Organic light emitting diodes (OLED) are efficient light sources based onorganic semiconductors. Unlike inorganic LEDs which are more or less pointsources, OLED are planar light sources with up to 1 m2 in area.By using organic materials, they are cheap to produce and economical to use.The determination of triplet exciton energy levels is of interest for thedevelopment of efficient OLED, based on the fact that electrical excitationusually creates three times as many triplets as singlets. Additionally, theknowledge of these energy levels is crucial for the design and choice ofemitter matrix materials and exciton blocking layers. These values arenormally determined by photoluminescence (PL) measurements in solution formaterials which show intersystem crossing (ISC) between singlet and tripletstates. For some materials, the triplet levels cannot be measured this waybecause some materials prohibit ISC. In this work, a method is presentedwhich allows the determination of the energy levels using low-temperatureelectroluminescence (EL) spectroscopy. The dependence on ISC is avoided bycreating triplets directly with electrical excitation and this allows tomeasure a large class of organic materials. A low-temperature EL spectrum ispresented forN,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (TPD) ina 3-phenyl-4-(1‘-naphthyl)-5-phenyl-1,2,4-triazole (TAZ) matrix (TPD/TAZ1:3) at 77 K. Triplet emission is only observed at very low charge carrierdensity (0.5 μA/mm2). Quenching processes are analyzed usingcombined EL and PL measurements and unipolar devices. Two factors can be thecause of the quenching: A strong quenching based on a low concentration ofelectrically activated impurities could explain the dependency. The otherexplanation points to a quenching based on electrons in the emitting layer.This might be explained with triplet-polaron quenching (TPQ). TPQ isproportional to the charge carrier density and contributes the dominant partto the quenching at low current densities.

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.