High Efficiency PPV-Based Polymer Light Emitting Diodes With Cs 2 CO 3 Cathode
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1115-H09-05
High Efficiency PPV-Based Polymer Light Emitting Diodes With Cs2CO3 Cathode Riikka Suhonen1,2, Ralf Krause1, Fryderyk Kozlowski1, Wiebke Sarfert1, Ralph Pätzold1 and Albrecht Winnacker2 1 Siemens AG, CT MM 1, Günther-Scharowsky-Str. 1, 91058 Erlangen, Germany 2 Department of Material Science VI, University Erlangen-Nuremberg, Martensstraße 7, 91058 Erlangen, Germany ABSTRACT In this study the phenomena resulting from the combination of a hole-dominant PPV-based light emitting polymer with an efficient electron injection layer of Cs2CO3 in light emitting diodes has been investigated. As a result, diodes with about 35 % higher efficiency were achieved with an optimized PPV-Cs2CO3 structure in comparison to the traditional PPV-Ba structure. Additionally to the increased efficiency, also the lifetime of the Cs2CO3-diodes is comparable to the Ba-diodes implying that the long-term stability of the diodes is not affected by the optimized Cs2CO3-cathode. In this study, the influence of the Cs2CO3 thickness on the electrical and optical properties before, during and after the electrical stressing of the diodes are discussed. INTRODUCTION The thin electron injection layers between the cathode and the light emitting polymer layer in polymer light emitting diodes (PLEDs) have been shown to have a big impact on the final device performance. Several salts like LiF, NaF, Ca(acac)2, Cs2CO3 and CsF have been shown to function as electron injection layers in light emitting devices [1-4]. From these, especially caesium carbonate (Cs2CO3) results into high efficiency diodes both as a solution processed electron injection layer in PLEDs [5-6] as well as an n-dopant in the electron transport layer in vacuum deposited small molecule based OLEDs [4,7-8]. The functional mechanism of Cs2CO3 as a pure interlayer is not yet fully understood. The proposed mechanisms include the n-doping of the organic layer with Cs2CO3 [9], the thermal decomposition of Cs2CO3 and following formation of caesium metal [10] or the formation of an n-doped CsO2 layer [6]. EXPERIMENT The PEDOT:PSS (Baytron® P VP CH 8000) was purchased from H. C. Starck, the Superyellow (SY) (PDA-132) was acquired from Merck KGaA and the caesium carbonate (Cs2CO3) was purchased from Sigma-Aldrich. The PLED device structure consisted of a glasssubstrate / ITO / PEDOT:PSS (120 nm) / SY (100 nm) / Ba or Cs2CO3 / Al. The Cs2CO3, Ba and Al electrodes were thermally evaporated. The applied Cs2CO3 thicknesses varied from 0.05 to 1.50 nm, the thickness of the reference barium layer being 3 nm. The luminance-current-voltage (LIV) characteristics were measured with a computer controlled Keithley 238 high current source measure unit and with a calibrated spectral camera Photo Research PR650. The PL-spectra was detected with a Perkin Elmer LS-50B Spectrometer and the EL spectrum was simulated with an optical simulation software etfos (FLUXiM). The impedance spectroscopy was performed with a Hewlett Packard 4192A LF Impedance Analyzer.
DISCUSSION Caesium Carbonate Thickness Variatio
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