Master Thesis Final Presentation Powerpoint

Evaluation von laserbearbeiteten Si-Nanopartikeldünnfilmen für denEinsatz in der Photovoltaik

Presentation to the Master Thesisby Levon Altunyan

Outline•Introduction and Motivation•Experiments and Results

▫Type I Cells▫Type II Cells

•Outlook

2

Problem Description and Solution

3

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

region5. Benefit in cost per watt

reduction

Fig: Schematic drawing of a solar cell withBSF

Particle Size Determination• Liquids of Si-nanoparticles:

▫ HWR.▫ p-doped (boron).▫ 5%wt and 10%wt.

4

Conclusions:Particles keep their size even after three weeks time.Graph fit – Gaussian distribution:

o Mean diameter valueµ = 100 d.nmo Standard diameter deviation σ = 9 d.nm.

10 100 1000

0

10

20

30

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

Mea

n N

umbe

r [%

]

Size [d.nm]

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

Layer Thickness Determination5

400,0479,0558,0637,0716,0795,0874,0953,0

0,0 0,5 1,0 1,5 2,00,0

0,5

1,0

1,5

2,0

y-Pos

ition

[cm

]

x-Position [cm]

Thickness [nm]

Conclusions: Average height hSiNp = 650nm (±25nm).Inhomogeneous thickness due to substrate size.Peak in middle due to deposition method/speed.Fig: Si-Layer Thickness vs. Position

on Substrate; Back Surface Top View.

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. no visible laser illumination;1. visible laser illumination/no change of the

surface;2. optimal = change to silver like color of the

surface;3. slightly scratched layer;4. ablation of cell's layer;5. layer is totally removed;

“Safe” Regions Determination6

0 2000 4000 6000 8000 1000015

20

25

30

35

40

45

"Eye" GuidelineOptimal IntensityOptimal Intensity ArgonOptimal Intensity Nitrogen

Lase

r Int

ensi

ty [%

]

Scan Velocity [mm/min]

Fig: IR laser system

Fig: Layer Thickness vs. Spin Speed, One Spin Phase

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

Reference Cell “Type I” with BSF

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Antireflex Coating

(SiN)

n-layer

p-layer

Al Paste (BSF)

Ag

Ag Ink

Ag Ink

“Ty

pe I

” w

ith B

SF

-3 -2 -1 0 1-0,04

-0,02

0,00

0,02

0,04

0,06

0,08

0,10

0,12

Reference Cell with Anti-reflex Coating and Al BSF;No Si-nano Particles, No Sintering;

Cur

rent

[A]

Voltage [V]

Illuminated Dark

Rs = 5,9 ;Rsh = 2060,19 ;

Fig: IV-Characteristic of Reference Cell Type I

“Type II”

“Type I”

Fig: Cell Types

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0 5 10 15 20 25 30

20

25

30

35

40a4

a5

a2

a1

a3

a7

a9

a10

a7

a7a8

Fig: Fill Factor vs Laser Intensity

FF [%

]

Laser Intensity [%]

a9a6

0 5 10 15 20 25 30-0,50,00,51,01,52,02,53,03,54,04,55,05,56,06,57,0

a5

a4

a5a2a1

a3

a9

a10

a7

a8

[%

]

Laser Intensity [%]

Fig: Cell Efficiency vs. Laser IntensityInitial Parameters – “Type I” Cells

Name Scan Parameters Fill Factor

Efficiency

a4 Laser Intensity, I = 1 ×15%;Scan Velocity, V =100 mm/min;

FF = 41 % η = 6,38 %

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Sample Treatment Procedure

Procedures Applied on “Type I” Cells

10

Final IV-Characterisations “Type I”

1 2 3 4 5 6 7 80,000

-0,005

-0,010

-0,015

-0,020

Jsc

[A/c

m2 ]

Combination [-]

1 2 3 4 5 6 7 8

0,00,10,20,30,40,50,6

Combination [-]

Voc

[V]

1 2 3 4 5 6 7 816

18

20

22

24

26

FF [%

]

Combination [-]

1 2 3 4 5 6 7 8

0,0

0,5

1,0

1,5

2,0

2,5

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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800 1000 1200 14000,01

0,1

1

10

2*10-3*exp{-1,59/(b*T)}*10-8 m2/s

2*10-3*exp{-1,6/(b*T)}*10-8 m2/s

6*10-5*exp{-1,15/(b*T)}*10-8 m2/s

Extrapolation of graph

Diff

usio

n C

oeffi

cien

t [m

2 /s]

Temperature [°C]

Fig: Diffusion Coefficient of Ag in Si vs Temperature

[43, 44][45, 46]

[47]

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

N

Si

1 µm

EDX Conciderations

Ag

Fig: EDX on the Front Surface Side of the Sample

TmeltAg = 961, 93 °C

n-type layer d=(0,3…

0,4)µm

TmeltSi = 1414 ° C

D = 3, 557 µm2/s

Tcritical = (1111 …1141) ° C

12

SEM Investigations

Highly reflective Non-reflective

Difference in colour!

10 µm

15 20 25 30 35 40 450,01

0,1

1

10

100 With Particles - Initial Study from 13.09.2011 With Particles - Samples from 16.09.2011 Without Particles - Samples from 16.09.2011

On-Off Current Ratio - Samples With and Without Nanoparticles

Rat

io [+

1/-1

]

Laser Intensity [%]

0 10 20 30 40 50 60 700,0

0,5

1,0

1,5

2,0

2,5

3,0

Laser Intensity [%]

Effi

cien

cy [%

]

Efficiency vs Laser Intensity

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IV-Characterisations “Type II” Cells

Fig: On-Off Ratio, Comparison of Cells With and Without Si-nanoparticles

Fig: Efficiency of Type II samples with Si-nanoparticles

•Lower laser intensities (low heating):•no particles - build-in defects removed;•with particles – high resistivity -> low on-off ratios;

•Higher laser intensities (increased heating):•decremental effect on the cell structure -> low on-off ratios;

->high on-off ratios; higher efficiency

->low on-off ratios; lower efficiency

Cell efficiency η = 2,95% (Type II) observed.

25 30 35 40 45 50 5510-4

10-3

10-2 Total Conductivity vs Laser Intensity

Con

duct

ivity

[

cm

]Laser Intensity [%]

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Conductivity Measurements

•Total conductivity, σtotal ≤ 2, 57 ×10-3 S/cm;•Conductivity for not laser treated particles, σtotal ≤ 3, 52 ×10-3 S/cm;

Fig: Four Point Measurement Schematic Picture

Fig: Conductivityof Si-nanoparticles Spin-coated on Intrinsic Si-wafers Irradiated for Different Laser Intensities

Where:U23 is the potential difference b/n the inner probes;I1 is a known current passing through the outer probes;A is the area through which current flows;dtotal is the total thickness of the measured wafer;s is the common contact length between the contact stripes;L is the distance between the inner contact stripes;σtotal is the total conductivity of the material under test;

total1

23

1

23total

1)L

sdtotal()I

U()LA()

IU(

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.

Thank you for your attention!!!

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