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A novel oligomer poly (phenylene vinylene) derivative containing oxadiazole segment
Synthetic Metals 111–112 Ž2000. 455–457 www.elsevier.comrlocatersynmet
A novel oligomer poly žphenylene vinylene/ derivative containing oxadiazole segment Xiaohui Yang a,b, Yulin Hua b,) , Shougen Yin a , Zhenjia Wang a , Yanbing Hou a , Zheng Xu a , Xurong Xu a,b, Junbiao Peng c , Wenlian Li c a
Institute of Optoelectronic, Northern Jiaotong UniÕersity, Beijing 100044, People’s Republic of China Institute of Material Physics, Tianjin Institute of Technology, Tianjin 300191, People’s Republic of China Laboratory of Excited States Process, Chinese Academy of Science, Changchun 130021, People’s Republic of China b
c
Abstract We have synthesized an oligomer derivative of poly Žphenylene vinylene. ŽOPPV. with oxadiazole in the main chain. Electroluminescence ŽEL. efficiency of single-layer OPPV device is about three to five times higher compared with single-layer poly Žphenylene vinylene. ŽPPV. device. The reason is attributed to the improvement of electron injection and transport ability in OPPV, which is shown by combining OPPV with hole-transport layer ŽPPV. and electron-transport layer ŽTb complex.. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Polymer electroluminescent device; Oxadiazole; Carrier transport characteristic; Bipolar transport material
1. Introduction Since the first polymer electroluminescent device ŽPELD. was reported by the Cambridge group w1x, research on PELD has been developed rapidly for the unique virtues of polymers, namely, easy processability, low cost, and the possibility of chemical tailoring to realize desired properties. Single-layer poly Žphenylene vinylene. ŽPPV. devices have relatively low efficiency due to the unbalanced carrier injection and transport Žmainly deficiency of electron.. Generally, two methods have been adopted to improve efficiency of the device: Ž1. utilizing low-work function metal such as calcium to facilitate electron injection from cathode w2x and Ž2. inserting additional layer as electron-transport layer to improve electron injection. Concerned about the reactivity of low-work function metal, it is highly desirable to fabricate PELD with a relatively high-work function metal. Greenham et al. w3x reported that cyano-substituted PPV ŽCN-PPV. increased the electron affinity of the material and reduced electron injection barrier while band offset at PPVrCN-PPV layer interface redistributed electric field, thus improving electron injec-
)
Corresponding author. E-mail address: [email protected] ŽY. Hua..
tion and efficiency of the device. However, owing to relatively smaller band gap of CN-PPV, emission comes from CN-PPV layer. Here, we report an OPPV derivative with oxidazole function segment in the backbone, namely, wpoly Ž2,5-diphenylene-1,3,4-oxadiazole. -4,4X -vinylenex. Photoluminescence ŽPL. and electroluminescence ŽEL. of this material were studied and carrier transport characteristic of OPPV was discussed in double-layer device consisting of OPPV and hole-transport layer or electron-transport layer, respectively.
2. Experimental Fig. 1a shows the molecular formula of OPPV whose structure was identified by infrared spectroscopy ŽIR. spectra and nuclear magnetic resonance ŽNMR.. Average molecular weight was determined to be 1000, which means that n s 4. OPPV was dissolved in chloroform and the concentration of solution was 5 mgrml. A PPV thin film was fabricated in standard precursor way w1x, while singlelayer OPPV and PPVrOPPV double-layer devices were built by spin-coating OPPV chloroform solution on precleaned indium–tin–oxide ŽITO. glass or precoated polymer layer, respectively. TbŽacetylacetonato. 3-Žmonophenathroline. w4x and aluminum were thermally evapo-
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 3 9 9 - 9
456
X. Yang et al.r Synthetic Metals 111–112 (2000) 455–457
Fig. 1. Ža. The chemical structure of OPPV. Žb. PL of OPPV in dilute solution Žsolid line., film Žtriangles., and EL spectrum of single-layer OPPV device Žsquares.. Inset: absorption spectrum of OPPV film.
rated. PL and EL spectra were measured by a Hitachi F-4000 spectrometer, while the room temperature absorption spectrum was measured by UVKON810 absorption spectrometer. The luminescence lifetime of an OPPV solid-state film was obtained by SLM4800S MultiFrequency Lifetime Spectrometer. The device brightness was measured by SA-16 radiance.
states are easily formed in this case. This assumption is further proved by time-resolved luminescence experiment, the decay time constants of luminescence are between 1 and 8 ns in film sample. Based on the above discussion, we believe that emission of OPPV originates from aggregated states w5x. The EL spectrum of single-layer OPPV device ŽITOrOPPV Ž70 nm.rAl. is basically identical to PL of OPPV film, as shown in Fig. 1b. The turn-on voltage of this device was about 5 V; current–voltage characteristic of the device had obvious diode-rectifying characteristic, with a rectification ratio at 5 V of about 100. Under the same injection current, the brightness of a single-layer OPPV device was about three to five times higher than that of a single-layer PPV device. Although, we think that the apparent increment of efficiency of OPPV device is related to the insertion of oxadiazole segment, but it cannot be simply assigned to any particular factor such as carrier transport characteristic, which we will discuss below. Since the luminescence efficiency of PLED is related to many factors such as PL efficiency, carrier injection, and recombination efficiency Že.g., relative higher PL efficiency for PPV oligomer than PPV and for the coincidence of EL and PL spectra, EL originates from specimen, which is responsible for PL; however, it is generally believed that chargetransfer state formation quenches luminescence.. OPPV was identified by the authors as having better electron-transport ability, due to the presence of electrondeficient nitrogen in oxadiazole w6x, which was our initial goal to synthesize these kinds of material. To prove this idea, we fabricated double-layer device: ITOrPPV Ž70 nm.rOPPV Ž30 nm.rAl Žtype I device.. A single-layer PPV device was also fabricated for comparison. The turn-
3. Results and discussion Optical characteristics of OPPV were studied, as displayed in Fig. 1b. For OPPV film absorption, the peak locates at 4.2 eV and PL peak lies at 2.48 eV. Comparing PPV oligomer with the same conjugated length, the relative large stokes shift and board structureless emission profile may imply that emission comes from aggregated states. We dispersed OPPV into a polyethylene oxide matrix, and found that the emission spectra blueshift with decreasing concentration of OPPV. When the concentration by weight is 0.5%, emission peak locates at 2.98 eV and does not shift anymore in accordance with PL spectrum of diluted OPPV solution Ž1 mgrl.. From the molecule structure, we can see that there is no bulky substitution in this material; molecules may have relatively large probability to stack with each other, and aggregated
Fig. 2. Ža. Current–voltage characteristics of PPVrOPPV double-layer device. Žb. Brightness–voltage characteristics of PPVrOPPV device Žtriangles. and single-layer PPV device Ždots..
X. Yang et al.r Synthetic Metals 111–112 (2000) 455–457
Fig. 3. The EL spectra of OPPVrTbŽAcAc. 3 Phen device driven at 8 V Ždotted line. and at 14 V Žsquares.. Spectra are normalised to the 544 nm peak of Tb3q. The solid line shows a decomposition of the 14 V EL spectrum.
on voltage of all these devices was about 6–7 V. It was only from the PPV layer that a double-layer device emission was observed. From our results, luminescence value reached 230 cdrm2 at high current density Žabout 110 mArcm2 ., while in single-layer PPV device at similar current density, the brightness was only about 10–20 cdrm2 , which indicates that the efficiency of the doublelayer device was improved by about 10 times. The current–voltage and brightness–voltage characteristics of the devices are presented in Fig. 2. We fabricated another type of device with structure: ITOrOPPV Ž70 nm.rTbŽAcAc. 3 Phen Ž40 nm.rAl Žtype II device.. It was fabricated to examine whether OPPV still had form er hole-transport ability. Single-layer TbŽAcAc. 3 Phen device was also fabricated for comparison. We argue that if OPPV has hole-transport ability in the device, carrier recombination zone of type device should locate at the interface of OPPV and TbŽAcAc. 3 Phen layer since TbŽAcAc. 3 Phen was reported to be an electron-transport material w4x; otherwise, emission from OPPV layer should be observed since it is generally accepted for electron-transport material carriers that recombine near the anode. Fig. 3 shows EL spectra of type II device. Under lower-driven voltage Ž8 V., EL spectrum
457
was mainly the characteristic emission of Tb 3q, while when the operating voltage was higher Ž14 V., emission from OPPV layer contributed to the total light emission. While in single TbŽAcAc. 3 Phen layer device, no emission or very faint and unstable emission was observed under a rather large current density of up to 50 mArcm2 , which suggests an almost unipolar carrier injection. The EL spectra of type I device are characteristic spectra of PPV, indicating that carrier recombination occurs in PPV layer. The improved efficiency of the device illustrates that OPPV may have the effect to facilitate electron injection; as a result, recombination ratio of hole and electron is greatly enhanced. In type device, the improved luminescence characteristic of Tb 3q and the carrier recombination zone indicate that the OPPV layer functions as hole-transport layer in this case. Based on the above discussion, we believe that by inserting electron that accepts characteristic oxadiazole function segment, we can balance the carrier transport ability of the material. In other words, OPPV is a kind of bipolar carrier transport material.
Acknowledgements This project was supported by NSFC grant no. 29992350, 69976021 and NSFT grant no. 983601511, 993600611. We would like to thank Professor Xiong Guangnan for valuable discussion.
References w1x J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.H. Marks, K. Mackay, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 Ž1990. 539. w2x D. Braun, A. Heeger, Appl. Phys. Lett. 58 Ž1991. 1982. w3x N.C. Greenham, S.C. Moratti, D.D.C. Bradley, R.H. Friend, A.B. Holmes, Nature 365 Ž1993. 628. w4x S. Gang, L. Wenlian, Z. Yu, Y. Yu, Z. Xu, L. Xingyuan, Y. Jiaqi, Z. Guozhu, Chinese Journal of Luminescence 17 Ž1996. 362, Žin Chinese.. w5x N.T. Harrison, D.R. Baigent, I.D.W. Samuel, R.H. Friend, A.C. Grimsdale, S.C. Moratti, A.B. Holmes, Phys. Rev. B 53 Ž1996. 15815, and references therein. w6x D.F. O’Brien, M.S. Weaver, D.G. Lidzey, D.D.C. Bradley, Appl. Phys. Lett. 69 Ž1996. 881.
A novel oligomer poly žphenylene vinylene/ derivative containing oxadiazole segment Xiaohui Yang a,b, Yulin Hua b,) , Shougen Yin a , Zhenjia Wang a , Yanbing Hou a , Zheng Xu a , Xurong Xu a,b, Junbiao Peng c , Wenlian Li c a
Institute of Optoelectronic, Northern Jiaotong UniÕersity, Beijing 100044, People’s Republic of China Institute of Material Physics, Tianjin Institute of Technology, Tianjin 300191, People’s Republic of China Laboratory of Excited States Process, Chinese Academy of Science, Changchun 130021, People’s Republic of China b
c
Abstract We have synthesized an oligomer derivative of poly Žphenylene vinylene. ŽOPPV. with oxadiazole in the main chain. Electroluminescence ŽEL. efficiency of single-layer OPPV device is about three to five times higher compared with single-layer poly Žphenylene vinylene. ŽPPV. device. The reason is attributed to the improvement of electron injection and transport ability in OPPV, which is shown by combining OPPV with hole-transport layer ŽPPV. and electron-transport layer ŽTb complex.. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Polymer electroluminescent device; Oxadiazole; Carrier transport characteristic; Bipolar transport material
1. Introduction Since the first polymer electroluminescent device ŽPELD. was reported by the Cambridge group w1x, research on PELD has been developed rapidly for the unique virtues of polymers, namely, easy processability, low cost, and the possibility of chemical tailoring to realize desired properties. Single-layer poly Žphenylene vinylene. ŽPPV. devices have relatively low efficiency due to the unbalanced carrier injection and transport Žmainly deficiency of electron.. Generally, two methods have been adopted to improve efficiency of the device: Ž1. utilizing low-work function metal such as calcium to facilitate electron injection from cathode w2x and Ž2. inserting additional layer as electron-transport layer to improve electron injection. Concerned about the reactivity of low-work function metal, it is highly desirable to fabricate PELD with a relatively high-work function metal. Greenham et al. w3x reported that cyano-substituted PPV ŽCN-PPV. increased the electron affinity of the material and reduced electron injection barrier while band offset at PPVrCN-PPV layer interface redistributed electric field, thus improving electron injec-
)
Corresponding author. E-mail address: [email protected] ŽY. Hua..
tion and efficiency of the device. However, owing to relatively smaller band gap of CN-PPV, emission comes from CN-PPV layer. Here, we report an OPPV derivative with oxidazole function segment in the backbone, namely, wpoly Ž2,5-diphenylene-1,3,4-oxadiazole. -4,4X -vinylenex. Photoluminescence ŽPL. and electroluminescence ŽEL. of this material were studied and carrier transport characteristic of OPPV was discussed in double-layer device consisting of OPPV and hole-transport layer or electron-transport layer, respectively.
2. Experimental Fig. 1a shows the molecular formula of OPPV whose structure was identified by infrared spectroscopy ŽIR. spectra and nuclear magnetic resonance ŽNMR.. Average molecular weight was determined to be 1000, which means that n s 4. OPPV was dissolved in chloroform and the concentration of solution was 5 mgrml. A PPV thin film was fabricated in standard precursor way w1x, while singlelayer OPPV and PPVrOPPV double-layer devices were built by spin-coating OPPV chloroform solution on precleaned indium–tin–oxide ŽITO. glass or precoated polymer layer, respectively. TbŽacetylacetonato. 3-Žmonophenathroline. w4x and aluminum were thermally evapo-
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 3 9 9 - 9
456
X. Yang et al.r Synthetic Metals 111–112 (2000) 455–457
Fig. 1. Ža. The chemical structure of OPPV. Žb. PL of OPPV in dilute solution Žsolid line., film Žtriangles., and EL spectrum of single-layer OPPV device Žsquares.. Inset: absorption spectrum of OPPV film.
rated. PL and EL spectra were measured by a Hitachi F-4000 spectrometer, while the room temperature absorption spectrum was measured by UVKON810 absorption spectrometer. The luminescence lifetime of an OPPV solid-state film was obtained by SLM4800S MultiFrequency Lifetime Spectrometer. The device brightness was measured by SA-16 radiance.
states are easily formed in this case. This assumption is further proved by time-resolved luminescence experiment, the decay time constants of luminescence are between 1 and 8 ns in film sample. Based on the above discussion, we believe that emission of OPPV originates from aggregated states w5x. The EL spectrum of single-layer OPPV device ŽITOrOPPV Ž70 nm.rAl. is basically identical to PL of OPPV film, as shown in Fig. 1b. The turn-on voltage of this device was about 5 V; current–voltage characteristic of the device had obvious diode-rectifying characteristic, with a rectification ratio at 5 V of about 100. Under the same injection current, the brightness of a single-layer OPPV device was about three to five times higher than that of a single-layer PPV device. Although, we think that the apparent increment of efficiency of OPPV device is related to the insertion of oxadiazole segment, but it cannot be simply assigned to any particular factor such as carrier transport characteristic, which we will discuss below. Since the luminescence efficiency of PLED is related to many factors such as PL efficiency, carrier injection, and recombination efficiency Že.g., relative higher PL efficiency for PPV oligomer than PPV and for the coincidence of EL and PL spectra, EL originates from specimen, which is responsible for PL; however, it is generally believed that chargetransfer state formation quenches luminescence.. OPPV was identified by the authors as having better electron-transport ability, due to the presence of electrondeficient nitrogen in oxadiazole w6x, which was our initial goal to synthesize these kinds of material. To prove this idea, we fabricated double-layer device: ITOrPPV Ž70 nm.rOPPV Ž30 nm.rAl Žtype I device.. A single-layer PPV device was also fabricated for comparison. The turn-
3. Results and discussion Optical characteristics of OPPV were studied, as displayed in Fig. 1b. For OPPV film absorption, the peak locates at 4.2 eV and PL peak lies at 2.48 eV. Comparing PPV oligomer with the same conjugated length, the relative large stokes shift and board structureless emission profile may imply that emission comes from aggregated states. We dispersed OPPV into a polyethylene oxide matrix, and found that the emission spectra blueshift with decreasing concentration of OPPV. When the concentration by weight is 0.5%, emission peak locates at 2.98 eV and does not shift anymore in accordance with PL spectrum of diluted OPPV solution Ž1 mgrl.. From the molecule structure, we can see that there is no bulky substitution in this material; molecules may have relatively large probability to stack with each other, and aggregated
Fig. 2. Ža. Current–voltage characteristics of PPVrOPPV double-layer device. Žb. Brightness–voltage characteristics of PPVrOPPV device Žtriangles. and single-layer PPV device Ždots..
X. Yang et al.r Synthetic Metals 111–112 (2000) 455–457
Fig. 3. The EL spectra of OPPVrTbŽAcAc. 3 Phen device driven at 8 V Ždotted line. and at 14 V Žsquares.. Spectra are normalised to the 544 nm peak of Tb3q. The solid line shows a decomposition of the 14 V EL spectrum.
on voltage of all these devices was about 6–7 V. It was only from the PPV layer that a double-layer device emission was observed. From our results, luminescence value reached 230 cdrm2 at high current density Žabout 110 mArcm2 ., while in single-layer PPV device at similar current density, the brightness was only about 10–20 cdrm2 , which indicates that the efficiency of the doublelayer device was improved by about 10 times. The current–voltage and brightness–voltage characteristics of the devices are presented in Fig. 2. We fabricated another type of device with structure: ITOrOPPV Ž70 nm.rTbŽAcAc. 3 Phen Ž40 nm.rAl Žtype II device.. It was fabricated to examine whether OPPV still had form er hole-transport ability. Single-layer TbŽAcAc. 3 Phen device was also fabricated for comparison. We argue that if OPPV has hole-transport ability in the device, carrier recombination zone of type device should locate at the interface of OPPV and TbŽAcAc. 3 Phen layer since TbŽAcAc. 3 Phen was reported to be an electron-transport material w4x; otherwise, emission from OPPV layer should be observed since it is generally accepted for electron-transport material carriers that recombine near the anode. Fig. 3 shows EL spectra of type II device. Under lower-driven voltage Ž8 V., EL spectrum
457
was mainly the characteristic emission of Tb 3q, while when the operating voltage was higher Ž14 V., emission from OPPV layer contributed to the total light emission. While in single TbŽAcAc. 3 Phen layer device, no emission or very faint and unstable emission was observed under a rather large current density of up to 50 mArcm2 , which suggests an almost unipolar carrier injection. The EL spectra of type I device are characteristic spectra of PPV, indicating that carrier recombination occurs in PPV layer. The improved efficiency of the device illustrates that OPPV may have the effect to facilitate electron injection; as a result, recombination ratio of hole and electron is greatly enhanced. In type device, the improved luminescence characteristic of Tb 3q and the carrier recombination zone indicate that the OPPV layer functions as hole-transport layer in this case. Based on the above discussion, we believe that by inserting electron that accepts characteristic oxadiazole function segment, we can balance the carrier transport ability of the material. In other words, OPPV is a kind of bipolar carrier transport material.
Acknowledgements This project was supported by NSFC grant no. 29992350, 69976021 and NSFT grant no. 983601511, 993600611. We would like to thank Professor Xiong Guangnan for valuable discussion.
References w1x J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.H. Marks, K. Mackay, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 Ž1990. 539. w2x D. Braun, A. Heeger, Appl. Phys. Lett. 58 Ž1991. 1982. w3x N.C. Greenham, S.C. Moratti, D.D.C. Bradley, R.H. Friend, A.B. Holmes, Nature 365 Ž1993. 628. w4x S. Gang, L. Wenlian, Z. Yu, Y. Yu, Z. Xu, L. Xingyuan, Y. Jiaqi, Z. Guozhu, Chinese Journal of Luminescence 17 Ž1996. 362, Žin Chinese.. w5x N.T. Harrison, D.R. Baigent, I.D.W. Samuel, R.H. Friend, A.C. Grimsdale, S.C. Moratti, A.B. Holmes, Phys. Rev. B 53 Ž1996. 15815, and references therein. w6x D.F. O’Brien, M.S. Weaver, D.G. Lidzey, D.D.C. Bradley, Appl. Phys. Lett. 69 Ž1996. 881.