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Influences of organic hole-transport layer on the emission of organic–inorganic heterostructure devices
ARTICLE IN PRESS
Journal of Luminescence 122–123 (2007) 617–619 www.elsevier.com/locate/jlumin
Influences of organic hole-transport layer on the emission of organic–inorganic heterostructure devices Yan Jiang, Shengyi Yang, Feng Teng, Zheng Xu, Yanbing Hou, Xurong Xu Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, China Available online 15 March 2006
Abstract In this paper, influences of poly (N-vinyl-carbazole) (PVK) on the emission properties of zinc selenide (ZnSe)-based organic–inorganic heterostructure devices have been studied. A broad band emission peak at 443 nm, with shoulders at 520 and 595 nm, was observed from ITO/PVK(80 nm)/ZnSe(120 nm)/Alq3(15 nm)/Al under electric fields and it consisted of emissions from PVK, ZnSe and Alq3 layers. Therefore, a charge carrier injection luminescence in ZnSe layer is suggested. By changing the thickness of PVK layer and/or ZnSe layer, the roles of PVK in the device were discussed. r 2006 Elsevier B.V. All rights reserved. Keywords: Organic–inorganic heterostructure; Electroluminescence (EL); Zinc selenide (ZnSe); Poly (N-vinyl-carbazole) (PVK)
1. Introduction Most polymer materials transport holes preferentially which cause imbalanced carrier injection in organic lightemitting diodes (OLEDs), as a result, holes may pass through the emissive layer without forming excitons with the oppositely charged carriers and sequentially lead to ohmic losses. Furthermore, the holes mobility is larger than that of electrons in most organic materials, so that the recombination zone of holes and electrons is close to cathode where excitons are easily quenched. On the other hand, most inorganic materials have higher electron mobility. Therefore, some attempts have been done to fabricate organic–inorganic heterostructure [1,2]. The mechanism in OLEDs is basically similar to that of inorganic electroluminescence (IEL). Additionally, we noticed that the electric field intensities of IEL and OLEDs are similar [3–5]. So it is possible to fabricate a hybrid EL device with inorganic and organic semiconductors. The organic–inorganic hybrid EL device is expected not only to permit a wide range selection of emitter and carrier transport materials but also to provide a new approach to construct high-performance EL device taking advantage Corresponding author. Tel.: +86 10 51683414; fax: +86 10 51683933.
E-mail address: [email protected] (S. Yang). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.240
of both the organic and inorganic semiconductors, such as high photoluminescence efficiency of organic materials and high carrier density, high carrier mobility and steady chemical property of inorganic semiconductors. In fact, some high-performance organic–inorganic hybrid EL devices have been reported [3,6,7]. However, the emission mechanism of these heterostructure devices varies from device to device [4,7]. Then it is necessary for one to find out the influence of organic hole-transport layer (HTL) on the emission performance of the heterostructure devices. As we know, as a good HTL with high holes mobility, poly (N-vinyl-carbazole) (PVK) is widely used in OLEDs, and zinc selenide (ZnSe) is usually used as the blue emitting material with an energy band gap of 2.7 eV. In this paper, by designing device structure of ITO/PVK/ZnSe/Alq3/Al and varying the thickness of PVK layer and ZnSe layer respectively, we try to find out. Respectively, we try to find out the influences of PVK on the emission of organic–inorganic diodes, and the roles of ZnSe in the device are discussed. 2. Experiments In our experiments, PVK (10 mg/ml in chloroform solution) was spin-coated onto the ITO (sheet resistance
ARTICLE IN PRESS Y. Jiang et al. / Journal of Luminescence 122–123 (2007) 617–619
50 O/sq) which was cleaned by sonication in detergent solution, deionized water, boiling in trichloroethylene, acetone and ethanol. ZnSe film was deposited by electron-beam evaporation at a rate of 1 A˚/s under high vacuum of 6 10 6 Torr and the substrate temperature was kept at 100 1C. The thickness of ZnSe layer was monitored by a quartz crystal thickness monitor placed near the substrate. PL and EL spectra were measured with Spex Fluorolog-3 spectrometer.
1.0
c b
0.8 Intensity (a.u.)
618
a
0.6 0.4
e
0.2
3. Results and discussion
d
4x105
440nm
3x105 Intensity (a.u.)
500 600 Wavelength (nm)
700
Fig. 2. Normalized EL spectrum of device ITO/PVK(80 nm)/ZnSe(100 nm)/ Alq3(15 nm)/Al (curve a) and its PL spectrum under excitation wavelength of 325 nm (curve d), as well as the normalized EL spectra of devices ITO/ Alq3(50 nm)/Al (curve b), ITO/PVK(80 nm)/Al (curve e) and ITO/ PVK(80 nm)/ZnSe(120 nm)/Alq3(15 nm)/Al (curve c).
428nm
1.0
Device D: PVK=100nm Device B: PVK=80nm Device C: PVK=50nm
0.8
0.6
0.4
0.2
0.0 350
400
450
500 550 Wavelength (nm)
600
650
700
Fig. 3. Normalized EL spectra of devices ITO/PVK/ZnSe(100 nm)/ Alq3(15 nm)/Al at the same applied voltage of 23 V.
33V
20V 2x105
1x105
595nm
0
400
400
Intensity (a.u.)
As shown in Fig. 1, a broad band emission peak at 443 nm, with shoulders at 520 and 595 nm, was observed from device ITO/PVK(80 nm)/ZnSe(120 nm)/Alq3(15 nm)/ Al (device A) under a forward applied bias (ITO as the positive electrode) and we believe that it combines emissions from PVK, ZnSe and Alq3 layers. Obviously, electrons injected from cathode recombine with holes injected from anode and radiate light in PVK, ZnSe and Alq3 layers, consequently, as the result of charge carriers transiting over the barriers formed at the ZnSe/Alq3 interface. The holes barrier is 0.7 eV and electrons barrier is 1.6 eV at the PVK/ZnSe interface. Furthermore, the mobility of electrons in ZnSe is much higher than that of holes and the mobility of holes in PVK is smaller than that of electrons in Alq3 [8]. Therefore, the emission from PVK layer is dominant since the recombination region mainly lies in PVK layer, and the combined emission peaks at 443 nm, therefore, we believe ZnSe plays roles of holetransporting layer, electron-transporting layer and emissive layer. Because of its weak fluorescence compared with that of PVK and Alq3, emission from ZnSe layer was too weak to be visible in the EL spectrum. In order to confirm our suggestion, we further reduced the thickness of ZnSe to 100 nm and fabricated device ITO/ PVK(80 nm)/ZnSe(100 nm)/Alq3(15 nm)/Al (device B). As shown in Fig. 2, we can see that its EL spectrum gets
0.0
500
600
700
Wavelength (nm) Fig. 1. EL spectra of device ITO/PVK(80 nm)/ZnSe(120 nm)/Alq3(15 nm)/ Al at different applied voltages.
narrower than that of device A, obviously, it implies the decreased intensity of emission from ZnSe bulk and Alq3 bulk because the thickness of ZnSe layer is reduced and the electric field in the device would redistribute. For comparison, normalized EL spectrum of ITO/Alq3/Al (peak at 520 nm) and ITO/PVK/Al (peak at 415 nm), as well as photoluminescence (PL) spectrum of device B are shown in Fig. 2. The PL of device B shows a maximum peak at 415 nm and a shoulder at 520 nm, emission peak at 415 nm origins from PVK excitons emission and the shoulder at 520 nm origins from Alq3 excitons emission. Furthermore, we changed the thickness of PVK and made other two kinds of devices ITO/PVK(50 nm)/ ZnSe(100 nm)/Alq3 (15 nm)/Al (device C) and ITO/ PVK(100 nm)/ZnSe(100 nm)/Alq3(15 nm)/Al (device D). The normalized EL spectra of devices B, C and D under
ARTICLE IN PRESS Y. Jiang et al. / Journal of Luminescence 122–123 (2007) 617–619
1.6
the stability of hybrid structure device and balance the injections of oppositely charged carriers by increasing electron injection barrier to 1.1 eV. Also, the I–V curve of device B under applied voltages (from 30 to 30 V) is shown in Fig. 4, which shows very good diode rectification characteristics.
-2.5eV -3.0eV
1.4 1.2 Current (mA)
-4.1eV 1.0
-4.6eV
619
-4.2eV
PVK Alq3
0.8 ZnSe
0.6
-6.1eV
0.4
4. Conclusions
-5.7eV -6.8eV
0.2 0.0 -0.2 -30
-20
-10
0
10
20
30
Applied voltage (V) Fig. 4. I–V characteristics of device ITO/PVK(80 nm)/ZnSe(100 nm)/ Alq3(15 nm)/Al. The inset shows the energy diagram of hybrid heterostructure device ITO/PVK/ZnSe/Alq3/Al.
a same applied voltage of 23 V are shown in Fig. 3. From the figure, we can see that all of them peak at 428 nm but the EL spectrum of device C is narrower than that of devices B and D and the EL spectrum of device B is a little broader than that of device D. As we know, the device spectra show the information of recombination region, the emission from PVK is dominant in the EL spectra of device C and no obvious emission from Alq3 layer is observed. For devices B and D, emission from Alq3 can be observed and it is more intensive from device B than that from device D, which is due to the charge carriers redistribution and consequentially electric fields redistribution across the different layers. Because holes can transport through ZnSe into Alq3 layer and electrons transport through ZnSe into PVK layer, oppositely charged carriers will encounter in ZnSe bulk and recombine to emit light. Therefore, it should be injection luminescence in ZnSe bulk. As shown in the energy diagram of ITO/PVK/ZnSe/Alq3/Al (see the inset in Fig. 4), we can see that the holes injection barrier from ITO into PVK is 1.5 eV and the electron injection barrier for electrons from Al into Alq3 is 1.2 eV. Therefore, it is easier for holes to inject into PVK by inserting PVK between the ZnSe and the ITO, in this way, the holes injection barrier reduces 0.7 eV.On the other hand, Alq3 layer between the cathode and the ZnSe layer can improve
In conclusion, influences of PVK on the emission of ITO/PVK/ ZnSe/Alq3/Al are discussed. A broad band emission peaking at 443 nm with shoulders at 520 and 595 nm was observed from the device ITO/PVK(80 nm)/ ZnSe(120 nm)/Alq3(15 nm)/Al under electric fields, and it consists of three parts of emissions from PVK, ZnSe and Alq3 layers. Therefore, injection luminescence in ZnSe layer is deduced. By changing the thickness of PVK layer and that of ZnSe layer, a conclusion can be drawn that PVK not only enhances holes injection but also acts as HTL and emissive layer. Acknowledgements This project was supported by the National Natural Science Foundation of China (no. 60406006 and 10434030), the ‘‘973’’ National Key Basic Research Special Foundation of China (no. 2003CB314707), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and the Science Foundation (no. 2003SM001 and LIJ03004) and the Paper Foundation of Beijing Jiaotong University. References [1] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Nature 374 (1990) 539. [2] V.L. Colvin, M.C. Schlamp, A.P. Allvisatos, Nature 370 (1994) 354. [3] X.H. Yang, X.R. Xu, Appl. Phys. Lett. 77 (2000) 797. [4] J. Kalinowaki, J. Phys. D 32 (1999) 179. [5] P.D. Rack, P.H. Holloway, Mater. Sci. Eng. R 21 (1998) 171. [6] N.D. Kumar, M.P. Joshi, C.S. Friend, P.N. Prasad, R. Burzynski, Appl. Phys. Lett. 71 (10) (1997) 1388. [7] S.Y. Yang, L. Qian, F. Teng, Z. Xu, Y.B. Hou, X.R. Xu, J. Appl. Phys. 97 (2005) 126101. [8] B.J. Chen, S.Y. Liu, Synthetic Met. 91 (1997) 169.
Journal of Luminescence 122–123 (2007) 617–619 www.elsevier.com/locate/jlumin
Influences of organic hole-transport layer on the emission of organic–inorganic heterostructure devices Yan Jiang, Shengyi Yang, Feng Teng, Zheng Xu, Yanbing Hou, Xurong Xu Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, China Available online 15 March 2006
Abstract In this paper, influences of poly (N-vinyl-carbazole) (PVK) on the emission properties of zinc selenide (ZnSe)-based organic–inorganic heterostructure devices have been studied. A broad band emission peak at 443 nm, with shoulders at 520 and 595 nm, was observed from ITO/PVK(80 nm)/ZnSe(120 nm)/Alq3(15 nm)/Al under electric fields and it consisted of emissions from PVK, ZnSe and Alq3 layers. Therefore, a charge carrier injection luminescence in ZnSe layer is suggested. By changing the thickness of PVK layer and/or ZnSe layer, the roles of PVK in the device were discussed. r 2006 Elsevier B.V. All rights reserved. Keywords: Organic–inorganic heterostructure; Electroluminescence (EL); Zinc selenide (ZnSe); Poly (N-vinyl-carbazole) (PVK)
1. Introduction Most polymer materials transport holes preferentially which cause imbalanced carrier injection in organic lightemitting diodes (OLEDs), as a result, holes may pass through the emissive layer without forming excitons with the oppositely charged carriers and sequentially lead to ohmic losses. Furthermore, the holes mobility is larger than that of electrons in most organic materials, so that the recombination zone of holes and electrons is close to cathode where excitons are easily quenched. On the other hand, most inorganic materials have higher electron mobility. Therefore, some attempts have been done to fabricate organic–inorganic heterostructure [1,2]. The mechanism in OLEDs is basically similar to that of inorganic electroluminescence (IEL). Additionally, we noticed that the electric field intensities of IEL and OLEDs are similar [3–5]. So it is possible to fabricate a hybrid EL device with inorganic and organic semiconductors. The organic–inorganic hybrid EL device is expected not only to permit a wide range selection of emitter and carrier transport materials but also to provide a new approach to construct high-performance EL device taking advantage Corresponding author. Tel.: +86 10 51683414; fax: +86 10 51683933.
E-mail address: [email protected] (S. Yang). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.240
of both the organic and inorganic semiconductors, such as high photoluminescence efficiency of organic materials and high carrier density, high carrier mobility and steady chemical property of inorganic semiconductors. In fact, some high-performance organic–inorganic hybrid EL devices have been reported [3,6,7]. However, the emission mechanism of these heterostructure devices varies from device to device [4,7]. Then it is necessary for one to find out the influence of organic hole-transport layer (HTL) on the emission performance of the heterostructure devices. As we know, as a good HTL with high holes mobility, poly (N-vinyl-carbazole) (PVK) is widely used in OLEDs, and zinc selenide (ZnSe) is usually used as the blue emitting material with an energy band gap of 2.7 eV. In this paper, by designing device structure of ITO/PVK/ZnSe/Alq3/Al and varying the thickness of PVK layer and ZnSe layer respectively, we try to find out. Respectively, we try to find out the influences of PVK on the emission of organic–inorganic diodes, and the roles of ZnSe in the device are discussed. 2. Experiments In our experiments, PVK (10 mg/ml in chloroform solution) was spin-coated onto the ITO (sheet resistance
ARTICLE IN PRESS Y. Jiang et al. / Journal of Luminescence 122–123 (2007) 617–619
50 O/sq) which was cleaned by sonication in detergent solution, deionized water, boiling in trichloroethylene, acetone and ethanol. ZnSe film was deposited by electron-beam evaporation at a rate of 1 A˚/s under high vacuum of 6 10 6 Torr and the substrate temperature was kept at 100 1C. The thickness of ZnSe layer was monitored by a quartz crystal thickness monitor placed near the substrate. PL and EL spectra were measured with Spex Fluorolog-3 spectrometer.
1.0
c b
0.8 Intensity (a.u.)
618
a
0.6 0.4
e
0.2
3. Results and discussion
d
4x105
440nm
3x105 Intensity (a.u.)
500 600 Wavelength (nm)
700
Fig. 2. Normalized EL spectrum of device ITO/PVK(80 nm)/ZnSe(100 nm)/ Alq3(15 nm)/Al (curve a) and its PL spectrum under excitation wavelength of 325 nm (curve d), as well as the normalized EL spectra of devices ITO/ Alq3(50 nm)/Al (curve b), ITO/PVK(80 nm)/Al (curve e) and ITO/ PVK(80 nm)/ZnSe(120 nm)/Alq3(15 nm)/Al (curve c).
428nm
1.0
Device D: PVK=100nm Device B: PVK=80nm Device C: PVK=50nm
0.8
0.6
0.4
0.2
0.0 350
400
450
500 550 Wavelength (nm)
600
650
700
Fig. 3. Normalized EL spectra of devices ITO/PVK/ZnSe(100 nm)/ Alq3(15 nm)/Al at the same applied voltage of 23 V.
33V
20V 2x105
1x105
595nm
0
400
400
Intensity (a.u.)
As shown in Fig. 1, a broad band emission peak at 443 nm, with shoulders at 520 and 595 nm, was observed from device ITO/PVK(80 nm)/ZnSe(120 nm)/Alq3(15 nm)/ Al (device A) under a forward applied bias (ITO as the positive electrode) and we believe that it combines emissions from PVK, ZnSe and Alq3 layers. Obviously, electrons injected from cathode recombine with holes injected from anode and radiate light in PVK, ZnSe and Alq3 layers, consequently, as the result of charge carriers transiting over the barriers formed at the ZnSe/Alq3 interface. The holes barrier is 0.7 eV and electrons barrier is 1.6 eV at the PVK/ZnSe interface. Furthermore, the mobility of electrons in ZnSe is much higher than that of holes and the mobility of holes in PVK is smaller than that of electrons in Alq3 [8]. Therefore, the emission from PVK layer is dominant since the recombination region mainly lies in PVK layer, and the combined emission peaks at 443 nm, therefore, we believe ZnSe plays roles of holetransporting layer, electron-transporting layer and emissive layer. Because of its weak fluorescence compared with that of PVK and Alq3, emission from ZnSe layer was too weak to be visible in the EL spectrum. In order to confirm our suggestion, we further reduced the thickness of ZnSe to 100 nm and fabricated device ITO/ PVK(80 nm)/ZnSe(100 nm)/Alq3(15 nm)/Al (device B). As shown in Fig. 2, we can see that its EL spectrum gets
0.0
500
600
700
Wavelength (nm) Fig. 1. EL spectra of device ITO/PVK(80 nm)/ZnSe(120 nm)/Alq3(15 nm)/ Al at different applied voltages.
narrower than that of device A, obviously, it implies the decreased intensity of emission from ZnSe bulk and Alq3 bulk because the thickness of ZnSe layer is reduced and the electric field in the device would redistribute. For comparison, normalized EL spectrum of ITO/Alq3/Al (peak at 520 nm) and ITO/PVK/Al (peak at 415 nm), as well as photoluminescence (PL) spectrum of device B are shown in Fig. 2. The PL of device B shows a maximum peak at 415 nm and a shoulder at 520 nm, emission peak at 415 nm origins from PVK excitons emission and the shoulder at 520 nm origins from Alq3 excitons emission. Furthermore, we changed the thickness of PVK and made other two kinds of devices ITO/PVK(50 nm)/ ZnSe(100 nm)/Alq3 (15 nm)/Al (device C) and ITO/ PVK(100 nm)/ZnSe(100 nm)/Alq3(15 nm)/Al (device D). The normalized EL spectra of devices B, C and D under
ARTICLE IN PRESS Y. Jiang et al. / Journal of Luminescence 122–123 (2007) 617–619
1.6
the stability of hybrid structure device and balance the injections of oppositely charged carriers by increasing electron injection barrier to 1.1 eV. Also, the I–V curve of device B under applied voltages (from 30 to 30 V) is shown in Fig. 4, which shows very good diode rectification characteristics.
-2.5eV -3.0eV
1.4 1.2 Current (mA)
-4.1eV 1.0
-4.6eV
619
-4.2eV
PVK Alq3
0.8 ZnSe
0.6
-6.1eV
0.4
4. Conclusions
-5.7eV -6.8eV
0.2 0.0 -0.2 -30
-20
-10
0
10
20
30
Applied voltage (V) Fig. 4. I–V characteristics of device ITO/PVK(80 nm)/ZnSe(100 nm)/ Alq3(15 nm)/Al. The inset shows the energy diagram of hybrid heterostructure device ITO/PVK/ZnSe/Alq3/Al.
a same applied voltage of 23 V are shown in Fig. 3. From the figure, we can see that all of them peak at 428 nm but the EL spectrum of device C is narrower than that of devices B and D and the EL spectrum of device B is a little broader than that of device D. As we know, the device spectra show the information of recombination region, the emission from PVK is dominant in the EL spectra of device C and no obvious emission from Alq3 layer is observed. For devices B and D, emission from Alq3 can be observed and it is more intensive from device B than that from device D, which is due to the charge carriers redistribution and consequentially electric fields redistribution across the different layers. Because holes can transport through ZnSe into Alq3 layer and electrons transport through ZnSe into PVK layer, oppositely charged carriers will encounter in ZnSe bulk and recombine to emit light. Therefore, it should be injection luminescence in ZnSe bulk. As shown in the energy diagram of ITO/PVK/ZnSe/Alq3/Al (see the inset in Fig. 4), we can see that the holes injection barrier from ITO into PVK is 1.5 eV and the electron injection barrier for electrons from Al into Alq3 is 1.2 eV. Therefore, it is easier for holes to inject into PVK by inserting PVK between the ZnSe and the ITO, in this way, the holes injection barrier reduces 0.7 eV.On the other hand, Alq3 layer between the cathode and the ZnSe layer can improve
In conclusion, influences of PVK on the emission of ITO/PVK/ ZnSe/Alq3/Al are discussed. A broad band emission peaking at 443 nm with shoulders at 520 and 595 nm was observed from the device ITO/PVK(80 nm)/ ZnSe(120 nm)/Alq3(15 nm)/Al under electric fields, and it consists of three parts of emissions from PVK, ZnSe and Alq3 layers. Therefore, injection luminescence in ZnSe layer is deduced. By changing the thickness of PVK layer and that of ZnSe layer, a conclusion can be drawn that PVK not only enhances holes injection but also acts as HTL and emissive layer. Acknowledgements This project was supported by the National Natural Science Foundation of China (no. 60406006 and 10434030), the ‘‘973’’ National Key Basic Research Special Foundation of China (no. 2003CB314707), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and the Science Foundation (no. 2003SM001 and LIJ03004) and the Paper Foundation of Beijing Jiaotong University. References [1] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Nature 374 (1990) 539. [2] V.L. Colvin, M.C. Schlamp, A.P. Allvisatos, Nature 370 (1994) 354. [3] X.H. Yang, X.R. Xu, Appl. Phys. Lett. 77 (2000) 797. [4] J. Kalinowaki, J. Phys. D 32 (1999) 179. [5] P.D. Rack, P.H. Holloway, Mater. Sci. Eng. R 21 (1998) 171. [6] N.D. Kumar, M.P. Joshi, C.S. Friend, P.N. Prasad, R. Burzynski, Appl. Phys. Lett. 71 (10) (1997) 1388. [7] S.Y. Yang, L. Qian, F. Teng, Z. Xu, Y.B. Hou, X.R. Xu, J. Appl. Phys. 97 (2005) 126101. [8] B.J. Chen, S.Y. Liu, Synthetic Met. 91 (1997) 169.