Investigation On The Self-Assembly And Optical Properties Of Organic Semiconductor Molecules

Organic semiconductor materials have been widely applied in a series of organic optoelectronic devices such as organic light-emitting diode(OLED),organic field-effect transistor(OFET),organic solar cells(OSCs),and organic solid-state lasers(OSSLs),due to the inherent merits such as tailor-made molecular structures,solution processing,the high solid photoluminescence(PL)quantum-efficiency and so on.In recent years,with the development of the miniaturized and integrated devices,the organic micro-/nano-crystals show high application potential.However,the self-assembly mechanism of the organic semiconductor molecules is complicated,which lead to the low controllability of the organic microcrystals in terms of morphology,optical properties and other physicochemical properties.Furthermore,the fabrication of the organic photonic devices based on the organic microcrystals is also challenging.In this thesis,based on the organic microcrystals as well as the photonic applications,we conducted our researches in terms of modulation of luminescence properties,morphologies and molecular packing modes,and the preparation of homo/hetero-structures microcrystals as well as the fabrication of waveguide devices.The main content can be divided into the following sections:In the first section,the modulation of the crystal luminescence properties based on the doping method was investigated.We successfully synthesized the green emissive tetracene and yellow emissive dibenzo[a,l]pentacene(DBP)into the 1,4-bis((E)-2-methylstyryl)benzene(o-MSB)plate microcrystals.Through the fluorescent resonant energy transfer(FRET),the blue emissive plate microcrystals can emit strong green light and yellow light.Under a proper DBP doping ratio(1.5%),the white emissive microcrystals can be successfully prepared(CIE coordinate:0.308,0.359).These white-emissive microplates can potentially be applied in the future optoelectronic system as the white emitters.In the second section,the modulation of the crystal morphologies by a protonic reaction was studied.The lone pair electrons on the nitrogen atom of the pyridine group were utilized to generate the protonic reaction with the protonic acid,which would finally change the original plate shape into the rod-like morphology.Meanwhile,the blue emission light can be converted to the yellow and red light.In the third section,we further successfully prepared two kinds of perylene crystal phases that belong to different molecular packing modes:H aggregate mode and J aggregate mode,which finally change the morphology and the optical properties.The emission colors of the two crystal phases are yellow and green,respectively,and the radiative rates are 0.02 ns-1 and 0.83 ns-1,which are consistent with the prediction of current molecular packing theory.What's more,we also prepared the doped perylene microcrystals by multi-doping of the two phases with adjustable evaporation rates.The investigation on the momlecular pakcing mode can offer deeper understanding on the properties-modulation of the orgaic microcrystalsIn the forth section,the controllable preparation of the homo-/hetero-structures was successfully demonstrated.Based on the previous research,we chose the high fluorescent molecule o-BCB,a derivative of oligomeric phenylene vinylene(OPV),and perylene to fabricate the complicated crystal structure.Through the room-temperature solution method,by modulating the supersaturation degree in the mixed solvent,the comb-like o-BCB secondary structure consisted of many single crystals can be prepared rather than the isolated single crystals.By adding the perylene into the solution,the complex structure composed of the perylene single crystals and the o-BCB microcrystals can be generated.What's more,the waveguide based on the o-BCB homo-/hetero-structures were successfully fabricated.The unique asymmetric waveguide properties in the homo-structures and the switchable out-coupled light wavelength in the heterostructures were foundIn summary,this thesis provided a systematic investigation on the modulation of physical/chemical properties of the organic micro/nano-crystals,such as the morphologies,the luminescence and the molecular packing modes.The performance of the corresponding waveguide devices was then studied.Our results offered a theoretical foundation of the rational design and controllable preparation of organic micro/nano-crystals for the practical applications.