High Performance Inverted Pyramidal Texture for Silicon Photovoltaics

An inverted pyramidal grating texture is known to reduce both surface reflection and to promote light trapping in crystalline silicon (c-Si) solar cells. However, these textures are not used in commercial solar cells mainly because of high fabrication costs, limited scalability of conventional fabrication techniques to thin wafers, and insufficient knowledge of the optimum grating parameters for silicon of different thicknesses. These issues become even more important as industry makes a transition to thinner Si wafers to reduce device cost. The objective of this this thesis is to address all of these issues. Firstly, a new process for inverted pyramidal texturing of c-Si has been developed that is compatible with thin silicon wafers and foils. Secondly, a theoretical study of the optimum inverted pyramidal grating parameters has been done for a wide range of Si wafer thicknesses. Finally, the optical performance of the optimal textures has been experimentally verified. The laser assisted texturing method produces high quality inverted pyramids in a non-cleanroom environment and is potentially scalable to mass production. Because of the contactless fabrication, the method can be used to texture fragile, ultra-thin Si foils. This approach also offers precise control of the patterned areas, which can exclude areas for front surface contacts on PV devices. The wave-optical study of the size dependent performance of inverted pyramidal textures identifies a 1000 nm period as being universally optimal for silicon thicknesses ranging from 2-400 microns. As a point of comparison, inverted pyramidal textures were also fabricated by electron beam lithography. The measured reflectances show that textures with micron scale periodicity outperform a submicron periodic texture, in agreement with trends predicted by the wave-optical simulations. Finally, a novel phenomenon of internal structuring within an optically transparent thin film is described that was discovered in the course of optimizing the laser processing parameters for making apertures in the hard mask layer for PV surface texturization. This phenomenon shows promise for varied applications such as marking of surfaces, and the production of buried channels within a thin film for lab on chip architectures.