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Research Papers

Femtosecond Laser-Induced Simultaneous Surface Texturing and Crystallization of a-Si:H Thin Film: Absorption and Crystallinity

[+] Author and Article Information
Hongliang Wang, Panjawat Kongsuwan, Gen Satoh, Y. Lawrence Yao

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

J. Manuf. Sci. Eng 134(3), 031006 (May 07, 2012) (10 pages) doi:10.1115/1.4006548 History: Received November 04, 2010; Revised March 27, 2012; Published May 04, 2012; Online May 07, 2012

Hydrogenated amorphous silicon (a-Si:H) thin films have been considered for use in solar cell applications because of their significantly reduced cost. Their overall efficiency and stability, however, are less than that of their bulk crystalline counterparts. Limited work has been performed on solving the efficiency and stability issues of a-Si:H simultaneously. In this study, both surface texturing and crystallization on a-Si:H thin film are achieved through one-step femtosecond laser processing. The nanoscale conical and pillar-shaped spikes formed on the surface of a-Si:H films by femtosecond laser irradiation in both air and water are presented and enhanced light absorption is observed due to light trapping based on surface geometry changes, while the formation of a mixture of hydrogenated nanocrystalline silicon (nc-Si:H) and a-Si:H after crystallization suggests that the overall material stability can potentially be increased. The relationship among crystallinity, fluence, and scan speed is also discussed. Furthermore, a comparison of absorptance spectra for various surface morphologies is developed. Finally, the absorptance measurement across the solar spectrum shows that the combination of surface texturing and crystallization induced by femtosecond laser processing is very promising for a-Si:H thin film solar cell applications.

Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) SEM image and (b) AFM image of surface of a-Si:H film laser irradiated in air (0.4 J/cm2 , 1 mm/s), showing texturing with conical spikes on the surface

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Figure 2

Comparison of textured spikes made by (a) scanning (0.4 J/cm2 , 1 mm/s) and (b) stationary (0.4 J/cm2 , 100 pulses) pulsing in air, showing nanoparticles distributed on the whole spike after laser scanning

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Figure 3

Comparison of textured spikes made by (a) scanning (1.2 J/cm2 , 2 mm/s) and (b) stationary (1.2 J/cm2 , 50 pulses) pulsing in water, showing similar pillar-shaped spikes textured on the both sample surfaces

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Figure 4

XPS spectra of a-Si:H sample surfaces of as-received, laser irradiated in air (0.4 J/cm2 , 1 mm/s) and in water (1.2 J/cm2 , 2 mm/s). The spectra have been shifted up for clarity. Note increase in oxygen and decrease in carbon after laser processing.

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Figure 5

Comparison of absorptance spectra of as-received and laser treated a-Si:H films at fluences of 0.4 J/cm2 (scan speed 1 mm/s) and 1.2 J/cm2 (scan speed 2 mm/s) in air and water by spectrophotometry, showing increase in absorptance over entire spectrum for both treated samples

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Figure 6

Schematic of cross-sectional absorption simulation model for textured a-Si:H film on glass substrate, total thickness is 1.6 μm, real and imaginary parts of refractive index of a-Si:H film are n and k, s and n0 are the refractive index of the substrate and air, respectively. Magnified image in the right shows the reference and local coordinate systems (X¯,Y¯,Z¯) and (X, Y, Z), where S denotes the spike surface that reflects the ray passing through point P0¯

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Figure 7

Comparison of simulated and measured absorptance spectra of the laser treated sample in air with the textured surface of conical spikes, with height of 900 nm and top angle of 90 deg

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Figure 8

Comparison of simulated and measured absorptance spectra of the laser treated samples in water with textured surface of pillar-shaped spikes with height of 400 nm height and a diameter of 100 nm

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Figure 9

Comparison of simulated absorptance spectra for the textured surfaces with two different densities of conical spikes, showing the surface with higher density spikes has higher absorptance

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Figure 10

Comparison of simulated absorptance spectra for textured surfaces with different heights of conical spikes, showing similar absorptance over the entire wavelength range

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Figure 11

X-ray diffraction spectra of as-received a-Si:H film, laser treated films at 0.4 J/cm2 , 1 mm/s and 1.2 J/cm2 , 2 mm/s in air and water, respectively. All the spectra show an “amorphous peak” at around 2θ = 25 deg and no signs of crystallinity for the untreated sample and that treated in water. Sample treated in air shows three different peaks for (111), (220), and (311) orientation of silicon. The spectra have been shifted for clarity.

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Figure 12

X-ray diffraction spectra of a-Si:H films laser treated at fluences of 0.5 J/cm2 and 1.2 J/cm2 , and scan speeds of 2 mm/s and 25 mm/s in water. An “amorphous peak” around 2θ = 25 deg is observed for all samples. Sample processed at 0.5 J/cm2 and 25 mm/s shows existence of two peaks for (111) and (220) orientation of silicon, showing the effect of different laser parameters on crystallinity

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Figure 13

X-ray diffraction spectrum of laser treated a-Si:H film at different fluences from 0.5 J/cm2 to 0.8 J/cm2 with the same scan speed of 25 mm/s in water. An “amorphous peak” around 2θ = 25 deg is observed for all samples. Existence of two overall different peaks for (111) and (220) orientation of silicon is found except for the condition at 0.8 J/cm2 , showing that the crystallinity decreases with increasing of fluence.

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Figure 14

(a) SEM image and (b) AFM image of surface of a-Si:H film laser irradiated in water (0.5 J/cm2 , 25 mm/s), showing texturing with randomly oriented spikes on the surface

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Figure 15

Comparison of absorptance spectra measured by spectrophotometry of as-received and laser irradiated a-Si:H films with and without crystallinity at different fluences and scan speeds (0.5 J/cm2 at 25 mm/s, 1.2 J/cm2 at 2 mm/s) in water, showing the effect of crystallinity on absorption

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Figure 16

Comparison of absorptance spectra measured by spectrophotometry of as-received and laser irradiated a-Si:H film in air (0.4 J/cm2 , 1 mm/s) and water (0.5 J/cm2 , 25 mm/s), showing the effect of different factors, such as surface geometry, crystallinity and processing medium on absorption

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