The characterization and enhancement of light scattering for thin solar cells

Abstract

Photovoltaic (PV) power is one of the most promising technologies for worldwide clean and sustainable energy production and as the technology begins to enter the mainstream the requirement for efficient use of materials becomes increasingly important. However, reducing material thickness typically lowers optical absorption, leading to lower cell efficiency. One proven method for enhancing absorption in a thin device is by texturing interfaces, typically achieved in the top transparent conducting oxide (TCO) of a thin-film design. This works by scattering transmitted light and therefore increasing its effective optical path length within the absorber layer. However, introducing rough surfaces to a PV device can lead to fabrication issues and also increases surface recombination which is detrimental to the electrical characteristics of the end device. In recent years, possible alternatives to reliance on random texturing have been found through the use of optimized diffraction gratings and the plasmonic effects of metal nanoparticles. In this work, comprehensive optical characterization has been carried out on a range of samples using traditional and novel techniques. In particular, a custom built wavelength and angle resolved scattering (WARS) measurement system has been developed and used to determine key characteristics that would remain undetected by conventional measurements. The investigation of several commercial and experimental TCO films has been carried out and clear links between topography and optical characteristics have been determined. These textured surfaces have also been modelled using finite difference time domain (FDTD) simulations which have shown good agreement with measurement results. This has allowed for further investigation of the effects of TCO topography through simulation which has revealed that scattering is best enhanced by increasing the aspect ratio of the texture rather than the overall scale. Periodic arrays of silver nanoparticles incorporated into a thin-film solar cell back-reflector design have also been extensively characterized and modelled and shown to provide scattering through both diffraction and plasmonic mechanisms, leading to an increase in useful absorption by up to 140% in comparison to a planar device.

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