enhancement exceed the Lambertian light trapping limit conventionally applied to wafer-based devices. Metallic diffraction gratings suffer from losses that place constraints on their structural parameters, limiting their light trapping efficiency. This work investigates the light trapping properties of wavelength-scale TiO2 diffraction gratings, exploring a range of designs and drawing direct comparisons with optimum silver diffraction gratings and the Lambertian light trapping limit. InGaAs/GaAs quantum well solar cells (QWSC) form the material system which facilitates this study, wherein the wavelengths longer than the bandgap of GaAs (873 nm), that are weakly absorbed by the quantum wells (QWs), are studied. Ultimately however, the theoretical and experimental investigations of this study aim to provide insights that can be considered more generally across a broad range of devices and material systems. Finite-difference time-domain (FDTD) simulations are used to study wavelength-scale TiO2 diffraction gratings for light trapping in InGaAs/GaAs QWSCs. The designs investigated include symmetric rectangular strip, symmetric square pillar and asymmetric skewed pyramid diffraction gratings. The ratio of this enhancement to the maximum achievable enhancement - that is, without transmission losses - is 33 %, 77 % and 75 % for the optimum TiO2 rectangular strip, square pillar and skewed pyramid diffraction gratings, respectively. The optimum TiO2 square pillar and skewed pyramid diffraction gratings perform comparably with relative enhancements greater than that of the optimum TiO2 rectangular strip diffraction grating and nearest that of the Lambertian light trapping limit (97 %). All three optimum designs outperform an optimum silver square pillar diffraction grating whose relative enhancement is 28 %. Experimentally, an InGaAs/GaAs QWSC structure is grown through metal organic chemical vapour deposition on a semi-insulating (SI) GaAs substrate. TiO2 and silver square pillar diffraction gratings are fabricated on the rear of InGaAs/GaAs QWSCs using a single layer, positive resist lithographic lift-off process with the techniques of electron beam lithography and electron beam evaporation. The structural parameters of the silver square pillar diffraction grating are optimal (50 nm in height), whereas those of the TiO2 square pillar diffraction grating (83 +/- 5 nm in height) represent a compromise between performance and what is practicably achievable with respect to the optimum height of approximately 500 nm that is calculated through FDTD simulations. The TiO2 and silver square pillar diffraction gratings generate enhancements in short-circuit current density of 0.2 mAcm-2 and 0.6 mAcm-2, respectively. For the TiO2 square pillar diffraction grating to provide enhancement that surpasses not only that of the silver square pillar diffraction grating but approaches the theoretical ideal defined through FDTD simulations new strategies for fabrication are required and discussed.






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