Lecture 34 Spt Dec

7/27/2019 Lecture 34 Spt Dec

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Solar Photovoltaic

Technologies

Prof. C.S. SolankiEnergy Systems Engineering

IIT Bombay

Lecture-34


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8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 2

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


7/458/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 7


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




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



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



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



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



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



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



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%



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?



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



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



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



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



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



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 ?



Integral Steps of PSLT

Porous silicon formation

Silicon


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SiliconSilicon



Active layer deposition

- Annealing

- CVD epitaxial layer


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Device fabrication

Silicon


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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




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Silicon ingot

Pt electrode

Electrolyte

Porous silicon

films


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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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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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.

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