Effect of thermal annealing on the performance of polysilane based organic light emitting diode
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Effect of thermal annealing on the performance of polysilane based organic light emitting diode Ranbir Singh, and Monica Katiyar Department of Materials and Metallurgical Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, India ABSTRACT Polysilanes, being σ bonded, are promising materials for emission in ultraviolet/near ultraviolet (UV/NUV) region and exhibit adequate charge mobility for fabricating practical devices. This study contributes to the understanding of the effects of thermal annealing on the performance of organic light-emitting diodes (OLEDs) made of poly (n-butylphenyl-silane) (PS4). OLEDs having indium tin oxide (ITO)/ poly (3, 4-ethylenedioxythiophene) poly (styrenesulfonate) PEDOT: PSS/poly (n-butylphenylsilane) (PS-4)/lithium fluoride (LiF)/Al device structure were fabricated. PS-4 is spin coated and annealed for one hour at different temperatures (90-120oC). EL Spectra from these devices consists of white emission along with the UV peak. Current density-voltage (J-V), photoluminescence (PL) and electroluminescence (EL) spectra are also measured. White emission is significantly suppressed when PS-4 is annealed at higher temperature and threshold voltage is lowest at 110oC annealing temperature. This is correlated with PL emission and structural properties of PS-4 films. Surface morphology of PS-4 was measured using atomic force microscopy (AFM). The results are explained in terms of effect of annealing of polymer films on interchain interactions.
INTRODUCTION With the commercial success of OLEDs in various display sectors, it is natural to expand the range of luminescent spectra from these devices to infrared and ultraviolet. Recently, we have demonstrated several polysilanes that can be used to obtain UV emission at room temperature [15]. High efficiency UV-OLED can be used in several applications including displays, lighting, water purification and bio-diagnostics. Lee and Park found that thermal annealing at a temperature above the glass transition temperature (Tg) of the emitting polymer before metal deposition or pre-annealing increases maximum light output but lowers the quantum yield [6]. However, they found that thermal annealing after Al deposition or post-annealing significantly enhances the device efficiency. They attributed the improvement to an altered polymer/Al interface. Niu et al. also demonstrated that by thermal annealing after cathode deposition just below the Tg of polyfluorene (PF) polymers, the device performance was greatly enhanced [7]. Prelipceanu et al. observed decrease in the turn on voltage with the increase in annealing temperature due to an interaction between the polymer chains by the change of the chain conformation [8]. The intermolecular interaction can be further reduced to enhance the exciton confinement [9]. It can also increase the thermal stability of the polymer thin film and its ability to endure the Joule heat generated during operation. The effect of the increased thermal stability of the device can play an important role
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