Light-emitting diodes based on polymers - Nature, Notas de estudo de Engenharia de Produção
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Light-emitting diodes based on polymers - Nature, Notas de estudo de Engenharia de Produção

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LETTERS TO NATURE properties of the neutrino are responsible for the solar-neutrino problem. One possible explanation is provided by neutrino oscillations resonantiy enhanced in their passage through matter —the MSW effect”, This implies neutrinos of finite mass and thus goes beyond the standard model of the electroweak theory”. o Receiver 6 June: soceptes 24 August 1990. 4 Clavere, A. I5298.6, Meo, CP. van der aa HE, & Roca Cortes, T. Nature 262, 591-594 asrai Deutmer, F.L. & Gough, D. 0. A Rev Astt. Astrcplys. 22, 593-619 (1954) Babes, . N. Froc. int. Conf on Neuttino Enystos and Astrantysies Vol 2 (eds Cence, R. 1. Ma E & Roberts, A.) 253 (University of Hawai 1681) Uirich, R.K, & Rhodes, E. 4X Astraphys. 2 268, 551-563 (1983) Sou, D. 0. & Novotay, E. Sol Prys. 128, 143.160 (1990) Christensen-Datsgasra, in Solar Interior ar! Atmosphere (eds Co, à N Livingston, W. 6. & Mathews, M (University of Arzona Press, in he press). Cox AN. Gueik 1 A. & Raby, S. Astrophys. ) 353, 698-711 (19901 Schatamen, E. & Maeder, A Astr. Astroptys 96, 1-16 (1981) Lebreton, Y, Berthomieu, G & Provost 5, Achances in Heli and Asteraseismology 14 123 (eds Christensen-Dalsgaard, À & Frandsen, 5) 85-98 [Reidel Dordrechr, 1988 19. Spergel,D.N & Press, W. H. Astrophys. 1 294, 663.673 (1685 11. Faulkner, 3. & Gulitong, RL. Astragtys, 1 289 9904-1000 (1965) 12. Davis, R, Uande, K, Lee, C. K, Cleveland, E, 1. & Ulman, inside the Sum (eds G. Berhomieu, 6. & Crer, hj 271 177 (lume. Dordrecht 1950) 13, Hr .S et al Inside the Sum (eds Berthonieu, G. & Criier.M4 LTS 186 (Kuner. Dordtecht, 1990 14, Dicke, RH & Goldenberg. HM. Astonhys, 4 Supp 27, 131-182 (1974) 15. Roberison. D. S. & Carter. WE. Nature 310, 572-574 (19841 16 Taylor, 1H, & Woisberg, ).M. Astrophys. 4 345, 434-450 (1969) 17. Dicko, RH, Ku 1.8. &ediblrecht, KG. Astohys. 1 328, 452 454 (1088) 18, Claveri, A. sao, 6 R. Meleod, GP, van der Iaay, H B, & Ruca Cortes, Y. Nature 293, 443 445 Ages 18. Wondord. MF. Nature 308, 530-532 (1584) 20. Baheal, 1 N. Neuttino Astraphysics Ch. 1 (Carmbndge Uiversiy Press, 1989) 21, Ciristensen-Dalsgaara 1 Proc Symo, Seismology of the Sun and Sun Stars led, Rorfe E. 2) 431-450 [European Space Agency, 1988) 22. Belhe, HA Prys. Rex Letr 68, 1905-1308 (19861 23. Leignton, RB. Noyes, RW. & Simon, G. W. Astranbys. J 435, 474-498 (19621 24, Beockes, 1 R. Isaah GR & van der Ray E. Mom Not R astr. Soc. 186, 1-11 (1078) 25, Elsworth Y. Howe, R, lsadk. G. R, MeLeod CP. & New, R. Nature 348, 322.324 (1990), 26. Woodard, MF. & Noyes R. W. Nature 318, 449-450 (1985) 27. Gol. B, Fossat, E & Grec, 6. Astr. Astroptys, 200, L29-131 (1988) 28. Pale P-L, Regulo, C. & Roca Corte, T. Astr Astropiys 224, 253-258, (1989) 28, Bancal 4. N. & Uri, R.K. fev. mod Phys, 6, 297-372 (1088) 30, Faulkner, J, Gough D. O. & Vahia, MN. Nature 321, 226-229 (1986 31 Déppem, W. Gilland R & Christensen Delsgaard 1 Niture 24, 226 231 (1966) 32 Jimenez. À et al, Advances à Helio and Astrosetssmology AU 123 (es Chvistensen-Dalsgaará, 1 & Frsndsen, 5) 205-209 (Neide, Dordrecht 1988) 33. Kuo, T.K & Pantoleone, À Rev mod Pis. 61, 837-979 (1988) ACHNOWLEDGEMENTS. We 1hark all members, past and present, of the Birmingham and Instituto de Astrofisica de Canários solar asoliatians grouns for their assistance. and 1 Christensen-Datsgaard, A.N. Cox end D. O Gough for their comments. This work was funded by SERC and by the CAICYT Light-emitting diodes based on conjugated polymers 3.H. Burroughes*;, D. D. C. Bradtey*, A. R. Brown*, R.N. Marks*, K. Mackay*, R. H. Fhend*, P. L, Burns? &A. B. Holmest * Cavendish Laboratory, Madingley Road, Cambridge CB3 OHE, UK + University Chemistry Laboratory, Lensfield Road, Cambridge CB2 1EW. UK CoNJUGATED polymers are organic semiconductors, the semicon- ducting behaviour heing associated with the 17 molecular orbitals delocalized along the polymer chain. Their main advantage over non-polymeric organic semiconductors is the possibility of process- ing the polymer to form useful and robust structures. The response df the system to electronic excitation is nonlinear—the injection of an electron and a hole on the conjugated chain can lead to a self-lucalized excited state which can then decay radiatively, sug- gesting the possibility of using these materials in electrolumines- cent devices. We demonstrate here that poly( p-phenylene vinylene), Prepared by way of a solution-processable precursor, can be used as the active element in a large-area light-emitting diode. The * Present address: IBM Thomas 1. Watson Research Centre, Vorktoum Heights, New York LOSS, USA. NATURE - VOL 347 - 41 OCTOBER 1990 combination of good structural properties of this polymer, its ease of fabrication, and light emission in the green-yellow part of the spectrum with reasonably high efficiency, suggest that the polymer can be used for the development of large-area lighi-emitting displays. There has been long-standing interest in the development of solid-state light-emitting devices. Efficient light generation is achieved in inorganic semiconductors with direct band gaps, such as GaAs, but these are noi easily or economically used in large-area displays. For this, systems based on polycrystalline ZnS have been developed, although low efliciencies and poor reliability have prevented large-scale production. Because of the high photoluminescence quantum yieids common in organic molecular semiconductors, there has long been interest in the possibitity of light emission by these organic semiconductors through charge injection under a high applied field (electro- luminescence)"”. Light-emitting devices are fabricated by vacuum sublimation of the organic layers, and although the efficiencies and selection of colour of the emission are very g00d, there are in general problems associated with the long-term stability of the sublimed organic film against recrystallization and other structural changes. One way to improve the structural stability of these organic layers is to move from molecular to macromolecular materials, and conjugated polymers are a good choice in that they can, in principle, provide both good charge transport and also high quantum efficiency for the luminescence. Much of the interest in conjugated polymers has been in their properties as conduct- ing materials, usually achieved at high levels of chemical dop- ing, and there has been comparatively little interest in their tuminescence. One reason for this is that polyacetylene, the most widely studied of these materials, shows only very weak phot luminescence. But conjugated polymers that have larger sem: conductor gaps, and that can be prepared in a sufliciently pure form to control non-radiative decay of excited states at defect sites, can show high quantum yicids for photoluminescence. Among these, poly(p-phenylenc vinylene) or PPY can be con- veniently made into high-quality films and shows strong photo- luminescence in a band centred near 2.2 eV, just below the threshold for x to 7w* interband transitions”!º. We synthesized PPV (1) using a solution-processable precur- sor polymer (II), as shown in Fig. 1. This precyrsor polymer is conveniently prepared from a,e'-dichloro-p-xylene (IH), through polymerization of the sulphonium salt intermediate (Ivy We carried out the polymerization in à water/melhano! mixture in the presence of base and, after termination, dialysed the reaction mixture against distilled water. The solvent was removed and the precursor polymer redissolved in melhanol. We find that this is a good solvent for spin-coating thin films of the precursor polymer on suitable substrates. After thermal conversion (typically 250 ºC, in vacuo, for 10h), the films of PPY (typical thickness 100 nm) are homogentous, dense and q ad cao -( Soma — a Me0H, 50 “€ GS cry €IVi 10H 2H 3. Dialysis 250“C, vacuum — x panda Erg 14 j m Sa Ch «ti an () FIG. 1 Synthetio route to PPV 539 LETTERS TO NATURE 25 Current (mA) 0.5 Voltage (V) FIG. 2 Current-voltage characteristio for en electroluminescent device hav- ing a PPY film 70 nm thick and active area of 2 mim?, a bottom contact of indium oxide, and a top contact of aluminium. The forward-bias regime is shown (indium oxide positive with respect to the aluminium electrode). uniform. Furthermore, they are robust and intractable, stable in air ai room temperature, and at temperatures >300ºC in à vaçuum!! Structures for electroluminescence studies were fabricated with the PPV film formed on a bottom electrode deposited on a suitable substrate (such as glass), and with the top electrode formed onto the fully converted PPY film. For the negative, electron-injecting contact we use materials with a low work function, and for the positive, hole-injecting contact, we use materials with a high work function. At least one of these layers must be semi-transparent for light emission normal to the plane of the device, and for this we have used both indium oxide, deposited by ion-beam sputtering!* and thin aluminium (typi- caliy 7-15 nm). We found that aluminium exposed to air to . 8 44 5 sq á : Ss sd . z . 8 o E q 44 . % . g “o E 4 . E e S . É 2d ê $ Rê 4 9 E E CO o 0.5 1 15 2 2.5 3 35 a Current (mA) FIG. 3 Integrated light output plotted against current for the electrolumines- cent device giving the current-voltage characterístic in Fig. 2. 540 allow formation of a thin oxide coating, gold and indium oxide can al; be used as the positive clecirode material, and that aluminium, magnesiom silver alloy and amorphous silicon hydrogen alloys prepared by radiofrequency sputtering are suitable as the negative electrade materials. The high stability of the PPY film atlows easy deposition of the top contact layer, and we were able to form this contact using thermal evaporation for metais and ion-beam sputtering for indium oxide. Figures 2 and 3 show typical characteristics for devices having indium oxide as the bottom contact and aluminium as the top contact. The threshold for substantial charge injection is just below 14 V, at a field of 2x 10º Vem”, and the integrated light output is approximately linear with current. Figure 4 shows the speetrally resolved output for a device at various temperatures. The spectrum is very similar to that measured in photolumines- cence, with a peak near 2.2 eY and well resolved phonon struc- ture”º, These devices therefore emit in the green-yellow part of the spectrum, and can be easily seen under normal laboratory lighting. The quantum efficiency (photons emitted per electron injected) is moderate, but not as high as reported for some of the structures made with molecular materials”. The quantum efficiences for our PPV devices were up to 0.05%. We found that the failure mode of these devices is usually associated with failure at the polymer/thin metal interface and is probably due to local Joule heating there. The observation and characterization of electroluminescence in this conjugated polymer is of interest in the study of the fundamenta! excitations of this class of semiconductor. Here, the concept of self-localized charged or neutral excited states in the nonlinear response of the electronic system has been a useful one. For polymers with the symmetry of PPV, these excitations are polarons, either uncharged (as the polaron exciton) or charged (singly charged as the polaron, and doubly charged as the bipolaron)%!%, We have previously assigned the photoluminescence in this polymer to rudiative recombination of the singlet polaron exciton formed by intrachain excitation?º and, in view of the identical spectral emission here, we assign the electroJuminescence to the radiative decay of the same excited state. The electroluminescence is generated by recombi- nation of the electrons and holes injected from opposite sides of the structure, however, and we must consider what the charge carriers are. We have previousty noted that bipolarons, the more stable of the charged excitations in photoexcitation and chemical doping studies, are very strongly self-localized, with movement of the associated pair of energy levels deep into the semiconduc- tor gap, to within 1 eV of each other”. In contrast, the movement of these levels into the gap for the neutral polaror exciton, which one-electron models predict to be the same as for the bipolaron'*, is measured directly from the photoluminescence p—— E 20K D SK . c 200K $ 8 BK S A ZE5K é E ; É / 31 E a 22 25 Photon Energy (ev) FIG. 4 Spectrally resolved output for an electroluminescent device at various temperatures. NATURE - VOL 347 - 11 OCTOBER 1990 LETTERS TO NATURE emission to be much smalier, with the levels remaining more than 2.2 €Y apart. For electroluminescence then, bipotarons are very unlikely to be the charge carriers responsible for formation of polaron excitons, because their creation requires coulescence of two charge carriers, their mobilities are low and the strong self-localization of the bipolaron evident in the positions of the gap states probably does not leave sufficient energy for radiative decay at the photon energies measured herc. Therefore, the charge carriers involved are probably polarons. The evidence that they can combine to form potaron excitons requires that the polaron gap states move no further into the gap than those ofthe polaron exciton and may account for the failure to observe the optical transitions associated “with the polaron. The photoluminescence quantum yield of PPV has been esti- mated to be 8%. It has been shown!” that the non-radiative processes that limit the efficiency of radiative decay as measured in photoluminescence are due to migration of the excited states to defect sites which act as non-radiative recombination centres, and also, at high intensities, to collisions between pairs of excited states. These are processes that can, in principle, be controlled esign of the polymer, and therefore there are excellent les for the development of this class of materials in à range of electroluminescence applications. q Received 21 August; accepted 18 September 1990. 1. Vincent ?. 5. Bartow, W. A. Hann, RA & Roberts, G. 6. Thin Soft Films 9, 475-488 (1962) 2. Teng, O. W. & VanSijee, S. À, App, Phys, Lott, 51, 913-946 (1987) 3. Tang, E. W. VanSiske, S.A & Chen, C.H. 3 apl Chys. 85, 3610-3616 (1089), 4, Adachi, C. Tokio, S, Tsutsui, T. & Saito, S. Jap. 4 apol Phys. 27, 59-61 (1988) 5. Adachi, C. Tsutsui, T. & Saito, 5. Apol Phys. Lert 55, 1489 1491 (1989) 6. Adachi C. Tsutsuk, T. & Saito, S. App, Poys, Lett, 56, 79-80] (1989) 7. Nohara. M. Hasegawa, M, Hosohawa, C. Tokeilin,. & Kusomato, T. Chem, Lett, 189 -190 (1950) & Basescu, N. et a Nature 327, 403-405 (1987) 9. Frend, RH, Bradley, D, D. C. & Townsend, P. O. 4 Phys DIO, 1367-1384 (1987) 10. Sradey,D. D.C. & Friend, R. H. 1 Phys; Condenscd Matter , 3671-3678 (1988) 11. Bradey.D.D.C. 1 Prys DZO, 1389-1410 (1987) 12. Murase. 1, Ohnishi, T, Noguchi, T. & Hiracka, M. Synthetiz Metais 17, 639 644 (1987), 13. Stenger-Smut, 1 D. Lenz, R. W. & Wegner, 6. Polymer 30, 1048-1053 (1989), 14. Belingham 1 R. Prilips, WA. & Adkins, C. 4 4 Pres; Condensed Matter 2, 6207-6221 (1980). 15. Fesser, K. Bishop, A. R. & Campbell, D. K. Phys. Rey, 827, 4804-4826 (19553) 16. Brazovski S. A. & Kirova, NN. EPT Lett 33, 4-8 (1981) 17. Bradey. D. D.C. et a! Springer Ser. Solid St Soi 76, 1097-142 (1987) ACKNOWLEOGEMENTS. We thank 1 R. Gelingham, C. 1. Adéns and W. A, Philips for their helg in preparing the Indium oxide Tkms. Wo thack SERG and Cambridge Research and Innovation Ltd for suport. Tropospheric lifetimes of three compounds for possible replacement of CFC and halons A. €. Brown, €. E. Canosa-Mas, A. D. Parr, K. Rothwell &R.P. Wayne Physical Chemistry Laboratory, South Pares Road, Oxford 0X1 302, UK CHLORINE and bromine have been implicated in the massive springtime depletion of stratospheric ozone over Antarctica”. As the source of these haiogens is anthropogenic emissions of halo- alkanes, the problem has acquired politica! and economic siguificance”. The chemical industry has been forced to consider urgentIy possible repiacements for conventional halocarbons, and the potential effects on the environment of proposed alternative compounds have been evaluated recently in the Alternative Fluorocarbon Environmental Acceptability Study (AFEAS/. Three such compounds that are not included in the AFEAS report are CF,BrH (halon 1201). CF;CFBrH (halon 2401) and CF;CF,CCLH (HCFC 225ca). Removal of these compounds from the atmosphere will occur primarily by reaction with the hydroxyl radical, OH (ref. 4). Here we determine, from laboratory studies, the absolute rate of reaction between these three species and OH. Such kinetic data are vital for assessing their viability as reptace- NATURE « VOL 347 - 14 OCTOBER 1990 ment compounds. We use these data, along with our measurements of ultraviolet absorption cross-sections, to estimate the tropos: pheric lifetimes of the halons and HCFC 228ca against removal by OH, and their potential for destroying ozone in the stratosphere. Our approach shows how laboratory measurements can provide a useful first estimate of the environmental acceptability of com- pounds of this sort. CF,BrR and CF;CFBrH are being considered as substitutes for CE;Br in fire extinguishers, and CF;CF,CCL,H has useful properties as a solvent. The efficiency with which a unit mass, of each halocarbon will destroy stratospheric ozone relative to the most important ozone-depleting molecules CEC-11 (CFCI,) and CFC-12 (CF;CL,) will depend strongly on the atmospheric lífetime of the compound. The daytime degradation of these hydrogen-containing molecutes, RH, will occur primarity by reaction with the OH radical? OH+RH —L, H,0+R om and it is therefore important to know the rates of the homogeneous gas-phase reactions of OH with RH. We have determined values of the second-order rate constants, k,, for the hydrogen-atom abstraction process (1) and their variation with temperature for the three hydrohalocarbon molecules. Our experimental procedure and data analysis are described fully elsewhere*”?. We used an absolute discharge flow technique with a movable injection point for the reactant to provide time resolution; OH concentrations were measured by resonance fluorescence. The variation of OH concentration with injector position was monitored with a known excess of hydrohalocarbon added. The logarithm of the OH signal was a linear function of calculated contact time, and the gradient was plotted against the concentration of hydrohalocarbon to give a straight line of slope k . Our values for k, are given in Table 1 along with the experimental conditions. We used a conventional Arrhenius equation, In k,=lIn A- E/RT, to analyse the tem- perature-dependence. Thus a plot of tn k, as a function of 1/T should give a straight line of slope E/R and intercept In A. Values of the activation energies E in the form E/R and pre-exponential factors A are given in Table 1. Errocs on the slope, calculated in a linear least-squares analysis, are the 95% confidence limits. The 95% confidence limits on the intercept were unrealistically large, as is often the case in caiculating Arrhenius parameters: we do not quote the errors here. The predictive capacity of the experimental Arrhenius expression is not properly represented by the individual errors on E and A, but rather by the combined expression incorporating both parameters. We paid particular attention to the possible presence of fast- reacting impurities in the hydrohalocarbon samples, which could seriously affect the measurement of k. AH of the “OL. so (Acre 1,090 77 (K) FIG. 4 Arrhenius plots for reactions of RH with OH.
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