Extremely Thin Absorber Solar Cells:EU Project Overview

Status quo and perspectives

A European Research Project (HPRN-CT-2000-00141)

GCEP - Stanford, 19.10.2004

Outline

• Energy Research Center of the Netherlands

• ETA solar cells: A PV technology for the future ?- Motivation - Materials and methods- Interface Design- Comparison of different systems

• Summary (status quo)

• Future Research Perspectives

Energy Research Center of the Netherlands

Bridge between academic research and commercial application

• Largest independent energy research center in NL (600 co-workers)

• R&D for/with private enterprises and governmental institutions, (national + international)

• Core Research:Sustainability in energy generation

and use

Research Priority Areas at ECN

- Solar Energy- Biomass- Windenergy- Fuel Cells-- Clean Fossil FuelsClean Fossil Fuels-- Energy Efficiency in IndustryEnergy Efficiency in Industry-- Building Integration of Renewable Building Integration of Renewable

EnergiesEnergies-- Policy StudiesPolicy Studies

ECN Solar Energy

Conventional and Advanced Silicon- New contact and module concepts- Ribbon grown Si

Thin Film Solar Cells- Thin Film Si (low T deposition on substrates)- Chalcopyrite

- Dye Sensitized Solar Cells (Electrolyte and Solid State)- Polymer- und Hybrid Solar Cells- ETA Solar Cells

EU Network Project : ETA Solar Cells

Project Partners:Energy Research Center of the Netherlands (ECN)Hahn Meitner Institute (HMI)Rijksuniversiteit Gent (UGent)Tallinn Technical University (TTU)Technical University Delft (TUD)Uppsala University (UU)Weizmann Institute of Science (WIS)

Project Coordinator: Dr. Jeannette Wienke (ECN)

Motivation for the projectQuestion: Is it possible to cope with absorber materials of low electronic quality (defects) ?

Implications: Short charge carrier lifetimesHigh (bulk) recombination probability

Proposed solution: Short distances to charge separating interface

Concept :Extremely Thin Absorbers (~ 2 - 50 nm)

Light

Contact (-)

Contact (+)

Transparent n- type

Inorg. absorber or mol. dye

Transparent p- type

3 component (nano)compositeWhy inorganic absorbers ?- Tunability of Eg(- Higher absorption)- Impact ionization

Könenkamp et. al., Appl. Phys. Lett. 75 (1999) 692O’Regan, Schwartz, Chem. Mater., 7 (1995) 1349

Concept :Extremely Thin Absorbers (~ 2 - 50 nm)

Transparent, n- type

p - Absorber Contact

(+)

Contact (-)

Light

2 component (nano)composite

Challenges

• Deposition techniques for thin (absorber) layers inside porous films

• Materials: n-type, p-type, absorber (band alignment, structural, defect

chemistry)

• Interface: Recombination

• Modelling and characterization(new device concept, complex geometry)

Deposition Techniques

• ALD: Atomic Layer Deposition M. Nanu - TUD(CuInS2, Al2O3, In2S3)

• CBD: Chemical bath deposition O. Niitsoo - WIS(CdS, CdSe, In(OH)xSy)

• ILGAR: Ion Layer Gas Reaction H. Muffler - HMI (Al2O3, CuInS2)

• Spray pyrolysis A. Katerski TTU (CuInS2)

• Solution casting (CuSCN) B. O‘Regan - ECN

• Electrodeposition (CdTe, CdHgTe) A. Belaidi - HMI

Atomic Layer Deposition (TU Delft)

A

B

C

D

Adsorption Compound 1 (CuCl, InCl3)

Monolayer Formation (compound 1)

Surface Reaction with compound 2(H2S)

M. Nanu, L. Reijnen, B. Meester, A. Goossens, and J.Schoonman, Thin Solid Films 431-432 (2003) 492

Transparent n-type materials

TiO2 films with different microstructures/electronic properties

Alternative materialslike ZnO, SnO2under development

- electron mobility- band alignment- defect chemistry

Micron-sized pores in a spray-pyrolized TiO2film

Nano-sized pores in a screen-printed TiO2film

Transparent p-type materials

CuSCN, CuI, (CuAlO2)

• Mandatory in dye sensitizedsolar cells

• Not always mandatory in ETA solar cells

B. O‘Regan - ECN1 µm thick CuSCN on top of a TiO2 layer

Absorbers

Inorganic binary compounds (Egap):PbS-q.s. (~1.1 eV) CdTe (1.45 eV)CdSe (1.8 eV)

Inorganic ternary compound (Egap):CuInS2 (1.5 eV)

Molecular metal-organic („Egap“):Ru(II)L2(NCS)2 “N3”-dye (~ 1.7 eV)

= best inorganic absorber of the project

Characterization Methods

Structural : SEM, TEM, BET, Raman, XPS

Crystallogr. : XRD

Electrical : Current-voltage, IMPS, IMVS, Surface Photovoltage, Transient Measurements (I,V)

Optoelectronic : DLTS, PL

Characterization I:Voltage Transient Measurements

Recombination dynamics: Example: TiO2/Dye/Electrolyte (CuSCN)

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Phot

ovol

tage

, nor

mal

ized

43210Time, milliseconds

CuSCN Cell

Electrolyte Cell

t ½ (µs)4 µm-CuSCN ~ 200 4 µm-Electrolyte ~ 4000

Faster recombination inCuSCN based dye cells

B. O‘Regan - ECN

Background Illumination: 1 sunFlash Illumination: ~ 10 %

Characterization II:XPS - Interface structure

CuI/Dye Interface

CuI / dyeTiO2 / dye

N 1s1

2

B. Mahrov - UU

Evidence for change of the chemical environmentof the N3-dye molecule at the CuI surface

Interface I:Modification by additional layers

In(OH)xSy by CBD, CuInS2 by Spray

without In(OH)xSywith In(OH)xSy

JSC : 10.7 mA/cm2

VOC : 450 mVff : 43 %η : 2 %

Example: TiO2 (flat) / In(OH)xSy / CuInS2

J. Wienke, M. Krunks, F. Lenzmann, Semicond. Sci. Technol. 18 (2003) 876

Interface II:Modification by additional layersExample: TiO2 (por) / Al2O3 / Dye / CuSCN

-8

-6

-4

-2

0

2

4

6

Cur

rent

mA

/cm

2

0.80.60.40.20.0-0.2Voltage (V)

with Al2 O3without Al203

JSC : 5.2 mA/cm2

VOC : 690 mVff : 59 %η : 2.1 %

VOC = 500 mV, JSC = 18 mA/cm2, FF = 45 %, η = 4%(AM 1.5, area: 3 mm2)

-0.9 -0.6 -0.3 0.0 0.3 0.6

-20

-10

0

10

20

30

40 SnO2:F/TiO2/Al2O3/In2S3/CuInS2/Au

curr

ent d

ensi

ty [m

A/c

m2 ]

voltage [V]500 600 700 800 900

0.0

0.2

0.4

0.6

0.8

exte

rnal

qua

ntum

effi

cien

cywavelength λ [nm]

Interface III:2 interface layers (Al2O3 & In2S3) ⇒ best cell with inorg. absorber

M. Nanu, TUD

M. Nanu, A. Goossens, J. Schoonman, Adv. Mater., 16 (2004) 453

Interface IV:Interface design in various systems

(1) TiO2 / Al2O3 / Dye / CuSCN (ECN, UU)

(2) TiO2 / Al2O3/In2S3 / CuInS2 (TUD, TTU, HMI)

(3) TiO2 / Al2O3/CdS / CdSe /Electrolyte (WIS)

(4) TiO2 / In(OH)xSy / PbS /PEDOT:PSS (HMI)Electrolyte (ECN)

n-type-layer absorber

= best device setup of the project

Modelling of the first ETA cells (TiO2/CdTe)

-15.0

-10.0

-5.0

0.0

5.0

10.0

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

curr

ent d

ensi

ty(m

A/c

m2 )

voltage (V)

eta-cell darkEta-cell lightSCAPS darkSCAPS light

S-shape

S-shaped I-V curve explained by conduction band offset (0.6 eV)

C. Grasso et. al, Proc. 17th Europ. PV Conf., Munich (2001) 211

Modelling the nanostructure

light

+-

1) Network Model:A network of small unit cells

M. Burgelman, C. Grasso J. Appl. Phys., 95(4) (2004) 2020

M. Burgelman, C. Grasso - RUGS. Ruehle - WISK. Fredin - UU

h+-SC

2) Effective Medium ModelOne homogeneous “effective” medium

CB TiO2

VB ETA EFp

EFnlight

e--SCSC = selective contact

Overview of the most efficient systems

TiO2/Al2O3/In2S3/CuInS2 4% (0.03 cm2) TUD

TiO2/Al2O3/Dye/CuSCN 2-2.5% (1 cm2) ECN

TiO2/CdTe 2% (< 1 cm2) HMI

TiO2/Dye/spiro-OMeTAD 3.5% (1 cm2) EPFL

TiO2/CdSe/CuSCN 2.3% (?) CNRS

General Summary

• Practical realization of working ETA solar cells

• Variable group of new systems (η = 2- 4%), competitive with other nanocompositeapproaches

• Interface design of primary relevance

Future Research Perspectives

• Deposition techniques (low cost)

• Optimizing the microstructure (absorber thickness/interface area)

- better performance (?)- easier & cheaper preparation

• Other materials ? (non-toxic, low cost)

Acknowledgements

Special thanks to:

• all our partners for their contributions

• European Commission for funding (HPRN-CT-2000-00141)

Thank you for your kind attention !

Extremely Thin Absorber Solar Cells:EU Project OverviewStatus quo and perspectivesOutlineEU Network Project : ETA Solar CellsMotivation for the projectConcept :Extremely Thin Absorbers (~ 2 - 50 nm)Concept :Extremely Thin Absorbers (~ 2 - 50 nm)ChallengesDeposition TechniquesAtomic Layer Deposition (TU Delft)Transparent n-type materialsTransparent p-type materialsAbsorbersCharacterization II:XPS - Interface structureInterface I: Modification by additional layersInterface II: Modification by additional layersInterface IV: Interface design in various systemsModelling of the first ETA cells (TiO2/CdTe)Overview of the most efficient systemsGeneral SummaryFuture Research PerspectivesInterface RecombinationTransient voltage measurements after flash illuminationMeasurement TechniqueTransient voltage measurementsat VOCSolid state vs. liquid electrolyte dye cell:Comparison of recombination dynamicsSolid state vs. liquid electrolyte dye cell: