cis / cigs solar cells

CIS/CIGS Solar Cells

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

Mark-Daniel Gerngroß, Julia Reverey02/04/2008 12:00 - 12:45

A. 241

Motivation

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

Contents

• Introduction

• Material Properties

• Growth Methods for Thin Films

• Development of CIGS Thin Film Solar Cells

• Fabrication Technology

• Conclusion & Prospect

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

Contents

• Introduction

• Material Properties• Phase diagram• Impurities & Defects





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: >1µm

• minority-carrier lifetime: several ns• electron diffusion length: few µm• electron mobility: 1000 cm2 V -1 s-1 (single crystal)

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

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: α-, β-, δ-phase and Cu2Se

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

Material Properties III

• α-phase (CuInSe2):– range @RT: 24-24.5 at% – optimal range for efficient thin film solar cells: 22-24 at %⇒ possible at growth temp.: 500-550°C, @RT: phase separation into α+β

• β-phase (CuIn3Se5)– built by ordered arrays of defect pairs

( ⟨VCu, InCu⟩ anti sites)

• δ-phase (high-temperature phase)– built by disordering Cu & In sub-lattice

• Cu2Se– built from chalcopyrite structure by Cu interstitials Cui & CuIn anti sites


Impurities & Defects I

problem: a-phase highly narrowed @RT

– solution: widening α-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 precursors

⇒ better film morphology

⇒ passivation of grain-boundaries

⇒ higher p-type conductivity

⇒ reduced defect concentration


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

Impurities & Defects III

• electrically neutral nature of structural defects– Ef

defect complexes < Efsingle defect

⇒ formation of defect complexes out of certain defects

⟨2VCu, InCu⟩ , ⟨CuIn, InCu⟩ and ⟨2Cui, InCu⟩

⇒ no energy levels within the band gap

• grain-boundaries electronically nearly inactive

Contents

• Introduction


• Growth Methods for Thin Films• Coevaporation process• Sequential process• Roll to roll deposition




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 300-550°C

– inverted three stage process:• evaporation of In, Ga, Se• deposition of (In,Ga)2Se3

on substrate @ 300°C• evaporation of Cu and Se deposition at elevated T• evaporation of In, Ga, Se

⇒ smoother film morphology

⇒ highest efficiency


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

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

Contents

• Introduction



• Development of CIGS Thin Film Solar Cells• Cross section of a CIGS thin film• Buffer layer• Window layer• Band-gap structure



Development of CIGS Solar Cells I

•

soda lime glasssubstrate 2mm

CIGS absorber 1.6 µm

Mo back contact 1µm

Zn0 front contact 0.5µm

CdS buffer 50nm

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

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/CIGS

interface

function: • misfit reduction between CIGS and ZnO layer• protection of CIGS layer


Development of CIGS Solar Cells III

Window layer: ZnO• band gap: 3.3 eV• bilayer high- / low-resistivity ZnO deposited by RF-sputtering / atomic

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

• high-resistivity layer:- layer thickness 0.5µm- intrinsic conductivity

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

- n-type conductivity

function:• transparent front contact

R.Menner, M.Powalla: Transparente ZnO:Al2O3 Kontaktschichten für CIGS Dünnschichtsolarzellen

Development of CIGS Solar Cells IV

band gap structure:

• i-ZnO inside space-charge region• discontinuities in conduction band structure

–i-ZnO/CdS: 0.4eV

–CdS/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: Relaxationsphänomene im elektrischen Transport von Cu(In,Ga)Se2, 1999.

Contents

• Introduction




• Fabrication Technology• Cell processing• Module processing


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

Fabrication Technology II

module processing:– packaging technology nearly identical to crystalline-Si solar cells

tempered glass as cover glass

Al frame

CIGS-based circuitjunction box with leads

soda-lime glass as substrate


ethylene vinyl acetate (EVA) as pottant

Contents

• Introduction






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

http://img.stern.de/_content/56/28/562815/solar1_500.jpgwww.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

Thank you for your attention!

sources:Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.Meyer, Thorsten: Relaxationsphänomene im elektrischen Transport von

Cu(In,Ga)Se2, 1999.Dimmler, Bernhard: CIS-Dünnschicht-Solarzellen Vortrag, 2006.

cis / cigs solar cells

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