- Email: [email protected]
Study of various evaporation rates of the mixture of Alq3: DCM in a single furnace crucible
Journal of Luminescence 147 (2014) 9–14
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Study of various evaporation rates of the mixture of Alq3: DCM in a single furnace crucible Zahra Abedi a, Mohammad Janghouri a, Ezeddin Mohajerani a,n, Masoud Alahbakhshi a, Amin Azari a, Afsoon Fallahi b a b
Laser and Plasma Research Institute, Shahid Beheshti University, G.C., Tehran 1983963113, Iran Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, 424 Hafez Avenue, P.O. Box 15875-4413, Tehran, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 1 June 2013 Received in revised form 28 September 2013 Accepted 7 October 2013 Available online 25 October 2013
The emitting color for a new organic light emitting diode (OLED) structure is tuned by doping an appropriate amount of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) orange dye into tris-(8-hydroxyquinoline) aluminum (Alq3) emissive layer. Here, the blend of Alq3: DCM is deposited in a single furnace crucible by various evaporation rates. The electro-optical behavior of organic light emitting diode devices is greatly influenced by varying the Alq3:DCM film composition. It is investigated that when the deposition rate increased from 0.6 to 5 Å/s, complete energy transfer occurred from Alq3 to DCM and Electroluminescence (EL) peak shifted to higher wavelength regions. The device with evaporation rate of 0.6 Å/s shows a luminance of 3532 cd/m2 and maximum efficiency of 0.82 cd/A at 20 V. These blends show excellent orange emission host–guest system properties with easier deposition rate control. & 2013 Elsevier B.V. All rights reserved.
Keywords: OLEDs Single furnace crucible Electroluminescence Dye doping
1. Introduction Organic light emitting devices (OLEDs) are promising candidates for displays and light sources due to their unique features such as high brightness, wide color range, high contrast, wide viewing angle, rapid response, and low fabrication cost. In addition, it is worth mentioning that they can be used to make thin and flexible displays [1–3]. Each kind of luminescent materials possesses its own characteristics which have to be compromised to show better optical and electrical characteristics [4,5]. Therefore, a lot of efforts have focused on the synthesis and using variety of improved synthetic electroluminescent (EL) materials for the construction of reliable OLEDs which can be highly suitable in different aspects like quantum efficiency, charge carrier mobility, thermal stability and processability. Among these materials Tris(8-hydroxyquinolinato)aluminum (Alq3) which, due to its specific properties, is used as a combined electron transport and emitter layer is a common component for organic light-emitting diodes [6–8]. Furthermore, Alq3 has also been shown to work as a host material in organic devices in which Alq3 provides a polar environment for a highly polar excited state of the dye molecule. Thus, it can cause the reduction of concentration quenching [9]. Among the blue, green and red-light-emitting materials required
n
Corresponding author. Tel.: þ 98 2129904003. E-mail address: [email protected] (E. Mohajerani).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.10.020
for full-color displays, red-light-emitting materials remain one of the greatest challenges. In a red OLED, Alq3 is used as host material and 4-dicyanomethylene-2-t-butyl-6-1,1,7,7-tetra- methyljulolidyl-9-enyl-4H-pyran(DCJTB) is used as dopant. However, the emission from such a host–guest system is often contaminated by the residual green emission fromAlq3 [10]. To improve the device performance, various methods have been developed, such as using 5,6,11,12-tetra- phenylnathacene(rubrene) as an assist dopant or using a cohost emitter system [11–14]. Electroluminescence with colors tunable from yellow–green to red can be obtained with DCM doped in Alq3 layers as the host material because of its high stability and good carrier-transport properties. [15,16]. Based on resonance energy transfer theory [17], an efficient Forster energy transfer requires a large overlap between host emission and guest absorption. For this reason, emission from Alq3:DCM can be achieved through direct excitation of the DCM fluorophores or through Forster energy transfer from the excited Alq3 host matrix [16]. The common mechanism for doping DCM in Alq3 layer is based on the usage of two separate furnace crucibles for Alq3 and DCM. Eventhough the effects of the evaporation rate of Alq3 on electroluminescence performance of OLEDs have been investigated in many researches [18–20], but there has been no report on the study of evaporation rate of dye doped Alq3 when a single crucible is applied. In our previous work, we reported the deposition of the mixture of Alq3 with porphyrin compounds with additional functional groups [21]. Indeed, the most serious
10
Z. Abedi et al. / Journal of Luminescence 147 (2014) 9–14
problem in using two separate crucibles is to monitor and control two evaporation rates simultaneously. In particular, the emitter with the lowest HOMO-LUMO energy has to be evaporated with a precise rate because of the fact that small fluctuations in its low concentration can significantly change the emitting color of its OLED device. In the present work, instead of evaporating Alq3 and DCM separately by two evaporative systems, the evaporation is occurring simultaneously in a single sublimation crucible. The advantage of this mixture of dyes with using single furnace crucible is the rate control. In the case of two crucibles, control of evaporation rate and the subsequent temperature control of the evaporation sources are very difficult, expensive and lowering the overall yield. In case of the single crucible, because the rate of the dye source is significantly higher, the difference in concentration of dyes does not need to be monitored but is determined by the weight ratio within the crucible. This allows for much easier evaporation rate control which potentially decreases the process complexity for large scale industrial production. Although using two separate crucibles to evaporate DCM and Alq3 has the advantages of better control on evaporation concentrations, however, low cost of fabrication and more homogenous layer are advantages of this method. Here, by changing the evaporation rate of Alq3:DCM, the emission wavelength and energy transfer between Alq3 and DCM have been investigated.
2. Experimental 2.1. Materials poly(3,4-ethylenedi-oxythiophene):poly(styrenesulfonate)(PEDOT: PSS), polyvinylcarbazole (PVK), tris(8-hydroxyquinolinato)aluminum (Alq3) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) were obtained from Sigma-Aldrich and used without any further purification. Fig. 1 gives the structure of the materials used in this study. 2.2. Physical measurements Thermal properties were measured by using differential scanning calorimetry (DSC)—Perkin Elmer Pyris Diamond—in alumina caps under a nitrogen flow at a scan rate of 15 1C min 1 over the temperature range of 35–430 1C. Thermal Gravimetric Analysis
(TGA) analysis was carried out by TGA 7 (Perkin-Elmer, Norwalk, CT) to monitor sample weight loss as a function of temperature. The heating rate was 10 1C/min ranging from 50 to 450 1C under nitrogen atmosphere to avoid oxidization. Thickness measurements were performed by DekTak 8000; EL and Photoluminescence(PL) of fabricated OLEDs were performed by USB2000 and HR4000 Ocean Optics. The current–voltage–luminance characteristics by a Keithley source meter 2400 model and optical meter Mastech-MS6612, respectively.
2.3. Fabrication of OLED Before film fabrication, all ITO substrates were ultrasonically cleaned in detergent, acetone, dichloromethane, ethanol, methanol for 20 min, respectively. Subsequently, cleaned ITO were rinsed in deionized water, dried and mounted in the vacuum chamber immediately. PEDOT:PSS as a hole injection layer was spin coated on clean ITO substrate to a thickness of 55 nm and was baked in an oven for 1 h at 120 1C. Afterwards, PVK was also spin coated over the sample to a thickness of 80 nm as a hole transport layer and was baked in the oven for 1 h at 120 1C to soften the sharp peaks of PVK layer and to achieve a more even surface. The main part in fabricating our OLED was preparation of light emitting layer (LEL). In order to prepare this layer, DCM and Alq3 were dissolved in dichloromethane. The solution was then left in ultrasonic bath for 15 min to make a homogeneous solution. This solution was poured in a quartz furnace crucible and exposed to heat at 30 1C for 15 min until the solvent was evaporated. Finally, the dried mixture of Alq3:DCM was coated in evaporation chamber to make layers with 95 nm in thickness. The aluminum cathode was deposited on the top of the structure through a shadow mask. The structure Table 1 The device structures. Device
Structure
Device 1
ITO/PEDOT:PSS(55 nm)/ PVK (80 nm)/Alq3:0.6 Å/s DCM (95 nm)/ Al(180 nm) ITO/PEDOT:PSS (55 nm)/PVK (80 nm)/Alq3: 3 Å/sDCM (95 nm)/Al(180 nm) ITO/PEDOT:PSS (55 nm)/PVK (80 nm)/Alq3: 5 Å/sDCM (95 nm)/Al(180 nm)
Device 2 Device 3
Fig. 1. The graphical structure of (a) PEDOT:PSS (b) PVK (c) DCM and (d) Alq3.
Z. Abedi et al. / Journal of Luminescence 147 (2014) 9–14
of the device and schematics of the fabricated film are shown in Tables 1 and 2 and Fig. 2.
3. Results and discussion The results of TGA reveals that Alq3 is quite stable up to 430 1C but DCM exhibit onsets for degradation in the range of 305–310 1C. The DSC data of the glass transition temperature (Tg) for the Alq3 was estimated at 204 1C and melting point (Tm) of 414 1C (Fig. 3), while for DCM a significant sharp Tm of 223 1C was detected (Fig. 4), indicating that it represents a more crystalline phase than Alq3 in the blend. Since Alq3 did not decompose until 430 1C, the onset temperatures of the weight loss at 290 1C must correspond to its sublimation onset point. Although it will continue up to 410 1C (from α- to δ-phase), before melting onset temperature [22,23]. Based on EL data of the blend and Fig. 5, collected from pure and the blend system, the blend sublimation temperature dropped drastically. We fabricated light-emitting layers consisting of a matrix material, Alq3 as green dye and DCM as red dye. The mixture of the two dyes was used for evaporation with a single evaporation source instead of employing two separate materials at two evaporation sources. The PL spectra of Alq3, DCM, and absorption spectrum of DCM are shown in Fig. 6(a). The PL spectrum of Alq3 is overlapped with absorption spectrum of DCM. This indicates that the energy transfer from Alq3 to DCM can effectively occur. The laser excitation light wavelength 405 nm was used for the PL measurements. The main absorption peak in the DCM film appeared at the 467 nm region. The PL spectrum of the mixture of Alq3:DCM solution is shown in Fig. 6(b). The PL spectrum of Alq3:DCM solution relative to PL spectrum of DCM, 28 nm blue shifted. For the solution there is one peak at orange emission region related to DCM, indicating that complete energy transfer occurred from Alq3 to DCM due to the overlap between PL emission of Alq3 and absorption of DCM (Fig. 6(a)). Fig. 7 shows
11
the EL spectra of the fabricated OLED with various evaporation rates of Alq3:DCM. Evidently, the device1 could promote the radiative recombination rate by balancing the distribution of electrons and holes. Thus an efficient evaporation rate exists for high EL intensity peak at 600 nm. We infer that the evaporation of
Fig. 3. Differential scanning calorimetry (DSC) curve for Alq3. Temperatures of phase transitions are also indicated on the plot. Inside: TGA of purchased solid Alq3 [22–23].
Table 2 Characteristics of the EL devices. Device
Device 1
Device 2
Device 3
Turn on voltage (V) Peak EL Driving voltage (V) Max luminescence (cd/m2)
7.1 573 4.7 3532
8.7 594 10.3 1876
9 612 15.4 564.3 Fig. 4. DSC curve for DCM.
Fig. 2. (a) the device structures and (b) fabrication of film.
12
Z. Abedi et al. / Journal of Luminescence 147 (2014) 9–14
Fig. 7. The EL spectra with different evaporation rate of DCM:Alq3. Fig. 5. Evaporation rate versus the crucible temperatures for Alq3 (left hand side peak), DCM (middle peak), and their blended system (right hand side peak).
Fig. 8. Current density–Voltage characteristic of three samples with different evaporation rate of DCM:Alq3.
Fig. 6. (a) Photoluminescence characteristic of Alq3, DCM and absorbance DCM, and (b) the PL spectra of the mixture of Alq3:DCM solution.
our premixed blend system will sublime in the form of spheres like droplets. The feature of this method is the existence of DCM molecules in droplets of Alq3, which are jumped up at evaporation temperature of Alq3. Therefore, any increment in the evaporation rate will enhance the amount of DCM molecules in Alq3 droplets and the EL peak shifts to higher wavelength regions. It is clear that the emission of the layer obtained at 0.6 Å/s is an overlap between the emission of Alq3 at around 530 nm and the emission of DCM at 600 nm. This can be explained by taking into account the exciton diffusion length on the order of 8 nm in organic thin films [24–26]. In evaporation rate 0.6 Å/s because the number of DCM molecules is lower in droplet Alq3, excitons formed at Alq3 molecules have to diffuse prior to energy transfer to the DCM and can recombine on Alq3 molecules, contributing to Alq3 spectral signature. But by increasing of deposition rate the number of DCM molecules in droplet Alq3 increases and energy transfer radius decreases and recombination of excitons at DCM molecules occur as result the emission of Alq3 in the El spectra (Fig. 7) was removed. Electrical characteristics of three samples at different evaporation rates of DCM doped into Alq3 with using single crucible is shown in Fig. 8. The results show an inverse relation between the increase of evaporation rate and current density changes at specific voltages.
Z. Abedi et al. / Journal of Luminescence 147 (2014) 9–14
This is due to the induction of the traps by DCM into Alq3. The existence of traps in bulk confine charge carriers in traps states which induces the space charge, which prevent further injections of carriers from electrodes to bulk [27]. The current density of the fabricated OLED with co-evaporated layer of Alq3:DCM ¼(0.6 Å/s) is more than that of for other three devices. Since the conductivity of Alq3 is higher than DCM, by increasing the evaporation rate, the number of DCM molecules in droplet Alq3 increases. As a result the trap density increases and the conductivity of the device decreases [28–33]. This phenomenon suggests that the film structure of a coevaporated layer plays an important role as the carrier transporter in an organic LED. The luminance–voltage properties of the OLEDs with evaporation rate of 0.6 Å/s Alq3:DCM is shown in Fig. 9. The turn-on voltage of this device is approximately 7.1 V. The device 1 also has a luminance of 3532 cd/m2 and maximum efficiency of 0.82 cd/A at 20 V which are the highest values among
13
the three devices. The Commission Internationale d'Eclairage (CIE) coordinates (Fig. 10.) were (0.47, 0.43) for the evaporation rate of device 1, (0.50, 0.42) for device 2 and (0.54, 0.38) for device 3 at 15 V applied voltage in aforementioned evaporation rate. Device 3 showed the purest orange emission among all fabricated devices. It is also evident that the deposition rate has a pronounced effect on the J–V characteristic. As the rate increases, the current density shifts to higher values for a constant bias voltage [18]. For two separate crucibles the effect of deposition rate on the J–V characteristic can be occurred. But control of the deposition rate for two separate crucibles is really difficult at the same time. This novel method can be used in doping processes for fabrication of homogenous stacks in thin film layers, which leads to more efficient OLEDs. However, in the two crucible method, each of the materials is evaporated in separate droplets and hence the uniformity and binding between guest and host molecules is much less.
4. Conclusion The optical and transport properties of charge carriers in doped amorphous Alq3 films fabricated using a single furnace crucible were investigated. Furthermore, the current–voltage, EL and PL were measured. Device 3 exhibited orange emission with high color purity. And also it could be observed that by increasing the evaporation rate, the driving voltage and the turn-on voltage will shift to higher voltages. In addition, the maximum luminescence was observed for the Alq3:DCM with an evaporation rate of 0.6 Å/s.
Acknowledgment The authors would like to thank the Vice-President's Office for Research Affairs of Shahid Beheshti University and the Iran National Science Foundation: INSF for supporting this work. References Fig. 9. The luminance–voltage relationship of OLEDs with evaporation rate of 0.6 A/s DCM:Alq3.
Fig. 10. Variation in the CIE 1931 chromaticity coordinate for devices with different evaporation rate of DCM:Alq3.
[1] C.W. Tang, S.A. Vanslyke, Appl. Phys. Lett. 51 (1987) 913. [2] J.H. Burroughes, D.D.C Bradley, A.R. Broun, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 (1990) 539. [3] J. Shi, C.W. Tang, Appl. Phys. Lett. 70 (1997) 1665. [4] M.A. Baldo, S. Lamansky, P.E. Burrows, M.E. Thompson, S.R. Forrest, Appl. Phys. Lett. 75 (1999) 4. [5] A. Köhler, J.S. Wilson, R.H. Friend, Adv. Mater. 14 (2002) 701. [6] C.H. Chen, J.M. Shi, Coord. Chem. Rev. 171 (1998) 161. [7] Q.L. Song, F.Y. Li, H. Yang, H.R. Wu, X.Z. Wang, W. Zhou, J.M. Zhao, X.M. Ding, C.H. Huang, X.Y. Hou, Chem. Phys. Lett. 416 (2005) 42. [8] P.C. Kao, S.Y Chu, H.H. Huang, Z.L Tseng, Y.-C. Chen, Thin Solid Films 517 (2009) 5301. [9] M.J. Currie, J.K. Mapel, T.D. Heidel, S. Goffri, M.A. Baldo, Science 321 (2008) 226. [10] C.H. Chen, C.W. Tang, J. Shi, K.P. Klubek, Thin Solid Films 363 (2000) 327. [11] N. Komiya, R. Nishikawa, M. Okuyama, T. Yamada, Y. Saito, S. Oima, K. Yoneda, H. Kanno, H. Takahashi, G.Rageswaran, M. Itoh, M. Boroson, T.K. Hatwar, in: Proceedings of the 10th InternationalWorkshop on Inorganic and Organic EL (EL'00), Hamamatsu, Japan, 4 December 2000, p. 347. [12] Yuji Hamada, Hiroshi Kanno, Tsuyoshi Tsujioka, Hisakazu Takahashi, Tatsuro Usuki, Appl. Phys. Lett. 75 (1999) 1682. [13] R.H. Young, C.W. Tang, A.P. Marchietti, Appl. Phys. Lett. 80 (2002) 874. [14] T.H. Liu, C.Y. Iou, C.H. Chen, Appl. Phys. Lett. 83 (2003) 5241. [15] C.W Tang, S.A VanSlyke, C.H Chen, J. Appl. Phys. 65 (1989) 3610. [16] S. Forget, S. Chenais, D. Tondelier, B. Geffroy, I. Gozhyk, M. Lebental, E. Ishow, J. Appl. Phys. 108 (2010) 064509. [17] T. Forster, Discuss. Faraday Soc. 27 (1959) 7. [18] C.B. Lee, A. Uddin, X. Hu, T.G. Anderssonb, Mater. Sci. Eng.: B 112 (2004) 14. [19] L.F. Cheng, L.S. Liao, W.Y. Lai, X.H. Sun, N.B. Wong, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 319 (2000) 418. [20] F. Zhang, Z. Xu, D. Zhao, W. Jiang, G. Yuan, D. Song, Y Wang, X. Xu, J. Phys. D: Appl. Phys 40 (2007) 4485. [21] M. Janghouri, E. Mohajerani, A. Khabazi, Z. Abedi, H. Razavi, J. Lumin. 140 (2013) 7. [22] C.P. Cho, C.Y. Yu, T.P. Perng, Nanotechnology 17 (2006) 5506.
14
Z. Abedi et al. / Journal of Luminescence 147 (2014) 9–14
[23] C. Pei Cho, T. Pyng Perng, Nanotechnology 17 (2006) 3756. [24] P.O. Anikeeva, C.F. Madigan, S.A. Coe-Sullivan, J.S. Steckel, M.G. Bawendi, V. Bulović, Chem. Phys. Lett. 424 (2006) 120. [25] E.B. Namdas, A. Ruseckas, I.D.W. Samuel, S.-C. Lo, P.L. Burn, Appl. Phys. Lett. 86 (2005) 091104. [26] N. Matsusue, S. Ikame, Y. Suzuki, H. Naito, Appl. Phys. Lett. 97 (2005) 123512. [27] B. Li, J. Li, Y. Fu, Z. Bo, J. Am. Chem. Soc. 126 (2004) 3430. [28] E. Aminaka, T. Tsutsui, S. Saito, J. Appl. Phys. 79 (1996) 8808.
[29] W.T. Higginsa, A.P. Monkman, H.G. Nothofer, U. Scherf, J. Appl. Phys. 91 (2002) 99. [30] G.Y. Zhong, J. He, S.T. Zhang, Z. Xu, Z.H. Xiong, H.Z. Shi, X.M. Ding, W. Huang, X.Y. Hou, Appl. Phys. Lett. 80 (2002) 4846. [31] Y. Sakakibara, S. Okutsu, T. Enokida, T. Tani, Appl. Phys. Lett. 74 (1999) 2587. [32] M. Pfeiffer, K. Leo, X. Zhou, J.S. Huang, M. Hofmann, A. Werner, J.B. Nimoth, Org. Electron. 4 (2003) 89. [33] A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem. 37 (1998) 428.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Study of various evaporation rates of the mixture of Alq3: DCM in a single furnace crucible Zahra Abedi a, Mohammad Janghouri a, Ezeddin Mohajerani a,n, Masoud Alahbakhshi a, Amin Azari a, Afsoon Fallahi b a b
Laser and Plasma Research Institute, Shahid Beheshti University, G.C., Tehran 1983963113, Iran Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, 424 Hafez Avenue, P.O. Box 15875-4413, Tehran, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 1 June 2013 Received in revised form 28 September 2013 Accepted 7 October 2013 Available online 25 October 2013
The emitting color for a new organic light emitting diode (OLED) structure is tuned by doping an appropriate amount of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) orange dye into tris-(8-hydroxyquinoline) aluminum (Alq3) emissive layer. Here, the blend of Alq3: DCM is deposited in a single furnace crucible by various evaporation rates. The electro-optical behavior of organic light emitting diode devices is greatly influenced by varying the Alq3:DCM film composition. It is investigated that when the deposition rate increased from 0.6 to 5 Å/s, complete energy transfer occurred from Alq3 to DCM and Electroluminescence (EL) peak shifted to higher wavelength regions. The device with evaporation rate of 0.6 Å/s shows a luminance of 3532 cd/m2 and maximum efficiency of 0.82 cd/A at 20 V. These blends show excellent orange emission host–guest system properties with easier deposition rate control. & 2013 Elsevier B.V. All rights reserved.
Keywords: OLEDs Single furnace crucible Electroluminescence Dye doping
1. Introduction Organic light emitting devices (OLEDs) are promising candidates for displays and light sources due to their unique features such as high brightness, wide color range, high contrast, wide viewing angle, rapid response, and low fabrication cost. In addition, it is worth mentioning that they can be used to make thin and flexible displays [1–3]. Each kind of luminescent materials possesses its own characteristics which have to be compromised to show better optical and electrical characteristics [4,5]. Therefore, a lot of efforts have focused on the synthesis and using variety of improved synthetic electroluminescent (EL) materials for the construction of reliable OLEDs which can be highly suitable in different aspects like quantum efficiency, charge carrier mobility, thermal stability and processability. Among these materials Tris(8-hydroxyquinolinato)aluminum (Alq3) which, due to its specific properties, is used as a combined electron transport and emitter layer is a common component for organic light-emitting diodes [6–8]. Furthermore, Alq3 has also been shown to work as a host material in organic devices in which Alq3 provides a polar environment for a highly polar excited state of the dye molecule. Thus, it can cause the reduction of concentration quenching [9]. Among the blue, green and red-light-emitting materials required
n
Corresponding author. Tel.: þ 98 2129904003. E-mail address: [email protected] (E. Mohajerani).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.10.020
for full-color displays, red-light-emitting materials remain one of the greatest challenges. In a red OLED, Alq3 is used as host material and 4-dicyanomethylene-2-t-butyl-6-1,1,7,7-tetra- methyljulolidyl-9-enyl-4H-pyran(DCJTB) is used as dopant. However, the emission from such a host–guest system is often contaminated by the residual green emission fromAlq3 [10]. To improve the device performance, various methods have been developed, such as using 5,6,11,12-tetra- phenylnathacene(rubrene) as an assist dopant or using a cohost emitter system [11–14]. Electroluminescence with colors tunable from yellow–green to red can be obtained with DCM doped in Alq3 layers as the host material because of its high stability and good carrier-transport properties. [15,16]. Based on resonance energy transfer theory [17], an efficient Forster energy transfer requires a large overlap between host emission and guest absorption. For this reason, emission from Alq3:DCM can be achieved through direct excitation of the DCM fluorophores or through Forster energy transfer from the excited Alq3 host matrix [16]. The common mechanism for doping DCM in Alq3 layer is based on the usage of two separate furnace crucibles for Alq3 and DCM. Eventhough the effects of the evaporation rate of Alq3 on electroluminescence performance of OLEDs have been investigated in many researches [18–20], but there has been no report on the study of evaporation rate of dye doped Alq3 when a single crucible is applied. In our previous work, we reported the deposition of the mixture of Alq3 with porphyrin compounds with additional functional groups [21]. Indeed, the most serious
10
Z. Abedi et al. / Journal of Luminescence 147 (2014) 9–14
problem in using two separate crucibles is to monitor and control two evaporation rates simultaneously. In particular, the emitter with the lowest HOMO-LUMO energy has to be evaporated with a precise rate because of the fact that small fluctuations in its low concentration can significantly change the emitting color of its OLED device. In the present work, instead of evaporating Alq3 and DCM separately by two evaporative systems, the evaporation is occurring simultaneously in a single sublimation crucible. The advantage of this mixture of dyes with using single furnace crucible is the rate control. In the case of two crucibles, control of evaporation rate and the subsequent temperature control of the evaporation sources are very difficult, expensive and lowering the overall yield. In case of the single crucible, because the rate of the dye source is significantly higher, the difference in concentration of dyes does not need to be monitored but is determined by the weight ratio within the crucible. This allows for much easier evaporation rate control which potentially decreases the process complexity for large scale industrial production. Although using two separate crucibles to evaporate DCM and Alq3 has the advantages of better control on evaporation concentrations, however, low cost of fabrication and more homogenous layer are advantages of this method. Here, by changing the evaporation rate of Alq3:DCM, the emission wavelength and energy transfer between Alq3 and DCM have been investigated.
2. Experimental 2.1. Materials poly(3,4-ethylenedi-oxythiophene):poly(styrenesulfonate)(PEDOT: PSS), polyvinylcarbazole (PVK), tris(8-hydroxyquinolinato)aluminum (Alq3) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) were obtained from Sigma-Aldrich and used without any further purification. Fig. 1 gives the structure of the materials used in this study. 2.2. Physical measurements Thermal properties were measured by using differential scanning calorimetry (DSC)—Perkin Elmer Pyris Diamond—in alumina caps under a nitrogen flow at a scan rate of 15 1C min 1 over the temperature range of 35–430 1C. Thermal Gravimetric Analysis
(TGA) analysis was carried out by TGA 7 (Perkin-Elmer, Norwalk, CT) to monitor sample weight loss as a function of temperature. The heating rate was 10 1C/min ranging from 50 to 450 1C under nitrogen atmosphere to avoid oxidization. Thickness measurements were performed by DekTak 8000; EL and Photoluminescence(PL) of fabricated OLEDs were performed by USB2000 and HR4000 Ocean Optics. The current–voltage–luminance characteristics by a Keithley source meter 2400 model and optical meter Mastech-MS6612, respectively.
2.3. Fabrication of OLED Before film fabrication, all ITO substrates were ultrasonically cleaned in detergent, acetone, dichloromethane, ethanol, methanol for 20 min, respectively. Subsequently, cleaned ITO were rinsed in deionized water, dried and mounted in the vacuum chamber immediately. PEDOT:PSS as a hole injection layer was spin coated on clean ITO substrate to a thickness of 55 nm and was baked in an oven for 1 h at 120 1C. Afterwards, PVK was also spin coated over the sample to a thickness of 80 nm as a hole transport layer and was baked in the oven for 1 h at 120 1C to soften the sharp peaks of PVK layer and to achieve a more even surface. The main part in fabricating our OLED was preparation of light emitting layer (LEL). In order to prepare this layer, DCM and Alq3 were dissolved in dichloromethane. The solution was then left in ultrasonic bath for 15 min to make a homogeneous solution. This solution was poured in a quartz furnace crucible and exposed to heat at 30 1C for 15 min until the solvent was evaporated. Finally, the dried mixture of Alq3:DCM was coated in evaporation chamber to make layers with 95 nm in thickness. The aluminum cathode was deposited on the top of the structure through a shadow mask. The structure Table 1 The device structures. Device
Structure
Device 1
ITO/PEDOT:PSS(55 nm)/ PVK (80 nm)/Alq3:0.6 Å/s DCM (95 nm)/ Al(180 nm) ITO/PEDOT:PSS (55 nm)/PVK (80 nm)/Alq3: 3 Å/sDCM (95 nm)/Al(180 nm) ITO/PEDOT:PSS (55 nm)/PVK (80 nm)/Alq3: 5 Å/sDCM (95 nm)/Al(180 nm)
Device 2 Device 3
Fig. 1. The graphical structure of (a) PEDOT:PSS (b) PVK (c) DCM and (d) Alq3.
Z. Abedi et al. / Journal of Luminescence 147 (2014) 9–14
of the device and schematics of the fabricated film are shown in Tables 1 and 2 and Fig. 2.
3. Results and discussion The results of TGA reveals that Alq3 is quite stable up to 430 1C but DCM exhibit onsets for degradation in the range of 305–310 1C. The DSC data of the glass transition temperature (Tg) for the Alq3 was estimated at 204 1C and melting point (Tm) of 414 1C (Fig. 3), while for DCM a significant sharp Tm of 223 1C was detected (Fig. 4), indicating that it represents a more crystalline phase than Alq3 in the blend. Since Alq3 did not decompose until 430 1C, the onset temperatures of the weight loss at 290 1C must correspond to its sublimation onset point. Although it will continue up to 410 1C (from α- to δ-phase), before melting onset temperature [22,23]. Based on EL data of the blend and Fig. 5, collected from pure and the blend system, the blend sublimation temperature dropped drastically. We fabricated light-emitting layers consisting of a matrix material, Alq3 as green dye and DCM as red dye. The mixture of the two dyes was used for evaporation with a single evaporation source instead of employing two separate materials at two evaporation sources. The PL spectra of Alq3, DCM, and absorption spectrum of DCM are shown in Fig. 6(a). The PL spectrum of Alq3 is overlapped with absorption spectrum of DCM. This indicates that the energy transfer from Alq3 to DCM can effectively occur. The laser excitation light wavelength 405 nm was used for the PL measurements. The main absorption peak in the DCM film appeared at the 467 nm region. The PL spectrum of the mixture of Alq3:DCM solution is shown in Fig. 6(b). The PL spectrum of Alq3:DCM solution relative to PL spectrum of DCM, 28 nm blue shifted. For the solution there is one peak at orange emission region related to DCM, indicating that complete energy transfer occurred from Alq3 to DCM due to the overlap between PL emission of Alq3 and absorption of DCM (Fig. 6(a)). Fig. 7 shows
11
the EL spectra of the fabricated OLED with various evaporation rates of Alq3:DCM. Evidently, the device1 could promote the radiative recombination rate by balancing the distribution of electrons and holes. Thus an efficient evaporation rate exists for high EL intensity peak at 600 nm. We infer that the evaporation of
Fig. 3. Differential scanning calorimetry (DSC) curve for Alq3. Temperatures of phase transitions are also indicated on the plot. Inside: TGA of purchased solid Alq3 [22–23].
Table 2 Characteristics of the EL devices. Device
Device 1
Device 2
Device 3
Turn on voltage (V) Peak EL Driving voltage (V) Max luminescence (cd/m2)
7.1 573 4.7 3532
8.7 594 10.3 1876
9 612 15.4 564.3 Fig. 4. DSC curve for DCM.
Fig. 2. (a) the device structures and (b) fabrication of film.
12
Z. Abedi et al. / Journal of Luminescence 147 (2014) 9–14
Fig. 7. The EL spectra with different evaporation rate of DCM:Alq3. Fig. 5. Evaporation rate versus the crucible temperatures for Alq3 (left hand side peak), DCM (middle peak), and their blended system (right hand side peak).
Fig. 8. Current density–Voltage characteristic of three samples with different evaporation rate of DCM:Alq3.
Fig. 6. (a) Photoluminescence characteristic of Alq3, DCM and absorbance DCM, and (b) the PL spectra of the mixture of Alq3:DCM solution.
our premixed blend system will sublime in the form of spheres like droplets. The feature of this method is the existence of DCM molecules in droplets of Alq3, which are jumped up at evaporation temperature of Alq3. Therefore, any increment in the evaporation rate will enhance the amount of DCM molecules in Alq3 droplets and the EL peak shifts to higher wavelength regions. It is clear that the emission of the layer obtained at 0.6 Å/s is an overlap between the emission of Alq3 at around 530 nm and the emission of DCM at 600 nm. This can be explained by taking into account the exciton diffusion length on the order of 8 nm in organic thin films [24–26]. In evaporation rate 0.6 Å/s because the number of DCM molecules is lower in droplet Alq3, excitons formed at Alq3 molecules have to diffuse prior to energy transfer to the DCM and can recombine on Alq3 molecules, contributing to Alq3 spectral signature. But by increasing of deposition rate the number of DCM molecules in droplet Alq3 increases and energy transfer radius decreases and recombination of excitons at DCM molecules occur as result the emission of Alq3 in the El spectra (Fig. 7) was removed. Electrical characteristics of three samples at different evaporation rates of DCM doped into Alq3 with using single crucible is shown in Fig. 8. The results show an inverse relation between the increase of evaporation rate and current density changes at specific voltages.
Z. Abedi et al. / Journal of Luminescence 147 (2014) 9–14
This is due to the induction of the traps by DCM into Alq3. The existence of traps in bulk confine charge carriers in traps states which induces the space charge, which prevent further injections of carriers from electrodes to bulk [27]. The current density of the fabricated OLED with co-evaporated layer of Alq3:DCM ¼(0.6 Å/s) is more than that of for other three devices. Since the conductivity of Alq3 is higher than DCM, by increasing the evaporation rate, the number of DCM molecules in droplet Alq3 increases. As a result the trap density increases and the conductivity of the device decreases [28–33]. This phenomenon suggests that the film structure of a coevaporated layer plays an important role as the carrier transporter in an organic LED. The luminance–voltage properties of the OLEDs with evaporation rate of 0.6 Å/s Alq3:DCM is shown in Fig. 9. The turn-on voltage of this device is approximately 7.1 V. The device 1 also has a luminance of 3532 cd/m2 and maximum efficiency of 0.82 cd/A at 20 V which are the highest values among
13
the three devices. The Commission Internationale d'Eclairage (CIE) coordinates (Fig. 10.) were (0.47, 0.43) for the evaporation rate of device 1, (0.50, 0.42) for device 2 and (0.54, 0.38) for device 3 at 15 V applied voltage in aforementioned evaporation rate. Device 3 showed the purest orange emission among all fabricated devices. It is also evident that the deposition rate has a pronounced effect on the J–V characteristic. As the rate increases, the current density shifts to higher values for a constant bias voltage [18]. For two separate crucibles the effect of deposition rate on the J–V characteristic can be occurred. But control of the deposition rate for two separate crucibles is really difficult at the same time. This novel method can be used in doping processes for fabrication of homogenous stacks in thin film layers, which leads to more efficient OLEDs. However, in the two crucible method, each of the materials is evaporated in separate droplets and hence the uniformity and binding between guest and host molecules is much less.
4. Conclusion The optical and transport properties of charge carriers in doped amorphous Alq3 films fabricated using a single furnace crucible were investigated. Furthermore, the current–voltage, EL and PL were measured. Device 3 exhibited orange emission with high color purity. And also it could be observed that by increasing the evaporation rate, the driving voltage and the turn-on voltage will shift to higher voltages. In addition, the maximum luminescence was observed for the Alq3:DCM with an evaporation rate of 0.6 Å/s.
Acknowledgment The authors would like to thank the Vice-President's Office for Research Affairs of Shahid Beheshti University and the Iran National Science Foundation: INSF for supporting this work. References Fig. 9. The luminance–voltage relationship of OLEDs with evaporation rate of 0.6 A/s DCM:Alq3.
Fig. 10. Variation in the CIE 1931 chromaticity coordinate for devices with different evaporation rate of DCM:Alq3.
[1] C.W. Tang, S.A. Vanslyke, Appl. Phys. Lett. 51 (1987) 913. [2] J.H. Burroughes, D.D.C Bradley, A.R. Broun, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 (1990) 539. [3] J. Shi, C.W. Tang, Appl. Phys. Lett. 70 (1997) 1665. [4] M.A. Baldo, S. Lamansky, P.E. Burrows, M.E. Thompson, S.R. Forrest, Appl. Phys. Lett. 75 (1999) 4. [5] A. Köhler, J.S. Wilson, R.H. Friend, Adv. Mater. 14 (2002) 701. [6] C.H. Chen, J.M. Shi, Coord. Chem. Rev. 171 (1998) 161. [7] Q.L. Song, F.Y. Li, H. Yang, H.R. Wu, X.Z. Wang, W. Zhou, J.M. Zhao, X.M. Ding, C.H. Huang, X.Y. Hou, Chem. Phys. Lett. 416 (2005) 42. [8] P.C. Kao, S.Y Chu, H.H. Huang, Z.L Tseng, Y.-C. Chen, Thin Solid Films 517 (2009) 5301. [9] M.J. Currie, J.K. Mapel, T.D. Heidel, S. Goffri, M.A. Baldo, Science 321 (2008) 226. [10] C.H. Chen, C.W. Tang, J. Shi, K.P. Klubek, Thin Solid Films 363 (2000) 327. [11] N. Komiya, R. Nishikawa, M. Okuyama, T. Yamada, Y. Saito, S. Oima, K. Yoneda, H. Kanno, H. Takahashi, G.Rageswaran, M. Itoh, M. Boroson, T.K. Hatwar, in: Proceedings of the 10th InternationalWorkshop on Inorganic and Organic EL (EL'00), Hamamatsu, Japan, 4 December 2000, p. 347. [12] Yuji Hamada, Hiroshi Kanno, Tsuyoshi Tsujioka, Hisakazu Takahashi, Tatsuro Usuki, Appl. Phys. Lett. 75 (1999) 1682. [13] R.H. Young, C.W. Tang, A.P. Marchietti, Appl. Phys. Lett. 80 (2002) 874. [14] T.H. Liu, C.Y. Iou, C.H. Chen, Appl. Phys. Lett. 83 (2003) 5241. [15] C.W Tang, S.A VanSlyke, C.H Chen, J. Appl. Phys. 65 (1989) 3610. [16] S. Forget, S. Chenais, D. Tondelier, B. Geffroy, I. Gozhyk, M. Lebental, E. Ishow, J. Appl. Phys. 108 (2010) 064509. [17] T. Forster, Discuss. Faraday Soc. 27 (1959) 7. [18] C.B. Lee, A. Uddin, X. Hu, T.G. Anderssonb, Mater. Sci. Eng.: B 112 (2004) 14. [19] L.F. Cheng, L.S. Liao, W.Y. Lai, X.H. Sun, N.B. Wong, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 319 (2000) 418. [20] F. Zhang, Z. Xu, D. Zhao, W. Jiang, G. Yuan, D. Song, Y Wang, X. Xu, J. Phys. D: Appl. Phys 40 (2007) 4485. [21] M. Janghouri, E. Mohajerani, A. Khabazi, Z. Abedi, H. Razavi, J. Lumin. 140 (2013) 7. [22] C.P. Cho, C.Y. Yu, T.P. Perng, Nanotechnology 17 (2006) 5506.
14
Z. Abedi et al. / Journal of Luminescence 147 (2014) 9–14
[23] C. Pei Cho, T. Pyng Perng, Nanotechnology 17 (2006) 3756. [24] P.O. Anikeeva, C.F. Madigan, S.A. Coe-Sullivan, J.S. Steckel, M.G. Bawendi, V. Bulović, Chem. Phys. Lett. 424 (2006) 120. [25] E.B. Namdas, A. Ruseckas, I.D.W. Samuel, S.-C. Lo, P.L. Burn, Appl. Phys. Lett. 86 (2005) 091104. [26] N. Matsusue, S. Ikame, Y. Suzuki, H. Naito, Appl. Phys. Lett. 97 (2005) 123512. [27] B. Li, J. Li, Y. Fu, Z. Bo, J. Am. Chem. Soc. 126 (2004) 3430. [28] E. Aminaka, T. Tsutsui, S. Saito, J. Appl. Phys. 79 (1996) 8808.
[29] W.T. Higginsa, A.P. Monkman, H.G. Nothofer, U. Scherf, J. Appl. Phys. 91 (2002) 99. [30] G.Y. Zhong, J. He, S.T. Zhang, Z. Xu, Z.H. Xiong, H.Z. Shi, X.M. Ding, W. Huang, X.Y. Hou, Appl. Phys. Lett. 80 (2002) 4846. [31] Y. Sakakibara, S. Okutsu, T. Enokida, T. Tani, Appl. Phys. Lett. 74 (1999) 2587. [32] M. Pfeiffer, K. Leo, X. Zhou, J.S. Huang, M. Hofmann, A. Werner, J.B. Nimoth, Org. Electron. 4 (2003) 89. [33] A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem. 37 (1998) 428.