lecture 34 spt dec
TRANSCRIPT
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Solar Photovoltaic
Technologies
Prof. C.S. SolankiEnergy Systems Engineering
IIT Bombay
Lecture-34
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Contents
Brief summary of the previous lecture
Various Thin film solar cell technologies
Low temperature deposition
High temperature deposition
Solar cell structures
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Classification of different approaches
A large number of different technologies are under parallel
development
A classification can be made based on different criteria:
According to Tmax during layer formation
According to grain size
According to cell structure
The R&D on the high-temperature routes is mainly driven
by considerations from classical bulk Si cells
Proven high efficiency and stability
The R&D on the low-temperature routes is mainly driven
by considerations from a-Si:H solar cells
low thermal budget processing
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Crystalline Si films: depositiontemperature
200 400 600 800 1000 1200 1400
KanekaPECVD
SanyoSPC
CanonVHF-
PECVD
ECNLPE
IMEC,CNRS-PHASE
CVD
ISE,MITSUBISHIZMR + CVD
Neuchtel,JlichVHF-
PECVD
(oC)
Low temperature deposition :
micro-crystalline Si ( g
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Crystalline Si materials
Type of Silicon Abbreviation Crystal SizeRange DepositionMethod
Single-crystalsilicon
sc-Si >10cm Czochralski,float zone
Multicrystallinesilicon mc-Si 1mm-10cm Cast, sheet,ribbon
Polycrystallinesilicon
pc-Si 0.1mm-1mm Chemical-vapor deposition
Microcrystallinesilicon
mc-Si
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Basic components of crystalline Si solarcells
Substrate
Active layer, 5 to50 m
EmitterARC
Diffusionbarrier
Base contact, if substrate isconductive
Substrate can be non-conductive, in that case both
the contact is taken from the front side
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Solar cell structures
p-type
n-type
1. Homo-junction solar cell
for instance Mono-crystalline andmulti-cystalline Si solar cells
p-type
n-type
2. Hetero-junction solar cellp-type and n-type are different material
more material choices some materialcan either be p-type or n-type
used for material (thin-films) thatabsorbs light better than Si
low series resistance window layercan be heavily doped
CdTe and CIS are the examples
in CdTe cell, CdS is used as windowlayer
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Solar cell structures
i-layer
n-type
p-type
3. p-i-n / n-i-p solar cell
Based on drift rather than diffusion
Absorption take place in thicker intrinsiclayer
p-type a-Si / int a-Si / n-type a-Si
4. Multijunction solar cell
Also called Tandem cells
Can acieve high efficiency by capturing
larger part of the spectrum individual cells with different bandgapsare stacked on top of one another
Mechanically stacked and Monolithic
Eg1 > Eg2 > Eg3
Eg1
Eg2
Eg3
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Diffusion vs drift in thin films
i
bidrift
L
VEL
q
kTDLdiff
High quality Crystalline-Si uses p-n junction
Carrier are transported by diffusion to the junction largediffusion length
junction is very thin
diffdrift LL *10
Low-quality material should use p-i-n structure
Diffusion length are small
Drift length is about 10 times greater than diffusion length
intrinsic layer is thicker
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Light trapping
"light trapping" in which the optical path length is several times the
actual device thickness Light trapping is usually achieved by changing the angle at which lighttravels in the solar cell texturing reduces reflection and increases optical path length
Following schemes are used for light trapping
2211sinsin nnSnells law
substrate substrate substrate
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Deposition techniques
Physical vapor depositionVacuum evaporationSputtering
Chemical deposition Chemical vapor deposition
(CVD)
Hot wire CVD
Plasma enhanced CVD Electro-deposition
Spray pyrolysis
Liquid phase deposition
Liquid phase epitaxy
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Contents
Motivation Different thin-film solar cell technologies
Why crystalline Si films?
Classification based on grain size
Thin-film solar cell structures
Deposition techniques
low temperature
High temperature approachesMono-crystalline Si thin films
Other concepts
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Low-temperature approaches
Property
Deposition temperature 200 550oC
Deposition technologies Plasma-enhanced (PECVD, VHF-PECVD,
microwave, ECR)Hot-wire CVD
Solid Phase crystallisation of a-Si:H
Si-precursor SiH4Dilution with H2 is necessary for PECVD
microcrystalline Si
Deposition rate 0.11 nm/s
1 nm/s (Kaneka), mostly below 0.5 nm/s
Cell structure Mostly p-i-n
Dual junction: Micromorph
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Low-temperature processes
Technology Main R&D-players Features / results
PECVDVHF-PECVD
IPV-Juelich
Neuchatel
(VHF-Technologies)
Kaneka Solartech
Pacific Solar
Systems (13.56, 27.12, 40.28 MHz, 4-chamber, 6-chamber system, 30x30 cm2)Micromorph cell: > 13%
Micromorph cell: 12%
Module: 9%
Micromorph cell: 14.5%Micromorph module: 10%
Module (30x30 cm2): 7%
!p-n polycrystalline Si solar cell!
Hot-wire CVD University UtrechtIPV-Juelich
Micromorph cell on stainless steel: 8%
Solid Phasecrystallisation
Sanyo Staring from n+-a-Si:H/a-Si:H-layerWith p+-a-Si:H HIT-emitter: 9.2%
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Low-temperature approaches: Strength/Weakness
Pros
Substrate Compatible with glass
Plastic
Efficiency Micromorph cell concept compatiblewith 15%
Upscalability Upscalability up to 1 m2 modulesseems feasible with cost < 1$/Wp
Cons
Deposition rate Best efficiencies are obtained withrates below 1 m/h
Stability Topcell degradation?
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Low temperature deposition:c-Si films
Features
Grain size ~ 100nm
Temperature < 600 C
Small Minority carrier diffusion length< I micron
P-I-N structure,
~ 10 %
Deposition techniques Solid phase crystallization
Plasma enhanced CVD
Hot wire CVD
Sputtering +
Metal induced crystallization
Substrates glass
SnO2/ZnO coated glass
metal : stainless steel
Back contact
c-Si i layer
front contactTCO
Glass/metal
n layer
p layer
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High-temperature approaches
Property
Deposition temperature 900 1300oC
Deposition technologies CVD
Solution Growth
ElectrodepositionChemical Vapor Transport (CVT)
IMEC, PHASE
ECN, Stuttgart
NRELNREL
Si-precursor SiH4, SiH2Cl2, SiHCl3
Deposition rate 1 10 m/min
Cell structure Always p-n Epitaxial cells
Interdigitated cells onnon-conductivesubstrate
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Technology Main R&D-players Features / results
CVD
RTCVD
Continuous CVD
IMEC, ISE,Stuttgart
PHASE
ISE
Monocrystalline epitaxial cells: 17.8%Multicrystalline epitaxial cells: 14%Polycrystalline Si solar cells: 6%
Chemical Vapor
Transport
NREL Based on iodine as transporting agent
Efficiency < 2%Solution Growth /Liquid PhaseEpitaxy
MPI-StuttgartECNUNSWANU
Monocrystalline epitaxial cells: 17.4%Multicrystalline epitaxial cells: 15%
Electrodeposition NREL Made from molten salts of Si
High-temperature approaches
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Pros
Efficiency High efficiency proven10.5% on Si:SiC, 8-9% on mullite, SiN,
Homogeneity/reproducibility
Because of the extreme conditions, small deviations duringrecrystallisation (thickness of ceramic, change in thermalproperties) can lead to unstable solidification and increaseddefect densities
Cons
Substrate Only very high-temperature resistant substrates: Si, SiN, mullite,Al2O3 Very strong requirements on TEC-match and purity Thick blocking layers
Process Rather complex process ( 4 additional steps to realise activelayer on ceramic)
Upscalability Quality of Si-layers, subjected to ZMR, decreases at
recrystallisation speeds above 10 cm/min
Zone Melting Recrystallisation
Hi h t t d iti
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High temperature deposition:Poly-Si films
Features
Grain size up to ~ several microns
Temperature > 600 C
diffusion length ~ 10s of microns
P-N structure,
~ 11 %
Deposition techniques:
Thermal CVD
Ion assisted deposition
Liquid phase epitaxy
(Zone melting recrystallization)
Substrate requirements
Cost-effective
Heat resistant
Chemically inert
Thermal expansion co-efficientmatching
Substrates: Alumina, mullite,graphite, low-cost Si
Diffusion barrier: SiC, oxide/nitride
Back contact
Front contactARC
Epi-Si filmp+
p
n+
Ceramic substrate
Diffusion barrier
CVD si layer
Conducting substrate
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Monocrystalline Si thin films Best possible thin-film solar cell performance
Thin mc-Si films are obtained using Layer transfer processes
Starting substrate is a Siwafer
surface conditioning forforming separation layer
thin-film transferto aforeign substrate
recycling of starting Sisubstrate
Device fabrication
How to form a separation layer?Example-2Intermediate Oxide layer
Example-1
Hydrogen implantation
Si
Si
Example-3
Porous Silicon layer
Si
Si
P Sili L T f
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Porous Silicon Layer Transfer(PSLT)
PSLT processes
ELTRAN (Canon, Japan)
SPS (Sony, Japan)
PSI (ZAE, Germany)
QMS (IPE, Germany)
LAST (IMEC, Belgium)
FMS (IMEC, Belgium)
High monocrystalline Si layer can be deposited.
Substrate can be re-used several times.
Porous silicon serves two purposes
Pores
Anodization of Si in HF results in the formation ofporous silicon, columns of Si etched out (p+ Si).
Layer porosity is a function of anodization parameters.
What is porous silicon ?
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Integral Steps of PSLT
Porous silicon formation
Silicon
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SiliconSilicon
Integral Steps of PSLT
Porous silicon formation
Active layer deposition
- Annealing
- CVD epitaxial layer
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Integral Steps of PSLT
Porous silicon formation
Active layer deposition
Device fabrication
Silicon
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Integral Steps of PSLT
Porous silicon formation
Active layer deposition
Device fabrication
Layer separation and transfer to
foreign substrate
Silicon
Substrate
Film Separation
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Film Separation:
One-step anodization
Porous SiliconFilm
Silicon Substrate
Porous Silicon Film
20 m film
Features Homogeneous film thickness
Film thickness from few microns toseveral tens of microns
Film area is limited byexperimental set up
Film thickness is functionanodization parameters
Separation occurs for limited set ofparameters
US patent # 6649485
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Silicon ingot
Electrolyte
Pt electrode
Continuous production of films
HF conc. resumes at the surface after film separation
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Silicon ingot
Pt electrode
Electrolyte
Continuous production of films
HF conc. resumes at the surface after film separation
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Continuous production of films
Silicon ingot
Pt electrode
Electrolyte
Porous silicon
films
HF conc. resumes at the surface after film separation
Patent pending
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FMS cell process
FMS (Freestanding Mono-Si) solar
cellsPS Film
Epitaxial layer
Epi layer after PS
removal
Two-side contactedcell structure
PS Film
Epi layer
Emitter
0
0.2
0.4
0.6
0.8
1
400 600 800 1000 1200Wavelength (nm)
IQE
FMS -1
FMS -2
Ref-20um
IQE analysis
0
5
10
15
20
25
30
35
0 0.2 0.4 0.6Voltage (Volts)
Current(mA/cm
2)
Voc: 602.6 Volts
Isc: 33.12 mA/cm2
FF: 60.18
Eff.: 12.01%
Area: 0.65 cm2
Film thickness: 20 m
I-V curve
Patent pending
Device is ready
9.6% FMScell with HITemitter
Oth
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Other processes:
a-Si/c-Si hetero-junction
This configuration has the following advantages:
potential for high efficiency;low processing temperatures.
low thermal budget for processing. Reduction of
energy pay back time;
Epi layer, p-type
Epi layer, p+ type Al backcontact
int. a-Si:H
n+, a-SiH
Front contactITO layer Combination of low
production cost ofamorphous cell
technology and high
efficiency of Mono-
crystalline Si cell
technology
Bandgaps: a-Si1.7 to
1.8 eV, C-Si 1.12eV
Oth :
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Other processes:Aluminum induced crystallization
for growing polycrystalline silicon (poly-Si) films on inexpensive glass
Films is formed by aluminum-induced crystallization (AIC) of amorphoussilicon (a-Si)
Annealing transforms an initial glass/Al/a-Si stack into a glass/poly-Si/Al(Si) below the eutectic temperature of the Al/Si system (Teu=577 C).
The poly-Si forms a continuous layer which consists of large grains with apreferential (1 0 0) orientation
Al
a-Si
Glass
Alcrystalline-Si
Glass
annealing
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Conclusions
Thin-film crystalline Si solar cells represent obvious way
to reduce costs PV
A large number of techniques are under investigation
There is a certain risk for subcritical R&D in this field
Crucial issues are clear:
Low-T techniques: increase of growth rate High-T techniques
Availability of ceramic substrate
Increase of recrystallisation speed for process
Improvement of nucleation control for process
without ZMR
On all of these questions there is a considerable R&D-activity
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Concentrator PV
systems
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Components of CPV systems
Solar cell
Heat sink
1 - Light collector
2 Solar Cell
3 Heat Sink
4 Sun tracker
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Light collectors
Refraction andreflection
Concentrationratio
Line focus &
point focus
Imaging & non-imaging
concentrator
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Multi-junction solar cells Bandgap engineering
Materials are manipulated to adjust the bandgap accordingto solar spectrum
Double and triple-junction solar cells
InGaP/GaAs/Ge on Ge substrate
0.5 1 1.5 2 2.5
Wavelength (m)
0.5
1.0
1.5
Sunlightintensity
(kW
/m2/m)
Eg1 > Eg2 > Eg3
Eg1
Eg2
Eg3
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Multi-junction solar cells
- 37.3 % (concentration@175 Suns, 2004) Worldrecord efficiency bySpectrolab
- 13% with 6 junction
Bandgap GaInP2 - 1.89eV
GaAs 1.42 eV
Ge 0.67 eV
Design challenge is to match current fromeach cell
Higher number of junction can achieve higherefficacies 40% is target by 2006
Potentially 45% by 2010 (Spectrolab)
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Comparison of technologies
Material t/ Disadvantages Advantages, perspectives
Mono-Si
300
m,15 -18 %
Lengthy productionprocedure, wafersawing necessary
Best researched solar cellmaterial in a next few years it
will dominate world market,especially there, wherehigh power/area ratio isrequired
Multi-c Si
300
m,13 -
15 %
lengthierproductionprocedure, wafer
sawing necessary
The most importantproduction procedure at
least for the next ten years
Polyc-Si
Transpare
nt
300m,10 %
Lower efficiency,special proceduresto achieve opticaltransparency
required
Attractive solar cells for differentBIPV applications. Possiblealso production of double sided
cells
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Comparison of technologies
Material t / Disadvantages
Advantages, perspectives
EFG
250m,
14 %
Limited use
of this
productionprocedure
Very fast crystal growth, nowafer sawing necessary,
significant decrease in productioncosts possible in the future
Riboon-Si
300m,
12 %
Limited use ofthis productionprocedure
No wafer sawing necessary,significant decrease in productioncosts possible in the future
a-Si
1 m,5 - 8%
Lower
efficiency,shorter lifespan.
No sawing necessary, possibleproduction in the flexible form. Itis a promising material in thefuture if long-term stability
increases
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Comparison of technologies
Material t / Disadvantages Advantages, perspectives
CdTe
2-3 m ,6 - 9 %
(mod.)
Poisonousraw
materials
Significant decrease inproduction costs possible in
the future
CIS
2-3 m,7,5 -9,5 %(mod.)
LimitedIndiumsupply innature
Significant decrease inproduction costs possible inthe future
HIT200 m,18 %
Limited useof thisproductionprocedure
Higher efficiency, bettertemperature coefficient andlower thickness.