Data and Time May 22 , 2012, 3:00-4:15 PM
Location Sanford Fleming Building (SF), Room B560
Host Alex Wong

Ultrafast Laser Direct Mask Writing for high Efficiency Crystalline Solar Cells

Kitty Kumar

The Department of Material Science and Engineering


ALow-cost high-efficiency crystalline silicon (c-Si) based photovoltaics (PVs) oblige the use of thinner wafers for cost reduction. Commensurate optical loss due to inadequate absorption of longer wavelength light requires the integration of an effective light-trapping scheme and reduction in surface reflection to compensate this loss. To minimize optical loss in commercial c-Si solar cells with wafer thickness on the order of 180-200µm, c-Si wafer surfaces are textured with random pyramids having size distribution of a few to ~10m. Further, a grating texture of 9µm inverted-pyramids with 10µm pitch has been used to demonstrate the highest efficiency 400m thick c-Si photovoltaic devices. The aforementioned textures however are not appropriate for thin wafers due to the high c-Si consumption during etching and their lack of effective light-trapping. Optical modeling studies have shown grating textures with periodicity and feature size comparable with the wavelength will benefit thin Si wafer solar cells by diffracting light at larger angles thus promoting lateral propagation of light, reducing surface reflection and leading to material savings during etching.

An innovative high resolution hard-mask laser writing technique to facilitate selective etching of c-Si into inverted-pyramidal (I.P.) texture at wavelength scale will be presented in this talk. The method enables engineered positional placement of I.P. which is necessary for the attainment of optimal high-efficiency texture design and is scalable for large area micro-fabrication of thin c-Si PVs. The technique permits control over the positional placement and size of the I.P. in the texture during processing, which is necessary for the attainment of optimal high-efficiency texture design. The specular reflectance (various incident angles; i) and total reflectance (i = 90º) spectra measured on fabricated texture with different I.P size at our highest pitch density revealed markedly enhanced optical performance compared with the state of art grid-less PERL cell at all incident angles. Wave-optical simulations suggest the best fabricated front-texture will lead to a moderate 2% relative increase in the photovoltaic conversion efficiency (PCE) relative to the PERL-cell texture on a 400µm thick wafer, while anticipating much higher gains of ~16.8% relative increase in PCE in thinner wafers of 20 µm thickness. With the prospect of large scale production of thin silicon foils using advanced wafering techniques within sight, the integration of wavelength scale inverted pyramid textures for high-efficiency ultra-thin silicon photovoltacis promises to be feasible.



Kitty Kumar is pursuing a PhD degree in the department of Material Science and Engineering. She is co-supervised by Profs. Jun Nogami, Nazir Kherani and Peter Herman.