Design, Synthesis, And Semiconducting Properties Of Indolo[3,2-b] Carbazole Derivatives

Organic semiconductors offer potential advantages over conventional inorganic semiconduxtor due to their easy processing,lower cost,and flexibility.Organic semiconductors based on arylamine have been extensively studied and applied as hole-transporting materials for optoelectronic applications.Among arylamine semiconductors,indolo[3,2-b]carbazole is an appealing system for studies because it has a relatively large and planar molecular structure to enable facile establishment of higher structural order for charge transport.It has a large band gap and lower lying HOMO than those of most semiconductors,and is therefore expected to be environmentally far more stable.The main contribution of this thesis is exploring the new organic semiconductors based on indolo[3,2-b]carbazole.Crystal structures of the indolo[3,2-b]carbazole-based organic semiconductors were determined and studied carefully.The electrical,optical and other properties and potential applications of these materials and devices in organic electronics were discussed in detail.The primary results are as follows;(1)We have designed and synthesized a series of new organic semiconductor materials based on indolo[3,2-b]carbazole through strategic peripheral substitutions. The introduction of long alkyl chains to N-5 and N-11 positions of indolo[3,2-b]carbazole can not only improve the solubility of the compounds in common solvents but also promote the molecular self-organization of indolo[3,2-b]carbazole derivatives to form high order films during vacuum deposition. The introduction of aryl groups to the 2,8-positions of indolo[3,2-b]carbazole can enhance the length ofÏ€-conjugation,then improve the charge transport performance.(2)Eight single crystals of indolo[3,2-b]carbazole-based organic semiconductors were obtained and their crystal structures were determined by X-ray crystallographic analyses to study the substituent effect on molecular conformation,packing characteristics,and the correlations between the crystal structure and the electrical characteristics.X-ray structure analysis of the single crystals reveal that the inner indolo[3,2-b]carbazole plane displays a quasi-planar structure with very small dihedral angles between each rings of indolo[3,2-b]carbazole.There exists two kinds of packing patterns in the packing diagrams of indolo[3,2-b]carbazole derivatives; herringbone packing andÏ€-Ï€packing.This packing character will facilitate charge transport.(3)The optical,electrical and thermal properties of the indolo[3,2-b]carbazole-based organic semiconductors were carefully studied.All the indolo[3,2-b]carbazole derivatives exhibited high thermal stability due to the large rigid molecular structure of indolo[3,2-b]carbazole.And they showed higher LUMO energy levels and lower HOMO energy levels,which indicated that they possessed excellent environmental stabilities.(4)Organic light-emitting diodes(OLEDs)using indolo[3,2-b]carbazole-based organic semiconductors as hole-transporting materials were fabricated.Alq₃ was used as electron-transporting and emitting layer.All the devices emitted green light.The devices using the triphenylamine-substituted or fluorene-substituted indolo[3,2-b]carbazole derivatives as hole-transporting materials showed good EL performance,while that using carbazole-substituted derivatives showed poor El performance.This results indicated that indolo[3,2-b]carbazole-based organic semiconductors can be served excellent hole-transporting materials for OLEDs and the substituents have great effects on the hole-transporting performance of indolo[3,2-b]carbazole-based hole-transporting materials.The results also indicated that the substituent positions have great effect on the hole-transporting performance.(5)Organic field-effect transistors(OFETs)using indolo[3,2-b]carbazole-based organic semiconductors as channel materials were fabricated.All the compounds exhibit p-type organic semiconductor characteristics with high mobility and high on/off ratio than those of unsubstituted indolo[3,2-b]carbazole compounds.The introduction of 4-trifluoromethylphenyl couldn't convert indolo[3,2-b]carbazole to be n-type semiconductor because the LUMO energy levels of indolo[3,2-b]carbazole could not be greatly lowered by 4-trifluoromethylphenyl groups.The charge transportat capabilities could benefit from theÏ€-extension of the indolo[3,2-b]carbazole unit by introducing aryl groups at the 2,8-positions of indolo[3,2-b]carbazole.