A NUMERICAL ANALYSIS OF PROCESSES IN ORGANIC ELECTROLUMINESCENT STRUCTURE WITH METAL NANOPARTICLES

Transcription

1 Journl of Optoelectronics nd Advnced Mterils Vol. 6, No., June 00, p A NUMERICAL ANALYSIS OF PROCESSES IN ORGANIC ELECTROLUMINESCENT STRUCTURE WITH METAL NANOPARTICLES A. V. Kukht *, E. E. Kolesnik, D. V. Ritchik, V. V. Glkin, I. I. Zholnerevich Institute of Moleculr nd Atomic Physics, F. Skryn Ave. 70, Minsk, 007, Belrus Belrusin Stte University, F. Skryn Ave., Minsk, 0080, Belrus A simple model for thin film orgnic electroluminescence in single lyered one-dimensionl cell with metl nnoprticles is developed, ssuming direct excittion of luminophor molecules y electron impct. The vlues nd ehviour of clculted curves for rightness nd efficiencies correlte with experimentl dt. The insertion of metl nnoprticles into ctive volume of electroluminescent cell leds to the increse of the verge electric field inside the cell nd to the decrese of the threshold of electroluminescence nd stimulted rdition due to improved conditions of electron tunneling s result of potentil rriers shpe chnge. (Received Ferury, 00; ccepted June, 00) Keywords: Orgnic electroluminescence, Nnoprticle, Electron impct, Efficiency. Introduction Orgnic solid thin film structures re very promising for the ppliction in mnifold optoelectronic devices []. One of the min requirements to them is high homogeneity of orgnic films. However, lot of nonuniform structures [-8] with high energy trnsformtion performnce ws offered. One of the methods of the nonuniform structure cretion is introducing of metl or semiconductor nnoprticles into n orgnic luminophor [8,9]. The presence of metl nnoprticles mkes the dielectric orgnic mteril conductive [9]. In such systems non-cvity lsing [0] ws otined. We hve developed n electroluminescent structure on the sis of MEH-PPV with silver nnoprticles with good light performnces. But, mechnism of electroluminescence threshold decresing nd efficiency incresing is still unknown. We offered erlier [,] physicl model descriing the processes of electric energy trnsformtion y orgnic luminophor molecules. This model is sed on pproch tht orgnic luminophor molecules excittion is given y inelstic interctions with electrons, emitted from the cthode s result of tunneling, nd ccelerted y n electric field. The otined dependences, for exmple, of the luminescent emission distriution in depth of the electroluminescent cell, s well s the electroluminescence energy efficiency dependence on the electric field hve shown good correspondence to the experimentl dt [,] tht is good confirmtion of our pproch dequcy. The clcultions hve llowed to conclude [5] tht it is possile to receive the stimulted rdition in thin electroluminescent cell on the sis of orgnic compounds t electricl excittion under sustntilly ccessile vlues of electric field intensity using low vlue of injection rrier. The purpose of the present work is the nlysis of processes in the electroluminescent cell with metl nnoprticles. First of ll, we re interested in their influence on the emissive properties, the electric current, nd on the possiility to otin the stimulted rdition.. Physicl model We consider the structure representing lyer of orgnic light-emitting sustnce with metl nnosized loes, y thickness d, plced in electric field etween metl electrodes with potentil * Corresponding uthor: kukht@imph.s-net.y

2 06 A. V. Kukht, E. E. Kolesnik, D. V. Ritchik, V. V. Glkin, I. I. Zholnerevich difference U. All los re distriuted over the cell evenly, i.e. plced on distnce d from ech other in directions long the electric field (x) nd perpendiculrly to it, nd hve identicl dimeter d. We ssume, tht luminophor molecules with concentrtion N re emedded into certin dielectric medium, nd n efficient energy trnsfer from the medium plying role of the energy ccumultor, on the impurity (luminophor) tkes plce in the system. The typicl exmples re doped orgnic crystls nd conjugted polymer compounds with emedded orgnic molecules. The supposed permittivity =.0 is typicl vlue for orgnic mediums. The presence of dielectric t the spce etween electrodes reduces n electric field. In the presence of dielectric the injection rrier is depressed on quntity of n electron ffinity of the dielectric, nd increses little owing to the reduction of imge forces overlpping. We suppose potentil injection rrier height tking into ccount the dielectric influence. The introduction of nnoprticles into the electroluminescent cell will result in redistriution of n electric field inside the cell. The cthode nd nnoprticle electron potentil rriers will hve finite width in this cse, nd t the defined d vlues the proility of electron tunneling through such rrier will e distinct from 0 (for simplifiction nd owing to preferred tunneling of electrons which kinetic energy long the electric field is close to Fermi level, we ssume tht the potentil rriers re one-dimensionl). Note, tht in the considered two-dimensionl conductive ensemle Coulom lockde will not ply such role s it tkes plce in single islnd chins [6]. In such multilevel structure two mechnisms of electron trnsfer re possile [7] such s the coherent electronic trnsport if the tunneling fctors re high enough, nd two steps trnsport if the resistnce of tunneling rriers is high. As we were interested in the cse of strong fields, we did not tke into ccount chrge effects. We ssume, tht electrons leve the cthode which surfce coincides with the plne x = x 0 = 0, nd then they re ccelerted y pplied electric field E, pssing chin of the potentil rriers etween metl nnoprticles. At such considered symmetry the excittion rte of luminophor molecules will e defined only y one sptil coordinte x. In the common view the luminophor molecule excittion rte in point X will e defined y the following expression: j( x, px, py, pz) W ( X ) = σ ( px, p y, pz ) dpxdpydpz, () e 0 where σ ( p, p, p ) is the molecule excittion cross-section s result of inelstic collision with x y z electron hving impulse projection vlues p x,p y,p z, j( x, px, py, p z ) is the current density in point X, e is the chrge of electron. The electron current density on metl/dielectric order is defined y the expression: where x0 y z j( p, p, p ) = f ( p, p, p ) D( p, E) v e, () x0 y z x0 y z x0 x0 f ( p, p, p ) is the distriution function of electrons in cthode metl, rrier permittivity coefficient for electrons with impulse p x0, x0 x / 0 D( p, E ) is the x0 v = p m is the electron velocity projection on x xis in point x=x 0. It is supposed in the formul () tht tunneling is crried out on vcnt levels of the next nnoprticle, i.e. nnoprticle Fermi levels ly elow the cthode Fermi level nd re lowered in the pplied field direction (distriution function for nnoprticle free re f o =0). Inverse tunneling t the presence of the pplied electric field is supposed to e sent. It is known, tht the metl nnoprticles presence deforms the potentil rrier shpes. In the cse of smll metllic prticle size nd distnces etween them the essentil superposition of imge forces tkes plce, nd, generlly, the shpe of rrier will hve rther complicted form depending on the sizes, shpes nd reltive loction of the nnoprticles [9]. It is often ccepted [8], tht rrier hs prolic form nd potentil rrier etween nnoprticles in the sence of electric field is defined y the expression: gx g( x x ) U ( x) =, ()

3 A numericl nlysis of processes in orgnic electroluminescent structure with metl nnoprticles 07 where x 0 =d is the rrier width. In electric field E rrier ecomes lower, its mximum is shifted: gx0 g( x x0) UE ( x) = eex. () However, such potentil rrier pproximtion cn e pplied only with some stipultions. The growth of such rrier width t the sttionry vlue of its height results in the prol incresing velocity reduction, nd t high enough electric fields it will give to tht clculted trnsprence vlues D nd consequently, conductnce current, will e higher for wider rriers, thn for nrrow (see Fig., ), tht is unrel. Thus, such potentil rrier representtion is legl t low electric field vlues only though the high vlues re more interesting for us, especilly regrding to rech the inverse density popultion of luminophor molecule levels. Therefore, we present the potentil rriers etween nnoprticles oth s prol nd rectngle with d width (see Fig., ), tht represents other extreme cse which is not tking into ccount the potentil rriers shpe chnges. x0 8 x0 x0 0 x0 - x0-8 x0 - x0-6 j, A /m E, V /m x0 8 x0 8 x0 8 x0 8 5x0 8 x0 8 x0 x0 0 x0 - x0-8 x0 - x j, A /m E, V /m 0 x0 8 x0 8 x0 8 x0 8 5x0 8 Fig.. Dependence of density of current j pssing through the prolic () nd rectngulr () rrier on pplied electric field; = ev. It is necessry to tke into considertion lso the electron emission from nnoprticles owing to tunneling electrons from them t high fields, nd electron thermo-emission s result of current wrming up. The electronic gs temperture in metl nnoprticles my chieve ev (i.e. out severl thousnds K), thus the film temperture my e close to environment temperture [8]. Nevertheless, it is known from experiments [8], tht the thermo-emission current vlue is some orders lower, thn the conductnce current. So, the thermionic emission current will not influence essentilly the excittion of molecules. Besides, only therml electrons re the result of thermoemission from metl while the electrons with energy of out 0 ev re most effective for excittion of orgnic compounds [0], so for reching such energy thermo-electrons hve to pss the comprle ccelerting potentil difference. It will require higher pplied voltges t which electron tunneling is more essentil. The work function of nnoprticles is essentilly higher, thn for the specil cthode, nd current hs exponentil dependence on the work function, so it is possile to expect, tht the tunneling current from nnoprticles will not essentilly deposit to luminophor molecules excittion. According to our clcultions this current differs t lest three orders in the cse of rrier height vlue = ev nd ev for nnoprticles nd cthode, correspondingly. Thus, the electron emission from nnoprticles lso does not essentilly ffect the luminophor molecule excittion. The clcultion of W(X) ws crried out y numericl integrtion of expression (), using function of degenerte Fermi gs s f ( px, p, ) 0 y p z. The efficiency η ws determined s rtio of the totl kinetic energy of the electrons leving metl, nd the energy of electron ccelertion in luminophor y electric field. ββ-dinphtilethelene with typicl for orgnic compounds spectrl nd kinetic chrcteristics [5] ws used s luminophor. The electroluminescent cell with thickness of 00 nm, dimensions of mm, with luminium cthode (E f =,8 ev) ws considered. The luminescence quenching y metl prticles [] ws not tken into ccount.

4 08 A. V. Kukht, E. E. Kolesnik, D. V. Ritchik, V. V. Glkin, I. I. Zholnerevich. Results nd discussion According to clcultions, the increse of pplied voltge etween nnoprticles results in disppernce of the dependence of potentil rrier width (i.e. vlues d ), curves for conductnce currents on Fig., re converged t vlues E~ V/m. The difference etween these two extreme cses of model rriers is essentil t smll fields while with the increse of E it ecomes less considerle. Fig. (-d) shows the clculted dependences of the cell volume verged excittion rte W of luminophor molecules nd fluence F of rdition from this cell in direction perpendiculrly to the pplied field, which re compred to the similr dt for the cell without nnoprticles [5]. It cn e seen tht stimulted rdition ppers t significntly smller pplied voltge, thn in electroluminescent cell without nnoprticles (curves on Fig. ). The most clerly it cn e seen t efficiency curve (e, f). Note, tht the zigzg ehviour of some clculted dependences in the region of low fields is connected with disproportionl chnge of electricl power consumption nd excittion rte of molecules. On the contrry (see Fig., f), the threshold voltge U th not so strongly depends on distnce etween nnoprticles (t the sme vlues of verged field). If we represent these dependences on electric field (Fig., rectngulr rrier), they ecome the sme eginning from some field vlue. The reson of such ehviour is pprently the increse of field in the cell with nnoprticles. The threshold of ppernce of stimulted emission is lower due to nonrectngulr rrier shpe (Fig. ). x0 6 x0 x0 - x0-6 x0-0 x W, s - U /ε, V W, s - x0 8 x0 x0 0 x0 - x0-8 x0 - x U /ε, V F, qunt/(s cm ) U/ε, V c F, qunt/(s cm ) U/ε, V d 0 - η η 0 - x0 - x e U/ε, V 0 - x0 - x U/ε, V f Fig.. Dependence of verged excittion rte W (, ), fluence F (c, d), nd efficiency η (e, f) on pplied voltge for electroluminescent cell with = ev, d=00 nm, d =d = (), 5 (),0 () nd 0 nm () for prolic (, c, e) nd rectngulr (, d, f) rrier.

5 A numericl nlysis of processes in orgnic electroluminescent structure with metl nnoprticles 09 F, qunt/(s cm ) E, V /m 0 0 x0 8 x0 8 x0 8 x0 8 5x x0 - x η E, V /m x0 8 x0 8 x0 8 x0 8 5x0 8 Fig.. Dependence of fluency F () nd efficiency η () on pplied electric field for the cell with = ev, d=00 nm, d =d = (), 5 (),0 () nd 0 nm () for rectngulr rrier. The distriution of excittion on the cell depth shows the strong inhomogeneity t low pplied voltges nd t smll vlues of distnce etween nnoprticles (Fig. ). It is pprently due to the electron pssge the severl potentil rriers with finite width. Note tht clculted dependences more dequtely correspond to the trnsient excittion, in sttionry cse t smll fields the clculted dependences should hve stepped chrcter owing to Coulom lockde. W,. u W,. u x, m.0x0-8.0x x x0-8.0x x, m.0 x 0-8.0x x x x0-7 W,. u..0 c x, m.0 x x x x x 0-7 Fig.. Normlized distriution of excittion rte on the cell depth: d =d = nm, U/ =0 (), 5 (), 0 () V; d =d =5 nm, U/ =5 (), 0 (), 5 () V; c d =d =0 nm, U/ =5 (), 0 (), 0 () V.

6 0 A. V. Kukht, E. E. Kolesnik, D. V. Ritchik, V. V. Glkin, I. I. Zholnerevich The chnge of nnoprticles dimensions t the sme distnce etween them pprently will result in the chnge of verge field in electroluminescent cell (Fig. 5, ), nd s consequence, it will influence the dependences of rdition chrcteristics on pplied voltge. However the size of nnoprticles ffects these dependences lso t the sme fields (compre curves nd 5 in Fig. 5). The smller nnoprticles concentrtion (greter nnoprticles dimeter) results in the decrese of tunneling current due to growth of potentil rrier nd the field drop in mny points on the cthode surfce. This effect is more noticele t low fields. Chnge of concentrtion of nnoprticles with the sme dimeter influences on electroluminescence chrcteristics t efore-threshold fields (fig.5,e); t high fields these curves ecome closer. W, s - x0 6 x0 x0 - x0-6 x U / ε, V x0 6 x0 x0 - x0-6 x0-0 W, s -, 0-0 x0 8 x0 8 x0 8 x0 8 5x0 8 E, V/ m η x0 - x c E, V / m 0 x0 8 x0 8 x0 8 x0 8 5x0 8 W, s - x0 8 x0 x0 0 x0 - x0-8 x0 - x d U / ε, V W, s - x0 8 x0 x0 0 x0 - x0-8 x0 - x e E, V/ m 0 x0 8 x0 8 x0 8 x0 8 5x0 8 η x0 - x x0 8 x0 8 x0 8 x0 8 5x0 8 f E, V/ m Fig. 5. Dependence of verged excittion rte W (,,d,e) nd stimulted rdition efficiency η (c, f) on pplied voltge (,d) nd electric field (,c,e,f) for cell with d =5 nm, d = (), 5 (), 0 () 0 () nm (-c), d =5 nm, d = (), 5 (), 0 () 0 () nm (d-f). Fig. 6 illustrtes the dependences of the mximl possile vlues of energy ( en) nd quntum ( q) luminophore molecule excittion yield verged over the cell volume. Both energy nd

7 A numericl nlysis of processes in orgnic electroluminescent structure with metl nnoprticles quntum yield re sensitive to vlue of Fermi level t rther low electric fields. Besides there is n optiml for reching pek efficiency rnge of pplied voltge, nd their numericl vlues decrese with electric field. It is connected with ccelertion of electrons up to the energies exceeding optimum, corresponding to mximl vlue of excittion cross section (0 ev [0]). The shift of mximums in the cse of increse of concentrtion of nnoprticles with the sme dimeter cn e explined y higher electric fields corresponding to optiml vlues of pplied voltge, nd some decrese of mximum heights is pprently connected with chnge of ctive cell volume. 0.0 η en E, V/m.0x0 8.0x x x0 8.0x η q E, V/m.0x0 8.0x x x0 8.0x0 9 Fig. 6. Dependence of energy () nd quntum () yield on pplied electric field: = ev, d =5 nm (-), E f =.8 (-,5), ev (), d = (), 5 (,), 0 (), 0 nm (5).. Conclusion The insertion of metl nnoprticles into ctive volume of electroluminescent cell leds to the increse of the verge electric field inside the cell nd to the decrese of the threshold of electroluminescence nd stimulted rdition due to improving conditions of electron tunneling s result of the chnge of potentil rriers shpe. References [] A. V. Kukht, J. Appl. Spectrosc. 70, 65 (00). [] A. V. Kukht, E. E. Kolesnik, A. M. Mozlev, A. G. Smirnov, G. H. Shkh, M. I. Toui, SID00 Digest, 65 (000). [] A. V. Kukht, G. G. Gorokh, E. E. Kolesnik, Y. A. Koshin, A. I. Mitkovets, M. I. Toui, A. M. Mozlev. Surf. Sci , 59 (00). [] A. V. Kukht, E. E. Kolesnik, T. A. Pvich, M. I. Toui Disply Reserches: Proc. Int. Conf., Plm Bech, 000, 55 (000). [5] V. Bliznyuk, B. Ruhstller, P. Brock, U. Scherf, S. Crter., Adv. Mt., 57 (999). [6] T. W. Lee, O. O. Prk, J. Yoon, J.-J. Kim, Synth.Met., 77 (00). [7] M. Grndstrom, M. Berggren, D. Pede, O. Ingns, M. R. Andersson, T. Hjerterg, O. Wennerstrom, Suprmol. Sci., 7 (997). [8] A. V. Kukht, E. E. Kolesnik, M. I. Toui, S. A. Voroyov, E. M. Grtsuev, A. I. Lesnikovich. Proc. Int. Workshop on Inorgnic nd Orgnic Electroluminescence (Ghent, 00), 57 (00). [9] A. D. Pomogilo, A. S. Rozenerg, U. E. Uflynd, Nnoscle metl prticles in polymers (in Russin), Khimi, Moscow (000).

8 A. V. Kukht, E. E. Kolesnik, D. V. Ritchik, V. V. Glkin, I. I. Zholnerevich [0] F. Hide, B. J. Schwrtz, M. A. Diz-Grci, A. J. Heeger, Chem. Phys. Lett. 56, (996). [] A. V. Kukht, E. E. Kolesnik, M. I. Toui, V. V. Glkin, I. I. Zholnerevich, J. Appl. Spectrosc. 69, 7 (00). [] A. Kukht, E. Kolesnik, M. Toui, V. Glkin. J. Optoelectron. Adv. Mter., 575 (00). [] J. Grüner, M. Remmers, D. Neher. Adv.Mt. 9, 96 (997). [] B. Chen, Y. Liu, C. Lee. Thin Solid Films 6, 7 (000). [5] A. V. Kukht, E. E. Kolesnik, V. V. Glkin, I. I. Zholnerevich. J. Appl. Spectrosc. 69, 8 (00). [6] P. G. Borsyk, O. E. Kiyyev, A. G. Numovets, R. D. Fedorovich. Ukr. Phys. J., 87 (998). [7] E. P. Friis, Yu. I. Khrkts, A. M. Kuznetsov, J. Ulstrup. J. Phys. Chem. 0, 785 (998). [8] R. D. Fedorovich, A. G. Numovets, P. M. Tomchuk. Phys. Rep. 8, 7 (000). [9] L. I. Trusov, V. A. Kholmynskii. Islnd Metl Films (in Russin). Metlurgy, oscow (97). [0] A. V. Kukht, J. Appl. Spectrosc. 65, 7 (998). [] H. Becker, S. E. Burns, R. H. Friend. Phys. Rev. B 56, 89 (997).