Materials for deep blue organic light emitting devices with ultra high thermal stability and charge mobility
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Materials for deep blue organic light emitting devices with ultra high thermal stability and charge mobility Soonnam Kwon, Kyung R. Wee, and Sang O. Kang Department of Materials Chemistry, Korea University, Sejong Campus, Chochiwon, Chungnam, 339-700, South Korea ABSTRACT The development of materials with high stability and high charge mobility is urgent for commercial application of blue phosphorescent organic light emitting devices (PHOLED). Silicon based inorganic-organic hybrid materials with ultra high glass transition temperature (over 150 oC) and high charge mobility (over 1.16 x 10-3 at 5 x 105 V/cm) were synthesized. These showed high external quantum efficiency of over 19%, and deep blue color coordinates of (0.15, 0.23), when they were used as a host materials in the PHOLED. The origin of the above interesting properties was investigated by experimental measurements complemented by DFT calculations. Estimations of the structure-property relationship of a molecule in an amorphous thin film would be presented INTRODUCTION Phosphorescent organic light emitting devices (PHOLEDs) have received tremendous attention, and have achieved revolutionary advance in efficiency and stability. As a result, red and green PHOLEDs are expected to be applied to commercial products such as active matrix OLED (AMOLED) in near future. However, in the case of blue PHOLED, stability should be developed further for full fledged applications. Deep blue PHOLEDs play a crucial role for commercializing highly efficient full colour flat panel displays and white general lighting with high color rendering index (CRI). For this reason, many research groups are working hard on the development of highly efficient and stable deep blue materials and devices. To achieve highly efficient and stable deep blue PHOLEDs, development of materials with high triplet energy (T1), high charge mobility, and high glass transition temperature (Tg) is necessary for use in the lightemitting layer (EML). However, it is commonly known that triplet energy and charge mobility are in a trade-off relationship, because a high T1 requirement renders conjugation breaking in a molecule, by which intermolecular molecular orbital overlap is reduced, resulting in low charge mobility. There are few materials which can meet all three requirements. To achieve high triplet energy, several synthetic approaches are used. One example is 4.4’-bis(9-carbazolyl)-2,2’dimethyl-biphenyl (CDBP), which achieve conjugation interrupting by interconnecting functional subunits in non coplanar way.9 Interconnection of subunits via meta-position is also well known method.2 And recently, high triplet energy materials are reported by utilizing phosphine oxides moiety as a point of interconnecting between active chromophores.10 Another well known method is separating chromophores using higher band gap blocks, such as Si atom.11, 12 The last method is kwon as resulting in low Tg or low charge mobility due to short
conjugation length.12 Recently, we found that cabazole based materials interconn
