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Styrene and quinoline groups are commonly incorporated into the organic fluorescent materials for organic light-emitting diodes (OLEDs). In this work, a type of small molecule derived from styrene and quinoline, with a chemical structure of 2,2'-(2,5-dimethoxy-1,4-phylenedivinylene)bis-8- acetoxyquinoline (MPV-AQ), is employed as the emitter and electron transporting material in the OLEDs, and its optoelectronic characteristics such as charge-carrier injection, transporting and recombination are investigated by the steady-state and transient technologies. It is found that the electron injection from the cathode into the MPV-AQ layer shows the Fowler-Nordheim (FN) tunneling characteristic in the N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB)/MPV-AQ bilayer OLED, which is different from the Richardson-Schottky (RS) thermionic emission in the electron-only device based on the MPV-AQ single-layer. The difference in electron injection is attributed to the bend of energy bands of MPV-AQ in the NPB/MPV-AQ device, which can be caused by the charge accumulation at the NPB/MPV-AQ interface. The accumulated charges should mainly be the holes on the side of NPB layer because the electron mobility of MPV-AQ is much lower than the hole mobility of NPB. Owing to the bending of lowest unoccupied molecular orbital (LUMO) of MPV-AQ, the tunneling distance for electrons is significantly reduced, which is favorable for the FN tunneling. The barrier height for electron injection is calculated to be 0.23 eV by fitting the current-voltage curve of the NPB/MPV-AQ bilayer OLED. And the electron mobility of MPV-AQ is determined by the delay time of transient electroluminescence (EL) and shows field-dependence with the value on the order of 10–6 cm2/(V·s). In addition, the electron-hole recombination coefficient is obtained from the long time component of the temporal decay of the EL intensity, and the coefficient is found to decrease with the applied voltage increasing, which is consistent with the efficiency roll-off in this bilayer OLED. This study may provide a foundation for understanding the electronic processes of carrier injection, transport and recombination in the OLEDs, which is helpful in improving the device performance.
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Keywords:
- organic light-emitting diodes /
- carrier injection /
- recombination coefficient /
- transient electroluminescence
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图 1 MPV-AQ和NPB的分子结构式
Figure 1. Molecular structures of MPV-AQ and NPB.
图 2 单载流子器件的电流-电压特性 (a) J-V曲线; (b) ln
$J\text{-}F^{1/2}$ 曲线Figure 2. Characteristics of current density and voltage in the electron-only and hole-only devices: (a) J-V curves; (b) ln
$J\text{-}F^{1/2}$ curves
图 3 (a) 不同MPV-AQ厚度时双层OLED的电流-电压特性; (b) 外加电压与MPV-AQ厚度的关系; (c) NPB和MPV-AQ层中平均电场(Fh和Fe)与电流的关系; (d) 双层OLED的能级结构示意图
Figure 3. (a) Characteristics of current density and voltage in the bilayer OLEDs with different MPV-AQ thickness; (b) relationship of applied voltage and MPV-AQ thickness; (c) relationship of average electric field (Fh and Fe) and current density in the NPB and MPV-AQ layers; (d) diagram of energy levels in the bilayer OLEDs.
图 4 NPB和MPV-AQ层中电流与电场(Fh和Fe)的关系 (a), (c) lnJ-F1/2曲线; (b), (d) lnJ-F–1曲线
Figure 4. Relationship of current density and electric field in the NPB and MPV-AQ layers (Fh and Fe): (a), (c) lnJ-F1/2 curves; (b), (d) lnJ-F–1 curves.
图 5 (a) 双层OLED中瞬态EL随电压的变化, 其中NPB和MPV-AQ的厚度都等于50 nm; (b) MPV-AQ电子迁移率与电场平方根的关系
Figure 5. (a) Voltage dependence of the transient EL from the bilayer OLED with the same thickness of 50 nm for NPB and MPV-AQ; (b) electron mobility of MPV-AQ as a function of the square root of electric field.
图 6 (a) 脉冲电压15 V时(ΦEL)–1/2与时间的关系, 插图为对应EL的衰减曲线; (b) 不同电压下的复合系数, 插图为不同电压下OLED的外量子效率
Figure 6. (a) EL decay at the falling edge of a 15 V pulse plotted in (ΦEL)–1/2 vs time scale. The inset shows the corresponding EL decay curve; (b) dependence of recombination coefficient on the voltage. The insert shows the external quantum efficiencies (EQEs) at various voltages in the OLED.
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[1] Tang C W, VanSlyke S A 1987 Appl. Phys. Lett. 51 913
Google Scholar
[2] Sun Y R, Giebink N C, Kanno H, Ma B W, Thompson M E, Forrest S R 2006 Nature 440 908
Google Scholar
[3] Uoyama H, Goushi K, Shizu K, Nomura H, Adachi C 2012 Nature 492 234
Google Scholar
[4] Lin T A, Chatterjee T, Tsai W L, Lee W K, Wu M J, Jiao M, Pan K C, Yi C L, Chung C L, Wong K T, Wu C C 2016 Adv. Mater. 28 6976
Google Scholar
[5] Fung M K, Li Y Q, Liao L S 2016 Adv. Mater. 28 10381
Google Scholar
[6] Ai X, Evans E W, Dong S Z, Gillett A J, Guo H Q, Chen Y X, Hele T J H, Friend R H, Li F 2018 Nature 563 536
Google Scholar
[7] Wu T L, Huang M J, Lin C C, Huang P Y, Chou T Y, Chen-Cheng R W, Lin H W, Liu R S, Cheng C H 2018 Nat. Photonics 12 235
Google Scholar
[8] Liu Y C, Li C S, Ren Z J, Yan S K, Bryce M R 2018 Nat. Rev. Mater. 3 18020
Google Scholar
[9] Kondo Y, Yoshiura K, Kitera S, Nishi H, Oda S, Gotoh H, Sasada Y, Yanai M, Hatakeyama T 2019 Nat. Photonics 13 678
Google Scholar
[10] Chan C Y, Tanaka M, Lee Y T, Wong Y W, Nakanotani H, Hatakeyama T, Adachi C 2021 Nat. Photonics 15 6
Google Scholar
[11] Jeon S O, Lee K H, Kim J S, Ihn S G, Chung Y S, Kim J W, Lee H, Kim S, Choi H, Lee J Y 2021 Nat. Photonics 15 9
[12] Xu Y W, Xu P, Hu D H, Ma Y G 2021 Chem. Soc. Rev. 50 1030
Google Scholar
[13] Ma D, Hummelgen I A, Jing X B, Hong Z Y, Wang L X, Zhao X J, Wang F S, Karasz F E 2000 J. Appl. Phys. 87 312
Google Scholar
[14] Baldo M A, Forrest S R 2001 Phys. Rev. B 64 085201
Google Scholar
[15] Goushi K, Yoshida K, Sato K, Adachi C 2012 Nat. Photonics 6 253
Google Scholar
[16] Tang X, Cui L S, Li H C, Gillett A J, Auras F, Qu Y K, Zhong C, Jones S T E, Jiang Z Q, Friend R H, Liao L S 2020 Nat. Mater. 19 1332
Google Scholar
[17] Lee J, Jeong C, Batagoda T, Coburn C, Thompson M E, Forrest S R 2017 Nat. Commun. 8 9
Google Scholar
[18] Ostroverkhova O 2016 Chem. Rev. 116 13279
Google Scholar
[19] Matsumura M, Jinde Y, Akai T, Kimura T 1996 Jpn. J. Appl. Phys., Part 1 35 5735
Google Scholar
[20] Matsumura M, Akai T, Saito M, Kimura T 1996 J. Appl. Phys. 79 264
Google Scholar
[21] Matsumura M, Jinde Y 1998 Appl. Phys. Lett. 73 2872
Google Scholar
[22] Koehler M, Roman L S, Inganas O, da Luz M G E 2002 J. Appl. Phys. 92 5575
Google Scholar
[23] Crone B K, Davids P S, Campbell I H, Smith D L 2000 J. Appl. Phys. 87 1974
Google Scholar
[24] Yang J, Shen J 2000 J. Phys. D:Appl. Phys. 33 1768
Google Scholar
[25] Tutis E, Bussac M N, Masenelli B, Carrard M, Zuppiroli L 2001 J. Appl. Phys. 89 430
Google Scholar
[26] Cai M, Zhang D, Xu J, Hong X, Zhao C, Song X, Qiu Y, Kaji H, Duan L 2019 ACS Appl. Mater. Interfaces 11 1096
Google Scholar
[27] Nabha-Barnea S, Gotleyb D, Yonish A, Shikler R 2021 J. Mater. Chem. C 9 719
Google Scholar
[28] Lee J H, Lee S, Yoo S J, Kim K H, Kim J J 2014 Adv. Funct. Mater. 24 4681
Google Scholar
[29] Tak Y H, Pommerehne J, Vestweber H, Sander R, Bässler H, Hörhold H H 1996 Appl. Phys. Lett. 69 1291
Google Scholar
[30] Weichsel C, Burtone L, Reineke S, Hintschich S I, Gather M C, Leo K, Lüssem B 2012 Phys. Rev. B 86 075204
Google Scholar
[31] Liu R, Gan Z, Shinar R, Shinar J 2011 Phys. Rev. B 83 245302
Google Scholar
[32] Barth S, Muller P, Riel H, Seidler P F, Riess W, Vestweber H, Bassler H 2001 J. Appl. Phys. 89 3711
Google Scholar
[33] Kalinowski J, Camaioni N, Di Marco P, Fattori V, Martelli A 1998 Appl. Phys. Lett. 72 513
Google Scholar
[34] Grüne J, Bunzmann N, Meinecke M, Dyakonov V, Sperlich A 2020 J. Phys. Chem. C 124 25667
Google Scholar
[35] Dong Q, Mendes J, Lei L, Seyitliyev D, Zhu L, He S, Gundogdu K, So F 2020 ACS Appl. Mater. Interfaces 12 48845
Google Scholar
[36] Yuan Q, Wang T, Wang R, Zhao J, Zhang H, Ji W 2020 Opt. Lett. 45 6370
Google Scholar
[37] Elkhouly K, Gehlhaar R, Genoe J, Heremans P, Qiu W 2020 Adv. Opt. Mater. 8 2000941
Google Scholar
[38] Xu M, Peng Q, Zou W, Gu L, Xu L, Cheng L, He Y, Yang M, Wang N, Huang W, Wang J 2019 Appl. Phys. Lett. 115 041102
Google Scholar
[39] Liang F, Chen J, Cheng Y, Wang L, Ma D, Jing X, Wang F 2003 J. Mater. Chem. 13 1392
Google Scholar
[40] Sze S M 1981 Physics of Semiconductor Devices (2nd Ed.) (New York: Wiley)
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