pvmaterials

7/29/2019 PVmaterials

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Session 1 Mon 10 Oct 2-5 pm

Prof J. Nelson The solar resource and solar energyconversion

Prof J. Nelson PV systems

Dr N. Ekins-Daukes Silicon solar cell technology

Session 2 Mon 17 Oct 25 pm

Dr N. Ekins DaukesProf J. Nelson

Concentrator Photovoltaics andConcentrated solar thermal power

Session 3 Mon 24 Oct 25 pm

Prof J. Nelson Photovoltaic materialsThin film PV materials

Organic photovoltaic materials

SEF01: Solar Energy Conversion (2011-12)

Objectives:

To understand the principles of solar photovoltaic energy conversion

To appreciate the role of materials in PV technology and the status of

established (silicon, then film) and emerging (organic) photovoltaic technologies

To be able to solve simple problems in the design of PV systems or applications

of PV


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Photovoltaic materials and future approaches:Outline

Strategies to reduce the cost per peak Watt

Cheaper (thin film) photovoltaic materials


Limits to power conversion efficiency


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THE FUTURE FOR PV

0.00

2000.00

4000.00

6000.00

8000.00

10000.00

12000.00

14000.00

16000.00

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

Installedcapacity/MWp

grid connected

off grid

Data: www.iea-pvps.org


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PERL cell

Status of Silicon PV

Maximum lab cell efficiency 24.9%

Module efficiency >15%

> 20 GWp capacity installed globally. Dominated by from polycrystalline andmonocrystalline silicon modules.

Module price ~ 3.47 $/Wp (US)or 3.09 Eu/Wp (http://www.solarbuzz.com) Cost of electricity (US) ~ 0.22 $/kWh (Commercial), 0.32 $/kWh (Residential)


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Strategies to cost reduction

Crystalline silicon solar cells are expensive

Use lessphotovoltaic

material?

Use cheaperphotovoltaic

material?

More work

per photon?


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Cheaper photovoltaic materials


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PV materials

1 2 310

4

105

106

107

108

109

Absorption

coefficient/m

-1

Photon energy / eV

Silicon

GaAs

aSi

absCIS

P3HT

Material Band gap

(eV)

Max Jsc

(mA cm-2

)

Type of gap Crystal size

Crystalline silicon

(c-Si)

1.1 42 indirect >10-3

m

Crystalline GaAs 1.4 32 direct >10-3

m

Polycrystalline Si

(p-Si)

1.1 42 indirect 10-4

m

Amorphous Si

(a-Si)

~1.7 ~ 23 ~ direct amorphous

CuInGaSe2 > 1.0 < 45 direct 10-6

m

Cd Te 1.4 42 direct 10-6

m

Polymer /

fullerene

1.6 24 Direct

(finite band

width)

amorphous


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PV materials

Material Band

gap(eV)

Theoretical

Jsc (mAcm-2)

Grain

size(um)

Voc (V) Jsc (mA

/cm2

)

FF (%) Efficiency

(%)

Crystalline silicon

(c-Si)

1.1 42 >104 0.696 42.0 83.6 24.9

Crystalline GaAs 1.4 32 >104 1.022 28.2 87.1 25.1

Polycrystalline Si

(p-Si)

1.1 42 10-100 0.628 36.2 78.5 19.8

Amorphous Si

(a-Si)

~1.7 ~ 23 Amor-

phous

0.887 19.4 74.1 12.7

CuInGaSe2 > 1.0 < 45 1 0.669 35.7 77.0 18.4

Cd Te 1.4 42 1 0.848 25.9 74.5 16.4

Organic (polymer /

fullerene)

1.6 24 Amor-

phous

~0.75 ~15 ~70 ~8


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Single Semiconductor Junction.

Amorphous / micro-crystalline material

Low materials usage

Low embedded energy

Low cost

Large unit size

- Efficiency

Thin film (2nd Generation) Photovoltaics

a-Si,

Richmond, U.K.

Dye-Sensitised Solar Cell 6%

Polymer, 4%10% CdTe, $0.98/W

6% a-Si, 6m2


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Thin-film Silicon

Direct deposition of Si onto

glass from SiH4

Typically 6% efficient

module

(14.7% research lab)

Low cost of materials

Large manufacturing Unit

Low embedded energy

Efficiency

6% a-c-Si, Applied materials ~ 6m2

4% semi-transparent Si, Kaneka


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Thin-film Silicon

Direct deposition of Si onto

glass from SiH4

Typically 10% efficient

module


Low cost of materials

Large manufacturing unit

Low embedded energy

X Growth rate (30nm/min)(PECVD)

ZnO

Micromorph

tandem

a-Si

Micromorph tandem

E.g. Oerlikon Solar


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Basic micromorph process steps

Clean glass Deposit TCO Laser scribe :1

Semiconductor

depositionLaser scribe :2Rear TCO

Laser scribe 3Contact, Encapsulate,

Test

Oerlikon production system


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Micro-morph tandem module performance

Initial module efficiency: 12.2%

Module efficiency after light soaking: 10.7%

U.Kroll et al., Thin film silicon PV: from R&D to

large area production equipment. Oerlikon Solar

Lab SA, Proc 37th IEEE Photovoltaics Specialists

Conference 2011.


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CdTe Thin-Film Modules

Glass laminateEthyl vinyl acetate

Aluminium

Nickel

CdTe

CdS

FTO

Glass Superstrate

3m

100nm

CdS & CdTe films grown by

vapour transport depositionExtremely fast deposition

rate >10m/min

10% module efficiency


Low cost of materials Large manufacturing unit

Low embedded energy

Abundance of Te

E.g. First Solar


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Basic CdTe process steps


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Molecular photovoltaic materials


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The goal

contacts active layer

Current andvoltage output

flexible substrate

barrier coatingLight



flexible substrate




flexible substrate


encapsulant

metal

deposition

solution

deposition

su

bs

tra

te

Target cost < 0.5 $/Wp

Rapid growth in production

capacity possible
http://www.google.co.uk/imgres?imgurl=http://www.envirogadget.com/wp-content/uploads/2009/11/Neuber-Energy-Sun-Bag.jpg&imgrefurl=http://www.solarfeeds.com/greentech-media/11455-konarka-lands-20m-for-organic-solar-cells&usg=__GevMJy4BmxRbnpNuiAH81xFJKzY=&h=500&w=728&sz=87&hl=en&start=1&zoom=1&um=1&itbs=1&tbnid=dSFvoEkUplr0zM:&tbnh=97&tbnw=141&prev=/images%3Fq%3Dorganic%2Bsolar%2Bbag%26um%3D1%26hl%3Den%26sa%3DN%26rlz%3D1T4GGLR_enGB270GB271%26tbs%3Disch:1


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conjugated

polymer

1 nm

Molecular electronic materials

Easily processable e.g. fromsolution

Abundant, non-toxic

materials

Tune properties via chemicaldesign

Excited states are localised:limited charge and excitonmobility

dye

conjugated molecule1nm

Konarka Technologies


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EB ~ 0.01 eV

Spontaneous charge pair generation

-

EB ~ 0. 1- 0.5 eV

Charges hard to dissociate

+

Inorganic semiconductor

+-

Molecular semiconductor

The main issue: charge separation

Cannot copy inorganic PV device structures!

n p

active region

dead region


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Donor-acceptor solar energy converters

Electron

acceptor

Electron

donorC60 Conjugated

polymer

Donor acceptor blend

active region

EB can be supplied by the free energy differencebetween donor and acceptor species


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Bulk heterojunction device structure

Donor-acceptor bulk heterojunction devices

Cathode

Donor-Acceptor blend

Anode

Substrate

h+e-

Active layer can be

100s of nm - limitedby charge diffusion

length

Domain size ca. 10 nm. ~exciton diffusion length

Blend layer deposited from solution


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Key steps in photocurrent generation

1

1. Photon absorption2

2. Exciton diffusion

-+

3

3. Exciton dissociationgeminate charge pair

-

+4

4. Geminate charge pair separation

5

5

5. Charge transport to contacts

Current generation

Exciton decay

Other excited statese.g. triplets

Geminate charge pair

recombination

Non-geminate charge

pair recombination

fl h l f


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-

+

Parameters influencing photovoltaic performance

Absorption

spectra

Excitondiffusion

lengths

Domain size

Order in molecular packing

Domain connectivity

microstructure

absorption

edge

energy levels

donor

acceptor(PCBM)

HOMO

HOMO

LUMO

LUMO

Eg DECS

DEe

open circuit

voltage

charge

separation


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0.0 0.2 0.4 0.6 0.8-60

-50

-40

-30

-20

-10

0

P3HT:PCBM solar cell

= 4 %

High efficiency silicon cell

= 24 %

Cur

rentdensity/mAcm-2

Voltage / V

Role of energy levels

donor

acceptor

(PCBM)

HOMO

HOMO

LUMO

LUMO

EgD

ECS

DEe

Donor HOMO- acceptor LUMO

gap controls maximum Voc

Donor optical gap

controls Jsc EFp

eVoc

EFn

S f h A


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State of the Art

Liangetal.,

Adv.Mater.22(2010)

electron

donorelectron

acceptor

HOMO

HOMO

LUMO

LUMO

Eg DECS

DEe

- 5.15 eV

- 3.31 eV

Combination of strategies (lower Eg and

deeper HOMO energy) leads to power

conversion efficiency of over 7% (now

8%)

S f h A


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State of the Art

Lifetimes of several years demonstrated with low to moderate

permeability barriers (WVTR of 10-5 10-2 g/m2/day).

Hauc

he

ta

l.,

So

l.Energy

Ma

ter.

So

l.Ce

lls92

,727

(2008)

Kime

tal.

,App

l.Phys.L

ett

.94

,16

6308(2009)

Food packaging ALD deposited alumina / organic layer

S f h A


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State of the Art

Cells combined into modules todeliver useful dc voltages

First consumer products being

tested on market

0.1 m2 1.3 Wp0.5 m2 9 Wp

Vmpp ~ 8 V

Impp = 0.16 1 A

P j d M k


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Projected Markets

When Efficiency

(module)

Lifetime Cost

(system)

Capacity

Consumer

electronics

2010-2013 1-3% 1-2 years 8-12 $/W 1 GWp /year

Mobile power 2012-2015 3-4% 1-3 years 2-4 $/W 1.5 GWp /year

Small scale BIPV 2015-2018 4-6% 3-5 years 1 $/W 5 GWp /year

Power

generation

2018-2025 6-9% 5-7 years 0.5 $/W 60 GWp /year

Nie

lsene

ta

l.,

So

l.Energy

Ma

ter.

So

l.Ce

lls94

,155

3(2010)

Also need to identify markets distinct from those of competing thin film PV

R h l


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Research goals

Efficiency:

materials for optimised light absorption and voltage generation

understanding charge pair separation

control of film microstructure

high mobility materials

selective electrodes

Lifetime:

materials with improved photostability

control of film microstructure

stable electrodes

Cost:

scalable and fast fabrication processes

cheap electrodes

stable photoactive materials and electrodes

Research programme at Imperial College London


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flexible substrate




flexible substrate




flexible substrate


Design and synthesise

materials with optimised

energy levels and self

organising properties

Study the microstructure

of photovoltaic thin films

Study the photophysics of

thin films

Study the electrical

properties of thin films

All as a function of chemical

structure and processing

Optimise PV device

performance and structure

Scale up fabrication

process

Develop models for

material and device design

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flexible substrate




flexible substrate




flexible substrate









thin films





Optimise PV device



process

Develop models for


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thin films





Optimise PV device



process

Develop models for


3 Departments

> 10 academics

> 50 researchers

Several m grant funding

National, international and

industrial collaborations

Summary
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Summary

The cost per Watt of PV electricity can be reduced by:

Reducing the cost of PV material using thin film materials Increasing the amount of work per photon

Reducing the amount of PV material per photon harvested

Inorganic thin films are amorphous or microcrystalline materials that can be

deposited in fast processes e.g. from vapour phase Leading options are amorphous Si, microcrystalline Si, CdTe and

CuInGaSe2

a-Si / micro-Si tandem structures offer 10% module efficiency but arelimited by slow growth

CdTe offers rapid growth but is potentially limited by Te availability

Organic thin films offer rapid and low cost production by printing or coating fromsolution

pvmaterials

Documents