Voltage-Tunable Multicolor Electroluminescence from Single-Layer

Chapter 14

Voltage-Tunable Multicolor Electroluminescence from Single-Layer Polymer Blends and Bilayer Polymer Films Downloaded by COLUMBIA UNIV on September 7, 2012 | http://pubs.acs.org Publication Date: August 17, 2004 | doi: 10.1021/bk-2005-0888.ch014

Maksudul M. Alam, Christopher J. Tonzola, Yan Zhu, and Samson A . Jenekhe* Departments of Chemical Engineering and Chemistry, University of Washington, Box 351750, Seattle, WA 98195-1750

Voltage-tunable orangeyellowgreen electroluminescence was observed from light-emitting diodes made from binary blends of n-type poly(2,2'-(3,3'-dioctyl-2,2'-bithienylene)6,6'-bis(4-phenylquinoline)) or poly(4-hexylquinoline) or poly(4-octylquinoline) with p-type poly(2-methoxy-5-(2'ethylhexyloxy)-1,4-phenylene vinylene). Bilayer poly(p -phenylene vinylene)/poly(9,9-dioctylfluorene) light-emitting diodes had voltage-tunable greenblue colors. A large enhancement in performance was observed in both the blend and bilayer color-tunable light-emitting diodes. The blend devices had a turn-on voltage as low as 5 V, a luminance of up to 1490 cd/m , and an external electroluminescence efficiency of up to 0.7%. Theseresultsdemonstrate that proper choice of electroluminescent polymers with favorable electronic structures can facilitate achievement of efficient multicolor light-emitting diodesfromdonor/acceptor polymer blends. 2

Conjugated polymers have good mechanical, electrical, and luminescent properties which lend them to the development of light-emitting diodes (LEDs) (1-14% photovoltaic cells (15-17), thin film transistors (18-20), imaging photoreceptors (21,22), and other device applications. LEDs based on conjugated polymers are being developed for various applications, including flat-panel displays and lighting (1-5). Conjugated polymer semiconductors offer many important advantages as the emissive materials in LEDs: low cost, light weight, large-area solution-processing of thinfilms,flexiblepanel displays with wide viewing angle, and full color emissive displays (1-6). Electroluminescence

188

© 2005 American Chemical Society In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

Downloaded by COLUMBIA UNIV on September 7, 2012 | http://pubs.acs.org Publication Date: August 17, 2004 | doi: 10.1021/bk-2005-0888.ch014

189 (EL) colors spanning blue, green, yellow, orange, and red have been obtained through synthetic manipulation of molecular and supramolecular architectures of conjugated polymers (7,8,23). In addition, multicomponent conjugated polymer systems such as multilayered thin films (5,9-11), blends (7,13,14), and block copolymers (24) offer additional capabilities and flexibilities in the design of polymer LEDs. Conceptually, in such multicomponent conjugated polymer systems, each polymer can contribute its emission spectrum by selective excitation depending on the extent of any intermolecular interactions that can lead to complications due to exciplex formation or energy transfer among the conçonents (21,25). Although color-tunable electroluminescence has previously been observed in multicomponent conjugated polymer systems (5,7-11), the most efficient devices reported were those comprised of multilayered conjugated polymers and other charge transport layers. Blending two or more emissive polymers seems to provide a simple and effective way to improve the EL efficiency and to obtain color-tunable EL emission. To date, polymer blend LEDs have been made from blends of the same electron donating (p-type) or electron accepting (n-type) components (7,13,14). In either the p-type blend or η-type blend cases, a separate charge transport layer is necessary in the devices. One avenue to improve the performance (i.e. lower turn-on voltage, higher E L efficiency and luminance) and stability of polymer LEDs is through blending of n-type (electron transport) polymers with p-type (hole transport) polymers. In the multilayered LEDs, the tunable colors were observed mainly in the redyellowgreen range (5,9-11). Proper selection of component E L polymers for other multilayered LEDs such as blue emitting poly(p-phenylene)s (1,4) or polyfluorenes (1,4) as one component can readily extend the accessible tunable multicolor EL throughout the CIE diagram. In this paper, we report voltage-tunable multicolor electroluminescence from devices made from binary blends of polyil^'-iS^'-dioctyl^^'bithienylene)-6,6'-bis(4-phenylquinoline)) (POBTPQ) or poly(4-hexylquinoline) (P4HQ) or poly(4-octylquinoline) (P40Q), as an n-type (electron transport) component with poly(2-methoxy-5(2'-ethyl-hexyloxy)-l,4-phenylene vinylene) (MEH-PPV) as a p-type (hole transport) component. These results represent the first achievement of efficient multicolor LEDs from electron donor/acceptor conjugated polymer blends. Bilayer LEDs made from green emitting poly(pphenylene vinylene) (PPV) and blue emitting poly(9,9-dioctylfluorene) (PFO) were also investigated to obtain greenblue color-tunable devices for the first time. The morphology of the blends was also investigated by atomic force microscopy (AFM).

Experimental Materials The polyquinolines, POBTPQ, P4HQ and P40Q, used in this study were synthesized in our laboratory. The synthesis, characterization, electrochemistry, In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

190 thinfilmprocessing, optical, and electroluminescent properties of POBTPQ (M ~ 64 900), P4HQ (M ~ 15 150), and P40Q (M ~ 10 250) were reported by our group (26,27). Both the MEH-PPV (M ~ 85 000) and the PFO (M ~ 10 000) samples used here were purchased from American Dye Source, Inc. Poly(ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT; solution in water) and the sulfonium precursor of PPV in ~ 1 wt % methanol solution were purchased from Aldrich Chemical Co. and Lark Enterprises, respectively. All solvents were of spectroscopic grade and were used as received. The molecular structures of the conjugated polymers used in mis study are shown in Chart 1. w

w

w


w

w

Chart 1

R = C H, (POBTPQ) 8

MeO

7

MEH-PPV

R = C H ;P4HQ C H ; P40Q 6

13

8

17

R- C H PFO 8

17

Preparation of Blends and Thin Films Binary blends of POBTPQ and MEH-PPV were prepared by dissolving them with appropriate weight percent ratio in chloroform in which both polymers are very soluble. The resulting solutions were homogeneous. Compositions of blends in this paper refer to weight percentage of MEH-PPV. Thin films of the homopolymers or binary blends were obtained by spin-coatingfromtheir CHC1 solutions (0.5 wt%). Similarly, the thinfilmsof binary P4HQ:MEH-PPV and P40Q.MEH-PPV blends were obtainedfromtheir blend solutions (0.5 wt%) in CHCI3. The thinfilmsused for optical absorption and photoluminescence (PL) measurements were spin-coated onto silica substrates. All thefilmswere dried in vacuum at 60 °C for 8 h to remove any residual solvent. The thinfilmsof binary blends were homogeneous and showed excellent optical transparency. No visible phase separation was observed. For morphological investigations, the polymer blends were spin coatedfromtheir solutions in CHCI3 onto polished silicon substrates and the resulting thinfilmswere dried at 60 °C in vacuum for 12 h. 3

In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

191 Photophysics and Surface Morphology Optical absorption spectra were obtained by using a Lambda-900 UV/vis/near-IR spectrophotometer (Perkin-Elmer). Photoluminescence (PL) studies were carried out on a Spex Fluolog-2 spectrofluorimeter. The thin films were positioned such that the emitted light was detected at 22.5° from the incident beam (5,5). For surface morphology of the binary polymer blends, tapping-mode Atomic Force Microscopy (AFM) images were obtained using a NanoScope III instrument (Digital Instruments Inc., Santa Barbara, CA).


Fabrication and Characterization of LEDs Electroluminescent (EL) devices were fabricated and investigated as sandwich structures between two electrodes where aluminum (Al) was used as the cathode and indium-tin oxide (ITO) was used as the anode. Schematic structures of the polymer EL devices are shown in Figure 1. The thinfilmsof homopolymers or their binary blends were spin coatedfromtheir solution in CHC1 onto pre-cleaned ITO-coated glass substrates (Delta Technologies, Ltd., Stillwater, MN; 1^ = 8-12 Ω/D) and dried at 60 °C in vacuum for 8 h. In some devices, a thin layer of poly(ethylenedioxythiophene):polystyrenesulfonate (PEDOT) (< 40 nm) wasfirstspin coatedfromits solution in water onto ITO and dried at 80 °C in vacuum for 10 h. Then, the homopolymers or their blend layers were spin coated onto the PEDOT layer and dried in vacuum at 60 °C for 8 h. For bilayer LEDs, PPV thin films were spin coated onto ITOfroma sulfonium precursor solution in methanol followed by thermal conversion in vacuum at 250 °C for 1.5 h (6). Thinfilmsof PFO were spin coatedfromits 1.0 wt % solution in CHC1 onto the PPV layer and then dried in vacuum at 60 °C for 10 h. The film thicknesses were measured by an Alpha-step profilometer (Model 500, KLA Tencor, San Jose, CA) with an accuracy of ± 1 nm and confirmed by an optical absorption coefficient technique. Finally, 100 - 120 nm thick aluminum layer was thermally deposited under high vacuum (~ 3 * 10" torr) onto the resulting polymer layer to form an active diode area of 0.2 cm (5mm diameter). 3

3

6

2

Figure 1. Schematic structure of (a) single-layer and (b) bilayer polymer LEDs. In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.


192 Electroluminescence (EL) spectra were measured on a SPEX Fluorolog-2 spectrofluorimeter. Electroluminescence microscopy of the LEDs was done by using a Leica fluorescence microscope. The true color images of light emitted from the LEDs under applied bias voltages was captured by a Hamamatsu Orca Π CCD camera (C4742-98) using an Openlab 2.2.5 imaging software in a PC. Current-voltage (I-V) and luminance-voltage (L-V) curves were recorded simultaneously by hooking up an HP4155A semiconductor parameter analyzer (Yokogawa Hewlett-Packard, Tokyo) together with a Grasby S370 optometer (Grasby Optronics, Orlando, FL) equipped with a calibrated luminance sensor head (Model 211) (5,6,8). The EL quantum efficiencies of the diodes were estimated by using procedures similar to that previously reported (5,6,8). All the fabrication and measurements were done under ambient laboratory conditions.

Results and Discussion Morphology and Photophysics of Binary Blends of Conjugated Polymers The 5 μηι χ 5 μηι AFM phase images of the surface morphology of a POBTPQ:MEH-PPV blend (60:40 wt% ratio) along with the homopolymers are shown in Plate 1. The AFM images of both homopolymers were featureless. Two distinct phases were observed in die morphologies of POBTPQ:MEH-PPV blend as exemplified by the AFM image of the 60:40 wt% blend shown in Plate 1. The phase separation length-scale was in the range of 90 to 120 nm. The nanophase separation observed for this blend is due to spinodal decomposition. Similar phase separation behavior was also observed in the surface morphologies of P4HQ:MEH-PPV and P40Q:MEH-PPV blend systems. The observed lengthscale of the phase-separation is in the range for which voltage-tunable EL color can be expected (J, 7,9,13).

Figure 2. Optical absorption and photoluminescence spectra of thin films: (a) POBTPQ and MEH-PPV; (b) P4HQ and P40Q. In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.


193

Plate 1. Tapping-mode AFM phase images ofPOBTPQ„POBTPQ:MEH-PPV blend (60:40 wt%) and MEH-PPV on polished silicon substrate.

Optical absorption and PL spectra of thinfilmsof POBTPQ and MEH-PPV homopolymers are shown in Figure 2a. The lowest energy η -π* transition bands of POBTPQ and MEH-PPV are at 414 and 500 nm, respectively. POBTPQ has green PL emission with peak at 530 nm whereas MEH-PPV has orange-red PL emission with peak at 585 nm. The PL spectrum of POBTPQ and the absorption spectrum of MEH-PPV overlap to a reasonable extent in the 450 - 600 nm region (Figure 2a)fromwhich energy transferfromPOBTPQ to MEH-PPV can be expected (14). Figure 2b shows the absorption and PL spectra of thinfilmsof P4HQ and P40Q. These two polymers have identical lowest energy π-π* transitions with peak maxima at 398 nm. They emit yellow light in thin films with emission peak at 550 nm. Similar energy transferfromP4HQ (or P40Q) to MEH-PPV can be expected due to their PL and absorption spectral overlap in the 460 - 660 nm region. Figure 3a shows die absorption spectrum of a binary blend thin film of POBTPQ;MEH-PPV (60:40 wt%). The absorption spectrum of die binary blend is a simple superposition of those of POBTPQ and MEH-PPV. No new absorption features were observed in the wavelengthrangeof 200 - 2000 nm, suggesting that the two blend components have no observable interactions in their electronic ground states. Similar results were observed in P4HQ:MEH-PPV (Figure 3b) and P40Q:MEH-PPV blends. the PL emission spectrum of a thinfilmof POBTPQ-.MEH-PPV blend (414 nm excitation) is shown in Figure 4. Only an orange PL emission band at 585 nm, characteristic of MEH-PPV with enhanced PL intensity compared to that of pure MEH-PPV, was observed. The enhancement in PL intensity of MEH-PPV is due to energy transferfromPOBTPQ to MEH-PPV. Similar energy transfer from P4HQ (or P40Q) to MEH-PPV was observed in the binary blend systems. These results are contrary to previously reported bipolar (n-type/p-type) conjugated polymer blends in which photoinduced charge transfer and luminescence quenching were significant (28). In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.


194

Wavelength (nm)

Wavelength (nm)

Figure 5. Optical absorption spectra of thinfilmsof (a) POBTPQ:MEH-PPV and (b) P4HQ: MEH-PPV blends. 410*.

450

,

500 550 600 650 700 750 000 Wavelength (nm)

Figure 4. PL spectra ofthinfilmsofPOBTPQ, MEH-PPV and a POBTPQMEH-PPV blend (60:40 wt%).

Voltage-Tunable Multicolor Polymer LEDs Green electroluminescence with a peak at 530 nm was observed for POBTPQ in LEDs of the type ITO/PEDOT/POBTPQ/A1 at all forward bias voltages. The observed EL emission spectrum of POBTPQ is identical to the PL emission spectrum discussed above. Yellow EL emission with a peak at 552 nm was observed for both P4HQ and P40Q diodes of the type ITO/PEDOT/P4HQ (or P40Q)/A1. Single-layer EL devices madefromPOBTPQ:MEH-PPV blends showed a continuous color change from green to yellow to orange under a varying bias voltage. Such blend LEDs, ITO/PEDOT/blend/Al, had a bright orange color at low bias voltages (10-12 V) and yellow to green colors at higher bias voltages. EL micrographs of a color-tunable POBTPQ:MEH-PPV blend In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

195

οι


450

550 600 650 700 750 Wavetength (nm)

450 500 550 600 650 700 Wavelength (nm)

Figure S. (a) EL Spectra of a POBTPQ.MEH-PPV blend (60:40 wt%) LED at various bias voltages: i) 11, ii) 14 and Hi) 16 V. The PL spectrum of MEH-PPV thinfilmis also shown, (b) EL spectra of a P4HQ.MEH-PPV blend (70:30 wt%) LED at various forward bias voltages.

Plate 2. EL micrographs (xlO) of a POBTPQ: MEH-PPV blend (60:40 wt%) diode at 11,14, and 16 V. (See Page 5 of color insert.)

(60:40 wt%) diode under different bias voltages are shown in Plate 2. The voltage-tunable EL spectra corresponding to the EL micrographs of Plate 2 are shown in Figure 5a. The orange EL emission (λ,^ * 585 nm) observed at low bias-voltages is identical with that of the single-layer MEH-PPV diodes which suggests that this EL emission was contributedfromthe MEH-PPV component in the blend. The green EL emission was similarly contributed from the POBTPQ component in die blend at the highest bias voltages (> 15 V). The yellow EL emission observed at an intermediate bias voltage in the blend LEDs is a result of simultaneous emission of green and orange lightsfromboth blend components, leading to the observed yellow color. Voltage-tunable multicolor EL emission (orange to yellow) was similarly observedfromP4HQ:MEH-PPV (Figure 5b) and P40Q:MEH-PPV blend LEDs.



196 The orange EL emission was contributedfromMEH-PPV component and the yellow EL emission was contributedfromthe P4HQ (or P40Q) component in these blend LEDs. Voltage-tunable EL emission was also observedfroma bilayer ITO/PPV(30 nm)/PFO(50 nm)/Al diode which switches colors reversiblyfromgreen at 10 14 V to blue at 17 V. The multicolor switching by the applied bias voltage was observed visually and also by EL microscopy as shown in Plate 3. The voltagetunable EL spectra corresponding to the EL micrographs of Plate 3 are shown in Figure 6. The EL spectrum at 12 V has peaks at 508 and 540 nm, showing die characteristic green PPV emission (5,6). The EL spectrum at 17 V, with dominant peaks at 436 and 465 nm, is due to the blue PFO emission (4). The EL spectra at intermediate bias-voltages have contributionfromboth PPV and PFO emission. The single-layer LEDs madefromPOBTPQ as an emissive material had a turn-on voltage of 9 V and a luminance of 53 cd/m at 17.5 V. The external EL quantum efficiency was 0.06 %. The P4HQ and P40Q LEDs had a turn-on voltage of 13-14 V and a luminance of 15-21 cd/m at 20 V. The external quantum efficiencies were 0.007% for P4HQ and 0.005% for P40Q. The poor electroluminescence efficiency of these polyquinoline LEDs is due to their low solid state PL quantum yield (27). Figure 7 shows the current-voltage and luminance-voltage curves of the POBTPQ.MEH-PPV blend LEDs of the type ITO/PEDOT/Blend/Al. A large enhancement in the performance of the blend LEDs was observed compared to the single-layer MEH-PPV and POBTPQ devices. The single-layer ΓΓΟ/ΜΕΗPPV/A1 diode had a turn-on voltage of 12 V and a maximum luminance of 186 cd/m at 16 V and current density of 500 mA/cm . The POBTPQ:MEH-PPV blend LEDs showed bright orange EL emission at low bias voltage (10 - 12 V) and yellow (14 V) to green (16 V) at higher bias voltages. The turn-on voltage of these blend LEDs was 5 V. The luminance of the orange color at 11 V was about 400 cd/m , the luminance of the yellow color at 14 V was about 1400 cd/m , and that of the green color at 16 V was about 1200 cd/m . The blend (60:40 wt%) diode had an external quantum efficiency of 0.7%, a factor of 35 times greater than the single-layer MEH-PPV diodes. Similar enhancement in LED performance was observed in devices madefromP4HQ:MEH-PPV (70:30 wt%) and P40Q:MEH-PPV (70:30 wt%) blends. The P4HQ:MEH-PPV blend (70:30 wt%) diode had a turn-on voltage of 7.0 V and a luminance of 700 cd/m at 17 V with a current density of 198 mA/cm . A turn-on voltage of 7.5 V and a luminance of 635 cd/m at 16 V were observed for the P40Q:MEH-PPV blend (70:30 wt%) diode. The external quantum efficiencies of these two blend diodes were 0.4 - 0.55%. The external quantum efficiency is thus enhanced by a factor of 20 - 27 times compared to that of the single-layer MEH-PPV diode. The enhanced performance (lower turn-on voltage, higher luminance, and higher EL 2

2

2

2

2

2

2

2

2

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197


1.2

Wavelength (nm)

Figure 6. EL spectra of a bilayer ITO/PPV(30 nm)/PFO(50 nm)/Al diode.

Plate 3. EL micrographs (x 10) of a bilayer ITO/PPV(30 nm)/PFO(50 nm)/Al diode. (See Page 6 of color insert.)



16

Figure 7. (a) Current-voltage and (b) luminance-voltage curves of ITO/PEDOT/Blend(POBTPQ:MEH-PPV; 60:40 wt%)/Al diodes.


00

199 efficiency) of these n-type/p-type binary polymer blend LEDs implies improved charge injection and recombination compared to that of the single-layer component polymer diodes. The performance of the color-tunable bilayer ITO/PPV(30 nm)/PFO(50 nm)/Al diode was also substantially enhanced compared to the conçonent single-layer devices. The turn-on voltage of the bilayer LED was 8 V . The maximum luminance was 420 cd/m at a current density of 236 mA/cm (17 V), and the external EL efficiency was 0.22%. The luminance of the PPV/PFO LED in the green state at 12 V was about 120 cd/m and the luminance in the blue state at 17 V was about 420 cd/m . In contrast, the single-layer PPV diode had a turn-on voltage of 9.S V and a luminance of 11 cd/m at 11 V with a current density of 500 mA/cm . The external quantum efficiencies of single-layer PPV and PFO diodes were 0.002% and 0.01%, respectively. The external quantum efficiency of the bilayer PPV/PFO diode is thus enhanced by a factor of 28 -110 times compared to the single-layer diodes. These results show that voltagetunable E L emissionfromthe bilayer diode is both efficient and bright. 2

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2

Summary The recent availability of organic-solvent soluble n-type conjugated polyquinolines such as POBTPQ, P4HQ and P40Q has allowed us to demonstrate nanophase-separated bipolar blends with p-type MEH-PPV, leading to efficient and bright voltage-tunable multicolor light emitting diodes. Voltagetumble green«-+blue E L emission was also observed in bilayer LEDs containing PPV and PFO layers. The tunable EL coldrs obtained from a bilayer LED by varying the applied voltage originate from the two different emissive layers. The observed performance (turn-on voltage, luminance level, and E L quantum efficiency) of both blend and bilayer LEDs was significantly enhanced conpared to the single-layer homopolymer LEDs.

Acknowledgement This research was supported by the U.S. Army Research Laboratory and the U.S. Army Research Office under Grant DAAD 19-01-1-0676 and in part by the Office of Naval Research.

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Voltage-Tunable Multicolor Electroluminescence from Single-Layer

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