pubs.acs.org/Langmuir © 2009 American Chemical Society
Spectral Tuning of Conjugated Polymer Colloid Light-Emitting Diodes Christopher F. Huebner and Stephen H. Foulger* Center for Optical Materials Science and Engineering Technologies and School of Materials Science and Engineering, Clemson University, Clemson, South Carolina 29634-0971 Received July 28, 2009. Revised Manuscript Received October 2, 2009 In recent years, the importance of the polymer light-emitting diode (PLED) has grown immensely, proving very desirable in numerous applications because of very high efficiencies, low power consumption, and ease of fabrication. Typically, these devices have been constructed in a layered, thin-film fashion consisting of either electron- and holetransport materials doped with a luminescent dye (Hebner, T. R.; Sturm, J. C. Appl. Phys. Lett. 1998, 73, 1775. Jiang, X.; Register, R. A.; Killeen, K. A.; Thompson, M. E.; Pschenitzka, F.; Hebner, T. R.; Sturm, J. C. J. Appl. Phys. 2002, 91, 6717. Yeh, K. M.; Chen, Y. Org. Electron. 2008, 9, 45-50. Oh, G. C.; Yun, J. J.; Park, S. M.; Son, S. H.; Han, E. M.; Gu, H. B.; Jin, S. H.; Yoon, Y. S. Mol. Cryst. Liq. Cryst. 2003, 405, 43-51. Lee, J. I.; Chu, H. Y.; Kim, S. H.; Do, L. M.; Zyung, T.; Hwang, D. H. Opt. Mater. 2003, 21, 205-210. Hwang, D.-H.; Park, M.-J.; Lee, C. Synth. Met. 2005, 152, 205-208) or a conjugated polymer that can be engineered to tune the emission of the PLED to particular wavelengths. Stable PLED aqueous colloidal dispersions were prepared containing poly[2-methoxy-5-(2-ethylhexyloxy)-1,4phenylenevinylene], (MEH-PPV), poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO), and a binary poly(9,9-di-n-octylfluorenyl-2,7-diyl)/poly(2,5-dioctyl-1,4-phenylenevinylene) (PFO/POPPV) particle dispersion. Red-, green-, and blue-lightemitting colloidal dispersions could then be combined to achieve color-tailored emissions spanning the visible spectrum.
1. Introduction The use of organic materials in solid state lighting is rapidly advancing in materials research because of ease of device fabrication, low materials cost, low environmental impact, facile synthesis routes, and high rates of improvement in luminous efficiency.1-9 In the early 1990s, Burroughes et al. developed the first polymer-based light-emitting diode (LED) device employing a π-conjugated poly(phenylene vinylene) polymer.10,11 Currently, numerous academic and private industry researchers are working toward highly efficient white-light-emitting devices focusing primarily on information display applications12-20 in which the pursuit of highly *To whom correspondence should be addressed. E-mail: foulger@ clemson.edu.
(1) Hebner, T. R.; Sturm, J. C. Appl. Phys. Lett. 1998, 73, 1775. (2) Jiang, X.; Register, R. A.; Killeen, K. A.; Thompson, M. E.; Pschenitzka, F.; Hebner, T. R.; Sturm, J. C. J. Appl. Phys. 2002, 91, 6717. (3) Yeh, K. M.; Chen, Y. Org. Electron. 2008, 9, 45–50. (4) Oh, G. C.; Yun, J. J.; Park, S. M.; Son, S. H.; Han, E. M.; Gu, H. B.; Jin, S. H.; Yoon, Y. S. Mol. Cryst. Liq. Cryst. 2003, 405, 43–51. (5) Lee, J. I.; Chu, H. Y.; Kim, S. H.; Do, L. M.; Zyung, T.; Hwang, D. H. Opt. Mater. 2003, 21, 205–210. (6) Hwang, D.-H.; Park, M.-J.; Lee, C. Synth. Met. 2005, 152, 205–208. (7) Mitschke, U.; Bauerle, P. J. Mater. Chem. 2000, 10, 1471. (8) Patel, N. K.; Cin0 a, S.; Burroughes, J. H. IEEE J. Sel. Top. Quant. 2002, 8, 346. (9) Kukhto, A. V. J. Appl. Spectrosc. 2003, 70, 165. (10) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539. (11) Braun, D.; Heeger, A. J. Appl. Phys. Lett. 1991, 58, 1982. (12) Misra, A.; Kumar, P.; Kamalasanan, M. N.; Chandra, S. Semicond. Sci. Technol. 2006, 21, R35. (13) Meerholz, K. Nature 2005, 437, 327. (14) Yan, B. P.; Cheung, C. C. C.; Kui, S. C. F.; Xiang, H. F.; Roy, V. A. L.; Xu, S. J.; Che, C. M. Adv. Mater. 2007, 19, 3599. (15) Lee, R. H.; Lin, K. T.; Huang, C. Y. J. Polym. Sci., Part B: Polym. Phys. 2007, 45, 330–341. (16) Al Attar, H. A.; Monkman, A. P.; Tavasli, M.; Bettington, S.; Bryce, M. R. Appl. Phys. Lett. 2005, 86. (17) Gong, X.; Wang, S.; Moses, D.; Bazan, G. C.; Heeger, A. J. Adv. Mater. 2005, 17, 2053. (18) Lee, P. I.; Hsu, S. L. C.; Lee, R. F. Polymer 2007, 48, 110–115. (19) Yan, B. P.; Cheung, C. C. C.; Kui, S. C. F.; Xiang, H. F.; Roy, V. A. L.; Xu, S. J.; Che, C. M. Adv. Mater. 2007, 19, 3599. (20) Zhou, Y.; Sun, Q. J.; Tan, Z.; Zhong, H. Z.; Yang, C. H.; Li, Y. F. J. Phys. Chem. C 2007, 111, 6862–6867.
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efficient, highly luminous white emission is desired. There is additional work in creating active matrix displays from organic materials,21,22 yet current device fabrication techniques do not allow for a large-scale production setting or a high-resolution display. Currently, there are very few groups showing interest in specialty applications that call for the emission of various colors within the visible spectrum.23-25 The ability to create-color tailored PLEDs has numerous applications, and industries including automotive lighting, decorative lighting, marketing (including point-ofpurchase advertising and product packaging) and other novelty luminaire manufacturing as well as pharmaceutical, medical, military, security, and sensory applications can all benefit from the advantages of color-tailored polymeric devices. Semiconductive π-conjugated polymers offer numerous processing benefits with the addition of electroluminescence (EL) from a single material as a result of the inherent semiconductivity of the material. Single-layer emissive conjugated polymer devices have been demonstrated without the need for various hole and/or electron injection/transport/blocking layers to facilitate balanced charge injection in functional devices.26, Characteristics of the EL spectra often exhibit strong luminosity with relatively narrow fullwidths at half-maximum intensity (fwhm) and otherwise featureless shapes. Conversely, devices fabricated with small organic molecules28-33 and quantum dots (QDs)34-37 require the use of (21) Andersson, P.; Nilsson, D.; Svensson, P. O.; Chen, M.; Malmstrom, A.; Remonen, T.; Kugler, T.; Berggren, M. Adv. Mater. 2002, 14, 1460–1464. (22) Zhao, Y. S.; Fu, H. B.; Hu, F. Q.; Peng, A. D.; Yao, J. N. Adv. Mater. 2007, 19, 3554. (23) Uchida, T.; Ichihara, M.; Tamura, T.; Ohtsuka, M.; Otomo, T.; Nagata, Y. Jpn. J. Appl. Phys. 2006, 45, 7126. (24) Gather, M. C.; Kohnen, A.; Falcou, A.; Becker, H.; Meerholz, K. Adv. Funct. Mater. 2007, 17, 191–200. (25) Uchida, T.; Ichihara, M.; Tamura, T.; Ohtsuka, M.; Otom, T.; Nagata, Y. Jpn. J. Appl. Phys., Part 1 2006, 45, 7126–7128. (26) Barford, W.; Bursill, R. J. Chem. Phys. Lett. 1997, 268, 535–540. (27) Malliaras, G. G.; Salem, J. R.; Brock, P. J.; Scott, C. Phys. Rev. B 1998, 58, 13411–13414. (28) Kido, J.; Kimura, M.; Nagai, K. Science 1995, 267, 1332–1334. (29) Gu, G.; Garbuzov, D. Z.; Burrows, P. E.; Venkatesh, S.; Forrest, S. R.; Thompson, M. E. Opt. Lett. 1997, 22, 396–398.
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multitudes of material layers to optimize the device, and these additional layers are often deposited via time-consuming and costly high-vacuum deposition techniques.38 Although QD-based devices have shown similar tailorability,39 the use of these materials is disadvantageous from a materials cost, synthesis, and environmental impact perspective. The incorporation of these materials into real world lighting applications would carry with it the need for extensive infrastructure to accommodate the proper disposal of potentially toxic waste materials. Organic light-emitting diode (OLED) devices fabricated from colloidal particles have been demonstrated previously40-44 and are composed of either a conjugated ladder-type poly(paraphenylene) polymer or a carbazole-oxadiazole luminescent dye system, yet in both examples, only a single color-emitting device was demonstrated. Through sequestering emissive species in colloidal particles, the ability to inhibit fluorescent resonance energy transfer (FRET)45 between emissive species enables the device engineer to create single-layer OLED devices that emit light in wavelengths across the visible spectrum for a variety of applications.46 These mixed colloidal dispersions can then be used in spin-cast device architecture or formed into inks suitable for a variety of conventional printing processes, such as inkjet,47-50 roll to roll,51,52 screen,53 and spray printing techniques. To tune the emission wavelength of individual devices effectively and easily, the enlisted conjugated polymer materials were (30) Gu, G.; Parthasarathy, G.; Tian, P.; Burrows, P. E.; Forrest, S. R. J. Appl. Phys. 1999, 86, 4076–4084. (31) Choong, V. E.; Shi, S.; Curless, J.; Shieh, C. L.; Lee, H. C.; So, F.; Shen, J.; Yang, J. Appl. Phys. Lett. 1999, 75, 172–174. (32) Choulis, S. A.; Choong, V. E.; Mathai, M. K.; So, F. Appl. Phys. Lett. 2005, 87. (33) Wantz, G.; Dautel, O. J.; Almairac, R.; Hirsch, L.; Serein-Spirau, F.; Vignau, L.; Lere-Porte, J. P.; Parneix, J. P.; Moreau, J. J. E. Org. Electron. 2006, 7, 38–44. (34) Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800–803. (35) Zhao, J. L.; Bardecker, J. A.; Munro, A. M.; Liu, M. S.; Niu, Y. H.; Ding, I. K.; Luo, J. D.; Chen, B. Q.; Jen, A. K. Y.; Ginger, D. S. Nano Lett. 2006, 6, 463– 467. (36) Huang, H.; Dorn, A.; Bulovic, V.; Bawendi, M. G. Appl. Phys. Lett. 2007, 90, 023110. (37) Huang, H.; Dorn, A.; Nair, G. P.; Bulovic, V.; Bawendi, M. G. Nano Lett. 2007, 7, 3781–3786. (38) Baldo, M.; Deutsch, M.; Burrows, P.; Gossenberger, H.; Gerstenberg, M.; Ban, V.; Forrest, S. Adv. Mater. 1998, 10, 1505. (39) Anikeeva, P. O.; Halpert, J. E.; Bawendi, M. G.; Bulovic, V. Nano Lett. 2007, 7, 2196–2200. (40) Piok, T.; Gamerith, S.; Gadermaier, C.; Plank, H.; Wenzl, F. P.; Patil, S.; Montenegro, R.; Kietzke, T.; Neher, D.; Scherf, U.; Land-fester, K.; List, E. J. W. Adv. Mater. 2003, 15, 800. (41) Piok, T.; Romaner, L.; Gadermaier, C.; Wenzl, F. P.; Patil, S.; Monetenegro, R.; Land-fester, K.; Lanzani, G.; Cerullo, G.; Scherf, U.; List, E. J. W. Synth. Met. 2003, 139, 609. (42) Piok, T.; Gadermaier, C.; Wenzl, F. P.; Patil, S.; Monetenegro, R.; Landfester, K.; Lanzani, G.; Cerullo, G.; Scherf, U.; List, E. J. W. Chem. Phys. Lett. 2004, 389, 7. (43) Heo, J. S.; Park, N. H.; Ryu, J. H.; Choi, Q. H.; Suh, K. D. Macromol. Chem. Phys. 2003, 204, 2002–2006. (44) Heo, J. S.; Park, N. H.; Ryu, J. H.; Suh, K. D. Adv. Mater. 2005, 17, 822. (45) Forster, T. Ann. Phys. 1948, 2, 55. (46) Huebner, C. F.; Carroll, J. B.; Evanoff, D. D.; Ying, Y. R.; Stevenson, B. J.; Lawrence, J. R.; Houchins, J. M.; Foguth, A. L.; Sperry, J.; Foulger, S. H. J. Mater. Chem. 2008, 18, 4942–4948. (47) Kamphoefner, F. J. IEEE Trans. Electron Devices 1972, ED19, 584. (48) Hebner, T. R.; Wu, C. C.; Marcy, D.; Lu, M. H.; Sturm, J. C. Appl. Phys. Lett. 1998, 72, 519–521. (49) Satoh, R.; Naka, S.; Shibata, M.; Okada, H.; Onnagawa, H.; Miyabayashi, T. Jpn. J. Appl. Phys., Part 1 2004, 43, 7395–7398. (50) Kamyshny, A.; Ben-Moshe, M.; Aviezer, S.; Magdassi, S. Macromol. Rapid Commun. 2005, 26, 281–288. (51) Gupta, C.; Mensing, G. A.; Shannon, M. A.; Kenis, P. J. A. Langmuir 2007, 23, 2906–2914. (52) Puetz, J.; Aegerter, M. A. Thin Solid Films 2008, 516, 4495–4501. (53) Shaheen, S. E.; Radspinner, R.; Peyghambarian, N.; Jabbour, G. E. Appl. Phys. Lett. 2001, 79, 2996–2998. (54) Landfester, K.; Monetenegro, R.; Scherf, U.; Guntner, R.; Asawapirom, U.; Patil, S.; Ne-her, D.; Kietzke, T. Adv. Mater. 2002, 14, 651.
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Figure 1. Structure of the emissive materials used in this work including (i) red-light-emitting MEH-PPV, (ii) blue-light-emitting PFO, and (iii) green-light-emitting POPPV.
formed into colloidal particles dispersed in an aqueous environment by the application of a miniemulsion technique.54-57 Conjugated polymers incorporated in this effort are red-orangelight-emitting poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) (cf. Figure 1i), blue-light-emitting poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) (cf. Figure 1ii), and a green-light-emitting poly(9,9-di-n-octylfluorenyl-2,7-diyl)/poly(2,5-dioctyl-1,4-phenylenevinylene) (PFO/POPPV) “hybrid” system58 (cf. Figure 1iii). Polymer colloid emission wavelengths, (λem), were ∼610, 424, and 507 nm, respectively. By exploiting the kinetics of the miniemulsion reaction, individual emissive polymers are able to be compartmentalized into colloids with a controllable size distribution of 20 to 500 nm.59 The facilitation of device fabrication is accomplished through the incorporation of a binder material in the colloidal dispersion. Casting a close-packed monolayer of polydisperse colloids with any hope of reproducibility is very unlikely, so to alleviate the probability of short-circuiting due to pinholes in the colloidal layer, a poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) dispersion is mixed in a certain ratio with the emissive particle suspension; the device architecture employed in this effort is illustrated in Figure 5. The concentration of PEDOT/ PSS is low enough to avoid any short-circuiting due to PEDOT/ PSS percolating across the electrodes, enabling charge injection to the emissive colloids and the production of visible light, as evidenced from the non-Ohmic characteristics in the currentvoltage plots (not presented).
2. Experimental Section Materials. The emissive polymers;MEH-PPV, PFO, and POPPV;and sodium dodecyl sulfate (SDS) were purchased from Sigma-Aldrich and used without any further purification. Chloroform was purchased from Fisher Scientific and used without any other purification. Deionized (DI) water was purified to a resistivity of 18.2 MΩ cm before use. Baytron P Formulation VP CH 8000 (PEDOT:PSS; H. C. Stark, Inc.) was filtered through a 0.450 μm syringe filter prior to use. Calcium (Ca) and aluminum (Al) (99.9 and 99.99% purity, respectively) were purchased from Kurt J. Lesker Co. and used without further purification. General Preparation of Aqueous PLED Dispersions. Preparation of the PFO/POPPV green-light-emitting particles is discussed elsewhere.58 One hundred milligrams each of PFO and MEH-PPV were dissolved in ca. 2 g of CHCl3 (1.43 mL). To this organic solution was added an aqueous solution containing SDS (100 mg in 25 mL H2O, 0.25 wt %). The two-phase solution was then emulsified using a tip sonicator (VirTis Virsonic (55) Kietzke, T.; Neher, D.; Landfester, K.; Montenegro, R.; Guntner, R.; Scherf, U. Nat. Mater. 2003, 2, 408. (56) Bechthold, N.; Landfester, K. Macromolecules 2000, 33, 4682. (57) Landfester, K.; Bechthold, N.; Tiarks, F.; Antonietti, M. Macromolecules 1999, 32, 5222. (58) Huebner, C. F.; Roeder, R. D.; Foulger, S. H. Adv. Funct. Mater. 2009, in press. (59) Landfester, K.; Schork, F. J.; Kusuma, V. A. C. R. Chim. 2003, 6, 1337.
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Figure 3. Scanning electron micrograph of prepared electroluminescent colloids. a thermally evaporated aluminum layer (100 nm) for stability and reduced risk of calcium oxidation. Device Characterization. Testing was conducted in an MBraun UniLab glovebox under an argon atmosphere. Devices were connected to a computer-controlled Keithley 228A voltage/ current source with the forward bias applied to the ITO anode. Current was recorded on a Keithley 2001 digital multimeter connected in series in the circuit. Luminescence data was recorded by a computer-controlled Konica-Minolta LS 110 luminescence meter. Electroluminescent spectra were recorded with a Horiba Jobin Yvon MicroHR monochromator with a Synapse CCD detector. All data were recorded at a temperature of 23 C unless otherwise noted.
3. Results and Discussion
Figure 2. Emission (0) and absorption (O) characteristics of (i) PFO (PL excitation = 380 nm) and PFO/POPPV and (ii) POPPV (PL excitation = 405 nm) and MEH-PPV. 600 Ultrasonic Cell Disruptor) at an operating power of 12 W for 4 min. After sonication, the colloidal dispersions were placed onto a hot plate at 60 ( 3 C and stirred for 3 h to evaporate any remaining CHCl3 present. PLED suspensions were then cleaned by dialysis to remove residual surfactant. Dialysis water was changed every 12 h until the conductivity of the water was less than 0.5 μS/cm. The final solids concentration of each dispersion was ca. 7 mg mL -1. Electroluminescent Material Preparation. PLED dispersions were mixed with electronic-grade PEDOT/PSS (F = 1 105-3 105 Ω cm) in a 4:1 (v/v) ratio. This ratio of EL particles to PEDOT/PSS consistently demonstrated a maximum luminous output for each system as previously observed.46 The resulting mixture was then concentrated to spin-cast 100-150 nm thin films. Analysis. Absorption spectra were collected with a PerkinElmer Lambda 900 UV/vis/NIR spectrophotometer, and photoluminescence (PL) and photoluminescence excitation (PLE) spectra were collected on a Horiba Jobin-Yvon Fluorolog FL-3-22/ Tau-3 Lifetime spectrofluorometer. Scanning electron microscopy was performed on a Hitachi S-4800 FESEM at a maximum accelerating voltage of 5.0 kV. Device Fabrication. ITO coated onto 12.5 mm 12.5 mm float glass (Delta Technologies, Stillwater, MN) was etched to a 4 mm 12.5 mm rectangular pattern and cleaned. The prepared EL mixture was then spin-cast onto the substrate to a thickness of ∼100 to 150 nm. A calcium electrode (ca. 33 nm) was thermally evaporated onto the emissive polymer layer and then capped with Langmuir 2010, 26(4), 2945–2950
The materials chosen in this work all have the appropriate spectral characteristics to act as a donor/acceptor pair in an energy-transfer mechanism, as illustrated in Figure 2. Energy transfer can take place through a Coulombic interaction (F€orster) or an electron exchange (Dexter) mechanism and occurs when an excited-state donor (e.g., a higher-energy emitter) transfers energy to a ground-state acceptor (e.g., a lower-energy emitter). To be an effective means of energy transfer, both the F€orster and Dexter mechanisms have a number of conditions that must be satisfied, but two major considerations include the spatial separation between the donor and acceptor molecules and the spectral characteristics of the donor’s emission and the acceptor’s absorption. A F€orster-type transfer can be potentially effective up to ca. 10 nm,45 whereas a Dexter-type transfer, though very dependent on the electronic configuration of the donor-acceptor, is roughly limited to distances of ca. 2 nm.60 In addition, both modes of energy transfer require a high level of spectral overlap of the donor’s emission and the acceptor’s absorption. By sequestering the emissive species into colloidal particles (cf. Figure 3) with radii exceeding this allowable distance, inhibition of this transfer is achieved by circumventing the proximity requirement, and systems composed of mixtures of variably emitting particles will exhibit ternary emissions from the devices, enabling color tuning. F€ orster energy transfer is described as a Coulombic interaction between an emissive excited-state donor material and an absorptive ground-state acceptor material and carries with it a greater length scale in donor and acceptor separation. The theory is defined as having the ability to transfer energy without the transfer of a photon, wherein energy is transferred from an excited-state donor molecule (D*) to a ground-state acceptor (A), resulting in the ground-state donor (D) and excited-state acceptor (A*). The transfer arises from the resonance between D (60) Levy, S. T.; Speiser, S. J. Chem. Phys. 1992, 96, 3585–3593.
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Figure 4. Photoluminescence of MEH-PPV (4), PFO (0), and PFO/POPPV (O) colloidal particles dispersed in deionized water. MEH-PPV excitation = 535 nm; PFO and PFO/POPPV excitation = 380 nm.
Figure 6. Electroluminescent spectra of primary-color-light-emitting devices. Red- (MEH-PPV, 4), blue- (PFO, 0), and green-lightemitting (PFO/POPPV, O) colloidal devices.
Figure 5. Device architecture of π-conjugated polymer-colloidbased devices. The active area is defined by depositing the EL mixture into a photolithographically defined feature. Film thicknesses: ITO (150 nm)/photoresist þ active material (150 nm)/Ca (30 nm)/Al (100 nm).
and A and is largely dependent on the alignment of the dipole moments of the two molecules. F€orster’s expression k ¼
9000ðln 10ÞK2 128π2 NA τD nr 4 RDA 6
Z dλ
FD ðλÞEA ðλÞ λ4
ð1Þ
(where κ is the orientational factor of the dipole moments, NA is Avogadro’s number, τD is the decay lifetime of the donor, nr is the refractive index of the matrix, λ is the wavelength in nanometers, FD(λ) is the emission spectrum of the donor, and ɛA(λ) is the molar extinction coefficient of the acceptor) describes the rate of transfer from D* to A within a matrix containing the D and A species and is derived from the radius equation, RDA 6 ¼
9000ðln 10ÞK2 ΦD, f 128π5 NA nr 4
Z
¥ 0
(61) Klopffer, W. J. Chem. Phys. 1969, 50, 2337.
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FD ðλÞEðλÞλ4 dλ
ð2Þ
Figure 7. Electroluminescent spectra showing a gradual tuning from red- to green-light emission in colloidal devices.
where ΦD,f is the quantum yield of the donor species, showing the dependence of the transfer characteristics on the D*-A separation.61-65 Becauase this is a resonance transfer and does (62) Itaya, A.; Okamoto, K.; Kusabayashi, S. B. Chem. Soc. Jpn. 1977, 50, 22–26.
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Figure 8. Electroluminescent spectra showing a gradual tuning from blue- to green-light emission in colloidal devices.
Figure 9. Electroluminescent spectra showing a gradual tuning from blue- to red-light emission in colloidal devices.
not rely on charge propagation, the length scale for this type of transfer is on the order of 2 to 10 nm.45 Colloidal particles used in this work exhibited a mean diameter of 59 ( 27 nm, based on a 300-member population. Photoluminescence (PL) studies of the polymer dispersions presented in Figure 4 illustrate the red-, green-, and blue-light (RGB) emissions of the various dispersions, presenting the ability to tailor the emission of dispersions or thin films through the application of a modified “rule of mixtures,” though what will be demonstrated here is a more subtle tuning about the gamut created by the RGB emissions of this system. PL data for the MEH-PPV and PFO dispersions were acquired normally, by direct excitations corresponding to the absorption maximum wavelengths of the materials (∼535 and 380 nm, respectively); however, in the case of the hybrid PFO/POPPV dispersion, the material was excited at the absorption maximum of the higherenergy emitter, PFO, and through a desired FRET a dominantly green emission is achieved. Optical properties of this hybrid dispersion have been reported elsewhere.58 Apparent in the EL spectra of the variety of devices created is the lack of FRET from higher-energy to lower-energy emissive species after compartmentalization in colloidal particles. Showing little difference in the shape of the spectra in EL or PL and the lack of
enhanced lower-energy emission (i.e., red tailing), it can be assumed that there is a negligible amount of electromer/electroplex formation occurring even though emissive polymer molecules have been forced into close proximity to one another during the miniemulsion procedure. Figure 6 presents the EL spectra of the RGB-lightemitting species from devices cast from independent dispersions. These devices were created to define the color gamut created by this system. The complete gamut is defined by the emission of the primary-color-light-emitting species, and upon blending of these emissions, any wavelength of light lying within these boundary wavelengths can be created, simplifying the color-tailoring process. Devices fabricated from the colloidal system allow for not only red-, green-, and blue-emitting devices but also the ability to create specific single-color-emitting devices over a broad range of wavelengths in the visible spectrum. Devices fabricated in this work encompassed a series of binary colloidal dispersions demonstrating color tuning across the visible spectrum. Figure 7 presents the EL spectra of devices composed of MEH-PPV and PFO/POPPV particles in varying concentrations, showing the tuning from red to green emission. Figure 8 presents the EL spectra of devices composed of PFO/ POPPV and PFO particles in varying concentrations showing the tuning from green to blue emission. Figure 9 presents the EL spectra of devices composed of PFO and MEH-PPV particles in varying concentrations showing the tuning from blue to red emission. Table 1 lists the dispersion concentrations and accompanying International Commission on Illumination (CIE) coordinates.
(63) Nguyen, T.-Q.; Wu, J.; Doan, V.; Schwartz, B. J.; Tolbert, S. H. Science 2000, 288, 652–656. (64) Xu, Q. H.; Wang, S.; Korystov, D.; Mikhailovsky, A.; Bazan, G. C.; Moses, D.; Heeger, A. J. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 530–535. (65) Gatica, N.; Marcelo, G.; Mendicuti, F. Polymer 2006, 47, 7397–7405.
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Table 1. Relative Dispersion Concentrations and Resultant CIE Coordinates mixture
%R
R/G R/G R/G R/G R/G R/G G/B G/B G/B G/B G/B G/B B/R B/R B/R B/R B/R B/R
100 90 75 50 25 10
%G 10 25 50 75 90 100 90 75 50 25 10
10 25 50 75 90
%B
CIEx
CIEy
10 25 50 75 90 100 90 75 50 25 10
0.608 0.608 0.561 0.498 0.389 0.368 0.270 0.254 0.242 0.214 0.210 0.186 0.162 0.191 0.278 0.300 0.516 0.542
0.391 0.386 0.395 0.409 0.426 0.417 0.485 0.422 0.402 0.342 0.315 0.293 0.178 0.224 0.292 0.294 0.385 0.394
Dispersions were mixed according to liquid volume, and because of the uniformity of solid content postminiemulsion (ca. 7 mg/mL -1) for all dispersions, negligible error was assumed. Plotting data derived from the EL spectra of devices fabricated from the colloidal suspensions defines the CIE coordinates for this system. A triangle drawn over the CIE space connecting our three primary color (RGB) emitting colloidal systems illustrates that upon combining the three in appropriate ratios, any color lying within this triangle is achievable for devices, defining the color gamut for this system. Although any perceived color falling within the limits of this gamut is possible, for this study only binary tuning about the border of the gamut was demonstrated. Tailoring devices to application-specific emitted colors would be a rather ambitious undertaking, and we feel that it is beyond the scope of this article. However it would certainly not be as costly or time-consuming as the development of a novel synthesis route and exotic polymer each time a new emission is required. The range of tailorable emissions will obviously be limited by the choice of EL polymers, in this particular case, that range being ca. 430 to 600 nm, encompassing most of the visible spectrum. Furthermore, we show that colored devices beyond the realm of white and the three primary colors are obtainable through RGB
2950 DOI: 10.1021/la9027749
Figure 10. International Commission on Illumination (CIE 1931) plot showing the color gamut created by this system.
mixing, eliminating the need to develop novel polymers with π-π* transitions that correspond to specific band gaps that produce photons of specific wavelengths every time a new colored emission is desired.
4. Conclusions In summary, we have shown that by using π-conjugated emissive polymers a tailored emissive device can be created by forming the polymers into colloidal particles and mixing the dispersions in particular RGB ratios to create the desired emission. Compartmentalization of the polymers into submicrometerscale particles effectively eliminates F€orster-type energy transfer among the emissive species, allowing the device engineer to precisely tailor the emission of a device through the judicious mixing of red-, green-, and blue-light-emitting dispersions. Acknowledgment. We thank DARPA (grant no. N66001-041-8933) and the National Science Foundation (CAREER award to S.H.F., grant no. DMR-0236692) for financial support.
Langmuir 2010, 26(4), 2945–2950
