Ultrafast Laser Textured Silicon Solar Cells
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1123-P07-09
Ultrafast Laser Textured Silicon Solar Cells Barada K. Nayak, Vikram Iyengar and Mool C. Gupta1 Charles L. Brown Department of Electrical and Computer Engineering, University of Virginia, Charlottesville, VA 22904 ABSTRACT A novel ultrafast laser texturing method has been developed to produce arrays of nano/micro surface textures in silicon. Laser processing conditions have been optimized for achieving appropriate optical and electronic properties for photovoltaic applications. The textured silicon surfaces absorb greater than 99% of incident light over the entire solar spectrum and the material appears complete black to bare eye. Textured silicon surfaces are characterized for surface morphology and optical properties. Chemical etching and thermal annealing steps have been performed to remove laser slag and induced defects. Finally, we report the total and external quantum efficiency results on photovoltaic devices fabricated on the textured silicon wafers, which can be further improved. INTRODUCTION Efficient light energy absorption to cause carrier generation in a photoactive material is one of the most important requirements for enhancing photovoltaic energy conversion in solar cells. Therefore, high efficiency solar cell devices require good reflection control to minimize optical loss at the front surface. This is typically achieved with a combination of surface texturing (using wet chemical process) and anti-reflection coating (by deposition of Si3N4 layer using PECVD method). Usually industry uses two types of texturing procedure: (a) Anisotropic etching for mono-crystalline silicon using alkaline solutions like NaOH and KOH at 70-80 ° C with addition of isopropanol [1]. This etching process produces randomly distributed pyramids and suffers from following shortcomings: lack of pyramid size control, poor reproducibility, presence of untextured regions, need for adequate surface preparation, and requirement of accurate control of temperature and isopropanol concentration. While this method is applicable for mono-crystalline materials, it could not be applied to multi-crystalline materials due to the anisotropic nature of the etchant. (b) Isotropic texturing for multi-crystalline silicon using acidic mixture of HF and HNO3 and organic additives [2]. This method relies on the residual saw damage that causes uneven etching of the surface that result in the texture formation. However, acidic texturing has several issues for multi-crystalline wafers due to: requirement of saw damage for effective texturing; accurate surface preparation, etc. Other methods of texturing like reactive ion etching, mechanical texturing and photolithography and wet 1
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etchings has been proposed but are not cost effective. On the other hand, lasers are unique energy sources that could texture surface by selectively removing material by ablation process. Since laser ablation is an isotropic process, texturing could be achieved irrespective of the crystallographic orientation
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