Light emitting, segmented block copolymers containing distyrylbenzene blocks connected through α,ω-nonamethylenedioxy chain spacer for applications in polymer light emitting diodes

Abstract

Two novel, soluble, semiconducting and light emitting segmented block copolymers, termed as NPPV1 and NPPV2 are synthesized using Horner-Emmons condensation polymerization. The chromophoric groups present in the copolymers are distyrylbenzene (DSB) units substituted with rigid, bulky cyclohexylmethoxy and cyclohexylethoxy groups. The rigid blocks are linked to flexible α,ω-nonamethylenedioxy chain spacer through ether linkage. The structural, morphological, thermal and photo-physical characteristics of the synthesized polymers are studied in detail. Noticeable differences in properties arising through the substituent effects are observed for both the polymers. Detailed studies bring out quite interesting structure-property relationships between the substituents introduced in the synthesized copolymers and their crystalline properties, thermal characteristics, optical properties, photoluminescence emission and fluorescence quantum yield. Polymer light emitting diodes (PLEDs) employing these segmented polymers as emissive layers are assembled and characterized to establish their suitability for lighting applications. Using the architecture, ITO/PEDOT:PSS/NPPV1/LiF-Al, polymer light emitting diodes (PLEDs) are assembled and as the first step of device characterization, their current-voltage characteristics are studied to establish the device behavior and find out the onset voltage. The emission characteristics of the devices are then studied using Ocean Optics USB 2000 spectrometer integrated to a computer using a dedicated program. The assembled devices show Schottky junction like behavior in the current-voltage characteristics and electroluminescence emission in the blue green region with appreciable intensity at the onset voltage of 8.8 V. It is highly desirable to develop polymers showing emission in the high frequency,blue region, using cost-effective synthesis routes. The novel approaches adopted in the synthesis of the monomers and the corresponding copolymers to achieve the anticipated structure-property relationships suitable for the effective designing of polymer light emitting diodes are the highlights of the present work.