有机发光掺杂器件中的反常磁效应
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摘要
采用不同浓度的热活化延迟荧光(thermally activated delayed fluorescence,TADF)材料2,3,5,6-四(3,6-二苯基-9-咔唑基)-对苯二腈(2,3,5,6-tetrakis(3,6-diphenylcarbazol-9-yl)-1,4-dicyanobenzene,4CzTPN-Ph)为掺杂剂,三(8-羟基喹啉)铝(tris-8-hydroxyquinoline aluminum,Alq3)为主体材料制备了发光层为Alq3:x%4CzTPN-Ph的有机发光二极管器件,并测量了室温下不同注入电流和不同掺杂浓度,以及固定某一电流和掺杂浓度在不同温度下器件的磁电致发光(magneto-electroluminescence,MEL)效应和磁电导(magneto-conductance,MC)效应.实验发现,在主客体掺杂类型器件中,与普通掺杂器件减小的磁效应相比,这种器件具有明显的反常磁效应——即表现出增强的MEL和MC幅值.以室温下注入电流为150μA的实验为例,发光层为Alq3:5%4CzTPN-Ph器件的MEL幅值在磁场为300m T处达到了10%左右,大约是参考器件(发光层为CBP:5%4CzTPN-Ph)的MEL幅值(~0.75%)的13倍,且该器件对应的MC幅值在磁场为300 m T处达到了6%左右,大约是此参考器件MC幅值(~0.12%)的50倍.此外,这种掺杂器件的MEL和MC明显受到掺杂浓度的调控,当掺杂浓度达15%左右时,MEL和MC幅值可达到最大值.在不同温度下,这种掺杂器件的MEL和MC值均随着温度的降低而减小.通过分析器件的能级结构和光谱可知,Alq3:4CzTPN-Ph器件具有主客体分子间特殊的能级排布,造成客体分子的能级陷阱较弱,外加磁场抑制三重态激子对电荷的散射作用(TQA)就可产生显著的MEL和MC幅值,从而得到不同于普通掺杂器件的反常磁效应.此外,由于TQA过程受三重态激子浓度与载流子浓度的影响,掺杂浓度和实验温度也能通过影响三重态激子浓度和载流子浓度来对TQA反应强弱进行调控,从而有效地调控这种反常磁效应.本研究工作有助于深入理解基于4CzTPN-Ph发光器件微观机制的演化过程,并将促进有机发光二极管在磁学器件方面的应用.
Organic magnetic field effects(OMFEs), which primarily includes magneto-electroluminescence(MEL) and magneto-conductance(MC), is a non-contact, non-destructive and sensitive tool for probing the mechanisms that occur within in organic light-emitting diodes(OLEDs). Not only OMFEs can be used to detect charge transport, exciton evolution and luminescence mechanisms in OLEDs, and can also be used in magnetic devices, including magnetic switches, magnetic storage and magnetic sensors, etc. Stable magnetic devices are usually required for practical applications. There is a wealth of research that shows that doped devices exhibit improved stability when compared with non-doped devices, however, they always show weaker OMFEs responses. Therefore, it is important to achieve strong OMFEs responses from stable, doped devices. In this work, we have fabricated OLEDs using the thermally activated delayed fluorescence(TADF) material 2,3,5,6-tetrakis(3,6-diphenyl-carbazol-9-yl)-1,4-dicyanobenzene(4 CzTPN-Ph) of different doping concentration as the dopant, and either tris-8-hydroxyquinoline aluminum(Alq3) and 4,4′-N,N′-dicarbazole-biphenyl(CBP) as the host materials. The light-emitting layer in these OLEDs was Alq3:x% 4 CzTPN-Ph(studied devices) or CBP:5% 4 CzT PN-Ph(reference device). The MEL and MC curves from these devices were measured with different injection currents and doping concentrations at room temperature, and with varying temperatures at a constant injection current and doping concentration. Compared with the reduced OMFEs observed from the reference device, the studied devices exhibited increased amplitudes in their MEL and MC responses. For example, at an injection current of 150 μA at room temperature, the amplitude of the MEL from the studied device reached ~10% when exposed to a magnetic field of 300 mT, which was approximately 13 times larger than that observed from the reference device(~0.75%). The corresponding amplitude from the MC from the studied device was ~6% at 300 mT, which was approximately 50 times larger than that observed from the reference device(~0.12%). In addition, the MEL and MC responses from the studied devices were determined by the doping concentration. At a doping concentration of approximately 15%, the MEL and MC values from the studied devices at 300 mT reached maximum values of 13.4% and 9.3%, respectively. Both MEL and MC values from the studied device reduced with decreasing temperatures. Analysis of the energy levels and optical spectra from these studied devices indicated that the special energy level arrangement between the guest and host molecules resulted in a weak energy level trap. The external magnetic field suppressed the scattering of charges by triplet excitons, a channel for triplet-charge annihilation(TQA), which generated significant MEL and MC responses. Therefore, OMFEs that were stronger than those from the reference device were obtained. In addition, since TQA is affected by the concentration and mobility of triplet excitons and carriers, the doping concentration and working temperature can also mediate TQA by modifying the concentration and mobility of triplet excitons and carriers, which eventually affects the OMFEs. This work gives insight into the mechanisms that occur within 4 CzTPN-Ph-based light-emitting diodes, and also importantly promotes practical applications of magnetic organic light-emitting diodes.
Organic magnetic field effects(OMFEs), which primarily includes magneto-electroluminescence(MEL) and magneto-conductance(MC), is a non-contact, non-destructive and sensitive tool for probing the mechanisms that occur within in organic light-emitting diodes(OLEDs). Not only OMFEs can be used to detect charge transport, exciton evolution and luminescence mechanisms in OLEDs, and can also be used in magnetic devices, including magnetic switches, magnetic storage and magnetic sensors, etc. Stable magnetic devices are usually required for practical applications. There is a wealth of research that shows that doped devices exhibit improved stability when compared with non-doped devices, however, they always show weaker OMFEs responses. Therefore, it is important to achieve strong OMFEs responses from stable, doped devices. In this work, we have fabricated OLEDs using the thermally activated delayed fluorescence(TADF) material 2,3,5,6-tetrakis(3,6-diphenyl-carbazol-9-yl)-1,4-dicyanobenzene(4 CzTPN-Ph) of different doping concentration as the dopant, and either tris-8-hydroxyquinoline aluminum(Alq3) and 4,4′-N,N′-dicarbazole-biphenyl(CBP) as the host materials. The light-emitting layer in these OLEDs was Alq3:x% 4 CzTPN-Ph(studied devices) or CBP:5% 4 CzT PN-Ph(reference device). The MEL and MC curves from these devices were measured with different injection currents and doping concentrations at room temperature, and with varying temperatures at a constant injection current and doping concentration. Compared with the reduced OMFEs observed from the reference device, the studied devices exhibited increased amplitudes in their MEL and MC responses. For example, at an injection current of 150 μA at room temperature, the amplitude of the MEL from the studied device reached ~10% when exposed to a magnetic field of 300 mT, which was approximately 13 times larger than that observed from the reference device(~0.75%). The corresponding amplitude from the MC from the studied device was ~6% at 300 mT, which was approximately 50 times larger than that observed from the reference device(~0.12%). In addition, the MEL and MC responses from the studied devices were determined by the doping concentration. At a doping concentration of approximately 15%, the MEL and MC values from the studied devices at 300 mT reached maximum values of 13.4% and 9.3%, respectively. Both MEL and MC values from the studied device reduced with decreasing temperatures. Analysis of the energy levels and optical spectra from these studied devices indicated that the special energy level arrangement between the guest and host molecules resulted in a weak energy level trap. The external magnetic field suppressed the scattering of charges by triplet excitons, a channel for triplet-charge annihilation(TQA), which generated significant MEL and MC responses. Therefore, OMFEs that were stronger than those from the reference device were obtained. In addition, since TQA is affected by the concentration and mobility of triplet excitons and carriers, the doping concentration and working temperature can also mediate TQA by modifying the concentration and mobility of triplet excitons and carriers, which eventually affects the OMFEs. This work gives insight into the mechanisms that occur within 4 CzTPN-Ph-based light-emitting diodes, and also importantly promotes practical applications of magnetic organic light-emitting diodes.
引文
1 Kalinowski J,Cocchi M,Virgili D,et al.Magnetic field effects on emission and current in Alq3-based electroluminescent diodes.Chem Phys Lett,2003,380:710-715
2 Obolda A,Peng Q,He C,et al.Triplet-polaron-interaction-induced upconversion from triplet to singlet:A possible way to obtain highly efficient OLEDs.Adv Mater,2016,28:4740-4746
3 Gillin W P,Zhang S,Rolfe N J,et al.Determining the influence of excited states on current transport in organic light emitting diodes using magnetic field perturbation.Phys Rev B,2010,82:195208
4 Clarke R H,Connors R E,Keegan J.Magnetic field effect on the low temperature triplet state population of an organic molecule.J Chem Phys,1977,66:358-359
5 Davis A H,Bussmann K.Large magnetic field effects in organic light emitting diodes based on tris(8-hydroxyquinoline aluminum)(Alq3)/N,N′-Di(naphthalen-1-yl)-N,N′-diphenyl-benzidine(NPB)bilayers.J Vac Sci Tech A,2004,22:1885-1891
6 Kalinowski J,Cocchi M,Virgili D,et al.Magnetic field effects on organic electrophosphorescence.Phys Rev B,2004,70:205303
7 Hu B,Yan L,Shao M.Magnetic-field effects in organic semiconducting materials and devices.Adv Mater,2009,21:1500-1516
8 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
9 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
10 Tang C W,vanSlyke S A,Chen C H.Electroluminescence of doped organic thin films.J Appl Phys,1989,65:3610-3616
11 Wang C M,Lei Y L,Zhang Q M,et al.Influence of exction recombination zone movement with the changing temperature on the magnetic field effect in OLED(in Chinese).Sci Sin Phys Mech Astron,2013,43:732-738[王春梅,雷衍连,张巧明,等.激子复合区随温度移动对OLED磁效应的影响.中国科学:物理学力学天文学,2013,43:732-738]
12 Zhang Q M,Chen P,Lei Y L,et al.Influence of BCP hole blocking layer on magnetoconductance effect in dye doped organic light emitting diodes(in Chinese).Sci Sin Phys Mech Astron,2010,40:1507-1513[张巧明,陈平,雷衍连,等.空穴阻挡层BCP对掺杂型有机发光二极管中磁电导效应的影响.中国科学:物理学力学天文学,2010,40:1507-1513]
13 Liu D Y,Chen L X,Xiang J,et al.Triplet induced competition between scattering and dissociation process in exciton-charge reaction(in Chinese).Chin Sci Bull,2017,62:3885-3893[刘冬玉,陈历相,向杰,等.三重态激子浓度对激子-电荷反应中散射和解离过程的调控.科学通报,2017,62:3885-3893]
14 Zou Y,Jia W Y,Chen Q S,et al.The influence of doping concentration and temperature on magnetic field effect of delayed fluorescence in organic light-emitting diode(in Chinese).Chin Sci Bull,2016,61:1679-1686[邹越,贾伟尧,陈秋松,等.掺杂浓度和温度对有机延迟荧光磁效应的影响.科学通报,2016,61:1679-1686]
15 Pan R H,Tang X T,Deng J Q,et al.Investigation of micro-mechanism in thermally activated delayed fluorescence quantum well devices by utilizing organic magnetic effects(in Chinese).Sci Sin Tech,2018,48,doi:10.1360/N092017-00289[潘睿亨,汤仙童,邓金秋,等.利用有机磁效应研究热活化延迟荧光量子阱器件中的微观机制.中国科学:技术科学,2018,48,doi:10.1360/N092017-00289]
16 Deng J Q,Jia W Y,Chen Y B,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
17 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]
18 Uoyama H,Goushi K,Shizu K,et al.Highly efficient organic light-emitting diodes from delayed fluorescence.Nature,2012,492:234-240
19 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]
20 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
21 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
22 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
23 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
24 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
25 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
26 Doubleday C,Turro N J,Wang J F.Dynamics of flexible triplet biradicals.Acc Chem Res,1989,22:199-205
27 Desai P,Shakya P,Kreouzis T,et al.Magnetoresistance and efficiency measurements of Alq3-based OLEDs.Phys Rev B,2007,75,094423
28 Hu B,Wu Y.Tuning magnetoresistance between positive and negative values in organic semiconductors.Nat Mater,2007,6:985-991
29 Wohlgenannt M,Vardeny Z V.Spin-dependent exciton formation rates inπ-conjugated materials.J Phys Condens Matter,2003,15:R83-R107
30 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 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]
31 Bruno A,Mauro A,Nenna G,et al.Insights on photophysical proprieties of DCM dye in PVK host matrix.Polym Compos,2013,34:1500-1505
32 Feng S W,Shih M C,Huang C J,et al.Impacts of dopant concentration on the carrier transport and recombination dynamics in organic light emitting diodes.Thin Solid Films,2009,517:2719-2723
2 Obolda A,Peng Q,He C,et al.Triplet-polaron-interaction-induced upconversion from triplet to singlet:A possible way to obtain highly efficient OLEDs.Adv Mater,2016,28:4740-4746
3 Gillin W P,Zhang S,Rolfe N J,et al.Determining the influence of excited states on current transport in organic light emitting diodes using magnetic field perturbation.Phys Rev B,2010,82:195208
4 Clarke R H,Connors R E,Keegan J.Magnetic field effect on the low temperature triplet state population of an organic molecule.J Chem Phys,1977,66:358-359
5 Davis A H,Bussmann K.Large magnetic field effects in organic light emitting diodes based on tris(8-hydroxyquinoline aluminum)(Alq3)/N,N′-Di(naphthalen-1-yl)-N,N′-diphenyl-benzidine(NPB)bilayers.J Vac Sci Tech A,2004,22:1885-1891
6 Kalinowski J,Cocchi M,Virgili D,et al.Magnetic field effects on organic electrophosphorescence.Phys Rev B,2004,70:205303
7 Hu B,Yan L,Shao M.Magnetic-field effects in organic semiconducting materials and devices.Adv Mater,2009,21:1500-1516
8 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
9 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
10 Tang C W,vanSlyke S A,Chen C H.Electroluminescence of doped organic thin films.J Appl Phys,1989,65:3610-3616
11 Wang C M,Lei Y L,Zhang Q M,et al.Influence of exction recombination zone movement with the changing temperature on the magnetic field effect in OLED(in Chinese).Sci Sin Phys Mech Astron,2013,43:732-738[王春梅,雷衍连,张巧明,等.激子复合区随温度移动对OLED磁效应的影响.中国科学:物理学力学天文学,2013,43:732-738]
12 Zhang Q M,Chen P,Lei Y L,et al.Influence of BCP hole blocking layer on magnetoconductance effect in dye doped organic light emitting diodes(in Chinese).Sci Sin Phys Mech Astron,2010,40:1507-1513[张巧明,陈平,雷衍连,等.空穴阻挡层BCP对掺杂型有机发光二极管中磁电导效应的影响.中国科学:物理学力学天文学,2010,40:1507-1513]
13 Liu D Y,Chen L X,Xiang J,et al.Triplet induced competition between scattering and dissociation process in exciton-charge reaction(in Chinese).Chin Sci Bull,2017,62:3885-3893[刘冬玉,陈历相,向杰,等.三重态激子浓度对激子-电荷反应中散射和解离过程的调控.科学通报,2017,62:3885-3893]
14 Zou Y,Jia W Y,Chen Q S,et al.The influence of doping concentration and temperature on magnetic field effect of delayed fluorescence in organic light-emitting diode(in Chinese).Chin Sci Bull,2016,61:1679-1686[邹越,贾伟尧,陈秋松,等.掺杂浓度和温度对有机延迟荧光磁效应的影响.科学通报,2016,61:1679-1686]
15 Pan R H,Tang X T,Deng J Q,et al.Investigation of micro-mechanism in thermally activated delayed fluorescence quantum well devices by utilizing organic magnetic effects(in Chinese).Sci Sin Tech,2018,48,doi:10.1360/N092017-00289[潘睿亨,汤仙童,邓金秋,等.利用有机磁效应研究热活化延迟荧光量子阱器件中的微观机制.中国科学:技术科学,2018,48,doi:10.1360/N092017-00289]
16 Deng J Q,Jia W Y,Chen Y B,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
17 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]
18 Uoyama H,Goushi K,Shizu K,et al.Highly efficient organic light-emitting diodes from delayed fluorescence.Nature,2012,492:234-240
19 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]
20 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
21 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
22 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
23 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
24 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
25 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
26 Doubleday C,Turro N J,Wang J F.Dynamics of flexible triplet biradicals.Acc Chem Res,1989,22:199-205
27 Desai P,Shakya P,Kreouzis T,et al.Magnetoresistance and efficiency measurements of Alq3-based OLEDs.Phys Rev B,2007,75,094423
28 Hu B,Wu Y.Tuning magnetoresistance between positive and negative values in organic semiconductors.Nat Mater,2007,6:985-991
29 Wohlgenannt M,Vardeny Z V.Spin-dependent exciton formation rates inπ-conjugated materials.J Phys Condens Matter,2003,15:R83-R107
30 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 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]
31 Bruno A,Mauro A,Nenna G,et al.Insights on photophysical proprieties of DCM dye in PVK host matrix.Polym Compos,2013,34:1500-1505
32 Feng S W,Shih M C,Huang C J,et al.Impacts of dopant concentration on the carrier transport and recombination dynamics in organic light emitting diodes.Thin Solid Films,2009,517:2719-2723