Research Papers

Removal Mechanism and Defect Characterization for Glass-Side Laser Scribing of CdTe/CdS Multilayer in Solar Cells

[+] Author and Article Information
Hongliang Wang

Department of Mechanical Engineering,
Columbia University,
New York, NY 10027
e-mail: hw2288@columbia.edu

Y. Lawrence Yao

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

Hongqiang Chen

Laser and 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 August 1, 2014; final manuscript received June 21, 2015; published online September 9, 2015. Assoc. Editor: Robert Gao.

J. Manuf. Sci. Eng 137(6), 061006 (Sep 09, 2015) (11 pages) Paper No: MANU-14-1417; doi: 10.1115/1.4030935 History: Received August 01, 2014

Laser scribing is an important manufacturing process used to reduce photocurrent and resistance losses and increase solar cell efficiency through the formation of serial interconnections in large-area solar cells. High-quality scribing is crucial since the main impediment to large-scale adoption of solar power is its high-production cost (price-per-watt) compared to competing energy sources such as wind and fossil fuels. In recent years, the use of glass-side laser scribing processes has led to increased scribe quality and solar cell efficiencies; however, defects introduced during the process such as thermal effect, microcracks, film delamination, and removal uncleanliness keep the modules from reaching their theoretical efficiencies. Moreover, limited numerical work has been performed in predicting thin-film laser removal processes. In this study, a nanosecond (ns) laser with a wavelength at 532 nm is employed for pattern 2 (P2) scribing on CdTe (cadmium telluride) based thin-film solar cells. The film removal mechanism and defects caused by laser-induced micro-explosion process are studied. The relationship between those defects, removal geometry, laser fluences, and scribing speeds are also investigated. Thermal and mechanical numerical models are developed to analyze the laser-induced spatiotemporal temperature and pressure responsible for film removal. The simulation can well-predict the film removal geometries, transparent conducting oxide (TCO) layer thermal damage, generation of microcracks, film delamination, and residual materials. The characterization of removal qualities will enable the process optimization and design required to enhance solar module efficiency.

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Fig. 1

Schematic of film removal of P2 laser scribing on CdTe-based solar cells under micro-explosion mechanism

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Fig. 2

(a) SEM image of film removal by a pulse irradiated at a fluence of 3 J/cm2; (b) magnified SEM image at square D; and (c) optical profilometry measurement along A

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Fig. 3

EDX line profile scanning along A in Fig. 2(a), showing a clean removal at the center, and small amount of remaining material is CdS and no interdiffusion occurs at the interface of TCO/CdS

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Fig. 4

Cross-sectional TEM image of as-received CdTe/CdS/TCO/glass samples

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Fig. 5

Cross-sectional TEM image of scribe boundary at square B shown in Fig. 2(a), showing microcrack is formed near the top surface at the scribe sidewalls

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Fig. 6

Cross-sectional TEM images of (a) near scribe boundary B and (b) scribe center region C in Fig. 2(a)

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Fig. 7

SEM image of film removal by a pulse irradiated at a fluence of 1 J/cm2, showing much more CdS remaining after processing compared to that processed at a fluence of 3 J/cm2

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Fig. 8

Cross-sectional TEM images of (a) scribe boundary at square B′ shown in Fig. 7 and (b) magnified image at the delamination tip, showing dislocations formed at both between the two layers and grains which may introduce further crack initiation

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Fig. 9

(a) TEM images taken at region C′ in Fig. 7 and (b) magnified TEM image at the interface between CdS and SnO2:F layers, showing nanobubbles formed due to the oxidation of sulfur during the laser processing

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Fig. 10

Dependence of scribe area and remaining CdS on laser irradiation conditions, error bars represent standard deviation

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Fig. 11

Simulation result of temperature distribution of CdTe/CdS/SnO2:F/glass multilayer thermal model at a fluence of 1 J/cm2

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Fig. 12

Simulation results of dependence of SnO2:F (TCO) temperature on fluence, showing the TCO layer will be damaged when the fluence reaches 5.5 J/cm2

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Fig. 13

Complete film removal with a fluence of 1 J/cm2 at 97.9 ns, the scribe radius is 28.7 μm, and delamination is 2.8 μm through the interface

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Fig. 14

Complete film removal with a fluence of 3 J/cm2 at 76.8 ns, the scribe radius is 34.8 μm, and microcrack with a length of 1.4 μm occurs near the top surface

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Fig. 15

Comparison of experimental and simulation results on scribe width, error bars represent standard deviation

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Fig. 16

Comparison of experimental and simulation results on residual CdS, error bars represent standard deviation

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Fig. 17

Simulation results of microcrack and delamination lengths after laser scribing

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Fig. 18

SEM images of (a) line scribing at a fluence of 3 J/cm2 and a speed of 2 mm/s and (b) magnified image at scribe boundary, showing macrocracks are removed due to the pulse overlapping

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Fig. 19

Dependence of sheet resistances on fluence. The high resistance at low-energy range is due to the residual CdS, and high resistance at high-energy range is caused by the TCO damage. Dashed line represents the sheet resistance of TCO material only and error bars represent standard deviations.




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