NANO SILICON DOTS EMBEDDED SIO 2 /SIO 2 MULTILAYERS FOR PV HIGH EFFICIENCY APPLICATION

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NANO SILICON DOTS EMBEDDED SIO

₂

/SIO

₂

MULTILAYERS FOR PV HIGH EFFICIENCY

APPLICATION

Olivier Palais, Damien Barakel, David Maestre, Fabrice Gourbilleau and Marcel Pasquinelli

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Outline

• Photovoltaic today “short view”

• 3G cells (case of tandem solar cells)_‏

• All Si tandem solar cells:Different possibilties for nc-Si fabrication (problem and interest of each)_‏

• nc-Si in multilayers SRSO

– A technique to grow Si nanostructures – Control of the size and the density

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Multijunction Concentrators

Three-junction (2-terminal, monolithic) Two-junction (2-terminal, monolithic)_‏

Crystalline Si Cells

Single crystal Multicrystalline Thin Si

Thin Film Technologies

Cu(In,Ga)Se₂ CdTe

Amorphous Si:H (stabilized)_‏

Emerging PV

Dye cells Organic cells

(various technologies)_‏

Photovoltaic today : Solar Cells technology

η

Claudio Pelosi and Matteo Bosi

Solar & Alternative Energy, SPIE Newsroom

2007

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3G photovoltaic cells: tandem cells

High efficiency(>39%) but very expensive (MOCVD)_‏‏‏‏

Typical III-V

Tandem stack cell

Use of nano-crystalline silicon for gap engineering: tandem solar cell 56% of solar spectrum lost with c-Si (1.12 eV)_‏‏‏‏

Power loss mechanisms in single band gap Solar Cells

Shorter wavelength Longer wavelength

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Silicon tandem cell

higher gap – Eg₁

lower gap – Eg₂

hν>Eg₁

Eg₁>hνννν>Eg₂

Quantum efficiency of tandem silicon cell versus band gaps

hν Eg₁ (eV)_‏ E g 2 (e V ) ‏ nanostructured silicon layer(s)

All Si tandem solar cells

5

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1 2 3 4 5 6 7 8 9 1,0 1,2 1,4 1,6 1,8 2,0 1200 1100 1000 900 800 700 600 L o n g u e u r d ' o n d e d u m a x i m u m d e P L ( n m )

Bande interdite du c-Si

E n e r g i e d u m a x i m u m d e P L ( e V )

Diamètre des grains (nm)

Schuppler et al., 1995 Fujii et al., 1998 Takagi et al., 1990 Takeoka et al., 2000 Ledoux et al., 2000 Ehbrecht et al., 1997 Garrido et al., 2002 Ledoux et al., 2000

(synthesis of experimental results fom Céline Ternon Thesis, Université de Caen, Décembre 2002). Silicon quantum dots: gap versus diameter

1.7

3

Our goal is to obtain 3nm diameter quantum dots

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Nano-silicon particles Silicon nano-wire

Si NP in SiNx matrix

(CVD VLS)

Various method for nc-Si elaboration

₊

-- Stability of diameter (use of colloïd)_‏ - Need of matrix - Conductivity (?)_‏ SiNx matrix (PECVD) SiO₂ matrix (RMS) - Stability of diameter - Modest NP density - Low temperature - Stable matrix - Stability of diameter, easy to select - High NP density - Time consuming 7

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Use of an argon

Use of an argon--hydrogen mixture to allow the incorporation hydrogen mixture to allow the incorporation of a Si excess in the growing film

of a Si excess in the growing film All Si tandem solar cells

GROWTH : Reactive Magnetron Sputtering

• Key parameters :

• Hydrogen rate r_H = P_H2/(P_H2+P_ar) • Substrate temperature

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Reactive Magnetron Sputtering T=500_{°C r}_H=80% T=500_{°C r}_H=50% T=500_{°C r}_H=25% T=60_{°C r}_H=80% e e e e 9

Effect of hydrogen and temperature

C O M P O S I T E M U L T I -L A Y E R S All Si tandem solar cells

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Si nanostructure for PV

Multilayers of 3nm-thick SRSO and 3nm thick SiO₂

(60 SRSO/SiO₂ pattern) c ro s s s e c ti o n plan view substrate SRSO Ar + H₂ Plasma 3 nm thick

Elaboration sequence for N multi-layers

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Multilayers of 3nm-thick SRSO and 3nm thick SiO₂ (60 SRSO/SiO₂ pattern) c ro s s s e c ti o n plan view Shutter + H₂ cut off substrate SRSO Ar + H₂ Plasma 3 nm thick Ar Plasma substrate SiO₂ 3 nm thick 11

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Multilayers of 3nm-thick SRSO and 3nm thick SiO₂ (60 SRSO/SiO₂ pattern) c ro s s s e c ti o n plan view Shutter + H₂ cut off substrate SRSO Ar + H₂ Plasma 3 nm thick Ar Plasma substrate SiO₂ 3 nm thick Shutter + H₂ entry

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• Presence of Si nanoparticles as evidenced by EFTEM experiments.

• Good homogeneity with Si size

• Distance Si-np→Si-np≈ 1.5 nm

(60 SRSO/SiO₂ pattern)

Filtered images at 16 eV (nc-Si)

c ro s s s e c ti o n plan view 13

TEM and Energy Filtered TEM

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• Presence of Si nanoparticles as evidenced by EF-TEM experiments.

• Good homogeneity with Si size

• Distance Si-np→Si-np≈ 1.5 nm

(60 SRSO/SiO₂ pattern)

Filtered images at 16 eV (nc-Si)

5 nm

5 nm

HRTEM plan view

c ro s s s e c ti o n plan view Si-np density 10+19_/cm-3

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effect of the post annealing temperature 400 500 600 700 800 500 600 700 800 900 400 500 600 700 800 900 3 2.5 2 1.5 3 2.5 2 1.5 3 2.5 2 1.5 R2 = 0,982 ×××× 40 Wavelength (nm) Wavelength (nm) P L i n te n s it y ( a .u .) 600°C R2 = 0,993 ×××× 35 700°C R2 = 0,997 ×××× 20 800°C R2 = 0,990 ××××₁₈ Wavelength (nm) 900°C R2 = 0,999 ×××× 10 1000°C R2 = 0,999 ×××× 1 1100°C R2 = 0,996 ×××× 10 1200°C

Energy (eV) Energy (eV) Energy (eV)

Best annealing treatment for the

optical properties : 1100_°C during

one hour

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Optical measurements 1.2 1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.6 102 103 104 105 A b so rp ti o n , α gra in ( cm -1 ) Energy (eV) 8 nm 6 nm 3 nm 1.5 nm Si nc diameter 300 350 400 450 500 550 600 0.0 5.0x103 1.0x104 1.5x104 2.0x104 2.5x104 3.0x104 3.5x104 4.0x104 4.5x104 5.0x104 In te n s it y ( a .u .) Raman Shift (cm-1) 8 nm 6 nm 3 nm 1.5 nm Si-nc diameter •

• Crystalline phase for Si-Crystalline phase for Si-ncnc having a diameter ranging from 3 to 8 nm having a diameter ranging from 3 to 8 nm •

• For For φφ_Si_Si_-_-_nc_nc = 1.5 nm, the system is amorphous = 1.5 nm, the system is amorphous

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1.2 1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.6 102 103 104 105 A b so rp ti o n , α gra in ( cm -1 ) Energy (eV) 8 nm 6 nm 3 nm 1.5 nm Si nc diameter •

• Similar absorption for Si Similar absorption for Si nanocrystalsnanocrystals ( 3nm to 8 nm)( 3nm to 8 nm) •

• Increase of the absorption coefficient with the nature of the SIncrease of the absorption coefficient with the nature of the Si i phase (amorphous) phase (amorphous)_‏‏ 300 350 400 450 500 550 600 0.0 5.0x103 1.0x104 1.5x104 2.0x104 2.5x104 3.0x104 3.5x104 4.0x104 4.5x104 5.0x104 In te n s it y ( a .u .) Raman Shift (cm-1) 8 nm 6 nm 3 nm 1.5 nm Si-nc diameter •

• Crystalline phase for Si-Crystalline phase for Si-ncnc having a diameter ranging from 3 to 8 nm having a diameter ranging from 3 to 8 nm •

• For For φφ_Si_Si = 1.5 nm, the system is amorphous = 1.5 nm, the system is amorphous

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Optical measurements

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Electrical measurements

Caractérisations techniques

Sheet resistance : Van Der Pauw Technique

I V

sheet

R

I

V

⇒

R_sheet around 250 kΩ Resistivity of ML around 100 Ω.cm

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I-V; Photoconductance 0 1 2 3 0.0 3.0x10-5 6.0x10-5 9.0x10-5 47.58 KΩΩΩΩ 36.98 KΩΩΩΩ I (A ) V (V) Darkness Illumination

ImprovementImprovement of the conductivityof the conductivity (~20%)

(~20%) underunder illuminationillumination

19 - Al Contacts ⇒_{500 ºC (15 h) N}

2

- I/V curves illumination and darkness

Green et al.

37 kΩΩΩΩ

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I-V; Photoconductance / effect of wavelength 0 1 2 3 0.0 3.0x10-5 6.0x10-5 9.0x10-5 47.58 KΩΩΩΩ 36.98 KΩΩΩΩ I (A ) V (V) Darkness Illumination 0 1 2 3 0.0 3.0x10-5 6.0x10-5 9.0x10-5 40.78 kΩΩΩΩ 46.20 kΩΩΩΩ I (A ) V (V) Darkness UV-Blue (λλλλ = 375 nm) IR-Red (λλλλ = 940 nm)

ImprovementImprovement of the conductivityof the conductivity (~20%)

(~20%) underunder white illuminationwhite illumination

•

• Dependence of the conductivity Dependence of the conductivity with

with λλ. . •

• No effect of the substrateNo effect of the substrate •

• Clear evidence of the contribution Clear evidence of the contribution of the

of the nanostructurednanostructured multilayersmultilayers

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• Transverse probe optical lifetime (Free Carrier Absorption)

- Values of ττττ_e_ff (∼∼∼∼30 ns) are rather low, even for µW-PCD (ττττ_e_ff ∼∼∼∼100 ns)

- Probably due to induced defect or contamination during thermal treatment

- Probe: optical (λ=1.55µm) - Pump : optical (λ=1,064 µm) A 481 Lased Diode λ= 1.55 µm Nd:YAG Laser λ= 1064 nm Detector

A.Irace (Dpt.E.T.E. Univ. Naples)

optical measurements

Parameter measured : Lifetime of free

carriers (ττττ_eff)

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Conclusion for the PV approach

• A reactive process allowing the growth of Si nanoclusters in multilayer structures

• Control of the Si size through the deposition parameters • High Si-nc density (~ 10+19 _cm-3₎

• Multilayers is a promising structure for the conduction (control of the SiO₂ sublayer thickness)

• Evidence of a photocurrent on 360 nm-thick SRSO/SiO₂

multilayers.

• Next step under progress :

– Effect of the Si-nc size on the photoconductance measurements. – p-n junction must be processed

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