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inorganic heterostructures for enhanced electroluminescence
Solid State Communications 142 (2007) 417–420 www.elsevier.com/locate/ssc
Organic/inorganic heterostructures for enhanced electroluminescence Jinzhao Huang, Zheng Xu ∗ , Suling Zhao, Yuan Li, Fujun Zhang, Lin Song, Yongsheng Wang, Xurong Xu Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, PR China Received 22 October 2006; received in revised form 15 January 2007; accepted 3 March 2007 by V. Pellegrini Available online 12 March 2007
Abstract In this work, organic/inorganic heterostructure electroluminescence devices are fabricated, in which the inorganic layers (ZnO, ZnS) act as the electron transfer layer and the hole blocking layer. Compared to single-organic-layer (MEH-PPV) devices, the heterostructure devices have a significantly enhanced luminous efficiency. The influences of the electric field and the barrier potential on the improvements in performance are analyzed in detail. The results are: (i) the introduction of the inorganic layer makes the injection of electrons easier; (ii) the hole current density is enhanced by the increase of the electric field in the organic layer due to a high dielectric constant of the inorganic layer and accumulation of electrons at the inorganic/organic interface; (iii) the electron–hole current density balance is improved. c 2007 Elsevier Ltd. All rights reserved.
PACS: 73.61.Ga; 71.35.-y; 78.60.Fi; 78.40.Me Keywords: A. Organic/inorganic heterostructure; C. Barrier potential; D. Electroluminescence; D. Electric field
1. Introduction Since the first organic electroluminescent device was proposed by Tang and Van Slyke [1], it has been a subject of intensive research for application in flat panel displays [2– 5]. Low power consumption and long life are essential in such applications, and in recent years a great deal of effort has gone into fabricating more efficient and stable devices [6, 7]. It was recognized that the efficiency of organic lighting emitting devices depends on the efficiency of carrier injection and of the carrier recombination as well as the balance of the hole and electron current densities. In general, the mobility of holes is much bigger than that of electrons in most organic materials, which causes imbalanced carrier injection in organic electroluminescence devices [8]. As a result, the efficiency has been decreased. On the other hand, inorganic semiconductors contain large numbers of carriers and the most important point
∗ Corresponding address: Institute of Optoelectronic Technology, Beijing
Jiaotong University 100044, Beijing, PR China. Tel.: +86 010 51688605; fax: +86 010 51683933. E-mail address: [email protected] (Z. Xu). c 2007 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2007.03.001
is that most inorganic materials have higher electron mobility. Additionally, the intensities of the electric field in organic and inorganic electroluminescence are similar [9]. So it is possible to fabricate a heterostructure electroluminescence device from organic and inorganic materials [10–12]. The organic/inorganic heterostructure electroluminescence device provides a new approach for constructing high performance electroluminescence devices which takes advantage of both the organic and inorganic properties such as the high luminescence efficiency of organic materials and the high carrier density, carrier mobility, steady chemical properties and physical strength of inorganic materials [13,14]. In this work, organic/inorganic heterostructure electroluminescence devices are fabricated. The influences of the electric field and barrier potential on the enhancement are analyzed in detail. 2. Experimental details Five kinds of devices have been fabricated: ITO/MEH-PPV (80 nm)/Al (device A); ITO/MEH-PPV(80 nm)/ZnO(40 nm)/Al (device B); ITO/MEH-PPV(80 nm)/ZnS(40 nm)/Al (device C); ITO/ZnO(40 nm)/Al (device D); ITO/ZnS(40 nm)/Al
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J. Huang et al. / Solid State Communications 142 (2007) 417–420
Fig. 1. The energy level diagram of the devices.
(device E). The energy level diagram of the devices is shown in Fig. 1. Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4phenylenevinylene] (MEH-PPV) was dissolved in chloroform with the concentration 3 mg/ml, and it was fabricated by the spin-coating method and the rotation rate was 2000 RPM (rotations per minute). The inorganic layer was prepared by electron beam evaporation under a high vacuum of 2 × 10−6 Torr and the growth rate is 0.1 nm/s controlled by a quartz crystal thickness monitor. In this process, the substrate temperature was kept at 160 ◦ C. The top Al electrode was prepared by thermal evaporation to about 100 nm, under the vacuum 2 × 10−6 Torr. The electroluminescence spectra were measured with a SPEX Fluorolog-3 spectrometer at room temperature. The optical power and current–voltage characteristics were measured with a U-I-L measurement system (Newport optical power meter 1830C and Keithley source meter 2410).
Fig. 2. Electroluminescence spectra of devices (normalized to the intensity of device C).
3. Results and discussion The electroluminescence spectrum of the devices A, B, and C is shown in Fig. 2. It is observed that the emission only takes place in the organic layer and no emission from the inorganic layer is observed for the devices B and C. Under a forward voltage, the electrons and holes are injected from the cathode (Al) and the anode (ITO) respectively. At the MEH-PPV/inorganic interface, the barrier potential of ZnO and ZnS for holes is about 1.7 eV and 1.9 eV respectively (Fig. 1). That is to say few holes can inject from the MEHPPV layer into the inorganic layer, and the holes are confined to the MEH-PPV layer. Besides, the mobility of electrons in the inorganic layer is higher than that of holes in the MEHPPV layer. So the recombination zone of electrons and holes is primarily restricted to the MEH-PPV layer. This is the reason that the emission is from MEH-PPV and the emission from the inorganic material is not observed in the devices B and C. The current–voltage curve of the devices A, B and C is shown in Fig. 3. The current–voltage characteristics of the devices B and C are similar to that of the device A, but the rectification characteristic of device C is not as good as those of the devices A and B, that is to say the current of device C is lower than those of devices A and B at a given voltage. The current of the heterostructure device is much less than that of the single-inorganic-layer devices D and E at a given voltage (Fig. 4). This means that the current–voltage characteristic of the heterostructure device is dominated by the organic layer.
Fig. 3. Current–voltage characteristic of the devices.
Fig. 4. Current–voltage characteristic of the devices.
That is to say, although the inorganic layer contains large numbers of electrons, the conductance of the heterostructure device is dominated by holes. Fig. 5 is the normalized luminous efficiency versus voltage of the devices B and C (normalized to the luminous efficiency of device A). It is found that the luminous efficiencies of the heterostructure devices B and C are higher than that of device A; in
J. Huang et al. / Solid State Communications 142 (2007) 417–420
JCt
419
3 3 2 2 = (Jh1 × Jh2 ) ∝ exp −A(ϕ1 + ϕ2 ) = exp [−1.32A] . (5)
From Eqs. (3)–(5), we find that the introduction of inorganic material makes the injection of electrons easier. From Figs. 3 and 4, it is observed that the current of the heterostructure device is much lower than that of the singleinorganic-layer device. That is to say, the organic layer has a barrier function for the electrons, and there is accumulation of electrons at the inorganic/organic interface. According to Ref. [16], dE h (t) ∝ −{[Jh (t) − Jh∗ (t)] − [Je (t) − Je∗ (t)]} dt Fig. 5. Normalized luminous efficiency versus voltage for devices (normalized to the luminous efficiency of device A).
particular, device B has higher luminous efficiency than device C when the device has applied a forward bias voltage. Obviously the introduction of the inorganic layer into the organic electroluminescence device greatly enhances the efficiency. We believe that the electric field and the barrier potential play a critical role in the enhancement of organic electroluminescence using inorganic material. Now we will give a detailed analysis of the enhancement as regards the two aspects mentioned above. According to the Fowler–Nordheim equation, the influence of the barrier potential on the injection of electrons from the cathode is estimated [15] as # " 3 1 8π(2m ∗ ) 2 ϕ 2 (1) Jt ∝ exp − 3hq E where Jt is the tunneling current density, h is the Planck constant, E is the electric field, ϕ is the zero-field barrier height, m ∗ is the effective mass of the carrier, q is the electron charge. 1
In the case of A = h i 3 Jt ∝ exp −Aϕ 2 .
8π(2m ∗ ) 2 3hq E
, Eq. (1) can be expressed as (2)
For device A, ϕ = 1.4 eV, # " 3 1 h i 3 8π(2m ∗ ) 2 ϕ 2 = exp −Aϕ 2 J At ∝ exp − 3hq E = exp [−1.67A] .
(3)
When the inorganic layer is introduced, the process of electron injection is separated into two steps: 3 3 Jt1 ∝ exp −Aϕ12 and Jt2 ∝ exp −Aϕ22 . For device B, ϕ1 = 0.8 eV, ϕ2 = 0.6 eV, 3 3 J Bt = (Jh1 × Jh2 ) ∝ exp −A(ϕ12 + ϕ22 ) = exp [−1.18A] . (4) For device C, ϕ1 = 1.1 eV ϕ2 = 0.3 eV,
(6)
where E h is the electric field in the hole transporting region, Jh is the injection current density of holes, Jh∗ is the leakage current density of holes, Je is the injection current density of electrons, Je∗ is the leakage current density of electrons. From Eq. (6) mentioned above, it is found that the accumulation of electrons at the inorganic/organic interface causes an enhancement of the anodic electric field. The intensity of the electric field can be calculated using Maxwell’s equation [17]: Ei =
εo Vtot εo di + εi do
(7)
where ε is the dielectric constant, d is the layer thickness, i stands for inorganic, o stands for organic, Vtot is the voltage across the total device. The dielectric constants for MEH-PPV, ZnO, ZnS are 2.50, 8.15 and 8.60 respectively [18]. According to the Eq. (7), the calculation results show that the intensity of the electric field in the organic layer is more than three times higher than that in the inorganic layer. It is known that the electric field dependent carrier mobility has the Poole–Frenkel form [19] √ µ = µ0 exp(α E). Here, µ0 is the zero-field mobility, α is the electric field dependent parameter, and E is the electric field. Obviously, the hole current density can be enhanced by the increase of the electric field in the organic layer due to a high dielectric constant of the inorganic layer and accumulation of electrons at the inorganic/organic interface. The electric field and the barrier potential play critical roles in the enhancement of organic electroluminescence using inorganic material, and they can affect each other. This influence tends to compensate for imbalanced electron–hole current densities due to different energy barriers and electric fields, and causes an increase of luminescence. 4. Conclusions On the basis of organic/inorganic heterostructure electroluminescence devices, we obtain significantly enhanced luminous efficiency at a given voltage compared to that for a single organic layer (MEH-PPV). The influence of the electric field and
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the barrier potential on the improvements in performance were analyzed in detail. The results show that the introduction of inorganic material makes the injection of electrons from the cathode into the organic layer easier, and the hole current density is enhanced by the increase of the electric field strength in the organic layer due to a high dielectric constant of the inorganic layer and accumulation of electrons at the inorganic/organic interface. Taking advantage of the two aspects mentioned above, the electron–hole current density balance is improved and then the efficiency is enhanced. Acknowledgements This work was supported by the National Natural Science Foundation of China (60576016), the National Key Basic Research Foundation of China (2003CB314707), the National High Technology Research and Development Program of China (2006AA03Z0412), the Beijing Natural Science Foundation of China (2073030), the Key Item of the National Natural Science Foundation of China (10434030), the Excellent Doctor’s Science and Technology Innovation Foundation of Beijing Jiaotong University (48010). References [1] C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913.
[2] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Br´edas, M. L¨ogdlund, W.R. Salaneck, Nature 397 (1999) 121. [3] I.H. Campbell, B.K. Crone, Appl. Phys. Lett. 88 (2006) 172113. [4] J.R. Sheats, J. Mater. Res. 19 (2004) 1974. [5] H.S. Yang, W.F. Xie, Y. Zhao, J.Y. Hou, S.Y. Liu, Solid State Commun. 139 (2006) 468. [6] Z.J. Wang, Y. Wu, Y.C. Zhou, J. Zhou, S.T. Zhang, X.M. Ding, X.Y. Hou, Z.Q. Zhu, Appl. Phys. Lett. 88 (2006) 222112. [7] G. Cheng, S. Qiu, F. Li, J. Feng, Y.G. Ma, S.Y. Liu, Jpn. J. Appl. Phys., Part 2 42 (2003) L376. [8] Y. Qiu, Y.D. Gao, P. Wei, L.D. Wang, Appl. Phys. Lett. 80 (2002) 2628. [9] 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. [10] F.S. Li, Z.J. Chen, W. Wei, Q.H. Gong, Org. Electron. 6 (2005) 237. [11] W. Rieß, H. Riel, P.F. Seidler, H. Vestweber, Synth. Met. 99 (1999) 213. [12] X.H. Yang, Y.L. Hua, F. Teng, Y.B. Hou, X.R. Xu, Z.H. Huang, Chin. Phys. Lett. 14 (1997) 946. [13] V.L. Colvin, M.C. Schlamp, A.P. Allvisatos, Nature 370 (1994) 354. [14] S. Coe, W.-K. Woo, M. Bawendi, V. Bulovic, Nature 420 (2002) 800. [15] C.C. Lee, M.Y. Chang, Y.D. Jong, T.W. Huang, C.S. Chu, Y. Chang, Jpn. J. Appl. Phys. 43 (2004) 7560. [16] D.V. Khramtchenkov, H. B¨assler, V.I. Arkhipov, J. Appl. Phys. 79 (1996) 9283. [17] W.G. Yu, Z. Xu, F. Teng, S.Y. Yang, Y.B Hou, L. Qian, C. Qu, S.Y. Quan, X.R. Xu, Phys. Lett. A 338 (2005) 402. [18] T. Yamamoto, Jpn. J. Appl. Phys., Part 2 42 (2003) L514. [19] F.J. Zhang, Z. Xu, S.L. Zhao, L. Liu, B. Sun, J. Pei, Physica B 381 (2006) 256.
Organic/inorganic heterostructures for enhanced electroluminescence Jinzhao Huang, Zheng Xu ∗ , Suling Zhao, Yuan Li, Fujun Zhang, Lin Song, Yongsheng Wang, Xurong Xu Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, PR China Received 22 October 2006; received in revised form 15 January 2007; accepted 3 March 2007 by V. Pellegrini Available online 12 March 2007
Abstract In this work, organic/inorganic heterostructure electroluminescence devices are fabricated, in which the inorganic layers (ZnO, ZnS) act as the electron transfer layer and the hole blocking layer. Compared to single-organic-layer (MEH-PPV) devices, the heterostructure devices have a significantly enhanced luminous efficiency. The influences of the electric field and the barrier potential on the improvements in performance are analyzed in detail. The results are: (i) the introduction of the inorganic layer makes the injection of electrons easier; (ii) the hole current density is enhanced by the increase of the electric field in the organic layer due to a high dielectric constant of the inorganic layer and accumulation of electrons at the inorganic/organic interface; (iii) the electron–hole current density balance is improved. c 2007 Elsevier Ltd. All rights reserved.
PACS: 73.61.Ga; 71.35.-y; 78.60.Fi; 78.40.Me Keywords: A. Organic/inorganic heterostructure; C. Barrier potential; D. Electroluminescence; D. Electric field
1. Introduction Since the first organic electroluminescent device was proposed by Tang and Van Slyke [1], it has been a subject of intensive research for application in flat panel displays [2– 5]. Low power consumption and long life are essential in such applications, and in recent years a great deal of effort has gone into fabricating more efficient and stable devices [6, 7]. It was recognized that the efficiency of organic lighting emitting devices depends on the efficiency of carrier injection and of the carrier recombination as well as the balance of the hole and electron current densities. In general, the mobility of holes is much bigger than that of electrons in most organic materials, which causes imbalanced carrier injection in organic electroluminescence devices [8]. As a result, the efficiency has been decreased. On the other hand, inorganic semiconductors contain large numbers of carriers and the most important point
∗ Corresponding address: Institute of Optoelectronic Technology, Beijing
Jiaotong University 100044, Beijing, PR China. Tel.: +86 010 51688605; fax: +86 010 51683933. E-mail address: [email protected] (Z. Xu). c 2007 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2007.03.001
is that most inorganic materials have higher electron mobility. Additionally, the intensities of the electric field in organic and inorganic electroluminescence are similar [9]. So it is possible to fabricate a heterostructure electroluminescence device from organic and inorganic materials [10–12]. The organic/inorganic heterostructure electroluminescence device provides a new approach for constructing high performance electroluminescence devices which takes advantage of both the organic and inorganic properties such as the high luminescence efficiency of organic materials and the high carrier density, carrier mobility, steady chemical properties and physical strength of inorganic materials [13,14]. In this work, organic/inorganic heterostructure electroluminescence devices are fabricated. The influences of the electric field and barrier potential on the enhancement are analyzed in detail. 2. Experimental details Five kinds of devices have been fabricated: ITO/MEH-PPV (80 nm)/Al (device A); ITO/MEH-PPV(80 nm)/ZnO(40 nm)/Al (device B); ITO/MEH-PPV(80 nm)/ZnS(40 nm)/Al (device C); ITO/ZnO(40 nm)/Al (device D); ITO/ZnS(40 nm)/Al
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J. Huang et al. / Solid State Communications 142 (2007) 417–420
Fig. 1. The energy level diagram of the devices.
(device E). The energy level diagram of the devices is shown in Fig. 1. Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4phenylenevinylene] (MEH-PPV) was dissolved in chloroform with the concentration 3 mg/ml, and it was fabricated by the spin-coating method and the rotation rate was 2000 RPM (rotations per minute). The inorganic layer was prepared by electron beam evaporation under a high vacuum of 2 × 10−6 Torr and the growth rate is 0.1 nm/s controlled by a quartz crystal thickness monitor. In this process, the substrate temperature was kept at 160 ◦ C. The top Al electrode was prepared by thermal evaporation to about 100 nm, under the vacuum 2 × 10−6 Torr. The electroluminescence spectra were measured with a SPEX Fluorolog-3 spectrometer at room temperature. The optical power and current–voltage characteristics were measured with a U-I-L measurement system (Newport optical power meter 1830C and Keithley source meter 2410).
Fig. 2. Electroluminescence spectra of devices (normalized to the intensity of device C).
3. Results and discussion The electroluminescence spectrum of the devices A, B, and C is shown in Fig. 2. It is observed that the emission only takes place in the organic layer and no emission from the inorganic layer is observed for the devices B and C. Under a forward voltage, the electrons and holes are injected from the cathode (Al) and the anode (ITO) respectively. At the MEH-PPV/inorganic interface, the barrier potential of ZnO and ZnS for holes is about 1.7 eV and 1.9 eV respectively (Fig. 1). That is to say few holes can inject from the MEHPPV layer into the inorganic layer, and the holes are confined to the MEH-PPV layer. Besides, the mobility of electrons in the inorganic layer is higher than that of holes in the MEHPPV layer. So the recombination zone of electrons and holes is primarily restricted to the MEH-PPV layer. This is the reason that the emission is from MEH-PPV and the emission from the inorganic material is not observed in the devices B and C. The current–voltage curve of the devices A, B and C is shown in Fig. 3. The current–voltage characteristics of the devices B and C are similar to that of the device A, but the rectification characteristic of device C is not as good as those of the devices A and B, that is to say the current of device C is lower than those of devices A and B at a given voltage. The current of the heterostructure device is much less than that of the single-inorganic-layer devices D and E at a given voltage (Fig. 4). This means that the current–voltage characteristic of the heterostructure device is dominated by the organic layer.
Fig. 3. Current–voltage characteristic of the devices.
Fig. 4. Current–voltage characteristic of the devices.
That is to say, although the inorganic layer contains large numbers of electrons, the conductance of the heterostructure device is dominated by holes. Fig. 5 is the normalized luminous efficiency versus voltage of the devices B and C (normalized to the luminous efficiency of device A). It is found that the luminous efficiencies of the heterostructure devices B and C are higher than that of device A; in
J. Huang et al. / Solid State Communications 142 (2007) 417–420
JCt
419
3 3 2 2 = (Jh1 × Jh2 ) ∝ exp −A(ϕ1 + ϕ2 ) = exp [−1.32A] . (5)
From Eqs. (3)–(5), we find that the introduction of inorganic material makes the injection of electrons easier. From Figs. 3 and 4, it is observed that the current of the heterostructure device is much lower than that of the singleinorganic-layer device. That is to say, the organic layer has a barrier function for the electrons, and there is accumulation of electrons at the inorganic/organic interface. According to Ref. [16], dE h (t) ∝ −{[Jh (t) − Jh∗ (t)] − [Je (t) − Je∗ (t)]} dt Fig. 5. Normalized luminous efficiency versus voltage for devices (normalized to the luminous efficiency of device A).
particular, device B has higher luminous efficiency than device C when the device has applied a forward bias voltage. Obviously the introduction of the inorganic layer into the organic electroluminescence device greatly enhances the efficiency. We believe that the electric field and the barrier potential play a critical role in the enhancement of organic electroluminescence using inorganic material. Now we will give a detailed analysis of the enhancement as regards the two aspects mentioned above. According to the Fowler–Nordheim equation, the influence of the barrier potential on the injection of electrons from the cathode is estimated [15] as # " 3 1 8π(2m ∗ ) 2 ϕ 2 (1) Jt ∝ exp − 3hq E where Jt is the tunneling current density, h is the Planck constant, E is the electric field, ϕ is the zero-field barrier height, m ∗ is the effective mass of the carrier, q is the electron charge. 1
In the case of A = h i 3 Jt ∝ exp −Aϕ 2 .
8π(2m ∗ ) 2 3hq E
, Eq. (1) can be expressed as (2)
For device A, ϕ = 1.4 eV, # " 3 1 h i 3 8π(2m ∗ ) 2 ϕ 2 = exp −Aϕ 2 J At ∝ exp − 3hq E = exp [−1.67A] .
(3)
When the inorganic layer is introduced, the process of electron injection is separated into two steps: 3 3 Jt1 ∝ exp −Aϕ12 and Jt2 ∝ exp −Aϕ22 . For device B, ϕ1 = 0.8 eV, ϕ2 = 0.6 eV, 3 3 J Bt = (Jh1 × Jh2 ) ∝ exp −A(ϕ12 + ϕ22 ) = exp [−1.18A] . (4) For device C, ϕ1 = 1.1 eV ϕ2 = 0.3 eV,
(6)
where E h is the electric field in the hole transporting region, Jh is the injection current density of holes, Jh∗ is the leakage current density of holes, Je is the injection current density of electrons, Je∗ is the leakage current density of electrons. From Eq. (6) mentioned above, it is found that the accumulation of electrons at the inorganic/organic interface causes an enhancement of the anodic electric field. The intensity of the electric field can be calculated using Maxwell’s equation [17]: Ei =
εo Vtot εo di + εi do
(7)
where ε is the dielectric constant, d is the layer thickness, i stands for inorganic, o stands for organic, Vtot is the voltage across the total device. The dielectric constants for MEH-PPV, ZnO, ZnS are 2.50, 8.15 and 8.60 respectively [18]. According to the Eq. (7), the calculation results show that the intensity of the electric field in the organic layer is more than three times higher than that in the inorganic layer. It is known that the electric field dependent carrier mobility has the Poole–Frenkel form [19] √ µ = µ0 exp(α E). Here, µ0 is the zero-field mobility, α is the electric field dependent parameter, and E is the electric field. Obviously, the hole current density can be enhanced by the increase of the electric field in the organic layer due to a high dielectric constant of the inorganic layer and accumulation of electrons at the inorganic/organic interface. The electric field and the barrier potential play critical roles in the enhancement of organic electroluminescence using inorganic material, and they can affect each other. This influence tends to compensate for imbalanced electron–hole current densities due to different energy barriers and electric fields, and causes an increase of luminescence. 4. Conclusions On the basis of organic/inorganic heterostructure electroluminescence devices, we obtain significantly enhanced luminous efficiency at a given voltage compared to that for a single organic layer (MEH-PPV). The influence of the electric field and
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the barrier potential on the improvements in performance were analyzed in detail. The results show that the introduction of inorganic material makes the injection of electrons from the cathode into the organic layer easier, and the hole current density is enhanced by the increase of the electric field strength in the organic layer due to a high dielectric constant of the inorganic layer and accumulation of electrons at the inorganic/organic interface. Taking advantage of the two aspects mentioned above, the electron–hole current density balance is improved and then the efficiency is enhanced. Acknowledgements This work was supported by the National Natural Science Foundation of China (60576016), the National Key Basic Research Foundation of China (2003CB314707), the National High Technology Research and Development Program of China (2006AA03Z0412), the Beijing Natural Science Foundation of China (2073030), the Key Item of the National Natural Science Foundation of China (10434030), the Excellent Doctor’s Science and Technology Innovation Foundation of Beijing Jiaotong University (48010). References [1] C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913.
[2] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Br´edas, M. L¨ogdlund, W.R. Salaneck, Nature 397 (1999) 121. [3] I.H. Campbell, B.K. Crone, Appl. Phys. Lett. 88 (2006) 172113. [4] J.R. Sheats, J. Mater. Res. 19 (2004) 1974. [5] H.S. Yang, W.F. Xie, Y. Zhao, J.Y. Hou, S.Y. Liu, Solid State Commun. 139 (2006) 468. [6] Z.J. Wang, Y. Wu, Y.C. Zhou, J. Zhou, S.T. Zhang, X.M. Ding, X.Y. Hou, Z.Q. Zhu, Appl. Phys. Lett. 88 (2006) 222112. [7] G. Cheng, S. Qiu, F. Li, J. Feng, Y.G. Ma, S.Y. Liu, Jpn. J. Appl. Phys., Part 2 42 (2003) L376. [8] Y. Qiu, Y.D. Gao, P. Wei, L.D. Wang, Appl. Phys. Lett. 80 (2002) 2628. [9] 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. [10] F.S. Li, Z.J. Chen, W. Wei, Q.H. Gong, Org. Electron. 6 (2005) 237. [11] W. Rieß, H. Riel, P.F. Seidler, H. Vestweber, Synth. Met. 99 (1999) 213. [12] X.H. Yang, Y.L. Hua, F. Teng, Y.B. Hou, X.R. Xu, Z.H. Huang, Chin. Phys. Lett. 14 (1997) 946. [13] V.L. Colvin, M.C. Schlamp, A.P. Allvisatos, Nature 370 (1994) 354. [14] S. Coe, W.-K. Woo, M. Bawendi, V. Bulovic, Nature 420 (2002) 800. [15] C.C. Lee, M.Y. Chang, Y.D. Jong, T.W. Huang, C.S. Chu, Y. Chang, Jpn. J. Appl. Phys. 43 (2004) 7560. [16] D.V. Khramtchenkov, H. B¨assler, V.I. Arkhipov, J. Appl. Phys. 79 (1996) 9283. [17] W.G. Yu, Z. Xu, F. Teng, S.Y. Yang, Y.B Hou, L. Qian, C. Qu, S.Y. Quan, X.R. Xu, Phys. Lett. A 338 (2005) 402. [18] T. Yamamoto, Jpn. J. Appl. Phys., Part 2 42 (2003) L514. [19] F.J. Zhang, Z. Xu, S.L. Zhao, L. Liu, B. Sun, J. Pei, Physica B 381 (2006) 256.