Advanced characterization of solar energy materials and ... · PDF fileAdvanced...

Advanced characterization of solar

energy materials and novel solar

cell concepts

Klaus Magnus JohansenHead of the Micro - and Nanofabrication lab at University of Oslo

Norwegian Micro and Nanofabrication Facility - NorFab

Outline

• Introduction to the open access national clean

room infrastructure NorFab

– Processing and characterization possibilities at UiO

• An overview and examples from the solar cell

activity at the UiO Micro and Nanotechnology

Laboratory

Norwegian Micro- and Nanofabrication Facility

NorFab in numbers:

•Project period: – NorFab I: 2010-2014

(7 M€ RCN support)

– NorFab II: 2015-2019

(14 M€ RCN-support)

•Ca. 90M€ total investment cost (60M€ buildings /30M€ equipment)

•10 M€ running cost/year

•22 engineers

•535 users/ 46476 user hours

•257 instruments

•2300m2 cleanroom area

•80 industrial users (41 companies)

www.norfab.no

Norway

Finland

Sweden

Denmark

NTNU NanoLab

Norwegian

University of

Science and

Technology

SINTEF

MiNaLab

MiNaLab

University

of Oslo

MST-lab

UC Buskerud

Vestfold

Trondheim

Oslo

Horten

Norway

Finland

Sweden

Nordic NanoLab Network (NNN)

Slide 4

MiNaLab

(SINTEF/UoO)

MST-lab

HBV

Electrumlab

KTH/Acreo

Ångström MSL

Uppsala

MC2 NFL

Chalmers

Lund Nano Lab,

Lund University

Denmark

NTNU

NanoLab

Cooperation of the national cleanroom infrastructures in the Nordic countries

Key numbers (2014):

• Open access to10 cleanrooms in 4 national

infrastructures serve over 2.000 users

• >1500 tools for micro- and nanofabrication

in over 10.000m2 cleanroom area

• Almost 300.000 user hours

• on the management level

• on the expert level

• on the user levelUniversity of Iceland, Reykjavik

Application submitted for ESFRI –European Strategy Forum on Research Infrastructures

2nd floor

UiO MiNaLab –

Novel semiconductors

Location: Oslo node

Area: 440 m2

Type: R&D, education

Staff: ~50 researchers, 3 engineers and

1 Administrator

The UiO MiNaLab facility is operated by

the LENS center of excellence at UiO.

Main competence:

•Semiconductor physics

•Ion beam modification and analysis

•Thin film manufacturing

•Electrical/chemical/optical defect

characterization

•Electronic devices

SiC

ZnO

In2O3

Si

GaN-ZnO

ZnSbCu2O

TiO2

NiO

Ga2O3

Material systems

• Wide bandgap materials:• Metal oxides• Metal oxynitrides

• Materials for LEDs, Solar cells, thermoelectrics andpower electronics

• Defect characterization• Optical • Electrical

UiO MiNaLab

Deposition- Magnetron sputtering- E-beam deposition- Thermal deposition- Metal organic vapor phase epitaxy(MOVPE)

- Atomic layer deposition (ALD)- Plasma-enhanced chemical vapor deposition (PECVD)

- Epitaxial sputter deposition

Available equipment

Processing- Ion implantation/

modification- Optical lithography- Chemical processing- Thermal processing; RTP

and furnaces up to1800°C

- Reactive ion etching (RIE)- Cross sectional polisher

- Electrical char.- Temperature dep. Hall effect

- Temperature dep.scanning probe microscopy(AFM/SPM)

- Spectrophotometer- Solar simulator

- Ellipsometry- Fourier transform infrared absorption (FTIR)

- High resolution x-ray diffraction (HRXRD)

- Chemical characterization- Rutherford backscattering spectroscopy (RBS/C)

- Cathodoluminesence

Characterization

Solar cell projects @ UiO MiNaLab

• The Norwegian Research Centre for Solar Cell Technology

(Solar United)

• Research Center for Sustainable Solar Cell Technology

(SUSOLTECH)

• Development of a Hetero-Junction Oxide-Based Solar Cell

Device (HeteroSolar)

• Longer lifetime and higher efficiency of CZTS thin-film solar

cells (PV-Life)

• Novel Semiconducting Alloys in Energy Technology (SALIENT)

• …

How does a solar cell work

• Sunlight shines on a pn-diode (i.e., with an internal electric field)

• Electron-hole pairs are generated and separated by the field

• Can be used to set up a current through an external circuit

• Key limiting factors

– The fraction of sunlight captured• Reduce reflection

• Reduce transmission

– Lifetime of the electron/hole• Avoid defect recombination

– Heat loss

– …

Solar Cell Research

@ MiNaLab UiO

Absorber

(Si)

Emitter (Si)

AR-coating

Commercial

solar cell

Solar Cell Research

@ MiNaLab UiO

AR-coating

• Silicon properties

• Defects

Commercial

solar cell

Absorber

(Si)

Emitter (Si)

Solar Cell Research

@ MiNaLab UiO

AR-coating

• Silicon properties

• Defects

• Shallow junction formation

• Diffusion

Commercial

solar cell

Absorber

(Si)

Emitter (Si)

Flash lamp annealing,

shallow B emittter formation

Riise et al. Appl. Phys. Lett.

107, 022105 (2015)

Solar Cell Research

@ MiNaLab UiO

AR-coating

• Silicon properties

• Defects

• Shallow junction formation

• Diffusion

• Replacing AR with TCO

• TCO as active emitterTCO

TCO

Commercial

solar cell

Next generation

solar cells

Absorber

(Si)

Emitter (Si)

Absorber

(Si)

Emitter (Si)

Absorber

(Si)

TCO = Transparent conductive oxide

Solar Cell Research

@ MiNaLab UiO

AR-coating

Tandem

cell

• Silicon properties

• Defects

• Shallow junction formation

• Diffusion

• Replacing AR with TCO

• TCO as active emitter

• Tandem cell concepts

TCO

TCO

Commercial

solar cell

Next generation

solar cells

Cu2O on glass ZnO/Cu2O on glass

ZnMgO:Al/Cu2O/Au on quartz

Absorber

(Si)

Emitter (Si)

Absorber

(Si)

Emitter (Si)

Absorber

(Si)

Solar Cell Research

@ MiNaLab UiO

AR-coating

Tandem

cell

• Silicon properties

• Defects

• Shallow junction formation

• Diffusion

• Replacing AR with TCO

• TCO as active emitter

• Tandem cell concepts

• Light conversion/

• Multi-carrier generation

• Novel concepts

TCO

TCO

Nano

Commercial

solar cell

Next generation

solar cells

Absorber

(Si)

Emitter (Si)

Absorber

(Si)

Emitter (Si)

Absorber

(Si)

«Why bother with solar cells beyond

Silicon?»

Si has been extremely successful:

- Cheap

- Reliable

- Well studied

- Optimized efficiencies

Image from: http://commons.wikimedia.org/ , photography by OhWeh

There are two main drawbacks with Si

• Small bandgap– Light with energy above ~1 eV

is converted to heat

– 1.34 eV would be optimal for a single pn-junction according to the Shockley–Queisser limit

– The limit for Si based cells is approximately 29 %

– Efficiency for biofuel < 1%

• Low absorption– Requires relatively thick solar cells to optimize the energy harvest.

Typically 100 – 500 µm

– No flexibility

Challenges for thin film solar cell materials

• Cost• CIGS (CuInGaS)

• InGaP / GaAs / Ge / InGaAs

• Toxicity• CdTe

• Perovskite (Often contains Pb)

• Reliability• Perovskite

• Organic

• Dye-sensitized

• Band gap

• Efficiency

Image from First Solar

CZTS (Copper Zinc Tin Sulfide)

Cu(In,Ga)Se2 (CIGS)

Indium is a rather

expensive metal

• Good absorption• 1 - 2 μm thick

Cu2ZnSnS4 (CZTS)In- free gives lower cost

• More research before industrial production

• Liquid phase non-vacuum deposition methods

are very successful → cheap production

Substrate

Mo

CdS

i-ZnO

ZnO:Al

Cu2ZnSnS4

Wide band gap

• CIGS current record device is 22.6 % (ZSW)

• CZTS current record is 13.7% with band gap grading (KIST)

Sigbjørn Grini

• Potential for tandem solar cell with a wide

band gap solar cell on top

CdS

Eg ~ 2.4 eV

Cu2ZnSn(S1-x,Sex)4

Eg ~ 1.0 - 1.5 eV

0x

Cu2ZnSnS4

CdS

Cu2ZnSn(S1-x,Sex)4

CdS x

0

Band gap grading

CdS

Band gap ~ 2.4 eV

Cu2ZnSnS4

Band gap ~ 1.5 eV

Band misalignment

Exchange sulfur with selenium

Se/S gradient?

Se atmosphere

450°C

10-30 minutes

32S

80Se

80Se

Cu2ZnSnS4

Cu2ZnSnS4

Cu2ZnSnSe4

N. Ross, J. Larsen, S. Grini, L. Vines, C. Platzer-Björkman, Practical limitations to selenium annealing of compound co-sputtered Cu2ZnSnS4 as a route to

achieving sulfur-selenium graded solar cell absorbers, Thin Solid Films, Volume 623, 1 February 2017, Pages 110-115, ISSN 0040-6090,

http://dx.doi.org/10.1016/j.tsf.2016.12.044.

Adapting the solar spectrum

Conduction band

Valence bande-

e-e-

e-e-

e-e-

e-

~1 e

V

The Si-cell

Photons ca 3 eV

Blue/UV-light

e-

Heat

We get a current, however, a

lot of the energy is lost as heat

Photonsplitting is one approach

e-e-e-e-e-e-e-e-

Conduction band

Valence bande-

e-e-

e-e-

e-e-

e-

~3 e

V

~1 e

V

Photon splitting material The solar cell material

Photons ca 3 eV

e-e-

Per-Anders Hansen,

TiO2 and europium (Eu)

The tandem concept is another approach

Cu2O – ZnO heterojunction

• Cu2O

– Cheap and reliable

– Direct band gap of

2.1 eV

– Well studied

• ZnO

– Cheap and reliable

– Direct band gap of

3.4 eV

– Well studied

Difficult to make n-type Cu2O

Heterojunction necessary

p - Cu2O n - ZnO

Lattice mismatch/Defects at interface

Kristin Bergum

Literature advances

• Until 2010, the highest efficiency was ~2%

Minami et al., Appl Phys Express, 9, 052301, 2016

8.1%

Cu2O:Na / Zn0.38Ge0.62O / AZO/MgF2

Oxidized Cu-metal sheet

Reactive magnetron sputtering

Reactive sputter deposition of Cu2O

Biccari, Francesco, «Defects and doping in Cu2O», PhD thesis

600 700 800 900

• Best solar cells (8%) are

from thermally oxidized

Cu sheets

• High crystallinity (mm-

sized grains)

– Us: ~100 - 150 nm

• Mobility of ~100 cm2/Vs

– Us: <20 cm2/Vs

• Carrier concentration of

~1013 cm-3

– Us: ~1015 cm-3

Annealing of Cu2O films

Annealing of Cu2O films –

Mobility and optical properties

400 500 600 700 800 9000

10

20

30

40

50

60

as-dep

Mob

ility

, (c

m2/V

s)

Annealing temperature, (°C)

Combination of a change in

both crystallinity and defects

Temperature dependent Hall reveals (at

least) two electrically active defects

• vCu’ and

• A still unknown defect

GaN – ZnO alloys

• Both have 3.4 eV direct bandgap

• The alloys are predicted to have a smaller

bandgap

• Of interest for, e.g.

– Water splitting

– Solar cells

Summary: Solar activity at UiO

• A modern open access cleanroom lab for

materials processing and characterization

• Activity related to several aspects of the the

Si-cell and beyond

– Transparent conductive oxides, ITO, AZO, GZO

– Oxide heterojunction cells

– CZTS-thin film cells

– Shallow emitter formation

– Defect characterization of solar grade Si

LENS – Light and electricity from novel semiconductors

The energy bandgap

• Sunlight can provide the energy to overcome the gap

• Different semiconductor materials can have different

gaps