Lifetime reliability of organic devices & applications

1. 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 Technology Lifetime reliability of organic devices & applications Limassol September 2014

2. Course Description 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. V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

3. 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

4. Course Content • 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 V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

5. Course Outline • Workload • Lecturing Time: 25 hours • Student Project Preparation Time: 20 hours • Preparation for the Exams: 10 hours • Exams Time: 3 hours • Total Estimated Workload: 58 hours • Grading • Student Project Presentation: 30% • Final Exam: 70% • Awarded ECTS: 2 • Learning 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

6. Introduction • Three major requirements for organic electronics meaningful commercialization The critical triangle for organic electronics [...] Long Lifetime High Efficiency Low Cost • 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 V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

7. 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. […] V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

8. Introduction • Example: Oxidation of top electrode out of Aluminum (Al) in normal OPVs • 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 […] V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

9. Introduction – Cost Parameter Remember the critical triangle for organic electronics [...] • Minimum system cost • Maximum initial performance • Minimum loss of performance over time Long Lifetime • Currently, Organic Electronics present low stability: • Low stability hinders organic electronics commercialization • 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    V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

10. Commercialization of OEs • 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 V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

11. Commercialization of OPVs V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

12. 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

13. 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

14. 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

15. Introduction – Lifetime Testing • 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 V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

16. V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis Roll-to-Roll (R2R) Κατασκευή & Παραγωγή OLED Εφαρμογές OPV Εφαρμογές AMOLED Konarka Power Plastic Monolithic Studios

17. Major degradation mechanisms in OPVs V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

18. 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

19. 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

20. Major degradation factors under heat/moisture/light • Degradation mechanisms caused by major degradation factors • Polymer degradation by oxygen or water leading to the formation of polymer/oxide composites • Susceptibility of the metal electrode to reactions with oxygen and water. • Photodegradation of the polymers • Water-induced degradation of ITO • 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

21. Electrode Degradation Metal Electrode 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. • 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

22. 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

23. 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

24. 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 CrOxas an interfacial layer prevents and minimizes the formation of Al–organic interface that is prone to oxidation. • P3HT:PCBM devices with CrOxinterfacial 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

25. 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

26. Electrode Degradation Charge Selective Contacts: 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. Hole Transport Layer • 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. V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

27. Electrode Degradation 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. V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

28. Electrode Degradation Hole Transport Layer • 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) […]. V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

29. Electrode Degradation Hole Transport Layer • 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

30. Electrode Degradation Hole Transport 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 […] V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

31. Electrode Degradation Hole Transport Layer • 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

32. Electrode Degradation Charge Selective Contacts V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

33. Electrode Degradation Transparent Electrode V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

34. Active layer Degradation V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

35. Module shading effects & hot spots V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

36. 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

37. Outdoor lifetime V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

38. International standards for measuring lifetime in organic devices V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

39. Major degradation mechanisms in OLEDs V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

40. OLED Degradation mechanism (Case Studies) V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

41. Degradation Related to Initial part of Lifetime Performance V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

42. Color Stability V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

43. Origin of Catastrophic Failure Syndrome (CSF) V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

44. Methods to improve long term stability of OLEDs V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

45. Dark Spots V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

46. Packaging and Encapsulation methods V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

47. Glass and Getter V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

48. Low cost packaging V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

49. Encapsulation specifications for long lifetime performance V. M. Drakonakis, A. Savva, P. Eleftheriou, and S. A. Choulis

50. 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, UllrichScherf, 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