Research Opportunities in
Laser Surface
Texturing/Crystallization of
Thin-Film Solar Cells
Y. Lawrence Yao
Columbia University
January 4th, 2011
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Outline
Overview of Photovoltaic (PV) Technology
Optical Confinement Methods
Laser Surface Texturing (LST) Applications
Simultaneous texturing/crystallization of aSi:H thin films
Research Opportunities
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Comparison of PV Absorbers
Absorber
Bulk crystalline
silicon
a-Si:H
Thin Films
Nanocrystalline
silicon
III-V (GaAs, InP)
CdTe/CdS
Chalcopyrite
compounds (I,II,VI)
CuInSe2, CuInS2,
CuGa1-xInxSe2
Dye sensitized and
organic
Pros
Stable, high efficiency
Low cost, has potential
Lowest cost can be
$0.5/watt
Stable, large-area
deposition
High efficiency and
absorption coefficient
Cons
High cost, low absorption coefficient (indirect
band gap) Cost is $2.50/watt
Unstable, low efficiency
Thicker than a-Si:H
nc-Si:H/a-Si:H (stable)
High cost of producing devices, easily cleaved
and weak, crystal imperfection, cannot use
lower-cost deposition method. For space
application, multi-junction devices. Cost of
electricity is ~1000 times of silicon cells.
High absorption
coefficient, a few micron
thick cell
High absorption
coefficient, a few micron
thick cell
Complex deposition process, efficiency is not
very high, cost is $0.98/watt
Low cost of both material
and substrate
Low efficiency, still under development
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higher cost of electricity than a-Si:H cells
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Performance Gaps in Efficiency
At ~1.4eV
highest
attainable  III-V (GaAs)
Si: 1.12eV
a-Si:H – 1.7eV
Performance gaps between best device efficiencies in the lab and attainable
efficiencies for several solar cell technologies (Kazmerski, 2005)
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a-Si:H –
largest
potential gain
in 
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Overview of Solar Cells
Thin films of more interest due to the large-area
manufacturing feasibility
a-Si:H has the lowest cost, however, it also
suffers low efficiency and instability (the StaeblerWronski Effect)
GaAs has the highest efficiency, however, it costs
1000 times to make as other thin film absorbers
III-V compound based multi-junction +
concentrator can achieve the best efficiency
(42.4%)
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Optical Confinement Methods
Anti-reflection coating (ARC)

Universally used
Chemical etching/texturing
KOH (c-Si) (D. Heslinga, 2008)
Anisotropic alkaline
and isotropic acid
 Not applicable for amorphous
and thin films

Mechanical texturing

Use mechanical dicing saws and
HF and HNO (polyc-Si) (D. Heslinga, 2008)
blades - damage
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Optical Confinement Methods
Reactive ion etching
Low throughput
Plasma (c-Si)
(D. Heslinga,
2008)
Laser surface texturing

Sharper surface features



Better absorption
More uniform absorption
Acid 2
Acid 1
Low throughput
not easy for scaling up
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LST Applications
Tribology
Biological
Other
applications
in PV
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Beyond Light Trapping (c-Si)
LST of c-Si in different
atmosphere
Below-band-gap abs.
(a) SF6, (b) N2, (c) Cl2, (d) air, (e) vacuum
all used fs laser
Sub dopant band
Band gap
Energy
Conduction band
Valence band
Carey, PhD Thesis, Harvard, 2004
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Beyond Light Trapping (c-Si)
c-Si, SF6, Crouch et al ,2004
 fs laser: recessed surface, smaller pitch (2 to 3 times of ,
interference), ns laser: protruded surfaced, larger pitch
(capillary wave generation)
 Below-band-gap absorption: ns-laser allows higher doping
concentration; annealing diffuses out dopants
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a-Si:H Thin Films
100
800nm,
130fs,
0.4J/cm2
Untreated a-Si:H
ns laser sample
fs laser sample
Absorptance (%)
80
Film
thickness
1.6 µm
60
40
20
0
0
500
1000
1500
2000
2500
Wavelength (nm)
248nm,
30ns,
0.4J/cm2
Film
thickness
1.6 µm
H. Wang, et al, 2009
• Feasible for thin films
• Below-band-gap absorption
enhancement without dopant
• nc-Si layer (1,100 nm)
• Increased defects
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nc-Si layer in a-Si:H film
2500
(111)
1500
(220)
(311)
1000
fs laser sample
500
Untreated
0
20
40
60
80
100
2
Cross-section TEM ns laser sample Cross-section TEM fs laser sample
1750
(111)
1500
1250
Intensity (arb. unit)
Intensity (a.u.)
2000
1000
(220)
750
(311)
500
ns laser sample
250
0
20
40
60
80
100
2 (degree)
• Texturing and surface crystallization in
one step (XRD, TEM, EBSD)
• ns laser induces more crystallinity
• Potential for stability improvement
• Crystalline structure to be further studied
• Cavities in ns laser to be studied
H. Wang, et al, 2009
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Research Needs
The one-step surface
texturing/crystallization of thin film

Understand laser type and process
conditions on resultant crystalline
structures

Understand how the partial
crystallization affecting stability of a-Si:H
cells

Simultaneous doping (e.g., sulfur) –how
does doping affect a-Si:H (minority
carrier mobility and lifetime)
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Research Needs
How to apply LST on III-V (e.g., GaAs)
and multijunction cells
MOCVD for crystalline GaAs thin films is very
expensive
 Low-cost MBD for amorphous GaAs is much
cheap –LST to surface texturing and
crystallization
 To address the high sensitivity to impurities
introduced during the process

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Research Needs
How to apply LST on III-V (e.g., GaAs)
and multi-junction cells (cont.)
LST can potentially be used for
texturing+crystallization+junction doping as a
one-step process for each junction
 Issues associated with complete crystallization
throughout film thickness instead of partial
crystallization
 Effects of the tunnel junctions

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Research Needs
Large-area, high-throughput LST
Effects of spatial and temporal characteristics of
laser irradiation
 Spatial: Homogeneous intensity/mask projection

(R. Delmdahl, et al,2010)
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Research Needs
 Temporal:
 longer laser pulse-width for
crystallization
 Double-peak pulse for high-
throughput crystallization
 But to address issues
associate with increased
HAZ and hydrogen
explosion
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