Electroluminescence From Polar Nonlinear Optical ... - IEEE Xplore

JOURNAL OF DISPLAY TECHNOLOGY, VOL. 7, NO. 4, APRIL 2011

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Electroluminescence From Polar Nonlinear Optical Chromophore With Low Turn-On Voltage Mohammad Taghi Sharbati, Alireza Gharavi, and Farzin Emami

Abstract—Structure property relationships have been studied for polar and non-polar electro-luminescent molecules. The polar chromophore with a large red-shift shows a lower turn-on voltage than its non-polar counterparts. The – curves show characteristic diode behavior. The external quantum efficiency measurements for the polar molecule shows a four times higher quantum efficiency than the other molecules. Index Terms—EL, low turn-on voltage, nonlinear chromophore, organic light-emitting diode (OLED).

I. INTRODUCTION

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ANY optical/photonic functionalities have been demonstrated using nonlinear optical polymers and chromophores. Devices such as optical switches [1], optical waveguides [2], photonic crystals [3], Mach-Zehnder interferometers [4] and attenuators [5] have been investigated in our laboratories and elsewhere. The mentioned devices have been fabricated in the integrated form using polymers and organic materials. Many of these optical materials contain a conjugated chromophore with electron withdrawing and electron donating groups on opposite ends. Many of these choromophores are conjugated and could have electroluminescent properties as well. With proper layering and biasing, the same material in an optical integrated circuit could act as a light source along with active and passive switches, waveguides, etc. In display technologies, organic light emitting diodes show advantages over liquid crystal displays (LCD’s) such as better resolution and wider viewing angles [7], no backlight, higher brightness and lower power consumption [8]. In this study, two chromophores with different donor/acceptor groups were used to study their electroluminescence (EL) characteristics. The chromophore’s polar constituents and pi-conjugated length determine the extent of its nonlinearity and luminescence properties [6]. II. EXPERIMENTAL SETUP AND RESULTS The chromophores were synthesized in our laboratories using standard synthetic procedures. We manipulated the electron-

Manuscript received July 11, 2010; revised November 22, 2010; accepted December 10, 2010. Date of current version March 04, 2011. M. T. Sharbati and F. Emami are with the Electrical and Electronic Engineering Department, Shiraz University of Technology, Shiraz, Iran (e-mail: m.t. [email protected]; [email protected]; [email protected]). A. Gharavi is with the Electrical and Computer Engineering Department, Shiraz University, Shiraz, Iran (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JDT.2010.2100366

Fig. 1. Molecular structure of the dyes used: (a), (b) non-polar chromophores; (c) polar chromophore; and (d) structure of a single layer OLED. holes and electrons are recombined in the organic layer producing light. Cathode is Mg and transparent anode is ITO.

donating groups in the choromophores and electron-accepting groups giving rise to choromophores [see Fig. 1(a)-(c)]. Here we studied three chromophores: two non-polar chromophores [Fig. 1(a) and Fig. 1(b)] which have two identical end-groups; and, a polar chromophore [Fig. 1(c)], with electron-donating and electron-withdrawing groups at opposite ends. The polar choromophore had a nonlinear-optical coeffi, of about 10.1 pm/V [6]. Usual structures of OLEDs cient, are based on multilayer organic materials sandwiched between two electrodes. In a simple OLED structure, a light-emitting organic material is sandwiched between two electrodes, as shown in Fig. 1(c), with the metal electrode as cathode and ITO as anode. Circular devices, with 3-mm diameters, were made on 2 cm 2 cm ITO-coated glass slides. The active area of each device was 7.1 mm . The materials were deposited using a thermal evaporator under vacuum conditions at 2 10 torr without any annealing. The deposition rates were about 1 nm/s for all compounds, and the thicknesses ranged from 20 to 100 nm. Finally, by means of a shadow mask, with 3-mm diameter circular holes, a 100-nm thick Mg cathode was deposited by thermal evaporation at a pressure of 2 10 torr using a deposition rate of 3 nm/s. The thickness of each layer was monitored by a quartz thickness monitor. The current (I) versus voltage (V),

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TABLE I COMPARISON OF THE EMISSION PEAKS OF THE TWO MOLECULES

Fig. 2. Spectral characterization and comparison of the polar and non-polar molecules; (1) absorption; (2) photo-luminescence; and (3) electroluminescence (EL).

absorption, photoluminescence (PL), and EL spectra were measured under ambient conditions at room temperature. The absorption spectra of the three molecules are shown in Fig. 2(1). We measured the PL spectra by illuminating the samples using a narrow-band light source with a peak at 400 nm. Fig. 2(2) shows a comparison between the PL spectra of the three molecules. There was a large spectral red-shift in the polar material with respect to the non-polar materials which was due to the strong dipole in the molecule [9]. With stronger groups this shift could be even larger [10]. The EL spectrum of glass/ITO/dye/Mg for each material is shown in Fig. 2(3). A study of EL and PL spectra for each molecule gave a measure of the band gap for each molecule which was about 2.48 for the non-polar sulfonated chromophore Fig. 1(a), 2.38 eV for the two non-polar molecules of Fig. 1(b), and 2.07 eV for the polar choromophore. As expected there was a close correlation between the EL and PL of each material. The spectral shifts in the chromophores can be explained

by the degree of conjugation and the charge transfer effect in the molecule. A combination of the degree of conjugation and electron contribution of the end groups produced the different band gaps observed in the systems studied. The electron withdrawing groups decreased aromaticity in the system (Molecule a), where the electron donating groups increased the aromaticity, lowering the HOMO-LUMO gap (Molecule b). Moreover, the push-pull effect of the end groups in Molecule C further reduced the band-gap in that molecule [13]. The comparative red shifts are shown in Fig. 2. The OLED optical characteristics of the three materials are summarized in Table I. We observed that the turn-on E-field calculated from the turn-on voltages for different thicknesses gave almost the same E-field strength for all turn-on voltages for each material. For the non-polar material the turn-on field strength was about 1.2 10 V/cm and for the polar material it was about 0.8 10 V/cm. We considered the turn-on voltage at a point on the – curve where the current started to increase. For thicker organic layers there was a higher bias voltage for the diode to turn on and higher intensities were produced compared to the thinner devices. But the E-field strength remained the same. By applying a bias voltage, the devices showed a characteristic diode behavior. Customarily, multilayer OLED structures are used to improve OLED performance, such as efficiency, and operating voltage, as compared with single layer structures [11], [12]. Choromophore (c) showed a much lower turn-on voltage than choromophores (a) and (b) along with a much lower band gap. As observed in Fig. 3, the polar molecule showed a lower turn-on voltage than the other two chromophores. The electron donating and withdrawing groups, chemically attached to the choromophore, facilitated hole injection and electron injection properties into the conjugated backbone respectively and consequently lowered, the turn-on voltage. In choromophore (b) the electron donating (nitrogen) groups acted as hole injecting (electron blocking) agents, which appeared at both ends of the molecule, causing a higher turn-on voltage than in the polar molecule (c). However, in addition to the hole injecting group (nitrogen) in molecule (c) a sulfone group was also present, which acted as an electron injecting group because it was electron rich (the sulfone is an electron withdrawing group). The same can be said about molecule (a) where the sulfone groups were electron injecting (hole blocking) and, therefore, the holes would have to tunnel through to recombine in the molecule. Fig. 3 shows the current versus voltage characteristics of several devices with different thicknesses for compounds (a), (b) and (c). By increasing the bias

SHARBATI et al.: EL FROM POLAR NONLINEAR OPTICAL CHROMOPHORE WITH LOW TURN-ON VOLTAGE

Fig. 3. Current density versus voltage for different thicknesses of molecules: (a), (b) non-polar chromophores and molecule; and (c) polar chromophore. The nitrogen groups at both ends of non-polar molecule (b) act as hole transport groups preventing efficient electron injection. The sulfone in molecule (a) acts as an electron transport group preventing efficient hole injection.

voltage of the diode the charge carriers injected into the emission layer increased, resulting in higher emission intensities. In Fig. 4, the measured EL spectra are plotted for different applied voltages. The same EL spectral peaks were observed at different bias voltages, showing stable spectral characteristics with voltage. Since the thicknesses of both samples (polar and nonpolar) are comparable ( 30 nm) and the spectra were collected under identical conditions, the intensity information gives us a measure of the relative quantum efficiency of the two molecules. In Fig. 5 the emission efficiency of the polar molecule (c) almost doubled for all voltages with respect to non-polar molecule (b). To measure the external quantum efficiency of our OLEDs we devised the setup of Fig. 5. We placed a large area, 1 cm 1 cm, silicon detector directly against the OLED substrate to collect most of the emitted light. We calculated the light still escaping through the sides to be about 10% of the solid angle.

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Fig. 4. Light emission spectra of the OLEDs at different voltages. (a) and (b) Non-polar chromophore. (c) Polar chromophore.

Fig. 5. External quantum efficiency measuring setup. (a) Glass substrate coated with ITO (2 cm 2 cm, 1 mm thick) (b) OLED pixel with 3 mm diameter (c) escaped light (d) Si detector with 1 cm 1 cm dimension.

2

2

We also attributed 10% more of light loss due to the interface reflections of the glass substrate and the detector. Therefore, a total of 20% of the photons lost were accounted for in the

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Fig. 6. Quantum efficiency of Si detector.

calculation. To calculate the quantum efficiency we obtained the following formula:

2

Fig. 7. The measured external quantum efficiency of OLEDs, 55 10 %, 9 10 % and 2 10 % for non-polar 1, non-polar 2, and polar chromophore respectively.

2

2

is the average of quantum efficiency for detector; where is the input injected current; : is the interface reflector : losses due to light escape. losses; Using these numbers we came up with a QE versus current density shown in Fig. 7 . While the measurement shows a QE for all three samples, the significance is that the polar chromophore showed a four times higher QE than the non-polar chromophore (a). III. CONCLUSION

where and are the external quantum efficiency of the OLED and detector respectively; : number of emitted photons from OLED; : number of injected electrons; : output current detected from sample. To simplify the calculation we replaced the wavelength dependent quantum efficiency of the detector with an average quantum efficiency calculated from the quantum efficiency graph of the detector, as shown in Fig. 6. In the range between 400 to 800 nm, the graph can be divided into two straight-line in the first portions. Between 400 to 500 nm the in portion of the graph. Between 500–800 nm the the second portion of the graph. The average at these two points of 0.7, which is used in our calculations. gives a total At the same time we could find the for the range of interest by integrating the QE graph of the detector in that range and divide by the number of points. Doing so gave the same . number 0.7 for

where

and

In summary, single-layer OLEDs have been fabricated with low turn-on voltages and nonlinear optical properties. The devices made with non-polar and polar materials emitted EL light with maximums at 523 and 600 nm, respectively. By changing the electron donating/withdrawing groups to produce a polar molecule, a large red-shift was observed in the EL spectrum, the turn on voltage was lowered by half and the quantum efficiency was increased. REFERENCES [1] E. Sarailou, A. Gharavi, S. Javadpour, and V. Shkunov, “Grating based electro-optic switch with azo nonlinear optical polymers,” Appl. Phys. Lett., vol. 89, pp. 171114–171117, 2006. [2] P. D. Parasad and D. J. Williams, Introduction to Nonlinear Optical Effect in Molecules and Polymers. New York: Wiley, 1991. [3] H. Karimi-Alavijeh, G.-M. Parsanasab, M.-A. Baghban, and A. Gharavi, “Two-dimensional photonic crystal for optical channel separation in azo polymers,” Appl. Opt., vol. 48, pp. 3250–3254, 2009. [4] A. Gharavi, “Biosensors with polymeric optical waveguides,” U.S. Patent 6 429 023, Aug. 6, 2002. [5] A. Gharavi, “Grating and polymeric based optical attenuators and modulators,” U.S. Patent 7016580, Mar. 21, 2006. [6] J. Y. Do, S. K. Park, J. J. Ju, M.-s. Kim, S. Park, M.-H. Lee, B. G. Kim, and M. Y. Jeong, “Alkyl sulfone-containing optical polyimide for an efficient wavelength conversion,” Opt. Mater., vol. 29, pp. 1563–1570, 2007. [7] O. Nuyken, S. Jungermann, V. Wiederhirn, E. Bacher, and K. Meerholz, Modern Trends in Organic Light-Emitting Devices (OLEDs). New York: Springer-Verlag, 2006, vol. 137, pp. 811–824. [8] C. W. Tang and S. A. Vanslyke, “Organic electroluminescent diodes,” Appl. Phys. Lett., vol. 51, pp. 913–916, 1987. [9] A. Chaieb, L. Vignau, R. Brown, G. Wantz, N. Huby, J. Francois, and C. Dagron-Lartigau, “PL and EL properties of oligo( p-phenylene vinylene) (OPPV) derivatives and their applications in organic light-emitting diodes (OLED),” Opt. Mater., vol. 31, pp. 68–74, 2008.

SHARBATI et al.: EL FROM POLAR NONLINEAR OPTICAL CHROMOPHORE WITH LOW TURN-ON VOLTAGE

[10] F. Wu, W. Tian, J. Sun, J. Shen, X. Pan, and Z. Su, “Study on the electronic structure of phenylene vinylene dimers with different substituents,” Mater. Sci. Eng., vol. B85, pp. 165–168, 2001. [11] M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, and S. R. Forrest, “1.55 reflection-type optical waveguide switch based on SiGe/Si plasma dispersion effect,” Appl. Phys. Lett., vol. 75, pp. 1–3, 1999. [12] C. Di, G. Yu, Y. Liu, X. Xu, Y. Song, and D. Zhu, “High-efficiency low operation voltage organic light-emitting diodes,” Appl Phys. Lett., vol. 90, pp. 133508–133511, 2007. [13] T. Vijayakumar, I. H. Joe, C. P. R. Nair, and V. S. Jayakumar, “Efficient electrons delocalization in prospective push-pull non-linear optical chromophore 4-N,N-dimethylamino-40-nitro stilbene (DANS): A vibrational spectroscopic study,” Chem. Phys., vol. 343, pp. 83–99, 2008. [14] H. Liu, F. Yan, W. Li, B. Chu, W. Su, Z. Su, J. Wang, Z. Hu, and Z. Zhang, “Remarkable increase in the efficiency of N,N -dimethylquinacridone dye heavily doped organic light emitting diodes under high crrent density,” Appl. Phys. Lett., vol. 96, pp. 083301–083301, 2010. [15] N. Hai, Z. Bo, and T. Xian-Zhong, “Significant improvement of OLED efficiency and stability by doping both HTL and ETL with different dopant in hetrojunction of polymer/small molecule,” Chinese Phys., vol. 16, p. 730, 2007.

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Mohammad Taghi Sharbati was born in Gorgan, Iran, on May 24, 1981. He received the B.S. degree in electronic engineering from the Faculty of Engineering, Mazandaran University, Babol, Iran, in 2006, and the M.S. degree from the Department of Electrical and Electronic Engineering, Faculty of Engineering, Shiraz University of Technology, Shiraz, Iran, in 2009. He is currently with the Optoelectronic Research Center, Shiraz University of Technology, Shiraz, Iran. His research interests are in the field of optoelectronics, organic light-emitting diodes (OLEDs), integrated optics, organic electronic and semiconductor devices.

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Alireza Gharavi was born in 1957 in Tehran, Iran. After graduating from high school majoring in math and sciences, he received the B.S. degree in electrical engineering and the M.S. degree in applied physics from the University of New Orleans, and the Ph.D. degree in chemistry from Tulane University, New Orleans, LA, in 1994. After a three year Post-Doctorate on NLO polymers at the University of Chicago with Professor Luping Yu, he moved on to establish two high-tech startup companies working on photonic devices using NLO polymers. His activities during the next five years resulted in six U.S. patents on devices and polymeric materials for photonic applications. In 2004, he joined the Electrical Engineering Department, Shiraz University, Shiraz, Iran, in an academic position, where has been teaching electrical and nonlinear optics courses, and continued his research while at Shiraz University, and published many articles on these subjects.

Farzin Emami was born in Shiraz, Iran, on July 30, 1967. He received the B.S. degree in electronic engineering from Shiraz (Pahlavi) University, Shiraz, Iran, in 1990, and the M.S. and Ph.D. degrees in laser optics from Khaje-Nasireddin Tousi and Tarbiyat Modares of Tehran-Iran in 1994 and 2002, respectively. From 2001 to 2003 he worked on semiconductor lasers simulations. In 2003 he was the head of the electronic department of Electronic Industrial University of Shiraz, where he worked on a variety of optoelectronic devices including optical modulators, optical detectors, semiconductor optical amplifiers, Raman amplifiers, photonic crystals, neutron transmission doping, and organic light emitting diodes. In September 2004, he joined the faculty of Shiraz University of Technology, Shiraz, Iran. At present, he is actively involved in optical telecommunication and has a few M.S. and Ph.D. students in this relation. He has more than 27 journal and conference papers.