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Silicon Thin Films Solar Cell: Its Recent Development

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Title: Silicon Thin Films Solar Cell: Its Recent Development


1
Silicon Thin Films Solar Cell Its Recent
Development
  • Swati RayEnergy Research Unit,
  • Indian Association for the Cultivation of
    Science,Jadavpur, Kolkata,
  • India.
  • E-mail ersr_at_iacs.res.in

2
Introduction
  • The primary objective of PV solar cell research
    and development is to reduce the cost of PV
    modules and systems to a level that will be
    competitive with conventional ways of generating
    electric power.
  • At present Solar cell used for large scale
    terrestrial use is generally made up of
    crystalline Silicon. Wafer based c-Si solar cells
    have relatively high efficiencies (12 - 16
    module efficiency).
  • These cells have already proven their excellent
    stability and reliability. The main disadvantage
    associated with the technology is high module
    price.
  • Attempts are being made to reduce the cost of
    c-Si cell by using thinner wafers but still there
    are many difficulties.
  • Development of thin film silicon solar cell
    technology perhaps is the only option.

3
PV Module Production 19902007
1990 47 MW
2007 3700 MW
4
Thin film market growth
5
The advantages of silicon films
  • The fabrication technology is relatively simple
    and inexpensive.
  • The product and process are free from
    environmental hazards.
  • Absorption coefficient of a-SiH is higher than
    that of c-Si. As much less material is required
    for a-SiH cell, they are lighter in weight and
    less expensive.
  • a-Si films can be deposited in one step on a
    large area and on a wide range of substrates,
    including flexible, curved and roll-away type.

6
The 3 generations
Solar Cell Technology
7
Structure of a-Si Solar Cell
Single Junction
Double Junction
8
Multijunction Solar cell
  • Multijunction structure enhances the efficiency
    of solar cell, since different active layers of
    stacked solar cell absorb wider range of solar
    spectrum.
  • Increase of built in field in the tandem cell can
    reduce metastable defect formation and hence
    stability of solar cell increases.

9
The materials developed by PECVD at IACS.
  • Undoped and doped amorphous silicon films
    (1.7-1.8 eV) (a-SiH, n-type SiH, p-type
    SiH, a-SiFH)
  • High band gap alloys of a-SiH, Silicon Carbide
    and Silicon Oxide (1.8-2.2 eV a-SiCH, p-type
    a-SiCH, a-SiOH, n-type a-SiOxH, p-type
    aSiOxH, a-SiCFH etc.)
  • Low band gap alloys of a-SiH specially amorphous
    silicon germanium (1.4 to 1.65eV).
  • Microcrystalline Si thin film and its alloys ( µ
    C-SiH, n-type p-type, µ C-SiC, µ C-SiCFH, n
    and p-type , undoped µ C-SiOxH ).
  • Nanocrystalline and Protocrystalline silicon.
  • Transparent conducting oxides (ZnO, SnO2, ITO)
    by magnetron sputtering method.

10
Status of Efficiencies for Si Thin-Film Solar
Cells at IACS, India
Structure Area Efficiency
Degradation (cm2) (Initial) Single
junction 1.0 10.8 20 -
25 Double junction 1.0
10.7
15 Double junction 4 8.0
stabilised (a-Si/ a-Si)
(Measured at NREL, USA) Triple junction
1.0 9.5
13 (a-Si/a-Si/a-SiGe) Single junction
760 7.5 module
(active area) Double junction
760 7.8 module
(active area)
11
Performance of single junction a-Si solar cell (1
cm1 cm)
Quantum Efficiency
Current Voltage characteristics
12
Spectral response of double junction a-Si solar
cells
13
I-V characteristic of double junction a-Si solar
cells
14
Multichamber PECVD System
15
Laser Scribing System
16
Flow Chart of Entire Process
17
Double Junction a-Si Module Fabricated in the
Prototype Line
Voc 14.82 V Isc 620.6 mA FF
0.627 Eff 7.59 Power 5.77 W
a-Si modules are fabricated with laser scribing
successively on the TCO, p-i-n and metal surfaces.
18
Current Voltage characteristics of 1 ft1ft
module
Quantum Efficiency
19
Light induced degradation of single junction
module (Area 30 cm 30 cm)
20
Stabilized efficiency and power of solar modules
21
Next generation thin film Si solar cell
  • Amorphous silicon solar cell technology is in
    commercial stage. R D on a-Si has reached its
    saturation.
  • During last few years, attention of the
    scientists in the field of Si thin films are
    drawn towards development of Microcrystalline and
    nanocrystalline based thin film solar cells.
  • Double junction cell combining a-Si with mc-Si
    represents very attractive thin film solar cell
    concept.

22
a-Si/mc-Si tandem solar cells by KANEKA
a-Si/mc-Si module, 13.5 (4141 cm2 area)
23
(No Transcript)
24
Present Status of Efficiencies for Si Thin-Film
Solar Cells
25
Striking features of microcrystalline silicon
solar cells
  • Stability of the efficiency under intense light
    soaking.
  • Enhanced extension of the spectral response at
    infrared wavelengths, as compared to a-Si.

Limitations
  • Low deposition rate of microcrystalline silicon
    layer.
  • Low absorption coefficient compared to a-Si.

Possible Solutions
  • High temperature deposition.
  • Use of Very High Frequency PECVD.
  • High pressure silane depleted RF PECVD.

26
Intrinsic micro-Si films
  • With increase of HD from 95 to 95.5 ?d
    increases by 4 order of magnitude indicating
    amorphous to microcrystalline transition in the
    silicon network.
  • At higher Ts transition occurs at a lower HD.

Fig. Variation of dark conductivity,
photosensitivity at different hydrogen dilutions.
(open symbols for Ts 340oC and closed symbol
for Ts 180oC)
27
Roughness (RMS) 8.40 nm
Roughness (RMS) 3.02 nm
AFM of silicon films prepared at 1800C
AFM of silicon films prepared at 3400C
28
Deposition Rate
  • High rate of nc-SiH depositions requires high
    flux of precursor radicals and sufficient flux of
    atomic hydrogen per monolayer deposition.
  • An increase in plasma excitation frequency leads
    to higher power transfer in the plasma, higher
    decomposition of silane, increased ion flux, but
    decrease in electron temperature.
  • Higher power increases growth rate but ion
    bombardment may create damage if deposited
    under low pressure.
  • High working pressure reduces electron
    temperature of the plasma and avoid damage of the
    film surface.

Fig. Deposition rate of Si films prepared at
different plasma excitation frequencies.
29
Structural properties
Fig. Grain size variations of Si films with HD
  • lt111gt and lt311gt orientations of c-Si are observed
    in case of high power deposition (gt0.3 W/cm2)
    whereas only lt220gt peak observed at low power.
  • The largest grain size is 35 nm using RF as well
    as VHF and the lowest size 5 nm is obtained at a
    chamber pressure of 9 Torr.

Fig. XRD spectra of Si films deposited at
different plasma excitation frequencies.
30

Fig. Crystalline volume fraction (Xc)
Fig. Raman spectra of the Si films deposited at
different plasma excitation frequencies.
31
54.24 MHz, 50 W
54.24 MHz, 70 W
32
Effect of p-layer on i-layer structure
i-layer with nanocrystalline p-layer
i-layer without p-layer
33
Light Induced Degradation
  • Degradation is significant even after thousand
    hours of light soaking when films is amorphous.
  • When the films becomes microcrystalline
    degradation is almost nil but the
    photosensitivity is very low (100).
  • For the films deposited at Y 95 98 i.e at
    transition region the films become stable after
    10 hours of light soaking and the
    photosensitivity is reasonably high (100).

Photo conductivities of the Si films after 1000
hours of light soaking
34
Fabrication of mc-Si solar cell
35
Effect of microstructure on the performance of
single junction microcrystalline silicon solar
cell
  • Open circuit voltage is highest at RF-PECVD vhphp
    condition where crystallite size is small.
  • Short circuit current is maximum for 105 MHz
    deposited film may be due to less carrier
    recombination at grain boundary.

36
Performance of solar cells prepared at 54.24 MHz
37
Light induced degradation
38
Fabrication of Si quantum dots
  • Quantum dots offer the potential to control the
    intermediate band energies.
  • Placing the appropriate quantum dot material of
    necessary size into an organized matrix in solar
    cell results in the formation of accessible
    energy levels .
  • Theoretically solar cells with quantum dots
    offer a potential efficiency of 63.2.

39
Nanocrystalline Si in SiOx matrix
Photoluminesence of SiOx films with different
silicon to oxygen ratio.
40
Conclusions
  • The low material cost and large area deposition
    capability of amorphous and nanocrystalline
    silicon alloys make this class of materials an
    attractive candidate for cost effective
    production of solar panels. Based on
    announcements by several manufacturers, Industry
    analysts project a production growth from about
    300MW in 2008 to 5000MW in 2012.
  • In Japan new NEDO program was initiated in June
    2008,aiming at very exciting target of 40
    including Thin film solar cells towards 2050.New
    program is aiming 30 by 2014 using 5 or 6
    junction thin film solar cells.
  • In India, Amorphous Solar cell technology has
    been developed. Work on more stable and efficient
    Nanocrystalline Silicon cell is in progress.
  • Extensive research and development work is
    necessary on 2nd and 3rd generation solar cell
    technology to make Solar PV viable for large
    scale use.

41
Silicon Thin Film Solar Cells Hokuto, Yamanashi
Roof-top Houses (Kubota Ecolony), KANEKA Hybrid
42
Thank you...
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