CARBAZOLE BASED ORGANIC MATERIALS FOR ELECTRONIC APPLICATION

Abstract

Carbazole and its derivatives have been the most effective heterocyclic building blocks for material scientists. Carbazole-based organic luminescent materials are extensively used in optoelectronic applications especially organic-light emitting diodes (OLEDs) as emissive as well as hole-transporting layer which enjoys its high charge mobility. The optical and electrical properties of these materials can be facilely tuned by multi-substitution of chromophores at C1, C2, C3, C6, C7, C8 and N9 positions of carbazole. Additionally, its sufficiently high triplet energy offers its utility as suitable host materials in electroluminescent devices. It’s self-emissive property, easy synthetic procedures, molecular rigidity and amorphous nature further proves its advantageous candidacy over other conventional fluorophores. In this thesis, we comprehensively examine the design, synthesis and optoelectronic properties of carbazole based functional materials. This thesis consists of seven chapters in total. The first chapter deals with the aim and scope of the work proposed on carbazole based organic materials for optoelectronic applications such as organic light-emitting diodes (OLEDs). The second chapter presented a detailed discussion on literature background based on synthesis and characterization of carbazole-containing organic fluorescent materials used for various applications such as fluorescent and phosphorescent OLEDs as well as molecular sensors. In chapter 3, a series of carbazole based derivatives (9-12) differing in the number of dicyanovinyl acceptors and donors (N-alkyl carbazole or N-phenyl carbazole) are designed, synthesized and characterized. The effect of donors on optical, electrochemical and thermal properties of carbazole-dicyanovinyl dyads is critically analysed. The dyes containing C3-linked carbazole donor showed red-shifted absorption and larger molar extinction coefficients when compared to the dyes containing N-phenylcarbazole attributable to the extended conjugation of C3-linked N-butylcarbazole. All the compounds showed positive solvatochromism in fluorescence attributable to intramolecular charge transfer in the excited state. The higher thermal stability of the dyes suggests the rigidity of the carbazole derivatives. Interestingly, all the dyes containing N-phenylcarbazole showed aggregation-induced emission properties in THF-H2O mixtures attributable to increased distortion in molecular configuration and restricted rotation in the solid-state. Furthermore, a green OLED device fabricated with the dye containing N-butyl carbazole donor as dopant emitter (3%) in CBP host achieved good performance with high external quantum efficiency (4.2%), current efficiency (13.1 cd A-1) and maximum brightness (Lmax) of 6180 cd/m2.