Failure Analysis and Long Term Stability of Thin Film Solar Cells and Modules Fehleranalyse und Langzeitstabilitätsmessungen von Dünnschichtsolarzellen und Modulen
Der Technischen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg
zur Erlangung des Doktorgrades Dr. Ing.
vorgelegt von Dipl.-Phys. Jens Adams
aus Berlin
i
Als Dissertation genehmigt von der Technischen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg Tag der mündlichen Prüfung: 21. Oktober 2015
Vorsitzender des Promotionsorgans: Prof. Dr. Peter Greil 1.Gutachter: Prof. Dr. Christoph J. Brabec 2.Gutachter: Prof. Dr. Vladimir Dyakonov
Zusammenfassung
Zusammenfassung
Fehleranalyse und Langzeitstabilitätsmessungen von Dünnschichtsolarzellen und Modulen Dünnschichtsolarzellen und –module, basierend auf organischen und anorganischen Halbleitermaterialen, wurden innerhalb der letzten 10 Jahre zu hoch effizienten Leistungsgeneratoren entwickelt. Diese Zellen bestehen aus einem Stapel verschiedener dünner Zwischenschichten, welche von einigen Nanometern bis hin zu einigen Mikrometern variieren können. Durch ungenaue Zellherstellung als auch durch umweltbedingte Alterungsprozesse können sich Defekte bilden, welche die Leistungsfähigkeit und Lebensdauer der Solarzellen beeinträchtigen. Aus diesem Grund ist ein genaues Verständnis des thermischen
und
elektrischen
Einflusses
von
unterschiedlichen
Verlustmechanismen entscheidend für eine Verbesserung der Zuverlässigkeit von Solarzellen und -module
Diese Arbeit wurde aus der Motivation heraus initiiert, das derzeitige Verständnis bezüglich des thermischen und elektrischen Einflusses von unterschiedlichen Defekten in Dünnschichtsolarzellen zu verbessern. Das Ziel dieser Arbeit ist es, mittels bildgebender Messverfahren unterschiedliche Verlustprozesse in Solarzellen zu charakterisieren und zu quantifizieren. Hierzu werden verschiedene Alterungsexperimente von organischen Solarzellen in Verbindung
mit
bildgebenden
Messverfahren
und
elektrischen
Charakterisierungsmethoden durchgeführt. Besonders die Kombination von elektrischen Charakterisierungen mit bildgebenden Messverfahren ermöglicht eine detaillierte Untersuchung verschiedener lokaler Rekombinations- und Verlustprozesses von licht- und strominduzierten Ladungsträgern.
i
Zusammenfassung Neben einer kurzen Einführung, welche auf die derzeitige Problematik bezüglich
leistungsminimierender
Verlustmechanismen
in
Dünnschichtsolarzellen hinweist, wird in Kapitel 2 der theoretische Hintergrund zur Beschreibung von organischen und anorganischen Dünnschichtsolarzellen präsentiert.
Kapitel
3
geht
anschließend
auf
die
unterschiedlichen
Verlustmechanismen von Ladungsträgern in Solarzellen ein und präsentiert zudem
eine
detaillierte
Beschreibung
des
technischen
Standes
von
bildgebenden Messverfahren, mit Hauptaugenmerk auf Lock-In Thermographie. Eine Zusammenfassung der unterschiedlichen Degradationsmechanismen von organischen Solarzellen wird in Kapitel 4 präsentiert. Die Bescheinigung der experimentellen Aufbauten sowie deren Messprinzipen, welche für diese Arbeit verwendet werden, ist in Kapitel 5 zu finden.
Im Detail wird im Kapitel 6 die Langzeitstabilität von organischen Tandemsolarzellen unter kontinuierlicher Beleuchtung mit sichtbarem Licht untersucht. Die untersuchten Testzellen zeigen eine Verlustleistung von 11% innerhalb der ersten 2000 Betriebsstunden. Die hohe Stabilität wird in erster Linie durch eine invertierte Zellenstruktur, der Verwendung von MoOx anstelle von PEDOT:PSS als Loch-Extraktions-Schicht, sowie der Modifikation der ZnO/Halbleitergrenzschicht mit Ba(OH)2 erzielt. An Hand von verschiedenen Alterungsexperimenten wird der Einfluss und die Bedeutung von ultravioletter (UV) Strahlung, für Zellen welche eine ZnO Zwischenschicht beinhalten, untersucht. Experimente mit unterschiedlichen Lichtbedingungen zeigen zu dem die Kinetik der S-Verformung der IV-Kennlinie. Des Weiteren kann der Ursprung der S-Verformung in der Tandemzelle genau lokalisiert werden.
Kapitel 7 bezieht sich auf die Charakterisierung der temperatur- und feuchtigkeitsbedingten organischen
Alterung
Solarzellen.
An
von Hand
invertierten der
und
glasverkapselten
Kombination
unterschiedlicher
bildgebender Infrarot- und Lumineszenzmessungen, sowie der elektrischen Charakterisierung
der
Testzellen
können
wertvolle
Erkenntnisse
von
feuchtigkeitsbedingten Alterungspfaden gewonnen werden. Für die präsentierte Studie werden glasverkapselte Testzellen bei unterschiedlichen kontrollierten Umweltbedingungen gelagert. Neben Lebenszeitdauern von ca. 20.000 ii
Zusammenfassung Stunden zeigen die Untersuchungen, dass die Diffusion von Feuchtigkeit einen Hauptgrund
für
temperaturen
eine
darstellt.
beschleunigte Die
Alterung
Quantifizierung
bei der
hohen
Umgebungs-
temperaturabhängigen
Beschleunigungsfaktoren, bestimmt nach dem Arrheniusmodel, zeigt zu dem eine Aktivierungsenergie der feuchtigkeitsbedingten Alterung von ~450 meV. Die Messung der ortsaufgelösten Photo- und Elektrolumineszenz offenbart, dass Wasser in erster Linie die Elektroden/Halbleitergrenzschicht der Testzellen beeinträchtigt und nicht das Halbleitermaterial selbst. Mittels unterschiedlicher Elektrodengeometrien wird gezeigt, dass Feuchtigkeit primär durch einer der Zellschichten diffundiert.
Neben der Charakterisierung unterschiedlicher Degradationspfade in organischen Solarzellen konzentriert sich Kapitel 8 auf den thermischen und elektrischen
Einfluss
von
produktionsbedingten
Defekten
in
Dünnschichtsolarmodulen. Dazu wurden 15 Testsolarmodule aus einer großindustriellen Produktionslinie für CIGS Module entnommen und für die Charakterisierung zur Verfügung gestellt. Jedes der einzelnen Testmodule enthält mehrere produktionsbedingte Fehlerstellen, welche die maximale Leistung
eines
Modules
beeinträchtigten.
Mit
„illuminated
lock-in
thermography“ (ILIT) kann jeder einzelne Defekt im Modul lokalisiert und dessen elektrischen Einfluss auf die ihm umgebene Zelle charakterisiert und quantifiziert werden. Um dies zu erreichen, wurde eine neue Methode zur Spannungsbestimmung einzelner Zellen im Module entwickelt. Das ermittelte Verhältnis aus IR-Emission eines Defektes
und dessen
verursachten
Spannungseinbruch der Zellspannung zeigt, dass über 95% der untersuchten Defekte lediglich eine Minimierung der Zellspannung von weniger als 20% verursachten. Es wird gezeigt, dass besonders starke Defekte eine schwache IR Emission aufweisen und gleichzeitig einen starken Spannungseinbruch verursachen. Mittels Computersimulationen kann das Phänomen bestätigt und auf eine Sättigung des Defektstromes zurückgeführt werden.
iii
Acknowledgement
Acknowledgement The first person I would like to thank is Professor Christoph J. Brabec who gave me the opportunity to make this thesis under his supervision. Thanks for fruit full discussions and for the right words at the right time. Furthermore, I would like to thank Professor Vladimir Dyakonov, who had agreed to act as a second supervisor for this thesis.
I would like to thank Dr. Monika M. Voigt, Dr. Claudia Buerhop-Lutz, Dr. Michael Salvador, and Dr. Hans-Joachim Egelhaaf for their guidance of the last five years. Special thanks go to Dr. Michael Salvador for sharing his experiences and knowledge of OPV as well as for asking questions which sometimes drove me crazy. Furthermore, I would like to thank Dr. Monika M. Voigt who introduced me to the world of OPV and who gave me the opportunity to see the world.
I would like to thank all members of the ZAE, I-Meet, and EnCN for their support and for a nice atmosphere. Special thanks go to Frank, Luca, George, Stefan, Andi, Simon, and Sebastian for supporting this thesis by making cells, simulations, measurements, and discussions. I thank George, Frank, Thomas, Fei, and Anastasia for an awesome time in the USA.
I want to thank Felix Hoga, Urs Bogner, and Karl Borutta for the technical support of this thesis and for good times in Nürnberg and Franken.
I want to thank my lovely wife Rena Hamabata, my family on both sides of the world and all my friends in Germany and Japan for their support during this time.
iv
Publications
Publications 2015
J. Adams, M. Salvador, Luca Lucera, George Spyropoulos, F.W. Fecher, M.M. Voigt, S. A. Dowland, A. Osvet, H. Egelhaaf, C.J Brabec “Water ingress in encapsulated inverted organic solar cells: correlating infrared imaging and photovoltaic performance”, Advanced Energy Materials, aenm.201501065 (submitted)
2014
J. Adams, A. Vetter, F. Hoga, F. Fecher, J.P. Theisen, C.J. Brabec, C. Buerhop-Lutz, “The influence of defects on the cellular open circuit voltage in CuInGaSe2 thin film solar modules—An illuminated lock-in thermography study”, Solar Energy Materials & Solar Cells 123 (2014) 159–165
J. Adams, G. D. Spyropoulos, M. Salvador, N. Li, S. Strohm, L. Lucera, S. Langner, F. Machui, H. Zhang, T. Ameri, M. M. Voigt, F. C. Krebs and C. J. Brabec, “Air-processed organic tandem solar cells on glass: toward competitive operating lifetimes”, Energy Environ. Sci., 2015,8, 169-176
J. Adams; F. W. Fecher ; F. Hoga ; A. Vetter; C. Buerhop, C.J. Brabec "IR-imaging and non-destructive loss analysis on thin film solar modules and cells", Proc. SPIE 9177, Thin Films for Solar and
Energy
Technology
VI,
917703
(October
3,
2014);
doi:10.1117/12.2061724; http://dx.doi.org/10.1117/12.2061724
v
Publications
T. R. Andersen, H. F. Dam, M. Hösel, M. Helgesen, J. E. Carlé, Thue T. Larsen-Olsen, S. A. Gevorgyan, J. W. Andreasen, J. Adams, N. Li, F. Machui, G. D. Spyropoulos, T. Ameri, N. Lemaître,. M. Legros, A. Scheel, D. Gaiser, K. Kreul, S. Berny, O. R. Lozman, S. Nordman, M. Välimäki, M. Vilkman, R. R. Søndergaard, M. Jørgensen, C. J. Brabec and F. C. Krebs, “Scalable,
ambient
atmosphere
roll-to-roll
manufacture
of
encapsulated large area, flexible organic tandem solar cell modules”, Energy Environ. Sci., 2014, 7, 2925–2933
F. Livi, Dr. R. R. Søndergaard, Dr. T. R. Andersen, B. Roth, Dr. S. Gevorgyan, Dr. H. F. Dam, Dr. J. E. Carlé, Dr. M. Helgesen, G. D. Spyropoulos, J. Adams, Dr. T. Ameri, Prof. C. J. Brabec, Dr. M. Legros, Dr. N. Lemaitre, Dr. S. Berny, Dr. O. R. Lozman, Dr. S. Schumann, Dr. A. Scheel, P. Apilo, Dr. M. Vilkman, Dr. E. Bundgaard, and Prof. F. C. Krebs, “Round-Robin Studies on
Roll-Processed
Combined
with
ITO-free
Organic
Inter-Laboratory
Tandem
Stability
Solar
Studies”,
Cells Energy
Technology. doi:10.1002/ente.201402095
F. W. Fecher, J. Adams, A. Vetter, C. Buerhop-Lutz, C. J. Brabec, “Loss analysis on CIGS-modules by using contactless, imaging illuminated
lock-in
thermography
and
2D
electrical
simulations“ Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th, 3331 – 3334, doi: 10.1109/PVSC.2014.6925648 2013
Vetter, F. Fecher, J. Adams, R Schaeffler, J. P. Theisen, C. J. Brabec & C. Buerhop, “Lock-in thermography as a tool for quality control of photovoltaic modules“, Energy Science and Engineering 2013; 1(1): 12–17
vi
Oral Presentation 2012
Cl. Buerhop, J. Adams, F. Fecher, C. J. Brabec, Lock-in Thermographie an Dünnschichtmodulen, ep Photovoltaik aktuell, no. 7/8 (2012) pp. 37-41
Oral Presentation 2015
J. Adams, M Salvador, F. W. Fecher, L. Lucera, S. Langner, A. Osvet, M. M. Voigt, H. J. Egelhaaf, C. J. Brabec, LOPEC 2015, Munich,
Germany,
“Characterization
of
moisture
induced
degradation of organic solar cells using non-destructive lock-in NIR/IR imaging techniques” 2014
J. Adams; F. W. Fecher ; F. Hoga ; A. Vetter; C. Buerhop, C.J. Brabec, SPIE 14, San Diego, USA, August, " IR-imaging and nondestructive loss analysis on thin film solar modules and cells"
J. Adams, F. W. Fecher, F. Hoga, S. Besold A. Vetter, C. Buerhop, M. M. Voigt, C. J. Brabec, ISFOE14, Thessaloniki, Greece, „ Loss analyses on organic solar cells and modules by using contactless and non-destructive infrared lock-in imaging techniques”
J. Adams.,1st Joint Workshop on Organic Electronics, May 2014, ZAE Bayern, Erlangen, Germany “Long term stability of OPV single junction and tandem cells“
F. W. Fecher, J. Adams, A. Vetter, C. Buerhop-Lutz, C. J. Brabec, IEEE 2014, Denver, USA, June, “Loss analysis on CIGS-modules by using contactless, imaging illuminated lock-in thermography and 2D electrical simulations“, given by A. Vetter
vii
Poster Presentation
H.-J. Egelhaaf, J. Adams, F. Fecher, M. Salvador, B. van der Wiel, T. Sauermann, A. Distler, U. Dettinger,H. Peisert, T. Chassé, X. Wang, D. Zhang, and C.J. Brabec, ISOS 7, Chambery, France, „Lifetime of Printed OPV – Aspects of Intrinsic Stability and Packaging”, given by M. Salvador
Poster Presentation 2013
J.
Adams,
L. Lucera,
M. M. Voigt,
C. J. Brabec,
ISOS
6,
Chambery, France, “ELLI-Imaging on degraded and inverted P3HT-PCBM solar cells” 2011
J. Adams, F. Fecher, R. Schäffler, R. Auer, C. J. Brabec, C. Buerhop-Lutz,
26th
European
Photovoltaic
Solar
Energy
Conference and Exhibition, Hamburg, Germany „Influence of Macroscopic Defects at Different Position in CIGS Modules on Optical Imaging Techniques“
Academic Work 2014
Chairman, SPIE14, San Diego, USA, Session 5: “Light trapping and anti-reflection”
viii
Table of Figures
Table of Figures Figure 2.1: Crystalline lattice structure of a CIGS chalcopyrite unit cell.
6
Figure 2.2: a) Optical band structure of an irradiated CIGS solar cell under short circuit condition.
7
Figure 2.3: 2 carbon atoms and 4 hydrogen (H) atom from an ethane molecule.
9
Figure 2.4: Chemical structure of two common used donor and acceptor materials used to form a BHJ absorber layer of an organic solar cell.
10
Figure 2.5: Illustration of the different physical processes which take place inside an organic and bulk hetero junction solar cell under irradiation.
11
Figure 2.6: a) Illustration of cell stack configurations used for single junction OPV devices.
12
Figure 2.7: Illustration of cell stacks used for multifunction OPV Devices with top and bottom cells connected in series.
14
Figure 2.8: Schematic module layout with monolithic interconnection of cells illustrated as top view (a) and cross section (b).
16
Figure 2.9: Replacement circuit of the one diode model
17
Figure 2.10: IV characteristics of a dark and irradiated organic solar cell solar cell
18
Figure 3.1: Generation and recombination processes of electron hole pairs in thin film solar cells under the Voc condition
22
Figure 3.2: Simplified illustrations of the different basic defect classes.
23
Figure 3.3: (a) Simulated distribution of the local electrical potential of the electrodes layers of a shunted CIGS solar cell under the Voc condition.
24
Figure 3.4: Simulated local electrical power density of a CIGS cell with a defect as a function of cell position.
26
Figure 3.5: DLIT images of a multicrystalline solar cell measured at (a) 0.5 V, (b) +0.5 V Image a and b are scaled to 0 mK (black) ix
Table of Figures to 5 mK (white). Image is reproduced with permission from [111].
29
Figure 3.6: DLIT images of the local distributed current density
30
Figure 3.7: a) Photograph of organic thin film solar modules with three in series connected cells.
32
Figure 3.8: DLIT measurement of a commercial available a-Si thin film solar module with randomly distributed defects.
33
Figure 3.9: DLIT image (left) and ILIT image (right) of a silicon solar cell.
34
Figure 3.10: Comparison between DLIT and Voc-ILIT images excited with different light spectra.
36
Figure 3.11: Relation between module maximum peak power Pmpp and ILIT defect value X for 103 thin film CIGS solar modules.
37
Figure 3.12: (left) Voc-ILIT measurement of flexible OPV modules with 10 cells connected in series.
38
Figure 3.13: Image of the local Rs distribution of a multicrystaline Si cell calculated using Eq 8. Image is reproduced with permission from [148].
40
Figure 3.14: (a) Photograph of the investigated module.
42
Figure 3.15: (left) EL images (top) and PL images (bottom) of a P3HT:PCBM solar cell measured after different dark storage times.
43
Figure 4.1: Different oxidation mechanism of P3HT polymer.
47
Figure 4.2: LBIC images of an inverted P3HT:PCBM solar cell with PEDOT:PSS as HTL at different aging stages.
50
Figure 4.3: Absorption and desorption of oxygen at the ZnO interface
52
Figure 5.1: Typical degradation characteristic of an OPV.
56
Figure 5.2: Photograph of a representative sample holder used for degradation studies, developed at ZAE-Bayern.
58
Figure 5.3: LED solar simulator used for illuminated JV characteristics
60
Figure 5.4: Irradiation spectra from the sun (AM1.5) and the LED solar simulator.
60
Figure 5.5: Emission spectrum of the LEDs used for photo aging.
62
x
Table of Figures Figure 5.6: Circuit diagram showing the open circuit voltage evaluation of a single thin film solar cell within a module with solar cells each connected in series.
63
Figure 5.7: Local cell voltages of a CIGS-module with 36 connected cells in series as measured by the above mentioned mask method (at 30 W/m² illumination power).
65
Figure 5.8: Schematic image of lock-in calculation.
67
Figure 5.9: Schematically sketch of Lock-in setup for DLIT, ELLI and ILIT applications
69
Figure 6.1: Operational stability of organic tandem solar modules
72
Figure 6.2: Absorption spectra of the active materials used for light absorption in the tandem cell.
73
Figure 6.3: Schematic device representation of the tandem and single cells investigated in the present photo-degradation study. Reproduced with permission from [198]
74
Figure 6.4: JV-characteristics of a representative Tandem and respective sub-cells measured at t=0 h. Reproduced with permission from [198]
76
Figure 6.5: Lifetime of the UV light soaking state under continuous photo-aging.
77
Figure 6.6: S-shape formation in the JV characteristic of a representative OPV tandem cell.
78
Figure 6.7: Long-term decay of the UV light soaking (LS) state in the dark.
79
Figure 6.8: Photo-aging of single and tandem OPV cells.
80
Figure 6.9: Relative change of device performance after 2000 h of continuous white light illumination.
81
Figure 6.10: Comparison of operating lifetime of P3HT:PC60BM cells with and without and additional Ba(OH)x interlayer.
82
Figure 6.11: Life time extrapolation from tandem PCE data presented in the experiment shown in Figure 6.7.
83
Figure 7.1: a) Cross section of the inverted P3HT:PCBM devices used in this study.
88
xi
Table of Figures Figure 7.2: Impact of relative humidity at 65 °C on the temporal evolution of Voc, Jsc, FF, and PCE for encapsulated P3HT:PCBM solar cells of inverted architecture.
90
Figure 7.3: Periodically measured JV characteristics of an inverted P3HT:PCBM solar cell stored in a climate chamber at 65°C/85%RH.
91
Figure 7.4: Long-term behavior of P3HT:PCBM solar cells stored at different temperatures (7 °C/51%RH, 20 °C/63%RH, 50 °C/20%RH, 65 °C/20%RH) in the dark.
92
Figure 7.5: Temperature dependence of the acceleration factor K for J sc, FF, and PCE as extracted from the experimental data shown in Figure 7.4.
94
Figure 7.6: ELLI images measured at different storage times using a pulsed voltage of 1 V (forward bias).
96
Figure 7.7: Top to bottom: ELLI, DLIT, and ILIT images of the same cell measured at different storage times.
97
Figure 7.8: Comparison of the electroluminescence (EL) and photoluminescence (PL) signal of a fresh and a degraded cell.
98
Figure 7.9: a) Water permeation model for inverted and encapsulated organic solar cells based on ITO/AlZnO/P3HT:PCBM/PEDOT:PSS/Ag.
102
Figure 8.1: a) Photograph of a 28 cm x 28 cm CIGS test module with 67 cells connected in series.
107
Figure 8.2: A defect (bright spot) identified by an ILIT-Voc measurement (illumination power of 30 W/m², 1Hz lock-in frequency, measuring time of 10 min): a) 0°-image, b) -90°-image, c) amplitude image and d) phase image. Reproduced with permission from [129]
109
Figure 8.3: a) Line scan (Sdiff) of a defective cell (amplitude image) in a CIGS solar module (see Figure 8.2).
110
Figure 8.4: Line scans of a defective cell (amplitude image) in CIGS solar module irradiated with different light intensity.
111
Figure 8.5: Simulated Voc,cell, current through the defect (defect current), and power dissipation of the defect depending on the defect resistance provided a defect size of 0.001 cm².
113
xii
Table of Figures Figure 8.6: a) Normalized open circuit voltage of different cells vs. the defect IR-emission.
114
xiii
Abbreviations
Abbreviations
Ag
silver
Al
aluminum
AL
active layer
AZO
aluminum doped zinc oxide
BHJ
bulk hetero junction
C
carbon
Ca
calcium
CCD
charge coupled device
CdS
cadmium sulfide
CIGS
copper indium gallium selenide
CO2
carbon dioxide
CuGaSe
copper gallium selenide
CuInSe
copper indium selenide
DLIT
dark lock-in thermography
EL
electroluminescence
ETL
electron transport layer
EQE
external quantum efficiency
HOMO
highest occupied molecular orbital
H2O
water
HP
high power
HTL
hole transport layer
ILIT
illuminated lock-in thermography
InGaAs
indium gallium arsenide
InSb
indium antimonide
IR
Infrared
ISOS
International
Summit
Stability
xiv
ITO
indium tin oxide
LED
light emitting diode
LiF
lithium fluoride
on
Organic
Photovoltaic
Abbreviations LIT
lock-in thermography
LS
light soak
LUMO
lowest unoccupied molecular orbital
MgF
magnesium fluoride
Mo
molybdenum
MoOx
molybdenum oxide
MoSe
molybdenum selenide
mpp
maximum power point
NETD
noise equivalent temperature difference
NIR
near infrared
O2
oxygen
OC
open circuit
OPV
organic photovoltaic
P3HT
Poly(3-hexylthiophen-2,5-diyl)
pDPP5T-2
diketopyrrolopyrrolequinquethio-phene
PCBM
phenyl-C60-butyric acid methyl ester
PEDOT
poly-3,4-ethylendioxythiophen
PL
photoluminescence
PSS
polystyrolsulfonat
QNR
quasi neutral region
S
camera signal
SC
short circuit
SCR
space charge region
Si
silicon
TCO
transparent conductive oxide
TFSC
thin film solar cell
TOF-SIM
time of flight secondary ion mess spectroscopy
TWI
thermal wave imaging
WOx
tungsten or wolfram oxide
ZnO
zinc oxide
xv
Symbols
Symbols
xvi
A
amplitude
c
speed of light
m/s
cT
specific heat
K
E
energy
eV
Ea
activation energy
eV
FF
fill factor
% or %/100
flock-in
lock-in frequency
Hz
h
Planck constant
Js
I
current
A or mA
J
current density
A/m² or mA/cm²
P
power
W
λ
wavelength of light
m or nm
λT
heat conductivity
W/K
Λ
heat diffusion length
m
PCE
photo conversion efficiency
%
R
resistance
Ω
ρ
material density
Kg/m³
r
distance
m or cm
T
temperature
K or °C
t
time
h or s
V
voltage
V or mV
ν
frequency of light
Hz
ω
frequency
Hz
φ
electric potential
Table of Content
Table of Content Zusammenfassung .............................................................................................. i Acknowledgement.............................................................................................. iv Publications......................................................................................................... v Oral Presentation .............................................................................................. vii Poster Presentation .......................................................................................... viii Academic Work ................................................................................................. viii Table of Figures ................................................................................................. ix Abbreviations ................................................................................................... xiv Symbols ........................................................................................................... xvi Table of Content .............................................................................................. xvii 1
Introduction .................................................................................................. 1
2
Thin film solar cells: concept and materials ................................................. 5 2.1 CIGS thin film solar cell and module: materials and structure .................. 5 2.2 Organic solar cells .................................................................................... 8 2.3 Cell structures of thin film and organic solar cells .................................. 12 2.4 Device architecture of thin film solar modules ........................................ 15 2.5 Electrical description of a solar cell ........................................................ 17
3
Loss analysis of solar cells using different imaging techniques ................. 21 3.1 Radiative and non-radiative recombination in thin film solar cells .......... 22 3.2 Electrical influence of macroscopic defects in thin film solar cells and modules........................................................................................... 23 3.3 Power dissipation of a defective cell under the Voc-condition ................. 25 3.4 Defect imaging with lock-in based IR imaging techniques...................... 27 3.4.1
Defect characterization with dark lock-in thermography ................ 28
3.4.2
Illuminated lock-in thermography .................................................. 33 xvii
Table of Content 3.4.3 4
Loss analysis with EL imaging ...................................................... 38
Degradation and Stability of OPV solar cells ............................................. 45 4.1 Degradation of active layer .................................................................... 46 4.2 Degradation of electrodes ...................................................................... 48
5
4.2.1
Metal electrode.............................................................................. 48
4.2.2
Degradation of hole transport layer ............................................... 49
4.2.3
Degradation of electron transport layer ......................................... 52
Methods for device characterization .......................................................... 55 5.1 Lifetime evaluation of OPV devices........................................................ 55 5.1.1
Sample holder for degradation cycles ........................................... 57
5.1.2
JV characterization ........................................................................ 59
5.1.3
Setup of photo-degradation ........................................................... 61
5.2 Voc-Measurement of thin film solar cells within a module ....................... 62 5.2.1
Cell voltage of modules with randomly distributed defects ............ 64
5.3 Lock-in Imaging ...................................................................................... 66
6
5.3.1
Theory of lock in based imaging.................................................... 66
5.3.2
Imaging setup................................................................................ 69
5.3.3
IR camera...................................................................................... 70
5.3.4
ELLI camera .................................................................................. 70
Photo-degradation of organic tandem solar cells ....................................... 71 6.1 Device fabrication and used materials ................................................... 72 6.1.1
Tandem cell layer deposition......................................................... 74
6.1.2
Device Encapsulation .................................................................... 75
6.2 Photo-aging without UV light .................................................................. 75 6.2.1
Photo-aging of tandem cells .......................................................... 75
6.2.2
Darkaging of OPV tandem cells .................................................... 78
6.3 Photo-aging with UV lights soaking ........................................................ 79 6.4 Conclusion ............................................................................................. 84 xviii
Table of Content 7
Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells ............................................................... 85 7.1 Test conditions and device fabrication ................................................... 87 7.1.1
Organic solar cell preparation and characterization ...................... 87
7.1.2
Imaging parameters and settings .................................................. 88
7.2 Moisture induced degradation of inverted OPC devices ........................ 89 7.2.1
Heat- and damp heat-induced degradation ................................... 89
7.2.2
Determination of the activation energy .......................................... 93
7.2.3
IR imaging of moisture-induced degradation at 65°C/85%RH ....... 95
7.2.4
Moisture diffusion in encapsulated devices ................................... 99
7.3 Conclusion ........................................................................................... 102 8
The electrical and thermal characterization of macroscopic defects in thin film solar modules ............................................................................ 105 8.1 Description of test modules .................................................................. 107 8.2 Influence of a defect on its surrounding cell measured by ILIT ............ 108 8.3 Relation between Voc,cell and IR- emission of a defect .......................... 112 8.4 Conclusion ........................................................................................... 116
9
Summary and Outlook ............................................................................. 119 9.1 Summary .............................................................................................. 119 9.2 Outlook ................................................................................................. 121
Appendix A ..................................................................................................... 123 Appendix B ..................................................................................................... 127 Appendix C ..................................................................................................... 138 Bibliography .................................................................................................... 141
xix
Table of Content
xx
1. Introduction
1 Introduction
In the future, the major economies will face great problems in both energy generation and supply. It is expected that current energy demand will increase from 18 TW (2013) to 30 TW in 2050 [1] and this increase will not be covered using only through conventional energy sources. The dominated use of fossil fuels and nuclear elements within these sources causes critical issues which affecting humans and our environment. The continuously increasing emission of carbon dioxide (CO2) results in climate change while the problem of a safe disposal of nuclear waste is still under discussion. In addition, the 2011 Fukushima disaster served as an impressive example of the very real risks to both humans and the environment which stem from nuclear power sources. Regarding these problems it is understandable that the call for clean and unlimited (renewable) energy continuously rises. Amongst the currently available renewable energy sources such as wind energy and hydropower, solar energy bears the largest potential of future energy supply [1]. One way to use the energy of the sun is the direct conversion of solar power into electricity by using solar cells. Based on the photoelectric effect, firstly described 1839 by Edmond Becquerel [2], the energy of light is absorbed by charge carriers which are lifted to a higher energy level within the cell. By connecting the terminals of the cell with an external load the light induced charge carriers contribute to a photocurrent and power can be extracted from the cell. Large efforts have been made to develop these cells into high performance energy converters [3]. Now a day, different solar cell technologies are available at the market while silicon (Si) based solar cells are the main technology [4]. However, due to the need of electrical grade silicon and the 1
1. Introduction
huge material loss during cell fabrication, the production of Si solar modules is both cost and energy intensive. To save costs and material, a new cell concept was developed which based on the direct deposition of thin films with thicknesses of some microns on rigid glass substrates [5–8]. During the last 15 years, the field of thin film solar cells (TFSC) has been highly active in developing new cell concepts (e.g. single-junction and multi-junction cells) and absorber materials [9]. Therefore, one needs to distinguish between both inorganic and organic solar cells (OPVs). Especially inorganic solar cells based on Copper Indium Gallium Selenide (CIGS) have been developed to highly advanced solar cells which are processed in a sequence of different coevaporation and chemical bath deposition steps. These devices show among all other technologies the currently highest values of efficiency for single cells (21.7%) and modules (15%-17%) [10–12]. However, the use of cost-intensive vacuum technology and the use of rare elements (e.g. indium, gallium etc.) limit the production of low-cost solar modules. Interestingly, in contrast to inorganic TFSCs, OPVs are made of cheap semiconductor materials based on conjugated polymers which can be either evaporated or processed from a solution to thin films. Due to the development of new OPV materials and cell concepts the efficiency of these devices could be continuously improved during the last decade. Current record efficiencies are in a range of about 12% and prove that OPV devices are competitive to standard thin film technologies such as amorphous silicon [9,12,13]. Furthermore, the prospect of implementing fully solution based device fabrication makes this solar technology uniquely suited for roll-to-roll printing applications with potentially very low costs. However, both thin film technologies show various problems with respect to their fabrication and long-term stability. Due to the thin film deposition, these cells and modules are very prone to production-related defects which limit an optimal power extraction and life time. The electrical influence of defects mainly depends on the size, type, position, and irradiation condition [14]. Especially, under low light conditions (e.g. 0.6 V is the current injection regime. This part is mainly limited by series resistances. The values of Rp and Rs can be determined from the dark IV characteristic by calculating the inverse slope at V = 0 for Rp and >0.8V for Rs.
19
2. Thin film solar cells: concept and materials
20
3. Loss analysis of solar cells using different imaging techniques
3 Loss analysis of solar cells using different imaging techniques
Thin film solar cells are made of large area semiconductors (typically several square centimeters) which are optimized for light absorption and charge carrier collection. However, these devices suffer from different loss processes which limit the cell with respect to both device performance and stability. An advanced characterization of spatial resolved loss mechanisms is therefore essential to understand recombination processes of charge carriers and degradation phenomena. Interestingly, if the operation mode of a cell is inverted (e.g. injection of current) different loss processes can be triggered and the injected charge carriers recombine either by the emission of photons (light) or by the transmission of phonons to the lattice (heat). With the development of bidimensional and focal plane array (FPA) detectors, this signals can be detected and converted into a spatially resolved image [97]. Infrared (IR) imaging techniques and near IR (NIR) imaging techniques such as dark lock-in thermography (DLIT), illuminated lock-in thermography (ILIT), electroluminescence lock-in (ELLI), and photoluminescence (PL) imaging are particularly suitable to provide a complementary insight of both localized power losses and recombination mechanisms. Especially IR imaging based on the lock-in technology has been widely explored for silicon cells as a fast, nondestructive, and contactless characterization tool for macroscopic defects (e.g., dark spots, electrode defects, shunts), changes in sheet resistance and for visualizing short circuit current distributions.
21
3. Loss analysis of solar cells using different imaging techniques
3.1 Radiative and non-radiative recombination in thin film solar cells Thin film solar cells are made of micro-crystalline, semi-crystalline or amorphous semiconductor materials with a high degree of disorder of electric states. This leads to different loss processes within the cell which limit an optimal formation of Voc, Jsc, and PCE. During the operation mode of the cell, light is absorbed by charge carriers (e.g. electrons) which are lifted into a higher energy level. If no charge carriers are extracted by an external load, the excess energy can be released either by the emission of photons (radiative) or the transmission of phonons (non-radiative). Regarding the literature different loss processes are discussed [98–106] in thin film solar cells and can be classified into three basic recombination processes (see Figure 3.1).
Figure 3.1: Generation and recombination processes of electron hole pairs in thin film solar cells under the Voc condition
Radiative recombination represents the inverse process of optical absorption. Here, the electron-hole pair recombines directly by the emission of a photon with the energy equal to Eg. Depending on the excitation, this recombination process can be trigger either with the injection of current (electroluminescence) or with the irradiation of light (photoluminescence). For light energies higher than Eg, the charge carrier thermalizes rapidly by energy transfer in form of lattice vibration (e.g transmission of phonons) to a metastable state (Ec). In case of Auger recombination, a third charge carrier might be accelerated by the absorption of a photon which rapidly thermalize to the charge band. The third 22
3. Loss analysis of solar cells using different imaging techniques
process refers to trap assisted recombination. In contrast to band-to-band recombination (radiative), the charge carriers recombine via defect related energy states (trap) within the band gap. Similar to thermalization, the excess energy is also transmitted by phonons, resulting in an increasing cell temperature and the emission of heat. Trap assisted recombination can take place at different sites and can occur as interface recombination, space charge recombination, neutral bulk recombination and back contact recombination [46,107,108]. In particular, thin film solar cells based on organic semiconductor materials show due to their semi-crystallinity a high degree of disorder of electric states. This results in a high density of trap states from the beginning which can increase if the cell is exposed to oxygen and water (see chapter 4.1) [18,26,109].
3.2 Electrical influence of macroscopic defects in thin film solar cells and modules Besides intrinsic losses due to unwanted recombination processes (section 3.1), macroscopic defects have the largest impact on cell performance and stability [35,110,111]. The origin of these defects can be manifold but is mainly related to both improper layer deposition and device fabrication or intrinsic material diffusions [112–115]. During the last decade, different types of defects in solar cells such as bulk/shunt defects [111,113,115,116], interface defects [26,117,118], and interconnection defects [116,119] have been identified and were intensively discussed in the literature. The different defect classes can be seen in Figure 3.2.
Figure 3.2: Simplified illustrations of the different basic defect classes.
23
3. Loss analysis of solar cells using different imaging techniques
Interestingly, the influence on the surrounding cell is strongly related to defect type, position, and irradiation condition. Due to the low defect resistance, a potential gradient between the two electrodes is induced which results in an inhomogeneous Voc distribution of the cell [120]. To gain a better understanding, several electrical simulations provide a complementary insight into local cell parameters and discuss the influence of a defect on the cell [14,121–124]. A simulated local voltage distribution of a single cell in a thin film solar module (CIGS) with an embedded low ohmic defect can be seen in Figure 3.3. The defect was placed in the center of the cell and is represented by a low ohmic resistor (see Figure 3.3 b). Regarding more details of the simulation model please refer to Fecher et al. [14]
Figure 3.3: (a) Simulated distribution of the local electrical potential of the electrodes layers of a shunted CIGS solar cell under the Voc condition. (b) Line scan through the electrical potential (black line in (a)). The equivalent circuit indicates the current (black arrow) and the drop of the electrical potential due to the resistive electrode. Image is reproduced with permission from [14]
The local voltage distribution Vi of an irradiated cell under the Voc condition (Figure 3.3) can be described as 𝑉𝑖 (𝑥, 𝑦) = (Φ 𝑇𝐶𝑂 (𝑥, 𝑦) − Φ𝑀𝑜 (𝑥, 𝑦))
Eq. 4
with Vi as junction voltage, Φ as the electrical potential distribution of the TCO electrode and of the Mo back electrode, and x,y as position parameters. Due to the low defect resistance, Vi is minimal in the center of the cell. In this case the defect acts as an internal load and attracts light induced charge carriers from the surrounding cell. This in combination with the local sheet resistances of the 24
3. Loss analysis of solar cells using different imaging techniques
electrodes induces a potential gradient leading to an inhomogeneous voltage distribution of Vi. Cell parts around the defect show a lower Vi (02000 K). The life times of theses light sources are limited (e.g. xenon lamp ~2000 h) and implications due to temperature degradation of the test samples need to be considered as well. One way to overcome these limitations is the use of high power light emitting diodes (HP-LEDs) as light sources for photo-degradation experiments as done in this study (see chapter 6). For all photo-degradation experiments in HP-LEDs from Bridgeluxe (BXRA-30E0800-B-00) were used. The spectral emission of the HP-LEDs ranges from 400 nm and 750 nm with a local maximum around 445 nm (see Figure 5.5). The emitted light is homogenized using a parabolic and highly reflective reflector. For the injection of current resulting in the emission of light two different standard power supplies made by Agilent (type: E3640A) and Hewlet Packert (type: E2632A) are used. To avoid overheating, the LEDs are mounted on a passive heat sink which is cooled by air. Before each photo-degradation experiment, the light intensity of a HPLED is adjusted to 1000 W/m² by using a test cell with an active layer material similar to the test samples. To neglect the spectral mismatch between the HPLEDs and an AM1.5 spectrum, the light intensity of the each LED was adjusted until Jsc,HP-LED of the test cell matched the Jsc value measured under the solar simulator at 1000W/m².
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5. Methods for device characterization
Figure 5.5: Emission spectrum of the LEDs used for photo aging. Inset: Photograph of the setup used for photo-aging of organic solar cells. One LED is placed above the investigated sample which is mounted in a sample holder. Image is reproduced with permission from [198]
5.2 Voc-Measurement of thin film solar cells within a module As described in section 3, a defect creates an alternative current path resulting in a decrease of the electrical power output and open circuit voltage of a cell (Voc,cell). The influence of the defect on the short circuit current (I sc) is negligible [38]. In order to quantify the defect induced drop of Voc a new method to measures the Voc,cell in a module was developed. During taking measurements of Voc,cell, the whole solar module except for one cell is covered by a house made mask. The mask is made of two polyester blades; each attached to a piece of black velvet and fitted to the required size to cover the module. The two parts of the mask are positioned in parallel with the cell of interest. The uncovered cell is irradiated with light while the module terminals are connected to a Keithley 2000 multimeter for measuring the open circuit voltage of single and uncovered cell. Figure 5.6 shows the equivalent circuit diagram of the setup used for the Voc,cell determination.
62
*This section is adopted with permission from ref [129] (Copyright Elsevier)
5. Methods for device characterization
Figure 5.6: Circuit diagram showing the open circuit voltage evaluation of a single thin film solar cell within a module with solar cells each connected in series. Extracted with permission from [129] (with the permission of Elsevier
The irradiated cell (1) is shown by the one-diode-module as found in the middle of Figure 5.6. The covered cells are represented by the same model, but without a current source (2). The multimeter acts as a high ohmic load (1∙10 7 kΩ in a voltage range of 0-1 V [199]) in the circuit and due to its high ohmic input resistance, a low photocurrent flows through the covered cells and the multimeter. In the covered cells however, no photocurrent is generated. Hence, the covered cells operate in a reverse direction resulting in a blocking of the diodes. Since the current only flows through the parallel and series resistances, the covered cells may be replaced by a high ohmic series resistance R sp,i. Thus, the resistance Rsp,i of a covered cell is calculated by: 𝑅𝑠𝑝,𝑖 = 𝑅𝑝,𝑖 + 𝑅𝑠,𝑖
Eq. 10
Replacing the cell under illumination by a voltage source having the voltage Voc,cell, the voltage measured by the multimeter Vm may be calculated by: 𝑉𝑚 = 𝑉𝑜𝑐,𝑐𝑒𝑙𝑙 ∙ 𝑅
𝑅𝑚
𝑁 𝑚 +∑𝑖=1 𝑅𝑠𝑝,𝑖
Eq. 11 63
5. Methods for device characterization
with Rm as input resistance of the multimeter and N as the number of covered cells. As long as the sum of Rsp 90% in a spectral range between 2 µm and 5 µm is used. A trigger signal provided by the camera is used to synchronize the lock-in algorithm with the pulsed excitation.
5.3.4 ELLI camera For EL investigation a highly advanced EQUUS 327NM EL camera (IRCAM GmbH, Erlangen, Germany) equipped with an FPA InGaAs with a spatial resolution of 640 x 512 pixels is used. The detector is cooled by a Pelletier element and has a spectral response between 0.9 µm and 1.7 µm. The frame rate is 100 Hz. As lens system a NIR objective with a focal distance of 30 mm and a transparency >90% in a spectral range between 0.6µm and 1.7µm is used. Similar to the IR camera a trigger signal is provided by the EL camera which can be used for lock-in measurements.
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6. Photo-degradation of organic tandem solar cells
6 Photo-degradation of organic tandem solar cells
Photovoltaic devices based on organic semiconductors (OPVs) hold great promise as a cost-effective renewable energy platform because they can be processed from solution and deposited on flexible plastics using roll-to-roll processing. Despite important progress and reported power conversion efficiencies of more than 11% the rather limited stability of this type of devices raises concerns towards future commercialization. The tandem concept allows for both absorbing a broader range of the solar spectrum and reducing thermalization losses. Recently, Anderson et al. presented first stability data of OPV tandem devices stored under irradiated and dark conditions (see Figure 6.1). Therefore, OPV tandem modules with efficiencies of about 1.7% with eight cells connected in series were investigated. The stability data are presented in Figure 6.1.
*This chapter is adopted from ref [198] with the permission of the Royal Society of Chemistry
71
6. Photo-degradation of organic tandem solar cells
Figure 6.1: Operational stability of organic tandem solar modules in the dark (top) and light (bottom). The modules had a Ag/HC PEDOT:PSS/ZnO/MH301:PCBM/PFN/HTL PEDOT:PSS/ZnO /MH306:PCBM/PFN/HTL PEDOT:PSS/HC PEDOT:PSS/Ag structure and were processed in an rollto-roll production line. Image was reproduced from [203]
Under Dark storage conditions, it can be seen that the shelf life time of the test devices demonstrates maintenance of Voc over 800 h while the loss in PCE is mainly governed by the decrease of Jsc. in the study it was assumed that the losses in FF and Jsc might be due the formation of gradual traps at the ZnO site. Under light, the degradation of all module parameters was observed to be accelerated and life times of 12 h for T80 or 140 h for Ts80 were calculated. However, current tandem OPV devices suffer from a rather poor long-term stability and are far beyond to be competitive with standard solar cell technologies. To understand the factors which limit the life time of OPV tandem devices an enhanced study with respect to the photo-stability and the influence of UV light on the device performance is presented.
6.1 Device fabrication and used materials All solar cells used for this study are based on an inverted tandem structure comprising sub-cells of the blends poly(3-hexylthiophene):phenyl-C60-butyric acid methyl ester (P3HT:PC60BM) and diketopyrrolopyrrolequinquethio-phene:phenylC70-butyric acid methyl ester (pDPP5T-2:PC70BM) as active layers. Due to its known stability and its absorption spectrum being complementary to that of the
72
6. Photo-degradation of organic tandem solar cells
low band gap polymer pDPP5T-2 [196] (see Figure 6.2), the standard P3HT:PC60BM as active layer of the bottom sub cell was used (see Figure 6.3).
Figure 6.2: Absorption spectra of the active materials used for light absorption in the tandem cell. The inset shows the chemical structure of the polymer pDPP5T-2. Reproduced with permission from [198]
As recombination layer zinc oxide (ZnO) in combination with poly(3,4ethylenedioxythiophene (PEDOT) was used. For charge carrier extraction aluminum-doped ZnO (AZO) layer was used as interface layer between ITO and active layer [72]. The interface between ZnO and pDPP5T-2:PCBM was modified by coating a thin barium hydroxide (Ba(OH)2) layer on top of ZnO to enhance the photovoltaic performance [204,205]. For a better understanding of the degradation behavior, inverted single OPV cells based on the polymer blends used in the tandem structure were fabricated and compared with the tandem cell data. To obtain representative variations of device performance vs. time five devices of each solar cell type were fabricated and encapsulated. To ensure a reliable and reproducible protection along with a minimization of extrinsic effects due to water and oxygen, a glass-on-glass encapsulation geometry was chosen. The quality of the encapsulation was probed using ELLI imaging before and after the photo-aging process (Figure A1). In the following
73
6. Photo-degradation of organic tandem solar cells
the fabrication parameters of the used tandem cells as well as the respective sub-cells will be explained.
6.1.1 Tandem cell layer deposition All photovoltaic devices were fabricated by doctor blading under ambient conditions using an inverted device structure (Figure 6.3).
Figure 6.3: Schematic device representation of the tandem and single cells investigated in the present photo-degradation study. Reproduced with permission from [198]
Laser-patterned ITO coated glass substrates (area of 2.5×2.5 cm2) were successively cleaned in an ultrasonic cleaner using acetone and isopropanol. After drying, a 40 nm thick AZO layer was coated on top of the substrates, which were then annealed for 10 minutes on a hot plate at 140 °C. For the tandem solar cells, a chlorobenzene based solution of P3HT:PC60BM (1:1 wt%, 32 mg/ml in total) was coated on top of the AZO layer to form a 130 nm thick bottom active layer. Subsequently, the intermediate layer was deposited by successively blading a 40 nm thick PEDOT HIL3.3 and 30 nm thick ZnO layer. These layers were dried at 70 °C for 5 min in air. The ZnO layer was modified by coating a very thin (20nm) Ba(OH)2 film on top (7 mg/ml in 2methoxyethanol). Afterwards, an 80 nm thick layer of pDPP5T-2:PC70BM (1:2 wt.%, dissolved in a solvent consisting of 90% chloroform and 10% dichlorobenzene with a total concentration of 24 mg/ml) was deposited as the top active layer. At the end, a 10 nm MoOx layer and 100 nm Ag layer were evaporated in order to form the top electrode. All fabricated solar cells had an active area of 10.4 mm². The active layer thicknesses in the tandem structure 74
6. Photo-degradation of organic tandem solar cells
were tuned to guarantee photocurrent balancing between the sub-cells (see Figure A2). The single solar cell devices were prepared in a way equivalent to the tandem solar cell device fabrication protocol described above. For intimate comparison, the thicknesses of the active layers were chosen to be the same as in the tandem structure, i.e., 130 nm for P3HT:PC60BM and 80 nm for pDPP5T2:PC70BM single cell devices. Optical simulation results for the tandem solar cell geometry based on the transfer matrix approach are depicted in the ESI (Figure. A2).
6.1.2 Device Encapsulation In order to neglect degradation due to extrinsic effects the devices were encapsulated in a glass-on-glass encapsulation geometry. Therefore a dispenser robot from I&J Fisnar Inc. (I&J 4100-LF) was used to distribute the adhesive Katiobond LP655 from DELO GmbH & Co KGaA on top of the completed OPV devices. After the adhesive deposition a second barrier glass with a thickness of about ~100 µm was used to complete the glass on glass encapsulation. Finally, the epoxy was cured for one minute inside a UVACUBE 100 from Hönle AG equipped with an iron doped lamp.
6.2 Photo-aging without UV light
6.2.1 Photo-aging of tandem cells The tandem cells as well as the corresponding single sub-cells were aged under continuous white light irradiation without UV component at ≈1000 W/m² (see section 5.1). The absence of UV light during photodegradation was essential for studying the effect of UV light treatment on the lifetime of the devices. During photo-degradation the open circuit condition was provided to the test cells. The initial device performance of the tandem cells and the respective sub-cells under AM1.5G conditions was 4.4% ±0.2% (tandem),
75
6. Photo-degradation of organic tandem solar cells
2.8%±0.1% (P3HT), and 4.1%±0.2% (pDPP5T-2), respectively (Figure A3). Representative JV characteristics of the investigated test devices can be seen in Figure 6.4.
Figure 6.4: JV-characteristics of a representative Tandem and respective sub-cells measured at t=0 h. Reproduced with permission from [198]
To eliminate the double diode effect which is intrinsic to devices incorporating ZnO and/or AZO [191,206], a single light soaking (LS) treatment by irradiating the test cells with UV light (365 nm) for the time of 10 s (prior to starting J-V characterization) was applied to the test cells. Afterwards the subsequent decay of the UV light soaking state under continuous photo-aging using white light was investigated.
76
6. Photo-degradation of organic tandem solar cells
Figure 6.5: Lifetime of the UV light soaking state under continuous photo-aging. The plots show the device parameters Voc, Isc, FF and PCE over time for the different types of single and tandem solar cells upon initial UV light soaking (365 nm, 10 s) and under continuous photo-aging using white light (400 – 750 nm) without UV component at 1000 W/m². UV light treatment was repeated after 60 hours. Each data point represents the average values of 5 solar cells and is normalized to the initial value at t = 0 hours. Reproduced with permission from [198]
Figure 6.5 shows two consecutive 60 h photo-degradation cycles for elucidating the effect of the initial light-soaking step. These cycles are within the burn-in period of photo-degradation, which typically follows an exponential decay of the initial device efficiency (see also Figure 6.8) [196]. Within the first cycle of continuous irradiation, the PCE of the tandem and the P3HT based sub-cells decreased to 60% and 50% of the initial value, respectively. In both cases, the loss in PCE is mainly dominated by losses in Jsc (≈15 – 20%) and FF (25 – 40%), which can be most likely related to a reduction in charge carrier extraction and the accumulation of carriers in the device, respectively [190,207]. The latter leads to the formation of an S-shape in the J-V-characteristics (see Figure 6.6) [191]. The Voc, on the other hand, remains fairly stable. Interestingly, under the same conditions the solar cell parameters and the overall PCE of the pDPP5T-2 based single cells remained almost intact throughout the same period of time. After 60 h the UV treatment was repeated. Notably, all solar cell 77
6. Photo-degradation of organic tandem solar cells
parameters were almost fully restored and followed the same degradation pattern as in the first cycle. This behavior suggests that the burn in period that is typically observed in the first hours of degradation is triggered by a reversible reaction in these types of devices. A plausible explanation would be that the conductivity of ZnO degrades with time due to the presence of traces of oxygen, while UV treatment can release oxygen and restore its electronic properties [22,190].
Figure 6.6: S-shape formation in the JV characteristic of a representative OPV tandem cell. The device performance was probed immediately upon UV light soaking (0 h, black curve), after 60 h under continuous photo-aging (red curve) and after repeated UV treatment (60 h, black dashed curve; LS means after light soaking). Reproduced with permission from [198]
6.2.2 Darkaging of OPV tandem cells In order to gain an additional insight into the effect of the UV light-soaking process, the transient behavior of the UV treatment in the dark was studied. Figure 6.7 shows the periodically measured change of the photovoltaic parameters for tandem cells upon a single UV light-soaking step. Here, the cells were stored in the dark between JV characterization. Upon UV light soaking, the FF increases dramatically (≈45%) while Jsc increases by about 5% and Voc barely changes. It is well documented in the literature that UV radiation can 78
6. Photo-degradation of organic tandem solar cells
improve the electronic properties (conductivity) of the ZnO layer as well as the contact at the ZnO interface [22,190,191]. This is most likely the reason for the JV characteristics translating from a double-diode type behavior (S-shape) to a diode behavior with high FF. The long-term behavior of UV treatment is much less documented.
Figure 6.7: Long-term decay of the UV light soaking (LS) state in the dark. Each data point represents the average value of 5 tandem cells. The filled symbols represent the condition after immediate light soaking, whereas the hollow symbols represent the temporal decay of the LS state. The data were extracted from J-V-measurements using an AM1.5 solar simulator and an illumination intensity of 1000 W/m². The tandem cells were stored in the dark in between J-V characterization. Reproduced with permission from [198]
In Figure 6.7 it can be seen that the light soaking state remains constant for about 10 h after which the photovoltaic performance decays sharply. Moreover, Figure 6.6 reveals that the light soaking state features a half-time of about 200 h and is, therefore, expected to contribute decisively to the burn-in period of OPVs containing ZnO.
6.3 Photo-aging with UV lights soaking From the previous result, the importance of UV light exposure during continuous 1-sun irradiation can be inferred. Therefore, a long-term and photo79
6. Photo-degradation of organic tandem solar cells
aging test, in which the solar cells were exposed to UV light for 10 s prior to each JV measurement, was designed. Figure 6.8 shows the average long-term evolution of Voc, Jsc, FF, and PCE for the tandem cells and respective sub-cells under continuous white light illumination with intermittent UV treatment.
Figure 6.8: Photo-aging of single and tandem OPV cells. The graphs show the average long-term temporal evolution of PCE, Voc, Jsc, and FF for the different single and tandem cells under continuous white light illumination. The photovoltaic parameters were extracted from J-Vmeasurements using an AM1.5 solar simulator at 1000 W/m². Before each J-V measurement the samples were UV treated (365 nm, 10 s). Each data point represents the average value of 5 tandem devices, 5 DPP devices, and 5 P3HT devices. Reproduced with permission from [198]
It can be seen that the burn-in period extends to ≈800 h, after which the decay of the PCE follows a close to linear trend (Figure 6.8). Remarkably, in the long-term measurements the tandem cells showed the most stable behavior by losing only 11% of the initial value after 2000 h. The PCE of P3HT and pDPP5T-2 based single sub-cells followed a similar decay with losses of 16% and 15%, respectively (see Figure 6.9)
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6. Photo-degradation of organic tandem solar cells
Figure 6.9: Relative change of device performance after 2000 h of continuous white light illumination. The relative change is calculated based on the performance of the single and tandem devices shown in Figure 6.8. The tandem cells showed better overall device stability. Reproduced with permission from [198]
Overall, the loss in PCE is mainly determined by a loss in FF (5 – 11%) and a modest decay in Jsc (0 – 8%). Impressively, in the case of the tandem and pDPP5T-2-based single cells almost no current losses throughout 2000 h of light exposure were observed. To put the results into perspective, PCE drops in the range of 10 – 20% for P3HT single cells and 25% for PCDTBT-based single cell devices under 1-sun exposure and within similar periods of time which have been reported in the past [19,67,196]. It is assumed that the reasons for the enhanced long-term stability of the presented solar cells are manifold. Specifically, an inverted device structure was used which enable elimination of reactive metal interfaces [208]. Additionally, the widely used but moisture sensitive and reactive PEDOT:PSS was replaced with MoOx as the top buffer layer. The benefit of using the chemically more inert MoOx for better device stability has been shown before [209,210]. Moreover, the ETL/pDPP5T-2 (ETL: electron transporting layer) interface was modified with a hole blocking Ba(OH)2 layer resulting in more stable tandem and pDPP5T-2 single sub-cells. Barium and Ba(OH)2 interlayers have been proven to reduce exciton quenching and trap induced recombination at cathode interfaces as well as improve the overall device efficiency in the case of OLEDs 81
6. Photo-degradation of organic tandem solar cells
and OPVs [204,205,211]. Based on the findings during the experiment, it is assumed that Ba(OH)2 could also contribute to stabilizing the ETL/polymer interface by reducing electronic trap formation and oxygen adsorption. Indeed, comparison of the temporal evolution of photovoltaic device performance in the case of P3HT:PCBM with and without Ba(OH)2 suggests that Ba(OH)2 may increase the operating lifetime (see Figure 6.10).
Figure 6.10: Comparison of operating lifetime of P3HT:PC60BM cells with and without and additional Ba(OH)x interlayer. The Ba(OH)x was deposited on top of the ZnO ETL to reduce interfacial recombination center. See also Figure A4 and Figure A5. Reproduced with permission from [198]
Furthermore, periodic UV light soaking during prolonged operation is expected to desorb oxygen trapped at the surface of ZnO and AZO. This step is likely to prevent significant conductivity losses of the ETLs ZnO and AZO, contributing to a larger FF throughout the lifetime measurements [190,212].
For an estimation of the lifetime of the tandem solar cells, a linear regression to the slowly, linearly decreasing PCE data points and extended this line up to 80% of the initial value was applied (see Figure 6.11).
82
6. Photo-degradation of organic tandem solar cells
Figure 6.11: Life time extrapolation from tandem PCE data presented in the experiment shown in Figure 6.7. The red line dependents and linear regression through slowly and linear decreasing PCE data points starting at t=1000 h. Reproduced with permission from [198]
In doing so, an extrapolated operating lifetime of 27000 h can be extracted. Considering an average 1500 hours of sunshine per year in central Europe, this represents, under the current conditions, a best case lifetime of 18 years. A more conservative lifetime for the test cells could be derived by accounting for the error bars of the experiment, which still resulted in a lifetime of 8 years. It is important to note that the presented operating lifetime was extrapolated from cells, which were aged under open circuit and under indoor conditions using a LED based solar simulator that does not emit radiation in the 180 – 400 nm wavelength range. The absence of UV light, which has been shown to accelerate degradation through bond scission and free radical formation in OPV semiconducting polymers, [27] may artificially increase the lifetime of our cells. Furthermore, for outdoor conditions in the field, there are influences from other sources such as natural thermal cycling, shading, and humidity cycling, which need to be taken into account.
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6. Photo-degradation of organic tandem solar cells
6.4 Conclusion In summary an organic tandem cell with a PCE loss of only 11% within the first 2000 h of operation was demonstrated. The stable tandem operation was possible by choosing active layer materials that can be processed in air and by adopting an inverted device geometry, in which MoOx as a replacement for PEDOT and Ba(OH)2 as hole blocking layer at the ETL/blend interface was used. Moreover, it was confirmed that the well-characterized importance of UV light treatment for devices based on ZnO is also an essential requirement for attaining long-term device stability, which requires periodic light soaking by UVphotons. This procedure mainly prevents the formation of S-shaped J–V performance. While the current generation of organic tandem devices does still require some UV light soaking, it is anticipated that the final OPV product will not rely on UV light treatment. As such, future studies should primarily foster materials development of new absorbers with enhanced photo and structural stability as well as alternative electron transport layers that are not subject to the requirement of photo-doping. Moreover, fully solution processed tandem OPVs on flexible plastics with state-of-the-art encapsulation need to be demonstrated for certifying market readiness. Assuming that the slow degradation rate observed in this work can be further improved, organic tandem solar cells with operating lifetimes comparable to traditional PV are thought possible in the forthcoming future.
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7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells
7 Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells
Solar cells and modules based on organic absorber materials have experienced a steady increase in power conversion efficiency [7,9]. Record PCEs for single OPV devices above 11% are reported and 21% are predicted [13,76,213,214]. The prospect of implementing fully solution based device fabrication steps makes this solar technology uniquely suited for roll-to-roll printing applications with potentially very low costs. In addition to the need for higher efficiencies, the long-term stability of OPVs is essential for practical and economically viable outdoor
applications
[198].
Investigating
the
underlying
degradation
mechanisms and macroscopic defects in organic solar cells is thus an indispensable condition for achieving competitive lifetimes [27,68,193].
*This chapter including all images is adopted from the paper „Water ingress in encapsulated inverted organic solar cells: correlating infrared imaging and photovoltaic performance“. The Manuscript is th
submitted (30 of May, 2015) to the scientific journal Advanced Energy Materials with the number of submission aenm.201501065.
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7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells
There are many aspects that can limit the lifetime of OPVs [27,166]. Most organic semiconducting materials are highly reactive in the presence of oxygen, water, high temperatures and light, which can induce thermal and photooxidation reactions as well as local trap formation and structure variations in the active layer [215]. In a device, adverse environmental conditions can lead to morphological changes, compositional gradients, and electronic defects at interfaces and electrodes [118,193]. While a clear correlation between observed performance loss and cause is often difficult, the underlying degradation mechanisms depend critically on the active materials and the device geometry.[68,193] It has been shown that conventional device architectures employing low work function metals typically break down of oxygen and water [171]. The diffusion of water preferentially occurs through grain boundaries and pinholes, leading to corrosion phenomena at the electrode and electrode/active layer interface, but also via the edges of the device. As a result, a drop in short circuit current and fill factor due to loss in contact area and the formation of resistive metal oxides is typically observed [24,68,168,181]. To overcome this problem, high barrier encapsulation materials with water vapor transmission rates (WVTR) much better than ~ 10 -3 g/(m²day) are commonly used in order to achieve module lifetimes of several thousands of hours [216–218]. Importantly, the presence of water may accelerate the aging process and could become a crucial degradation factor in climates with high levels of humidity. Specifically, it is known that the presence of water the fill factor of the JV characteristics due to S-shape formation, which originates from deleterious chemical reactions at the active layer/interface. In this study both the kinetics of water ingress and the primary reaction site of water in inverted P3HT:PCBM solar cells with thick glass barriers are investigated. In detail, the temporal evolution of the solar cell parameters V oc, Jsc, FF, and PCE at the temperatures (relative humidity, RH) 7 °C (51%), 20 °C (63%), 50 °C (20%), and 65 °C (20%) with a moisture variability of ± 5% was investigated. Samples maintained at 7 °C were kept in a refrigerator while samples maintained at 20 °C, 50 °C and 65 °C were stored in a heating oven with controlled heating performance.
86
7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells In a second set of experiments, the solar cell performance under controlled temperature (65°C) and moisture (85% RH) settings by storing the devices in a climatic chamber (CTS) was investigated. Temperature and moisture fluctuations were kept below 1%. For each degradation setting, a set of 10 cells was fabricated and averaged of the performance. All cells were kept in the dark to minimize photo-oxidation reactions.
7.1 Test conditions and device fabrication
7.1.1 Organic solar cell preparation and characterization All interface and active layers were processed in ambient atmosphere using doctor blading. The test cells were prepared on ITO-coated glass substrates (25 mm x 25 mm x 1.3 mm from Osram) with a sheet resistance of 5 Ω/sq [192]. The substrates were successively sonicated in acetone and isopropanol for 10 min each. After blow-drying with nitrogen, the aluminum-zincoxide (AZO) precursor was coated on top of ITO. Hydrolysis of the AZO precursor was achieved by annealing the samples at 140 °C for 10 min [72]. Solutions of the photovoltaic blend layer components P3HT (Rieke Metals) and PC61BM (Solenne BV) were prepared separately in chlorobenzene at a concentration of 2 wt.% and stirred for 120 min at 60°C. Afterwards, the P3HT:PCBM (1:1) blend solution was deposited on top of the AZO layer. As electron blocking/hole extraction layer, a 100 nm thick PEDOT:PSS film was doctor-bladed on top of the P3HT:PCBM film. The whole stack was annealed on a hot plate at 140 °C for 10 min. A silver (Ag) film with a thickness of 100 nm was evaporated as top electrode. The active area of one cell was 10.4 cm². Figure 7.1a shows the device geometry and the final solar cell stack layout. For encapsulation, a thick (0.7 mm) glass barrier and an ultra violet curable epoxy adhesive from DELO (Katiobond LP 655) was used. For current-voltage characterization, the test cells were removed from the climate chamber and the measurements were performed in ambient atmosphere using an Agilent 2900A source measure unit in a two-wire configuration. The cells were irradiated with a 87
7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells light emitting diode (LED) solar simulator featuring an AM 1.5G spectrum at 1000 W/m² (FUTURELED GmbH, Berlin, Germany) [197]. The intensity of the LEDs was calibrated with an ISE Fraunhofer certificated silicon calibration photodiode.
Figure 7.1: a) Cross section of the inverted P3HT:PCBM devices used in this study. Water ingress is symbolized with red arrows. b) Schematic top view of a test device as shown in a).
In order to satisfy comparability between cells exposed to different conditions a rigorous selection protocol was developed, in which solar cells with a short circuit current density (Jsc) of less than 7.7 mA/cm², open circuit voltage (Voc) of less than 0.58 V, a fill factor (FF) of less than 60, and PCE of less than 2.5% were discarded from the degradation studies. A representative JV curve for one test solar cell that complies with the selection criterion can be seen in Figure B1.
7.1.2 Imaging parameters and settings During the degradation study electroluminescence (EL) and infrared (IR) imaging based on ELLI, DLIT, ILIT as well as PL imaging was carried out. For ELLI and DLIT measurements, two Equus 327k NM IR cameras (IRCAM GmbH, Erlangen, Germany) equipped with a cooled indium-gallium-arsenide (InGaAs) FPA detector (640x512) and a cooled indium-antimonite (InSb) based focal plane array (FPA) detector (640 x 512), respectively, were used. To avoid perturbations owing to temperature drift, the cells were allowed to reach thermal equilibrium prior to applying a voltage. In order to minimize implications due to the heat diffusion length the lock-in frequency was set to 88
7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells 7 Hz for all imaging techniques [219]. The image acquisition time for DLIT and ILIT was 45 min, while the acquisition time for ELLI was 60 seconds. Solar cells investigated with ILIT were measured under Voc condition and excited with pulsed white light coming from an LED-array (FUTURELED GmbH, Berlin, Germany). The spectral emission of the LEDs extends from 400 nm to 750 nm with a spectral peak at around 440 nm. The light intensity was set to 1000 W/m², adjusted with an ISE Fraunhofer certified silicon calibration photodiode. Due to glass thickness constraints the cells were irradiated from the front (ITO) side and investigated from the backside for all ILIT measurements. Solar cells investigated with DLIT and ELLI measurements were excited with a pulsed voltage of 1 V (forward bias). In this case, the IR and EL radiation of the solar cells was detected from the front side (see Figure B2). For measuring the PL signal, the cells were scanned with an Argon ion laser using 488 nm wavelength. The PL emission was detected by a germanium detector (ADC 403L) and integrated over the wavelength range 610 – 840 nm. Details regarding the PL setup have been described before [220].
7.2 Moisture induced degradation of inverted OPC devices
7.2.1 Heat- and damp heat-induced degradation In order to study the impact of moisture on the stability of the encapsulated and inverted OPV devices, an experiment in which 10 cells stored at 65 °C and 85% relative humidity (65 °C/85% RH) and 65 °C and 20% RH, respectively, was carried out. For this purpose, the cells were stored in a dark climate chamber with controlled relative humidity and temperature. The device parameters at 65 °C/85% RH decay quickly within the first 300 h compared to the reference measurement at 65 °C/20%RH (Figure 7.2). This fast decrease is mainly due to the emergence of an S-shape deformation in the JV characteristics of the test cells (see Figure 7.3). Analysis of the acceleration factors by evaluating the slopes of the traces of FF (for details see further 89
7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells below) reveals a faster degradation rate at Jsc and FF, for cells stored under 65 °C / 85% RH than compared to those stored at 65 °C /20% RH. Considering that the temperature is the same in both experiments, the change in oxygen concentration can be neglected. Taking into account the absolute values of humidity dissolved in the air at 65 °C (136.2 g/m³ and 32.1 g/m³ at 85% RH and 20% RH, respectively) the result from Figure 7.2 strongly suggests that higher content of moisture is mainly responsible for the fast aging process and Sshape deformation in this type of devices.
Figure 7.2: Impact of relative humidity at 65 °C on the temporal evolution of V oc, Jsc, FF, and PCE for encapsulated P3HT:PCBM solar cells of inverted architecture.
90
7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells
Figure 7.3: Periodically measured JV characteristics of an inverted P3HT:PCBM solar cell stored in a climate chamber at 65°C/85%RH.
To study the thermal stability of inverted P3HT:PCBM solar cells under varying moisture settings, a new experiment in which a set of test cells were stored at different temperature settings was carried out. Figure 7.4 shows the long-term behavior of glass-encapsulated P3HT:PCBM solar cells for different storage temperatures in the dark. Each data set is normalized to the initial value at 0 h and represents the average value from 10 test cells. All devices were stored in the dark at room temperature for 45 min prior to JV characterization to minimize temperature-induced variations in the JV performance.
91
7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells
Figure 7.4: Long-term behavior of P3HT:PCBM solar cells stored at different temperatures (7 °C/51%RH, 20 °C/63%RH, 50 °C/20%RH, 65 °C/20%RH) in the dark. Each data point represents the average value for 10 solar cells and is normalized to the initial value at t = 0 h. The solar cell parameters were extracted from J-V measurements under 1000 W/m² using an AM1.5G solar simulator spectrum.
The lifetime performance shown in Figure 7.4 can be categorized into two regimes: a regime for T ≤ 20 °C where degradation is slowed down and a regime of accelerated degradation for T ≥ 50 °C. Below 20 °C, a decelerated decrease in device performance of the encapsulated solar cells is observed. This is valid even after more than 28 months (20,000 h) of storage. For instance, after 20,000 h and at T = 20 °C (7 °C), Jsc decreases by 15% (2%), the FF drops by 17% (12%), and the overall PCE decreases by about 28% (15%). Remarkably, the Voc remains almost unchanged for all temperatures, suggesting that the morphology and recombination characteristics of the cells are not significantly altered throughout the long investigation time. By applying a linear regression to the data points of PCE for 20 °C (7 °C), a shelf lifetime of about 2.2 (3.5) years is extrapolated (80% of the initial PCE, Figure B3). At temperatures ≥ 50 °C, the photovoltaic performance is dominated by early losses, mainly in terms of Jsc and FF, which decrease to 79% (78%) and 57% (67%) of the initial value, respectively, for 50 °C (65 °C) after 6200 h (2100 h). Overall, the PCE drops by about 52% after 6200 h for solar cells stored at 50 °C and 46% after 2100 h for cells stored at 65 °C. At the end of the investigations, nearly all JV curves reveal a distinct S-shape (Figure 7.3). This observation is not fully understood and may result from several causes, such as the loss of 92
7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells conductance in the ZnO layer [22,68] or moisture-induced aging [181,221]. It is assumed that the fast decrease of device performance for T > 50 °C is mainly related to the diffusion of water through the edges of the glass–glass encapsulation. Ongoing from 50 °C to 65 °C, aging accelerates by a factor of five, which is associated with a combined effect of temperature and moisture. Note that the difference in oxygen concentration at this small temperature difference can be considered negligible [222].
7.2.2 Determination of the activation energy Based on the experimental data shown in Figure 7.4, the quantification of the activation energy of the performance loss from the temperature depending acceleration factors of the investigated test cells is attempt by applying the Arrhenius model: E
k deg = A ∙ exp (− k aT), Eq. 20 B
where Ea is the activation energy (in eV) of the degradation mechanism, kB the Boltzmann constant, T the temperature, and A a pre-exponential factor. The reaction rate coefficient kdeg is obtained from applying a linear degradation kinetics to the experimental Jsc data of Figure 7.4a [223]: Jsc = Jsc (0) ∙ (1 − k deg ∙ t). Eq. 21
Following a procedure by Brabec et al., the acceleration factor K for degradation is extracted from the ratio of the degradation rate constants, i.e., by combining Eq. 20 and Eq. 21, [223]:
K=k
k` deg
E
1
1
= exp [k a (T − 𝑇`)]. Eq 22 B
93
7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells
Figure 7.5: Temperature dependence of the acceleration factor K for J sc, FF, and PCE as extracted from the experimental data shown in Figure 7.4. The reference temperature T’ is 293 K.
Figure 7.5 shows the acceleration factor K = kdeg`/kdeg vs. 1/T, referenced to T = 293 K, as determined from the temperature dependent losses of Jsc (Figure 7.4 a). The same approach was applied for analyzing the temporal decay of FF and PCE, giving rise to similar activation energies of 440 meV (Jsc), 470 meV (FF), and 460 meV (PCE). According to Figure 7.5, the degradation occurs more than 10 times faster at T = 65 °C than at T = 20 °C. Generally, the degradation rate is determined by a superposition of effects originating from the presence of intrinsic (e.g., radicals, metal traces) and extrinsic (e.g., O 2, H2O) contaminants. Since water has been identified as the dominant degradation reactant, Ea can be associated with the activation energies for diffusion and reaction of water with solar cell components as well as with temperature dependence of the amount of water dissolved in the encapsulated test cells, which in turn comprise the factor of humidity and solubility. Previously, Brabec et al. reported activation energies of 300 – 350 meV for encapsulated, MDMOPPV:PCBM based solar cells of regular architecture [223]. Kinloch et al. reported activation energies for the displacement of different adhesives by water in the order of 170 meV – 400 meV [178,179]. However, a direct comparison is only meaningful for identical device geometries when measured under the same storage conditions. Here, it is emphasized that the acceleration 94
7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells factor can be primarily considered as a phenomenological measure for establishing a stability metrics, comparing the optoelectronic robustness of the material system and/or the quality of the sealing.
7.2.3 IR imaging of moisture-induced degradation at 65°C/85%RH The data presented in the previous sections show the global change of the solar cell specific parameters. However, it would be highly desirable to correlate the loss in device performance with local changes in device behavior. To further elucidate the effects of moisture diffusion on the local change of OPV device performance different imaging techniques including ELLI, DLIT, and photoluminescence imaging were applied on the same device area. All lock-in images are represented as the 0° (in-phase) signal. Technical details regarding the setup and the interpretation of the signals are presented in section 5 and the Supporting Information (Appendix B). At first the ELLI measurements of the test cells (Figure 7.6) will be discussed. EL radiation was previously identified as originating from interfacial charge-transfer (CT) state luminescence [226]. Here electroluminescence imaging was possible using a sensitive indium-gallium-arsenide (InGaAs) focalplane array with a spectral response matching the CT emission of the P3HT:PCBM blend (Figure B4) At time zero, the emitted EL-radiation is nearly homogeneous over the active areas of the cells. After ≈160 h of storage in the dark at 65°C/85%RH, the emitted EL-radiation starts to bleach out from the outside towards the center of the substrate while maintaining homogenous intensity in the remaining active areas of the devices (see also Figure 7.7). Particularly, the three cells at the top of the 160 h image show a strong and localized reduction of the emitted ELLIsignal. The concentric damage is clearly discernible after a storage time of > 500 h. At this point, only the center shows EL radiation while close to the corners and edges the EL is almost fully suppressed.
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7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells
Figure 7.6: ELLI images measured at different storage times using a pulsed voltage of 1 V (forward bias). The cells were kept in the dark at 65°C/85% RH.
In order to reveal if IR thermography might be a more sensitive tool than ELLI, DLIT, ILIT, and ELLI images were measured (Figure 7.7) [227] of the same solar cell as a function of storage time under 65 °C/85%RH. In general DLIT and ILIT measurements reveal local thermal loss processes which can be attributed to non-radiation recombination mechanism induced by injected current or light. In both cases the DLIT and ILIT measurements show a distinct intensity change in the degraded areas similar to the EL disruption observed at the ELLI measurements. In addition to the loss of EL and IR emission in degraded areas, a relative increase of the EL and IR radiation in the unaffected parts of the cell is observed (Figure 7.7, top and center). This is clearly apparent from line scans taken along the long side of the active area of the solar cell (Figure B5). The increased signal might be attributed to decreasing active area leading in an increasing potential difference at the electrode of the unaffected cell parts. Hence, the bias point at the dark characteristic of the non-defective cell parts is shifted to a slightly higher applied voltage resulting in an increased injected current under the dark condition. The result would be an increased ELLI signal of the cells. Note that the scratch discernible in all images of Figure 7.7 does not affect the electrical properties of the solar cell and that the disruption and increase of the ELLI signal is also apparent in other devices (see Figure 7.6). Notably, ILIT shows an opposite IR 96
7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells dissipation behavior compared with the DLIT and ELLI measurements, i.e., the ILIT signal increases in the defective cell parts, which could be related to a disruption at the electrode/active layer interface. It is generally possible that charge carriers under open circuit conditions flow through the electrode to recombine via the shunt resistance of the solar cell [14,228]. In case of a disturbed interface, however, the electrodes are not accessible and the charge carriers are likely to recombine in the active layer, leading to a local temperature increase. Considering the notably higher signal to noise ratio of ELLI images at relatively short acquisition times (60 s compared to 45 min for DLIT and ILIT), ELLI emerges as the most promising technique for fast, in-situ imaging of OPV cells and modules.
Figure 7.7: Top to bottom: ELLI, DLIT, and ILIT images of the same cell measured at different storage times. The sample was stored in the dark at 65 °C/85%RH. For ELLI and DLIT measurements the cell was biased in forward direction with a pulsed voltage of 1 V.
While the diffusion of moisture is apparent in EL and IR images (Figure 7.7), the signals can be affected by both, electro-optical changes of the semiconductor material and the quality of the contact, making it difficult to identify the layer where the damage actually takes place. To discriminate 97
7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells between degradation effects occurring in the bulk active layer and at the active layer interface ELLI images and PL scans of fresh and degraded solar cells were recorded (Figure 7.8 and Figure B6). PL, when performed under open circuit conditions is exclusively related to the light induced radiative recombination behavior of the light induced exactions inside the semiconductor [129]. The potentially large volume of active material sampled by diffusing excitons, makes PL spectroscopy a very sensitive tool for the detection of morphological changes or trap formation in bulk heterojunction layers. A change in the PL signal can, therefore, be employed as probe for changes in exciton and charge carrier dynamics in the bulk active layer.
Figure 7.8: Comparison of the electroluminescence (EL) and photoluminescence (PL) signal of a fresh and a degraded cell. The cells were kept in the dark at 65 °C/85% RH (see also Figure B7). PL was excited at 488 nm and the detection range was 610 – 840 nm (see Figure B4).
As opposed to ELLI, the PL map of the degraded solar cell remains relatively homogenous over the whole active area (Figure 7.8 and Figure B7). Importantly, the PL signal does not follow the same behavior as the ELLI signal. Since the PL radiation originates directly from the active layer and is mostly decoupled from influences related to the electrodes [229], it is concluded that diffusion of moisture mainly affects the electrode/active layer interface, while the P3HT:PCBM layer remains mostly intact. 98
7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells For the solar cells shown in Figure 7.8, the active EL emitting area is correlated with the photovoltaic performance as a function of the degradation time (Figure B8). For the first 150 h the active EL area and the photovoltaic parameters remain unchanged, corresponding to the diffusion time of water through the rim of adhesive. After 150 h, moisture starts to affect device performance and the ELLI area starts to drop rapidly, even faster than the photovoltaic parameters. Interestingly, there is little change in Jsc with decreasing active ELLI area, while Voc and FF decrease noticeably with degradation time. This suggests that the traces of water diffusing into the solar cell and turning off EL emission do barely affect net charge generation and charge carrier extraction. Furthermore, a close to constant Jsc with simultaneous decrease in Voc indicates that the work function of the affected contact is likely to undergo change under damp heat conditions.[192,230] Recently, Reinhardt et al. showed that a change in work function, more specifically, a reduced charge selectivity at the contacts due to surface recombination, can lead to a concomitant decrease of Voc and EL intensity [99]. This is consistent with our observations and further suggests that the degradation is occurring at the contact rather than in the bulk active layer. Overall, comparison of electroluminescence imaging and photovoltaic performance during accelerated temperature and humidity testing of OPVs reveals that ELLI features much better sensitivity for predicting moistureinduced device failure than I-V measurements.
7.2.4 Moisture diffusion in encapsulated devices As shown in the previous section, the presence of moisture at the semiconductor/electrode interfaces is responsible for the observed S-shape formation in our organic solar cells. To find countermeasures for preventing moisture-induced failure modes in encapsulated organic photovoltaics, it is important to understand quantitatively the transport of moisture through the packaged device. In the following, it will be shown that classical diffusion theory is applicable for describing isothermal moisture transport upon side ingress of water into the packaging of our samples. To that end, encapsulated large area 99
7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells (1.44 cm²) OPV devices and Ca corrosion tests of the same geometry were prepared and degraded at 65 °C/85% RH (Figure B9). The diffusion of water in the calcium samples is measured optically by monitoring the transition of the films from opaque to transparent, while in the case of OPV devices detecting the decrease of the electroluminescent area using ELLI allows to analyze the moving waterfront. In order to understand the kinetics of moisture diffusion inside encapsulated OPV devices the diffusion coefficient D for water in the adhesive was calculated by applying Fick’s second law of diffusion to a calcium test: x
c(x, t) = cs ∙ (1 − erf (2√Dt)), Eq. 23 where cs = c(x=0,t) is the saturation concentration and erf(x) is the Gauss error function. The diffusion coefficient reflects the speed at which the moisture diffuses through the adhesive. Equation 23 can be rearranged to solve for D:
D=
2 dx⁄ d√t ( c ) , 2∙erf−1 (1− k )
Eq. 24
cs
The concentration ck represents the critical concentration of water necessary for consuming the calcium film and was estimated to be 4∙10 -4 mol/cm3 for a 100 nm thick film. The saturation concentration c s was measured gravimetrically to be 3.2×10-3 mol/cm³ (Table B1). The thickness of the adhesive film was 15 µm. The calcium test (Figure 7.9 and figure B9), which featured the same encapsulation architecture as the test devices, shows that the position x of the waterfront as a function of √t follows a linear behavior, as expected from Fick’s law, with dx/d√t = 3.3×10-6 m/√s. From the optical calcium test D = 2.1∙10-12 m²/s using Eq. 24 is extracted. Alternatively, D is calculated using the water vapor transmission rate (WVTR) of the adhesive as provided by the manufacturer (Appendix B), giving rise to D = 1.4×10-12 m2/s, which matches the value from the calcium test closely. Similar diffusion coefficients for water in lamination adhesives have been recently presented by Michels et al. [231] ELLI images of large area OPV devices show a similarly linear x vs. √t, also obeying 100
7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells Fick‘s law of diffusion. Determination of the diffusion coefficient of water in the device requires specific knowledge of ck or WVTR in the case of a full device and was outside the scope of the present study. The value of the diffusion coefficient is important when considering the water permeation path inside encapsulated devices. To test whether the degradation kinetics is mainly determined by the diffusion of water through the adhesive or through one of the active layers of the device solar cells with grid finger electrodes as well as with different thicknesses of silver were prepared (Figure B11). Metal electrodes may offer barrier protection against vertical diffusion when water is primarily transported via the adhesive. ELLI images of devices with grid finger electrodes show that the EL signal emerging from areas in direct contact with adhesive and from areas protected by finger electrodes deteriorates with very similar kinetics, suggesting that the waterfront progresses through one of the active layers of the device stack under the silver electrode (Figure B12 and B13). Additionally, the observation that EL degradation patterns for solar cells with thick silver metal electrodes resemble those with thin electrodes further eliminates the possibility of water diffusing through the electrodes (Figure S14). Considering the device geometry and previous reports using similar device layouts, PEDOT:PSS emerges as the most likely candidate for efficient transport of moisture in inverted devices [178,180]. In fact, Feron et al. recently measured the diffusion coefficient of water in PEDOT:PSS to be D = 5.0×10-10 m2∙s-1, i.e., approximately two orders of magnitude larger than D for our adhesive [182]. To confirm this hypothesis and identify the site of predominant degradation due to damp heat full devices and samples missing PEDOT:PSS layer and top electrode (i.e., ITO/ZnO/P3HT:PCBM) were exposed to damp heat conditions (Figure S15). The results show that the degradation effects are strongest in complete devices while only minor impact is observed in the case of ITO/ZnO/P3HT:PCBM samples with fresh PEDOT:PSS/silver top electrode. Since there is no indication of corroded silver electrodes or silver interfaces it is conclude that degradation of the PEDOT:PSS contact is primarily responsible for early device performance losses under damp heat conditions. Finally, assuming that the linear behavior shown in Figure 7.9 is valid at longer times, the shelf life of an OPV device for a given adhesive geometry and environment by extrapolating the kinetics of water transport in the protective 101
7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells adhesive can be predicated (Figure 9). By extending dx/d√t, a rim of adhesive of 9.3 cm would be sufficient to achieve a lifetime of t≈100,000 h under 65 °C/85%RH.
Figure 7.9: a) Water permeation model for inverted and encapsulated organic solar cells based on ITO/Al-ZnO/P3HT:PCBM/PEDOT:PSS/Ag. b) Experimentally determined kinetics of water diffusion in epoxy adhesive using calcium test: penetration distance vs. t1/2. The error bars are approximated reading errors. The slope dx/d√t is dx/d√t = 3.3∙10-6 m/√s. Similar results were obtained with large area calcium tests (Figure B10)
7.3 Conclusion Considering that the diffusion of moisture during manufacturing, storage, or operation is thought to be one of the major reliability concerns in encapsulated organic electronic devices, identifying the underlying degradation mechanisms represents a prerequisite for developing strategies towards longlived organic photovoltaics. Here, it was shown that by combining spatial 102
7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells information on charge transfer luminescence and charge carrier recombination with JV measurements as a function of controlled aging conditions important insight into possible degradation paths in OPVs can be gained. By investigating the temporal JV performance under controlled temperature and moisture settings water was identified as the main promoter for performance loss in inverted P3HT:PCBM based solar cells. The degradation related acceleration factors and the activation energy for the diffusion of water were quantified by applying the Arrhenius model (≈450 meV). Furthermore, it was shown that selective electrical and optical excitation using lock-in imaging in the IR allows to visualize the loss in active area due to the permeation of water and, importantly, to distinguish between degradation occurring in the bulk heterojunction and at the electrode/active layer interface. By comparing ELLI, DLIT, ILIT and PL measurements it was proved that the presence of moisture mainly affects the local electroluminescence and the active layer electrode interface of inverted organic solar cells. This suggests that in the presence of moisture charge carrier extraction is most likely inhibited, which could be one possible cause for the temporal decay in photovoltaic performance. Under damp heat PEDOT:PSS was shown to degrade more faster than ZnO. The hypothesis is that water upon diffusing through the rim of adhesive is quickly transferred to the PEDOT:PSS layer at the contact area between adhesive and PEDOT:PSS. The water diffusion then progresses via the PEDOT layer. As a result, the charge carrier extraction is most likely inhibited in area affected by water ingress, which could be on reason of S-shape formation and temporal decay of photovoltaic performance. This mechanism is expected to be valid as long as the diffusion coefficient of water in the adhesive is much more smaller than the diffusion coefficient of water in the PEDOT:PSS layer. Furthermore, the diffusion of water can be approximated using Fick`s law of diffusion Finally, among the IR imaging techniques studied in this work, ELLI represents the fastest and most responsive optical imaging modality. It thus bears the potential to be employed as a standard characterization technique for both studying the local radiative recombination behavior and the quality of barrier materials in organic solar cells. This result further strengthens the importance of IR imaging techniques not only as a quality control tool but also 103
7. Temperature and moisture induced degradation of inverted and organic P3HT:PCBM solar cells as a means for localizing, studying and preventing the origin of local degradation effects in organic solar cell performance, particularly when combined with spectroscopic information.
104
8. The electrical and thermal characterization of macroscopic defects in thin film solar modules
8 The electrical and thermal characterization of macroscopic defects in thin film solar modules
Due to the opportunity to print organic solar cells on light weight and flexible substrates, OPV devices bear the potential for a cost-efficient mass production. In order to guarantee a successful commercialization of OPV modules, these devices need to fulfill different requirements which focus on the efficiency, longterm stability, and fabrication. With efficiencies above 10% OPV cells already proved their potential to be competitive with standard thin film technologies [232]. Furthermore, several groups as well as the findings presented in section 6 show that life times of several years are possible [196,233]. In terms of manufacturing, the main challenges are mainly related to upscaling (transition from small area single solar cells to large area solar modules) [234]. Here, the introduction of performance reducing defects due to improper device fabrication is one of the major concerns of manufacturers. The amount of the performance reduction depends predominantly on the defect type, defect size, and defect position in the cell as well as the illumination conditions [14]. Especially, under low light irradiation condition (< 100 W/m²) even “weak” defects may lead to a pronounced loss of electrical cell output.
* This chapter including all images beginning from Figure 8.2 is adopted with permission from ref [129] (Copyright Elsevier)
105
8. The electrical and thermal characterization of macroscopic defects in thin film solar modules During the last decade different thermographic methods have been developed to characterize several thermal losses in thin film solar cell technologies. ILIT measurements have been proven to be sensitive enough for a contactless, fast, and non-destructive characterization of defects in organic solar modules [32]. The possibility of a contactless module characterization during different production steps and allows an early sorting of defective devices and an improvement of the production line’s throughput rate. To do so, it is crucial to have a firm physical and electrical understanding of defects and their impact on the surrounding cell. Especially a highly advanced analysis of both the quantitative
influence
on
device
performance
and
quantitative
defect
parameters such as resistance are essential. Recently, Besold et al. presented a study in which randomly distributed and fabrication related defects in OPV modules were assigned with a distinct ohm value [122]. From JV characteristics at different light intensities, the group demonstrated that bulk heterojunction solar cells exhibit a photo shunt, which means that the overall value of the parallel resistance depends reciprocally on light intensity and decreases with increasing irradiation. Cells containing one or more defects did not followed this reciprocal trend. The defects could be, therefore, attributed to purely ohmic. Based on ILIT measurement at different irradiation conditions and 2 dimensional LT spice simulations each defect was associated with a distinct ohm value in which the IR emission of a defect was set in relation with its simulated electrical power dissipation. The authors mentioned that the findings of the study can be easily transferred to other thin film solar technologies and clarify the importance of quantitative analyses of defect related losses in solar modules. However, organic solar modules are still in the pre-commercialization stage and large improvements need to be done for market readiness. Therefore, for an efficient and quantitative analysis of defects, one needs to focus on already highly advanced thin film solar technologies. Especially solar cells and modules based on CuInGaSe2 (CIGS) have been successfully developed into high performance power converters of solar energy with a global market share of about 2% [4]. These devices belong to the highest advanced thin film solar technologies and show among all other technologies the currently highest values of PCE for single cells (21.7%) and modules (15%-17%) [10–12]. Similar to OPV devices, 106
8. The electrical and thermal characterization of macroscopic defects in thin film solar modules CIGS solar modules suffer from a large variety of fabrication related defect which can be possibly introduced to the module during the different layer deposition steps. The end result is that defects cause a decrease in the module electrical output. Therefore a firm physical and electrical understanding of these defects is essential to improve both module efficiency and fabrication.
8.1 Description of test modules The investigated test samples consisted of 15 non encapsulated CIGS modules with 67 solar cells connected in series. The modules were fabricated at a large industrial production line for CIGS modules and had a size of 28 cm x 28 cm. The electrical power output (Pmpp) of each module was 9-10 W at 1000 W/m² with an irradiation spectrum of AM 1.5G. A photograph and an ILIT measurement of one test modules can be seen in Figure 8.1
Figure 8.1: a) Photograph of a 28 cm x 28 cm CIGS test module with 67 cells connected in series. b) ILIT measurement (amplitude signal) of the test module presented in in the photograph. The brightest spot (marked with arrows) indicates defects which reducing the maximal power output of the module c) Schematic cross section of a CIGS solar module. The interlayer between the ZnO window layer and CIGS absorber layer represents the CdS buffer layer. Illustration is simplified and not in scale. Image a) and b) are reproduced and modified from [120]
107
8. The electrical and thermal characterization of macroscopic defects in thin film solar modules At this point only a short and summarized description of the device fabrication will be given. A detailed description of the different production steps can be found in [38,235–238]. As back contact of the individual cells, a molybdenum (Mo) layer was used, which was sputtered on a 3 mm thick glass substrate (soda-lime glass). To separate the back contacts of the cells, a first patterning step (P1) was realized with a laser scribe. Next, the CIGS absorber layer was deposited by co-evaporation on top of the Mo back electrodes. This was followed by a buffer layer of CdS deposited using a chemical bath deposition process. A second patterning step (P2) ensures the series connection between the cells by mechanical scribing down to the Mo layer. The front contact was then deposited by sputtering subsequently an intrinsic zinc oxide (ZnO) and an aluminum doped zinc oxide (ZnO:Al) window layer. A final patterning step (P3) separates the cells by mechanical scribing again down to the Mo layer (see figure 8.1 c).
8.2 Influence of a defect on its surrounding cell measured by ILIT Figure 8.2 shows a magnification of a representative defect from one of the test modules, visualized by an ILIT-Voc measurement. According to the lock-in thermography principles it is possible to display four images: the 0° image (inphase), the -90°image (out of phase), the amplitude image A(x,y), and the phase image Φ(x,y). Details about the different signals obtained by DLIT and ILIT measurements can be found in Breitenstein et al [128].
108
8. The electrical and thermal characterization of macroscopic defects in thin film solar modules
Figure 8.2: A defect (bright spot) identified by an ILIT-Voc measurement (illumination power of 30 W/m², 1Hz lock-in frequency, measuring time of 10 min): a) 0°-image, b) -90°-image, c) amplitude image and d) phase image. Reproduced with permission from [129]
The small vertical and dark lines which can be seen in Figure 8.3 a), b), and c) are the patterning lines while the bright spot represents the defect. The vertical cell region around the defect shows a lower IR-emission compared to non or less-defective regions. This region appears darker because of the attraction of light induced charge carriers as described in section 3.3. It can be seen from the images a), b) and c) in Figure 8.3 that the IR-emission of the adjacent neighbor cells is slightly increased (“cross talk”) when compared with to the outer neighbor cells. This behavior can be attributed to a charge carrier injection from the defect into the adjacent neighbor cells [14] resulting in an increased non-radiative recombination of charge carriers in the neighbor cells around the defect. In order to qualitatively confirm the influence of the defect on its surrounding cell and the recombination behavior of light induced charge carriers Figure 8.3 a) shows a referenced line scan over the defective cell presented in Figure 8.2. The ILIT signal was referenced to an ILIT line scan of an ideal neighbor cell, resulting in a referenced IR-emission Sdiff(x) = S(x) – Sreference(x), with x as the position in the cell. The reference cell was chosen as close as possible to the defective cell in order to minimize both the spatial inhomogeneity of illumination and the local variations of material properties. However, to avoid 109
8. The electrical and thermal characterization of macroscopic defects in thin film solar modules referencing with an affected neighbor cell (“cross talk”), the second ideal neighbor cell was chosen as the reference cell.
Figure 8.3: a) Line scan (Sdiff) of a defective cell (amplitude image) in a CIGS solar module (see Figure 8.2). b) Simulation of the local electrical power density of a CIGS cell with a defect.
The cell length of 28 cm is equivalent to 475 pixels of the camera detector. This results in a spatial resolution of about 0.6 mm x 0.6 mm per pixel. Figure 8.3 a) shows a defect as a distinct maximum caused by highly localized IR-emissions at 18 cm. As can be seen in the graph within the first 6 cm (Figure 8.3 a) left part), the IR-emission is approximately zero meaning that there is no difference between the IR-emissions of the defective cell and the reference cell. In these regions the heat generation is mainly induced by thermalization and trap state assisted recombination and not influenced by the defect. Continuing after the first 6 cm the influence of the defect becomes noticeable which is reflected in the negative values in Figure 8.3 a). The IR-emission decreases continuously towards the defect, where a large peak exists (joule heating). The data from the experiment shows a similar behavior (a distinct peak and slightly negative values for the signal around the defect) when compared with a line scan of the simulated electrical power density of a cell with an embedded defect (Figure 8.4.b). A closer description of the cell parameters and simulation model used for the simulations can be seen in the appendix and in reference [14], 110
8. The electrical and thermal characterization of macroscopic defects in thin film solar modules respectively. The simulated ohmic defect had an area of 0.001 cm² with a resistance of 0.1 Ω∙cm² and was placed at the same position as in the experiment. It can be seen that electrical power density shows a similar nonsymmetric behavior to the data found from experiment, indicating that the applied model incorporates the main processes for heat dissipation. Here, it should be mentioned that the aim of the simulation was not to calculate the IRemission precisely but rather to gain a better understanding of the dissipation processes in dependency of the charge carrier generation. In a further study the influence of light on the impact of a defect on its surrounding cell was investigated. Therefore Figure 8.5 shows two line scans over a defective cell with an embedded defect close to the cell center. The two line scans show that with increasing light intensity from 50 W/m² to 380 W/m² the influence length decreased from around 10 cm to 5 cm. Especially, under low light irradiation conditions the influence of the defect goes over the entire cell length.
Figure 8.4: Line scans of a defective cell (amplitude image) in CIGS solar module irradiated with different light intensity. The module was investigated for both experiments with a lock-in frequency of 1Hz and a measuring time of 10 min.
This decreasing influence of a defect in dependency of increasing light intensity can be attributed to a shunt screening behavior which is explained in 111
8. The electrical and thermal characterization of macroscopic defects in thin film solar modules detail in the section 3.2 the references [14,120]. DLIT and ELLI measurements in which the injected current was adjusted to the respective photocurrent of the ILIT measurement showed the same behavior (see Appendix C). One conclusion from these results is that the defects in thin film modules have an influence region from where they attract electrons. This influence region can be quantified by an influence distance. However, defects are generally randomly distributed on the solar cell. As the influence distance is rather large if the sample is irradiated with lower light intensity (90% (IRCAM GmbH, Erlangen, Germany).
Calcium deposition:
The
Ca
layers
were
thermal
evaporated
glass
substrates
(25 mm x 25 mm x 1.3 mm) similar to the glass substrates used for the cell fabrication. The substrates were successively sonicated in acetone and isopropanol for 10 min each. Thermal evaporation of the Ca layers with a thickness of 100 nm was done inside a glove box with a nitrogen protection atmosphere. After evaporation the samples were encapsulated using a thick (0.7 mm) glass barrier and an ultra violet curable epoxy adhesive from DELO (Katiobond LP 655).
*This part including all images is adopted from the paper „Water ingress in encapsulated inverted organic solar cells: correlating infrared imaging and photovoltaic performance“. The Manuscript is submitted (30
th
of May, 2015) to the scientific journal Advanced Energy
Materials with the number of submission aenm.201501065.
127
Appendix B Table B1: Data to calculate D from Eq 6. Data
Value
Saturation concentration of adhesive cs
3.7∙10-3 mol/cm³
(65°C/ 85%RH) Saturation concentration of adhesive cs
3.2∙10-3 mol/cm³
(60°C/ 90%RH) dx/d√t
4∙10-6 m/√s
Determination of D using first law of Fick
In order to understand the diffusion kinetic of moisture inside the encapsulated OPV devices the diffusion constant D was calculated by applying Fick’s 1st law which can be written as
J = −D ∙
∆c l
, Eq. S1
where J (mol cm-2s-1) is the diffusion flux of moisture, D (m2/s) is the diffusion coefficient, and Δc as the concentration difference between of saturation concentration cs and material concentration c0 across the membrane thickness l. The Diffusion coefficient reflects the speed in with the moisture diffuses though the adhesive of our sample and can be written by rearranging of Eq 1 as:
D0 = (c
J∙l
, Eq. S2
s −c0 )
For the diffusion flux of moisture through the adhesive (DELO Katiobond LP655) we used the water vapor transmission rate (WVTR = 6.1 g m-2 d-1 at 60°C/90%RH) as given from the technical data sheet [239]. The saturation concentration cs was measured gravimetrically to be 3.2∙10-3 mol/cm³ (SI). The thickness of our adhesive film was measured with 15 µm (cell stack ~150 nm). Using these input parameters Eq. S2 gives rise to D0 = 1.05∙10-12 m2∙s-1. The calculated diffusion constant refers to water diffusion through the adhesive at 60° C and a relative humidity of 90%. For our test conditions (65°C/85%RH) D 128
Appendix B needs to be corrected. We use the determined E a for the diffusion process and a modification of Eq. S1. In this case D is defined as
𝐸
1
1
𝐷 = 𝐷0 ∙ 𝑒𝑥𝑝 ( 𝑅𝑎 ∙ (𝑇 − 𝑇 )). Eq. S3 0
and can be calculated to D = 1.37∙10-12 m2∙s-1, which is in close agreement to the value reported in the main text.
Figure B1: JV characteristic of a test device which fulfills the section criteria mentioned in section 7.1. The table show averaged values of Jsc, Voc, FF, and PCE of the test cells at t = 0 h.
129
Appendix B
Figure B2. Schematic illustration of the lock-in setup used for ELLI, DLIT and ILIT investigations
Figure B3. Extrapolated lifetime of inverted OPV cells. Long-term PCE decay of inverted P3HT:PC60BM based solar cells. Each data point represents an average value of 10 devices. For estimating the accelerated lifetime, we applied a linear fit to the data points and extended the fit to where the efficiency drops to 80% of the initial value (red and black line).
130
Appendix B
Figure B4. ELLI line scans (top to bottom) of the solar cell shown in Figure 7.6 as a function of the storage time at 65°C/85 %.
Figure B5 Normalized PL and EL spectra of the encapsulated and inverted P3HT:PCBM solar cell. For the excitation of PL a light beam of an argon ion laser with an wavelength of 488nm was used. EL radiation was measured with an injection current of 48 mA/cm².
131
Appendix B
Figure B6. ELLI images of fresh and aged a pulsed voltage of 1 V (forward bias). The cells were kept in the dark at 65°C/85%RH. The excitation conditions were similar to those described in section 2.3. The difference in contrast to the measurements presented in Fig 7.6 arises from larger distance between sample and camera during the ELLI measurements.
Figure B7. Spatial distribution of PL signal of a fresh (left) and degraded (right) cell presented in Figure 7.8.
132
Appendix B
Figure B8: Active cell area vs. different cell parameters. For the analysis the left bottom cell presented in Figure B6 was taken into account.
Figure B9: Comparison of the diffusion kinetics of moisture though the adhesive of Ca test and OPV device. The P3HT:PCBM cell was fabricated under the same condition as described in section 7.1.1. For both experiments the samples were stored in dark at 65°C/85%. In terms of a better comparison of the moisture diffusion through the adhesive the active area of a cell was set to 1.44 cm². The red square angle represents the position of the active area. During the experiment a change of contrast was observed at the Ca test. This change might be due to a changing layer thickness and can be related to a primary diffusion of moisture through the adhesive material.
133
Appendix B
Figure B10: Experimentally determined kinetics of water diffusion in epoxy adhesive using calcium test: penetration distance vs. t1/2. The error bars are approximated reading errors. The slope dx/dh is dx/d√t = 3.9∙10-6 m/√s
Figure B11: Degradation characteristics of cell parameters of an encapsulated and inverted P3HT:PCBM solar cell with a grid finger electrode made of silver. The cell was stored under controlled moisture and temperature setting at 65°C/85% RH. The cell was ELLI investigated with a pulsed injection current of 80 mA, a lock in frequency of 1 Hz, and a measure time of 60 s.
134
Appendix B
Figure B12: (left) ELLI image of a fresh OPV device with a grid finger electrode. (right) Line scans of local ELLI emission of a fresh and aged device. Both ELLI images were measured with an injection current of 80 mA. The increased ELLI emission of the aged OPV device can be attributed to an increased current density of the active area which is due to the decreasing functionality of the electrode/active layer interface.
Figure B13: (left) ELLI images of an aged OPV device with a grid finger electrode. The continuous and dashed lines refer to line scans presented in the right graph. (right) Line scans of local ELLI emission. The ELLI image was measured with an injection current of 80 mA.
135
Appendix B
Figure B14: ELLI images of encapsulated and inverted P3HT:PCBM solar cells measured after different degradation stages. The samples were stored under controlled temperature and moisture settings at 85°C/85%RH. Each sample was investigated with a lock in frequency of 1 Hz, measure time of 15 s, and an injection current of 10 mA
136
Appendix B
0.8
8
0.6
0.4
Voc
Jsc
6
4
2
JSC VOC
0.2
0.0 before DH
after DH
AL DH
Conditions (DH= 85°C, 85% RH Damp Heat) 0.8 3
0.6
FF
PCE
2
1
0.4
PCE FF
0
0.2 before DH
after DH
AL DH
Current Density [mA/cm2]
Conditions (DH= 85°C, 85% RH Damp Heat)
10
Complete device before 85/85 after 85/85 (2hrs) Incomplete device 85/85 after AL (2hrs)
0
-10
-0.4
0.0
0.4
0.8
Bias [V]
Figure B15: Degradation of full (ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag) and partial (AL: ITO/ZnO/P3HT:PCBM) devices under damp heat (DH: 85°C/85%RH). Full devices were measured before and after DH. Partial devices were finalized and measured after exposure to DH.
137
Appendix C
Appendix C
Figure C1: Shunt screening behavior of a defect in a CIGS cell investigated with ELLI, DLIT, and ILIT. For all measurement the same imaging settings were used. The injection current for DLIT and ILIT measurements was adjusted to the Isc of the respective light intensity used for the ILIT investigations
138
Appendix C
Figure C2: Comparison of shunt screening measurements at different excitation modes. The injection currents are related to the Isc of the respective light intensity
Simulation model:
We developed 2D-simulations (finite element method) by using the software COMSOL Multiphysics and a multi-diode network model. Specifically, we simulated a submodule with 5 cells in order to take into account the influence of the adjacent neighbor cells on the middle cell's electrical properties. A 2Dnetwork was generated using a geometric dependent mesh, with mesh 139
Appendix C refinement towards small objects such as defects. The electrical parameters (such as ideality factor and parallel resistance) were found using the IV-curves found through our measurements. The sheet resistances were 18 Ω/sq. for ZnO:Al and 1.25 Ω/sq. for Mo. The simulated cells were 28 cm long and 0.4 cm wide which was essentially equal to the original cell area of our samples. The ohmic defect was set in the center of the cell in the center of the submodule with an area of 0.001 cm² and a resistance of 0.1 Ω∙cm²
140
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