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Evaluation von laserbearbeiteten Si-Nanopartikeld端nnfilmen f端r den Einsatz in der Photovoltaik Presentation to the Master Thesis by Levon Altunyan


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Outline • Introduction and Motivation • Experiments and Results ▫ Type I Cells ▫ Type II Cells

• Outlook


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Problem Description and Solution Classical solutions negative impact on cells:

Suggested Solution:

− Different expansion coefficients of Al and Si

1. Spin-coated Si – nanoparticles

− Warping of the cell observed

2. Controlled, brief, local heating

− Difficulties in subsequent production

3. Sintered with the Silicon layer

− Increased probability of breakage

4. Create highly doped p+-type region 5. Benefit in cost per watt reduction

Fig: Schematic drawing of a solar cell with BSF


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Particle Size Determination

Conclusions: Particles keep their size even after three weeks time. Graph fit – Gaussian distribution: o Mean diameter value µ = 100 d.nm o Standard diameter deviation σ = 9 d.nm.

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Mean Number [%]

• Liquids of Si-nanoparticles: ▫ HWR. ▫ p-doped (boron). ▫ 5%wt and 10%wt.

Filtered particles Measurement 3 weeks before rest of curves 45 min 2000 rpm 75 min 3000 rpm

20

10

0 10

100

1000

Size [d.nm]

Fig: Determination of the Si-nanoparticle size via DLS measurement


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Layer Thickness Determination Thickness [nm] 953,0 874,0 795,0 716,0 637,0 558,0 479,0 400,0

y-Position [cm]

2,0 1,5 1,0

Conclusions:

0,5 0,0 0,0

 Average height hSiNp = 650nm 0,5

1,0

1,5

2,0

x-Position [cm]

Fig: Si-Layer Thickness vs. Position on Substrate; Back Surface Top View.

(±25nm). Inhomogeneous thickness due to substrate size. Peak in middle due to deposition method/speed.


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“Safe” Regions Determination 45

Laser Intensity [%]

40

Fig: IR laser system

35 30 25

"Eye" Guideline Optimal Intensity Optimal Intensity Argon Optimal Intensity Nitrogen

20 15

0

2000

4000

6000

8000

10000

Scan Velocity [mm/min]

Crystallization using an IR laser: Wavelength λ= 808 nm; Pulse length = continuous; Pulse profile: 13mm × 50µm; Power (max) ~ 452 W; Process Chamber: Volume V chamber=(1…2)l;

0. 1. 2. 3. 4. 5.

Fig: Layer Thickness vs. Spin Speed, One Spin Phase no visible laser illumination; visible laser illumination/no change of the surface; optimal = change to silver like color of the surface; slightly scratched layer; ablation of cell's layer; layer is totally removed;


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Ag Ink

Reference Cell “Type I” with BSF Reference Cell with Anti-reflex Coating and Al BSF; 0,12 No Si-nano Particles, No Sintering;

Fig: Cell Types

n-layer

Al Paste (BSF) Ag Ink

II”

p-layer

Illuminated Dark

0,10 0,08

Current [A]

Antireflex Coating (SiN)

“Type

“Type I” with BSF

“Type I”

Ag

0,06

Rs = 5,9 Ω ; Rsh = 2060,19 Ω ;

0,04 0,02 0,00 -0,02 -0,04

-3

-2

-1

0

1

Voltage [V]

Fig: IV-Characteristic of Reference Cell Type I

•Fill factor, FF = 59,21%. •Cell efficiency, η = 12, 93%. •Low series (Rs) and high shunt (Rsh) resistances


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Initial Parameters – “Type I” Cells

Fig: Cell Efficiency vs. Laser Intensity

Fig: Fill Factor vs Laser Intensity a3

40

a4 a7

35 a6 a7

30

a10 a8 a9

η [%]

FF [%]

a7

a9 25

a1 a2

20

a5 0

5

10

15

20

25

30

7,0 6,5 6,0 5,5 5,0 4,5 4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0 -0,5

a4 a3

a5

a1 0

a2 5

Laser Intensity [%]

10

a5

15

a7

a8

a9 a10 20

Laser Intensity [%]

Name

Scan Parameters

Fill Factor

Efficiency

a4

Laser Intensity, I = 1 ×15%; Scan Velocity, V =100 mm/min;

FF = 41 %

η = 6,38 %

25

30


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Sample Treatment Procedure

Procedures Applied on “Type I” Cells


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Final IV-Characterisations “Type I” 5

6

7

8

0,6 0,5 0,4 0,3 0,2 0,1 0,0

5

6

7

8

2,5 1,5 1,0 0,5 0,0

26 24

-0,015

22

-0,005

4

2,0

-0,020 -0,010

Combination [-]

20 18 16

0,000 1

2

3

4

5

Combination [-]

6

7

8

[%]

4

3

η

3

2

Voc [V]

2

1

FF [%]

2

Jsc [A/cm ]

1

Combination [-]

1

2

3

4

5

6

7

8

Combination [-]

a.) Open Circuit Voltage and Short Circuit Current; b.) Fill Factor and Cell Efficiency; •Random distribution of data points; •Difficult extraction of pronounced trend; •Further investigations using different cell structure needed.


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EDX Conciderations Fig: Diffusion Coefficient of Ag in Si vs Temperature 10

2

Diffusion Coefficient [µm /s]

1 µm Fig: EDX on the Front Surface Side of the Sample

N

1

Tcritical = (1111 …1141) ° C

0,1

-8

2

2*10 *exp{-1,59/(κb*T)}*10 µm /s

[45, 46]

2*10 *exp{-1,6/(κb*T)}*10 µm /s

[47]

0,01 800

-3

[43, 44]

-3

-8

-5

-8

2

6*10 *exp{-1,15/(κb*T)}*10 µm /s Extrapolation of

1000

2

1200

graph

n-type layer d=(0,3…0,4)µm

D = 3, 557 µm2/s

1400

TmeltAg = 961, 93 °C Temperature [°C] TmeltSi = 1414 ° C Si

Ag

• Possible diffusion of front Ag contacts into n-layer. • Probability that front contacts get even further - to the p-layer.


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SEM Investigations Highly reflective

Difference in colour!

10 Âľm

Non-reflective


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IV-Characterisations “Type II” Cells Efficiency vs Laser Intensity

On-Off Current Ratio - Samples With and Without Nanoparticles With Particles - Initial Study from 13.09.2011 With Particles - Samples from 16.09.2011 Without Particles - Samples from 16.09.2011

100

3,0 2,5

Efficiency [%]

Ratio [+1/-1]

10

1

2,0 1,5 1,0

0,1

0,01

15

20

Cell efficiency η = 2,95% (Type II) 25 30 observed. 35 40 45

Laser Intensity [%] Fig: On-Off Ratio, Comparison of Cells With and Without Si-nanoparticles

0,5 0,0

0

10

20

30

40

50

60

70

Laser Intensity [%] Fig: Efficiency of Type II samples with Si-nanoparticles

•Lower laser intensities (low heating): ->high on-off ratios; higher efficiency •no particles - build-in defects removed; •with particles – high resistivity -> low on-off ratios; •Higher laser intensities (increased heating): ->low on-off ratios; lower efficiency •decremental effect on the cell structure -> low on-off ratios;


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Conductivity Measurements -2

Total Conductivity vs Laser Intensity

−1

−1

Conductivity [Ω cm ]

10

Fig: Four Point Measurement Schematic Picture

ρtotal = (

U23 A U23 dtotal × s 1 )×( ) = ( )×( )= I1 L I1 L σtotal

-3

10

-4

10

25

30

35

40

45

50

55

Laser Intensity [%]

Where: Fig: Conductivityof Si-nanoparticles Spin-coated U23 is the potential difference b/n the inner probes; on Intrinsic Si-wafers Irradiated for Different Laser Intensities I1 is a known current passing through the outer probes; A is the area through which current flows; dtotal is the total thickness wafer; conductivity, σtotalof ≤ the 2, 57measured ×10-3 S/cm; •Total s is the commonfor contact length between the contact not laser treated particles, σtotal ≤ stripes; 3, 52 ×10-3 S/cm; •Conductivity L is the distance between the inner contact stripes;


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Summary • Size and stability of the particles inside the dispersion was determined. • The characteristic curves of different treated samples were examined. • Fill Factor of FF = 41%; cell efficiency η = 6,38% (Type I) was obtained. • Fill Factor of FF = 27%; cell efficiency η = 2,95% (Type II) was observed. • Estimated doping depth to at least hBSF = 5 µm (SEM). • An initial work with thin-film Kapton® foils was carried out.


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Outlook • More thorough studies of the regions characterized by a highly reflective surface. • Further investigations of the correlation between crystallinity and diode behavior. • Remove native silicon surface oxide with hydrouoric acid before laser treatment. • Use more scans at higher intensity. • Pulsed UV-Laser treatment on Kapton® foils.


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Acknowledgements

THANK YOU to: Prof. Dr. Roland Schmechel for giving me the opportunity to work on this exciting topic. Dr. Niels Benson and Dipl.Ing. Martin Meseth for their time and guidance during the development of this work. Their advices contributed to the pleasant and fruitful experience that I obtained during this time. The whole team of the NST department for their support concerning my work in the laboratory.


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Thank you for your attention!!!

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Master Thesis Final Presentation Powerpoint