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High Efficiency PPV-Based Polymer Light Emitting Diodes With Cs2CO3 Cathode

Published online by Cambridge University Press:  01 February 2011

Riikka Suhonen ,
Ralf Krause ,
Fryderyk Kozlowski ,
Wiebke Sarfert ,
Ralph Päetzold  and
Albrecht Winnacker
Riikka Suhonen
Affiliation:
r.a.suhonen@gmail.com, Siemens AG, CT MM1, Erlangen, Germany
Ralf Krause
Affiliation:
krause.ralf@siemens.com, Siemens AG, CT MM1, Erlangen, Germany
Fryderyk Kozlowski
Affiliation:
fryderyk.kozlowski@siemens.com, Siemens AG, CT MM1, Erlangen, Germany
Wiebke Sarfert
Affiliation:
wiebke.sarfert@siemens.com, Siemens AG, CT MM1, Erlangen, Germany
Ralph Päetzold
Affiliation:
ralph.paetzold@siemens.com, Siemens AG, CT MM1, Erlangen, Germany
Albrecht Winnacker
Affiliation:
albrecht.winnacker@ww.uni-erlangen.de, University Erlangen-Nuremberg, Department of Material Science VI, Erlangen, Germany
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Abstract

The thin electron injection layers between the cathode and the lightemitting polymer layer in polymer light emitting diodes (PLEDs) have beenshown to have a big impact on the final device performance. Usually, inPLEDs low work function metals like Ba, Mg or Ca are used to reduce theenergy barrier between the cathode and the polymer thus providing a betterelectron injection from the cathode. Also salts like LiF, NaF, Cs2CO3 and CsF have recently been shown to functionas electron injection layers in light emitting devices. From these,especially caesium carbonate (Cs2CO3) results intohigh efficiency diodes both as a solution processed electron injection layerin PLEDs, as well as an n-dopant in the electron transport layer in vacuumdeposited small molecule based OLEDs. The functional mechanism of Cs2CO3 asa pure interlayer is not yet fully understood. The proposed mechanismsinclude the n-doping of the organic layer with Cs2CO3,the thermal decomposition of Cs2CO3 and followingformation of caesium metal or the formation of an n-doped CsO2layer. In this study the phenomena resulting from the combination of ahole-dominant alkoxy-phenyl-substituted poly(phenylene vinylene) (PPV) basedlight emitting polymer with a highly efficient electron injection layer of Cs2CO3 in light emitting diodes has beeninvestigated. As a result, diodes with about 35 % higher efficiency wereachieved with PPV-Cs2CO3 structure in comparison tothe traditional PPV-Ba structure. Additionally to the increased efficiency,also the lifetime of the Cs2CO3-diodes is comparableto the Ba-diodes implying that the long-term stability of the diodes is notaffected by the optimized Cs2CO3-cathode. The strongincrease in the electron injection of the Cs2CO3diodes is apparently caused by a highly conductive, n-doped layer resultingfrom the charge transfer reaction between Cs2CO3 andPPV, where the magnitude of the reaction and resulting effects stronglydepend on the amount of the applied Cs2CO3. Theconclusion of the n-doped layer can be drawn from the LIV, impedance andphotoluminescence measurements of the diodes with Ba and Cs2CO3 cathodes before, during and after electricalstressing.

Keywords

dopantelectrical propertiesoptoelectronic

Information

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 1115: Symposium H – Physics and Technology of Organic Semiconductor Devices , 2008 , 1115-H09-05
Copyright
Copyright © Materials Research Society 2009

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References

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