Insights into charge balance and its limitations in simplified phosphorescent organic light-emitting devices |
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| Affiliation: | 1. Department of Chemistry, Indian Institute of Technology Bombay, Mumbai 400076, India;2. Centre of Excellence in Nanoelectronics, Indian Institute of Technology Bombay, Mumbai 400076, India;3. National Centre for Photovoltaic Research and Education, Indian Institute of Technology Bombay, Mumbai 400076, India;4. National Centre of Excellence in Technologies for Internal Security, Indian Institute of Technology Bombay, Mumbai 400076, India;1. Department of Chemistry, Chemistry Institute for Functional Materials, Pusan National University, Busan 690-735, Republic of Korea;2. Department of Physics, Pukyong National University, Busan 608-737, Republic of Korea;3. Department of Physics, Ulsan University, Ulsan 680-749, Republic of Korea;4. Department of Industrial Chemistry, Pukyong National University, Busan 608-739, Republic of Korea;1. CAS Key Laboratory of Biomedical Diagnostics, Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Science (CAS), 88 Keling Road, Suzhou 215163, China;2. Institute of Theoretical Chemistry, Jilin University, Changchun 130023, China;3. Department of Chemical and Biomolecular Engineering, Lafayette College, Easton 18042, PA, USA;1. Instituto de Ciencia Molecular (ICMOL), Universidad de Valencia, Calle Catedrático José Beltrán, 2, 46980 Paterna, Spain;2. Skolkovo Institute of Science and Technology, 143025 Skolkovo, Moscow Region, Russia;3. Canatu Oy, Konalankuja 5, FI-00390, Helsinki Finland;4. Department of Applied Physics, Aalto University, School of Science, P.O. Box 15100, FI-00076 Aalto, Finland;5. St. Petersburg Polytechnic University, 195251, St. Petersburg, Russia;1. Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan 430072, China;2. Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China |
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| Abstract: | Simplified phosphorescent organic light-emitting device (PHOLED), which utilizes only two organic layers, showed record-high efficiency when first introduced. It is quite surprising that this device can have such high efficiency without the use of complex carrier and exciton confinement layers that are common in the state-of-the-art PHOLEDs nowadays. Therefore, it is important to understand how good charge balance is in simplified PHOLED and why. In this work, we study the effects of altering charge balance in simplified PHOLED through means of changing layer thickness in the hole transport layer (HTL) and electron transport layer (ETL) as well as intentionally doping hole and electron traps in the HTL and ETL, respectively, on device efficiency. The results show that when using high carrier mobility charge transport materials, changing layer thickness does not impact charge balance appreciably. On the other hand, introducing charge traps in a thin layer within the HTL or ETL can, in comparison, influence charge balance more significantly, and proves to be a more effective approach for studying the factors limiting charge balance in these devices. The results reveal that simplified PHOLEDs are generally hole-rich, and that the leakage of electrons to the counter electrode is also a major mechanism behind the poor charge balance and efficiency loss in these devices. In order to optimize charge balance in simplified PHOLED, it is important to reduce hole transport in the device so that e-h ratio can be brought closer to unity, as well as eliminate electron leakage. Finally, we show that by simply using an electron blocking HTL, the efficiency of the device can be enhanced by as much as 25%, representing the highest reported for simplified PHOLEDs. |
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| Keywords: | Simplified PHOLED Charge balance Hole-rich Electron leakage Delayed EL |
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