The nanostructure of silicon thin films for solar cells

1
The nanostructure of silicon thin films for solar
cells

• Andreja Gajovic1,2, Davor Gracin2, Miran Ceh1

1Institute Joef Stefan, Ljubljana,
Slovenia 2Rudjer Bokovic Institute, Zagreb,
Croatia
2
OUTLINE

• Why Si nanostructure is important?
• Experimental
• - Preparation of Si thin films
• - Methods of characterization
• Raman spectroscopy and HRTEM results
  (correlation)
• Conclusions

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PHOTOVOLTAIC SOLAR CELL
4
SOLAR IRRADIANCE SPECTRA
EG nc-Si
EG c-Si
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SILICON THIN FILMS

• Si in different structural forms improved the
  efficiency of devices
• efficient absorption of light in broader
  wavelength interval is achieved

6
Si thin films preparation

• Plasma Enhanced Chemical Vapour Deaposition
  (PECVD)
• - in RF glow discharge
• - using silane (SiH4) diluted with hidrogene
  (H2)
• Different crystallinity and crystallite sizes in
  nc-Si layer
• - changing the RF plasma power density
• - changing the SiH4/H2 ratio.

7
Methods of characterization

• BULK PROBE
• Raman spectroscopy (RS)
• MICRO PROBE
• High Resolution Transmission Electron Microscopy
  (HRTEM)

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Raman bands of Si (cubic, Fd3m)
Intensity normalized to ITO
Wavenumber (cm-1)
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Si active layer
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Raman spectra



Crystal fraction
also HRTEM study
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Analysis of Raman spectra 1. APPROACH fit ? 4
Gaussian 2 Voigt
?TO
Cryst. fract.ATOcr/(ATOamATOcr) Dcrys
const./(??)1/2 ?? 520 cm-1 - ?TO
TO-cryst
TO-amorf.
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Analysis of Raman spectra 2. APPROACH difference
? (amorph.cryst.) amorph.scatter. factor
small crystals
large crystals
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Analysis of Raman spectra 2. APPROACH difference
? (amorph.cryst.) amorph.scatter. factor
small crystals
large crystals
VII VI V IV III II I



also HRTEM study
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HRTEM sample II
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HRTEM sample II
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RS - HRTEM
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HRTEM sample IV
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HRTEM sample IV
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RS - HRTEM
IV (37 cryst.fr.)
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HRTEM sample V
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HRTEM sample V
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RS - HRTEM
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?
?
?
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HRTEM ? RS for large crystallites ?

• estimation from RS suppose sferical crystallites
  
• compressive stress also affect the TO Raman band
  by a blue shift
• Smaller lattice distances (d) are
  observed in HRTEM
• (cubic, Fd3m, Si (JCPDS No. 27-1402), d for
  (111) is 3.13550 Å)

2.961 Å 3.067 Å 2.987 Å 2.923 Å
3.005 Å 3.061 Å 3.052 Å 3.092 Å
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Conclusions

• Reliable structural results only by combining RS
  and HRTEM measurements
• Bimodal distribution of the crystallite sizes
• Crystalline sizes estimated from RS
• excellent agreement with HRTEM for small
  crystallites
• as crystallites are larger the agreement with
  HRTEM are worst
• Explanations
• crystallite shapes
• compressive stress in the crystallites

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Distribution of absorption coefficients

• Thanks to Krunoslav Juraic (Rudjer Bokovic
  Institute, Zagreb, Croatia) for
  UV-VIS-transmitance and reflectance measurements