Study on Organic Triplet Exciton Emission and Quenching Processes by Low-temperature Photo- and Electroluminescence Spectroscopy

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.