Chapter 6: Conclusions and Perspective
6.2 Perspective
In TADF emitters, the material design to accelerate kRISC is crucial to reduce efficiency roll-off and even extent the operational stability of TADF OLEDs by suppression of exciton annihilation. However, the RISC process is not fully understood so far. Generally, RISC is a forbidden process because the spin multiplicity is not
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conserved, but ΔEST can be small enough to make it possible. At present, the design principle for TADF molecules mainly focuses on the ΔESTs. Most TADF emitters show the kRISC lower than 106 s−1, which limit their further application in organic electronics, such as organic lasers and TADF assisted fluorescence OLEDs. It is difficult to enchance the kRISC of TADF molecules by just modulating their ΔESTs. Thus, other strategies should be explored to further improve kRISC, such as increase the spin–orbit coupling and mixed state between S1 and T1. According to El-Sayed rules, the rate of intersystem crossing is relatively large if the radiationless transition involves a change of orbital type.
For example, 1π, π* → 3n, π* is faster than 1π, π* → 3π, π* and 1n, π* → 3π, π* is faster than 1n, π* → 3n, π*. Thus, 3CT states mixed with 3LE states is a potential strategy to accelerate the kRISC of TADF molecules. In addition, although there are many efficient TADF emitters for visible emission, highly efficiency infrared TADF emitters are rarely reported in the literature. I believe that the infrared TADF emitters have great potential applications in optical signal processing, night-vision technologies, bioimaging and photodynamic therapy (Figure 6-1). In a word, the research in TADF technology is still in its infancy and there is a need to resolve many issues which keep this field far away from applications.
151 Figure 6-1. TADF molecules for future applications.
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Acknowledgements
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Acknowledgements
It has been almost three years since I stepped onto the land of Fukuoka, all images reappeared in my mind as if it happened just yesterday. During the evolution of this dissertation, I am deeply obliged to a number of people who have guided and supported me through my academic journey.
First and foremost, I want to extend my heartfelt gratitude to my supervisor Professor Chihaya Adachi for his sincere support and valuable advice in my research process. He gives me an unforgettable memory of his benevolence, intelligence, diligence and erudition, which inspires me a lot both in academic study and daily life, these will definitely be of great treasure for my future career and academic research. Also, I highly enjoy the scientific freedom he offers me.
I am deeply grateful to Professor Hiroyuki Furuta and Professor Sunao Yamada for helpful discussions and constructive comments regarding this dissertation.
I would like to thank Associate Professor Hajime Nakanotani for his careful review of my dissertation and their valuable comments. He gave me substantial suggestions for the refinement of my presentation during preparation of the oral defense.
I also would like to thank Associate Professor Ryota Kabe for being so patient with me during my doctoral studies. I was unfamiliar with equipment, laboratory rules and regulations when I came to Adachi lab. I give him a lot of trouble, but I never got punished.
I am grateful to Dr. William J. Potscavage, Jr. for helping me to improve my manuscripts. He is always very careful to check my manuscripts, and gives me many valuable comments and suggestions.
I would like to thank the TADF group members, Dr. Yan Geng, Chin-Yiu Chan; Mr Jong Uk Kim, Naoto Notsuka, Masaki Tanaka and Masayuki Shiochi for the inspiring discussions and crucial comments. I am also grateful to Ms. Nozomi Nakamura and Keiko Kusuhara for technical support.
I want to thank Associate Professor Toshinori Matsushima, Sandanayaka S. D.
Atula, Chuanjiang Qin and Youichi Tsuchiya; Assistant Prof. Kenichi Goushi, Takeshi
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Komino, Masashi Mamada, and Jean-Charles Ribierre; Dr. Ko Inada, Fatima Bencheikh, Piotr Sleczkowski, Li Zhao; Mr. Hiroyuki Mieno, Hao Ye, Noda Hiroki, MD ASHADUL ISLAM, Zesen Lin, Masayuki Yokoyama, Yu Esaki, Ryo Nagata, Momoka Miyajima, Kazuya Jinnai, Yu Shiihara, Fumihiro Otaka and Shibin Ruan.
Next, I would like to thank the former member Associate Professor Qisheng Zhang for guiding me during the first year of my PhD. I also want to thank Dr. Kou Yoshida for sharing the joy with me and giving me selfless help.
I also want to think all staffs of Adachi‟s group for taking care of my daily life in laboratory. Especially, I thank Ms. Rei Sasagawa for my Pd.D. degree application. I will bear all these in mind forever.
Furthermore, I would like to acknowledge my domestic supervisor, Professor Liangsheng Liao in Soochow University, for giving me the opportunity to study abroad and continued support of my research.
Finally, I am deeply indebted to my parents, whose everlasting encouragement and silent support motivate me to move on. In the meanwhile, I owe special thanks to my fiancée, I would like to acknowledge her endless love and understanding. They gave me the mighty incentive to pursue the career and enjoy the wonderful life, your love is so important to me!
Fukuoka June, 2017
Linsong Cui
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Appendixes
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List of Chemical Compounds
HAT-CN 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile TAPC 1,1-bis[4-[N,N-di(p-tolyl)amino]phenyl]cyclohexane α-NPD N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,10-biphenyl-4,4′-diamine TCTA 4,4′,4′′-tris(N-carbazolyl)triphenylamine Tris-PCz 9,9′,9″-triphenyl-9H,9′H,9″H-3,3′:6′3″-tercarbazole mCP N,N′-dicarbazolyl-3,5-benzene mCBP 3,3-di(9H-carbazol-9-yl)biphenyl CzSi 9-(4-tert-butyl phenyl)-3,6-bis(triphenylsilyl)-9H-carbazole DPEPO bis(2-(diphenylphosphino)phenyl)ether oxide PPT 2,8-bis(diphenylphosphoryl)dibenzo[b,d]thiophene TmPyPB 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene TPBi 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene TSPO1 diphenyl-4-triphenylsilylphenylphosphine oxide Bebq2 bis(10-hydroxybenzo[h]quinolinato)beryllium Liq 8-hydroxyquinolatolithium LiF lithium fluoride 4CzIPN 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene DPA-AQ 2,6-bis(4-(diphenylamino)phenyl)anthracene-9,10-dione 3Cz-TRZ 9′-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9′H-9,3′:6′,9′′-tercarbazole FIrpic bis[(4,6-difluorophenyl)pyridinato-N,C2](picolinato) iridium Ir(dmp)3 tris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f] phenanthridine] iridium Al aluminum fac-Ir(iprpmi)3 fac-tris[(2,6-diisopropylphenyl)-2-phenyl-1H′-imidazo[e]]iridium(III)
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List of Abbreviations
BDE bond dissociation energy B3LYP Becke, three-parameter, Lee-Yang-Parr functional CE current efficiency CIE commission internationale de l‟Eclairage CV cyclic voltammetry DFT density functional theory DSC differential scanning calorimetry EA electron affinity EBL electron blocking layer EIL electron injection layer EL electroluminescence EML emissive layer EQE external quantum efficiency ETL electron hole transport layer eV electron volt FMO frontier molecular orbital HBL hole blocking layer HIL hole injection layer HOMO highest occupied molecular orbital HTL hole transport layer IC internal conversion IP ionization potential IQE internal quantum efficiency
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ISC intersystem crossing ITO indium tin oxide LCAO linear combination of atomic orbital LUMO lowest unoccupied molecular orbital MS mass spectrum NMR nuclear magnetic resonance OLEDs organic light emitting diodes PE power efficiency PL photoluminescence PLQY photoluminescence quantum yield RISC reverse intersystem crossing SCLC space charge limited current SOC spin-orbit coupling SOP spin-orbit perturbation TADF thermally activated delayed fluorescence TDDFT time-dependent density functional theory TGA thermogravimetric analysis TIR total internal reflection TPA triplet-polaron annihilation TSDD triplet spins density distribution TTA triplet-triplet annihilation UV ultraviolet
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List of Important Symbols
S total spin angular momentum S0 singlet ground state S1 lowest singlet excited state T1 lowest triplet excited state λ wavelength ε(λ) molar absorption coefficient f oscillator strength kr radiative decay rate knr non-radiative decay rate kISC intersystem crossing rate kRISC reverse intersystem crossing rate kPh radiative decay rate of phosphorescence τS singlet lifetime τT triplet lifetime ΦPL fluorescence quantum yield ΦPh phosphorescence quantum yield Φp prompt fluorescence quantum yield Φd delayed fluorescence quantum yield μ charge carrier mobility J current density V voltage kB Boltzmann constant 𝜼𝐄𝐐𝐄 external quantum efficiency
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𝜼𝐫 charge carrier balance factor 𝝌 spin statistics factor 𝜼𝐨𝐮𝐭 outcoupling factor
∆EST singlet-triplet energy splitting T50 half lifetime nm nanometer ns nanosecond μs microsecond
°C degrees Celsius Å Ångstrom h hour K kelvin β Poole–Frenkel factor L luminance Tg glass transition temperature Tm thermal decomposition temperature
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List of Publications and Symposiums Original Papers
1) Lin-Song Cui, Shi-Bin Ruan, Ryo Nagata, Lei Zhang, Ko Inada, Hajime Nakanotani, Liang-Sheng Liao,* Chihaya Adachi* “Long-lived efficient delayed fluorescence organic lightemitting diodes using n-type hosts” Submitted.
2) Lin‐Song Cui, Hiroko Nomura, Yan Geng, Jong Uk Kim, Hajime Nakanotani, Chihaya Adachi,* “Controlling Singlet–Triplet Energy Splitting for Deep‐Blue Thermally Activated Delayed Fluorescence Emitters” Angew. Chem. Int. Ed. 2017, 129, 1593-1597.
3) Lin‐Song Cui, Ya‐Li Deng, Daniel Ping‐Kuen Tsang, Zuo‐Quan Jiang, Qisheng Zhang,* Liang‐Sheng Liao,* Chihaya Adachi,* “Controlling Synergistic Oxidation Processes for Efficient and Stable Blue Thermally Activated Delayed Fluorescence Devices” Adv. Mater. 2016, 28, 7620-7625.
4) Lin‐Song Cui, Jong Uk Kim, Hiroko Nomura, Hajime Nakanotani, Chihaya Adachi,* “Benzimidazobenzothiazole‐Based Bipolar Hosts to Harvest Nearly All of the Excitons from Blue Delayed Fluorescence and Phosphorescent Organic Light‐
Emitting Diodes” Angew. Chem. Int. Ed. 2016, 55, 6864-6868.
Joint Papers
1) Jong Uk Kim, Saripally Sudhaker Reddy, Lin-Song Cui, Hiroko Nomura, Sunbin Hwang, Dae Hyeon Kim, Hajime Nakanotani, Sung‐Ho Jin,* Chihaya Adachi,*
“Thermally activated delayed fluorescence of Bis(9,9-dimethyl-9,10-dihydroacridine) dibenzo[b,d]thiophene 5,5-dioxide derivatives for organic light-emitting diodes” J. Lumin. Accepted.
2) Yan Geng, Lin-Song Cui, Jong Uk Kim, Hajime Nakanotani, Chihaya Adachi,*
“Molecule Design for Blue Thermal Activated Delayed Fluorescence Materials:
Substitution Position Effect”Submitted.