SNM: Scalable Surface Corrugation of Silicon Surfaces for Enhanced Light Trapping in Solar Cells

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Solar photovoltaics market is growing rapidly on a global scale. Notwithstanding the rapid growth, the cost of installed photovoltaic solar energy system must be further reduced for a wide distribution of solar power usage in the marketplace. The difficulty has been to reduce, by a significant degree, both the material cost and the "soft cost", such as transportation and installation of solar photovoltaic modules. Addressing this market challenge, this Scalable NanoManufacturing (SNM) award will provide manufacturing solutions to reduce the material cost by first making use of thin, flexible crystalline silicon substrates, as crystalline silicon accounts for as much as 30-40 percent of a typical solar module cost. The use of thin substrates would also reduce the soft cost by enabling cells to be supported on a lightweight flexible platform. Lightweight translates to reduced transportation and installation costs. While the cost benefits are clear, maintaining the same photovoltaic efficiency from thin silicon solar cells requires significantly improved light trapping and absorption within the thin layer. The technical solution to be explored is to use periodic, nanoscale surface features with reduced symmetry to effectively couple the sunlight into the underlying silicon substrate. A manufacturable, cost-effective, high-throughput process will be developed to fabricate such nanostructures on thin silicon films. This new process will provide uniformity over a wafer-scale, bridging six orders of magnitude in length scale. The project will also help the public appreciate alternative energy sources by developing educational tools using web-based immersive interactive visualization.Fabricating nanoscale features uniformly over a wafer scale poses significant engineering challenges. Various lithographic techniques exist today to define submicron features. However, the success of any one technique will depend on its large-scale performance and manufacturing cost. For instance, the conventional deep-UV optical steppers are highly suitable for sub-micron light-trapping features in solar cells but are overly expensive for wafer-scale applications. The research team will develop processes to scale up phase-mask-based interference lithography, where a coherent beam is projected on a pre-patterned grating mask, and the diffracted plane waves from the mask interfere with each other to make periodic patterns. The scale-up lithography processes will also involve wet etching steps to fabricate efficient light-trapping structures on thin silicon solar cells. A multiscale, multiphase transport and reaction model of the wet etch process will be developed to solve the scale-up engineering challenge. The photovoltaic characteristics of the large-area solar cells fabricated by the developed processes will be investigated on a device level.