Singlet Exciton Quenching by Radical Cations of Aromatic Diamines as an Electron Donor in Organic Electroluminescent Dev
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Spectroelectrochemical Study of the Formation of Radical Cations of 4,4’-bis[N-(1naphthyl)-N-phenyl-amino]-biphenyl as a Hole Transport Semiconductor Material
Sharavsambuu Baasanjav, Gendensvren Bolormaa, Batjargal Naranbileg, Munkhbat Battulga, and Chimed Ganzorig Center for Nanoscience and Nanotechnology and Department of Chemical Technology, School of Chemistry and Chemical Engineering, National University of Mongolia, Ulaanbaatar, Mongolia
ABSTRACT Spectroelectrochemical study on a new absorption band of radical cations of 4,4’-bis[N-(1naphthyl)-N-phenyl-amino]-biphenyl (α-NPD) as an electron-donor hole-transporting material used in organic electronics is reported in this work. UV-visible spectroscopic and cyclic voltammetric properties for α-NPD in solution are also examined. We find that the results are attributed to quenching process for blue fluorescence from α-NPD by excess α-NPD+· radical cations accumulated in the emission region in the organic light-emitting devices related to a relatively large overlap between the fluorescence spectrum of α-NPD and the absorption spectrum of α-NPD+· radical cations. The band gap energy for α-NPD is calculated from the UVvisible spectroscopic data.
INTRODUCTION At present, development of high performance organic electroluminescence (EL) device with blue emission is not satisfactory. However, there are many studies focused on blue organic EL devices, and a variety of blue luminescent materials has been developed [1-3]. The hole-blocking layer is essentially needed to develop a new type of blue organic EL devices. Blue light-emitting materials have lower efficiencies than green or red light-emitting materials because of their larger energy band gaps, which may deter charge injection from electrodes. Enhanced hole and electron injection is greatly important to improve power and EL efficiencies [4-9]. More recently, Ganzorig and Fujihira have reported that luminance increased more than linearly with an increase in current density of tris(8-hydroxyquinoline)alumininum based EL devices when hole and electron injection was well balanced [9]. In order to maximize the EL efficiency, we have to consider the loss mechanisms in the EL efficiency. In this paper, we report that quenching process for blue fluorescence from 4,4’-bis[N-(1naphthyl)-N-phenyl-amino]-biphenyl (α-NPD) by an excess α-NPD+· radical cations accumulated in the emission region in the organic light-emitting devices. This quenching process related to a relatively large overlap between the fluorescence spectrum of α-NPD and the absorption spectrum of α-NPD+· radical cations.
EXPERIMENTAL DETAILS 10-5 M of α-NPD was dissolved in the chloroform. Adsorption spectra of prepared solution were measured by UV-Visible spectrophotometer. Therefore, we have calculated energy gap from the fluorescence spectra of α-NPD solution. All the chemical were of analytical grade. Electrochemical measurements were performed with a Bipotentiostat/Galvanostat µStat 400 (DropSens, Spain). Cyclic voltammetric (CV) was performed in a seal
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