Time-resolved electroluminescence from single and bilayer LEDs based upon substituted poly-arylenevinylenes

Chemical Physics ELSEVIER

Chemical Physics 212 (1996) 471-485

Time-resolved electroluminescence from single and bilayer LEDs based upon substituted poly-arylenevinylenes Y.-H. Tak

a

H. Vestweber a H. B~issler a , * A. Bleyer b, R. Stockmann b H.-H. HSrhold b

a Fachbereich Physikalische Chemie und ZentrumJfir Materialwissenschafien der Philipps-Universitiit, Hans-Meerweinstrasse, D-35032 Marburg, Germany b Institfit ffir Organische Chemie und Makromolekulare Chemie der Friedrich-Schiller-Universitfit Jena, Humboldstrasse 10, D-07743 Jena, Germany

Received 3 April 1996

Abstract

Light emitting diodes (LEDs) have been fabricated on the basis of poly-arylenevinylenes substituted at the vinylene double bond and blended with either polycarbonate (PC) or polyvinylcarbazole (PVK). Contacts were indium-tin-oxide (ITO) and aluminum (AI). The transient behavior of both single layer as well as bilayer devices with oxadiazole dispersed in polystyrene being the second (cathodic) layer was studied upon application of rectangular voltage pulses. The external quantum efficiency of single layer and bilayer devices is of order 10-2% and 1%, respectively. Detailed studies were performed on Poly[1,4-phenylene-l,2-di(4-phenoxyphenyl)vinylene] (DPOP) because this system showed evidence for a change of the j(E) characteristic from being space charge limited to injection limited upon replacing PVK as blend material by PC. This illustrates the profound effect the binder material can have on injection as well as charge transport in a LED based upon polymer blends. In the single layer LED delayed onset of the light emission reflects the transit time of injected holes, while in bilayer LED it is determined by the time minority carriers (electrons) need to reach the internal interface. 1. Introduction

For obvious reasons the efficiency, defined as the number of photons emitted per charge carrier injected, is a key parameter for organic light emitting diodes [1]. High power consumption would be economically prohibitive for the use of large area devices, and sample heating due to high current flow may shorten their lifetime. There are several factors that affect the LED efficiency, the most obvious one being the luminescence yield of the emitter material. Because of quenching effects specific for the solid * Corresponding author. Fax: +6421 28 8916.

state this quantity may be considerably smaller than the photoluminescence yield of the same chromophore in liquid solution. Reducing the efficiency of excited state quenching due to inadvertent impurities is, therefore, one strategy to improve the LED light output [2-4]. The other one, no less important, however, is to enhance the charge density inside the device. Recombination of the injected carriers is a bimolecular rate process. It scales with the product of the stationary hole and electron densities while non-radiative discharge at the exit contact is a monomolecular loss process. Therefore any enhancement of the former is likely to improve the LED yield [5].

0301-0104/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH S0301 -01 04(96)002 18-2

472

Y.-H. Tak et al. / Chemical Physics 212 (1996) 471-485

Apart from minimizing the injection barriers for charge carriers at the contacts [6-8] - which is a problem related to energy level matching - there are two additional ways to increase the number of charge carriers momentarily present in a LED. The first one, applicable to single layer devices, is to reduce the charge carrier mobility, for instance by diluting the charge transporting component in a composite system [9] and by charge confinement [10]. The second one is to introduce an internal interface that impedes passage of one sort of carriers. This is realized in bilayer devices [11-14]. The guiding principle behind both strategies is that an injected carriers will recombine with unit probability only if the stationary density of opposite carriers inside the device is comparable to the capacitor charge [5]. In this paper we present the results of a comparative experimental study of single and bilayer LEDs based upon polyphenylenevinylene substituted at the vinylene double bond. The second layer in the bilayer LEDs is a molecular dispersion of an oxadiazole in polystyrene (PS). The experimental techniques include (i) standard j(V) characterization, (ii) time-resolved studies of onset and delay of both the cell current and the light output upon application of rectangular voltage pulses of variable duration [15,16], (iii) time-resolved recording of the electroluminescence (EL) spectra, and (iv) measurements of the extemal quantum efficiency. The aim of the study has been to delineate the rate limiting processes in the bilayer as opposed to the single layer system and to derive principles for cell optimization.

2. Experimental The electro-optically active materials used for fabricating LEDs were poly[ 1,4-phenylene- 1,2-di(4phenoxyphenyl)vinylene] (DPOP), poly[6-phenoxy10-(4-phenoxyphenyl)-2,9-phenanthrenylene] (CDPOP), poly[ 1,4-phenylene- 1,2-di(4-methoxyphenyl)vinylene] (DMOP), and poly[4,4'-diphenylene- 1,2-di(3,4-dimethoxyphenyl)vinylene] (TMOP). For structure formulae as well as abbreviations see Table 1. The synthesis of the polymers DMOP, DPOP, TMOP and of the precursor polymer DPOPPmPV has been conducted by dechlorination polymerization of appropriately substituted bis(dichloro-

methyl)arylenes using chromium(II)acetate as the reducing agent [ 17-19]. The intramolecular cyclization of DPOP-PmPV to form the phenanthrene based polymer C-DPOP proceeds under mild conditions with ferric chloride hexahydrate in methylene chloride solution at room temperature [19]. The polymers are amorphous and soluble in toluene, chloroform, chlorobenzene, dioxane etc. Transparent films can be obtained by spin coating or by casting from solution. The macromolecular properties of the investigated PPV derivatives are given in Table 2. Photoluminescence properties were taken from dilute solution in dioxane. In solid state the photoluminescence quantum yield exceeds that in solution (e.g. for DPOP ~PL = 60% [20]). LEDs were prepared by dissolving equal amounts (by weight) of the active material and either polyvinylcarbazole (PVK) or polycarbonate (PC) in chloroform and by spin casting a film onto ITO glass (Baltracon, Balzers). The layer thickness was typically 90 to 100 nm. Single layer devices were completed by vapor deposition of an aluminum contact. The active area was either 0.018 or 0.07 cm 2. To fabricate bilayer devices a 20:80 blend of oxadiazole dispersed in polystyrene was spin cast on top of the single layer using cyclohexane as a solvent. Its thickness varied between 40 and 60 nm. Again, A1 was used as a top contact. Current voltage curves were taken with a Keithley 236 source measure unit. To follow the temporal evolution of the electroluminescence upon application of a rectangular voltage pulse the light output of the device was recorded synchronously with the cell current employing a storage oscilloscope (Tektronix 2430). The RC time constant of the circuit was of order of a few Ixs depending on the choice of the load resistor and sample capacitance. The repetition rate of the voltage pulses was 1 Hz, the pulse length varied between 3 and 500 ms. Electroluminescence as well as photoluminescence spectra were recorded with an optical multichannel analyzer which allowed spectra monitoring within selected time windows, e.g. during the afterglow period following turn-off of the driving voltage. A calibrated radiometer equipped with a diffuser and filter (Polytec, IL 1700) was used for measuring the light output of the LED. In case of a TMOP: PCIPBD : PS bilayer LED the result has been

473

Y.-H. Taket al. / Chemical Physics 212 (1996) 471-485

checked by comparing it with the read-out of an integrating sphere.

3. Results

Fig. 1 shows j(V) curves for DPOP:PVK and D P O P PC single layer devices. The influence of the binder material is obvious. At a given electric field

current densities obtained with the PVK-blend system are typically one order of magnitude larger and the increase with increasing electric field is less pronounced. It should also be noted that in the DPOP:PVK system the current values measured with a virgin sample were somewhat less than those measured in successive runs indicating that some conditioning process had occurred. As documented by Fig. 2 EL is not observed until a delay time td has

Table 1 List of compounds

~ O

o Poly[ 1,4-phenylene- 1,2-di(4-phenoxyphenyl)vinylene] DPOP

Poly[6-phenoxy- I 0-(4-phenoxyphenyl)-2,9-phenanthrenylene] C-DPOP

OCH 3

OCH 3 CH30~ j

OGH 3 Poly[ 1,4-phenylene- 1,2-di(4-methoxyphenyl)vinylene] DMOP

)~

OCH 3 Poly[4,4'-diphenylene- ] ,2-di(3,4-dimcthoxyphcnyl)vmylenc] TMOP

474

Y.-H. Tak et al. / Chemical Physics 212 (1996) 471-485

Table 2 Macromolecular and absorption/photoluminescence properties of the polymers in solution Light emitting polymer

Mn(VPO) (g/mol)

Tg(DSC) (°C)

Abs. )tmax (rim)

Em. )tmax (nm)

t~p L (%)

DPOP C-DPOP DMOP TMOP

20000 20900 19000 3000

163 180 242

363 385 373 373

530 524 532 525

6 I0 3 5

Mn(VPO): number average molecular weight determined by vapor pressure osmometry. Tg(DSC): glass transition molecular weight determined by differential scanning calorimetry. Abs. Areax : wavelength of the absorption band maximum (dioxane). Em. Areax : wavelength of the emission band maximum (dioxane). ~beL: photoluminescence quantum yield (dioxane).

elapsed. It increased from 40 to 100 ixs upon decreasing the bias voltage from 28 to 16 V and was usually slightly longer in virgin samples. An interesting correlation exists between onset of EL and the decay of the cell current. For times exceeding the RC time constant of the circuit the current decayed to an equilibrium value after featuring a kink that coincided with onset of EL at t = t d (Fig. 3). The kink was not always as well developed as it is in Fig. 3 but usually at least a shoulder was discernible that correlated with t 0. For t > t d the E L intensity rose until a plateau value was attained after a time ta, henceforth denoted as onset time. The

latter decreased with increasing voltage in the same fashion as t o did. After turning off the driving voltage the EL intensity decayed on a time scale of 10 Ixs, virtually independent o f the cell current (Fig. 4). The time response of a 50: 50 D P O P : PC device was much faster than that of a D P O P : P V K cell. A n y delay time was masked by the RC time of the circuit, which in that case was ~ 5 I~s (Fig. 5). The steady state currents in bilayer devices were significantly smaller than in the corresponding single layer LEDs and followed a In j oc E - i behavior at high voltages (Fig. 6). Build-up of EL was also associated with a delay time t d and saturated at a

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Y.-H. Tak et al./ Chemical Physics 212 (1996) 471-485 120

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476

Y.-H. Tak et al. / Chemical Physics 212 (1996) 471-485

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Y.-H. Tak et al./ Chemical Physics 212 (1996) 471-485

477

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Y.-H. Tak et al. / Chemical Physics 212 (1996) 471-485

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Y.-H. Tak et al./ Chemical Physics 212 (1996) 471-485

time t a which was roughly 3 times td if one inferred t d from the intersection of tangents (Fig. 7). The delay time td correlated with the time after which the cell current reached its equilibrium value (Fig. 8). The EL spectrum of the DPOP : PVK bilayer system showed only one broad structureless band that agreed with the photoluminescence spectrum of DPOP. In order to clarify whether or not the emission after turning-off the cell voltage maintains to be due to intrinsic emission states, the EL spectrum was also measured at variable delay time after the voltage pulse. It turned out to be identical with the steady state EL spectrum as documented by Fig. 9. The light output increased linearly with cell current, both with the single and bilayer DPOP:PVK system. However, the yield in the latter case was two orders of magnitude larger (Fig. 10).

479

ated. In the former case, relevant for LEDs, the current flowing can either be injection limited or, if one contact is able to act as an inexhaustible reservoir for charge carriers, is limited by the space charge built up by the migrating carriers. In the latter case the electric field at the injection contact drops to zero. Space charge limited (SCL) current flow is, therefore, bound to the condition that injection does not require a high electric field to be efficient. For unipolar SCL conduction Child's law predicts [21] 9

JSCL = gggO/XeffV

2/d3

(1)

'

where e is the dielectric constant, d the sample thickness and /'Left is the effective carrier mobility. If at high fields JSCL begins to exceed the maximum current the contact can sustain a transition from SCL to injection limited current flow occurs. The total current can exceed the value predicted by Eq. (1) only if the internal space charge is partially compensated by counter charges injected from the back-electrode. If one injects a g-shaped sheet of charge carriers, comprising only a small fraction of the capacitor charge CV, into a dielectric, one observes a transit time tt~ = dZ/tzV. If, on the other hand, a rectangu-

4. Discussion

4.1. Single layer devices Undoped organic solids are insulating materials unless charge carriers are injected or optically gener-

10-5 10 -6

1 0 -7

E vN

1 0 .8.

10_ 9

10 -10 1 0 .5

i

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[]

bi l a y e r

•

single

-

-

layer

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1 0 .3

i ( A / c m 2) Fig. 10. Comparison of the light output from a single and a bilayer LED based upon DPOP: PVK as a function of the current density.

480

Y.-H. Tak et al. / Chemical Physics 212 (1996) 471-485

lar voltage pulse is applied to the same sample but equipped with an ohmic electrode able to sustain a SCL current, the transient current will feature a cusp already at

10 -5

10 q

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(2)

JSCL. m [] [] . . . .

10 "2 ' •

because of the increase of the electric field the leading edge of the carrier packet will experience while drifting towards the counterelectrode [22]. Inserting /Zeff inferred experimentally via Eq. (2) into Eq. (1) yields

E o

IEXPk n B

B ro

[]

<

~a

10-7

l O - a -:

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JSCL tcusp ~" 0.9ee0 E.

10-6

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V VvV

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Usually current-field characteristics of LEDs are much steeper than compatible with SCL current flow. They often follow F o w l e r - N o r d h e i m ' s law for tunneling injection, at least at fields exceeding l 0 6 V c m - s [6,7]. The single layer LED with DPOP: PVK is an exception of that rule. The following experimental facts support the notion that in this case the current is actually limited by the space charge set up by the injected holes. Upon application of a voltage pulse (i) electroluminescence can occur not until the fronts of hole and electron clouds injected from anode and cathode begin to interpenetrate. This occurs at time d/E(tz++ ix_). If, as is usually assumed to be the case,/z+ >>/z_ this time practically equals the transit time of the holes. Identifying the delay time t d with the carrier arrival time under SCL conditions (i.e. t~osp) Eq. (2) allows to estimate the maximum unipolar current JscL the sample can sustain. Fig. 11 shows a plot of JSCL versus voltage calculated from Eq. (2) for an average dielectric constant ~ = 3.5. The actual cell current approaches JSCL asymptotically. For the highest applied field calculated and experimented values of the current density are identical. The deviation from an exact j cz E 2 behavior can be accounted for by a field dependence of the carder mobility as indicated in Fig. 11. (ii) The cell current features a shoulder, in some cases a cusp, at a time equal to t a, superimposed into a transient signal that decays by typically a factor of 5 within the time interval RC _< t _< t a. The straightforward explanation is that the ITO anode is able to act as an ohmic contact. Since the transporting material is disordered, carriers are not yet in dynamic equilibrium immediately after injection but relax energetically towards tail states of the

10-4

10-8 10

2O

30

40

V o l t a g e (V)

Fig. 11. Comparison between the space charge limited current density JSCLcalculated on the basis of the hole transit time inferred from the delay time to and the experimental current density for a DPOP: PVK single layer LED. Triangles indicate the hole mobility as a function of electric field inferred from ta via eq. (2).

distribution of hopping states. This causes the current to decay as a function of time. Since under SCL conditions the electric field acting on the leading edge of the packet of carriers increases as it migrates [23] a cusp appears that marks the arrival of the fastest carriers at the exit contact. The effective hole mobility derived from t d ranges from 3.8 X 10 -8 c m 2 / V s at E = 2 . 0 X 106 V cm -1 to 5.0X 10 -8 c m 2 / V s at E = 3.1 X 106 V cm - I . These values are significantly less that those estimated for pure P V K on the basis of existing data for lower electric fields [24]. Recall that P V K itself is not a particularly efficient hole transporting material, probably because of trapping due to incipient carbazole dimers known to give rise to excimer fluorescence [25]. Neither can hole injection into P V K be expected to be particularly efficient because the energy barrier at a contact with a workfunction of 4.7-4.8 eV is too high [26,27]. On the other hand the above mobility value cannot be exclusively associated with the DPOP either since in a DPOP: PC blend the transient LED behavior is much faster than in D P O P : P V K . Note that Fig. 5 does not bear out a delay time in excess of the RC time of the circuit (see Fig. 5). This

Y.-H. Tak et al. / Chemical Physics 212 (1996) 471-485

nan y

O

.

m

Fig. 12. Schematic illustration of the energetic situation encountered when a ITO electrode is in contact with a material of given oxidation potential yet present in a state of weak (1) and large (2) energetic disorder.

translates into a lower bound for the hole mobility of 10 - 6 c m 2 / V s and a lower bound for the hypothetical space charge limited current that is a factor of 20 larger than for DPOP: PVK. Since, on the other hand, the actual cell current is typically one order of magnitude less than that measured with DPOP : PVK it cannot be space charge limited. In fact, the X E ) curve reproduces as a straight line if plotted on In j versus E - t scale suggestive of tunneling injection. From the slope an injection barrier of 0.35 eV is calculated. The obvious conclusion is that the polymeric binder material has a profound influence on both injection and transport of charge carriers. Consistent with previous results [7] both effects can be rationalized on the premise that substituting polycarbonate as a matrix by PVK broadens the energetic distribution of hopping states DPOP provides for charge transport as illustrated schematically in Fig. 12. Since in a disordered material the charge cartier

481

mobility decreases exponentially with the square of the disorder potential [28] this reduces the average carrier mobility. It simultaneously ensures that there are more localized states at the interface into which ITO can inject holes with smaller energy barriers to be overcome. The open question concerns the nature of the specific interaction between DPOP and PVK. One possibility is that due to steric effects additional disorder is introduced into DPOP. In any event is this an illustration on the profound effect of the matrix on transport as well as injection. In this present case it even causes a transition from injection to transport limited behavior. In single layer devices this would be a desired effect since it enhances the density of majority carriers stored inside the sample under steady state conditions. The rise of the EL in single layer devices for t > t d is likely to be controlled by a superposition of two effects. The most important one will be motion of injected electrons into the bulk thus establishing a stationary recombination zone. If one identifies the time ta, defined by the intersection of asymptotes, with the time needed by electrons to migrate over a distance of 50 n m - previously shown to be a typical electron range [29] - one can obtain an estimate of the electron mobility. The values range from 0.8 X 10 -8 cmZ/V s of a field at 1.8 x 10 6 V c m - I tO 1.4 X 10 -8 c m 2 / V s at 3.1 X 106 V cm - I , They are approximately a factor of 5 less than the hole mobilities inferred from to. Decay of the EL signal after the end of the voltage pulse must reflect the kinetics of electronhole recombination in the absence of an external electric field. Although in principle a bimolecular reaction, it should feature quasi-monomolecular kinetics since there is a surplus of holes. However under SCL conditions the hole density is determined by the capacitor charge. It will not, therefore, vary by more than 50% in the field range under consideration. Under this circumstance one would expect an exponential decay of the EL signal with a time constant that scales inversely with the applied voltage provided that the system were homogenous. Analysis of the decay function in Fig. 4 indicates that it rather follows a stretched exponential with an exponent close to 0.5. This can be taken as a signature of the disorder of the system that renders the diffusivity of a carrier time dependent [30].

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Y.-H. Tak et al. / Chemical Physics 212 (1996) 471-485

,

/

• i. I*

A!

9'=0 ¢ --

ITO

°"

~+ i /

[

'÷ I÷

Fig. 13. Schematic view of energy bias levels and currents operated under forward in a bilayer LED. The distribution of the energies of the transport levels is neglected for reasons of clarity.

4.2. Bilayer devices In a bilayer LED with DPOP and PBD being the active anodic and cathodic materials an interfacial barrier for hole transport must exist whose magnitude is no less than 1 eV. (On the basis of cyclic voltammetry the HOMO of PBD has been located - 6.3 eV below vacuum while that of DPOP cannot be much below - 5 . 1 eV as inferred from the injection properties of an ITO contact [14].) Holes migrating towards to cathode will therefore accumulate at the interface• This causes a redistribution of the electric field inside the cell. In general, the electric field at the cathode will increase while the anodic field is shielded• Being dependent on the anodic field the majority current will therefore be lower as compared with the single layer device, while the minority current will be increased. The situation is illustrated in Fig. 13. The total cell current Jtot is composed of four contributions, i.e. the anodic injection current of majority carriers (j+), the leakage current of majority carriers injected across the internal interface into the cathodic cell compartment (j'+), the cathodic injection current of minority carriers ( j _ ) , and the leakage current of minority carriers into the anodic cell compartment ( f _ ) . The condition of current continuity implies that under steady state condition

J+ + f- =J'+ + J - = J t o t ,

the difference j + - f÷ = j _ - f_ being a measure of the carriers recombining at or near the interface. Since under the present conditions the energy barriers for hole crossing at the interface (~b~_) and for electron injection at the cathode (~b_) will exceed those for hole injection at the anode and for electron crossing, at the interface (~b'_ = 0), the total cell current will no larger be controlled by majority carrier injection but by the currents f+ and j_ injected into the cathodic cell compartment. Since t .¢ ~b+ < qS'_, j+ will be the dominant contribution. Given by the fact that on the basis of results from cyclic voitammetry. ~b~_> ~b'_, recombination will take place at the anodic side of the interface consistent with the observation of DPOP emission only• The j ( E ) c u r v e m e a s u r e d with the DPOP: PVK ]PBD : PS bilayer LED is in accord with the above notion. Plotting the j(V) data on a log j scale versus the reciprocal average field E = V/d, d = d a + d r being the sum of the thicknesses of anodic and cathodic layers, a straight line is observed over 3 decades• If analysed in terms of tunneling across a rectangular barrier predicting [31] q t Z reef f )

J =j°exp -

3he

(p

'

(4)

m~ff being the effective mass inside the barrier and q5 the barrier height, qSif= 0.35 eV is obtained for the interfacial barrier. If, on the other hand, one assumes that the entire potential drops across the PBD:PS layer one arrives at ~bif= 0.6 eV. The actual value will be somewhere in between since neither of the extreme cases is realized. The energy barrier is less than the above estimate based upon the location of the HOMO levels. This is to be expected since Eq. (4) ignores barrier lowering at high fields, disorder induced broadening of the energy spectra of charge donating and accepting states as well as the finite thickness of the interfacial layer. The rise of EL in the DPOP:PVK[PBD:PS system must reflect (i) the redistribution of the electric field with a concomitant increase in the electron injection current and (ii) transport of the electrons towards the emitter (DPOP). From the single layer LED it is known that the hole transit time across the D P O P : P V K layer occurs on a time scale which is roughly one tenths of the delay time td(b~ in the

483

Y.-H. Tak et al. / Chemical Physics 212 (19961 471-485 4E-9

!

/

/

7 >

/

E

¢3

/

1E-9

/

/

/ 1000 . . . .

1500 ~ . . . .

20100 . . . .

2500

E1/2 ( V / c m ) l / 2 Fig. 14. Electron mobility in the PBD: PS layer inferred from the rise time t a under the extreme cases that (i) there is no field distortion by interfacial charging (full squares) and (ii) the entire potential drops across the PBD:PS layer (open squares).

bilayer. Since the hole current in that system is space charge limited, charging of the interface must be completed in a time t << #db) and forced electron injection becomes operative. However, EL cannot occur until electrons have traversed the PBD:PS layer. In the bilayer LED the delay time t~b) must, therefore, be associated with the arrival of the leading edge of the electron cloud at the DPOP interface while the time t~b), defined by the intersection of the asymptotes of the rise function, indicates attainment of steady state conditions. In fact, the ratio t~b)/t~ b) is virtually independent of the applied voltage which it should if both times are controlled by electron motion and if the relative field distribution is independent of the absolute field. Fig. 14 shows values for electron mobility calculated for the two extreme cases that (i) the entire field is across the PBD:PS layer and (ii) the field is constant across the entire sample and given by V / d . In both cases In/z ot E ~/2

behavior is found, the slope parameters being 1.2 X 10 -3 ( c m / V ) 1/2 (case i) and 1.7 × 10 -3 ( c m / V ) 1/2 (case ii), respectively, while the absolute values are of order 10 -9 cm2/W s. Both the type of field dependence as well as the slope parameter are consistent with what is known about charge transport in random photoconductors [28]. The message that follows from this analysis is that the frequency at which a bilayer LED can be operated is basically controlled by the electron mobility in the cathodic layer. It seems appropriate to mention that PBD tends to crystallize even if present at a concentration of 20% only. This will cause failure of the LED. Prior to cell failure, however, the response time of the cell turns out to become shorter. Apparently aggregation establishes good transport paths in the PBD:PS that eventually become lethal for the cell. A comparison of the extemal quantum efficiencies (Table 3) confirms that presence of a cathodic blocking layer increases the efficiency by a factor 10 to 100 and values of the order I% can the achieved even with AI as cathode material. Observing this effect with DPOP: PVKIPBD : PS indicates that this increase must largely be due to the enhancement of the electric field at the cathode due to interracial charging with concomitant enhancement of electron injection. Since in that case the majority current is space charge limited insertion of an internal hole blocking layer cannot enhance the stationary hole density any further and the increase of the yield can only be due to an increase of the number of electrons available for recombination. The quantum efficiencies are similar for all materials investigated. In

Table 3 External electroluminescence yield (%) of single layer and bilayer LEDs

TMOP DMOP DPOP C-DPOP

PC PVK PC PVK PC PVK PC PVK

Single layer LED

Bilayer LED

0.045 0.015 0.008 0.007 0.005 0.012 0.015 0.007

1.87 0.25 0.55 0.55 0.20 0.90 0.26 0.26

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Y.-H. Tak et a l . / Chemical Physics 212 (1996) 471-485

particular, the previous observation that DMOP is inactive in a LED [32] could not be confirmed.

5. Concluding remarks This work demonstrates that bilayer LEDs with an external quantum efficiency of order 1% can be fabricated on the basis of substituted poly-arylenevinylenes and aluminum as cathode material. It further shows that the type of current-field characteristics depends in a subtle way on the active material. While in a single layer LED with a DPOP: PVK blend the total current is controlled the space charge established by the migrating majority carriers (holes) it becomes injection limited it upon replacement of PVK by polycarbonate. The transient LED behavior indicates that in the latter system hole transport is faster and that under this circumstance the ITO anode is no longer able to act as an inexhaustible reservoir for holes. The microtopology of the structure is of crucial importance for charge carrier mobility in the blend because it affects the shape of the density of states function which has been known to control charge carrier hopping in random organic media. Studying the time dependence of both electroluminescence and cell current upon addressing the cell with rectangular voltage pulses allows to delineate the time limiting processes. In single layer devices the initial delay time of EL reflects arrival of holes at the cathode and the subsequent rise time is controlled by electron motion into the recombination zone. In bilayer devices accumulation of holes at an interfacial blocking layer occurs followed by enhanced electron injection from the cathode under the action of the increased electric field in the cathodic cell compartment. Which process will ultimately limit the cell response depends on whether or not the time required for establishing the inteffacial charge accumulation exceeds the transit time of electrons across the hole blocking layer.

Acknowledgements This work has been supported by the Bundesministerium for Bildung and Forschung (BMBF).

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