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Microscopic Characterisation of Solar Cells

The presentation I used for my Ph.D. thesis defence on September 6th, 2013. Best viewed with a 1280:800 screen resolution.
by

Timo Wätjen

on 16 September 2013

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Transcript of Microscopic Characterisation of Solar Cells

Overview
Microscopic Characterisation of Solar Cells

ZnO:Al

An Electron Microscopy Study of Cu(In,Ga)Se₂
and Cu₂ZnSn(S,Se)₄ Solar Cells
J. Timo Wätjen
Ångström Laboratory
Uppsala, Sweden
2013-09-06
"Buffer layer" (n-type)
Paper I: Zn Sn O Buffer Layers
Paper VI: CZTS Annealing Process
Back contact
Transparent front contact
ZnO
CdS
CIGS
Paper III: Laser Interconnects in Solar Cell Modules

Electrical Characterization
[Solibro]
Interconnect
[Westin, By Means of Beams: Laser Patterning and Stability in CIGS Thin Film Photovoltaics]
What is a solar cell module?

Consumer product
Large area
Many solar cells in series
Laser Micro-Weld
Micro-Weld Zone
CdS
Absorber layer (p-type)
Mo
Thin Film Solar Cell
Glass substrate
Several thin layers
Nanometers thick

Influence performance
Material Characterization
Topview
Cross-Section
Paper VIII: TEM Artifacts in Kesterites
Paper VII: Back contact reaction in CZTS
Anneal

~500 °C
Small grains
Uniform composition
Rapid grain growth
Zn-S, Cu-S and Sn-S
Low Temperature Precursor
Annealed Absorber
explain image: STEM, EDS, Mo-S EDS and Why EELS
what impact could secondary phases have?
Sample preparation
ions cause damage
energy dependent
chemical binding
Ion damage
Cu rich damage layers


Better sample preparation

Results:
CZTS Cu source
Cu in MoS is an artifact
FIB Cross Section
Combine FIB, TEM & EBIC
Microscopic electrical characteristics

Avoid secondary phases
Explains dark JV
What is missing?
Solar Cell JV Characteristics
A
A
B
B
Microscopic Solar Cells
Uniform CIGS

Transparent mask (SiO )

Light source
Contact
Microscopic JV Characteristics
Paper V: CZTSe Solar Cell
Paper IV & V

Series Connection:

3 lines
Mechanical scribing
Interrupts deposition process
Chipping / dead area
No chipping
Fast laser pulses
Heat material locally

Transform material
Conductive weld zone
Difficult to characterize
Focused Ion Beam (FIB)
Working scribe (50 kHz):

Resolidified melt
New phases
Se poor absorber layer


Cu Se phase (~1:1)
Conductive phase
Explains electrical properties
x
CIGS or CZTS
Why use TEM?

Thickness (rough interface)
Phase segregation (ZnO & SnO?)
Local compositional profiles (EDS)
1-x
x

Results:
Nucleation problems
Uniform coverage & composition

Poor performance JV

Band gap ~ 1 eV
Low current

Homogeneously poor?
Contact Cell
Local current generation
Electrical properties

Electron Beam Induced Current
EBIC
SE
3D Model
New perspective
Combine FIB, TEM & EBIC
Clear link to ZnSe
What about JV?
2 Cu ZnSnS + Mo
2
4
2 Cu S + 2 ZnS + 2 SnS + Cu-MoS
2
2
Single phase region
Decomposition with time
Back contact reaction

Secondary phases
MoS & SnS using Raman
2
Agrees with theoretical calculation
CZTS less stable than CIGS
Change back contact material
Weld Zone
Solar Cell
Cell 2
Cell 1
Mo & MoSe
CZTSe
ZnO
2
Identify area
Pt-Mask
Ion etch
Wet etch
Contact
Microscopic Devices
x
Illuminated JV
Low Voc
Low FF
2
4
5
6
7
8
9
10
12
11
13
14
15
16
17
18
19
20
21
22
23
[Westin, By Means of Beams: Laser Patterning and Stability in CIGS Thin Film Photovoltaics]
Solar cell efficiency


Parallel solar cells
Influence each other
Limited by small regions

x
__
__
__
__
2
30 keV
5 keV
Transmission Electron Microscope
Thin sample (<100 nm)
High resolution (Å)
Energy Dispersive Spectroscopy
Energy filter
Electron Energy Loss Spectroscopy
Light elements
Inhomogeneous!
Dead area (~25 %)
3
Two parallel cells
Active area underestimated
Imaging
Electron Diffraction
Material Characterization
Edited from
[www.youtube.com/watch?v=_g7c2dswglc]
[http://www.todaysphoto.org/potd/large/earth-sun-iss.jpg]
[http://www.todaysphoto.org/potd/large/earth-sun-iss.jpg]
Full transcript