Polymer Layer Ordering of Polyaniline Derivatives in Pled Devices: Surface Adsorption and Characterization
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ABSTRACT The fabrication and characterization of polyaniline (PANI) derivatives deposited on ITO coated glass is investigated as possible hole injection layers for MEH-PPV based polymer light emitting diode (PLED) devices. This involved multilayer ordering by the alternate polyelectrolyte adsorption of polyaniline and sulfonated poyaniline with an oppositely charged polyelectrolyte from solution. A combination of spectroscopic and microscopic techniques was utilized to determine the layer ordering, film structure, morphology, and homogeneity. The deposition process generally showed a linear behavior for all pairs as shown by ellipsometry and UV-vis spectroscopy. However, surface plasmon spectroscopy (SPS) and AFM revealed that thicker films are accompanied by increased surface roughness regardless of concentration. Comparison in performance was made between bare ITO and PANI or SPANI coated devices. Initial investigations of PLED performance showed significant improvements in lifetime and efficiency compared to bare ITO. INTRODUCTION The optimization of polymer light emitting diode (PLED) device performance requires the proper combination of materials and processing protocols. The goal being to increase the quantum efficiency, lifetime, lower the operating voltage, and increase the luminance levels. A common processing protocol involving the ITO/MEH-PPV/Ca system [1], has been widely investigated by the UNIAX, IBM, and HP groups. These generally showed poor behavior due to oxidation or catasthropic failures. In contrast, a device formed by the incorporation of a polyaniline complex (PANI-CSA or PanaquaTM) by spin coating, prior to MEH-PPV has been shown to have dramatic improvements in both efficiency and lifetime. [2] These layers are typically several hundred up to a thousand A's thick. However, at very thin films of less than two hundred A, the formation of pinhole defects is evident. Investigations at these thicknesses are believed to be important in determining the mechanisms of device performance and failure. [3] A question arises as to the critical thickness by which these layers play an important role in the hole injection process. Thinner layers are also believed to result in lower turn-on voltages.[3] Rubner et. al. has initially reported the formation of heterostuctures of polyaniline and sulfonated polyaniline with a number of other conducting polymers using alternate polyelectrolyte adsorption.[4] In this work, we initially report our investigations on the layer ordering, characteristics, and film properties of polyelectrolyte adsorbed polyaniline (PANI) and sulfonated polyaniline (SPANI) layers on the ITO/ MEH-PPV/Ca device configuration. A number of surface sensitive 115
Mat. Res. Soc. Symp. Proc. Vol. 488 0 1998 Materials Research Society
spectroscopic and microscopic techniques such as surface plasmon spectroscopy, ellipsometry, UV-vis spectroscopy, atomic force microscopy (AFM) and x-ray reflectivity were applied. The absence of pinhole defects is critical in determining the performance of thes
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