Numerical Modeling of Laser-silicon Interactions during Formation of Selective Emitters

Open Access
Author:
Blecher, Jared Jacob
Graduate Program:
Materials Science and Engineering
Degree:
Master of Engineering
Document Type:
Master Thesis
Date of Defense:
April 20, 2012
Committee Members:
  • Todd Palmer, Thesis Advisor
  • Dr Tarasankar Deb Roy, Thesis Advisor
  • Suzanne E Mohney, Thesis Advisor
Keywords:
  • laser doping
  • solar cell
  • photovoltaics
  • laser diagnostic
  • beam characterization
Abstract:
The formation of highly doped selective emitters improves the efficiency of silicon-based solar cells by lowering the contact resistance at the interface between the silicon front surface and the metal contacts. The use of laser doping in which a laser locally melts the silicon substrate and rapidly incorporates the dopant into the silicon can provide faster processing speeds, lower energy inputs, and higher diffusion rates than other methods of forming selective emitters. A mathematical model, which solves for the temperature, velocity, and dopant concentration fields, is used to investigate the characteristics of the molten region during laser doping with a continuous-wave green laser (λ = 532 nm) moving over the sample at speeds up to 8 m/s and laser output powers ranging from 5 to 20 W. The calculated liquid pool widths and dopant distribution profiles are validated with published experimental results. The effects of fluid flow and diffusive movement on the final shape of the dopant distribution profiles are discussed, and figures are provided to illustrate each effect. Process maps are produced to illustrate how the size of the molten pool dimensions, the average dopant concentration, the relative shape of the dopant distribution, and silicon sheet resistance are impacted by the combination of power and scan speed. The molten pool depth and width are important in determining the diffusion profile and the front-side contact width during plating. High power solid state fiber and disk lasers combine faster processing speeds, deeper weld penetrations, and lower levels of work piece distortion. Both transmissive optics and reflective optics systems are commonly used. When using transmissive optics at high laser powers and prolonged periods of operation, changes in the focal length and beam diameter have been observed, and these changes adversely affect consistency of the processed materials. In this study, the properties of beams delivered using both transmissive and reflective optics systems from the exit of the process fiber through the final focusing optics have been characterized using commercial diagnostic tools. In the transmissive optics system, changes of nearly 8 mm in focal length have been measured with a 500 mm focal optic at 12 kW output power over several minutes of continuous operation. At powers above 4 kW, damage to the anti-reflective coating on a transmissive collimator resulted in a doubling in the beam diameter at the original focal position when using a 200 mm focal optic and a quadrupling of the beam diameter with a 500 mm focal optic. On the other hand, the performance of the reflective optics was not impacted by either increases in power or time at powers up to 12 kW during prolonged laser operation.