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Speaker ：陳永祈

Organic Luminescent Molecule with Energetically Equivalent Singlet and Triplet Excited States for Organic Light-Emitting Diodes (OLED) . Keigo Sato, Katsuyuki Shizu , Kazuaki Yoshimura, Atsushi Kawada, Hiroshi Miyazaki, and Chihaya Adachi

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Speaker ：陳永祈

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  1. Organic Luminescent Molecule with Energetically Equivalent Singlet and Triplet Excited States for Organic Light-Emitting Diodes (OLED) • Keigo Sato, KatsuyukiShizu, Kazuaki Yoshimura, Atsushi Kawada, Hiroshi Miyazaki, • and Chihaya Adachi • Phys. Rev. Lett. 110, 247401 (2013) • Published June 10, 2013 Speaker：陳永祈 Report Date：10月24日 http://dx.doi.org/10.1103/PhysRevLett.110.247401

  2. Outline • Introduction • Experiment method • Result and discussion • summary

  3. Introduction

  4. Organic luminescent materials are widely used in various consumer products such as highlighters, detergents . In addition to their use as chemical products, recen t development of organic materials from a new perspective has opened up a novel possibility for their use in optoelectronic devices . ex：display 有機基板顯示器 螢光筆 zh.wikipedia.org

  5. In particular, the luminescence and unique semiconducting characteristics of organic materials can be combined, leading to the rapid development of organic semiconductor devices such as organic light emitting diodes (OLEDs) OLED電視 LED 可撓性OLED zh.wikipedia.org

  6. OLEDs possess attractive features such as 1. high EL efficiency . ηEQE≈20% 2. high contras t . OLED無論是顏色對比度或者是發光強度都比LCD來得好 3. low weight and low cost . 4. It possess the flexible of charactertistic . cck1616tw.pixnet.net

  7. 分子放光的方式可以有兩種： 電致發光 光致發光

  8. After extensive study of OLEDs for the past 20 years, electroluminescent(EL, 電致發光) materials have usually been classified by one of two mechanisms, fluorescence(螢光) or phosphorescence(磷光), where radiative decay occurs from singlet or triplet excited states, respectively .

  9. Electroluminescent(電致發光)： 當電子和電洞在發光層內“相遇“時，就會產生激子，因而放光 • e：電荷量 • d：發光層的厚度 • J：電流密度 • kS：單重激子的輻射速率 • η S：單重激子數與總激子數的比值 • η out：內部產生的激子數與外部流動的電子數之比值

  10. Spin multiplicity ：2S＋1 T1 S1 2S+1=3 S=1 2S+1=1 S=0 χ11=|↑↑> χ00=1/√2(|↑↓> ―|↓↑>) χ10=1/√2(|↑↓> + |↓↑>) χ1-1=|↓↓> S1：Singlet excited state (單重激發態) T1：Triplet excited state (三重激發態) ＊激子也可以叫做是激發態!!!

  11. 電洞注入 電子注入 三重態激子 τ ＝ 10-6~101 s 單重態激子 τ ＝ 10-9~10-8 s 螢光 τ ＝ 10-6~101 s 磷光 kr：the fluorescence decay rate kp： the phosphorescence decay rate knp： the nonradiative decay rate from the T1 excited state kISC：the intersystem crossing rate

  12. Photoluminescent(PL, 光致發光)： Photoluminescent materials have usually been classified by one of two mechanisms, fluorescence or phosphorescence, where radiative decay occurs from singlet or triplet excited states, respectively . 光致發光 Intersystem crossing S1 T1 螢光 磷光 S1 S0 T1 S0

  13. Energy State(能階) 電子的躍遷是否發生， 取決於state與state之間的偶合程度 <S0∣u∣S1> LUMO S1 HOMO S0 S0：Singlet ground state S1：Singlet excited state T1：Triplet excited state S0 S1 T1 HOMO ：highest occupied molecular orbital (最高電子佔據軌域) 2S+1=1 S=0 2S+1=3 S=1 LUMO ：lowest unoccupied molecular orbital (最低電子未佔據軌域)

  14. τ ＝ 10-6~101 s 受光激發 螢光 磷光 τ ＝ 10-6~101 s τ ＝ 10-9~10-8 s kr：the fluorescence decay rate kp： the phosphorescence decay rate knp： the nonradiative decay rate from the T1 excited state kISC：the intersystem crossing rate

  15. 螢光和磷光最大的不同就是輻射生命期的長短：螢光和磷光最大的不同就是輻射生命期的長短： • 螢光輻射生命期：10-9~10-8 s • 磷光輻射生命期：10-6~101 s 奎寧水因吸收紫外光之後而放出螢光 夜明珠所放出的磷光 zh.wikipedia.org/zh-tw

  16. The use of phosphorescent iridium 銥(Ir) complexes overcame the limitation of exciton production efficiency through spin-orbit coupling, resulting in devices with very high external EL efficiency (EQE)of over 20% . In fact, the high EQE of such phosphorescentcomplexes prompted commercialization of OLEDs. Unfortunately, rare metals such as Ir and Pt are not the ideal choice for OLED emitters. They are unevenly distributed resources, expensive, rather low solubility and produce toxic waste . spin-orbit coupling → <L · S>＝value <L · S>的值越大，就越有可能放出磷光 iridium 銥(Ir) complexes

  17. To produce OLED emitters without these drawbacks, we recently prepared novel light-emitting materials that realize high efficiency without using rare metal complexes by taking advantage ofvery efficientthermally activated delayed fluorescence (TADF) .

  18. 螢光 受光激發 磷光 電洞注入 電子注入 螢光 磷光

  19. Although some molecules are already known to exhibit TADF, their efficiencies are generally quite low . In the last few years, they have surveyed various candidates as TADF emitters and found the molecules that exhibit significant TADF .

  20. SnF2OEP CC2TA ΔEST＝0.24eV ΔEST＝0.06eV ηEQE＝0.1% ηEQE＝11% PIC-TRZ ACRFLCN ΔEST＝0.1eV ηEQE＝5.3% >2% ΔEST＝0.1eV ηEQE＝10% considering only the photoluminescence (PL) efficiency of ΦPL＝39% related to conventional fluorescence J. Chem. Theory Comput. 2013, 9, 3872−3877

  21. Recently, they found an advanced molecule with an ultimately small ΔEST ≈ 0eV, virtually zero gap. They believe that this is the first report on the molecules having zero-gap between the singlet and triplet excited states. An OLED containing this molecule exhibited a high ηEQE of 14% ±1% S1 △E≈0 eV T1 S0

  22. In their successive design of TADF molecules, taking advantage of the backbone of PIC-TRZ, they systematically changed the number and position of an indolocarbazole unit through quantum mechanism calculation . Indolocarbazole unit PIC-TRZ PIC-TRZ2

  23. Experimentmethod

  24. DFT(Density Function Theoy) • 計算方法：DFT/PBE0/6-31G(d) PBE0： 氫原子的分子軌域 Setting up of parameter during calculation 6-31G(d) ：A kind of basis set Φi： electron basis function ci： coefficient Ψ：the molecular orbital of wavefunction

  25. HOMO LUMO PIC-TRZ HOMO and LUMO are separated incompletely ΔEcalc＝0.08eV HOMO LUMO PIC-TRZ2 HOMO and LUMO are completely separated ΔEcalc＝0.003eV

  26. Their goal is to achieve high TADF performance in a solid film, aimed for application in OLEDs, so they focused on host-guestsystems. Dope(摻雜) host guest PIC-TRZ2

  27. Figures 3(a) and 3(b) show transient PL decay and fluorescence (black line) and delayed fluorescence , 延遲螢光(red line) spectra of6 wt %-PIC-TRZ2 doped(摻雜) into a 1,3-bis(carbazol-9-yl)benzene (mCP) host layer at T=5K. λ ＝475nm (S1＝2.61 eV) Spectra corresponded well with each other, indicate that the delayed component is caused by TADF .

  28. Result and discussion

  29. In the case of PIC-TRZ, although we clearly observed the appearance of phosphorescence at low temperature, demonstrating a characteristic emission spectrum with clear vibrational peaks , no such emission spectrum was observed for PIC-TRZ2. 有磷光產生 at T=5K 沒有磷光產生

  30. They conclude that the delayed component is related to TADF, and up-conversion occurs in PIC-TRZ2 even at T ＝ 5K, indicating it possesses virtually zero gap .

  31. The transient PL decay characteristics of PIC-TRZ2 at T＝300 Kat Figure (c) Fast PL transient lifetime τf ＝83 ns slow PL decay τd ＝2.7 μs iridium 2-phenylpyridine They note that the latterτd is comparable to that of conventional Ir complexes such as iridium 2-phenylpyridine [Ir(ppy)3] derivatives with τ＝1~5 μs , indicating that PIC-TRZ2 has comparable transient performance of delayed fluorescence to those of Ir complexes .

  32. They measured the temperature dependence of the radiative decay rate of the delayed fluorescence(kr(TADF)) ＊阿瑞尼士方程式(Arrhenius equation)告訴我們：溫度越高，kr的反應速率就越快 !

  33. To maximize the TADF efficiency of films doped with PIC-TRZ2, they optimized the host materials.

  34. PLQE ： PL quantum efficiencies Total PLQE ：總放光之效率 PromptPLQE ：放螢光之效率 Delayed PLQE ：放延遲螢光之效率 學者發現說PIC-TRZ摻雜其他不同的host materials之後，會有不同大小的發光效率 ＊總放光效率＝放螢光之效率＋放延遲螢光之效率

  35. 學者猜測三重激子會在TAPAC分子待上一段很長的時間，而後再將能量釋放出來，並轉移至PIC-TRZ，使得PIC-TRZ的激子數增多，學者猜測三重激子會在TAPAC分子待上一段很長的時間，而後再將能量釋放出來，並轉移至PIC-TRZ，使得PIC-TRZ的激子數增多， 放光效率因而提升。 Back energy transfer energy transfer PIC-TRZ2

  36. Transient PL measurements revealed that Triplet excitation energy ：host materials ＞PIC-TRZ2 Energy transfer host materials PIC-TRZ2 → 延遲螢光會發生 Triplet excitation energy ：host materials ＜PIC-TRZ2 Energy transfer host materials PIC-TRZ2 → 能量會以非輻射(nonradiation)的方式流失， 例如：熱 。因而造成不必要的能量浪費。 當然!這是我們不希望的!!!

  37. if energy transfer occurs readily, we can expect a further enhancement of EQE, suggesting a theoretical maximum of 11.8%. They tried to optimize the device parameters to maximize ηEQE due to imperfect triplet exciton confinement.

  38. device V device VI device VII LiF/Al Cathode LiF/Al Cathode LiF/Al Cathode (45nm) TmPyPBi DPEPO TPBi (20nm) PYD2：6wt％PIC-TRZ2 PYD2：6wt％PIC-TRZ2 PYD2：6wt％PIC-TRZ2 TAPAC TAPAC TAPAC (40nm) ITO Anode ITO Anode ITO Anode Metal Cathode Electron Transport Layer Light Emitting Polymer Hole Transport Layer ITO Anode

  39. ? Device VII LiF/Al Cathode 雖然本篇Paper沒有顯示出Device I、Device II、Device III 以及Device IV的結構，但唯一能夠確定的是；無論是效率 或著是穩定性都沒有比右邊的三種Device來得好! 並且效率最大的是Device VII TmPyPBi PYD2：6wt％PIC-TRZ2 TAPAC ITO Anode

  40. 1. They expect that improved understanding of molecular structures will reveal undeveloped functions that will further enhance OLED performance. Summary ： 2. While PIC-TRZ2 has nearly zero-gap energy, a rather high f of 60% was obtained. This is because PIC-TRZ2 has appreciable oscillator strength (振子強度)→<S0∣u∣S1>, leading to highly efficient TADF. ＊振子強度的數值越大，兩個State之間的躍遷就越容易發生 3. The radiative decay rate of the delayed fluorescence(kr(TADF)) dependence of temperature .

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