Západočeská univerzita v Plzni, Česká republikaVýzkumné centrum Nové technologie

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STUDIUM ZMĚNY OPTICKÝCH VLASTNOSTÍ V ZÁVISLOSTI NA ZMĚNĚ STRUKTURY a-Si:H

Marie Netrvalová, Jarmila Mullerová, Pavol Šutta

Lucie Prušáková, Veronika Vavruňková,

This presentation is co-financed by the European Social Fund and the state budget of the Czech Republic.

Materiály a technologie (MAT)projekt 1M06031

Materiály a komponenty pro ochranu

životního prostředí

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Introduction Introduction ::

Tandem solar cell is one of the concepts well established as a Tandem solar cell is one of the concepts well established as a way how to improve solar cell performance beyond that of a way how to improve solar cell performance beyond that of a single cellsingle cell..

This concept needs semiconductor materials with different This concept needs semiconductor materials with different band-gaps, which are stacked on top of another, in order to filter band-gaps, which are stacked on top of another, in order to filter the photons of different energies passing through the stackthe photons of different energies passing through the stack..

One of the possibilities how to solve this problem is taking One of the possibilities how to solve this problem is taking advantage from the well established silicon technology (a-Si:H, advantage from the well established silicon technology (a-Si:H, μc-Si:H and poly-Si materials)μc-Si:H and poly-Si materials)..

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Introduction Introduction ::

One of the possibilities how to obtain poly-Si films of a good One of the possibilities how to obtain poly-Si films of a good quality consists generally of two steps:quality consists generally of two steps:

DDeposition of a-Si or a-Si:H thin films by means of PVD eposition of a-Si or a-Si:H thin films by means of PVD or CVDor CVD

technologies at low temperatures andtechnologies at low temperatures and

SSubsequent reubsequent re--crystallization of the films from solid crystallization of the films from solid phase by thermalphase by thermal treatment at temperatures near to treatment at temperatures near to 600°C600°C..

0 2 4 6 8 10

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d in

teg

rate

d in

ten

sity

[-]

Thermal treating time [hours]

580°C 590°C 600°C 610°C 620°C

a-Si:H to uc-Si:H transition

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Deposition techniqueDeposition technique::

PE-CVD SAMCO PD 220N unitPE-CVD SAMCO PD 220N unit

deposition temperature 250°Cdeposition temperature 250°C

RF power 40 WRF power 40 W

at constant presure of 67 Paat constant presure of 67 Pa

Precursors: SiH4 (10% in Ar), H2 Precursors: SiH4 (10% in Ar), H2

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Outline of the experimentOutline of the experiment::

Dilution SiH + Ar flow

[sccm]

H flow

[sccm]

Volume content [%] Dimension of coherently diffracting

domain [nm]

Thickness [nm]

Silicon Silicon hydride

Silicon Silicon hydride

R 0 250 0 - - - - 170

R 20 83 167 - - - - 120

R 30 62.5 187.5 - - - - 140

R 40 50 200 53 47 6 18 140

R 50 42 208 32 68 9 14 180

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2 R = [H ] / [SiH ] 4

XRDXRD::

Panalytical X’Pert PROX-ray powder diffractometer

Applications:• Qualitative and quantitative phase analysis• Residual stress analysis• Texture analysis• Analysis of changes in the crystal structure• Ultra fast data collection with using

X’CeleratorTheta-Theta goniometer

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Substrate: Corning glassDeposition temperature: 250 °CDilution: R = 0, 20, 30, 40, 50 (R = H2/SiH4)Working gas: Argon (90 %) / silane (10 %)

XRD patterns - in range of 15 – 65 degrees of 2 measured on attachment *) with asymmetric geometry

- semi-quantitative XRD phase analysis was carried out from all significant diffraction lines

XRDXRD::

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XRDXRD::

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XRDXRD::

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Raman spectroscopyRaman spectroscopy::

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Yvon Labram Raman Spectrometer

Micro-Raman spectra excited with a laser generating the wavelength of 532 nm

Shift of the Raman peaks due to different hydrogen dilution was observed

400 450 500 550 600 650

0

500

1000

1500

Inte

nsi

ty [a

.u.]

Raman shift [cm-1]

520

Crystalline Si

Influence of hydrogen Influence of hydrogen ::

15 20 25 30 35 40 45 50 55 60 65

0

1 000

2 000

3 000

4 000

Si4H

Inte

nsi

ty (

cou

nts

)

2 (degrees)

Si (111)

Si (220)

Si (311)Corning glass

Resultant patternsa-Si:H/glassR = 40

Si4H

Experimental data

Dilution Volume content [%] Dimension of coherently

diffracting domain [nm]

Thickness [nm]

Silicon Silicon hydride

Silicon Silicon hydride

R 40 53 47 18 6 140

R 50 68 32 14 9 180

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Optical spectrophotometryOptical spectrophotometry::

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• UV / VIS Spectrophotometer- spectral region 190 – 1100 nm- measurement of transmittance, absorbance

and reflectance in dependence on the wavelength

• Accessories- Absolute Reflectance Attachment (determine

the absolute reflectance of reflecting films)- Variable Angle Reflectance Attachment

(determine refractive index of solid samples)- Integrating Sphere (for the measurement of

transmittance and diffuse reflectance)

SPECORD 210

400 500 600 700 800 900 1000 1100

0

20

40

60

80

100

Tra

nsm

itta

nce

(%

)

Wavelength (nm)

t = 170 nm t = 340 nm t = 520 nm

R = 0

• Spectral refractive indices and absorbtion coefficients were extracted from measured transmittance spectra using the Delphi-based program based on an optimization procedure using genetic algorithm

• The optimization procedure minimizes differences between the experimental and theoretical transmittance i the broad spectral region including the region in the vicinity of the absorption edge.

Optical propertiesOptical properties::

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The refractive index shifts towards the values typical for single-crystalline silicon (n ~ 3.5) with increasing R.

Increased hydrogen dilution shifts the absorption edge to the higher energies (lower wavelengths).

The optical band-gap energies for the films with lower hydrogen dilution (R≤20) are 1.75 - 1.9 event. In case of R≥30 Eg = 2.15 eV.

Outline of the experimentOutline of the experiment::

Series number

Thickness [nm]

Re-crystallization temperature [°C]

Comments1 2 3 4 5 6

580 590 600 610 620 ref.

A0035 2400 42 42 22 18 18 1* The thicknesses of the samples were evaluated from the optimization procedure during the optical spectra processing

A0036 1860 42 42 22 18 18 1*

A0037 1200 42 42 22 18 18 1*

A0038 600 - 42 22 18 18 1*

- Σ = 126 168 88 72 72 4* ΣΣ = 530 XRD records

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Substrate: Corning glassDeposition temperature: 250 °CDilution: R = 0 (R = H2/SiH4)Working gas: Argon (90 %) / silane (10 %)Isothermal heating was usedLinear temperature starting-up (50°C/min.) was appliedPressure in a high-temperature chamber was 0.1 PaTemperature decay - exponential shape (only by irradiation)

XRD patterns – in range of 15 – 65 degrees of 2 in initial state measured on attachment *)

– lines (111) during the heating– in range of 20 – 65 degrees of 2 after the heat treatment

Outline of the experimentOutline of the experiment::

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„„In situ“ XRD monitoring of a-Si:H poly-Si re-crystallization processIn situ“ XRD monitoring of a-Si:H poly-Si re-crystallization process Parameters of the experimentParameters of the experiment

Sample Sample numbernumber TemperatureTemperature Number of XRD Number of XRD

measurementsmeasurementsDuration Duration [hours][hours]

11 580°C580°C 40+240+2 2020

22 590°C590°C 40+240+2 2020

33 600°C600°C 20+220+2 1010

44 610°C610°C 16+216+2 88

55 620°C620°C 16+216+2 88

66 roomroom beforebefore --

0 2 4 6 8 10 12 14 16 18 20

0

2500

5000

7500

10000In

teg

rate

d in

ten

sity

[a.u

.]

Thermal treatment time [hours]

590°C

600 nm

1200 nm

2400 nm

0 2 4 6 8 10 12 14 16 18 20

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d in

teg

rate

d in

ten

sity

[-]

Thermal treating time [hours]

580°C 590°C 600°C 610°C 620°C

2400 nm

X-ray diffraction – symmetric X-ray diffraction – symmetric -- geometry geometry ::

20 25 30 35 40 45 50 55 60 650

1x104

2x104

3x104

4x104

Inte

nsi

ty (

cou

nts

)

2 (degrees)

600 nm

(111)

(220)(311)

20 25 30 35 40 45 50 55 60 650

1x104

2x104

3x104

4x104

Inte

nsi

ty (

cou

nts

)

2 (degrees)

(111)

(220)

(311)

1200 nm

20 25 30 35 40 45 50 55 60 650

1x104

2x104

3x104

4x104

Inte

nsi

ty (

cou

nts

)

2 (degrees)

(111)

(220)

(311)

2400 nm

20 25 30 35 40 45 50 55 60 650

1x104

2x104

3x104

4x104

Inte

nsi

ty [c

ou

nts

]

2 [degrees]

1860 nm

(111)

(220)

(311)

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Raman spectroscopy Raman spectroscopy ::

400 450 500 550 600 650

0

500

1000

1500

Inte

nsi

ty [a

.u.]

Raman shift [cm-1]

520

Crystalline Si

475 500 525 5500.0

4.0x104

8.0x104

1.2x105

1.6x105

580°C590°C600°C610°C620°C

Inte

nsi

ty (

a.u

.)

Raman shift [cm-1

]

519

475 500 525 5500.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d in

ten

sity

Raman shift [cm-1

]

519580°C590°C600°C610°C620°C

400 450 500 550 6000.0

3.0x104

6.0x104

9.0x104

1.2x105

1.5x105

Inte

nsi

ty [

a.u

.]

Raman shift [cm-1]

re-crystallizedpoly-Si films

519 cm-1

as-deposited a-Si:H film

480 cm-1

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Refractive indices Refractive indices ::

500 600 700 800 900 1000 11003.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

4.6R

efr

act

ive

ind

ex

[-]

Wavelength [nm]

590°C 600°C 610°C 620°C Initial state

500 600 700 800 900 1000 11003.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

4.6

Re

fra

ctiv

e in

de

x [-

]

Wavelength [nm]

580°C 590°C 600°C 610°C 620°C Initial state

500 600 700 800 900 1000 11003.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

4.6

Ref

ract

ive

ind

ex

[-]

Wavelength [nm]

580°C 590°C 600°C 610°C 620°C Initial state

600 700 800 900 1000 11002,5

3,0

3,5

4,0

4,5

5,0

Re

frac

tive

inde

x [-

]

Wavelength [nm]

580°C 610°C 590°C 620°C 600°C Initial (small) Initial (large)

1860 nm

600 nm 1200 nm

2400 nm

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UV-VIS spectrophotometry UV-VIS spectrophotometry ::

300 400 500 600 700 800 900 1000 1100

0

20

40

60

80

100

Tra

nsm

itta

nce

[%

]

Wavelength [nm]

Initial state580°C590°C600°C610°C620°C

200 300 400 500 600 700 800 900 1000 1100

0

20

40

60

80

100Initial state590°C600°C610°C620°C

Tra

nsm

ittan

ce [%

]

Wavelength [nm]

400 500 600 700 800 900 1000 1100

0

20

40

60

80

100Initial state580°C590°C600°C610°C620°C

Tra

nsm

ittan

ce [%

]

Wavelength [nm]

400 500 600 700 800 900 1000 1100

0

20

40

60

80

100Initial state580°C590°C600°C610°C620°C

Tra

nsm

ittan

ce [%

]

Wavelength [nm]

600 nm 1200 nm

2400 nm1860 nm

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Absorption coeficient Absorption coeficient ::

1,2 1,3 1,4 1,5 1,6 1,7 1,8 1,9 2,0100

101

102

103

104

Ab

sorp

tion

co

efii

cie

nt [

cm-1

]

Photon energy [eV]

580°C 590°C 600°C 610°C 620°C initial state (small) initial state (large)

amorphous state

polycrystalline state

1.2 1.4 1.6 1.8 2.0 2.2 2.4100

101

102

103

104

105

Ab

sorp

tion

co

effi

cie

nt [

cm-1

]

Photon energy [eV]

580°C 590°C 600°C 610°C 620°C initial state

2400 nm

1200 nm

1.2 1.4 1.6 1.8 2.0 2.2 2.4100

101

102

103

104

105

Abso

rptio

n c

oeff

icie

nt [

cm-1

]

Photon energy [eV]

580°C 590°C 600°C 610°C 620°C Initial state

1,2 1,4 1,6 1,8 2,0 2,2 2,4100

101

102

103

104

105

Ab

sorp

tion

co

effi

cie

nt [

cm-1

]

Photon energy [eV]

590°C 600°C 610°C 620°C Initial state

1860 nm

600 nm

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Absorption coeficient Absorption coeficient ::

1,2 1,3 1,4 1,5 1,6 1,7 1,8 1,9 2,0100

101

102

103

104

Ab

sorp

tion

co

efii

cie

nt [

cm-1

]

Photon energy [eV]

580°C 590°C 600°C 610°C 620°C initial state (small) initial state (large)

amorphous state

polycrystalline state

1.2 1.4 1.6 1.8 2.0 2.2 2.4100

101

102

103

104

105

Ab

sorp

tion

co

effi

cie

nt [

cm-1

]

Photon energy [eV]

580°C 590°C 600°C 610°C 620°C initial state

2400 nm

1200 nm

1.2 1.4 1.6 1.8 2.0 2.2 2.4100

101

102

103

104

105

Abso

rptio

n c

oeff

icie

nt [

cm-1

]

Photon energy [eV]

580°C 590°C 600°C 610°C 620°C Initial state

1,2 1,4 1,6 1,8 2,0 2,2 2,4100

101

102

103

104

105

Ab

sorp

tion

co

effi

cie

nt [

cm-1

]

Photon energy [eV]

590°C 600°C 610°C 620°C Initial state

1860 nm

600 nm

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A.V.Shah A.V.Shah et alet al., Prog., Progress inress in Photovolt Photovoltaics:aics:ResResearch andearch and Appl Applications, ications, 2004, 12, 113 – 1422004, 12, 113 – 142

1.2 1.4 1.6 1.8 2.0 2.2 2.4100

101

102

103

104

105

Abs

orpt

ion

coef

ficie

nt

[cm

-1]

Photon energy [eV]

580°C 590°C 600°C 610°C 620°C initial state

Our resultsOur results

ComparisonComparison

Absorption coeficient Absorption coeficient ::

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Conclusions Conclusions ::

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• Amorphous and polycrystalline silicon films were obtained using PE-Amorphous and polycrystalline silicon films were obtained using PE-CVD. Amorphous phase a-Si:H - RCVD. Amorphous phase a-Si:H - R≤20≤20;; polycrystalline polycrystalline c-Si:H - R≥30.c-Si:H - R≥30.

• Polycrystalline silicon films were obtained using subsequent thermal Polycrystalline silicon films were obtained using subsequent thermal processing of the films at temperatures near 600°C.processing of the films at temperatures near 600°C.

• PE-CVD / PE-CVD / re-crystallized poly-Si films still containing re-crystallized poly-Si films still containing 30-5030-50 / / 21-25% 21-25% residual amorphousresidual amorphous (disordered) (disordered) phase, which was confirmed by phase, which was confirmed by the the Raman spectroscopyRaman spectroscopy..

• Average crystallite size obtained from the PEAverage crystallite size obtained from the PE-CVD / -CVD / re-crystallization re-crystallization process was process was 15-20 / 15-20 / 40-50 nm without particular dependence on heat 40-50 nm without particular dependence on heat treatment temperature usedtreatment temperature used..

• Significant optical absorption in re-crystallized Significant optical absorption in re-crystallized siliconsilicon films films compared compared with awith a--SiSi:H:H waswas observed between 1.65 – 1.85 eV photon energies observed between 1.65 – 1.85 eV photon energies..

• These results indicate that the films under study could be considered These results indicate that the films under study could be considered as convenient material for tandem solar cells technologies.as convenient material for tandem solar cells technologies.

Pilsner TCO´s Pilsner TCO´s ::

XRD patterns for sputtered ZnO:Al filmsStandard - ZnO powder

All samples – polycrystalline structure All samples – polycrystalline structure

Crystallite size – 68 to 109 nmCrystallite size – 68 to 109 nm

Low resistivity – Low resistivity –

High transparency - High transparency -

High reproducibility of sputtering processHigh reproducibility of sputtering process

Dependence of Resistivity on Biaxial stressDependence of Resistivity on Biaxial stress

Texture in Texture in [[001001]] direction perpendicular to the substrate direction perpendicular to the substrate

0,0046 0,0046 ΩΩ.cm.cm

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> 9> 90%0%

Acknowledgements Acknowledgements ::

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0.0 0.2 0.4 0.6 0.8 1.0-0.0020

-0.0015

-0.0010

-0.0005

0.0000

Asahi U-type = 6.93 % ZnO:Al (etch) = 6.89 %

Cur

rent

den

sity

[A

/m2]

Voltage [V]0.0 0.2 0.4 0.6 0.8 1.0

-0.0020

-0.0015

-0.0010

-0.0005

0.0000

ZnO:Ga = 5.51 % ZnO:Al (etch) = 6.89 %

Cur

rent

den

sity

[A

/m2]

Voltage [V]

Efficiency Efficiency 9.979.97% - ASAHI% - ASAHI

Efficiency 6.89% - AZO 4Efficiency 6.89% - AZO 4

Efficiency η[%]

5,26

5,51

5,18

6,93

6,55

6,35

5,14

6,44

6,89

6,17

6,73

GZO

GZO

GZO

ASAHI

ASAHI

ASAHI

AZO 3

AZO 3

AZO 4

AZO 4

AZO 4

1ln

.

0J

J

q

TkV phBoc

ocsc VI

VIFF

.

. maxmax

Acknowledgements Acknowledgements ::

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This work was supported by the project of MSMT of the This work was supported by the project of MSMT of the Czech RepublicCzech Republic No. 1M06031 and by the No. 1M06031 and by the SlovakSlovak Grant Grant Agency under the project Agency under the project No. No. 2/0070/10 and by the 2/0070/10 and by the Slovak Research and Development Agency under the Slovak Research and Development Agency under the project APVV-0577-07.project APVV-0577-07.

The authors would like to thank to M. Ledinsky from The authors would like to thank to M. Ledinsky from CASCAS for Raman spectra measurements. for Raman spectra measurements.

Thank you for Your interest in Thank you for Your interest in photovoltaic applicationsphotovoltaic applications