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Device Physics of Organic Light-Emitting Diodes : Interplay Between Charges and Excitons

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Device Physics of Organic Light-Emitting Diodes : Interplay Between Charges and Excitons Device physics of organic light-emitting diodes : interplay between charges and excitons Citation for published version (APA): Eersel, van, H. (2015). Device physics of organic light-emitting diodes : interplay between charges and excitons. Technische Universiteit Eindhoven. Document status and date: Published: 16/11/2015 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement: www.tue.nl/taverne Take down policy If you believe that this document breaches copyright please contact us at: [email protected] providing details and we will investigate your claim. Download date: 04. Oct. 2021 Device physics of organic light-emitting diodes: interplay between charges and excitons PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de rector magnificus prof.dr.ir. F.P.T. Baaijens, voor een commissie aangewezen door het College voor Promoties, in het openbaar te verdedigen op maandag 16 november 2015 om 16:00 uur door Herman van Eersel geboren te Klundert Dit proefschrift is goedgekeurd door de promotoren en de samen- stelling van de promotiecommissie is als volgt: voorzitter: prof.dr. H.J.H. Clercx 1e promotor: prof.dr. R. Coehoorn 2e promotor: prof.dr.ir. R.A.J. Janssen co-promotor(en): dr. P.A. Bobbert leden: prof.dr. A.B. Walker (University of Bath) prof.dr. B. Ruhstaller (Zürcher Hochschule für Angewandte Wissenschaften) prof.dr.ir. W.M.M. Kessels adviseur(s): dr. S. Reineke (TU Dresden) Het onderzoek of ontwerp dat in dit proefschrift wordt beschreven is uitgevoerd in overeenstemming met de TU/e Gedragscode Wetenschapsbeoefening. ii harm van eersel DEVICEPHYSICSOF ORGANICLIGHT-EMITTINGDIODES: interplay between charges and excitons colophon Cover design: Harm van Eersel Printed by: Gildeprint Drukkerijen A catalogue record is available from the Eindhoven University of Technology Library ISBN: 978-90-386-3949-9 This thesis is part of NanoNextNL, a micro and nanotechnology innova- tion consortium of the Government of the Netherlands and 130 partners from academia and industry. More information on www.nanonextnl.nl Copyright © 2015, Harm van Eersel CONTENTS 1 introduction 1 1.1 Organic Electronics . 2 1.2 Organic light-emitting diodes . 5 1.3 Modeling OLEDs . 13 1.4 Scope of this thesis . 17 2 electronic and excitonic processes in organic ma- terials 25 2.1 Empirical charge transport models . 26 2.2 Energy levels and the density of states . 28 2.3 Charge carriers and charge carrier transport . 29 2.4 Empirical models for exciton dynamics . 30 2.5 Excitons, charge transfer states and photophysical processes . 31 2.6 Energy transfer . 34 2.7 Exciton loss processes . 36 3 a kinetic monte carlo model for charge transport and excitonic processes in organic materials and devices 51 3.1 Box, sites and species . 52 3.2 Physical processes . 56 3.3 Algorithm . 59 3.4 Extracting results from the simulation . 60 3.5 Error margins on the simulations results . 61 3.6 Other approaches . 62 4 molecular-scale simulation of electroluminescence in a multilayer white organic light-emitting diode 71 4.1 The OLED stack: electrical and optical characteristics . 73 4.2 Monte Carlo simulation of charge and exciton dynamics . 77 4.3 Simulation results: current density, exciton genera- tion, and light emission . 80 4.4 Conclusion . 82 5 monte carlo study of efficiency roll-off of phos- phorescent organic light-emitting diodes: evidence for dominant role of triplet-polaron quenching 93 5.1 Introduction . 94 5.2 Device, approach, and results . 95 v vi contents 5.3 Conclusion and outlook . 101 6 kinetic monte carlo study of triplet-triplet anni- hilation in organic phosphorescent emitters 117 6.1 Introduction . 118 6.2 Monte Carlo simulation of steady-state triplet-triplet an- nihilation . 122 6.3 Monte Carlo simulation of PL transients . 128 6.4 Summary and conclusions . 138 7 effect of förster-mediated triplet-polaron quench- ing and triplet-triplet annihilation on the efficiency of organic light-emitting diodes 145 7.1 Introduction . 146 7.2 Method . 148 7.3 Results – red and green devices . 151 7.4 Results – idealized devices . 152 7.5 Conclusions and outlook . 160 8 kinetic monte carlo modeling of the efficiency roll- off of a multilayer white oled 165 8.1 Introduction . 166 8.2 Stack, approach, and results . 166 8.3 Summary and conclusions . 174 9 kinetic monte carlo study of the sensitivity of oled efficiency and lifetime to materials parameters 177 9.1 Introduction . 178 9.2 Simulation method . 180 9.3 The uniform density model – the relationship between roll-off and lifetime . 188 9.4 Sensitivity of the roll-off to material parameters . 194 9.5 Sensitivity of the OLED lifetime to the Ir-dye concentration 201 9.6 Summary, conclusions and outlook . 203 10 conclusions and outlook: the future of mechanis- tic oled device modeling 209 10.1 Conclusions and next steps . 210 10.2 Potential additional applications of 3D MC OLED device modeling . 215 10.3 Next-generation OLEDs . 225 10.4 Next-generation OLED modeling methods . 226 INTRODUCTION 1 Organic light-emitting diodes (OLEDs) are the commercially most suc- cessful example of organic electronics today, with a significant fraction of the high-end smartphone displays being based on OLED technology. Also OLED products for signage and lighting have reached the market in the past years. It was, however, not always evident that OLEDs would be a breakthrough technology. The success of OLEDs was made possible, firstly, due to an extensive body of research focusing on device physics and the design of new device architectures and materials. In this chapter, we will introduce the field of organic electronics and organic light-emitting diodes, explain the concepts necessary for a basic understanding of OLED device physics, sketch the context of the work presented in this thesis and define the scope of the thesis as presented in the following chapters. 1 2 introduction 1.1 organic electronics In the past five years, organic electronic devices have had a successful introduction to the market in the form of organic light-emitting diodes (OLEDs), with a over one third of the revenue of mobile phone screens 1 coming from OLED displays in 2014.[ ] Nowadays, OLEDs are also com- mercially available in the form of signage and light-emitting tiles. For other fields of organic electronics, such as organic photovoltaics (OPV), (light-emitting) organic field-effect transistors, and organic lasers, mar- ket introduction is still in an earlier state and most research focuses on realizing and improving lab-scale prototypes. What all these devices have in common, is that the active layer is based on organic materials, i.e. materials that consist of molecules mainly based on carbon and hydrogen atoms. Life as we know it is based on organic molecules, hence the adjective “organic”. However, also materials that we do not immediately associate with life, such as polymers (plastics), are also a member of this class of organic compounds. A subclass of organic materials – both small molecules and polymers – exhibit semi- conducting and conducting properties. 2 3 Already in the 1950s and 1960s,[ , ] organic materials were found with good electrical properties. Also electroluminescence was already observed 4 in organic materials quickly after.[ ] Research into the field of organic electronics has continued thereafter. For their pioneering work on con- ductive polymers, the Nobel prize in Chemistry was awarded to Alan J. 5 Heeger, Alan G. MacDiarmiid, and Hideki Shirakawa in 2000.[ ] Most of these (semi)conducting organic compounds have in common that their molecular structure contains a conjugated substructure. This substructure consists of mainly carbon atoms with alternating single and double bonds. This pattern results in an overlap of the p-orbitals and allows for delocalization of the p-electrons over a large part of the molecule, hence facilitating conduction. Figure 1.1 shows the molecu- lar structure of several compounds used in modern OLEDs, which are based on small molecules. The upper two molecules are used for charge transport, the lower two for light emission. The alternating single and double bonds are clearly visible. The energy levels of these molecules (Figure 1.2) show a gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molec- ular orbital (LUMO). The energy difference between these levels is de- fined as the gap energy, Eg.
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