Emission properties of thin films of electroactive doped polymers

Abstract

We have studied the effect of the intensity of the exciting radiation and the temperature on the emission properties of two kinds of thin-film samples based on blends of two types of organic electroactive materials: polyfluorene + iridium triphenylpyridinate and polyepoxypropylcarbazole + zero-th order PAMAM dendrimer with eosin. We have shown that an increase in the excitation intensity leads to an increase in the intensity of the luminescence of the polymer matrices and the iridium complex up to a power density of 300 kW/cm 2 , and the emission of the dendrimer is rapidly saturated and does not return to the initial value when the excitation level decreases. Heating up to 170 o C followed by cooling causes an increase in the intensity for all the components except the dendrimer. The data obtained show that annealing is an important method for improving the emission efficiency of the proposed thin-film structures, due to a change in the packing of the activator molecules in the polymer matrix leading to more efficient transfer of the excitation energy. Molecules of the studied dendrimer are not stable when exposed to optical radiation and temperature.Introduction. Organic electroactive materials, owing to their optical and electrophysical properties, have become widely used in designing electroluminescent diodes, displays, sensors, solar cells, etc.[1]. Data are available on achieving amplification [2] and designing electrically pumped thin-film lasers [3,4]. Thin-film lasers and radiation converters based on organic electroactive materials have also been widely studied [5][6][7]. The most attractive for these purposes from both a functional and a technological point of view are blends of electroactive polymers and electroactive low molecular weight organic materials which are activators. In such blends, the polymer matrix serves as a solid "solvent" for the activator. Under appropriate conditions, energy and charge transfer processes can occur in these systems, improving the transport and emission properties. During operation of such devices, as a result of electric current flow the organic materials used heat up, which causes a change in their optical and electrophysical properties [8][9][10] and also their morphology. Widespread use of organic light emitters that are promising for application in devices for display of video information (televisions, computer displays, mobile phones, digital cameras, etc.) and also in photovoltaic and sensor devices has been significantly held back by insufficient photostability and temperature stability. These parameters cannot be improved without obtaining additional information about the processes and effects responsible for the temperature and degradation stability of organic semiconductors. Such information will make it easier to achieve operating characteristics for organic semiconductor emitters sufficient for their application in commercial devices for displaying information. For their reliable operation, we need to choose not only activators with appropriate propertie...