Effects of polymeric hole transporting layer on the stability of organic light emitting device

초록

A polymeric hole transporting layer (HTL) in organic light emitting devices (OLED) helps oppositely charged carriers to meet away from the cathode, so that the emitting efficiency is increased, and can also help the constituent layers to become thermally and mechanically more stable. Two experimental results concerning the operation of polymeric HTL layer, made of poly [N-(p-diphenylamine) phenylmethacrylamide] (denoted as PDPMA), are presented and analyzed. Firstly, OLED devices composed of ITO/PDPMA/Alq 3/Al were prepared and heat-treated systematically to study the influence of the resulting structural changes in the polymeric layer on the device performance. It was observed that the power efficiency of the device could be improved, especially at higher voltage, by annealing at a temperature above the glass transition temperature of PDPMA. The results are thought to come from the rearrangement of the polymer chains in the film, that reduces the vacant volume present within the amorphous chains and helps chains to be packed close to each other, which is expected to be advantageous for charge transport and also for ensuring better thermal stability. Secondly, a fluorescent dopant (Nile red) was added in the light emitting Alq 3 layer (ITO/PDPMA/dye doped Alq 3/Al) and on the other hand in the PDPMA layer (ITO/dye doped PDPMA/Alq 3/Al), so that the energy could be transferred to the dopant molecules. The lifetimes of the devices were compared, and it was shown that the device with the dye doped PDPMA had superior stability to the one with dye doped Alq 3 layer.

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