Research Opportunities in Laser Surface Texturing/Crystallization of Thin-Film Solar Cells

Research Opportunities in

Laser Surface Texturing/Crystallization of

Thin-Film Solar Cells

Y. Lawrence YaoColumbia University

January 4th, 2011

Research Opportunities in Energy Manufacturing 2011 CMMI Grantees Conference1

Outline

Overview of Photovoltaic (PV) Technology

Optical Confinement Methods

Laser Surface Texturing (LST) Applications

Simultaneous texturing/crystallization of a-Si:H thin films

Research Opportunities


Comparison of PV Absorbers

Absorber Pros ConsBulk crystalline silicon

Stable, high efficiency High cost, low absorption coefficient (indirect band gap) Cost is $2.50/watt

a-Si:H Low cost, has potentialLowest cost can be $0.5/watt

Unstable, low efficiency

Nanocrystalline silicon

Stable, large-area deposition

Thicker than a-Si:H nc-Si:H/a-Si:H (stable)

III-V (GaAs, InP) High efficiency and absorption coefficient

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.

CdTe/CdS High absorption coefficient, a few micron thick cell

Complex deposition process, efficiency is not very high, cost is $0.98/watt

Chalcopyrite compounds (I,II,VI)CuInSe2, CuInS2,

CuGa1-xInxSe2

High absorption coefficient, a few micron thick cell

higher cost of electricity than a-Si:H cells

Dye sensitized and organic

Low cost of both material and substrate

Low efficiency, still under development

Th

in F

ilms


Performance gaps between best device efficiencies in the lab and attainable efficiencies for several solar cell technologies (Kazmerski, 2005)


Performance Gaps in Efficiency

At ~1.4eV highest attainable -III-V (GaAs)

Si: 1.12eV

a-Si:H – 1.7eV

a-Si:H – largest potential gain in

Overview of Solar CellsThin 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 Staebler-Wronski 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%)



Anti-reflection coating (ARC) Universally used

Chemical etching/texturing Anisotropic alkaline

and isotropic acid Not applicable for amorphous

and thin films

Mechanical texturing Use mechanical dicing saws

and blades - damage

KOH (c-Si) (D. Heslinga, 2008)

HF and HNO3 (polyc-Si) (D. Heslinga, 2008)



Reactive ion etchingLow throughput

Laser surface texturing Sharper surface features

Better absorption More uniform absorption

Low throughput not easy for scaling up

Plasma (c-Si) (D. Heslinga, 2008)

Acid 1

Acid 2


LST Applications

Tribology BiologicalOther applications in PV


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

Carey, PhD Thesis, Harvard, 2004


En

erg

y

Valence band

Conduction band

Sub dopant band

Ban

d

gap

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


800nm, 130fs, 0.4J/cm2

Film thickness 1.6 µm

• Feasible for thin films• Below-band-gap absorption

enhancement without dopant• nc-Si layer (1,100 nm)• Increased defects

H. Wang, et al, 2009

a-Si:H Thin Films

248nm, 30ns, 0.4J/cm2

Film thickness 1.6 µm

0 500 1000 1500 2000 25000

20

40

60

80

100

Abs

orpt

ance

(%

)

Wavelength (nm)

Untreated a-Si:H ns laser sample fs laser sample


nc-Si layer in a-Si:H film

20 40 60 80 1000

500

1000

1500

2000

2500

Inte

nsi

ty (a.u

.)

2

(111)

(220)

(311)

fs laser sample

Untreated

• 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 studiedH. Wang, et al, 2009

20 40 60 80 1000

250

500

750

1000

1250

1500

1750

Inte

nsi

ty (

arb

. unit)

2 (degree)

(111)

(220)

(311)

ns laser sample

Cross-section TEM ns laser sample Cross-section TEM fs laser sample


Research NeedsThe 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)


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


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


Research NeedsLarge-area, high-throughput LST Effects of spatial and temporal

characteristics of laser irradiation Spatial: Homogeneous intensity/mask

projection

(R. Delmdahl, et al,2010)


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


Research Opportunities in Laser Surface Texturing/Crystallization of Thin-Film Solar Cells

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