two-layer light emitting diodes prepared by the sol–gel route

C. R. Acad. Sci. Paris, t. 1, Série IV, p. 479–491, 2000Physichimie/Physical chemistry

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Two-layer light emitting diodesprepared by the sol–gel routeTony DANTAS DE MORAIS a,b, Frederic CHAPUT a, Jean-Pierre BOILOT a, Khalid LAHLIL a,Bruno DARRACQ c, Yves LÉVY c

a Groupe de chimie du solide, Laboratoire de physique de la matière condensée, UMR CNRS 7643, Écolepolytechnique, 91128 Palaiseau, FranceE-mail: [email protected]; [email protected];[email protected]; [email protected]

b Saint-Gobain recherche, 39, quai Lucien-Lefranc, BP 135, 93303 Aubervilliers cedex, Francec Laboratoire Charles-Fabry de l’institut d’optique, UMR CNRS 8501, Bâtiment 503, Université d’Orsay-Paris

XI, BP 147, 91403 Orsay cedex, FranceE-mail: [email protected]; [email protected]

(Reçu le 28 janvier 2000, accepté le 3 mars 2000)

Abstract. We have elaborated organic–inorganic hybrid light-emitting diodes (HLED). These devicesemitting in the green are formed of two hybrid thin layers, exhibiting different function-alities, which are sandwiched between indium–tin oxide (ITO) and metallic electrodes.These layers have been prepared from silane precursors modified with hole transportingunits and light-emitting naphthalimide moieties by the sol–gel technique. The hole trans-porting sol–gel layers exhibit about the same charge mobility as organic polymers havingequivalent active units. The maximum external quantum efficiency of the best diode usingLiF/Al cathode is about 1% and the luminance reaches 4000 cd·m−2. 2000 Académiedes sciences/Éditions scientifiques et médicales Elsevier SAS

hybrid light-emitting diodes (HLED) / green emission / multilayer devices / holetransporting gels / sol–gel precursors / grafting of molecules

Diodes électroluminescentes bi-couches élaborées par voie sol–gel

Résumé. Des diodes électroluminescentes hybrides organiques–inorganiques, émettant dans le vert,sont réalisées en insérant deux films hybrides avec différentes fonctionalités entre uneélectrode transparente d’oxyde (ITO) et une électrode métallique. Les films sont préparéspar la technique sol–gel en utilisant des précurseurs alkoxysilanes modifiés soit avec desunités conductrices de trous, soit avec des unités émettrices de type naphthalimide. Lesmobilités de charges dans les films sol–gel conducteurs de trous sont voisines de cellesobservées dans les polymères organiques portant des unités actives identiques. La diodela plus performante, utilisant une cathode LiF/Al présente un maximum de rendementquantique externe proche de1% et un maximum de luminance de4000 cd·m−2. 2000Académie des sciences/Éditions scientifiques et médicales Elsevier SAS

diodes électroluminescentes hybrides (HLED) / émission dans le vert / diodes multi-couches / gels conducteurs de trous / précurseurs sol–gel / greffage de molécules

Note présentée par Guy LAVAL .

S1296-2147(00)00146-3/FLA 2000 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS. Tous droits réservés. 479

T. Dantas De Morais et al. ORGANIC ELECTROLUMINESCENCE

Version française abrégée

De nombreux matériaux hybrides organo–minéraux peuvent être préparés par polymérisation sol–gelà partir de précurseurs alcoxydes ou alcoxysilanes. Ces matériaux sont obtenus à température ambianteet sous différentes formes (films ou monolithes). Les caractéristiques optiques et mécaniques de ceshybrides permettent leur utilisation comme matrice solide pour piéger des espèces moléculaires à propriétésoptiques spécifiques. Pour la réalisation de diodes électroluminescentes, en comparaison avec les polymèreset les petites molécules organiques, les alkoxysilanes permettent d’élaborer facilement des structuresmulticouches.

Dans cette étude, différents alkoxysilanes fonctionnalisés (figure 1) avec des unités actives pour letransport de charges et l’émission de lumière sont synthétisés et caractérisés (résonances RMN, spectresd’absorption et de photoluminescence, détermination des niveaux d’énergie HOMO et LUMO). Des filmssol–gel sont préparés par hydrolyse–condensation des alcoxysilanes et dépôt par centrifugation sur dessubstrats d’ITO.

Nous avons déterminé les mobilités électroniques des films sol–gel par la technique classique dite de« temps de vol » qui consiste à mesurer le temps de transit, dans le matériau, de charges photogénérées àl’aide d’un laser. Des films épais ont été préparés pour les transporteurs de trous (épaisseur comprise entre3 et 4,5µm). Le PVK a été utilisé comme matériau de référence et une molécule de type pérylène, connuepour ses propriétés de photoconduction de typep, a été utilisée comme photogénérateur.

Pour l’ensemble des transporteurs de trous sol–gel, les variations de mobilitéµ en fonction du champélectriqueE sont ajustées par une loi de type Poole–Frenkel (figure 3). La mobilité la plus forte est obtenuepour Si-TPD : elle est de 5,7·10−5 cm2·V−1·s−1 à 5·105 V·cm−1. Avec le dérivé de carbazole (Si-2KH),la vitesse des porteurs est réduite d’environ un facteur 20 à même champ électrique. De façon générale,les mobilités de charges dans les films sol–gel conducteurs de trous sont voisines de celles observées dansles polymères organiques portant des unités actives identiques et en même concentrations massiques. Danstous les matériaux, la mobilité de trous est fortement dépendante de la densité de molécules dans la matrice.

Des diodes électroluminescentes hybrides organiques–inorganiques, émettant dans le vert, sont réaliséesen insérant deux films hybrides avec différentes fonctionalités entre une électrode transparente d’oxyde(ITO) et une électrode métallique (Al). Nous avons tout d’abord déposé une couche de transporteur detrous (épaisseur comprise entre 40 et 60 nm), puis une couche émettrice (55 nm) contenant des unitésnaphthalimide (Si-NABUP). Après dépôt, tous les films sont étuvés une heure à 100C. Les caractéristiquespour chaque dispositif réalisé sont répertoriées dans letableau 4. La couche de transport de trous, élaboréepar voie sol–gel, à partir d’un monomère de carbazole (Si-2KH) donne des diodes un peu moins efficacesque le PVK (figure 4). En fait, l’utilisation de deux équivalents de TEOS (tétraéthoxysilane) par précurseurcarbazole, pour conduire à une matrice très condensée, a pour conséquence la réduction d’un facteur 5 de lamobilité des trous, expliquant probablement les performances légèrement inférieures de la couche sol–gel.

Les performances de la diode avec Si-TPD sont plus surprenantes. En effet, ce précurseur permetd’obtenir des matrices dont la mobilité de trous est supérieure d’un ordre de grandeur à celle du PVK et deplus, son niveau HOMO est de−5,27 eV contre−5,67 eV (injection de trous plus favorable). Toutes lesconditions semblent a priori réunies pour obtenir une diode plus efficace. Expérimentalement, on constateque la luminance maximale et le rendement à 100 cd·m−2 sont divisés par un facteur quatre quand onles compare à ceux obtenus avec le PVK comme transporteur de trous. En fait, les spectres d’absorptionet de photoluminescence (réduction d’intensité et déplacement de la bande d’émission vers le rouge —figure 7) suggèrent l’existence d’une interaction électronique de type « exciplexes » entre les unités Si-TPDet naphthalimides, à l’origine d’une diminution des performances en électroluminescence.

Après optimisation des épaisseurs des deux couches sol–gel, nous avons ensuite montré l’utilité d’unecathode double de LiF (1,2 nm)/Al (70 nm) sur une diode bi-couches PVK (35 nm)/Si-NABUP (75 nm). Lescaractéristiques de cette diode sont reportées sur lafigure 8. Pour cette structure, le rendement quantique

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ORGANIC ELECTROLUMINESCENCE Two-layer light emitting diodes prepared by the sol–gel route

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d’électroluminescence est 1% à 100 cd·m−2, la tension de démarrage est d’environ 10 V et la luminance laplus élevée atteint 4000 cd·m−2 à 27 V.

1. Introduction

Since the report of bright emission from thin two-layer molecular dye films by Tang and VanSlyke [1],there is a growing interest in using organic electroluminescent devices (OLEDs) in display applicationsbecause of their low operating voltage, large viewing angle and high efficiency and brightness. A variety ofmaterials have been proved to be useful for LEDs: low molecular weight molecules, conjugated polymers,main or side chain polymers and molecules dispersed in polymers. These families show very promisingadvantages but also some drawbacks. The main difference between small molecules and the latter is theirmanufacturing technology: the former are evaporated under vacuum whereas the latter are spin-coated.Because of low Tg, the small molecules tend to recrystallize when the OLEDs work (Joule heating), whilepolymers — despite a good and low cost processibility — are sometimes quite difficult to synthesize.Furthermore, the fabrication of multilayer devices (that allows well-balanced double injection for holes andelectrons) using different polymers remains uncommon because of redissolution.

The sol–gel technique provides a low temperature attractive approach to the preparation of hybridorganic–inorganic matrices [2]. Sol–gel chemistry involves the sequential hydrolysis and condensation ofsilicon alkoxides or alkoxysilanes initiated by acidic or basic aqueous solution in presence of a mutualcosolvent:

≡Si-OR+ H2O → ≡ Si-OH+ ROH (hydrolysis)

≡Si-OH+ HO-Si≡ → ≡Si-O-Si≡+ H2O (condensation)

The mild synthesis conditions offered by the sol–gel technique allow for the incorporation of opticallyactive organic molecules into the glassy matrix to form doped gels with specific optical properties [3].A variety of shapes, including thin films and monoliths, can be prepared. They exhibit good optical quality(transmission in visible range) and mechanical strength (easy machining) required for optical applications.

In considering electroluminescent devices, alkoxysilanes functionalized with active units (for chargetransport and light emitting) have many advantages over polymers and small molecules: (a) material designis very flexible because one can benefit from the experience gained with low molecular weight molecules;(b) sols can be prepared in common polar solvent making their spin-coating easy; (c) the reticulationof layers provides chemical stability, mechanical strength and insoluble films allowing multilayer devicestructures as previously shown [4].

In this paper, we present the preparation and characterization of silane precursors modified with holetransporting units and light-emitting naphthalimide moieties. These precursors are then used to prepareorganic–inorganic hybrid light-emitting diodes (HLED) formed from two sol–gel thin layers sandwichedbetween indium–tin oxide (ITO) and metallic electrodes.

2. Synthesis and characterizations of functionalized precursors

Silylated precursors are small organic molecules that have been chemically altered to providealkoxysilane functionality. This allows the silane modified molecules to participate in the hydrolysis andcondensation reaction similar to other alkoxide precursors. Two kind of reactions were used to functionalizesmall molecules: (a) hydrosilylation between a vinyl-terminated molecule and the triethoxysilane (HTEOS).This coupling reaction gives a C-Si bond; (b) addition of a hydroxyl group of the molecule on the 3-(isocyanatopropyl)triethoxysilane (ICPTEOS) which results in a carbamate link. Monomers resulting fromthese reactions were purified by chromatography and/or recrystallization. For electroluminescent devices,

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Si-NABUP Si-2KH

Si-TPD

Si-OXDN

Figure 1. Molecularstructures of the

functionalized sol–gelprecursors.

three active modified silanes were synthesized for hole transport and one other to provide green emittinglayer (figure 1).

Organic oligomers and polymers containing carbazole units [5] such as the well-known PVK (polyvinyl-carbazole) have proved to be useful in organic LED devices where they were used as a hole-transport layer.Carbazole attached alkoxysilane (Si-2KH) was obtained as a colorless oily product by hydrosilylation.

The interest in tetraphenylphenylenediamine (TPD) derivatives [6] is due to their low HOMO energyvalues in the range of−5.0 to −5.2 eV (thus closed to the work function of ITO) and to theirhigh carrier mobility (from 10−5 to 10−2 cm2·V−1·s−1). We also synthesized an alkoxysilane-typemonomer Si-TPD with TPD-OH unit, prepared by a two step procedure. First, [2-phenyl-4-(4′-[3-methyl(phenyl)anilino[1,1′-biphenyl]-4-ylanilino)phenyl] methanol (TPD-OH) was synthesized using amethod previously published [7]. Second, Si-TPD was obtained by reacting the hydroxy groups of theformer molecule and the isocyanate group from the 3-(isocyanatopropyl)triethoxysilane.

Oxadiazoles are primarily known for their electron transport ability [8]. However, the incorporation ofelectron-releasing substituents, such as diethylamino or diphenylamino, into the oxadiazole skeleton isknown to enhance hole-transport characteristics [9]. This let us to synthesize tetra-functionalized Si-OXDNusing 2,5-bis(4-dipropyl-aminophenyl)-1,3,4-oxadiazole.

For green emissive layers, we selected a fluorescent molecule belonging to the naphthalimide familywhich exhibits a high photoluminescence quantum yield in solution and good electron affinity [10].

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Table 1.Absorption and photoluminescence maxima in solution (dichloromethane)

Molecule Max. absorption (nm) Max. emission (nm)

PVK 230, 261, 294 and 344 373 and 415 (exc. 290)

Si-TPD 312 and 350 397 (exc. 312)

Si-2KH 238, 264, 295 and 346 352 and 369 (exc. 290)

Si-OXDN 359 416 (exc. 359)

Si-NABUP 404 505 (exc. 404)

Table 2.Electrochemical data (versus ferrocene/ferricinium redox couple) and HOMO/LUMO levels offunctionalized alkoxysilanes

Molecule Eox (V) Ered (V) Optical gap (eV) HOMO (eV) LUMO (eV)

PVK +0.87 − 3.45 −5.67 −2.20

Si-TPD +0.47 − 3.10 −5.27 −2.17

Si-2KH +0.85 − 3.46 −5.65 −2.19

Si-OXDN +0.53 −3.04 2.91 (3.57* ) −5.33 −2.42

Si-NABUP +0.74 −1.94 2.49 (2.68* ) −5.54 −2.86* value of the ‘electrochemical gap’

Si-NABUP was obtained by coupling N-(4-butylphenyl)-4-[(N-2-hydroxyethyl)(methyl) amino]naphthali-mide and ICPTEOS.

The expected NMR resonances of well-purified Si-2KH, Si-OXDN, Si-NABUP and Si-TPD monomerswere observed on1H (200 MHz) and13C spectra [11]. Absorption spectra were recorded on a ShimadzuUV-Vis 160A spectrophotometer. Photoluminescence (PL) spectra were obtained on a Hitachi FluorescenceSpectrophotometer F4500. Absorption and photoluminescence maxima are summarized intable 1. Theseabsorption and photoluminescence maxima are almost identical to those of corresponding unfunctionalizedmolecules. From this result, it can be assumed that the electronic structures were not significantly modifiedby the functionalization. Si-NABUP shows a characteristic green emission (505 nm) under excitation at404 nm.

Oxidation potentials (Eox) and reduction potentials (Ered) were measured by the cyclic voltammetrytechnique with a platinum working electrode, a platinum counter electrode and a calomel referenceelectrode under an argon atmosphere (sweep rate 500 mV·s−1). Tetra-n-butyl-ammonium tetrafluoroborate(0.25 mol·l−1) was used as a supporting electrolyte. Molecular concentration was about 6 mmol·l−1

in tetrahydrofuran. Assuming an energy level of−4.8 eV for ferrocene [12] the absolute positionsof the HOMO and/or LUMO levels can be deduced from the electrochemical levels measured versusferrocene/ferricinium redox couple. WhenEox andEred could not be measured, cyclic voltammetry wascoupled with UV-Visible spectrometry [13] in order to determine HOMO and LUMO levels (see table 2).

3. Device preparations

Devices were prepared on glass substrates precoated with patterned ITO electrode (R = 20 Ω/square).Prior to deposition, the substrates were ultrasonically cleaned and dried in a flow of nitrogen. Functionalizedmolecules were copolymerized with phenyltriethoxysilane (φ-TEOS) or tetraethoxysilane (TEOS) usedas crosslinking agents according to a procedure previously published [14]. In a typical sol preparation,silylated molecules were dissolved in a common solvent.φ-TEOS or TEOS were added before introducingacidic water (pH= 1 except for Si-OXDN for which pH= 1.6 was used) (H2O/Si-OR= 1). After two

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hours of magnetic stirring, a small amount of pyridine was added to neutralize the acidity of the mediumand hence increase the condensation reaction rate. The as-prepared hybrid sol was then passed througha 0.45µm filter before deposition. The angular velocity range of the spinner was 2000–4000 rpm. Afterdeposition the sample plates, were heated at 130C for 1 hour. Film thickness was estimated using a SloanDektak III ST stylus profilometer. For time-of-flight experiments (TOF), the thickness ranged from 3 to4.5µm and residual solvent was removed by heating the filmin vacuofor 10 h at 56C. For OLEDs, 30to 70 nm thick layers were prepared. When manufacturing electroluminescent bilayer devices, we checkedthat the solvent used to spin-coat the upper emitting layer did not dissolve the lower hole transport layer.Metallic electrodes (aluminium) were then evaporated under5 · 10−6 Torr vacuum at a rate of 5−10 Å·s−1

(thickness about 1000 Å) through a shadow mask which defines the 0.2 cm2 active area. For all the devices,the emitting layer (55 nm) consisted of Si-NABUP/φ-TEOS (1 : 1). It was successively associated with thedifferent sol–gel hole transport layers (50 nm): Si-TPD/TEOS (1 : 2), Si-OXDN or Si-2KH/TEOS (1 : 2).PVK films were also prepared and used for reference studies because the hole transport in PVK has beenexamined extensively [15]. Commercial PVK (Sigma-Aldrich,Mw = 1.1 · 106 g·mol−1) was purified bytwofold precipitations from dichloromethane solutions by methanol. Appropriate amounts of polymer weredissolved in benzene chloride in order to obtain PVK spin-coated films of thickness equal to 3.5µm forTOF experiments and 50 nm for OLEDs.

4. Hole transport in sol-gel materials

Charge transport in amorphous organic solids is a hopping process. In sol–gel films, one may considertransport to be dominated by conditions for charge exchange between neighboring hopping sites (molecularunits grafted on the gel network). Hole transport has been measured in different sol–gel films using thetime-of-flight (TOF) technique [16]. The TOF experiment involves photogeneration of charge carriers byillumination of a sample sandwiched between two electrodes. The drift of carriers under an external biasto the collecting electrode results in a time-dependent current that is monitored across an external loadresistor. The transit timeτ , for the arrival of carriers is related to the carrier mobilityµ via the relationµ = d/(τ × E) whered is the film thickness andE the external electric field. For investigation of holemobility, the sign of the applied bias should be selected negative on ITO, corresponding to holes driftingacross the film. For of photocurrent enhancement, a perylene derivative (PERY-H [17]) was used as a carrierphotogeneration material. It is assumed to possessp-type semiconducting characteristics and high holemobility [18]. A PERY-H layer (thickness= 50 nm) and aluminium top electrode were then successivelyvacuum-deposited on spin-coated sol–gel and PVK films. For carrier generation, a frequency-doubledNd:YAG laser (532 nm, 10 Hz) giving intense short light pulses (10 ns) was used as a light source. Sol–geland PVK films do not absorb the laser light while a PERY-H layer does (0.18 at 532 nm). Consequently,carriers are generated only in the PERY-H layer by laser irradiation. The transient current was measuredacross the load resistor using a digital storage oscilloscope.

A typical hole current recorded on a Si-TPD film at a fieldE = 4 · 105 V·cm−1 is shown infigure 2.The initial current spike is followed first by a very clear constant current plateau (extending from about 5to 20 ms). This corresponds to the hole transport with a time-independent drift velocity. The subsequentdecrease in the current is caused by the holes reaching the ITO electrode at which they recombine withelectrons. As previously observed for PVK, all the compounds exhibit a slow decrease of the current,revealing a dispersive transport.

In this study, the time dependence of the hole current curves was adjusted by the method proposed byScott et al. [19] in which Gaussian distribution of transit times is considered. The average mobility fordifferent bias fields was then deduced by using the average transit times. Assuming a Poole–Frenkel typebehavior for the mechanism of hole transport in these materials, the mobility in the different sol–gel layerswas plotted as a function of the square root of the bias field [20].Figure 3 shows that the characteristiclinear dependence is well established for all the compounds which exhibit a similar field dependence of

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Figure 2. (a) A typical transient photocurrent for a Si-TPD sol–gel film (thickness= 3.8 µm). Measurements wereperformed at 300 K and for an applied field of4 · 105 V·cm−1; (b) experimental arrangement for the time-of-flight

technique.

Figure 3. Field dependence ofthe hole mobility for sol–geland PVK layers. The data areplotted as a function of thesquare root of the external

bias field.

the mobility. This indicates that the investigated sol–gel and PVK materials show about the same degree ofboth energetic and positional disorder for the transport active sites.

The highest hole mobility is observed for the Si-TPD layer:5.7 ·10−5 cm2·V−1·s−1 at a field strength ofE = 5 · 105 V·cm−1. For the best carbazole compound (Si-2KH), at the same field, the mobility is found tobe about twentyfold lower than for the Si-TPD. Finally, the lowest mobility is observed for the Si-OXDNlayer:5 · 10−7 cm2·V−1·s−1 at the same field strength.

Table 3summarizes the hole mobility at the same field observed, both for the sol–gel materials andfor previously studied organic polymers formed from the same active units. For Si-2KH and Si-OXDN,hole mobilities are about the same as those observed for the equivalent organic polymers. Concerningthe Si-TPD, the hole mobility is lower than the one measured in the TPD molecular film. This could beattributed to a lower concentration of the molecular units in the sol–gel material as suggested in a previousstudy where a decrease of mobility was observed by diluting TPD in a polycarbonate matrix [21,22].

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Table 3.Hole mobilities in different sol–gel and PVK matrices at a field strength of5 · 105 V·cm−1. These values arecompared with the ones previously measured for organic systems containing the same molecular units

Layer type Hole mobility (cm2·V−1·s−1) Reference material mobility (cm2·V−1·s−1)

PVK 1.5 · 10−6 2 · 10−6

PVK (previous studies) [18,15]

Si-2KH/TEOS (1 : 0.25) 3.3 · 10−6 4 · 10−6

P-CARSILOX [23]

Si-TPD/TEOS (1 : 0.25) 5.7 · 10−5 10−3

TPD molecular film [22]

Si-OXDN1.1 · 10−6 (7.5 · 105 V·cm−2)

1.1 · 10−7 (2 · 105 V·cm−2)

9.8 · 10−8 [9]

1.7 · 10−7 [27]OXDN in polycarbonate

30 wt% [9] and 50 wt% [27]

The same effect is observed for Si-2KH layers by changing the mass fraction of carbazole units.Therefore, a sol–gel layer prepared from the Si-2KH/TEOS (molar ratio1 : 2) composition (i.e. containing38 wt% of carbazole units) results in a mobility decrease of one order of magnitude (at the same field) incomparison with that which contains 58 wt% of carbazole (molar ratio1 : 0.25). However, this behaviorcontrasts with the PVK one whose the hole mobility seems independent of the concentration of carbazoleunits [23]. This is probably due to the formation of excimers in PVK which act as traps for transport viacarbazole groups.

5. Electroluminescent device characterizations

Electroluminescence (EL) measurements were performed under nitrogen atmosphere where oxygenand water contents were kept below 1 ppm. The devices were operated in a continuous dc mode.The I–V characteristics were recorded using a Keithley 238 High Current Source Measure Unit. Theelectroluminescence intensity was deduced from a 2.5 cm2 Si photodiode directly applied on the device (thephotocurrent was measured by a Keithley 617 Programmable Electrometer). A Photo Research SpectrascanPR 704 spectro photocolorimeter was used both to record the EL spectra and to measure luminance andradiance (measuring angle:2). We define the turn-on-voltage (at which the light emission starts) as 25

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Table 4.Electrical and luminance properties of bilayer devices with 55 nm Si-NABUP/φ-TEOS (1 : 1) layer and50 nm hole transport layer (cathode: aluminium)

Hole transport Performance at 100 cd·m−2 Highest luminance

layer ηEL (%) Voltage (V) Luminance (cd·m−2) Current (mA·cm−2) Voltage (V)

Si-2KH/TEOS (1 : 2) 0.30 19.5 1 620 140 26

Si-OXDN 0.14 19.5 365 155 25

Si-TPD/TEOS (1 : 2) 0.11 19 500 265 25

PVK 0.57 18.5 2 010 185 26

(a) (b)

Figure 4. Current density and luminance versus voltage with PVK (a) and Si-2KH (b) as hole-transport layer(cathode: aluminium).

times the background noise level of the photodiode. The EL efficiencyηlm (lm·W−1) and the externalquantum efficiencyηEL (ratio of the number of emitted photons to the number of injected charges) werecalculated assuming a lambertian emission [24].Table 4summarizes the effect of the hole transport layeron the performances of OLEDs, such as maximum brightness, voltage at 100 cd·m−2 and external quantumefficiency.

Current density and luminance are plotted infigure 4, as a function of the applied voltage, for bilayerdiodes containing PVK and Si-2KH hole transport layers. For the PVK diode, the turn-on voltage ofluminance is about 10 V (≈ 1 · 108 V·m−1), the maximum luminance of 2000 cd·m−2 is observed at25 V, the highest value before significant degradation of the diode occurs. The external quantum yield is0.57%. Similar results are obtained with Si-2KH: a maximum luminance of 1600 cd·m−2 is reached at thesame voltage (ηEL = 0.30%). Surprisingly, luminances and efficiencies were reduced to values lower than500 cd·m−2 and 0.15% respectively for diodes containing Si-TPD and Si-OXDN hole transport layers.

Electroluminescence spectra are shown infigure 5. As generally observed for polymer LEDs, the ELspectrum, with PVK or Si-2KH as a hole transporting layer, is similar to that of PL indicating that the sameexcitations are involved in both cases. The emission peak occurs in the green region at around 535 nmshowing that the EL originates from naphthalimide units. However, one can note a red shift of the emissionpeak (bathochromic effect) for diodes containing Si-OXDN and Si-TPD layers.

From cyclic voltammetry measurements, it is clear that Si-TPD and Si-OXDN have the lowest barrierfor oxidation at the ITO/HTL interface (figure 6). Thus, the previous diode characteristics suggests

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Figure 5.Photoluminescence (PL)

spectra of Si-NABUP layerand electroluminescencespectra of diodes (EL).

Figure 6. Schematic energy level diagram ofbilayer devices (seetable 2for ionization

energies of HTL).

that highest external quantum efficiencies could be achieved with hole-transporting materials having thehighest oxidation potentials. In fact, interactions between molecules can occur at interfaces. If we supposea molecule in its excited state is coupled with another different molecule in its ground state, the resultingexcited state is called an exciplex. For this exciplex formation, one of the molecule should have a smallionization potential so that it can donate an electron to the second molecule, which should have a highelectron affinity. In films made of1 : 1 mixture of Si-TPD and Si-NABUP or Si-2KH and Si-NABUP,a bathochromic effect and a reduction of the photoluminescence yield were observed upon exication at420 nm (this wavelength corresponds to the maximum absorbance of naphthalimide units, seefigure 7). Inthese cases, we can conclude that the electroluminescence of OLED was due to a complex formed at theinterface between the naphthalimide and the hole transport layers. It is therefore reasonable to assume thatthe exciplex formation takes place after a local excitation of one of the two molecules.

The thickness ratio of the hole transport layer over the emitting layer can be optimized in order to increasethe quantum efficiency [25]. Optimal results are obtained when PVK is 35 nm thick and Si-NABUP layeris 75 nm. Moreover, to achieve the best device performance, it is advantageous to use metals having a lowwork function for electron injection into organic materials. However, metals like calcium or magnesiumare sensitive to atmospheric oxidation. Using a bilayer cathode LiF (1.2 nm)/Al (80 nm), the maximumof luminance of such a device reaches 3900 cd·m−2 at 27 V and a current density of 185 mA·cm−2

( figure 8). At 100 cd·m−2, which requires a voltage of 16.5 V, the electroluminescence quantum yieldis about 1% (ηlm = 0.65 lm·W−1). So, the presence of the thin LiF layer at the gel–Al interface enhances

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Figure 7. Absorption (a) and photoluminescence (b) spectra of films which compositions are Si-TPD/Si-NABUP(1 : 1) and Si-2KH/Si-NABUP (1 : 1).

Figure 8. Current density andluminance versus voltage for a bilayerdevice (PVK/Si-NABUP) with LiF/Al

as a cathode material.

electron injection. Many mechanisms [26] have been previously invoked to explain this effect: large dipolemoment of LiF that decreases the surface potential of aluminium, chemical reactions at the LiF/Al interface,and a decrease in the electric field strength within the emissive layer which reduces the field-induceddissociation of excitons.

6. Conclusion

For the first time, hole mobility has been displayed in sol–gel materials prepared from a series of newsilane modified molecules. The influence of these materials on the performance of hybrid organic–inorganicelectroluminescent bilayer diodes has also been studied. The maximum external quantum efficiency ofthe best diode using LiF/Al cathode is about 1% and the luminance reaches 4000 cd·m−2. Exciplexformation between the emitter and a hole transport material was found to significantly decrease theelectroluminescence efficiency. In these bilayer devices, exciplexes are only formed at the interface whichallows the emission position to be defined exactly.

Acknowledgements.We gratefully acknowledge P. Le Barny (Thomson-CSF, Palaiseau) for supplying butyl-naphthalimide molecule. We are indebted to H. Facoetti and M. Vergnolle (Thomson-CSF, Palaiseau) for ELmeasurements and to B. Geoffroy and A. Lorin (CEA-LETI, Saclay) for LiF/Al electrode evaporation.

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