Optical Properties of Polymers by TDDFT
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Optical Properties of Polymers by TDDFT Nobuhiko Akino1 and Yasunari Zempo2 Tsukuba Material Development Laboratory, Sumitomo Chemical Co. Ltd., 6 Kitahara Tsukuba, 300-3294 Japan 2 Hosei University, 3-7-2 Kajino Koganei, Tokyo, 184-8584, Japan 1
ABSTRACT TDDFT in real-space and real-time approach has been applied to study the optical property of poly(9,9'-dioctyl-fluorene) in the gas phase and in the solid state in order to clarify the effect of aggregation on the spectrum. We show that the spectrum of polymer chain is basically the same in linear and helical shape when it is in the gas phase, however, in solid state phase, the spectrum of aligned aggregate model is different from the spectrum sum of its constituent single chains, suggesting the existence of inter-chain interaction and its effect on the spectrum. INTRODUCTION Polymer light emitting diodes (P-OLEDs) have been of great interest for display and lighting applications during decades. The performances such as efficiency and lifetime seem to be reaching a sufficient level for the commercialization. Polymers for P-OLEDs are readily soluble to ink solvents, hence they are considered as appropriated materials for a wet/printing process like ink-jet, die-coating, etc., which are considered to be essential for the cost-effective fabrication of large-scale devices. The simplest P-OLED consists of the polymer layer sandwiched between the cathode and the anode. The radiative recombination of injected electrons and holes in the polymer emitting layer gives the light emission. The emission color is determined by the nature of polymers and the device optimization requires us to understand the fundamental physics of charge injection, charge transport, and recombination. Thus, it is essential to study from both the material design and the device optimization to achieve better P-OLEDs. In this study, we have focused on the spectrum of polymer, which is one of the most important material properties as it determines the emission color. Although many theoretical studies on polymer spectra have so far been performed, they are mostly based on the gas phase. Light emitting polymers in P-OLEDs are in the solid-state where each of the polymers can be considered to be in a solid solution of the same or different materials. In order to study the spectra of polymers, we have employed the time dependent density functional theory (TDDFT) , which is one of the most prominent and widely used methods for calculating excited states of medium-to-large molecules, and it is recognized as a powerful tool for studying electronic transition of molecules[1]. However, since a real polymer is too large to handle, we have limited to perform the calculations of oligomer with small number of repeating units [2,3]. In this study, we could have simulated relatively large system on K-computer to study the optical properties of polymers, especially focusing on the difference between in the gas phase and in the solid-state. In the next section we describe the theory with our numerical details. Then, t
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