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Improved performance of mixed single layer top-emission organic light emitting devices using capping layer
Solid-State Electronics 56 (2011) 155–158
Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Improved performance of mixed single layer top-emission organic light emitting devices using capping layer Zhaokui Wang ⇑, Shigeki Naka, Hiroyuki Okada Graduate School of Science & Technology, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
a r t i c l e
i n f o
Article history: Received 10 June 2010 Received in revised form 29 September 2010 Accepted 2 November 2010 Available online 24 November 2010 The review of this paper was arranged by Dr. Y. Kuk Keywords: Top-emission organic light emitting device (TEOLED) Mixed single layer Capping layer Viewing angle dependence
a b s t r a c t The performance of top-emission organic light emitting devices (TEOLEDs) can be improved by using a thin capping layer on top of the semitransparent metal electrode. We investigated the emission properties of inverted mixed single layer TEOLEDs with the same device structure but different capping materials. The thickness of capping layer was optimized by calculation. The power efficiency of device was 2.5 times enhanced when 45 nm TPD capping layer was added. The enhancement is not simply dependent on the transmittance and reflectance of the top contact, but also on other complex phenomena such as the interference effects in the device. The results of properties and dependence of EL spectra on viewing angle for all devices indicated that the large enhancement factor may be related to the complex interference phenomenon in our mixed single layer devices due to the emitter center and recombination region is different from conventional heterojunction devices. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Organic light emitting devices (OLEDs) are currently one of the most promising technologies since their future application in large area flat panel displays. However, the currently using rather expensive glass substrate with transparent electrode materials such as indium tin oxide (ITO) are a disadvantage of the usual bottom emitting OLEDS which emit through the substrate. To overcome this issue, it would be preferable to fabricate top-emitting organic light emitting devices (TEOLEDs) on opaque and flexible substrates such as metal foils. In addition, TEOLEDs are intensively investigated due to their large aperture ratio, high pixel resolution, and high information content in active matrix displays [1–4]. During fabrication process of TEOLEDs, metal films are usually used as the semitransparent top electrode because of the advantage of no damage to the organic layers during the thermal deposition [5–8]. However, the light outcoupling was usually limited by the poor transparence of the metal films. This issue can be solved by depositing a light outcoupling layer onto the top metal electrode [5,9–12]. Actually, the capping effect of dielectric thin films is not simply dependent on the transmittance of the top contact, but rather on the overall performance of the device. Recently, a mixed organic layer OLED has attracted wide attention because it ⇑ Corresponding author. Tel.: +81 76 445 6731. E-mail address: [email protected] (Z. Wang). 0038-1101/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2010.11.007
potentially provides an approach to improve the device performance [13–17]. Particularly, Liu et al. [18] investigated the carrier mobility in mixed layer and found that it was sensitive to the compositional fraction of organic materials, which presents direct evidences on the role of the mixed layer in these devices. In this study, we investigated the effects of different capping layers on the performance of inverted TEOLEDs with a mixed single layer by mixing of electron transport materials tris-(8-hydroxy-quinoline) aluminum (Alq3) and 2,5-bis(60 -(20 ,200 -bipyridyl))-1,1-dimethyl3,4-diphenylsilole (PyPySPyPy), hole transport material 4,40 -bis[N-(1-napthyl)-N-phenyl-amion] biphenyl (a-NPD) and dopant material 5,6,11,12-tetraphenylnaphthacene (rubrene).
2. Experiment An aluminum–neodymium alloy (AlNd, Al:Nd = 98:2, Kobelco Research Institute) is selected as cathode in order to flatten the electrode surface and avoid the short-circuiting [19]. Gold is used as a semitransparent anode. The mixed single layer TEOLED had a structure of Glass/AlNd (50 nm)/PyPySPyPy + Alq3 + a-NPD + rubrene (100 nm)/MoO3 (50 nm)/Au (20 nm). Glass substrate used is alkaline earth boro-aluminosilicate glass (Corning 1737). The mixing ratio of PyPySPyPy, Alq3, a-NPD and rubrene is optimized with a ratio of PyPySPyPy:Alq3:a-NPD:rubrene = 25:50:25:1. Mixed organic materials are evaporated at a pressure of about 4 106 Torr at a
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Z. Wang et al. / Solid-State Electronics 56 (2011) 155–158 0
(a)
3. Results and Discussion
1 Alq3 MoO3 TPD
MoO3 50nm/Au 20nm/Capping layer x nm
0.8
T, R, A
T 0.6
0.4
R
0.2
A
0 0
20
40
60
80
100
Cappling layer thickness (nm) Fig. 1. Calculated transmittance (T), reflectance (R) and absorptance (A) of the top contact (MoO3 50 nm/Au 20 nm/capping layer x nm) at a wavelength of 560 nm depending on the capping layer thickness.
102 101 100 10-1 10-2 Without Capping Layer With Alq 50nm 3 With MoO 40nm 3 With TPD 45nm
10-3 10-4 10-5 10-6 10-7
0
10
20
Applied voltage V (V)
(b) Luminance L (cd/m2)
105 104 103 102 101 Without Capping Layer With Alq3 50nm With MoO3 40nm With TPD 45nm
100 10-1 -2 10
10-1
100
Current density
(C)
10
EL efficiency η (lm/W)
TEOLEDs usually show stronger microcavity effects compared to bottom-emission OLEDs due to its top-emission mechanism [20–23]. For our mixed single layer TEOLED, it contains a reflective bottom electrode and a semitransparent top electrode for light outcoupling. This may be considered as a Fabry–Perot cavity embedded with a source. To get maximal luminance from TEOLEDs, the highest possible reflectance R1 for bottom electrode and a reflectance R2 for top electrode matching R1 should be adopted. In view of this, it should be made the top electrode as transparent as possible. This can be realized by depositing a capping layer on the top metal electrode. Here, we used Alq3 (n = 1.7), TPD (n = 1.9) and MoO3 (n = 2.1) as capping layer. Fig. 1 shows the calculated transmittance (T), reflectance (R) and absorptance (A) of top contact capped with different thin films with varied thickness at 560 nm for rubrene emitting wavelength. We can see that maximum transmittance and minimum reflectance corresponding to capping layer Alq3, TPD and MoO3 reaches to 0.758, 0.091 at 50 nm, 0.775, 0.078 at 45 nm, and 0.801, 0.032 at 40 nm, respectively. With the equation of A = (1TR), the absorptance of three materials can be calculated easily as a result of 0.151, 0.147 and 0.167 corresponding to capping layer Alq3, TPD and MoO3. Compared with transmittance 0.471 and reflectance 0.434 in the case of no capping layer, it is obvious that the optical properties of top contact are largely modulated with the thickness of any capping materials. Capping layers with higher refractive indices usually have the advantage of giving stronger modulation in optical characteristics and thus possibly higher luminance enhancement. Experiments were conducted on mixed single layer TEOLED with above optimized capping layer thickness. Fig. 2a–c shows the current density vs voltage (J–V), the luminance vs current density (L–J), and the power efficiency vs current density (g–J) characteristics of mixed single layer TEOLED capped with and without different thin films. Table 1 lists the comparison of TEOLEDs performance with and without capping layer. It is clearly seen that the capping layer has a significant influence on the power effi-
103
Current density J (mA/cm2)
rate of 1–3 Å A/s. The AlNd cathode and Au anode were evaporated using a metal mask. The device area was 2 2 mm2. Device characteristics were measured using semiconductor parameter analyzer (HP 4155B) and luminance meter (Topcon BM-3). All measurements were carried out in air at room temperature. The optical properties of top contact were calculated using Optical Spectrum Simulation Software SCOUT of Techno-Synergy, Inc.
101
102
103
(mA/cm2)
2
101 100 10-1
Without Capping Layer With Alq 3 50nm With MoO3 40nm With TPD 45nm
10-2 10-3 10-2
10-1
100
Current density
101
102
103
(mA/cm2)
Fig. 2. (a) Current density vs voltage (J–V), (b) luminance vs current density (L–J) and (c) power efficiency vs current density (g–J) characteristics of mixed single layer TEOLED with different capping layers.
ciency and luminance of the TEOLED. The device without capping layer shows a power efficiency of 0.26 lm/W and luminance 1500 cd/m2 at 100 mA/cm2. With 45 nm TPD capping layer applied, the power efficiency and luminance at 100 mA/cm2 reached 0.78 lm/W and 3700 cd/m2, respectively. The device performance was improved significantly, showing the maximum enhancement by a factor of 2.5. From Fig. 2a, the J–V characteristics of the devices with and without capping layers are almost identical, e.g., the driving voltage of devices at 100 mA/cm2 is 17.8 V, 17.6 V, 17.2 V and 14.8 V corresponding to devices without capping layer, with Alq3, with MoO3 and with TPD, respectively. The driving voltage of the mixed single layer device is larger high than that of the heterostructure device, which is consistent with our previous report [24]. The increase of the operational voltage in the mixed single layer device may be related to the larger hopping distance for hole transport with respect to heterostructure device [25]. Another possible reason of decrease of current density is the decrease of hole transport at Au/mixed layer because of mixing hole transport material into mixed single layer. There is not nearly large difference for the driving voltage in four devices with mixed single layer
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Z. Wang et al. / Solid-State Electronics 56 (2011) 155–158 Table 1 Electrical and optical characteristics of mixed single layer TEOLEDs with and without different capping layer. Sample
V (V) @ 100 mA/cm2
L (cd/m2) @ 100 mA/cm2
L (cd/m2) Max
g (lm/W) @ 100 mA/cm2
g (lm/W)
Without capping layer With Alq3 (50 nm) With MoO3 (40 nm) With TPD (45 nm)
17.8 17.6 17.2 14.8
1500 2120 2520 3700
7000 8520 10,770 20,000
0.26 0.38 0.45 0.78
3.45 5.30 6.20 7.52
excluding the device with TPD capping layer. The lesser difference in J–V characteristics in these devices is supposed to little variations on the thickness of the mixed layer due to asynchronous experiments as reported by Kalinowski [26]. With capping layer of 45 nm TPD, the driving voltage of device is decreased with 20.2%. While, from Fig. 2b and c, and Table 1, the luminance and power efficiency at 100 mA/cm2 is improved with 146.7% and with 200.0%, respectively. This proves that the enhancement of optical outcoupling is mainly associated with the capping thin films with high refractive index. More noticeable, with optimized thickness, the device with 40 nm MoO3 capping layer only shows an enhancement factor of 1.7 although the top contact with it has the highest transmittance and lowest reflectance as shown in Fig. 1. It is clearly seen that the largest enhancement is achieved in the case of top contact having the lowest absorption, which is consistent with the result form other groups [27,28]. Obviously, the enhancement is not simply dependent on the transmittance and reflectance of the top contact, but also on other complex phenomena such as the interference effects in the device. It is well known that the efficiency enhancement of OLEDs by capping layer could be attributed either to the redistribution of the emitted light, or to the improved outcoupling efficiency as a result of modified optical structure by the capping layer [29]. For our mixed single layer TEOLED, the emitter center and recombination region is different from conventional heterojunction device, resulting in more complex interference phenomenon. This may be one reason why our mixed single layer device with capping layer has larger enhancement factor compared to conventional heterojunction device. In addition, the wavelength dependence of transmittance for all devices with and without different capping layer is shown in Fig. 3. We can see that device transmittance is improved when different capping layer is added. The transmittance is strongly dependent on the wavelength in visible light. The difference of transmittance between three devices capped different materials is small when wavelength less than 500 nm. Up to 500 nm, the light outcoupling effects of MoO3 is preceded to other two materials. The grey color of MoO3 material may be another rea-
son of higher device efficiency happening with TPD capping layer although its refractive index is smaller than that of MoO3. The electroluminescence (EL) spectrum of device also exhibits a large dependence on the capping layer as shown in Fig. 4. By applying capping layer, the position of the peak and shoulder remains almost unchanged. However, the full width at half maximum (FWHM) of device EL spectrum is changed. Without capping layer, FWHM of EL spectrum is 42 nm. With optimized capping layer applied, FWHM increases and reaches 72 nm when 45 nm TPD films capped, which agrees with the result from other groups [11,12]. The FWHM can be estimated by [30,31]
FWHM ¼
pffiffiffiffiffiffiffiffiffiffi k2 1 R1 R2 ; 2L pðR1 R2 Þ1=4
ð1Þ
where L is the optical length between the two mirrors for a microcavity and here is represented by the thickness of mixed single organic layer. As seen in Fig. 1, the reflectance (R2) of the top contact is decreased largely when optimized capping layer is applied. This, in a certain degree, weakens the microcavity effect, resulting in larger FWHM of the EL spectrum. Fig. 5 depicts the angular distributions of EL intensity (normalized to the 0° intensity of device without capping layer) for four devices. Because of the microcavity effect in TEOLEDs, the emission profiles of all devices are not Lambertian. The devices with capping layer show more directed emission and enhanced luminance in the range of 0°–30°. Such enhancement leads to increased forwarddirection luminance and efficiency, as can be seen in Fig. 2 and Table 1. Representatively, Fig. 6a and b shows the EL spectra at viewing angles of 0°, 20°, 40°, 60°, and 80° off the surface normal for the mixed single layer TEOLED without and with 45 TPD capping layer, respectively. As can be seen in Fig. 5 and insets, device without capping layer shows a large change in color (red-shift) with varying observation angle from 0° (0.44, 0.53) to 80° (0.48, 0.46). Device with 45 nm TPD capping layer exhibits relatively
1
Transmittance
0.8 0.6 0.4 Without cap layer With Alq 3 50 nm With MoO 3 45 nm With TPD 40 nm
0.2 0
400
500
600
Without cappling layer With TPD 45 nm With MoO3 40 nm With Alq3 50 nm
EL intensity (arb. units)
1
700
Wavelength λ (nm) Fig. 3. The wavelength dependence of devices transmittance with and without different capping layer.
Max
0 500
550
600
650
700
750
Wavelength λ (nm) Fig. 4. The EL spectra of mixed single layer TEOLEDs with and without capping layer.
Z. Wang et al. / Solid-State Electronics 56 (2011) 155–158
EL Internsity (arb.units)
158
by redistribution of the emitted light duo to the different interference effects.
0
1.2 30
1
30
4. Conclusions
0.8 0.6 60
60
0.4 0.2 90
0 Without Capping Layer With 50 nm Alq3 With 40 nm MoO3 With 45 nm TPD
Fig. 5. The angular distribution of EL intensity for mixed single layer TEOLEDs with and without capping layer.
(a) 00
EL intensity (arb. units)
1
Acknowledgements
0.6
CIE y
200 400
This work was supported by regional innovation of R&D projects, overseen by the Ministry of Economy, Trade and Industry (METI), Japan. We also thank Kobelco Research Institute for providing AlNd and Chisso Corporation for supplying PyPySPyPy.
00
0.5 800
0.4 0.4
600
0.5 CIE x
0.6
References [1] [2] [3] [4] [5] [6]
800
0 500
550
In summary, we have investigated the effects of capping layers on the performance of inverted TEOLEDs with a mixed single layer. It is demonstrated that the luminance and power efficiency in forward direction of devices applying capping layer can be improved significantly. Especially, the device with 45 nm TPD capping layer shows an enhancement factor of 2.5. The results of properties and dependence of EL spectra on viewing angle for all devices indicate that the large enhancement factor may be attributed to redistribution of the emitted light because of complex interference phenomenon in our mixed single layer device, in which the emitter center and recombination region is different from conventional heterojunction device. More details research is necessary in the future.
600
650
700
750
Wavelength λ (nm)
(b)
[7] [8] [9] [10]
20
0
0.6
CIE y
EL intensity (arb. units)
1
00
0.5
[11]
800
0.4 0.4
400
[12] [13]
00
0.5 CIE x
[14] [15] [16] [17]
0.6
[18] [19] [20]
600 800
[21]
0 500
550
600
650
700
750
Wavelength λ (nm) Fig. 6. EL spectra at varying viewing angles for (a) without capping layer and (b) with 45 nm TPD capping layer. Inset: CIE color coordinates (0°–80°) belonging to the EL spectra.
[22] [23] [24] [25] [26] [27]
stable color coordinates originated from rubrene molecules for different observation angles. In all, the showing performance and dependence of EL spectra on viewing angle for all devices indicate that the performance enhancement is not maximum at highest transmittance of the top contact but that rather it is determined
[28] [29] [30] [31]
Riel H, Karg S, Beierlein T, Ruhstaller B, Rieb W. Appl Phys Lett 2003;82:466. Han S, Feng X, Lu Z H, Johnson D, Wood R. Appl Phys Lett 2003;82:275. Hsu SF, Lee C-C, Hu AT, Chen CH. Curr Appl Phys 2004;4:663. Kim H-K, Lee K-S, Kwon JH. Appl Phys Lett 2006;88:0121031. Riel H, Karg S, Beierlein T, Rieb W, Neyts K. J Appl Phys 2003;94:5290. Ma GL, Ran GZ, Xu AG, Qiao YP, Zhao WQ, Zhang BR, et al. Appl Surf Sci 2006;252:3580. Lee CJ, Pode RB, Han JI, Moon DG. Appl Phys Lett 2006;89:123501. Xie Z, Hung L-S, Zhu F. Chem Phys Lett 2003;381:691. Chen S, Jie Z, Zhao Z, Cheng G, Wu Z, Zhao Y, et al. Appl Phys Lett 2006;89:043505. Chen S, Zhao Y, Cheng G, Li J, Liu C, Zhao Z, et al. Appl Phys Lett 2006;88:153517. Huang Q, Walzer K, Pfeiffer M, Lyssenko V, He G, Leo K. Appl Phys Lett 2006;88:113515. Chen S, Xie W, Meng Y, Chen P, Zhao Y, Liu S. Appl Phys Lett 2008;103:054506. Naka S, Shinno K, Okada H, Onnagawa H, Miyashita K. Jpn J Appl Phys 1994;33:L1772. Aziz H, Popovic ZD, Hu N-X, Hor AM, Xu G. Science 1999;283:1900. Cao Y, Parker ID, Zhang C, Heeger AJ. Nature London 1999;397:414. Pfeiffer M, Forrest SR, Leo K, Thompson ME. Adv Mater 2002;14:1633. Liu SW, Huang CA, Lee JH, Yang KH, Chen CC, Chang Y. Thin Solid Films 2004;453–454:312. Liu SW, Lee JH, Lee CC, Chen CT, Wang JK. Appl Phys Lett 2007;91:142106. Miyashita T, Naka S, Okada H, Onnagawa H. Jpn J Appl Phys 2005;44:3682. Lu M-H, Weaver MS, Zhou TX, Rothman M, kwong RC, Hack M, et al. J Appl Phys Lett 2002;81:3921. Chen C-W, Hsieh P-Y, Chiang H-H, Lin C-L, Wu H-M, Wu C-C. Appl Phys Lett 2003;83:5127. Gu G, Bulovic V, Burrow PE, Forrest SR, Thompson ME. Appl Phys Lett 1996;68:2006. Parthasarathy G, Burrows PE, Khalfin V, Kozlov VG, Forrest SR. Appl Phys Lett 1998;72:2138. Wang ZK, Naka S, Okada H. Jpn J Appl Phys 2010;49:01AA02. Ma D, Lee CS, Lee ST, Hung LS. Appl Phys Lett 2002;80:3641. Kalinowski J, Cocchi M, Giro G, Fattori V, Marco PD. J Phys D: Appl Phys 2001;34:2274. Raychaudhuri PK, Madathil K, Shore JD, Van Slyke SA. J Soc Inf Disp 2004;12:315. Lin C-L, Lin H-W, Wu C-C. Appl Phys Lett 2005;87:021101. Huang H, Walzer K, Pfeiffer M, Leo K. J Appl Phys 2006;100:064507. Schubert EF, Hunt NEJ, Micovic M, Mali RJ, Sivco DL, Cho AY, et al. Science 1994;265:943. Dodabalapur A, Rothberg LJ, Miller TM, Kwock EW. Appl Phys Lett 1994;64:2486.
Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Improved performance of mixed single layer top-emission organic light emitting devices using capping layer Zhaokui Wang ⇑, Shigeki Naka, Hiroyuki Okada Graduate School of Science & Technology, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
a r t i c l e
i n f o
Article history: Received 10 June 2010 Received in revised form 29 September 2010 Accepted 2 November 2010 Available online 24 November 2010 The review of this paper was arranged by Dr. Y. Kuk Keywords: Top-emission organic light emitting device (TEOLED) Mixed single layer Capping layer Viewing angle dependence
a b s t r a c t The performance of top-emission organic light emitting devices (TEOLEDs) can be improved by using a thin capping layer on top of the semitransparent metal electrode. We investigated the emission properties of inverted mixed single layer TEOLEDs with the same device structure but different capping materials. The thickness of capping layer was optimized by calculation. The power efficiency of device was 2.5 times enhanced when 45 nm TPD capping layer was added. The enhancement is not simply dependent on the transmittance and reflectance of the top contact, but also on other complex phenomena such as the interference effects in the device. The results of properties and dependence of EL spectra on viewing angle for all devices indicated that the large enhancement factor may be related to the complex interference phenomenon in our mixed single layer devices due to the emitter center and recombination region is different from conventional heterojunction devices. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Organic light emitting devices (OLEDs) are currently one of the most promising technologies since their future application in large area flat panel displays. However, the currently using rather expensive glass substrate with transparent electrode materials such as indium tin oxide (ITO) are a disadvantage of the usual bottom emitting OLEDS which emit through the substrate. To overcome this issue, it would be preferable to fabricate top-emitting organic light emitting devices (TEOLEDs) on opaque and flexible substrates such as metal foils. In addition, TEOLEDs are intensively investigated due to their large aperture ratio, high pixel resolution, and high information content in active matrix displays [1–4]. During fabrication process of TEOLEDs, metal films are usually used as the semitransparent top electrode because of the advantage of no damage to the organic layers during the thermal deposition [5–8]. However, the light outcoupling was usually limited by the poor transparence of the metal films. This issue can be solved by depositing a light outcoupling layer onto the top metal electrode [5,9–12]. Actually, the capping effect of dielectric thin films is not simply dependent on the transmittance of the top contact, but rather on the overall performance of the device. Recently, a mixed organic layer OLED has attracted wide attention because it ⇑ Corresponding author. Tel.: +81 76 445 6731. E-mail address: [email protected] (Z. Wang). 0038-1101/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2010.11.007
potentially provides an approach to improve the device performance [13–17]. Particularly, Liu et al. [18] investigated the carrier mobility in mixed layer and found that it was sensitive to the compositional fraction of organic materials, which presents direct evidences on the role of the mixed layer in these devices. In this study, we investigated the effects of different capping layers on the performance of inverted TEOLEDs with a mixed single layer by mixing of electron transport materials tris-(8-hydroxy-quinoline) aluminum (Alq3) and 2,5-bis(60 -(20 ,200 -bipyridyl))-1,1-dimethyl3,4-diphenylsilole (PyPySPyPy), hole transport material 4,40 -bis[N-(1-napthyl)-N-phenyl-amion] biphenyl (a-NPD) and dopant material 5,6,11,12-tetraphenylnaphthacene (rubrene).
2. Experiment An aluminum–neodymium alloy (AlNd, Al:Nd = 98:2, Kobelco Research Institute) is selected as cathode in order to flatten the electrode surface and avoid the short-circuiting [19]. Gold is used as a semitransparent anode. The mixed single layer TEOLED had a structure of Glass/AlNd (50 nm)/PyPySPyPy + Alq3 + a-NPD + rubrene (100 nm)/MoO3 (50 nm)/Au (20 nm). Glass substrate used is alkaline earth boro-aluminosilicate glass (Corning 1737). The mixing ratio of PyPySPyPy, Alq3, a-NPD and rubrene is optimized with a ratio of PyPySPyPy:Alq3:a-NPD:rubrene = 25:50:25:1. Mixed organic materials are evaporated at a pressure of about 4 106 Torr at a
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Z. Wang et al. / Solid-State Electronics 56 (2011) 155–158 0
(a)
3. Results and Discussion
1 Alq3 MoO3 TPD
MoO3 50nm/Au 20nm/Capping layer x nm
0.8
T, R, A
T 0.6
0.4
R
0.2
A
0 0
20
40
60
80
100
Cappling layer thickness (nm) Fig. 1. Calculated transmittance (T), reflectance (R) and absorptance (A) of the top contact (MoO3 50 nm/Au 20 nm/capping layer x nm) at a wavelength of 560 nm depending on the capping layer thickness.
102 101 100 10-1 10-2 Without Capping Layer With Alq 50nm 3 With MoO 40nm 3 With TPD 45nm
10-3 10-4 10-5 10-6 10-7
0
10
20
Applied voltage V (V)
(b) Luminance L (cd/m2)
105 104 103 102 101 Without Capping Layer With Alq3 50nm With MoO3 40nm With TPD 45nm
100 10-1 -2 10
10-1
100
Current density
(C)
10
EL efficiency η (lm/W)
TEOLEDs usually show stronger microcavity effects compared to bottom-emission OLEDs due to its top-emission mechanism [20–23]. For our mixed single layer TEOLED, it contains a reflective bottom electrode and a semitransparent top electrode for light outcoupling. This may be considered as a Fabry–Perot cavity embedded with a source. To get maximal luminance from TEOLEDs, the highest possible reflectance R1 for bottom electrode and a reflectance R2 for top electrode matching R1 should be adopted. In view of this, it should be made the top electrode as transparent as possible. This can be realized by depositing a capping layer on the top metal electrode. Here, we used Alq3 (n = 1.7), TPD (n = 1.9) and MoO3 (n = 2.1) as capping layer. Fig. 1 shows the calculated transmittance (T), reflectance (R) and absorptance (A) of top contact capped with different thin films with varied thickness at 560 nm for rubrene emitting wavelength. We can see that maximum transmittance and minimum reflectance corresponding to capping layer Alq3, TPD and MoO3 reaches to 0.758, 0.091 at 50 nm, 0.775, 0.078 at 45 nm, and 0.801, 0.032 at 40 nm, respectively. With the equation of A = (1TR), the absorptance of three materials can be calculated easily as a result of 0.151, 0.147 and 0.167 corresponding to capping layer Alq3, TPD and MoO3. Compared with transmittance 0.471 and reflectance 0.434 in the case of no capping layer, it is obvious that the optical properties of top contact are largely modulated with the thickness of any capping materials. Capping layers with higher refractive indices usually have the advantage of giving stronger modulation in optical characteristics and thus possibly higher luminance enhancement. Experiments were conducted on mixed single layer TEOLED with above optimized capping layer thickness. Fig. 2a–c shows the current density vs voltage (J–V), the luminance vs current density (L–J), and the power efficiency vs current density (g–J) characteristics of mixed single layer TEOLED capped with and without different thin films. Table 1 lists the comparison of TEOLEDs performance with and without capping layer. It is clearly seen that the capping layer has a significant influence on the power effi-
103
Current density J (mA/cm2)
rate of 1–3 Å A/s. The AlNd cathode and Au anode were evaporated using a metal mask. The device area was 2 2 mm2. Device characteristics were measured using semiconductor parameter analyzer (HP 4155B) and luminance meter (Topcon BM-3). All measurements were carried out in air at room temperature. The optical properties of top contact were calculated using Optical Spectrum Simulation Software SCOUT of Techno-Synergy, Inc.
101
102
103
(mA/cm2)
2
101 100 10-1
Without Capping Layer With Alq 3 50nm With MoO3 40nm With TPD 45nm
10-2 10-3 10-2
10-1
100
Current density
101
102
103
(mA/cm2)
Fig. 2. (a) Current density vs voltage (J–V), (b) luminance vs current density (L–J) and (c) power efficiency vs current density (g–J) characteristics of mixed single layer TEOLED with different capping layers.
ciency and luminance of the TEOLED. The device without capping layer shows a power efficiency of 0.26 lm/W and luminance 1500 cd/m2 at 100 mA/cm2. With 45 nm TPD capping layer applied, the power efficiency and luminance at 100 mA/cm2 reached 0.78 lm/W and 3700 cd/m2, respectively. The device performance was improved significantly, showing the maximum enhancement by a factor of 2.5. From Fig. 2a, the J–V characteristics of the devices with and without capping layers are almost identical, e.g., the driving voltage of devices at 100 mA/cm2 is 17.8 V, 17.6 V, 17.2 V and 14.8 V corresponding to devices without capping layer, with Alq3, with MoO3 and with TPD, respectively. The driving voltage of the mixed single layer device is larger high than that of the heterostructure device, which is consistent with our previous report [24]. The increase of the operational voltage in the mixed single layer device may be related to the larger hopping distance for hole transport with respect to heterostructure device [25]. Another possible reason of decrease of current density is the decrease of hole transport at Au/mixed layer because of mixing hole transport material into mixed single layer. There is not nearly large difference for the driving voltage in four devices with mixed single layer
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Z. Wang et al. / Solid-State Electronics 56 (2011) 155–158 Table 1 Electrical and optical characteristics of mixed single layer TEOLEDs with and without different capping layer. Sample
V (V) @ 100 mA/cm2
L (cd/m2) @ 100 mA/cm2
L (cd/m2) Max
g (lm/W) @ 100 mA/cm2
g (lm/W)
Without capping layer With Alq3 (50 nm) With MoO3 (40 nm) With TPD (45 nm)
17.8 17.6 17.2 14.8
1500 2120 2520 3700
7000 8520 10,770 20,000
0.26 0.38 0.45 0.78
3.45 5.30 6.20 7.52
excluding the device with TPD capping layer. The lesser difference in J–V characteristics in these devices is supposed to little variations on the thickness of the mixed layer due to asynchronous experiments as reported by Kalinowski [26]. With capping layer of 45 nm TPD, the driving voltage of device is decreased with 20.2%. While, from Fig. 2b and c, and Table 1, the luminance and power efficiency at 100 mA/cm2 is improved with 146.7% and with 200.0%, respectively. This proves that the enhancement of optical outcoupling is mainly associated with the capping thin films with high refractive index. More noticeable, with optimized thickness, the device with 40 nm MoO3 capping layer only shows an enhancement factor of 1.7 although the top contact with it has the highest transmittance and lowest reflectance as shown in Fig. 1. It is clearly seen that the largest enhancement is achieved in the case of top contact having the lowest absorption, which is consistent with the result form other groups [27,28]. Obviously, the enhancement is not simply dependent on the transmittance and reflectance of the top contact, but also on other complex phenomena such as the interference effects in the device. It is well known that the efficiency enhancement of OLEDs by capping layer could be attributed either to the redistribution of the emitted light, or to the improved outcoupling efficiency as a result of modified optical structure by the capping layer [29]. For our mixed single layer TEOLED, the emitter center and recombination region is different from conventional heterojunction device, resulting in more complex interference phenomenon. This may be one reason why our mixed single layer device with capping layer has larger enhancement factor compared to conventional heterojunction device. In addition, the wavelength dependence of transmittance for all devices with and without different capping layer is shown in Fig. 3. We can see that device transmittance is improved when different capping layer is added. The transmittance is strongly dependent on the wavelength in visible light. The difference of transmittance between three devices capped different materials is small when wavelength less than 500 nm. Up to 500 nm, the light outcoupling effects of MoO3 is preceded to other two materials. The grey color of MoO3 material may be another rea-
son of higher device efficiency happening with TPD capping layer although its refractive index is smaller than that of MoO3. The electroluminescence (EL) spectrum of device also exhibits a large dependence on the capping layer as shown in Fig. 4. By applying capping layer, the position of the peak and shoulder remains almost unchanged. However, the full width at half maximum (FWHM) of device EL spectrum is changed. Without capping layer, FWHM of EL spectrum is 42 nm. With optimized capping layer applied, FWHM increases and reaches 72 nm when 45 nm TPD films capped, which agrees with the result from other groups [11,12]. The FWHM can be estimated by [30,31]
FWHM ¼
pffiffiffiffiffiffiffiffiffiffi k2 1 R1 R2 ; 2L pðR1 R2 Þ1=4
ð1Þ
where L is the optical length between the two mirrors for a microcavity and here is represented by the thickness of mixed single organic layer. As seen in Fig. 1, the reflectance (R2) of the top contact is decreased largely when optimized capping layer is applied. This, in a certain degree, weakens the microcavity effect, resulting in larger FWHM of the EL spectrum. Fig. 5 depicts the angular distributions of EL intensity (normalized to the 0° intensity of device without capping layer) for four devices. Because of the microcavity effect in TEOLEDs, the emission profiles of all devices are not Lambertian. The devices with capping layer show more directed emission and enhanced luminance in the range of 0°–30°. Such enhancement leads to increased forwarddirection luminance and efficiency, as can be seen in Fig. 2 and Table 1. Representatively, Fig. 6a and b shows the EL spectra at viewing angles of 0°, 20°, 40°, 60°, and 80° off the surface normal for the mixed single layer TEOLED without and with 45 TPD capping layer, respectively. As can be seen in Fig. 5 and insets, device without capping layer shows a large change in color (red-shift) with varying observation angle from 0° (0.44, 0.53) to 80° (0.48, 0.46). Device with 45 nm TPD capping layer exhibits relatively
1
Transmittance
0.8 0.6 0.4 Without cap layer With Alq 3 50 nm With MoO 3 45 nm With TPD 40 nm
0.2 0
400
500
600
Without cappling layer With TPD 45 nm With MoO3 40 nm With Alq3 50 nm
EL intensity (arb. units)
1
700
Wavelength λ (nm) Fig. 3. The wavelength dependence of devices transmittance with and without different capping layer.
Max
0 500
550
600
650
700
750
Wavelength λ (nm) Fig. 4. The EL spectra of mixed single layer TEOLEDs with and without capping layer.
Z. Wang et al. / Solid-State Electronics 56 (2011) 155–158
EL Internsity (arb.units)
158
by redistribution of the emitted light duo to the different interference effects.
0
1.2 30
1
30
4. Conclusions
0.8 0.6 60
60
0.4 0.2 90
0 Without Capping Layer With 50 nm Alq3 With 40 nm MoO3 With 45 nm TPD
Fig. 5. The angular distribution of EL intensity for mixed single layer TEOLEDs with and without capping layer.
(a) 00
EL intensity (arb. units)
1
Acknowledgements
0.6
CIE y
200 400
This work was supported by regional innovation of R&D projects, overseen by the Ministry of Economy, Trade and Industry (METI), Japan. We also thank Kobelco Research Institute for providing AlNd and Chisso Corporation for supplying PyPySPyPy.
00
0.5 800
0.4 0.4
600
0.5 CIE x
0.6
References [1] [2] [3] [4] [5] [6]
800
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In summary, we have investigated the effects of capping layers on the performance of inverted TEOLEDs with a mixed single layer. It is demonstrated that the luminance and power efficiency in forward direction of devices applying capping layer can be improved significantly. Especially, the device with 45 nm TPD capping layer shows an enhancement factor of 2.5. The results of properties and dependence of EL spectra on viewing angle for all devices indicate that the large enhancement factor may be attributed to redistribution of the emitted light because of complex interference phenomenon in our mixed single layer device, in which the emitter center and recombination region is different from conventional heterojunction device. More details research is necessary in the future.
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Wavelength λ (nm)
(b)
[7] [8] [9] [10]
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[11]
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[12] [13]
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[14] [15] [16] [17]
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[18] [19] [20]
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[21]
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Wavelength λ (nm) Fig. 6. EL spectra at varying viewing angles for (a) without capping layer and (b) with 45 nm TPD capping layer. Inset: CIE color coordinates (0°–80°) belonging to the EL spectra.
[22] [23] [24] [25] [26] [27]
stable color coordinates originated from rubrene molecules for different observation angles. In all, the showing performance and dependence of EL spectra on viewing angle for all devices indicate that the performance enhancement is not maximum at highest transmittance of the top contact but that rather it is determined
[28] [29] [30] [31]
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