Lifetime reliability of organic devices & applications
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Transcript of Lifetime reliability of organic devices & applications
Lifetime reliability of organic devices & applications
Vasileios M. Drakonakis, Achilleas Savva, Polyvios Eleftheriou and Stelios A. Choulis
Molecular Electronics & Photonics Laboratory Department of Mechanical Engineering and Materials Science and Engineering
Cyprus University of TechnologyLimassol
September 2014
Course Description (2 ECTS)Lifetime is an equally important factor for commercialization of organic electronic devices. This course describes the main degradation mechanisms of organic opto-electronic devices. OLED degradation mechanisms are presented as case studies. The degradation of OPVs under heat/humidity/light is also covered in details. In addition, the standard tests for lifetime performance evaluation are presented. Finally, packaging requirements and encapsulation methods are exhibited at later stages.
Course Description
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Course Content
Introduction The importance of Lifetime Performance on the product
development targets of organic electronic devices
Major degradation mechanisms in Organic Photovoltaic (OPVs) devices Major degradation factors under heat/moisture/light Electrode Degradation Active layer Degradation Module shading effects and hot spots Accelerated Lifetime Outdoor lifetime International standards for measuring lifetime in organic devices
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Major degradation mechanisms in Organic Light Emission Diodes (OLEDs) devices OLED Degradation mechanism (Case Studies) Degradation Relater to Initial part of Lifetime Performance Color Stability Origin of Catastrophic Failure Syndrome (CSF) Methods to improve long term stability of OLEDs Dark Spots
Packaging and Encapsulation methods Glass and Getter Low cost packaging Encapsulation specifications for long lifetime performance
Course Content
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Course Outline
Workload Lecturing Time: 25 hours Student Project Preparation Time: 20 hours Preparation for the Exams: 10 hours Exams Time: 3 hoursTotal Estimated Workload: 58 hours
Grading Student Project Presentation: 30% Final Exam: 70%
Awarded ECTS: 2Learning Outcomes
Understanding the major degradation mechanisms of organic opto-electronic devices
Understanding the importance of Lifetime on the product development targets of Organic Opto-electronic devices
Examples from the industry: learn to solve research and development problems
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Introduction
Long Lifetime can be fundamentally established by repeating the following steps:
Identification of major and minor degradation mechanisms Categorize mechanisms for Interfaces and Active layer Degradation Increase device lifetime by
Working on improvement of device materials and configuration Working on improvement of device packaging materials and system
Three major requirements for organic electronics meaningful commercialization
The critical triangle for organic electronics [...]
Long Lifetime
High Efficiency
Low Cost
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Introduction
Example: Oxidation of top electrode out of Aluminum (Al) in normal OPVs Identification of major and minor degradation mechanisms
As soon as the electrode is introduced to air without any encapsulation, oxidation occurs rapidly within hours.
The rapid formation of aluminum oxide deteriorates the device long before any other mechanism initiates.
Oxidation causes charge collection impedance. It can be observed with optical microscopy and it occurs both on the top surface of the electrode as well as within the interface with the active layer.
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V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
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Categorize mechanisms for Interfaces and Active layer Degradation Mechanism is categorized to degradation mechanisms for Interfaces Since the consequences upon its initiation are rapid and fatal for the device
performance, it is considered as a major degradation mechanism
Increase device lifetime by Working on improvement of device materials and configuration Try different metals or mixtures with lower susceptibility to oxidation Try buffer interlayers within the metal-active layer interface in order to slow
down oxidation
Working on improvement of device packaging materials and system Use encapsulation. Compare lifetime of devices with simple encapsulation
and with more complex systems such as getters or other materials
Example: Oxidation of top electrode out of Aluminum (Al) in normal OPVs
Introduction
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Introduction – Cost ParameterRemember the critical triangle
for organic electronics [...] Minimum system cost Maximum initial performance Minimum loss of performance over time
Long Lifetime
Organic electronics present much more degradation mechanisms when exposed to the environment compared to crystalline Si-based solar cells or light emitting diodes and light crystal displays
Lifetime comparison: Si-based/LED, LCD 15-25 years, OPVs/OLEDs 2-3/5-6 years
Nevertheless, lifetime and efficiency should not be considered individually when competing with other technologies
All three of the parameters need to be simultaneously taken into account
Currently, Organic Electronics present low stability:
Low stability hinders organic electronics commercialization
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Low-cost manufacturing and processing is the strong advantage of OPVs and OLEDs
Simple printing and coating techniques can be utilized
Such techniques enable fabrication of flexible electronics for a wider range of applications than conventional nonflexible solar cells
It has been shown that such devices can be manufactured with electricity cost as low as 8.1€ per Watt-peak (Wp)
In the future, upon large-scale production processing optimization, organic solar cells can become even more cost-effective and can be manufactured at a cost as low as 1€/Wp
Commercialization of OEs
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Commercialization of OPVs
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Developing cost-effective OEs
Cost estimation (Example OPV)
It is estimated that the manufacturing cost of purely organic solar cells ranges between $50 and $140/m2, depending on the materials and processes used
These manufacturing costs lead to electricity costs ranging between 49¢ and 85¢/kWh
A more competitive electricity cost is around 7¢/kWh with the same production costs
This requires OPVs efficiency of 15% and lifetime between 15 and 20 years
Is this feasible with the current techology?
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Developing cost-effective OEs
OPV technology is not yet mature enough to attain these efficiency and lifetime goals, as such limitation of the cells the production costs is required, without compromising PCE
Approximately half of the overall production costs originate from the materials required for OPV production
Approximately 10% of overall production costs typically originate from the packaging material
In order to keep OPV production costs as low as possible, it is important to use as low-cost packaging materials as possible, on the other hand, packaging is directly related to the lifetime of the devices
Packaging materials must keep the production cost down as well as provide effective sealing of the device, and thus impeding its degradation
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Introduction – Lifetime Testing
Environmental factors influencing lifetime
Illumination
Heat
Relative Humidity
Oxygen
Depending on the structure, the environmental factors initiate several degradation mechanisms that fatally affect the organic electronics
Degradation mechanisms can occur simultaneously and their propagation can vary in terms of size and time
Device engineering and effective packaging are the two keys in preventing degradation mechanisms initiation or propagation
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Accelerated lifetime testing Simulation of environmental factors Isolate environmental factors and define the source of degradation for each
mechanism Acceleration of degradation mechanisms propagation through continuous and intense
exposure of the devices to the simulated environment Standardization feasibility and communication between research labs based on
references establishment Translate the measured accelerated lifetime to real time organic electronics lifespan Outdoor lifetime testing Real time examination of organic electronics lifespan Dependence on local weather parameters Cannot isolate environmental factors and define the source of degradation It is more time consuming More reliable for addressing the exact lifetime of organic electronics at certain
environmental conditions
Introduction – Lifetime Testing
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
OPV Εφαρμογές
Konarka Power Plastic
OLED Εφαρμογές
Monolithic Studios
AMOLED
Roll-to-Roll (R2R) Κατασκευή & Παραγωγή
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Major degradation mechanisms in OPVs
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Major degradation factors under heat/moisture/light
Degradation mechanisms usually occur due to degradation factors
The most common degradation factors are: Heat Light Moisture Oxygen Contamination from fabrication Morphology abnormalities from fabrication
From those factors, the ones dependent on fabrication can be limited by improving our fabrication environment.
Degradation Mechanisms commonly met in OPVs due to fabrication environment:
Abnormalities in morphologies of layer or interfaces due to deposition
Contamination through remaining molecules from fabrication
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Major degradation factors under heat/moisture/light
Cleaning the fabrication area, Calibrating the fabrication equipment, and Storing materials properly
are some of the actions that can be taken in order to limit degradation from fabrication factors
Improving the fabrication environment depends exclusively on the human factor.
However, the rest of the degradation factors, Heat Light Moisture Oxygen
do not depend on the human factor and comprise major degradation factors for impeding OPVs long lifetime.
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Major degradation factors under heat/moisture/light
Susceptibility of the metal electrode to reactions with
oxygen and water.
Polymer degradation by oxygen or water leading to the formation
of polymer/oxide composites
Photodegradation of the polymers
Water-induced degradation of ITO
Degradation mechanisms caused by major degradation factors
Other Degradation Mechanisms commonly met in OPVs: Interaction of active layer with the cathode and/or
the anode layers Interaction of active layer with the interfaces
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Electrode Degradation
Metal Electrode Certain metals such as Al, Ca and Ag are commonly
used as electron selective contacts in OPV devices because of their:
high electrical conductivity, work function properties, and ability for deposition at very thin layers.
Metal electrode
Two main degradation mechanisms of the metal electrode have been identified:
primarily its oxidation at the metal/polymer interface and/or at the upper
surface of the metal layer [...], and secondarily its chemical interaction with polymers at the interface with
the active layer [...].
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Electrode Degradation
Metal Electrode First mechanism:
The degradation at the electrode/polymer interface, can result in the formation of an oxidation layer at the metal/polymer interface […].
This oxidation layer hinders the charge selectivity of the electrode, thus reducing device performance.
For Ca/Al electrodes it has been reported that their degradation in air is due to considerable changes at the metal–organic interface […].
Cross-sectional TEM studies have revealed the formation of void structures to be the primary degradation mechanism for Ca/Al contacts.
These structures grow as the electrode ages and becomes oxidized, as shown in the Figure.
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Electrode Degradation
Metal Electrode Ag contacts become similarly oxidized and an interfacial layer of silver oxide is formed
over time, but its formation is a much longer process compared to Al-based electrodes […].
The second degradation mechanism of the metal electrode, the chemical reaction with the active layer, involves the chemical interaction of the thiophenes in the P3HT with the top metal electrodes […].
For example:
Cu electrodes have been found to react with sulfur sites on P3HT during the deposition process […].
It has also been observed that aluminum penetrates into the active layer, gradually forming aluminum – carbon bonds.
A diffused organic-Al interface is formed, which then results in a large oxidized interfacial area upon air exposure, causing reduced charge transport and device performance […].
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Electrode Degradation
Metal Electrode The existence of an ultrathin layer between the metal electrode and the active layer
has been proven to act as a barrier, which prevents the reaction between the metal and the polymer. Layers such as Al2O3 […] LiF […] and CrOx […].
For example: The oxidation of Al leads to the formation of a charge-blocking layer, however,
the use of CrOx as an interfacial layer prevents and minimizes the formation of Al–organic interface that is prone to oxidation.
P3HT:PCBM devices with CrOx interfacial layer exhibit more than 100 times higher stability than reference devices
Other barrier interfaces that have demonstrated increased lifetime in the literature are:
C60/LiF […], CuOx […], C6H5COOLi […], Cs2CO3 […], and TiOx […]. V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Electrode Degradation
Metal Electrode The utilization of barrier interfaces on both the upper and the lower surfaces of the
active layer isolates the active layer, preventing the penetration of oxygen and humidity and ultimately reducing the degradation of the active layer.
Another mechanism promoted by the metal oxide interfacial layers is that they tend to create bonds with atmospheric oxygen, which as a result protect the metal electrode from oxidation […]. (TiOx in the Figure [...])
-OH groups and -OR functionalities within the oxide are activated with UV radiation and are photo-oxidized, consuming O2 and producing CO2 and H2O in the form of gas.
The photo-activation of these films leads to O2 scavenging and opens new horizons for thin films.
Oxygen is trapped when the device is exposed to light and as a result:
oxidation of the metal electrode is much slower reaction
the penetration of oxygen within the active layer is inhibited […].
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Electrode Degradation
Hole Transport Layer
In most OPV devices, poly(ethylenedioxythiophene) poly(styrenesulfonic acid) (PEDOT:PSS) is used for the transfer of holes between the transparent electrode and the active layer for normal structures and between the metal electrode and the active layer for inverted structures.
Materials such as MoO3 [...], V2O5 [...], WO3 [...], and NiO [...] have also been used in literature as improved hole transport layers.
Even though the hole transport layer is essential to the efficient function of OPV devices, the degradation of PEDOT:PSS can shorten the lifetime of the devices, deteriorating the other layers.
Charge Selective Contacts: Hole Transport Layer
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Hole Transport Layer Thermal degradation
PEDOT:PSS is highly vulnerable
Heat treatment of PEDOT:PSS films for up to 10-20 min is beneficial for the electrical properties
However, prolonged exposure to high temperatures may cause thermal degradation
For example exposure at 120°C for more than 55 min significantly reduces the electrical conductivity of the PEDOT:PSS film [...]. This occurs due to shrinking of PEDOT cinductive grains.
Annealing of PEDOT:PSS films at lower temperatures and for shorter periods can help increase their electrical conductivity due to thermal activation of the carriers and improvement of the crystallinity.
Electrode Degradation
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Hole Transport Layer
Degradation due to moisture and oxygen absorption
Atmosphairic air has dentrimental effects on the electrical conductivity of PEDOT:PSS [...]
Electrode Degradation
PEDOT:PSS is highly hygroscopic. Upon water absorption, its conductivity decreases and consequently device lifetime shortens.
Figure shows the change in conductivity of PEDOT:PSS films with respect to heating time, under inert atmosphere (blue) and in air (red) […].
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Hole Transport Layer
Electrode Degradation
The PEDOT:PSS layer can also increase the degradation of other layers of OPV devices.
It has also been observed that water absorbed by the PEDOT:PSS layer can diffuse through the device all the way to the metal cathode accelerating the its degradation […].
PEDOT:PSS layer can increase the degradation of the active layer.
It has been observed that the effect of water absorption in PEDOT:PSS is to increase the sheet resistance of the PEDOT:PSS/blend layer interface […]. In this work the blend layer was MDMO-PPV/PCBM
It has also been reported that the PEDOT:PSS layer can induce the degradation of the active layer in P3HT:PCBM OPVs, through a decrease in the absorbance and the formation of aggregates in the active layer [...].
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Hole Transport Layer
Electrode Degradation
Processing additives to PEDOT:PSS have demonstrated significant enhancement to the hole carrier selectivity in inverted solar cells […].
Comparison between normal and inverted OPVs under ambient illumination has shown that:
In the case of normal OPVs, degradation is much quicker due to top metal oxidation (such as Al) […].
In the case of inverted OPVs, it has been shown that the main degradation mechanism for inverted OPVs under dark ambient environment is due to the phase separation of PEDOT:PSS (water and oxygen molecules absorbance) as well as the interaction at the active layer/PEDOT:PSS interface […].
By using reverse engineering methods it has been also proved that the PEDOT:PSS hole selective contact is the major degradation mechanism for inverted OPVs under accelerated lifetime humidity conditions […]
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Hole Transport Layer
Electrode Degradation
Improvement:
It has been found that water-based PEDOT:PSS is more susceptible to degradation than IPA-based PEDOT:PSS […]
Substitution with metal oxides (MoO3 [...], V2O5 [...], WO3 [...], and NiO [...]), which are also compatible with roll-to-roll processing.
Maintain low cost in manufacturing
Device performance improvement
Increased device lifetime
Metal oxides deposition process in some cases is more time consuming than PEDOT:PSS deposition
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Electrode Degradation
Charge Selective Contacts
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Electrode Degradation
Transparent Electrode
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Active layer Degradation
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Module shading effects & hot spots
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Accelerated Lifetime
LIFETIME DEFINITION:
Lifetime (T80) is considered as the time needed for the power conversion efficiency of an OPV (E0) to degrade to the 80% (E80) of its initial value
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Outdoor lifetime
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
International standards for measuring lifetime in organic devices
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Major degradation mechanisms in OLEDs
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
OLED Degradation mechanism (Case Studies)
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Degradation Related to Initial part of Lifetime Performance
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Color Stability
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Origin of Catastrophic Failure Syndrome (CSF)
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Methods to improve long term stability of OLEDs
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Dark Spots
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Packaging and Encapsulation methods
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Glass and Getter
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Low cost packaging
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Encapsulation specifications for long lifetime performance
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis
Bibliography
• Hoth, C.N., Schilinsky, P., Choulis, S.A, Balasubramanian, S., Brabec, C.J., Solution-processed organic photovoltaics, In: Cantatore, E. (Ed.) Applications of Organic and Printed Electronics - A Technology-Enabled Revolution, Springer: Boston, 2013.
• Frederik C. Krebs, Fabrication and processing of polymer solar cells: A review of printing and coating techniques, Solar Energy Materials & Solar Cells 93 (2009) 394–412.
• Nelson Jenny, The Physics of Solar Cells, Imperial Collage Press, 2003.
• Klaus Müllen, Ullrich Scherf, Organic Light Emitting Devices: Synthesis, Properties and Applications, Wiley (2006) DOI: 10.1002/3527607986.
• F. So, B. Krummacher, M.K Mathai, D. Poplavskyy, S.A Choulis, V.E Choong, Recent progress in solution processable organic light emitting devices, Journal of Applied Physics 102 (9), 091101, (2007)
V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis