Flexible light-emitting three-terminal device with color-controlled emission

Organic Electronics 10 (2009) 426–431

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Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Flexible light-emitting three-terminal device with color-controlled emission Sung Heum Park a,b, Shinuk Cho a, Jae Kwan Lee a, Kwanghee Lee a,b,*, Alan J. Heeger a,b,* a b

Center for Polymer and Organic Solids, University of California at Santa Barbara, Santa Barbara, CA 93106, USA Heeger Center for Advanced Materials, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, South Korea

a r t i c l e

i n f o

Article history: Received 14 October 2008 Received in revised form 6 January 2009 Accepted 8 January 2009 Available online 22 January 2009

PACS: 72.80 Le 78.60 Fi 85.60 Jb

a b s t r a c t A new architecture for flexible light-emitting devices with color-controlled emission is described. The various layers in these novel three-terminal devices are fabricated by casting soluble conjugated polymers from solution. The light-emitting three-terminal device (LET) can be described as comprising two polymer light-emitting diodes (LEDs) connected back-to-back with an internal common electrode. By controlling the bias voltages between the common buried electrode and the two outer electrodes, the LET can be turned on/off, and the emission color can be switched and modulated. Thus, the LET architecture provides a route to flexible displays that can be fabricated by printing technology with pixels that are color-switchable and color-tunable. Ó 2009 Elsevier B.V. All rights reserved.

Keywords: Flexible devices Light-emitting three-terminal device Colortunable devices Polymer light-emitting diode

1. Introduction High information content displays for future applications must be lower in cost, thinner, lighter and mechanically flexible. These demanding features are required to enable paper-like, portable ‘‘roll-up” and disposable displays [1–3]. Moreover, since such displays must have a device structure that is sufficiently simple to be compatible with low-cost manufacturing, it is important to reduce the number and complexity of circuit elements by integrating multiple functions into the components within each pixel.

* Corresponding authors. Address: Center for Polymer and Organic Solids, University of California at Santa Barbara, Santa Barbara, CA 93106, USA. E-mail addresses: [email protected] (K. Lee), [email protected] (A.J. Heeger). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.01.008

We report here that we have successfully demonstrated a new architecture for flexible polymer-based, three-terminal devices with all layers cast from solution. The light emitting three-terminal device (LET) can be described as comprising two polymer light-emitting diodes (LEDs) connected back-to-back with an internal common electrode. By controlling the bias voltages between the common buried electrode and the two outer electrodes, the device can be turned on/off, and the emission color can be switched and modulated. Plastic electronic devices made from semiconducting and metallic polymers offer a number of potential advantages, including lightweight, flexibility, and fabrication by printing/coating methods that enable low-cost manufacturing [4–6]. Moreover, multifunctional devices have been demonstrated, including light-emitting transistors [7,8], light-emitting solar cells (LESCs) [9] and light-emitting photo-diodes [10].

S.H. Park et al. / Organic Electronics 10 (2009) 426–431

The pixels within high information content active matrix electroluminescent displays require two main functions: light generation from a LED and on/off switching by a field-effect transistor (FET). Full color is obtained by dividing each pixel into three sub-pixels which emit red, green and blue, respectively. Demonstration of a flexible three-terminal light emitting device that can be turned on/off and color-controlled by applied voltages is, therefore, of interest. Voltage controlled color change has been demonstrated by using polymer blends [11] or multilayer stacks [12,13] for EL, or by using voltage controlled thermochromism [14]. Vertical-type color-controlled light emitting devices (CCLEDs) have been demonstrated using independently stacked electrode and emitting layers [15,16]. Light emitting transistors were shown to function as an LED and a transistor in series [17,18]. \Vertical-type CCLEDs are particularly interesting because the emission of each emitting layer can be controlled independently, thereby maximizing display resolution with high color purity. In addition, they also exhibit a high aperture ratio for light-emission compared with a device having a narrow light emitting region [7,18]. In spite of such advantages, however, known vertical-type CCLEDs are hard to consider as components for next generation flexible displays because of the inflexibility of their components and the relative complexity of fabrication (vapor deposition of many layers of small molecule organic semiconductors). With the LET described in here, we have successfully addressed the challenge of creating flexible CCLEDs fabricated by solution processing.

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2. Results and discussion 2.1. LET structure Fig. 1 shows the structure of the LET together with the molecular structures and the electronic structures of the component materials. The LET is equivalent to two polymer LEDs connected back-to-back with poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate), PEDOT:PSS, as the internal common electrode. The first diode with structure Al/poly [2-(4-(3’,7’-dimethyloctyloxy)-phenyl)-p-phenylenevinylene](P-PPV)/PEDOT:PSS (Bottom) emits green light. The second diode with structure PEDOT:PSS/poly(2methoxy, 5-(2,-ethyl-hexoxy)-1,4-phenylene vinylene) (MEH-PPV)/Al (Top) emits orange-red light. The function of the PEDOT:PSS layer is analogous to that of the polyaniline network layer in the polymer grid triode (PGT) [19,20]. Since the PEDOT:PSS electrode is continuous, there is no direct current path between the two outer electrodes. Thus, the LET described here is equivalent to two back-to-back diodes in contrast to the polymer grid triode (the latter can, in principle, exhibit gain). Three different conducting materials were used as electrodes, each with a different work function: Al (4.3 eV), Ba:Al (2.7 eV) and PEDOT:PSS (5.2 eV) (see Fig. 1b). Semitransparent Al with a thickness of 10 nm is used as a low work function metal for injecting electrons into the bottom PLED, while Ba:Al (100 nm) is used for injecting electrons into the top PLED. The high work function semitransparent PEDOT:PSS conducting polymer is used as the internal anode for hole injection into both the top and bottom PLEDs. Because the Al electrode is grounded, the Ba:Al

Fig. 1. (a) Schematic of the device cross-section: the device consists of two polymer light emitting diodes, Al/P-PPV/PEDOT:PSS and PEDOT:PSS/MEH-PPV/ Ba:Al, connected back-to-back. The P-PPV diode emits green light; the MEH-PPV diode emits red-orange light. (b) Energy level diagram.

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and PEDOT:PSS electrodes can also be switched to perform either the functions of anode or cathode under different applied bias conditions. The water-soluble PEDOT:PSS layer enables the fabrication of the multilayered device architecture. Because P-PPV and MEH-PPV are insoluble in water and PEDOT:PSS is insoluble in organic solvents, the various layers remain intact with no intermixing. The emission from MEH-PPV in the top LED must pass through the PEDOT:PSS layer and the P-PPV layer; the former is semitransparent and the absorption edge of the latter is at a photon energy above that of the emission from MEH-PPV. Earlier work has demonstrated that P-PPV shows high luminous intensity in single devices using Al as the cathode material [21,22]. The semitransparent Al electrode (thickness of 10 nm) was deposited on a polyethersulfone (PES) substrate by thermal evaporation in a vacuum of about 5  10 7 Torr. The P-PPV was spin-cast (2000 rpm) from solution [1 wt.% in chlorobenzene (CB)] on top of the Al electrode, and baked at 80 °C for 60 min in a glove box. Because the P-PPV film is hydrophobic, we have deposited a thin layer of TiOx [23–25] onto the P-PPV film before depositing hydrophilic PEDOT:PSS layer. The dilute TiOx precursor solution was spin-cast onto the P-PPV emitting layer at 6000 rpm for only 5 s and the PEDOT:PSS solution was immediately dropped onto the TiOx film. Since a continu-

ous film of TiOx is not typically formed under such spincasting conditions, the TiOx (amphiphilic) acts principally as a ‘‘glue” that assures adhesion between the P-PPV (hydrophobic) and PEDOT:PSS (hydrophilic) layers. Highly conductive PEDOT:PSS (Baytron PH 500) solution mixed with DMSO solvent [26] was dropped on the TiOx-treated P-PPV film without substrate spinning and then baked at 30 °C for 3 h. This results in higher conductivity for PEDOT:PSS (the extended annealing at 30 °C increases the degree of crystallinity of PEDOT:PSS). The MEH-PPV solution was spin-cast on top of the PEDOT:PSS layer. Finally, Ba (10 nm):Al(100 nm) was deposited by thermal evaporation in a vacuum of about 5  10 7 Torr. Since the LET has three-terminals, there are three different current pathways: I1, I2 and I3 as shown Fig. 2a. I1 is the current between the PEDOT:PSS and Al electrode, I2 is the current between the PEDOT:PSS and Ba:Al electrode, and I3 is the current between the Ba:Al and Al electrode. I1, I2 and I3 a are determined by the electric field (E-field) generated by the combination of VA voltage (Ba:Al to Al electrode) and VB voltage (PEDOT:PSS to Al electrode). In the zero-bias condition shown in the energy diagram of Fig. 2b, the E-field in each layer results from the difference between the work functions of the two neighboring electrodes (EG for P-PPV and ER for MEH-PPV). We assume

Fig. 2. (a) Schematic of current paths in the device. I1, I2 and I3 represent the hole currents between the PEDOT:PSS and Al electrode, PEDOT:PSS and Ba:Al electrode, and Ba:Al and Al electrode, respectively. (b) Energy band diagram for zero-bias condition.

Fig. 3. Energy band diagram for different bias conditions (see text).

S.H. Park et al. / Organic Electronics 10 (2009) 426–431

a

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the direction of E-field as positive from PEDOT:PSS to both outside electrodes. Fig. 3 shows four different combinations of EG and ER in the device. Fig 3a represents zero (or small) bias; Fig. 3a is similar to Fig. 2b in that both diodes are in reverse bias. Under the conditions described by Fig. 3b, the first diode is back-biased while the second diode is forward biased. In Fig. 3c, both diodes are in forward bias. In Fig. 3d, the first diode is in forward bias and the second diode is in reverse bias.

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2.2. LET performance This device operating concept described above is verified by the measurements of current density (I) – voltage (V) characteristics at various external biases. A plot of IA (current from Al to Ba:Al, see Fig. 2a) versus VA voltage under various VB is shown in Fig. 4a. For VB = 0, the I–V curve exhibits general PLED characteristics except symmetric curve shape [27]. The current increases with increasing voltage. Control of the IA–VA characteristics by the application of VB is shown in Fig. 4a. With VB = 10 V (red line), IA is negative below VA < 9 V and IA – 0 even for VA = 0 V. In contrast, when VB is biased at 10 V (blue line), IA is positive above VA > 7 V and current flows even at VA = 0 V. This tendency is also observed in the control of the IB– VB (current and voltage from Al to Ba:Al, see Fig. 2 a) characteristics by the application of VA in Fig. 4b. For VA = 10 V (red line), IB is negative below VB < 8 V and IB – 0 even for VA = 0 V. In contrast, when VA is biased at 10 V (blue line), IB is positive above VB > 9 V and current flows even at VB = 0 V. Since IA is determined by the combination of VA and VB (EG and ER), these current behaviors can be understood by the comparison of the energy level diagrams shown in Fig. 3a–d. For VB = 10 V and VA < 9 V in Fig. 4a, both EG and ER are positive (corresponding to Fig. 3c). Consequently holes are injected from PEDOT:PSS to Ba:Al. This corresponds to IA < 0 because Ba:Al is the anode. For VB = 10 V and VA > 7 V in Fig. 4a, therefore, both EG and ER are negative (corresponding to Fig. 3a). On the other hand, IB has two current pathways, I1 and I2 (see Fig. 2a). Thus holes from PEDOT:PSS can be injected to Al [I1] and Ba:Al [I2]. For VA = 10 V and VB > 9 V in Fig 4b, the holes are injected from PEDOT:PSS to Ba:Al (corresponding to Fig. 3b). Under high +VB, especially, the holes are injected from PEDOT:PSS to both Al and Ba:Al. This corresponds to the same E-field configuration shown in Fig. 3c. For VA = 10 V and VB < 9 V in Fig. 4b, holes are withdrawn at PEDOT:PSS from Ba:Al. This hole injection from Ba:Al is caused by the strongly reversed bias ER (see Fig 3a). Under these conditions, IB < 0. When VB increases from negative to positive, the negative IB current decreases. Moreover, when VB > 9 V, EG > 0 (corresponding to Fig 3d) and holes are injected from PEDOT:PSS to Al. Therefore IB changes sign from negative to positive. These experimental results are fully consistent with the device operating concept shown in the energy diagrams of Fig. 3a–d. The data clearly indicate that one can not only control IA by controlling VB at fixed VA, but also IB by controlling VA at fixed VB.

MEH-PPV P-PPV

2.3. Color switching

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Since the energy barrier corresponding to the difference between the workfunction of PEDOT:PSS and the LUMO levels of two emitting polymers is much higher than those of PEDOT:PSS and the HOMO levels, electron injection from the PEDOT:PSS layer into the emitting layers is inhibited [27]; i.e. the device can usually emit the light under positive EG or positive ER.

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Wavelength (nm) Fig. 5. (a) Electroluminescent spectra of the device for various VA and VB combinations. The emission spectrum can be switched from that of P-PPV to that of MEH-PPV. (b) Photos of color switching from the LET. The LET fabricated on PES is flexible. (c) EL spectra for various VB at fixed VA voltage. The EL spectra are dramatically changed for different VB. (d) Evolution of color coordinates. The chromaticity coordinates on a CIE diagram can be changed from x = 0.5, y = 0.37 to x = 0.37, y = 0.5 by controlling the VA and VB voltage.

Because both EG and ER can be changed in both sign and magnitude by the combination of VA and VB, the device can function as a color switch. Fig. 4c shows the luminance (LB) – voltage (VB) characteristics at various external VA biases. For VA = 10 V (red line) and VB = 10 V (ER < 0, EG > 0) in Fig. 4c, holes are injected into the P-PPV layer and electrons are injected into the P-PPV layer from Al. Consequently the device emits light from the P-PPV layer. For VA = 10 V (blue line) and VB = 0 V (ER > 0, EG < 0), however, holes are injected from PEDOT:PSS into MEH-PPV and electrons are injected into MEH-PPV from Ba:Al. Thus the device emits light from MEH-PPV layer. Since the emission of each layer is governed by a combination of both IA and IB currents, the calculation of luminous efficiency defined by luminance intensity for injected current is non-trivial. Here, for simplicity, we use the current IB as the basis for calculating the luminous efficiency. For VB = 15 V, the luminous efficiency was 0.05 cd/A for the P-PPV layer and 0.03 cd/A for the MEH-PPV layer. Fig. 5a shows the electroluminescent spectra of the device with various combinations of VA and VB. When EG > 0 and ER < 0 (VA = 10 V and VB = 10 V), the LET exhibits the typical green emission of P-PPV. However,

when EG < 0 and ER > 0(VA = 10 V and VB = 0 V), the emission color switches to orange-red corresponding to that of MEH-PPV. The LET does not emit under EG < 0 and ER < 0. Fig. 5b directly illustrates the color switching with color photos obtained by varying VA and VB (orange-red color or green color from a single LET). In addition, since the device consists of ductile metals and polymers layer fabricated on top of the plastic PES substrate, the LET is flexible (total device thickness of 200 lm). Interestingly, for EG > 0 and ER > 0, the device can emit both colors simultaneously and the intensity of each color can be controlled by varying VB and VA. Because the emission color can be changed continuously by varying the ratio of emission from MEH-PPV and P-PPV(see the blue line in Fig 4c), the LET can function as a color-tunable device. The EL spectra of the device for various VB at fixed VA are shown in Fig. 5c. The spectra change dramatically for different VB. As a consequence, the color of the device gradually changes continuously from orange-red to green. The corresponding chromaticity coordinates on a CIE (Commission Internationale de l’Eclairage) 1931 diagram, see Fig. 5d, change from x = 0.5, y = 0.37 to x = 0.37, y = 0.5.

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3. Conclusion We have successfully demonstrated a flexible, light emitting and color-switchable polymer-based three-terminal devices. The LET functions as a color switch and a color modulator. By varying the bias voltages between the common internal electrode and the two outer electrodes, the LET can be turned on/off, the emission color can be switched, and the color (and brightness) can be modulated. Moreover, the multilayer LET structure can be fabricated with each layer cast from solution. Acknowledgement The research was supported by a grant from Samsung Advanced Institute of Technology. S. H. Park was partially supported by the Korea Research Foundation funded by the Korean Government (KRF-2007-357-D00076). References [1] S.R. Forrest, Nature 428 (2004) 911. [2] T. Sekitani, M. Takamiya, Y. Noguchi, S. Nakano, Y. Kato, T. Sakurai, T. Someya, Nature 6 (2007) 413. [3] D.J. Gundlach, Nature 6 (2007) 173. [4] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C.D. Taliani, D.C. Bradley, D.A. Dos Santos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Nature 397 (1999) 121. [5] G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colaneri, A.J. Heeger, Nature 357 (1992) 477.

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