CYANOCARBAZOLE-BASED ORGANIC MATERIALS FOR ELECTRONIC APPLICATIONS

Abstract

Organic materials have received immense attention in the last few decades owing to their utility in optoelectronic applications particularly organic light emitting diodes (OLED) and photovoltaics. The OLED serves as potentially energy saving technology when compared to their inorganic counterpart light sources such as liquid crystal displays and incandescent sources. Further, they provide low cost and environmental sustainability over their hazardous inorganic counterparts. The wide range of availability of organic chromophores and their facile functional modification leads to the development of library of organic emitters exhibiting primary colors such as red, green and blue. Despite of benchmark red and green emitters, blue emitters showing high efficiency and long durability are scarce in the literature owing to their associated wide band gap and improper charge injection and transportation. Thus, the pursuit of developing highly efficient blue emitters exhibiting good charge transport and better morphological stability is indispensable. In general, the efficacy of functional chromophore decides the fate of OLED device in terms of efficiency and durability. Carbazole is considered as a potential building block for the construction of fluorescent emitters since they possess high charge carrier mobility, good quantum yields, wide band gap and good electronic conjugation. The facile functionalization at various nuclear position of carbazole opens promising pathway for developing number of organic materials with tunable properties. The materials exhibiting bipolar character are desirable for realizing high efficiency because they provide balanced charge mobility. Generally, the incorporation of electron withdrawing group on carbazole results in bipolar nature. The modification at 3,6-position of carbazole gives good redox properties and the insertion of electron withdrawing group induces donor-acceptor interaction. The detrimental donor-acceptor interaction must be controlled otherwise which would quench the emission. On the other hand, 2,7-substitution of carbazole suppresses donor-acceptor interaction while providing extended conjugation. So, the carbazole-based materials containing 2,7-substitution got significant attention. This thesis is classified into seven chapters. Chapter 1 presents the review of functional derivatives of carbazole in OLED applications as hole transporter, fluorescent emitter, thermally activated delayed fluorescence (TADF) emitter and phosphorescent host. Carbazole substitution at 3,6-position provides good redox properties, 2,7-substitution gives extended conjugation while 1,8-position results good morphological stability. However, N-functionalization did not iv alter conjugation length, though increased the solubility of the materials. Chapter 2 deals with the aim and scope of the thesis in which cyanocarbazole-based derivatives are developed for fluorescent OLED applications. Chapter 3 to Chapter 6 presents the carbazole-based materials containing electron withdrawing cyano group and different chromophoric moieties as fluorescent emitters. The highlight of the thesis can be summarized as below. Chart 1. Pictorial representation of the work done in this thesis. In chapter 3, asymmetrically 2,7-substituted carbazole derivatives are presented as fluorescent emitters. The photophysical properties of the dyes are dependent on the nature of chromophore attached. Further, the effect of acetylene/vinyl spacer between cyanocarbazole and the chromophore on optical, electrochemical and electroluminescent properties is elucidated. The absorption maxima of the compounds depending on the linker are in the order of direct < acetylene < vinyl. Carbazole/triphenylamine substituted dyes showed positive solvatochromism in emission spectra which is due to intramolecular charge migration from donor to acceptor. Electrochemical studies revealed high oxidation propensity for v triphenylamine substituted compounds among the substituents and for vinyl spacer among three different linkers. The high onset thermal decomposition temperature of carbazole substituted dyes is attributed to the presence of two rigid carbazole molecular back bones. All the dyes showed deep blue electroluminescence emission with the CIE coordinate of 0.15≤ x ≤0.17 and 0.04≤ y ≤0.10 while vinyl derivatives carbazole, triphenylamine and pyrene showed 0.12≤ y ≤0.33. The best electroluminescence performance is realized for the device fabricated with a dye containing triphenylamine chromophore directly connected to cyanocarbazole chromophore exhibiting the maximum EQE of 4.1%. In chapter 4, cyanocarbazole end-capped rigid and rod-like fluorophores containing fluorene or carbazole bridge are described. The optical and electroluminescence properties of the dyes are tuned by altering the bridge and the position/mode of linkage. The absorption maxima depends on the nature of bridge and follows the order 3,6-carbazole < 2,7-fluorene < 2,7-carbazole. The linear carbazole dyes displayed high thermal robustness when compared to fluorene analogs which is ascribed to three rigid carbazole units. Linear dyes showed typical π-π* transition while V-shaped carbazole dyes showed intramolecular charge transfer transition. The largest Stokes shift is observed for directly connected dyes which indicate the involvement of high degree of structural reorganization of the dye molecules in the excited state. Despite of similar HOMO energy levels, dyes containing acetylene spacer showed low lying LUMO over directly connected dyes. It supports the electron withdrawing nature of acetylene spacer. The cyano end capping is found to be helpful in realizing balanced charge transport and thus better electroluminescence performance when compared to the known compounds in the literature. The devices fabricated with the dyes as emitting dopants in CBP host system showed deep blue emission with CIEy of 0.04. The electroluminescence performances of these dyes are superior to their analogous dyes containing only one cyanocarbazole unit (Chapter 3). In chapter 5, pyrene appended cyanocarbazoles connected through acetylene spacer are presented. The optical and electroluminescence properties of the dyes are drastically changed as the number of cyanocarbazole units increased on the pyrene core. Particularly, 1,6- and 1,8-disubstituted dyes exhibited different optical and electroluminescence properties. 1,6-disubstitued dye showed hypsochromic absorption maxima when compared to mono-substituted dye while 1,8-disubstituted dye showed bathochromic shift. The solvent independent absorption and emission profiles of the dyes indicate the absence of donor-acceptor interactions. The vi electroluminescence emission is tuned from deep blue to green color depending on cyanocarbazole density. The power and current efficiency of the dyes are linearly increased as the number of cyanocarbazole units increased on the pyrene core. It clearly indicates the role of cyanocarbazole in realizing the balanced charge transport and thus better device performance. In chapter 6, a series of donor-acceptor configured arylamine donor substituted cyanocarbazoles linked through acetylene spacer is developed. The π electronic conjugation of triphenylamine dyes is increased as compared to their carbazole analogs. This leads to enhancement in thermal stability for them over rigid carbazole counterparts. The compounds containing arylamine acetylene at 2,7-position of core carbazole displayed negative absorption solvatochromism while the other isomers showed solvent independent absorption profiles. The positional variation of donor and acceptor tuned the optical and electroluminescence properties of the dyes. Despite of similar HOMO energy levels, 1,8-arylamine substituted dyes showed wide band gap over 2,7-substitued congeners which is due to the steric nature of the former. The high lying HOMO energy level of triphenylamine substituted dyes align well with the neighboring charge transporting layer which favors facile charge injection. As a result, low turn-on voltage and superior electroluminescence performance is observed. The best performance is obtained for the dye containing 3,6-dicyano and triphenylamine at 2,7-position of carbazole (3 wt%) exhibiting power efficiency, current efficiency and EQE of 5.3 lm/W, 6.7 cd/A and 4.1% respectively. In chapter 7, summary of thesis is presented. The structure-property relationships among the dyes established in the thesis is critically analyzed. The outcome is used as a basis to predict the development of conceptually advanced emitters.