Title: The nanostructure of silicon thin films for solar cells
1The 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
2OUTLINE
- Why Si nanostructure is important?
- Experimental
- - Preparation of Si thin films
- - Methods of characterization
- Raman spectroscopy and HRTEM results
(correlation) - Conclusions
3PHOTOVOLTAIC SOLAR CELL
4SOLAR IRRADIANCE SPECTRA
EG nc-Si
EG c-Si
5SILICON THIN FILMS
- Si in different structural forms improved the
efficiency of devices - efficient absorption of light in broader
wavelength interval is achieved
6Si 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.
7Methods of characterization
- BULK PROBE
- Raman spectroscopy (RS)
- MICRO PROBE
- High Resolution Transmission Electron Microscopy
(HRTEM)
8Raman bands of Si (cubic, Fd3m)
Intensity normalized to ITO
Wavenumber (cm-1)
9Si active layer
10 Raman spectra
Crystal fraction
also HRTEM study
11Analysis 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.
12Analysis of Raman spectra 2. APPROACH difference
? (amorph.cryst.) amorph.scatter. factor
small crystals
large crystals
13Analysis 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
14HRTEM sample II
15HRTEM sample II
16RS - HRTEM
17HRTEM sample IV
18HRTEM sample IV
19RS - HRTEM
IV (37 cryst.fr.)
20HRTEM sample V
21HRTEM sample V
22RS - HRTEM
23?
?
?
24HRTEM ? 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
25Conclusions
- 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
26Distribution of absorption coefficients
- Thanks to Krunoslav Juraic (Rudjer Bokovic
Institute, Zagreb, Croatia) for
UV-VIS-transmitance and reflectance measurements