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


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


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


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


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.

[…]


[…]

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


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


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


Commercialization of OPVs


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?


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


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


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


OPV Εφαρμογές

Konarka Power Plastic

OLED Εφαρμογές

Monolithic Studios

AMOLED

Roll-to-Roll (R2R) Κατασκευή & Παραγωγή


http://www.google.com/url?sa=i&source=images&cd=&cad=rja&uact=8&docid=nFnTrZGUrSpySM&tbnid=vsUTXzOJSt_VzM:&ved=0CAgQjRw49QI&url=http://hardforum.com/showthread.php?t=1787425&page=2&ei=iF02VLPjIMPV7gavo4G4AQ&psig=AFQjCNFQiagLV83LssrhufSh2ocy9ewnDw&ust=1412935432631218

Major degradation mechanisms in OPVs


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



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.



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


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 [...].



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.



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 […].



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


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 […].



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


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.




Degradation due to moisture and oxygen absorption

Atmosphairic air has dentrimental effects on the electrical conductivity of PEDOT:PSS [...]


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) […].




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 [...].




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 […]




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



Charge Selective Contacts



Transparent Electrode


Active layer Degradation


Module shading effects & hot spots


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


Outdoor lifetime


International standards for measuring lifetime in organic devices


Major degradation mechanisms in OLEDs


OLED Degradation mechanism (Case Studies)


Degradation Related 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


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)


lifetime reliability of organic devices & applications

Documents

organic devicesv

degradation of opvs

introduction long lifetime

oled degradation mechanisms

course description

polyvios eleftheriou

achilleas savva

identification of major