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The nanostructure of silicon thin films for solar cells

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The nanostructure of silicon thin films for solar cells – PowerPoint PPT presentation

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Title: 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

3
PHOTOVOLTAIC SOLAR CELL
4
SOLAR IRRADIANCE SPECTRA
EG nc-Si
EG c-Si
5
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)

8
Raman bands of Si (cubic, Fd3m)
Intensity normalized to ITO
Wavenumber (cm-1)
9
Si active layer
10
Raman spectra



Crystal fraction
also HRTEM study
11
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.
12
Analysis of Raman spectra 2. APPROACH difference
? (amorph.cryst.) amorph.scatter. factor
small crystals
large crystals
13
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
14
HRTEM sample II
15
HRTEM sample II
16
RS - HRTEM
17
HRTEM sample IV
18
HRTEM sample IV
19
RS - HRTEM
IV (37 cryst.fr.)
20
HRTEM sample V
21
HRTEM sample V
22
RS - HRTEM
23
?
?
?
24
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
25
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

26
Distribution of absorption coefficients
  • Thanks to Krunoslav Juraic (Rudjer Bokovic
    Institute, Zagreb, Croatia) for
    UV-VIS-transmitance and reflectance measurements
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