利用有机磁效应探究基于DCJTB电荷转移态发光器件的微观机制
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摘要
能量转移(energy transfer,ET)和直接电荷捕获(direct charge trapping,DCT)是掺杂型有机发光二极管(organic light-emitting diode,OLED)中两种相互竞争的微观过程.最近研究表明,磁电致发光(magnetoelectroluminescence,MEL)可作为探测OLED中微观机制的有效手段.本文以电荷转移(charge transfer,CT)态材料4-(二氰基亚甲基)-2-叔丁基-6-(1,1,7,7-四甲基久罗尼定基-4-乙烯基)-4H-吡喃(4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran,DCJTB)为客体,以具有不同三重态激子能量的三(8-羟基喹啉)铝、4,4-二(9-咔唑)联苯(4,4′-bis(N-carbazolyl)-1,1′-biphenyl,CBP)和2,4,6-三[3-(二苯基膦氧基)苯基]-1,3,5-三唑(2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine,PO-T2T)为主体制备了3种OLED,并测量了它们的MEL.实验发现器件的MEL都由低场(|B|<5 mT)和高场(5 mT≤|B|<300 mT)两部分组成,其高场部分均表现为由三重态激子-电荷反应和三重态激子对湮灭导致的随磁场正相关或负相关的MEL.但它们的低场变化则完全不同:Alq3:DCJTB器件表现为主体极化子对(polaron-pair,PP)的系间窜越过程(PPT)导致的正MEL,而CBP:DCJTB器件和PO-T2T:DCJTB器件则表现为客体电荷转移态的反向系间窜越过程导致的负MEL.这两类器件分别对应ET和DCT过程,再加之主体三重态能级的高低对反向系间窜越过程的影响,从而导致不同器件中多变的MEL线型;另外,低温环境更有利于DCJTB中三重电荷转移态参与的反向系间窜越和三重态激子对湮灭过程的发生.本研究工作对基于DCJTB发光器件的内部微观机制有了更深入的理解.
Although energy transfer(ET) and direct charge trapping(DCT) are two competing mechanisms that are known to occur in dye-doped organic light-emitting diodes(OLEDs), which of these processes governs the electroluminescence is not clear. Additionally, each process influences the formation of excitons, and the interactions between excitons, differently. If ET is the dominate process, the mechanism that governs an OLED involves both the host and guest materials. In contrast, if DCT is the dominant process, the mechanism that governs an OLED will only involve the guest material. Magneto-electroluminescence(MEL) is an effective technique for exploring and understanding the mechanisms that occur within OLEDs. It exhibits sensitive fingerprint responses to intersystem crossing(ISC), reverse intersystem crossing(RISC), triplet-triplet annihilation(TTA) and triplet-charge annihilation(TQA). In this work, we fabricated three different OLEDs and measured their MEL curves. Tris-8-hydroxyquinoline aluminum(Alq3), 4,4′-N,N′-dicarbazolebiphenyl(CBP) and 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine(PO-T2T) were used as the host materials, each of which had a different triplet exciton energy. 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4 H-pyran(DCJTB) was used as a charge-transfer(CT) dopant that had a small energy difference between the singlet and triplet excited states. By analyzing the energy level structure of the different devices, the absorption spectrum of the dopant, and the emission spectra of the host materials, we determined that ET was the dominant process when DCJTB was used as a dopant with Alq3. However, DCT dominated when DCJTB was used as a dopant with both CBP and PO-T2T. The current-and temperature-dependent MEL curves from all devices were composed of low field components(|B|<5 m T) and high field components(5 mT≤|B|<300 mT). The high field components of the MEL curves exhibited a positive or negative correlation with B because both TTA and TQA were competing processes, but their low-field changes were completely different. The MEL curves from the DCJTB/Alq3 device exhibited a positive MEL because polaron-pair(PP) states can occur on the host material via ISC(PPS→PPT). Conversely, the DCJTB/CBP and DCJTB/PO-T2T devices exhibited negative MEL responses because RISC(1 CT←3 CT) occurred in the CT states of the dopant. However, the triplet energy levels of the host material simultaneously impacted RISC. Hence, these devices exhibited a diverse range of MEL curves. Low temperatures enhanced RISC and TTA of the triple CT state within DCJTB, which was attributed to the increased concentration and lifetime of triplet excitons at low temperature. This work serves as a reference for improving the internal quantum efficiency of fluorescent OLEDs by enhancing the use of triplet states, and importantly gives a deeper understanding of the mechanisms that govern light-emission from devices that incorporate DCJTB CT materials.
Although energy transfer(ET) and direct charge trapping(DCT) are two competing mechanisms that are known to occur in dye-doped organic light-emitting diodes(OLEDs), which of these processes governs the electroluminescence is not clear. Additionally, each process influences the formation of excitons, and the interactions between excitons, differently. If ET is the dominate process, the mechanism that governs an OLED involves both the host and guest materials. In contrast, if DCT is the dominant process, the mechanism that governs an OLED will only involve the guest material. Magneto-electroluminescence(MEL) is an effective technique for exploring and understanding the mechanisms that occur within OLEDs. It exhibits sensitive fingerprint responses to intersystem crossing(ISC), reverse intersystem crossing(RISC), triplet-triplet annihilation(TTA) and triplet-charge annihilation(TQA). In this work, we fabricated three different OLEDs and measured their MEL curves. Tris-8-hydroxyquinoline aluminum(Alq3), 4,4′-N,N′-dicarbazolebiphenyl(CBP) and 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine(PO-T2T) were used as the host materials, each of which had a different triplet exciton energy. 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4 H-pyran(DCJTB) was used as a charge-transfer(CT) dopant that had a small energy difference between the singlet and triplet excited states. By analyzing the energy level structure of the different devices, the absorption spectrum of the dopant, and the emission spectra of the host materials, we determined that ET was the dominant process when DCJTB was used as a dopant with Alq3. However, DCT dominated when DCJTB was used as a dopant with both CBP and PO-T2T. The current-and temperature-dependent MEL curves from all devices were composed of low field components(|B|<5 m T) and high field components(5 mT≤|B|<300 mT). The high field components of the MEL curves exhibited a positive or negative correlation with B because both TTA and TQA were competing processes, but their low-field changes were completely different. The MEL curves from the DCJTB/Alq3 device exhibited a positive MEL because polaron-pair(PP) states can occur on the host material via ISC(PPS→PPT). Conversely, the DCJTB/CBP and DCJTB/PO-T2T devices exhibited negative MEL responses because RISC(1 CT←3 CT) occurred in the CT states of the dopant. However, the triplet energy levels of the host material simultaneously impacted RISC. Hence, these devices exhibited a diverse range of MEL curves. Low temperatures enhanced RISC and TTA of the triple CT state within DCJTB, which was attributed to the increased concentration and lifetime of triplet excitons at low temperature. This work serves as a reference for improving the internal quantum efficiency of fluorescent OLEDs by enhancing the use of triplet states, and importantly gives a deeper understanding of the mechanisms that govern light-emission from devices that incorporate DCJTB CT materials.
引文
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2 Luo Y,Aziz H.Correlation between triplet-triplet annihilation and electroluminescence efficiency in doped fluorescent organic lightemitting devices.Adv Funct Mater,2010,20:1285
3 Chiang C J,Kimyonok A,Etheringyon M K,et al.Ultrahigh efficiency fluorescent single and bi-layer organic light emitting diodes:The key role of triplet fusion.Adv Funct Mater,2013,23:739
4 Uoyama H,Goushi K,Shizu K,et al.Highly efficient organic light-emitting diodes from delayed fluorescence.Nature,2012,492:234-240
5 Goushi K,Adachi C.Efficient organic light-emitting diodes through up-conversion from triplet to singlet excited states of exciplexes.Appl Phys Lett,2012,101:023306
6 Endo A,Ogasawara M,Takahashi A,et al.Thermally activated delayed fluorescence from Sn~(4+)-porphyrin complexes and their application to organic light emitting diode―A novel mechanism for electroluminescence.Adv Mater,2009,21:4802
7 Zhang Q,Li J,Shizu K,et al.Design of efficient thermally activated delayed fluorescence materials for pure blue organic light emitting diodes.J Am Chem Soc,2012,134:14706-14709
8 Goushi K,Yoshida K,Sato K,et al.Organic light-emitting diodes employing efficient reverse intersystem crossing for triplet-to-singlet state conversion.Nat Photon,2012,6:253
9 Chen Y B,Yuan D,Xiang J,et al.Analysis of triplet dissociation and electron scattering in the rubrene-based devices by utilizing magneto-conductance(in Chinese).Sci Sin Technol,2016,46:61-67[陈颖冰,袁德,向杰,等.利用有机磁电导分析rubrene发光器件中三重态激子解离和电子散射过程.中国科学:技术科学,2016,46:61-67]
10 Thompson N J,Hontz E,Congreve D N,et al.Nanostructured singlet fission photovoltaics subject to triplet-charge annihilation.Adv Mater,2014,26:1366-1371
11 Hu B,Wu Y.Tuning magnetoresistance between positive and negative values in organic semiconductors.Nat Mater,2007,6:985-991
12 Goushi K,Yoshida K,Sato K,et al.Organic light-emitting diodes employing efficient reverse intersystem crossing for triplet-to-singlet state conversion.Nat Photon,2012,6:253-258
13 Chen P.Study on magneto-electroluminescence of charge transfer states in organic light emitting diodes(in Chinese).Doctor Dissertation.Changchun:Jilin University,2014[陈平.基于电荷转移态的有机磁电亮度变化研究.博士学位论文.长春:吉林大学,2014]
14 Peng Q M,Gao N,Li W J,et al.Investigation of energy transfer and charge trapping in dye-doped organic light-emitting diodes by magneto-electroluminescence measurement.Appl Phys Lett,2013,102:193304
15 Ding B F,Yao Y,Wu C Q,et al.Using magneto-electroluminescence as a fingerprint to identify the carrier-to-photon conversion process in dye-doped OLEDs.J Phys Chem C,2011,115:20295-20300
16 Lee J H,Lee S,Yoo S J,et al.Langevin and trap-assisted recombination in phosphorescent organic light emitting diodes.Adv Funct Mater,2014,24:4681-4688
17 Luo Y C,Aziz H,Klenkler R,et al.Temperature dependence of photoluminescence efficiency in doped and blended organic thin films.Chem Phys Lett,2008,458:319-322
18 Huang W,Mi B X,Gao Z Q.Organic Electronics(in Chinese).Beijing:Science Press,2011.94-98[黄维,密保秀,高志强.有机磁电子学.北京:科学出版社,2011.94-98]
19 Liu R,Zhang Y,Lei Y L,et al.Magnetic field dependent triplet-triplet annihilation in Alq3-based organic light emitting diodes at different temperatures.J Appl Phys,2009,105:093719
20 Lei Y L,Zhang Y,Liu R,et al.Driving current and temperature dependent magnetic-field modulated electroluminescence in Alq3-based organic light emitting diode.Org Electron,2009,10:889-894
21 Chen P,Xiong Z H,Peng Q M,et al.Magneto-electroluminescence as a tool to discern the origin of delayed fluorescence:Reverse intersystem crossing or triplet-triplet annihilation?Adv Opt Mater,2014,2:142-148
22 Lei Y L,Song Q L,Xiong Z H.Progress in the magnetic field effects in organic semiconductor devices(in Chinese).Chin Sci Bull(Chin Ver),2010,55:2361-2370[雷衍连,宋群梁,熊祖洪.有机半导体器件中的磁场效应研究进展.科学通报,2010,55:2361-2370]
23 Deng J Q,Tang X T,Pan R H,et al.Abnormal magnetic field effects in doped organic light-emitting devices(in Chinese).Chin Sci Bull,2018,63:2974-2984[邓金秋,汤仙童,潘睿亨,等.有机发光掺杂器件中的反常磁效应.科学通报,2018,63:2974-2984]
24 Doubleday C,Turro N J,Wang J F.Dynamics of flexible triplet biradicals.ACC Chem Res,1989,22:199-205
25 Desai P,Shakya P,Kreouzis T,et al.Magnetoresistance and efficiency measurements of Alq3-based OLEDs.Phys Rev B,2007,75:094423
26 Jia W,Chen Q,Chen L,et al.Molecular spacing modulated conversion of singlet fission to triplet fusion in rubrene-based organic light-emitting diodes at ambient temperature.J Phys Chem C,2016,120:8380-8386
27 Liu S,Li B,Zhang L,et al.Enhanced efficiency and reduced roll-off in nondoped phosphorescent organic light-emitting devices with triplet multiple quantum well structures.Appl Phys Lett,2010,97:083304
28 Sheng Y,Nguyen T D,Veeraraghavan G,et al.Hyperfine interaction and magnetoresistance in organic semiconductors.Phys Rev B,2006,74:045213
29 Liu W,Chen J X,Zheng C J,et al.Novel strategy to develop exciplex emitters for high-performance OLEDs by employing thermally activated delayed fluorescence materials,Adv Funct Mater,2016,26:2002-2008
30 Peng Q M,Li A W,Fan Y X,et al.Studying the influence of triplet deactivation on the singlet-triplet inter-conversion in intra-molecular charge-transfer fluorescence-based OLEDs by magneto-electroluminescence.J Mater Chem C,2014,2:6264-6268
2 Luo Y,Aziz H.Correlation between triplet-triplet annihilation and electroluminescence efficiency in doped fluorescent organic lightemitting devices.Adv Funct Mater,2010,20:1285
3 Chiang C J,Kimyonok A,Etheringyon M K,et al.Ultrahigh efficiency fluorescent single and bi-layer organic light emitting diodes:The key role of triplet fusion.Adv Funct Mater,2013,23:739
4 Uoyama H,Goushi K,Shizu K,et al.Highly efficient organic light-emitting diodes from delayed fluorescence.Nature,2012,492:234-240
5 Goushi K,Adachi C.Efficient organic light-emitting diodes through up-conversion from triplet to singlet excited states of exciplexes.Appl Phys Lett,2012,101:023306
6 Endo A,Ogasawara M,Takahashi A,et al.Thermally activated delayed fluorescence from Sn~(4+)-porphyrin complexes and their application to organic light emitting diode―A novel mechanism for electroluminescence.Adv Mater,2009,21:4802
7 Zhang Q,Li J,Shizu K,et al.Design of efficient thermally activated delayed fluorescence materials for pure blue organic light emitting diodes.J Am Chem Soc,2012,134:14706-14709
8 Goushi K,Yoshida K,Sato K,et al.Organic light-emitting diodes employing efficient reverse intersystem crossing for triplet-to-singlet state conversion.Nat Photon,2012,6:253
9 Chen Y B,Yuan D,Xiang J,et al.Analysis of triplet dissociation and electron scattering in the rubrene-based devices by utilizing magneto-conductance(in Chinese).Sci Sin Technol,2016,46:61-67[陈颖冰,袁德,向杰,等.利用有机磁电导分析rubrene发光器件中三重态激子解离和电子散射过程.中国科学:技术科学,2016,46:61-67]
10 Thompson N J,Hontz E,Congreve D N,et al.Nanostructured singlet fission photovoltaics subject to triplet-charge annihilation.Adv Mater,2014,26:1366-1371
11 Hu B,Wu Y.Tuning magnetoresistance between positive and negative values in organic semiconductors.Nat Mater,2007,6:985-991
12 Goushi K,Yoshida K,Sato K,et al.Organic light-emitting diodes employing efficient reverse intersystem crossing for triplet-to-singlet state conversion.Nat Photon,2012,6:253-258
13 Chen P.Study on magneto-electroluminescence of charge transfer states in organic light emitting diodes(in Chinese).Doctor Dissertation.Changchun:Jilin University,2014[陈平.基于电荷转移态的有机磁电亮度变化研究.博士学位论文.长春:吉林大学,2014]
14 Peng Q M,Gao N,Li W J,et al.Investigation of energy transfer and charge trapping in dye-doped organic light-emitting diodes by magneto-electroluminescence measurement.Appl Phys Lett,2013,102:193304
15 Ding B F,Yao Y,Wu C Q,et al.Using magneto-electroluminescence as a fingerprint to identify the carrier-to-photon conversion process in dye-doped OLEDs.J Phys Chem C,2011,115:20295-20300
16 Lee J H,Lee S,Yoo S J,et al.Langevin and trap-assisted recombination in phosphorescent organic light emitting diodes.Adv Funct Mater,2014,24:4681-4688
17 Luo Y C,Aziz H,Klenkler R,et al.Temperature dependence of photoluminescence efficiency in doped and blended organic thin films.Chem Phys Lett,2008,458:319-322
18 Huang W,Mi B X,Gao Z Q.Organic Electronics(in Chinese).Beijing:Science Press,2011.94-98[黄维,密保秀,高志强.有机磁电子学.北京:科学出版社,2011.94-98]
19 Liu R,Zhang Y,Lei Y L,et al.Magnetic field dependent triplet-triplet annihilation in Alq3-based organic light emitting diodes at different temperatures.J Appl Phys,2009,105:093719
20 Lei Y L,Zhang Y,Liu R,et al.Driving current and temperature dependent magnetic-field modulated electroluminescence in Alq3-based organic light emitting diode.Org Electron,2009,10:889-894
21 Chen P,Xiong Z H,Peng Q M,et al.Magneto-electroluminescence as a tool to discern the origin of delayed fluorescence:Reverse intersystem crossing or triplet-triplet annihilation?Adv Opt Mater,2014,2:142-148
22 Lei Y L,Song Q L,Xiong Z H.Progress in the magnetic field effects in organic semiconductor devices(in Chinese).Chin Sci Bull(Chin Ver),2010,55:2361-2370[雷衍连,宋群梁,熊祖洪.有机半导体器件中的磁场效应研究进展.科学通报,2010,55:2361-2370]
23 Deng J Q,Tang X T,Pan R H,et al.Abnormal magnetic field effects in doped organic light-emitting devices(in Chinese).Chin Sci Bull,2018,63:2974-2984[邓金秋,汤仙童,潘睿亨,等.有机发光掺杂器件中的反常磁效应.科学通报,2018,63:2974-2984]
24 Doubleday C,Turro N J,Wang J F.Dynamics of flexible triplet biradicals.ACC Chem Res,1989,22:199-205
25 Desai P,Shakya P,Kreouzis T,et al.Magnetoresistance and efficiency measurements of Alq3-based OLEDs.Phys Rev B,2007,75:094423
26 Jia W,Chen Q,Chen L,et al.Molecular spacing modulated conversion of singlet fission to triplet fusion in rubrene-based organic light-emitting diodes at ambient temperature.J Phys Chem C,2016,120:8380-8386
27 Liu S,Li B,Zhang L,et al.Enhanced efficiency and reduced roll-off in nondoped phosphorescent organic light-emitting devices with triplet multiple quantum well structures.Appl Phys Lett,2010,97:083304
28 Sheng Y,Nguyen T D,Veeraraghavan G,et al.Hyperfine interaction and magnetoresistance in organic semiconductors.Phys Rev B,2006,74:045213
29 Liu W,Chen J X,Zheng C J,et al.Novel strategy to develop exciplex emitters for high-performance OLEDs by employing thermally activated delayed fluorescence materials,Adv Funct Mater,2016,26:2002-2008
30 Peng Q M,Li A W,Fan Y X,et al.Studying the influence of triplet deactivation on the singlet-triplet inter-conversion in intra-molecular charge-transfer fluorescence-based OLEDs by magneto-electroluminescence.J Mater Chem C,2014,2:6264-6268