能量传输对基于4CzTPN-Ph发光器件磁效应的调控
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摘要
把三重态激子(T_1)与单重态激子(S_1)能量接近的典型热辅助延迟荧光材料4CzTPN-Ph作为掺杂客体,以具有不同T_1能量的材料分别作为掺杂主体、空穴传输层和电子传输层,制备了一系列基于4CzTPN-Ph掺杂的有机发光二极管,并测量了这些器件在室温下的磁电致发光效应(Magneto-Electroluminescence, MEL)和磁电导效应(Magneto-Conductance, MC),以及器件随温度变化的MEL和MC.实验发现:室温下,当空穴传输层、电子传输层和掺杂主体分别选用T_1能量高低不同的材料时,各器件的MEL和MC在低磁场范围(|B|<20 mT)分别呈现出不同变化规律的线型,具体表现为当器件各功能层同时都选用较高T_1能量的材料时,器件MEL的幅度在低磁场范围内表现出随注入电流的减小而变小的反常行为,并出现了由正到负的转变, MC曲线则表现出符号为负且其幅度随磁场的增加而变大的RISC属性;而当器件的空穴传输层、电子传输层或掺杂主体材料的三重态能量较低时, MEL和MC表现出减弱的RISC过程;并且,当电子传输层或掺杂主体选用三重态能量与4CzTPN-Ph接近的Alq_3时, MEL和MC直接表现出类似未掺杂的Alq_3荧光器件的线型.分析器件的能量传输过程可知, T_1能量高低不同的空穴传输层、电子传输层或掺杂主体材料对4CzTPN-Ph三重态能量的束缚能力不同,造成各器件中T_1激子不同的传输通道和能量损失,从而使各器件在低磁场范围出现了不同的MEL和MC线型.本研究不仅丰富了能量传输对4CzTPN-Ph发光器件内部机制的认识,同时也对TADF器件中三重态激子的可控应用提供了一定的理论参考.
4CzTPN-Ph is a kind of typical thermally activated delayed fluorescence(TADF) material with a small energy gap between its singlet and triplet levels of the charge-transfer states(CTs). In this paper, we used 4CzTPN-Ph as dopant and materials with different triplet exciton(T_1) energy as host, hole-transporting layer(HTL) or electron-transporting layer(ETL) to fabricate a series of 4CzTPN-Ph-doped organic light-emitting diodes(OLEDs). The magneto-electroluminescence(MEL) and magneto-conductance(MC) from these devices were measured with different bias currents at room temperature, and with various temperatures at a fixed injection current. The experimental results demonstrated that when the materials with different T_1 energy are selected as the HTL, ETL and doping host, the MEL and MC of these devices exhibited various line-shapes with different variation tendency within low magnetic fields(|B|<20 mT), respectively.When each functional layer of the OLEDs with high T_1 energy, the amplitude of MEL curves showed abnormal behavior that reduced with decreasing injection currents at low magnetic field, and could convert from positive to negative. At the same time, the MC traces showed typical RISC process where the MC value was negative and rapidly enhanced with increasing magnetic field. However, when the HTL, ETL or host were selected with low T_1 energy materials, the MEL and MC of devices showed weakened RISC process. Especially, when Alq_3, whose T_1 energy is close to that of 4 CzTPNPh, was used as the ETL or host, the MEL and MC traces of devices directly presented the line-shape that was similar to that of pure Alq_3-based fluorescent devices. By analyzing the energy transfer process of spin-pair states in these studied OLEDs, we found that the HTL, ETL and host materials with different T_1 energy could limit the T_1 energy of 4CzTPNPh with different intensity, causing different energy transfer channel and energy loss of T_1 in devices. Consequently,MEL and MC of these devices were diverse from each other in low magnetic fields. Not only could this work enrich the understanding of the intrinsic physical mechanisms of 4CzTPN-Ph-based OLEDs, but also provide a theoretical reference for the controllable applications of T_1 in TADF devices.
4CzTPN-Ph is a kind of typical thermally activated delayed fluorescence(TADF) material with a small energy gap between its singlet and triplet levels of the charge-transfer states(CTs). In this paper, we used 4CzTPN-Ph as dopant and materials with different triplet exciton(T_1) energy as host, hole-transporting layer(HTL) or electron-transporting layer(ETL) to fabricate a series of 4CzTPN-Ph-doped organic light-emitting diodes(OLEDs). The magneto-electroluminescence(MEL) and magneto-conductance(MC) from these devices were measured with different bias currents at room temperature, and with various temperatures at a fixed injection current. The experimental results demonstrated that when the materials with different T_1 energy are selected as the HTL, ETL and doping host, the MEL and MC of these devices exhibited various line-shapes with different variation tendency within low magnetic fields(|B|<20 mT), respectively.When each functional layer of the OLEDs with high T_1 energy, the amplitude of MEL curves showed abnormal behavior that reduced with decreasing injection currents at low magnetic field, and could convert from positive to negative. At the same time, the MC traces showed typical RISC process where the MC value was negative and rapidly enhanced with increasing magnetic field. However, when the HTL, ETL or host were selected with low T_1 energy materials, the MEL and MC of devices showed weakened RISC process. Especially, when Alq_3, whose T_1 energy is close to that of 4 CzTPNPh, was used as the ETL or host, the MEL and MC traces of devices directly presented the line-shape that was similar to that of pure Alq_3-based fluorescent devices. By analyzing the energy transfer process of spin-pair states in these studied OLEDs, we found that the HTL, ETL and host materials with different T_1 energy could limit the T_1 energy of 4CzTPNPh with different intensity, causing different energy transfer channel and energy loss of T_1 in devices. Consequently,MEL and MC of these devices were diverse from each other in low magnetic fields. Not only could this work enrich the understanding of the intrinsic physical mechanisms of 4CzTPN-Ph-based OLEDs, but also provide a theoretical reference for the controllable applications of T_1 in TADF devices.
引文
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29 Hu B,Wu Y.Tuning magnetoresistance between positive and negative values in organic semiconductors.Nat Mater,2007,6:985-991
30 Luo Y,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
31 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]
2 Reineke S,Thomschke M,Lüssem B,et al.White organic light-emitting diodes:Status and perspective.Rev Mod Phys,2013,85:1245-1293,arXiv:1302.3435
3 Brocklehurst B.Formation of excited states by recombining organic ions.Nature,1969,221:921-923
4 Schulten Z,Schulten K.The generation,diffusion,spin motion,and recombination of radical pairs in solution in the nanosecond time domain.JChem Phys,1977,66:4616-4634
5 Karabunarliev S,Bittner E R.Spin-dependent electron-hole capture kinetics in luminescent conjugated polymers.Phys Rev Lett,2003,90:057402
6 Rothberg L J,Lovinger A J.Status of and prospects for organic electroluminescence.J Mater Res,1996,11:3174-3187
7 Baldo M A,O’Brien D F,You Y,et al.Highly efficient phosphorescent emission from organic electroluminescent devices.Nature,1998,395:151-154
8 Baldo M A,Thompson M E,Forrest S R.High-efficiency fluorescent organic light-emitting devices using a phosphorescent sensitizer.Nature,2000,403:750-753
9 Endo A,Ogasawara M,Takahashi A,et al.Thermally activated delayed fluorescence from Sn4+-porphyrin complexes and their application to organic light emitting diodes-A novel mechanism for electroluminescence.Adv Mater,2010,21:4802-4806
10 Uoyama H,Goushi K,Shizu K,et al.Highly efficient organic light-emitting diodes from delayed fluorescence.Nature,2012,492:234-238
11 Li J,Nakagawa T,MacDonald J,et al.Highly efficient organic light-emitting diode based on a hidden thermally activated delayed fluorescence channel in a heptazine derivative.Adv Mater,2013,25:3319-3323
12 Wu S,Aonuma M,Zhang Q,et al.High-efficiency deep-blue organic light-emitting diodes based on a thermally activated delayed fluorescence emitter.J Mater Chem C,2013,2:421-424
13 Lee S Y,Yasuda T,Yang Y S,et al.Luminous butterflies:Efficient exciton harvesting by benzophenone derivatives for full-color delayed fluorescence oleds.Angew Chem Int Ed,2014,53:6402-6406
14 Im Y,Lee J Y.Above 20%external quantum efficiency in thermally activated delayed fluorescence device using furodipyridine-type host materials.Chem Mater,2014,26:1413-1419
15 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
16 Li C S,Ren Z J,Yan S K.Thermally activated delayed fluorescence materials in OLEDs devices:Design,synthesis and applications(in Chinese).Chin Sci Bull,2015,60:2989-3004[李晨森,任忠杰,闫寿科.热活性延迟荧光材料的设计合成及其在有机发光二极管上的应用.科学通报,2015,60:2989-3004]
17 Sato K,Shizu K,Yoshimura K,et al.Organic luminescent molecule with energetically equivalent singlet and triplet excited states for organic light-emitting diodes.Phys Rev Lett,2013,110:247401
18 Hu B,Yan L,Shao M.Magnetic-field effects in organic semiconducting materials and devices.Adv Mater,2009,21:1500-1516
19 Desai P,Shakya P,Kreouzis T,et al.The role of magnetic fields on the transport and efficiency of aluminum tris(8-hydroxyquinoline)based organic light emitting diodes.J Appl Phys,2007,102:073710
20 Bloom F L,Wagemans W,Kemerink M,et al.Correspondence of the sign change in organic magnetoresistance with the onset of bipolar charge transport.Appl Phys Lett,2008,93:263302
21 Lee S Y,Yasuda T,Nomura H,et al.High-efficiency organic light-emitting diodes utilizing thermally activated delayed fluorescence from triazine-based donor-acceptor hybrid molecules.Appl Phys Lett,2012,101:093306
22 Wang H,Xie L,Peng Q,et al.Novel thermally activated delayed fluorescence materials-thioxanthone derivatives and their applications for highly efficient oleds.Adv Mater,2014,26:5198-5204
23 Deng J,Jia W,Chen Y,et al.Guest concentration,bias current and temperature-dependent sign inversion of magneto-electroluminescence in thermally activated delayed fluorescence devices.Sci Rep,2017,7:44396
24 Desai P,Shakya P,Kreouzis T,et al.Magnetoresistance and efficiency measurements of Alq3-based OLEDs.Phys Rev B,2007,75:094423
25 Deng J Q,Jia W Y,Chen Y B,et al.The magnetic field effects of electroluminescence in thermally activated delayed fluorescence devices(in Chinese).Sci Sin-Phys Mech Astron,2016,46:087011[邓军权,贾伟尧,陈颖冰,等.热活化延迟荧光器件中的发光磁效应.中国科学:物理学力学天文学,2016,46:087011]
26 Baldo M A,Forrest S R.Transient analysis of organic electrophosphorescence:I.Transient analysis of triplet energy transfer.Phys Rev B,2000,62:10958-10966
27 Chen Y B,Yuan D,Xiang J,et al.Analysis of triplet dissociation and electron scattering in the rubrene-based devices by utilizing magnetoconductance(in Chinese).Sci Sin Tech,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]
28 Doubleday Jr.C,Turro N J,Wang J F.Dynamics of flexible triplet biradicals.ACC Chem Res,1989,22:199-205
29 Hu B,Wu Y.Tuning magnetoresistance between positive and negative values in organic semiconductors.Nat Mater,2007,6:985-991
30 Luo Y,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
31 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]