Is it possible to enhance solar energy conversion ... · GaInP/GaAs/Ge 32.0 % 1.5AM Spectrolab 2003 GaInP/GaInAs/GaAs 40.8 % 326 suns NREL 2008 a-Si/µc-Si 11.7 % 1.5AM Kaneka 2004

High-Efficiency Concepts for Photovoltaicsbased on Silicon Quantum Structures

Bert Stegemann, Thomas Lußky, Andreas Schöpke, Manfred Schmidt

Helmholtz Center Berlin for Materials and Energy GmbH* Department of Silicon Photovoltaics (SE1)

* formerly Hahn-Meitner-InstituteBert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt

Silicon Quantum Structures for Photovoltaics

2

Is it possible to enhance

solar energy conversion efficiencies by

utilization of quantum size effects ?

Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt


3

Si Nanostructures for PV – Research at Helmholtz Center Berlin

Nano-Physicsexploring atomic scale matterbasic material research

Nano-Geometrynano-composite structuresnanostructured solar cells

Quantum-Technologyreduction of thermalizationutilization of quantum size effectsrealization of high-efficiency concepts

First BMBF joint project (2006 – 2008): „Band Structure Design: Charge Carrier Transport in Silicon-based quantum structures for high-efficiency solar cells“

New BMBF joint project (2009 – 2011) SINOVA:„Silicon-based nanostructured thin-film materials asinnovative functional elements in next generation solar cells“

20… ?



4

Outline1. Introduction

- loss mechanisms- beyond Shockley Queisser- tandem cell concept

2. Si/SiO2 Multi Quantum Wells- quantum size effect- structure and photoconductivity

3. High Quality Si/SiO2 Single Quantum Wells- preparation- interface gap state spectroscopy- in situ interface passivation- photoelectrical properties

4. Conclusions and Outlook



5

Loss mechanisms in photovoltaic energy conversion

• Aim of 3rd generation concepts: exceeding the Shockley Queisser limit (single junction efficiency)

transform excess energy intoelectrical rather than thermal energy

0

20

40

60

80

100

hν > Eg

%

usable energy

hν < Eg

excess energy

further losses

> 50 %

Shockley-Queisser limit: max. single bandgap efficiency: 32.7 %

Record cell: 24.7 %(mono-Si, „PERL“, UNSW 1994)

W. Shockley and H.J. Queisser, J. Appl. Phys. 32 (1961), 51 Zhao J, Wang A and Green M 1999 Prog. Photovolt. 7 471


Silicon Quantum Structures for PhotovoltaicsBeyond Shockley-Queisser : Approaches

• Energy bandgap engineering Tandem Cells• Hot Carrier Conversion

- Extraction, collection, and utilization of hot carriers- Impact ionization / multiple exciton generation

• Intermediate Band Solar Cell• Thermophotonic Solar Cells• Down-Conversion and Up-Conversion• …

See:

• M. Green: Third Generation Photovoltaics, Springer, 2003

• P. Würfel: Solarzellen der dritten Generation, Phys. J., 2003

• A. Marti / A. Luque: Next Generaton Photovoltaics, 2002

6

Quantum Structures



7

Multijunction Solar Cells

wavelength / nm

spec

tral

irrad

ianc

e/ W

/m2 µ

m

• Multiple junctions (tandems or stacks)Jackson, E. D., Proc. Conf. on the Use of Solar Energy, Tuscon, AZ, (1955),122

solar cells with decreasing bandgap are stacked on each othereach of the cells converts photons from a certain energy rangetheoretical limits: N = 2: 42%, N = 3: 49%, …, N = ∞: 68%



8

Multijunction Solar Cells

Considerations

• match the terrestrial solar spectrum

• match the current throughput

• control of band offsets

• matching of the lattice constants



9

Multijunction Solar CellsRealizations

GaInP/GaAs/Ge 32.0 % 1.5AM Spectrolab 2003GaInP/GaInAs/GaAs 40.8 % 326 suns NREL 2008

a-Si/µc-Si 11.7 % 1.5AM Kaneka 2004a-Si/a-SiGe/a-SiGe 10.4 % 1.5AM USSC 1998

III-V based Multijunctions:

a-Si based Multijunctions:

☺ highest efficiencies, light weightcomplex, expensive, > 20 layers/ interfaces, need high concentration

☺ abundent material, large scale productionlow efficiencies

energy band gap engineering by making use of quantum size effects ?



10

Size QuantizationIncrease of bandgap by decrease of Si layer thickness

spatial carrier confinementquantization of wave functionsincrease of effective bandgap

c-Si

SiO2 3.2 eV

1.1 eV

4.7 eV

SiO2

bandgap tuningincreased absorptionindirect to direct conversion



11

Silicon Based Tandem Cell

Si Quantum Well Solar Cell:⇒ Si/SiO2 superlattices with absorberstacks of different thickness and size-dependent bandgap energy

cf. M.A. Green: Third Generation Photovoltaics(Springer, Berlin, 2005).



12








1313

Si/SiO2 Multi-QWs by R-PECVD

4 nm Si-QWs / 3 nm SiO2 Superlattice, RTA: 30‘ @ 1000°C

TEM cross-section

1.0 1.2 1.4 1.6 1.80.0

0.2

0.4

0.6

0.8

1.0

20 x 1 nm

10 x 2 nm

7 x 3 nm

PL in

tens

ity [a

.u.]

Photon energy [eV]

T = 75 K

5 x 4 nm

Photoluminescence

R. Rölver , B. Berghoff, D. Bätzner, B. Spangenberg, H. Kurz, M. Schmidt, B. Stegemann: Thin Solid Films 516, 6763 (2008)



14

Lateral vs. vertical transport in QWs

14

Dark I-V characteristics of 20x 3 nm/3 nm Si/SiO2 MQWs

0 5 10 15 201E-9

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

curr

ent d

ensi

ty [A

/cm

²]

voltage [V]

vertical

lateral

~3 orders of magnitude higher conductivity in the lateral configuration

R. Rölver , B. Berghoff, D. Bätzner, B. Spangenberg, H. Kurz, M. Schmidt, B. Stegemann: Thin Solid Films 516, 6763 (2008)

interface recombination

tunneling



15

Photocurrent Measurements

⇒ Determination of quantum efficiencies:• Yint (hν) ~ η · μ · τeff• YextR (hν) ~ α (hν) , δn, δp = const

main onset of PC at bandgap of a-Si

a-Si

Spectral dependence of Yint

1 2 3 4 5

10-8

10-7

10-6

10-5

Intern-Yield-LateV.OPJ

Y int

.PC

hν / eV

10x MQW: 5nm Si / 5nm SiO2, (1000°C, 30s)

M. Schmidt, R. Rölver , B. Stegemann: in prep.

IphU

T

RΦ0 hν



16

Crystallinity

Zacharias et. al. Phys. Rev. B, 62 (2000) 8391

low crystallinity: < 25%decreasing crystallinity with decreasing a-

Si layer thickness

in accordance with Zacharias Model:

„thin Si layers are more difficult to crystallize, due to increased interface binding energies“

data from: R.Roelver, PhD thesis RWTH Aachen

0 2 4 6

0

20

40

60

80

100

Ram

an c

ryst

allin

ity /

%

QW thickness

Si/SiO2-MQW - R-PECVD, RTA

/ nm



17

Summing up: MQWs by R-PECVD ... so far

Photoconductivity:a-Si contribution dominates transportno quantum size effect

Photoluminescence:blue shift of PL signal due to quantum size effectoriginates from Si nano-crystals

Raman:crystalline fraction: < 25%nano-crystals embedded in a-Si matrix

complementaryprocesses !

need high-quality material



18








19

ultrahigh vacuum preparation and analysis of thebuilding block of multiple-quantum wells and superlattices:

SiO2/Si/SiO2 single quantum well

B. Stegemann, A. Schoepke, M. Schmidt: J. Non-Cryst. Sol. 354 (2008) 2100

room temperature1 Å / sec.

300°Cneutral atomic oxygenwith thermal impact

crystallization

1000°C anneal

Si/SiO2 Quantum Wells: UHV Preparation



2020

• Si single QW 7SiO2 barrier (7 nm / 2 nm)• uniform layers• structurally abrupt interfaces• atomic resolution of the QW

• nano-crystalline structure• high crystallinity: ~80%

TEM Raman

Si/SiO2 Single Quantum Wells: Characterization

B. Stegemann, A. Schoepke, M. Schmidt: J. Non-Cryst. Sol. 354 (2008) 2100



2121

• photocurrent detectable in Si QWs



25

0.0 0.5 1.0

10-10

10-9

10-8

10-7

0.0 0.5 1.0

EV

EF

E - EV / eV

SiO2/Si(111) +energetic H plasma +1000°C annealing

Yin

t

SiO2/Si(111) +thermal H plasma +1000°C annealing

EV

EF

Si/SiO2 Interface Defect Passivation

Thermal impact:• decrease of midgap states

passivation of dangling bonds• no interface degradation

Energetic impact:• increase of interface states

generation of interface defectstates due to bond breaking

B. Stegemann, A. Schoepke, D. Sixtensson, B. Gorka, T. Lussky, M. Schmidt: Physica E (2008) doi:10.1016/j.physe.2008.08.012 Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt


26

1 2 3 4 5 6

10-8

10-7

10-6

10-5

10-4

Y in

t, P

C

hν / eV

after hydrogen passivation without hydrogen passivation

a-Sic-Si

Photoconductivity of Si/SiO2 QWs: Hydrogen Passivation

26

Spectral dependence of Yint

• UHV H-plasma treatment improves Yint ~10× due to passivation of defect states (dangling bonds) at Si/SiO2 interfaces (and at intralayer grain boundaries)• db passivation results in increased carrier lifetimes and lower recombination velocity

B. Stegemann, D. Sixtensson, T. Lussky, A. Schoepke, I. Didschuns, B. Rech, M. Schmidt: Nanotechnology 19 (2008) 424020

µ · τeff ≈ 2×10-7 cm2V-1



27


- loss mechanisms- beyond Shockely Queisser- tandem cell concept






28

• complete UHV cycle for producing high-qualitiy ultrathin-SiO2/Si interfaces

• key point: RF plasma oxidation and H passivation with nearly thermalimpact energies (< 1eV)

98 100 102 104 106

XPS

Si 2

p si

gnal

/ a.

u.

EB / eV

Si(100)

0.0 0.5 1.0

10-10

10-9

10-8

10-7

Yin

t

E-EV / eV

Si(111)7x7 SiO2/Si(111) SiO

2/Si(111):H thermal plasma (UHV)

SiO2/Si(111):H energetic plasma (CVD)EV

EF

• abrupt interfaces:

electronicallychemicallystructurally

preparation interfacepassivationgap state

spectroscopyinterface

passivationgap state

spectroscopy

Conclusions I



2929

• successful preparation of

Si/SiO2 single/multi quantum wells (and quantum dots)

ultra-thin SiO2 layers

• correlation of structural

chemical and

electronic Si/SiO2 interface properties

• detection of lateral photocurrents in single QWs (and QD layers)

• determination of µ·τeff product ⇒ strong influence of Si/SiO2 interfaces

• improvement by passivation with hydrogen

SummaryConclusions II



3030

• B. Rech• D. Sixtensson, D. Patzek, • L. Korte, M. Schulz, B. Gorka, I. Didschuns, K. Jacob, A. Scheu• U. Bloeck, P. Schubert-Bischoff• Financial Support: BMBF 03SF0308

Acknowledgements

Project partners• IHT - RWTH Aachen (Prof. H. Kurz, Dr. B. Spangenberg)• IEF5-PV - FZ Jülich (Prof. J.H. Werner, Prof. U. Rau)• BTU Cottbus (Prof. M. Kittler)• ERC/GfE Jülich/Aachen (Prof. J. Mayer)• IFTO - FSU Jena (Prof. F. Bechstedt)• IZM-MLU Halle (Dr. H. Leipner)

Coorperations:• Center of Excellence, UNSW, Australia (Prof. M. Green)• Academy of Science, Czech Republic (Prof. Dr. J. Kocka)

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Is it possible to enhance solar energy conversion ... · GaInP/GaAs/Ge 32.0 % 1.5AM Spectrolab 2003 GaInP/GaInAs/GaAs 40.8 % 326 suns NREL 2008 a-Si/µc-Si 11.7 % 1.5AM Kaneka 2004