Gerngross Reverey Presentation Ws 08

8/6/2019 Gerngross Reverey Presentation Ws 08

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CIS/CIGS Solar Cells

Mawi Seminar WS 07/08 Prof. Dr. H. Fll

Mark-Daniel Gerngro, Julia Reverey

02/04/2008 12:00 - 12:45

A. 241


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Motivation

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http://world.honda.com/environment/2006report/05010000_12.jpg

http://www.photon-magazine.com/news/news_2004-03%20ap%20sn%20Honda_big.jpg

problem: global warming and climate changeproblem: short running oil resources and raising power demandsolution: solar cells, especially CIS/ CIGS solar cells


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Contents

Introduction

Material Properties Growth Methods for Thin Films

Development of CIGS Thin Film Solar Cells

Fabrication Technology

Conclusion & Prospect


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Introduction

CIS = CuInSe2 (copper indium diselenide)

CIGS = CuInxGa1-xSe2 (copper indium gallium diselenide)

compound semiconductor ( I-III-VI)

heterojunction solar cells high efficiency (19% in small area, 13% in large area modules)

very good stability in outdoor tests

applications:

solar power plants

power supply in aerospace

decentralized power supply

power supply for portable purposes

http://www.baulinks.de/webplugin/2007/i/0732-wuerthsolar1.jpghttp://www.copper.org/innovations/2007/05/images/civilian_flex_panel.jpghttp://www.esa.int/images/ISS_2004_web400.jpghttp://www.rgp.ufl.edu/publications/explore/v12n2/images/thin-film.jpg


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Contents

Introduction

Material Properties

Phase diagram

Impurities & Defects

Growth Methods for Thin Films





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Material Properties I

crystal structure:

tetragonal chalcopyrite structure

derived from cubic zinc blende structure

tetrahedrally coordinated

direct gap semiconductor

band gap: 1.04eV 1.68eV

exceedingly high adsorptivity

adsorption length: >1m minority-carrier lifetime: several ns

electron diffusion length: few m

electron mobility: 1000 cm2V-1 s-1 (single crystal)

Shiyou Chen and X. G. Gong: Physical Review B 75, 205209 2007Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.


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Material Properties II

simplified version of the ternary phase diagram

reduced to pseudo-binary phase diagram along the red dashed line

bold black line: photovoltaic-quality material

4 relevant phases: E-, F-, H-phase and Cu2Se

Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.


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Material Properties III

E-phase(CuInSe2):

range @RT: 24-24.5 at%

optimal range for efficient thin film solar cells: 22-24 at %

possible at growth temp.: 500-550C, @RT: phase separation into E+F

F-phase(CuIn3Se5)

built by ordered arrays of defect pairs

( VCu, InCuanti sites)

H-phase(high-temperature phase)

built by disordering Cu&In sub-lattice

Cu2Se

built from chalcopyrite structure by

Cu interstitials Cui & CuIn anti sites



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Impurities & Defects I

problem: a-phase highly narrowed @RT

solution: widening E-phase region by impurities

partial replacement of In with Ga

20-30% of In replaced Ga/(Ga+In) }0.3

band gap adjustment

incorporation of Na

0.1 at % Na by precursorsbetter film morphology

passivation of grain-boundaries

higher p-type conductivity

reduced defect concentration



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Impurities & Defects II

doping of CIGS with native defects:

p-type:

Cu-poor material, annealed under high Se vapor pressure

dominant acceptor: VCu

problem: VSe compensating donor

n-type:

Cu-rich material, Se deficiency

dominant donor: VSe

electrical tolerance to large-off stoichiometries

nonstoichiometry accommodated in secondary phase

off-stoichiometry related defects electronically inactive


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Impurities & Defects III

electrically neutral nature of structural defects

Efdefect complexes < Efsingle defect

formation of defect complexes out of certain defects

VCu, InCu, CuIn, InCu and 2Cui, InCu no energy levels within the band gap

grain-boundaries electronically nearly inactive


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Contents

Introduction

Material Properties


Coevaporation process

Sequential process

Roll to roll deposition





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Growth Methods for Thin Films I

coevaporation process: evaporation of Cu, In, Ga and Se from elemental sources

precise control of evaporation rate by EIES & AAS or mass spectrometer

required substrate temperature between 300-550C

inverted three stage process:

evaporation of In, Ga, Se

deposition of (In,Ga)2Se3

on substrate @ 300C

evaporation of Cu and Sedeposition at elevated T

evaporation of In, Ga, Se

smoother film morphology

highest efficiency



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Growth Methods for Thin Films II

sequential process: selenization from vapor:

substrate: soda lime glass coated with Mo

deposition of Cu and In, Ga films by sputtering

selenization under H2Se atmosphere

thermal process for conversion into CIGS

advantage: large-area deposition

disadvantage: use of toxic gases (H2Se)


annealing of stacked elemental layers

substrate: soda lime glass coated with Mo

deposition of Cu and In, Ga layers by sputtering

deposition of Se layer by evaporation

rapid thermal process

advantage: large-area deposition

avoidance of toxic H2Se


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Growth Methods for Thin Films III

roll to roll deposition: substrate: polyimide/ stainless steel foil coated with Mo

ion beam supported low temperature deposition of Cu, In, Ga & Se

advantages: low cost production method

flexible modules and high power per weight ratio

disadvantages: lower efficiency

http://www.solarion.net/images/uebersicht_technologie.jpg

Mo Cu,Ga,In,Se CdS ZnO


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Contents

Introduction

Material Properties



Cross section of a CIGS thin film

Buffer layer

Window layer

Band-gap structure




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Development of CIGS Solar Cells I

soda lime glasssubstrate 2mm

CIGS absorber 1.6 m

Mo back contact 1m

Zn0 front contact 0.5m

CdS buffer 50nm

www.kolloquium-erneuerbare-energien.uni-stuttgart.de/downloads/Kolloq_2006/Dimmler_EEKolloq-290606.pdf


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Development of CIGS Solar Cells II

Buffer layer: CdS deposited by chemical bath deposition (CBD)

layer thickness: 50 nm

properties:

band gap: 2.5 eV

high specific resistance

n-type conductivity

diffusion of Cd 2+ into the CIGS-absorber (20nm)

formation of CdCu- donors, decrease of recombination at CdS/CIGSinterface

function: misfit reduction between CIGS and ZnO layer

protection of CIGS layer



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Development of CIGS Solar Cells III

Window layer: ZnO band gap: 3.3 eV

bilayer high- / low-resistivity ZnO deposited by RF-sputtering / atomiclayer deposition (ALD)

resistivity depending on deposition rate (RF-sputtering)/flow rate (ALD)

high-resistivity layer:

- layer thickness 0.5m

- intrinsic conductivity

low-resistivity layer:- highly doped with Al (1020 cm-3)

- n-type conductivity

function:

transparent front contactR.Menner, M.Powalla: Transparente ZnO:Al2O3 Kontaktschichten fr CIGS Dnnschichtsolarzellen


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Development of CIGS Solar Cells IV

band gap structure:

i-ZnO inside space-charge region

discontinuities in conduction band structure

i-ZnO/CdS: 0.4eVCdS/CIGS: - 0.4eV 0.3eV

depends on concentration of Ga

positive space-charge at CdS/CIGS

huge band discontinuities of

valance-band edge

electrons overcome heterojunction

exclusively

heterojunction: n+ip

Meyer, Thorsten: Relaxationsphnomene im elektrischen Transport von Cu(In,Ga)Se2, 1999.


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Contents

Introduction

Material Properties




Cell processing Module processing



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Fabrication Technology I

cell processing:

substrate wash #1

deposition of metal base electrode

patterning #1

formation of p-type CIGS absorber


deposition of buffer layer

patterning #2

deposition of n-type window layer

patterning#3

substrate

deposition Ni/Al collector grid

deposition of antireflection coating

monolithical integration:

during cell processing

fabrication of complete modules


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Fabrication Technology II

module processing:

packaging technology nearly identical to crystalline-Si solar cells

tempered glass as cover glass

Al frame

CIGS-based circuit

junction box with leads

soda-lime glass as substrate


ethylene vinyl acetate (EVA) as pottant


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Contents

Introduction

Material Properties






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Conclusion & Prospects

conclusion:

high reliability

high efficiency (19% in small area, 13% in large area modules)

less consumption of materials and energy monolithical integration

high level of automation

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www.kolloquium-erneuerbare-energien.uni-stuttgart.de/downloads/Kolloq_2006/Dimmler_EEKolloq-290606.pdf

prospects:

increasing utilization (solar parks, aerospace etc.)

optimization of fabrication processes

gain in efficiency for large area solar cells possible short run of indium and gallium resources


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Thank you for your attention!

sources:


Meyer, Thorsten: Relaxationsphnomene im elektrischen Transport von

Cu(In,Ga)Se2, 1999.

Dimmler, Bernhard: CIS-Dnnschicht-Solarzellen Vortrag, 2006.

Gerngross Reverey Presentation Ws 08

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