langevin temperature dependent electron-hole recombination in polym

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Temperature dependent electron-hole recombination in polymer light-emitting diodes

P. W. M. Blom, M. J. M. de Jong, and S. Breedijk

Citation:Applied Physics Letters 71, 930 (1997); doi: 10.1063/1.119692

View online: http://dx.doi.org/10.1063/1.119692

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Temperature dependent electron-hole recombination in polymerlight-emitting diodes

P. W. M. Blom,a) M. J. M. de Jong, and S. Breedijkb)

Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands

Received 17 March 1997; accepted for publication 13 June 1997

The current densityvoltage characteristics of polydialkoxyp -phenylene vinylenebased polymerare investigated as a function of temperature. Model calculations show that the differences betweensingle and double carrier devices can be well understood by taking into account a bimolecularrecombination process. It is found that the bimolecular recombination is thermally activated with anidentical activation energy as measured for the charge carrier mobility. This demonstrates that therecombination process is of the Langevin type, and explains why the conversion efficiency photon/carrierof a polymer light-emitting diode is temperature independent. 1997 American Instituteof Physics.S0003-69519702133-5

The discovery that conjugated polymers can be used asthe active material in light-emitting diodes LEDs hasopened the route towards easy processable and mechanicallyflexible large-area applications, based on polymer LEDsPLEDs.1,2 Attention has especially been focused on the

polyp-phenylene vinylene PPVderivatives as a result oftheir large conversion efficiency of 1% to 2% photons/chargecarrier. For an optimization of the device performance, athorough understanding of the device properties is required.Recently, we have demonstrated that in our PPV based de-vices at low electric fields the transport of holes is limited byspace charge effects in the polymer layer.3 At high electricfields, the hole transport is governed by a combination ofspace-charge effects and a field dependent mobility,4 whichis thermally activated. The electron current on the other handis found to be limited by the presence of traps.3 Thus incontrast to conventional semiconductor LEDs, the chargetransport in a PLED is strongly dependent on temperature.

Let us first discuss the influence of temperature on thedevice performance of a PLED without traps and with afield-independent mobility. Due to space-charge effects, thesingle-carrier hole-only current Jis given by5

J9

8 p

V2

L3, 1

with0rthe permittivity of the polymer andpthe holemobility. For a double-carrier device, two additional phe-nomena become of importance, namely recombination andcharge neutralization. We will assume that the recombination

is bimolecular, i.e., that its rate is proportional to the productof the electron and hole concentrations. Due to charge neu-tralization, the total charge may far exceed the net charge. Asa result, the current density in a double-carrier device can beconsiderably larger than in a single-carrier device. In theplasma limit, the current is given by5

J 981/2

2qpnpn B

1/2 V2

L3, 2

with nthe electron mobility and B the bimolecular recom-bination constant. Upon increasingB , the amount of neutral-ization decreases, so that Jbecomes smaller. The differencein current between a single- and a double-carrier device pro-vides direct information about the strength of the recombina-tion process. Let us assume that the mobility is thermallyactivated and that pn . The temperature dependence ofthe hole-only current Eq. 1 is directly governed by pwhereas the LED current Eq. 2 is determined byp

3/2/B1/2. Thus for a constant B , the relative difference be-tween a single- and a double-carrier device is expected todecrease with decreasing temperature.

In the present study, the temperature dependence of thecharge transport in polymer devices is investigated by per-forming J Vmeasurements on both hole-only and double-carrier devices. We demonstrate that there is a bimolecularrecombination mechanism of the Langevin type, where the

rate limiting step is the diffusion of oppositely charged par-ticles towards each other in their mutual coulomb field. Theoccurrence of Langevin recombination clarifies why the con-version efficiencyphoton/charge carrierof a PLED is tem-perature independent, in spite of the thermally activatedcharge transport. Langevin recombination may be expectedfor organic hopping conductors6 and has recently been in-cluded in an analytical model for PLEDs.7 However, we donot know of any temperature-dependent experiments whichare essential to demonstrate its occurrence in PLEDs.

The devices under investigation consist of a single poly-mer layer sandwiched between two electrodes on top of aglass substrate. The polymer is soluble polydialkoxy-p-phenylene vinylene8 PPV and is spin coated on top ofthe patterned indium-tin-oxide ITO bottom electrode. Forthe hole-only devices, an evaporated Au contact is used, soboth the electrodes have a work function close to the valenceband of the conjugated polymer. For the double-carrier de-vices, an evaporated Ca contact is used, which has a workfunction close to the conduction band of the PPV. The J Vmeasurements are performed in a nitrogen atmosphere in thetemperature range of 200300 K using a HP 4145A semi-conductor parameter analyzer. In Fig. 1, the J Vcharacter-istics of both an ITO/PPV/Au device and an ITO/PPV/Cadevice are shown for various temperatures. The thickness of

aElectronic mail: [email protected] address: Van der Waals-Zeeman Laboratorium, Universiteit van

Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands.

930 Appl. Phys. Lett .71 (7), 18 August 1997 0003-6951/97/71(7)/930/3/$10.00 1997 American Institute of PhysicsThis article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to

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values predicted by Eq. 9. A possible explanation for thissmall discrepancy might be the fact that recombination fromtrapped electrons may play a role as well. In organic LEDsbased on metal chelate complexes Alq, it has recently beenproposed that the recombination originates from trappedelectrons.11 Inclusion of trapped electrons in our modelthrough Eq. 5gives rise to an enhancement of the recom-bination strength which leads to a decrease ofB in order todescribe the double-carrier J V characteristics of Fig. 1.

However, our model calculations show that a full participa-tion of the trapped electrons leads to large deviations fromthe experimental results. Nevertheless, a small contributionof the trapped electrons to the recombination cannot be ex-cluded and will be the subject of further research.

It should be noted that by including Langevin recombi-nation Eq. 9 into Eq. 2 the double-carrier current J(np)

1/2, which forpnmeans that Jis proportionalto pinstead ofp

3/2 . An equal temperature dependence forthe hole-only and the double-carrier device is the result, inagreement with the experimental results shown in Fig. 1. Infact, the temperature independence of the difference betweenthe single- and double-carrier current is a direct demonstra-

tion that the recombination process is proportional to thecharge-carrier mobility.According to Eq. 6, the charge carrier mobility in a

PLED is enhanced by the applied electric field. Interestingly,we find that the Langevin recombination constant B is inde-pendent on electric field. Clearly, an applied electric fieldonly enhances the diffusion of electrons and holes towardseach other in one direction. The diffusion in the directionsperpendicular to the field is not enhanced which is the rate-limiting step for recombination at high fields.

Finally, we discuss the consequences of Langevin re-combination for the device performance of a PLED. In Fig.3, the measured conversion efficiency CE, defined as

photon/charge carrier, is shown for a PLED with L110 nm at various temperatures. It appears that the CE of aPLED is independent of the temperature, in contrast to con-ventional semiconductor LEDs. This typical behavior of aPLED is a direct consequence of the occurrence of Langevinrecombination. In a space-charge limited device as ourPLED, the number of charge carriers is mainly determinedby the applied voltage and not by the temperature. As aresult, the device current is only dependent on temperatureby means of the charge carrier mobility. Since the recombi-nation mechanism is of the Langevin type, both the recom-bination strength and the device current are thermally acti-vated by means of the charge carrier mobility. Since in PPVthe photoluminescence efficiency is not dependent12 on tem-perature in the range of 200300 K, the resulting CE of aPLED is temperature independent. The CE increase at lowvoltages, where electrons are trapped close to the electroninjecting contact, is suggested to originate from additionalnonradiative recombination losses at the cathode interface.9

Furthermore, our model calculations reveal that the re-combination efficiency in a PLED resulting from Langevin

recombination approaches unity, making PLEDs efficient de-vices in agreement with earlier estimates by Albrecht andBassler.13 By comparing the total number of recombinationsinside the PLED with the actual light output, we find thatonly about 5% of the total recombinations actually contrib-utes to the light output of the device. This large contributionof nonradiative processes is mainly responsible for the rela-tively low external CE of 2%2.5%, which also includes outcoupling losses.

In conclusion, from the temperature dependence of theJ V characteristics, we have demonstrated that the recom-bination mechanism in a PLED is of the Langevin type. Dueto this diffusion controlled recombination, PLEDs exhibit, incontrast to conventional inorganic LEDs, a temperature in-dependent recombination efficiency. Enhancement of thePLED performance can be obtained by decreasing the non-

radiative recombination processes, which limit the maximumexternal conversion efficiency.

1 J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K.Mackey, R. H. Friend, P. L. Burn, and A. B. Holmes, Nature London347, 539 1990.

2 D. Braun and A. J. Heeger, Appl. Phys. Lett. 58, 1982 1991.3 P. W. M. Blom, M. J. M. de Jong, and J. J. M. Vleggaar, Appl. Phys. Lett.68, 3308 1996.

4 P. W. M. Blom, M. J. M. de Jong, and M. G. van Munster, Phys. Rev. B55, R656 1997.

5 M. A. Lampert and P. Mark, Current Injection in Solids Academic, NewYork, 1970.

6 U. Albrecht and H. Bassler, Phys. Status Solidi B 191, 455 1995.7 J. C. Scott, S. Karg, and S. A. Carter unpublished.8 D. Braun, E. G. J. Staring, R. C. J. E. Demandt, G. J. L. Rikken, Y. A. R.

R. Kessener, and A. H. J. Venhuizen, Synth. Met. 66, 75 1994.9 P. W. M. Blom, M. J. M. de Jong, C. T. H. F. Liedenbaum, and J. J. M.

Vleggaar, Synth. Met. 85, 1287 1997.10 P. Langevin, Ann. Chem. Phys. 28, 289 1903.11 P. E. Burrows, Z. Shen, V. Bulovic, D. M. McCarty, S. R. Forrest, J. A.

Cronin, and M. E. Thompson, J. Appl. Phys. 79, 7991 1996.12 M. Furukawa, K. Mizuno, A. Matsui, S. D. D. V. Rughooputh, and W. C.

Walker, J. Phys. Soc. Jpn. 58, 2976 1989.13 U. Albrecht and H. Bassler, Chem. Phys. 199, 207 1995.

FIG. 3. External quantum efficiency photons/carrier vs voltage measuredfrom an ITO/PPV/Ca double-carrier device with a thickness ofL110 nmat T300, T261, and T242 K. The maximum quantum efficiencyamounts to 2% and is independent of temperature.

932 Appl. Phys. Lett., Vol. 71, No. 7, 18 August 1997 Blom, de Jong, and BreedijkThis article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to

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langevin temperature dependent electron-hole recombination in polym

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