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Electroluminescence devices made with poly(alkylthiophenes)
ELSEVIER
Synthetic
Electroluminescence
Metals 85 (1997)
1285-1286
devices made with poly(alkylthiophenes)
Y. Q. Liu, X. Z. Jiang, Q. L. Li, Y. Xu, and D. B. Zhu Institute of Chemistry Chinese Academy of Sciences, Beijing 100080, P. R. China
Abstract In our previous papers, we reported the synthesis of various poly(3-alkylthiophenes) derivatives with different side chain length, fabrication of their Langmuir-Blodgett films, conductivities and Schottky diodes. Here, we extend our research to their electroluminescent properties. Multilayer light-emitting diodes composed of poly(3-octylthiophene) as light-emitting layer and hole transport layer are constructed. The current-voltage and brightness-voltage characteristics as well as the electroluminescent spectra are investigated and discussed. Keywords: Electroluminescence;
Poly(3-alkylthiophenes);
Spin casting
1. Introduction Since the discovery of electroluminescence (EL) in semiconducting conjugated polymer poly(p-phenylene vinylene) (PPV) [l], electroluminescent devices based on thin films of conjugated polymer have attracted considerable interest and have enjoyed an extremely big and rapid progress due to their potential application in various display. Up to now, a lot of conjugated polymers have been studied. Among these conjugated polymers, poly(3-alkylthiophenes) (P3AT) are thought to be promise candidate materials due to their good solubility and chemical stability. It has been demonstrated that polymeric light-emitting diodes (LEDs) utilizing P3AT as active layers can span the gamut of the visible spectrum [2]. In our previous papers, we reported the synthesis of various P3AT derivatives with different side chain length, fabrication of their Langmuir-Blodgett ftis, conductivities and Schottky diodes [3]. Here, we extend our research to their EL behavior. Multilayer LEDs composed of poly(3-octylthiophene) (P3OT) as light-emitting layer and hole transport layer are fabricated. The structures of the devices are ITO/PPV/P3OT/Al (Type I) , ITO/P3OT/Alq/Al (Type II) , ITO/PPV/P3OT/Alq/Al (Type lIl), ITOlPVWP3OT/Alq/Al (Type IV), and ITO~PVKJ’P~OTI PMMA:PBD/Al (Type V) , respectively. The current-voltage (I-V) and brightness-voltage (B-V) characteristics as well as the EL spectra are investigated and discussed. 2. Experimental
substrate, a metallic mask was used to obtain 4 samples with active area of about 15 mm*, EL spectra were recorded on a Per&n-Elmer LSSOB luminescence spectrometer with the LED devices forward biased. The luminance was measured with a LS-1 portable luminance meter. I-V characteristics were measured with a programable Solartron 7081 Precision Voltmeter interfaced to a personal computer through an IEEE 488 interface board. All the measurements were performed under ambient atmosphere at room temperature. 3. Results and discussion Fig. 1 shows the I-V characteristics of Type I - Type IV devices. For Type I device (inset), the current increases very quickly with the forward biased voltage. Its red-orange EL can be seen clearly at a voltage about 2 V in a dark room. But the intensity of the emssion was too low and drops too quickly (in a few seconds) to be detected by our Perkin-Elmer LSSOB luminescence spectrometer, which uses an optical fibre to transfer the emission and therein causes a serious lost of the
1 -m-A -- e--B .. .. c
f 8 )
details
The IT0 substrates with sheet resistance less than 100 C?/O were thoroughly cleaned ultrasonically with a series of organic solvents before use. The synthesis, purification, film fabrication and convertion of PPV precursor were described in our previous paper [4]. P30T, PVK and PMMA:PBD blend films were spin-cast from a toluene solution (5.0 mg/ml), a 1,2dichloroethane solution (4.0 mg/ml) and a 1,2-dichloroethane solution (PMMA:PBD=4: 1 in weight), respectively. The speed of spin-casting was about 2500 rpm. Alq and Al were deposited by vacuum deposition at a pressure below 2x10” Pa. For each 0379-6779197B17.00 0 1997 Elsevier PII SO379-6779(96)04360-3
Science S.k
All rights reserved
Fig. 1. I-V characteristics of fabricated Type I; A: Type II; B: Type III; C: Type IV.
devices. Inset:
1286
Lirl et al. /Synthetic Metals 85 (1997) 1285-I 286
Y.Q.
emission intensity and requires the emission more intense to be detected. When reverse biased, it can also emit red-orange EL, while the voltage is higher. This is in consistence with the results of other authors [5]. It should be noted that for Type IV device, the turn on voltage is lower than that of Type II and III. This suggests that the hole injection and transport ability of PVK is higher than that of PPV and P3OT. But it is surprising that the Type II and Type lIl devices almost have the same I-V characteristics. It is possible that the P30T has poorer hole injection and transport ability. Therefore, the P30T layer plays a key role in controlling the current level in Type II and Type III devices. However, if this is true, why Type III and Type IV devices have significantly different I-V characteristics? Further investigation of this difference is under way. Fig. 2 shows the I-V and B-V characteristics of Type V device. The brightness increases superlinearly with the applied voltage, just as the current density. The maximum brightness was 18 cd/m* at an applied forward bias voltage of 22 V. The highest luminous efficiency of the EL calculated by 17~ = x B/(J*V) (lm/W’) is 0.1% hw’W at an applied forward bias voltage of 20 V. That is, the brightness and the efficiency don’t reach the maximum values at the same applied voltage, but the efficiency reaches the maximum value first. This was also found in devices of other structure and with different matierilas [6].
3000
-
E ‘2 2000
-
100
-
, 20
I 4ooot
2
the electrons occurs in both P3OT and Alq layer. So the P3OT layer serves as both hole transport layer and emissive layer. From the spectrum, we can also conclude that the Alq layer contributes more to the emision. However, when the PPV layer in Type III devices is substituted by PVK, that is, in Type IV devices, the emission ( spectrum C, identical with that of Type II.) comes from Alq layer dominantly. Almost no contribution from the P3OT is observed. Therefore, the P30T layer only plays a role of hole transport layer. Compared with Type III devices, this also suggests that PVK is a better hole injection and transport material than PPV. In Type V devices, when using a electron transport layer of PMMAPBD, the emission (spectrum D) comes from P30T layer.
I15
I
1
I
1
t
I
450
500
550
600
650
700
Vhelength
(nm)
Fig. 3. EL spectra of fabricated devices. A: Type II; B: Type III; C: Type IV; D: Type V. 4. Conclusions
8 F E 1000 5
We have constructed five types of LEDs with P30T as emitting layer or hole transport layer. When the structure of the LED devices is so designed that the P3OT serves as the emissive layer, red-orange EL is observed. The maximum brightness of Type V LED devices is 18 cd/m*, with a relatively low maximum luminous efficiency of 0.1% lm/W.
*.’
0
a..*.*
t I 0
. ..
1 5
. . .. . .. y
0
J
1 15
I 10 Voltege
I 20
i
I 25
(v)
Fig. 2. I-V(circle+dot) and B-V(square+solid) ITO/PVK/P3OT/PMMA:PBD/Al
characteristics
of
Fig. 3 shows the EL spectra of Type II, Type III, Type IV and Type V. In Type II, the emission (spectrum A) comes from the Alq layer. This is a little abnormal. Usually, in organic LEDs, exciton will transfer to a narrow bandgap material and emssion comes from the narrow bandgap material. The tentative explanation of this abnormal phnomenon is as follows: the holes inject more easily and transport more ‘quickly, so the recombination zone mainly locates in the Alq layer. And the thickness of the Alq layer is greater than the diffusion length of the exciton. Therefore, the exciton can’t diffuse to the P30T layer. It is also possible that the recombination of the holes and the electrons does occur in the P3OT layer, but the EL efficiency of P3OT is too low, or maybe the EL is quenched, so the EL of P3OT is not observed. In Type ICI devices, the emission ( spectrum B) comes from both P3OT and Alq layer. That is, the holes injected from PPV can transport through P30T and into Alq layer, while the electron injected from Alq can reach P3OT layer, and the recombination of the holes and
Acknowledgements This project was supported by a Key Program of Academia Sinica and NNSFC. References [l] J. H Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R, H. Friend, P. L. Burns, and A. B. Holmes, Nature, 347 (1990) 539.
[2] M. Berggren, 0. Inganas, G. Gustafsson, M. R. Andersson , T. Hjertberg, Nature, 372 (1994) 444. [3] Y. Q. Liu, Y. Xu, J. Wu and D. B. Zhu, Solid State Commun., 95(10) (1995) 695. 141 X. Z. Bang, Y. Q. Liu, X. Q. Song and D. B. Zhy Solid State Commun., 99(3) (1996) 183. [5] F. Garten, k R. Schlatmamr, R. E. Gill, J. Vrijmoeth, T. M. Klapwijk and G. Hadziioannoy Appl. Phys. Lett., 66(19) (1995) 2540. [6] X. Z. Jiang, Y. Q. Liu, Q. L. Li, X. Q. Song and D. B. Zhu, to be submitted.
Synthetic
Electroluminescence
Metals 85 (1997)
1285-1286
devices made with poly(alkylthiophenes)
Y. Q. Liu, X. Z. Jiang, Q. L. Li, Y. Xu, and D. B. Zhu Institute of Chemistry Chinese Academy of Sciences, Beijing 100080, P. R. China
Abstract In our previous papers, we reported the synthesis of various poly(3-alkylthiophenes) derivatives with different side chain length, fabrication of their Langmuir-Blodgett films, conductivities and Schottky diodes. Here, we extend our research to their electroluminescent properties. Multilayer light-emitting diodes composed of poly(3-octylthiophene) as light-emitting layer and hole transport layer are constructed. The current-voltage and brightness-voltage characteristics as well as the electroluminescent spectra are investigated and discussed. Keywords: Electroluminescence;
Poly(3-alkylthiophenes);
Spin casting
1. Introduction Since the discovery of electroluminescence (EL) in semiconducting conjugated polymer poly(p-phenylene vinylene) (PPV) [l], electroluminescent devices based on thin films of conjugated polymer have attracted considerable interest and have enjoyed an extremely big and rapid progress due to their potential application in various display. Up to now, a lot of conjugated polymers have been studied. Among these conjugated polymers, poly(3-alkylthiophenes) (P3AT) are thought to be promise candidate materials due to their good solubility and chemical stability. It has been demonstrated that polymeric light-emitting diodes (LEDs) utilizing P3AT as active layers can span the gamut of the visible spectrum [2]. In our previous papers, we reported the synthesis of various P3AT derivatives with different side chain length, fabrication of their Langmuir-Blodgett ftis, conductivities and Schottky diodes [3]. Here, we extend our research to their EL behavior. Multilayer LEDs composed of poly(3-octylthiophene) (P3OT) as light-emitting layer and hole transport layer are fabricated. The structures of the devices are ITO/PPV/P3OT/Al (Type I) , ITO/P3OT/Alq/Al (Type II) , ITO/PPV/P3OT/Alq/Al (Type lIl), ITOlPVWP3OT/Alq/Al (Type IV), and ITO~PVKJ’P~OTI PMMA:PBD/Al (Type V) , respectively. The current-voltage (I-V) and brightness-voltage (B-V) characteristics as well as the EL spectra are investigated and discussed. 2. Experimental
substrate, a metallic mask was used to obtain 4 samples with active area of about 15 mm*, EL spectra were recorded on a Per&n-Elmer LSSOB luminescence spectrometer with the LED devices forward biased. The luminance was measured with a LS-1 portable luminance meter. I-V characteristics were measured with a programable Solartron 7081 Precision Voltmeter interfaced to a personal computer through an IEEE 488 interface board. All the measurements were performed under ambient atmosphere at room temperature. 3. Results and discussion Fig. 1 shows the I-V characteristics of Type I - Type IV devices. For Type I device (inset), the current increases very quickly with the forward biased voltage. Its red-orange EL can be seen clearly at a voltage about 2 V in a dark room. But the intensity of the emssion was too low and drops too quickly (in a few seconds) to be detected by our Perkin-Elmer LSSOB luminescence spectrometer, which uses an optical fibre to transfer the emission and therein causes a serious lost of the
1 -m-A -- e--B .. .. c
f 8 )
details
The IT0 substrates with sheet resistance less than 100 C?/O were thoroughly cleaned ultrasonically with a series of organic solvents before use. The synthesis, purification, film fabrication and convertion of PPV precursor were described in our previous paper [4]. P30T, PVK and PMMA:PBD blend films were spin-cast from a toluene solution (5.0 mg/ml), a 1,2dichloroethane solution (4.0 mg/ml) and a 1,2-dichloroethane solution (PMMA:PBD=4: 1 in weight), respectively. The speed of spin-casting was about 2500 rpm. Alq and Al were deposited by vacuum deposition at a pressure below 2x10” Pa. For each 0379-6779197B17.00 0 1997 Elsevier PII SO379-6779(96)04360-3
Science S.k
All rights reserved
Fig. 1. I-V characteristics of fabricated Type I; A: Type II; B: Type III; C: Type IV.
devices. Inset:
1286
Lirl et al. /Synthetic Metals 85 (1997) 1285-I 286
Y.Q.
emission intensity and requires the emission more intense to be detected. When reverse biased, it can also emit red-orange EL, while the voltage is higher. This is in consistence with the results of other authors [5]. It should be noted that for Type IV device, the turn on voltage is lower than that of Type II and III. This suggests that the hole injection and transport ability of PVK is higher than that of PPV and P3OT. But it is surprising that the Type II and Type lIl devices almost have the same I-V characteristics. It is possible that the P30T has poorer hole injection and transport ability. Therefore, the P30T layer plays a key role in controlling the current level in Type II and Type III devices. However, if this is true, why Type III and Type IV devices have significantly different I-V characteristics? Further investigation of this difference is under way. Fig. 2 shows the I-V and B-V characteristics of Type V device. The brightness increases superlinearly with the applied voltage, just as the current density. The maximum brightness was 18 cd/m* at an applied forward bias voltage of 22 V. The highest luminous efficiency of the EL calculated by 17~ = x B/(J*V) (lm/W’) is 0.1% hw’W at an applied forward bias voltage of 20 V. That is, the brightness and the efficiency don’t reach the maximum values at the same applied voltage, but the efficiency reaches the maximum value first. This was also found in devices of other structure and with different matierilas [6].
3000
-
E ‘2 2000
-
100
-
, 20
I 4ooot
2
the electrons occurs in both P3OT and Alq layer. So the P3OT layer serves as both hole transport layer and emissive layer. From the spectrum, we can also conclude that the Alq layer contributes more to the emision. However, when the PPV layer in Type III devices is substituted by PVK, that is, in Type IV devices, the emission ( spectrum C, identical with that of Type II.) comes from Alq layer dominantly. Almost no contribution from the P3OT is observed. Therefore, the P30T layer only plays a role of hole transport layer. Compared with Type III devices, this also suggests that PVK is a better hole injection and transport material than PPV. In Type V devices, when using a electron transport layer of PMMAPBD, the emission (spectrum D) comes from P30T layer.
I15
I
1
I
1
t
I
450
500
550
600
650
700
Vhelength
(nm)
Fig. 3. EL spectra of fabricated devices. A: Type II; B: Type III; C: Type IV; D: Type V. 4. Conclusions
8 F E 1000 5
We have constructed five types of LEDs with P30T as emitting layer or hole transport layer. When the structure of the LED devices is so designed that the P3OT serves as the emissive layer, red-orange EL is observed. The maximum brightness of Type V LED devices is 18 cd/m*, with a relatively low maximum luminous efficiency of 0.1% lm/W.
*.’
0
a..*.*
t I 0
. ..
1 5
. . .. . .. y
0
J
1 15
I 10 Voltege
I 20
i
I 25
(v)
Fig. 2. I-V(circle+dot) and B-V(square+solid) ITO/PVK/P3OT/PMMA:PBD/Al
characteristics
of
Fig. 3 shows the EL spectra of Type II, Type III, Type IV and Type V. In Type II, the emission (spectrum A) comes from the Alq layer. This is a little abnormal. Usually, in organic LEDs, exciton will transfer to a narrow bandgap material and emssion comes from the narrow bandgap material. The tentative explanation of this abnormal phnomenon is as follows: the holes inject more easily and transport more ‘quickly, so the recombination zone mainly locates in the Alq layer. And the thickness of the Alq layer is greater than the diffusion length of the exciton. Therefore, the exciton can’t diffuse to the P30T layer. It is also possible that the recombination of the holes and the electrons does occur in the P3OT layer, but the EL efficiency of P3OT is too low, or maybe the EL is quenched, so the EL of P3OT is not observed. In Type ICI devices, the emission ( spectrum B) comes from both P3OT and Alq layer. That is, the holes injected from PPV can transport through P30T and into Alq layer, while the electron injected from Alq can reach P3OT layer, and the recombination of the holes and
Acknowledgements This project was supported by a Key Program of Academia Sinica and NNSFC. References [l] J. H Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R, H. Friend, P. L. Burns, and A. B. Holmes, Nature, 347 (1990) 539.
[2] M. Berggren, 0. Inganas, G. Gustafsson, M. R. Andersson , T. Hjertberg, Nature, 372 (1994) 444. [3] Y. Q. Liu, Y. Xu, J. Wu and D. B. Zhu, Solid State Commun., 95(10) (1995) 695. 141 X. Z. Bang, Y. Q. Liu, X. Q. Song and D. B. Zhy Solid State Commun., 99(3) (1996) 183. [5] F. Garten, k R. Schlatmamr, R. E. Gill, J. Vrijmoeth, T. M. Klapwijk and G. Hadziioannoy Appl. Phys. Lett., 66(19) (1995) 2540. [6] X. Z. Jiang, Y. Q. Liu, Q. L. Li, X. Q. Song and D. B. Zhu, to be submitted.