Chromogenic Phenomena in Polymers - American Chemical Society

Chapter 16

Polarized Electroluminescence from Double-Layer LEDs with Active Film Formed by Two Perpendicularly Oriented Polymers Downloaded by UNIV OF GUELPH LIBRARY on September 7, 2012 | http://pubs.acs.org Publication Date: August 17, 2004 | doi: 10.1021/bk-2005-0888.ch016

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A. Bolognesi , C. Botta , D. Facchinetti , C. Mercogliano , M . Jandke , P. Strohriegl , K. Kreger , A. Relini , and R. Rolandi 2

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Istituto per lo Studio delle Macromolecole, CNR, Via E. Bassini 15, 20133 Milano, Italy Bayreuther Institüt fur Makromoleküforschung, Universität Bayreuth, D-95440 Bayreuth, Germany Istituto Nazionale di Fisica della Materia and Dip. Fisica Università di Genova, Via Dodecaneso 33, 16146 Genova, Italy 3

The aim of this work is to study the feasibility of a double layer device, where the two active materials are formed, respectively, by two oriented polymers, whose orientation direction is orthogonal, and which can emit simultaneously in different regions of the visible spectrum. We demonstrate that the anisotropy of the polymers is not lost when they are perpendicularly oriented, obtaining polarized light in a large spectral region, extending from the green to the red. The emitted light, observed through a polarizer, can be varied from green to red by simply rotating the polarization axis, obtaining polarized light of variable colour. This peculiar device design is particularly appealing as it can increase the versatility of organic LEDs providing polarized light with easily variable colour emission.

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

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212 In recent years, organic light-emitting diodes (OLEds) are gathering a lot of interest. Polymers used as active materials in OLEDs have conjugated chains which can be aligned by low-cost techniques and so that it's easy to prepare well oriented thin films giving polarized electroluminescence (EL) (1). Polarized EL is useful for application such as backlights in liquid crystal displays (2). The most used methods for aligning polymeric films have been recently described in a review by Grell and Bradley (3). Depending on the optical properties of the polymers and on the orienting technique employed, different emission colours and EL polarization ratios (R L) were obtained. For emission in the blue, disubstituted polyfluorene aligned by LC selforganization on pre-oriented substrates gives a polarization ratio of about 15 in EL(4). A further improvment in the orientation of polyfluorene has been obtained by orienting monodomains of poly(9,9-dioctylfluorene) on an alignment layer of segmented poly(p-phenylenevinylene) (PPV), reaching the highest polarization ratio of25inEL#). For g reen e mission, u nsubstituted Ρ ΡV orientedb y t he r ubbing technique gives a value of R « 12 (6). Red polarized emission with a dichroic ratio R L « 8 has been obtained with a poly(3-alkylthiophene) derivative oriented by a combination of rubbing and thermal annealing (7). In this paper we report the preparation and characterization of a device in which the active material is composed of two layers of two different polymers emetting in the yellow-green and in the red, oriented perpendiculary to each other.

Downloaded by UNIV OF GUELPH LIBRARY on September 7, 2012 | http://pubs.acs.org Publication Date: August 17, 2004 | doi: 10.1021/bk-2005-0888.ch016

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Results The yellow-green emitting polymer is a segmented PPV (Fig. 1), obtained as reported in ref. 8 and 9.

Figure 1: Structure ofsegmented PPV used in this work

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

213 The red emitting polymer is methoxyhexyl)thiophene] (P60Me, Fig. 2).

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regioregular

(CH ) OCH


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poly[3-(6-

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Figure 2: Structure of P60Me The differential scanning calorimetry (DSC) trace of P60Me powders, reported in Fig. 3, consists of two broad endotherms whose maxima are at 110°C and 170°C. The higher temperature transition has been attribuited to the melting of the ρ olymer. At 110°C a thermal phase transition takes ρ lace from a three dimensional ordered semicrystalline phase to a liquid crystalline phase (7).

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Figure 3: DSC scan ofP60Me P60Me can be aligned by means of the rubbing technique. Higher dichroic ratios were obtained by heating the oriented samples at 105°C in vacuum, for 1 h, followed by a slow cooling (7). The procedure to prepare the heterostructures was: 1

deposition of PPV in its precursor form onto ITO (indium-tin-oxide) coated glass substrate, orientation and conversion, as described in ref. 6;


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spin coating of P60Me from chloroform solution on the top of the converted and oriented PPV fdm; rubbing of the surface of the P60Me film, with a cloth mounted on a rotating cylinder (7) in the orthogonal direction with respect to the orientation of the PPV chains; thermal annealing of the films, as previously described (7) for P60Me single layer devices.

The final thickness of the P60Me layer was 10-15 nm. In Fig. 4 we report a sketch of the PPV/P60Me interface.

Figure 4: Sketch of the PPV/P60Me interface


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The heterostructure of the active double layer LED was been characterized by atomic force microscopy (AFM) (10): domains oriented orthogonally are clearly recognized. The polarized UV-Vis absorption spectra at room temperature of the heterostructures, prepared following the above reported steps, are shown in Fig.5.

Wavelength (nm)

Figure 5: Polarized absorption spectra of a double-layerfilm of oriented PPV/P60Me for polarization parallel (dotted line) and orthogonal (solid line) to the PPV orientation during the preparation procedure: a) stepU b)step 2; c)step3, d)step 4.



216 We have verified that the anisotropy of the PPV layer is not reduced after the deposition (see Fig. 5b) and rubbing (see Fig. 5c), orthogonally to the PPV orientation, of the P60Me layer, provided that the rubbing is soft enough. The thermal annealing of the films increases the anisotropy of the P60Me layer without affecting the PPV layer (see Fig. 5d). The increase in the PAT orientation, due to the thermal treatment, is independent of the direction of the PPV orientation, as we observed the same increase in polarization ratio after rubbing of the PAT layer in the same direction of PPV. The alignment procedure used for P60Me permits a two-layer film to be prepared with any tipe of relative orientation of the layers, because alignment of spin-coated P60Me on preoriented polymeric substrates by thermal annealing does not occur. Even though the degree of the anisotropy reached for this double layer system is lower than that found for the single polymeric layers, it shows photoluminescence and electroluminescence emission with sensitive polarization depencences. In Fig. 6 we report the low temperature (100 K) polarized photoluminescence measurements for a double layer of PPV and PAT oriented perpendicularly.

Figure 6: Polarized photoluminescence of a double layerfilmof oriented PPV/P60Me. Thefilmsare excited at 363 nm (solid line) and 514 nm (dotted line). The PL polarization ratio is 5 and 3 for PPV and PAT, respectively. The PL of PôOMe was obtained by exciting the film at 514.5 nm, where PPV has a negligible absorption, while by exciting at 363 nm the PL spectrum corresponds to the PPV emission, with a weak contribution from PAT only for the light analysed for polarization orthogonal to the PPV orientation. This is consistent with the presence of negligible energy transfer from PPV to PAT.


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EL measurements were performed on a simple device structure ITO/PPV/P60Me/Al where the PPV layer consists of two isotropic layers of PPV and a third thin layer of oriented PPV, that represents the interface with the perpendicularly oriented thin layer of P60Me. Figure 7 shows polarized EL from the two-layer device, at different voltages.

Wavelength (nm)

Figure 7: Polarized electroluminescence of a double layer device, at different voltages. The light is analized with a polarizer with axis parallel to the PPV (solid line) and P60Me (dotted line) orientations. At low voltages (< TV) only the red emission of PôOMe is detected, showing an anisotropy of about 3. By increasing the voltage the emission from the PPV layer is also observed, with an onset voltage of about 8V. The EL



218 emission from PPV increases, with respect to that from P60Me, by increasing the bias voltage, reaching nearly the same intensity at 11-12V. The polarization properties of the ELfromthe double structure allows to clearly observe a variation in the colour from the red to the green, at high voltages, by simply rotating the axis of a polarizer. In conclusion, we have realized a simple heterostructure device that can emit light changing reversiblyfromred to orange-green by increasing the voltage from 4 to 12 V. Moreover, the polarization properties of the emitted light allow to tune the colour from red to green by simply rotating the axis of a polarizer, while the device is operating at 10-12V.

Experimental PPV was prepared from a precursor polymer containing acetate side groups, which partially remain in polymer during thermal elimination (6). Orientation of the PPV layer was obtained by rubbing during thermal elimination, and subsequently converting to PPV by annealing at 180°C for 2 h. Details on the preparation of P60Me have been reported previously (11). The weight average molecular weight, Mw, as detected from GPC, and referred to a calibration curve on polystyrene standards was 24.000 with a Mw/Mn of 1.6. All the data reported in the text are referred to the polymer obtained as residue to hot acetone extraction. The regioregularity of the polymer, as determined by ^ - N M R investigation, was 98% (11). After deposition by spin coating of the PAT layer, the films were repeatedly rubbed by a velvet cloth on a rotating cylinder. After this treatment the samples were heated at 105°C in vacuum (10" mmHg) for 1 h. The temperature was then slowly decreased down to room temperature in 1 h Polarized absorption spectra were measured with a Cary 2400 spectrometer using a couple of sheet polarizers on both the sample and the reference beam, whose baseline was previously recorded for both the polarizations. Polarized PL and EL were obtained with a SPEX 270M polychromator equipped with a liquid N cooled CCD detector. The emission was analysed with a sheet polarizer and a polarization scrambler was used to avoid polarization dependence of the gratings and detection system. The sample was kept under nitrogen atmosphere during the PL measurements, performed in the backscattering geometry by exciting with the 514.5 and 363.8 nm lines of an Ar ion laser. The spectra were corrected for the spectral response of the instrument, measured with a calibrated lamp. The devices were obtained by spin coating the chloroform solution of P60Me (13mg/ml) onto ITO (50O/cm ) coated glass which was previously covered with PPV. The film on ITO was then rubbed and annealed as described above. The second electrode was formed by aluminium evaporated (10 mmHg) 3

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219 on the top of the rubbed-annealed film (aluminium thickness 100 nm). The onset voltages of the LEDs are in the range of 3 - 4.5 V and the external efficiencies are about 10" %. 4


References (1) Bradley, D. D. C.; Friend, R. H.; Lindenberger, H.; Roth, S. Polymer 1986, 27, 1709. (2) Friend, R. H . ; Gymer, R. W.; Holmes, A . B . ; Burroughes, J. H.; Matks, R. N . ; Taliani, C.; Bradley, D . D . C.; Dos Santos, D . Α.; Brédas, J. L . ; Lögdlun, M.; Salaneck, W. R. Nature 1999, 397, 121 (3) Grell, M.; Bradley, D. D. C. Adv. Mater. 1999,11,895 (4) Grell, M.; Knoll, W.; Lupo, D.; Meisel, Α.; Miteva, T.; Neher, D.; Nothofer, H . G.; Scherf, U . ; Yasuda, A . Adv. Mater. 1999, 11, 671. (5) Whitehead, K . S.; Grell, M.; Bradley, D. D. C.; Jandke, M.; Strohriegl, P. Appl. Phys. Lett. 2000, 20, 2946. (6) Jandke, M.; Strohriegl, P.; Gmeiner, J.; Brütting, W.; Schwoerer, M . Adv. Mater. 1999,11,1518. (7) Bolognesi, Α.; Botta, C.; Martinelli, M.; Porzio, W . Organic Electronics 2000, 1, 27. (8) Herold,M.;Gmeiner, I.; Schwoere, M. Acta Polymerica 1994, 45, 392. (9) Loerner, E.; Meier, M.; Gmeiner, I.; Herold, M.; Brütting, W.; Schowere, M . Opt. Mater. 1998, 9, 109. (10) Bolognesi, Α.; Botta, C.; Facchinetti, D.; Jandke,M.;Kreger, Κ.; Strohriegl, P.; Relini, Α.; Rolandi, R.; Blumstengel, S. Adv. Mater. 2001, 13, 1072. (11) Bolognesi, Α.; Porzio, W.; Bajo, G . ; Zannoni, G . ; Fanning, L . Acta Polymerica 1999, 50, 151.


Chromogenic Phenomena in Polymers - American Chemical Society

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