Nanoimprint lithography for light trapping applications in solar cells
In this thesis, substrate conformal imprint lithography (SCIL) process is adopted to fabricate efficient light trapping structures for thin-film solar cells. The SCIL process can achieve economical and large area patterns with high fidelity sub-micron resolution over non-planar surfaces without involving intense curing processes. We study and compare both plasmonic particle arrays and dielectric diffraction grating arrays placed on the rear of thin-film cells for light trapping, and we enhance the effect further by integrating a detached silver (Ag) mirror that creates a Fabry-Perot resonance effect. The effect of the Ag nanoparticle size distribution on the performance of plasmonic polycrystalline silicon (Si) thin-film solar cells is studied. The short-circuit current enhancement for cells with a back reflector is 34% and 30% with a multi-disperse array and the mono{u00AD}dispersed array respectively, compared to 13% enhancement due to the reflector alone. The better performance of multi-disperse Ag nanoparticle arrays is attributed to a broader scattering cross-section of the array owing to a broad particle size distribution and a higher nanoparticle coverage. We present an experimental demonstration of photocurrent enhancement in thin-film recrystallised silicon solar cells using titanium dioxide (Ti0{u2082}) pillar arrays fabricated on the rear of the cells using nanoimprint lithography. A short-circuit current enhancement of 19% is measured experimentally, and excellent agreement with numerical simulations is obtained. We show numerically that by replacing the Ag capping present on the cells with a detached rear Ag back reflector the enhancement could reach 37%. We numerically investigate the light trapping properties of two-dimensional diffraction gratings formed from Ag disks or Ti0{u2082} pillars, placed on the rear of Si thin-film solar cells. By optimizing the grating geometry and the position of a planar reflector, we predict short circuit current enhancements of 45% and 67% respectively for the Ti0{u2082} and Ag nanoparticles. Furthermore, we show that Fabry-Perot resonance effect between the grating and the detached planar mirror can significantly enhance, or suppress, the light trapping performance. In order for the Fabry-Perot resonance effect to be experimentally feasible, we numerically simulate a planar Ag mirror sitting on top of a sol-gel planarized silicon dioxide (Si0{u2082}} layer, where the enhancement could reach 53%, and a conformal Ag mirror sitting on sputtered Si0{u2082} layer could give similar enhancement value. From absorptance measurements on thin-films, we predict that the short-circuit current density (Jsc) enhancement will be around 1.8 mA/cm{u00B2} less than the simulated value, which is due to the morphology differences between the experimental structures and the simulated structures. Nanoimprint lithography with various deposition techniques has also been adopted to fabricate blazed line and cone gratings as well as ring type gratings. In this thesis, the best performing structure is mono-disperse plasmonic arrays with integrated detached Ag mirror. From simulation, the structure can enhance the Jsc by over 50% from the planar case and the structure can be fabricated cheaply and easily. From the simulations and experimental investigations, we conclude that it would be feasible to adopt nanoimprint lithography for light trapping applications in solar cells.
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