Application of Nanocrystalline Silicon in
Third generation thin film Solar cells
Swati Ray
Indian Association for the Cultivation of Science
Kolkata-700032, India.
Solar Cell Technology
US$0.10/W
100
US$0.20/W
US$0.50/W
Thermodynamic
limit
Efficiency,%
80
Thin-film
60
US$1.00/W
40
III
Present limit
20
I
II
0
100
200
300
US$3.50/W
400
Cost, US$/m2
500
Introduction:
Third generation photovoltaics have been developed to overcome
the efficiency limit of conventional solar cell. According to Shockley
and Queisser model efficiency of standard cell is limited to 31%.For
3rd generation solar cell targeted efficiency is more than 40%.
A possible route to overcome the loss of energy by photogenerated
carriers is to use of multilayer
solar cell consisting of absorber
layers with different bandgap.
 An alternative way is to engineer wide bandgap materials by
utilizing quantum size effects in silicon based nanocrystalline
materials.
Multichamber PECVD System
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.
Nanocrystalline/microcrystalline based solar cells:
Back Reflector
c-Si
Bottom Cell
Interlayer
a-Si Top Cell
Textured TCO
Glass Sub
Sun-Light
a-Si/mc-Si module, 13.5%
(4141 cm2
area)
98%
Hydrogen
95%
Hydrogen
Raman spectra:
XC=60%
Intensity (a.u.)
Intensity (a.u.)
XC=62%
400
440
480
520
-1
Wave number (cm )
Power= 100 Watt
560
400
440
480
520
-1
Wave number (cm )
Power= 270 Watt
560
Effect of microstructure on the performance of
single junction microcrystalline silicon solar cell
Depositio
n
condition
Voc
(V)
Jsc
(mA/cm2)
FF

(%)
Crystallite
size
(nm)
Xc
(%)
RF
(vhphp)
0.64
16.2
0.63
6.5
6.7
65
RF
(vhphp)
0.58
19.5
0.62
7.01
8.1
78
VHF
(105 MHz)
0.55
20.7
0.59
6.7
15
45
54.24 MHz
0.52
19.8
0.64
6.6
12
64
Light induced degradation
0
Fc~48%
Solar cell
00
-3
Light soaking time (hr)
500
TF
400
00
300
-2
200
TF
100
00
0
-1
5.0
2
TF
5.2
4
00
Xc=85%
0
-3
5.4
2
TF
5.6
4
00
5.8
0
-2
6.0
TF
Xc=62%
2
-1
00
6.2
0
4

Efficiency (%)
6.4
2
TF
Xc=59%
Jsc
6.6
FF
% of degradation in 500 hr
6.8
Voc
4
Fc~60%
Silicon Quantum Dot Solar cell:
The
ultimate
objective
is
to
enhance
the
efficiencies of solar cell by making use of quantum
size effect. Quantum confinement occurs when size
of the nanocrystals in a high bandgap dielectric
matrix is comparable to the Bohr radius (~5 nm for
Si) in bulk c-Si.
Fabrication of Si quantum dots
SiOx, SiyNx, SiCx
SiO2, Si3N4, SiC
SiOx, SiyNx, SiCx
SiO2, Si3N4, SiC
100nm

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 more than 40%.
Schematic of superlattice structure
Up to n layers
Up to n layers
SiOx
SiO2
SiOx
SiO2
SiOx
Annealed
at 1000 oC
SiO2
SiO2
Substrate
Substrate
 The SiOx/SiO2 superlattice thickness depends on the thickness of each SiOx layer and
SiO2 layer, and is also related to the number of periods (n) of the superlattice. In our
case n = 50. The SiO2 layer was about 2 nm and thickness of the SiOx layer was varied
from 3 to 7 nm.
 The structure of the bilayer which consists of a SiOx and SiO2 is relaxed. At higher
temperatures, the annealing of the amorphous SiOx films results in a phase separations
described by:
2 SiOx
x SiO2 + (2-x)Si (x changes from 0 to 2)
High resolution transmission electron microscopy
SRSO films deposited at Y = 2.0
 In SRSO films nano crystalline silicon quantum dots of average size 5 – 8 nm
surrounded by amorphous silicon oxide network have been observed.
Photoluminescence
 Presence of
visible PL peak at
room temperature of SRSO materials
can be explained by the quantum
confinement effect theory.
400
1
PL Intensity
300
200
 PL band ~730 nm is due to nanometer silicon grains embeded in SiOx
network .
2
3
100
0
700
710
720
730
740
Wavelength (nm)
1. Co = 8.3 at%
2. Co = 9.3 at%
3. Co = 10.7 at%
750
760
 With increase of oxygen content
oxygen rich region increases and
number density of Si nanocrystals
decreases. Thus the PL intensity
decreases.
Raman study of superlattice structure
o
Superlattice annealed at 800 C
Super lattice as-deposited
SiOx = 3 nm
3nm
5nm
7nm
SiOx = 5 nm
Intensity (a.u.)
Intensity (a.u.)
200
400
600
800
1000
Raman shift (cm-1)
SiOx = 7 nm
1200
1400
FWHM
3nm: 5.386
5nm: 5.36
7nm: 5.00
460
480
500
520
540
Raman shift (cm-1)
Quantum dot size was calculated from the formula: dRaman = 2π(B/ω)
B= 2.0 cm-1; ω= peak shift for nc-Si:H compared to c-Si
560
Raman studies:
2
1: SL-1
2: SL-2
3: SL-3
Intensity (a.u)
Intensity (a.u)
3
2
3
1
1
400
400
4
1 : SL-1
2 : SL-2
3 : SL-3
4 : SL-4
420
440
460
480
500
520
Raman shift (cm-1)
Superlattice annealed at
540
800oC
560
420
440
460
480
500
520
540
560
Raman shift (cm-1)
Superlattice annealed at 900oC
 For the sample SL-1, annealed at 800oC, a-Si phase dominates and a weak
signal of nc-Si has been detected near 515 cm-1. In SL-1, N2 is incorporated both in
SiOx and SiO2 layer.
 In case of SL-2 and SL-3 sharp peaks were observed indicating highly crystalline
growth. In these two SL’s N2 has been incorporated only in SiO2 layers. In SL-3
SiOx sublayer thickness is 3 nm whereas it is 5 nm for SL-2.
X-ray diffraction studies:
<111>
<111>
3
<220> <311>
3
2
1
1
30
40
50
60
70
 degree
Superlattice annealed at 800oC
: SL-1
: SL2
: SL-3
: SL-4
<220>
<311>
4
2
20
1
2
3
4
: SL-1
: SL-2
: SL-3
Intensity (a.u)
Intensity (a.u)
1
2
3
20
30
40
50 60
 degree
70
Superlattice annealed at 900oC
 When annealed at 800oC, SL-1 does not show any peak as N2 is incorporated in
both SiOx and SiO2 sublayers. SL-2 and SL-3 have signature of <111>, <220> and
<311> peaks of c-Si. When SL-1 was annealed at 900oC the three peaks with low
intensity appeared.
High resolution transmission electron microscopy:
SL-3 annealed 800oC
SL-3 annealed 800oC
SL-2 annealed 900oC
SL-2 annealed 800oC
 Nanocrystalline silcons are formed
in the amorphous silicon oxide layer
with average size of 4 – 6 nm in
diameter.
SL-3 annealed 800oC
SL-2 annealed 900oC
SL-2 annealed 900oC
SL-2 annealed 900oC
Fabrication of P-I-N Structure using a Si-QDSL
Konagai et al fabricated the solar cells using Si-QDSL as a light absorption layer. From the
I-V characteristics, the open circuit voltage and short circuit current were estimated to
be 165 mV and 1.3 x 10-2 mA/cm2.
Later using N-containing Si-QDSL as an intrinsic layer and after passivation of interfaces Voc
increased to 389mV and then 518mV.
n-type Si-QDSL/p-type c-Si photovoltaic device
The best cell parameters
obtained by Green et al
were an open-circuit
voltage Voc of 556 mV,
short-circuit current Jsc of
29.8 mA/cm2, fill factor FF
of 63.8 %, and conversion
efficiency of 10.6 % from 3
nm Si-QDs with a 2 nm
SiO2
layer.
The target structure of the all silicon tandem cell
based on Si-QDSLs is as follows
Conclusions:
 High efficiency can be realized using multijunction Si solar cell
structure.
 The electrical and optical properties of material change when the size
of the crystal becomes less than the Bohr excitation radius.
 SiOx/ SiO2 or SiyNx / Si3N4 superlattices can been developed by
RFPECVD technique by varying the deposition conditions and
annealing temperatures.
 Presence of nanocrystals have been confirmed from Raman study,
high resolution transmission electron microscopy and X-ray
diffraction studies.
 Ultimately the QDSL is a potential material for high efficiency low
cost solar cell. To obtain high performance, extensive research work is
needed and it will take sometime to realize high efficiency solar cell
containing the third generation concepts.