Department of Energy Science and Engineering
Indian Institute of Technology Bombay
Si Quantum Dots for Solar Cell Applications
Chetan S. Solanki
IRCC Award Talk
18th Aug. 2010
Acknowledgements Dr. Ashish Panchal
Dharmendra Rai
Paresh Kale
Dr. Pravin Narwankar, Applied Materials
Department of Energy Science and Engineering
Applied Materials
Nano-electronics Project
My family
Chetan S Solanki, IIT Bombay 3
Contents Solar PV scenario: World and India
Why Si nanomaterials?- Solar PV potential, Cost and efficiency
Si quantum dots for solar cells
- All-Si multi-junction cells
- Obtaining Si QDs: HWCVD and Porous Si
- Results and Analysis
Conclusion and future work
Generation of Power with Solar Cell• A Solar cell is a device that convert light into electricity
It requires
- Absorption of a photon
- Separation of a electron-hole pair
- Collection of the charges at electrodes
P-N Jn –separation force
Metal contact
Metal contact
• Sun light as input energy
• No moving parts, long life, moderate efficiency
• Solar cell are being manufactured since 1954
Solar Energy Potential in India 300 days of clear sky average 2000 kWh/m2/year
Our capita energy consumption is about 600 kWh/capita/year
India’s annual energy consumption is about 4 trillion kWh/year
India’s solar energy resource is 5000 trillion kWh/year
Characteristics of Solar PV technologies are:
- Clean, maintenance free
- Modular
- Distributed generation
Worldwide PV Module Production
PV market is growing with over 35% rate since last decade
Learning curve for PV is -18%
Solar PV in India
Chetan S Solanki, IIT Bombay 7
Current cell and module manufacturing capacity is about 400 and 750 MWp respectively
Jawaharlal Nehru National Solar Mission (JNNSM) has been announced to promote solar PV electricity generation in India
SIPS scheme for promoting solar PV manufacturing in India
Target is to install 20,000 MW of Solar power in India by 2022 stirred lot of activities in the country
The Mission document mentions NCPRE at IIT Bombay: “setting up of a National Centre for Photovoltaic Research & Education
at IIT Bombay, drawing upon its Department of Energy Science & Engineering and its Centre of Excellence in Nanoelectronics”
Solar PV Module Prices
Source: www.solarbuzz.com
Price fluctuation due to demand-supply balance, increase in production volume
Cost of conventional power is ~1$/Watt
Solar PV technology is expensive
Si Shortage
Challenges to PV Technologies
High price per unit watt (high cost of material)
1.5 to 4 $ / Watt, should be about 1$/Watt
Moderate efficiencies 14 to 16% for c-Si, 6 to 9% for thin film, higher is better
Availability of material Should be abundant
Long term stability Minimum acceptable life is 25 years
Long energy pay back period (high processing cost)
2 to 3 years for c-Si, < 1 year for thin-film, should be low
Long money pay back period Depends on region, 5 to 12 years
Technology Attributes Features
Chetan S Solanki, IIT Bombay 10
Efficiency and Cost
in
scoc
P
FFIVEfficiency =η
• Efficiency is defined as the ratio of energy output from the solar cell to input energy from the sun.
IscI
Vm
Im
Pm
X
Voc
Power
===2
2$
$
mWatt
mWatt
Cost Production cost
Efficiency
Raw material cost, cell and module processing
Quality of material, technology understanding, cell size
Chetan S Solanki, IIT Bombay 11
Photovoltaic Generations: 1st, 2nd & 3rd
1st generation: Si wafer based technologies
3rd generation: Advanced nanostructure based concepts
2nd generation: Thin-film technologies
0 100 200 300 400 500
Cost, US$/m2
80
60
40
20
Effi
cien
c y, %
100
US$0.10/W US$0.20/W US$0.50/W
US$1.0/W
US$3.50/W
III
III
Ref: M.A Green, Progress In Photovoltaics, 9 (2000) 123.
Chetan S Solanki, IIT Bombay 12
Si is good but expensive
How to minimize Si consumption?
Chetan S Solanki, IIT Bombay 13
Si is Expensive
Metallurgical grade Si (MGS)
Initial
ReactionCholorosilanes
Separation and
purificationPure SiHCl3
Deposit solid Si
H2
Pu
re p
oly
-E
GS
EGS ingot
Grow single crystal
Si wafers
HC
l
Solid
Gas Gas
Solid
LiquidSolid
QuarziteCoal +Liquid
Melting
Solid
Pu
re p
oly
-E
GS
Chetan S Solanki, IIT Bombay 14
Approaches to Si PV cost reduction
High cost due to large volume consumption of high purity material
Si
Concentrators
Thin film C-Si
Nanostructures of Si
Chetan S Solanki, IIT Bombay 15
Contents Why Si nanomaterials
- Solar PV potential- Cost and efficiency
Si nanomaterials for solar cells
- All-Si multi-junction cells
- Obtaining Si QDs: HWCVD and Porous Si
- Results and Analysis
Conclusion and future work
Chetan S Solanki, IIT Bombay 16
Concept of multi-junction cells
How to obtain higher efficiency?
Chetan S Solanki, IIT Bombay 17
Single junction cells are inefficient
200 J 50 J100 J 100 J
Required work
Chetan S Solanki, IIT Bombay 18
18
Single junction cells are inefficient
Losses in energy conversion inefficient utilization of solar spectrum
1-transmission, 4- contact losses
2-thermalization, 5. recombination losses
3-junctionn loss
23%
33%
energyPhoton
doneWork
EffPhoton
=
.
Eg
Electrons
Holes
Photon efficiency is different for different energy
Chetan S Solanki, IIT Bombay 19
Strategies for higher efficiencyApproach- I Band gap matching with spectrum (splitting spectrum over several
materials/cells)• tandem cells • intermediate band cells
Approach- III: Reshaping the solar spectrum
(Adopting Solar spectrum for one host material)
up- and down-conversion
Approach- II: Reduced thermalization losses (adopting one host material for solar spectrum)
hot electron cells
0.5 1 1.5 2 2.5Wavelength (µm)
0.5
1.0
1.5
Su
nlig
ht
inte
ns i
ty (
kW/m
2/µ
m)
Multi-junction cell approach
Chetan S Solanki, IIT Bombay 20
All Si Multi-junction solar cells
How to change band gap?
Chetan S Solanki, IIT Bombay 21
Quantum confinement and Band gap
D Si D Si D Si
Quantum confinement occurs when the crystal size becomes less than the Bhor exciton radius (4.9 nm for Si)
SiO2, SiN, SiC could be used as possible dielectric matrix
Band gap of the Si QD depends on size of the dots and quantum confinement parameter
Effective band gap
Chetan S Solanki, IIT Bombay 22
Wide band gap material
Top cell Middle cell c-Si cell
Si-QDs for PV applications
Si-QD
2 eV 1.5 eV 1.1 eV
Multi-junction solar cell of Si with control over Si-QD size is possible
Theoretical efficiency of triple junction cell is about 64%
dEh
mTe
∆−=∗
2
8exp16
Tunneling Probability
2)()(
d
kbulkEeVE gg +=
Band gap variation
Si QDs using porous SiTop down approach
Two approaches for obtaining Si QDs:
- Porous Si
- Thin film deposition (HWCVD)
Chetan S Solanki, IIT Bombay
Porosity, HF conc. & Current Density
HF solution
Silicon
H2
x = 0
x
ft(t)
Porosity (p) as a function of HF conc. and current density
Provides control over size of nanoparticles
Larger porosity results in smaller Si particle size
HFHF
HF
HF
15
35
55
75
0 50 100 150
Current Density (mA/cm2)
Po
rosi
ty (
%)
10%
12.5%
35%
25%
16.6%
HF
Chetan S Solanki, IIT Bombay
Porous Silicon (PS) Thin Films
Porous Silicon films between 5 to 15 micrometers are obtained
Time (min or sec)
Current Density(mA / cm2)
t1 t2
J1
J2
Pores
Pore diameter can be from nano-meter to micro-meter range
Use of heavily doped P-type Si results in nano-porous Si
Chetan S Solanki, IIT Bombay
2 Hr Sonication.
Particle size in range of
10nm.
TEM results
100 nm
Plan is:
Dispersion of Si particle in suitable dielectric
Spinning on substrate to deposit Si-QD layer
Control over particle size
Distribution of particles
Chetan S Solanki, IIT Bombay
Chetan S Solanki, IIT Bombay 27
Si Quantum dots using HWCVD
Bottom up approach
(Structural and optical characterization)
Hot Wire CVD
Wafer size 2 inch
Substrate temperature Room temperature to 800oC
Filament temperature Up to 2000oC
Gases SiH4, NH3, H2, B2H6, N2
Chamber pressure Up to 10-7 mbar
Distance between filament and substrate
3-5 cm
• SiH4 & NH3 cracked at hot filament
• Gas utilization:
~10% in PECVD
~80% in HWCVD
• Deposition rate as high as 2 nm/sec
5 cm
substrate
hot filamentsilane
ammonia
If
Principle of HWCVD
Chetan S Solanki, IIT Bombay
Variation in HWCVD parameters
Depo. parameters SiNx deposition Si-nS deposition
Basic Pressure ~ 10-6 mbar ~ 10-6 mbar
Gas pressure ~ 10-2 mbar ~ 10-3 mbar
Filament Temp. 1900oC 1900oC
Substrate Temp. 250oC 200-400-600-650-700-750oC
Gas Flow 1:20 sccm (SiH4:NH3) 1-2-4 sccm (SiH4)
Dep. Time 75 sec 60-40-20 sec
Deposition of Si-nS with variation in parameters to :
Substrate temperature
SiH4 & NH3 gas flow rate
Deposition time
to see the effects in Si-nS size
• With increasing above parameters, the Si-nS size expected to increase
Superlattice of SiNx/a-Si Using the optimized conditions 40 alternate layers of SiNx and a-Si are deposited
1 deposition takes about 5 hr
A novel deposition technique is developed where the flow of NH3 is interrupted for pre-determined time
Substrate
SiNxa-Si
SiNxa-Si
SiNxa-Si
40 layers
Substrate
SiNx
SiNx
SiNx40 layers
Annealinga-Si
a-Si
a-Si
Chetan S Solanki, IIT Bombay
Effect of annealing Temp.: 1st order Raman spectra
Annealing temperature of sample
(oC)
a-Si peak position
(cm-1)
Si-QD peak position
(cm-1)
Intensity ratio for 2nd order
Raman spectra
As-deposited 482 505 -
800 495 507 0.52
850 498 509 0.61
900 500 508 0.83
950 disappeared making shoulder
0.88
With increasing annealing temp.- a-Si phase reduces - Asymmetric shoulder appears
2nd order Raman spectra
Regain c-Si shape & increased intensity
Asymmetric shift & reduced intensity
With increasing annealing temp.
the 2nd order Raman peak for Si increases, shows the growth of Si-QD
Chetan S Solanki, IIT Bombay
PL Analysis
• Room temp. PL performed with He-Cd laser 325 nm
• PL peak shifts from blue to red wavelength as the Si-QD size increases as annealing temp. increases
• Estimated band gap between 2.3 to 2.6 eV
PL
inte
nsity
(a
.u.)
• With PL presence of quantum dots can be established
Chetan S Solanki, IIT Bombay
TEM analysis
a-Si/SiNx multilayer as-deposited
at 250oC and annealed at 850oC
for 30 min.
40 alternate layers are deposited
Thickness of layers in 4 to 6 nm
range
Optimized deposition and
annealing conditions were used
SIMS analysis is done for Si/N
variation in layers
Chetan S Solanki, IIT Bombay
TEM: as deposited multilayer
as-deposited a-Si/SiNx ML at 250oC
No Si-QD formation
Chetan S Solanki, IIT Bombay
Annealing of ML @850oC
annealed a-Si/SiNx multilayer at 850oC
Si-QD with av. Inter-dot distance < 5 nm
Optimized temp. for Si-QD growth
No Si precipitation in SiNx layer after annealing
Chetan S Solanki, IIT Bombay
Annealing of ML @900oC
annealed a-Si/SiNx multilayer at 900oC
Inter-dot distance < 1 nmmaking continuous nC Si film
Chetan S Solanki, IIT Bombay
38
Effect of deposition time of a-Si layer
A60AS
A20AS
A40AS
PL HRTEM
A20AS
A40AS
A60AS
a-Si ~3.0 nm
a-Si ~3.47 nm
No ML
2.44 eV
2.67 eV
At smaller deposition time, there is no continuity in a-Si layers
39
Solar cell with Si-QD/SiNx ML
QD
SiNx layer
n-Si substrate
Back Al
Front Al
i-type Si-QD/SiNx
p-type a-Si layer (30 nm)
Front Al
QD
QD
QD
QD
Solar cell device structure with Si quantum dots
Whole device fabricated in HWCVD
40
Cell with A60AS and A60AN850 40 ML
Voc
(mV)
Isc
(pA)
80 6.37
70 34.3
50 50
340 804
90 1.17
20 1.23
Hydrogenation for 40 min at 420oC after Al contact evaporation40 Si-QD/SiNx ML
as-deposited on
different devices
illuminated
40 Si-QD/SiNx ML
annealed gives no
I-V characteristics
with illumination
Best cell
Conclusion and Future Work Successful formation of less than 10 nm Si dots using PS
Successful development of alternate 40 layers of a-Si/SiNx in single HWCVD
Formation of Si-QD 3-5 nm with optimized annealing temp. 850oC
No Si-QD formed in as-deposited samples
Charge transport increased due to light shining on the device
Best cell with 340 mV of open circuit voltage
Effect of band gap enhancement on open circuit voltage is yet to be demonstrated
Current transport through quantum dot structure is under investigation
Chetan S Solanki, IIT Bombay 42
Thank you for your
attention
“It isn't new energy that will make such a difference in the next millennium. The power to run ……………... It's new technology that will bring proven ways of generating power ………………..”