Organic light emitting diodes (OLED) are efficient light sources based on organic semiconductors. Unlike inorganic LEDs which are more or less point sources, 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 the development of efficient OLED, based on the fact that electrical excitation usually creates three times as many triplets as singlets. Additionally, the knowledge of these energy levels is crucial for the design and choice of emitter matrix materials and exciton blocking layers. These values are normally determined by photoluminescence (PL) measurements in solution for materials which show intersystem crossing (ISC) between singlet and triplet states. For some materials, the triplet levels cannot be measured this way because some materials prohibit ISC. In this work, a method is presented which allows the determination of the energy levels using low-temperature electroluminescence (EL) spectroscopy. The dependence on ISC is avoided by creating triplets directly with electrical excitation and this allows to measure a large class of organic materials. A low-temperature EL spectrum is presented for N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (TPD) in a 3-phenyl-4-(1‘-naphthyl)-5-phenyl-1,2,4-triazole (TAZ) matrix (TPD/TAZ 1:3) at 77 K. Triplet emission is only observed at very low charge carrier density (0.5 μA/mm2). Quenching processes are analyzed using combined EL and PL measurements and unipolar devices. Two factors can be the cause of the quenching: A strong quenching based on a low concentration of electrically activated impurities could explain the dependency. The other explanation points to a quenching based on electrons in the emitting layer. This might be explained with triplet-polaron quenching (TPQ). TPQ is proportional to the charge carrier density and contributes the dominant part to the quenching at low current densities.

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. Usually, in PLEDs low work function metals like Ba, Mg or Ca are used to reduce the energy barrier between the cathode and the polymer thus providing a better electron injection from the cathode. Also salts like LiF, NaF, Cs2CO3 and CsF have recently been shown to function as electron injection layers in light emitting devices. From these, especially caesium carbonate (Cs2CO3) results into high efficiency diodes both as a solution processed electron injection layer in PLEDs, as well as an n-dopant in the electron transport layer in vacuum deposited small molecule based OLEDs. 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, the thermal decomposition of Cs2CO3 and following formation of caesium metal or the formation of an n-doped CsO2 layer. In this study the phenomena resulting from the combination of a hole-dominant alkoxy-phenyl-substituted poly(phenylene vinylene) (PPV) based light emitting polymer with a highly 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 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. The strong increase in the electron injection of the Cs2CO3 diodes is apparently caused by a highly conductive, n-doped layer resulting from the charge transfer reaction between Cs2CO3 and PPV, where the magnitude of the reaction and resulting effects strongly depend on the amount of the applied Cs2CO3. The conclusion of the n-doped layer can be drawn from the LIV, impedance and photoluminescence measurements of the diodes with Ba and Cs2CO3 cathodes before, during and after electrical stressing.