Surface Texturing by rapid scanning of

pulsed laser beam

Di Liu, Matthew Quigley

Advisor: Dr. David Hwang

Annual Student Research Poster

Symposium, May 10, 2013

Department of Mechanical

Engineering

College of Engineering and

Applied Sciences (CEAS)

Surface texturing in micro/nanoscale plays an important role in many application fields

including enhanced light trapping in solar cell, surface anti-icing by super hydrophobicity in

wind turbine and airplane wings, and enhanced convective and radiative heat transfer from

surface. In this study, we demonstrate high flexibility and control precision offered by the

pulsed laser scanning method. Compared with conventional laser-based approach,

nanosecond laser scanning method we tested corresponds to a highly cost-effective and

uniform means for the texturing of arbitrary sample area and shape. Relevant texturing

mechanisms are explained based on advanced characterization by scanning electron

microscope and Raman spectroscopy.

Acknowledgement

Summary • Rapid laser scanning enabled superior textured surfaces for solar & military applications. • Detailed texturing mechanisms were investigated in relation to target device performance. • For solar cell, light trapping was greatly enhanced with optimal crystalline structures. • For military applications, desired stealth and thermo-fluidic functions were enabled.

*. The research is supported by Yuco optics corp. (Bohemia, NY) through SPIR program, and Advanced Energy Research and Technology Center (Stony Brook, NY). *. SEM characterization was performed at the Center for Functional Nanomaterials, Brookhaven National Laboratory supported by US Department of Energy.

New Approach : Rapid scanning of pulsed laser beam

Conventional surface texturing techniques

By D. Neuhaus et al. (2007) By D. Yong Wu et al. (2007) By L. Dobrzaǹski et al. (2008) By SiOnyx

Ti c-Si c-Si c-Si

Chemical Wet Etching • Limited flexibility as

crystalline plane dictates

Laser Ablative Texturing • Loose structural: issues on carrier transport and

limit in current extraction

Ultrafast Laser Texturing • High laser cost, lack of

arbitrary scale uniformity

Mechanical Roughing • Quality limited due

to friction with abrasive particles

Experimental setup

Effect of laser parameters : 3 representative regimes

Ablation dominant • Excellent light trapping • Structural issues

Melting dominant • Excellent light trapping • Favorable structures

Mild melting • Insufficient light trapping • Low textures

Spectral reflectance data show Performance of Black Si depending on laser parameters

Silicon surface texturing for enhanced light trapping

Si surface morphology results comparison *Different optical properties are achieved by different surface texture structures

*Brown Silicon is formed and dominated by ablative phase change *Shorter wavelength gives smaller scale of spikes and shallower affected depth

Brown Si Black Si

λ=355nm

Super black metal surface texturing by laser scanning

Black Cu SEM image

http://en.wikipedia.org/wiki/F-117_Nighthawk http://thegundeck.blogspot.com/2011/01/how-much-for-pentagon.html

Black Silicon for advanced Si solar cells

• Laser-based texturing • High flexibility and control precision

Light trapping enhancement

Stealth function : minimal reflection of UV, visible and IR illumination

Enforcing Pool boiling for improved

heat transfer by phase change

http://www.81.net/hangkongdongtai/0426230192013_3.html

http://isnps.unm.edu/research/facilities/

IR image

Cu

Blackbody-like Thermal Emission

Key Advantages • Size independent uniformity • Precision texture control： active control of unit features dictated by local melting and thermo-capillary effect • Simultaneous annealing for improved carrier diffusion • Cost-effective laser solution

Photograph image

SEM image

• Ablation dominant regime is superior in light trapping but with low crystalline quality • Melting dominant regime show good light trapping and excellent crystalline quality

Improved heat transfer and control of phase change (cooling, anti-freezing)

Transient behavior during laser process

Necessary ablation

for spikes formation

Secondary annealing for efficient current extraction

On Laser focus

Off Laser focus

Laser focal position effects on secondary annealing Extended

laser beam for annealing

Cross section Raman spectroscopy

Anti icing texturing

Non-textured Textured

http://www.gao.pitt.edu/antiicing.html

522cm-1

(Single crystal Si peak)

Black Si

λ=532nm Raman shift /cm-1

Efficient Current

Extraction

*Good Crystallinity

*Minimal stress

Focused on military and defense applications

Black Cu

Colorized Steel Reflect specific wavelength