Electroluminescence Spectral Imaging in Polymer Blend Light Emitting Diodes
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Electroluminescence Spectral Imaging in Polymer Blend Light Emitting Diodes Noriyuki Takada, Kiyohiko Tsutsumi and Toshihide Kamata Photonics Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8565, JAPAN ABSTRACT The spectral imaging for electroluminescence (EL) characterization in the light emitting diode (LED) based on blends of poly[2,7-(9,9-di-n-octylfluorene)] (PFO) and poly[2,7-(9,9-di-n-octylfluorene)-alt-(1,4-phenylene-((4-sec-butylphenyl)amino)-1,4-phenylene) ] (TFB) was performed using the two dimensional imaging micro-spectroscopy system. We found that EL spectral images varied with increasing applied voltages. The origin for such variation of EL spectral images will be discussed in this report. INTRODUCTION Recently, light-emitting diodes (LEDs) based on conjugated polymers have attracted much attention for the application of flexible display etc. They now have sufficient brightness and range of colors. However, the emission efficiency and lifetime are still not high and long in comparison with the low molecular based-organic electroluminescent devices, respectively. To improve electroluminescence (EL) performance, the fabrication of LEDs based on blends of polymers has been commonly tried [1, 2]. The combination of blend components can allow for efficient bipolar charge transport and luminescence in solution-processed films. On the other hand, the phase separation between the blend polymers and the variation of morphology largely influence EL performance. Therefore, it is important to elucidate morphologies of blend polymers on microscopic scale for achieving high EL performance. Traditionally, atomic force microscopy (AFM) and transmission electron microscopy (TEM) have been utilized to characterize the morphology [3]. Several techniques, such as optical microscopy [4], scanning near-field optical microscopy (SNOM) [5], Raman microscopy [6] and fluorescence lifetime imaging (FLIM) [7], have been also employed to study degradation or morphology of the luminescent materials in organic LEDs. We here demonstrate the spectral imaging for EL characterization (EL spectral imaging) in a polymer blend LED using the imaging micro-spectroscopy system, which has functions both micro-spectroscopy measurement and imaging analysis. This imaging technique is used to elucidate what and where emission species are located within an emission surface. EXPERIMENTAL DETAILS AND EL SPECTRAL IMAGING The polymer blend LED used in this study is composed of ITO/PEDOT:PSS/PFO:TFB/Ca/Al [8]. The 70nm thickness poly(3,4-ethylenedioxy thiophene):poly(styrenesulphonic acid) (PEDOT:PSS) layer and 150nm thickness poly[2,7-(9,9-di-n-octylfluorene)] (PFO) blended with poly[2,7-(9,9-di-n-octylfluorene) -alt-(1,4-phenylene-((4-sec-butylphenyl)amino)-1,4-phenylene)] (TFB) (weight ratio PFO:TFB = 3:2) as the emission layer were put down onto the ITO coated glass substrates by conventional spin coating procedures and then baked at about 100oC. Next, calsium and aluminium were
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