Determination of triplet excitons in organic semiconductor materials
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Determination of triplet excitons in organic semiconductor materials S. Döring1*, T. Riedl2, T. Rabe1 and W. Kowalsky1 1 Institut für Hochfrequenztechnik, Technische Universität Braunschweig, Schleinitzstr. 22, 38106 Braunschweig, Germany 2 Bergische Universität Wuppertal, Lehrstuhl für Elektronische Bauelemente, Rainer-GruenterStr. 21, 42119 Wuppertal, Germany ABSTRACT The formation of triplet excitons in semiconducting organic materials plays an important role for the operation of organic optoelectronic devices. Triplet excitons are difficult to investigate spectroscopically at room temperature due to their non-radiative character. Here we show the measurement of the triplet decay dynamics by a highly sensitive time-resolved measurement of the triplet state absorption using pump and probe experiments within a waveguide configuration. Pump and probe pulse have to be separated spatially and in time to ensure a segregation of singlet and triplet excitons. The non-radiative triplet excitons are detected at room temperature by their absorption. A variation of the time delay between pump and probe pulse allows for the investigation of the dynamics of the excitons. Former experiments made use of the photoluminescence of the material under investigation itself as source of probe light. But since there is a spectral shift between the photoluminescence band and the triplet absorption band the spectral bandwidth of probe light has to be broadened for the identification of the triplet absorption. Here we show how a widening of the probe light band up to red part of the spectrum is achieved by an application of extra emission layers. With the help of this technique the triplet exciton dynamics of the hole transport material 2,2',7,7'-tetrakis(diphenylamino)-9,9'spirobifluorene (SpiroTAD) was measured and analyzed. INTRODUCTION The determination of the properties of optoelectronic materials is a key issue in the development of modern display and light source technologies. It is reported that the formation of triplet states is a major drawback for the production of efficient optoelectronic systems [1,2]. Triplet excitons can be generated by charge recombination in electroluminescent materials suitable for the production of OLEDs or by intersystem crossing in luminescent materials suitable for laser applications [3]. In OLED materials the ratio of triplet generation compared to singlet generation is 3:1 due to quantum energy statistics. A potential way for an increase of the efficiency of fluorescent OLED materials is enabled by the generation of additional singlet excitons by triplet-triplet annihilation [4,5]. At the same time singlet-triplet quenching decreases the power efficiency [6]. At the other hand highly efficient triplet emitters are used but they suffer from a luminance roll-off at high driving current densities [7]. In organic lasers the waveguide losses increase due to the overlap of the T1-TN absorption with S1 emission and the optical gain reduces due to singlet quenching. This is one reason why electrical driven
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