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Research Papers

Predictive Modeling for Glass-Side Laser Scribing of Thin Film Photovoltaic Cells

[+] Author and Article Information
Hongliang Wang

e-mail: hw2288@columbia.edu

Y. Lawrence Yao

Department of Mechanical Engineering,
Columbia University,
New York, NY 10027

Magdi N. Azer

Laser & Metrology System Lab,
GE Global Research,
Niskayuna, NY 12309

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received November 27, 2012; final manuscript received May 21, 2013; published online September 11, 2013. Assoc. Editor: Y. B. Guo.

J. Manuf. Sci. Eng 135(5), 051004 (Sep 11, 2013) (11 pages) Paper No: MANU-12-1351; doi: 10.1115/1.4024818 History: Received November 27, 2012; Revised May 21, 2013

Laser scribing of multilayer-thin-film solar cells is an important process for producing integrated serial interconnection of mini-modules, used to reduce photocurrent and resistance losses in a large-area solar cell. Quality of such scribing contributes to the overall quality and efficiency of the solar cell, and therefore predictive capabilities of the process are essential. Limited numerical work has been performed in predicting the thin film laser removal processes. In this study, a fully-coupled multilayer thermal and mechanical finite element model is developed to analyze the laser-induced spatio-temporal temperature and thermal stress responsible for SnO2:F film removal. A plasma expansion induced pressure model is also investigated to simulate the nonthermal film removal of CdTe due to the micro-explosion process. Corresponding experiments of SnO2:F films on glass substrates by 1064 nm ns laser irradiation show a similar removal process to that predicted in the simulation. Differences between the model and experimental results are discussed and future model refinements are proposed. Both simulation and experimental results from glass-side laser scribing show clean film removal with minimum thermal effects indicating minimal changes to material electrical properties.

Copyright © 2013 by ASME
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Figures

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Fig. 1

Illustration of the glass-side laser scribing model for SnO2:F and CdTe film removal. Lasers with wavelength of 1064 nm and 532 nm are adopted for SnO2:F and CdTe scribing, respectively.

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Fig. 2

Temperature distribution in the SnO2:F/glass multilayer system under laser irradiation at a fluence of 3 J/cm2. A large penetration depth of laser energy allows for a uniform temperature distribution along film thickness. Snapshot is taken at 36 ns. 10× Deformation scale for viewing clarity.

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Fig. 3

Fully coupled thermal stress analysis of SnO2:F removal by laser irradiation at a fluence of 3 J/cm2 at 38 ns. Absorption of laser energy induces local thermal expansion and thermal stress. Elements experiencing a principal stresses larger than the failure strength are deleted from calculation. A 2 μm opening has been generated accordingly. 10× Deformation scale for viewing clarity.

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Fig. 4

Principal stress and heat flux history in an element at SnO2:F film center. The element deletion occurs at 38 ns. Heat flux drops to zero due to instantaneous dependence between thermal and mechanical analyses.

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Fig. 5

The result of SnO2:F removal by 3 J/cm2 laser irradiation based on the fully coupled thermal stress analysis. An 8.3 μm opening is generated. The snapshot is taken at 200 ns. Deformation scale is 10× for viewing clarity.

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Fig. 6

Temperature and stress history in a deleted element at SnO2:F film center treated at a fluence of 1 J/cm2. The element is subjected to a compressive stress followed by a tensile stress. The element fails when the tensile failure stress is met at 1430 ns.

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Fig. 7

Comparison of depths and widths of the removed SnO2:F films obtained in simulation and experiments. The film is completely removed in depth for all the conditions used in the simulations.

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Fig. 8

(a) SEM image of the film removal by single pulse processed SnO2:F samples from glass side at a fluence of 127 J/cm2; (b) removal line profile along A measured by optical profilometry; (c) SEM image of scribe sidewall; (d) EDX line profile scanning along A

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Fig. 9

(a) 3D scanning of the removal film profile by optical profilometry and (b) SEM image of the sidewall of film removal by single pulse processed SnO2:F samples from film side at a fluence of 127 J/cm2

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Fig. 10

Dependence of removal depth and width on laser fluence. Error bars indicate standard deviation.

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Fig. 11

Temperature distribution of CdTe/SnO2:F/glass multilayer system under laser irradiation at a fluence of 0.2 J/cm2

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Fig. 12

Temporal distribution of the plasma pressure at different fluences from 0.2 J/cm2 to 0.8 J/cm2

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Fig. 13

Micro-explosion model with a pressure input at the CdTe/SnO2:F interface and the plasma dimension is 10 μm in width. A layer of cohesive elements is defined between the CdTe layer and SnO2:F layer. The CdTe film deforms due to the plasma expansion. The snapshot is taken at 10 ns. Deformation scale is 10× for viewing clarity.

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Fig. 14

S22 stress distribution of the magnified area A in Fig. 13 at the same moment. The cohesive elements have been deformed due to S22 stress. Deformation scale is 10× for viewing clarity.

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Fig. 15

S22 stress distribution of the region shown in Fig. 14 at the later stage (20 ns), showing some cohesive elements have been deleted. Deformation scale is 10× for viewing clarity.

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Fig. 16

Stress evolution of the failed elements at the top center and bottom center of the CdTe layer. Tensile stress occurs on the top, while compressive stress occurs at the bottom. The stresses drop to zero once the Coulomb-Mohr criterion is met.

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Fig. 17

Typical evolution of stresses and the quadratic nominal stress ratio defined in Eq. (7) of the removed cohesive elements. Nodal displacement of the cohesive element is also shown.

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Fig. 18

Maximum principal stress distribution at 66 ns. The film has been completely removed with an opening width of 12.5 μm. Both brittle failure and film delamination contribute to the film removal. Deformation scale is 2× for viewing clarity.

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