Preparation And Photoelectronic Performance Of High-quality Cyano Group Substituted Phenylene Ethylene Organic Single Crystals

In recent years,organic single crystals?OSCs?have been widely studied in optoelectronic devices such as organic light-emitting diodes,field-effect transistors and organic lasers on account of many advantages.First,OSCs are more stable than the polycrystalline and amorphous films states of the same molecule structure in both thermodynamical and photochemical properties.Second,the OSCs own higher carrier mobilities for the reason of their long-range ordered structure.Last but not least,organic crystals with accurate structures can provide exact models for us to study the relationships between the material structures and their optoelectronic properties.In this thesis,OSCs based on cyano-substituted p-phenylene ethylene compounds were used as the research objects.Starting from the growth and formation aspect of crystals,optical properties and the performance of organic light emitting transistor devices based on OSCs were studied in details.The works can be summarized into the following three parts.In the first chapter,the growth methods of crystal,growth theories of crystal,laser properties of organic crystals and their applications in field-effect transistors?FETs?were introduced.In the second chapter,the preparations of OSCs and their growth mechanisms were discussed.High-quality OSCs of two kinds of materials PBNA:?2Z,2'Z?-3,3'-?1,4-phenylene?bis?2-?naphthalen-2-yl?acrylonitrile?and BPCNDSB:?2Z,2'Z?-3,3'-?[1,1'-biphenyl]-4,4'-diyl?bis?2-phenylacrylonitrile?were prepared by the physical vapor transport?PVT?method and their structures were resolved.By preparing crystals from PBNA crystals,a new kind of crystal with regular distorted structure was obtained for the first time.This special structure was carefully discussed and the possible formation mechanism was proposed.The organic molecules might had formed their stable four-clusters in the vapor phase and then moved towards to the crystal phase by the structure of clusters,to form the new crystal.In the third chapter,the optical properties of the high-quality OSCs prepared were studied.To investigate the effects of molecular stacking structures on the optical properties,two different phases of PBNA crystals and crystals of BPCNDSB were taken as research models to investigate the amplified spontaneous emission?ASE?properties under the optical pump?high energy pulsed laser?.The orange PBNA crystals in distorted structures did not exhibit the ASE phenomenon under optical pump.While the green PBNA crystals exhibited excellent ASE properties with the full width at half maximum?FWHM?of 4.41 nm and threshold value of 21?J cm^-2.BPCNDSB crystals also exhibited outstanding ASE properties with the FWHM of 4.80nm and threshold value of 1.9?J cm^-2.By the ultrafast time-resolved transient absorption spectra?TA?,we investigated the photophysical processes of crystals in the excited states.By kinetics analysis of spectra,we discovered that the BPCNDSB crystals in the excited states had experienced the intramolecular electron transfer processes and the recombination processes,by which we can accumulate the basic optical physical data of crystals in such molecular structures.It is expected to be helpful for the design and optimization of such materials in the future.In the fourth chapter,the electronic properties of PBNA and BPCNDSB crystals were studied.The same structrue field-effect transistors devices based on two phases PBNA crystals were fabricated,to explore the electronic differences influenced only by structural differences of molecular packing.Through the study,we discovered that the two phases of PBNA crystals both exhibited bipolar charge transport property in the devices and the green phase crystals preferred to exhibit the electron transportation property.The mobilities of the orange phase crystals were two orders of magnitude smaller than that of the green ones by calculation.It was probably caused by the twisted structure in orange phase crystal broke the long-range molecular packing and hindered the charge transport ability.Besides,in the devices of green crystals of PBNA and crystals of BPCNDSB,the electroluminescence phenomena were observed.When OLETs based on the green crystals of PBNA operated under ambipolar model,the emission line was able to be controlled within the channels by carefully adjusting the applied gate and drain voltages.The maximum external quantum efficiency?EQE?of electroluminescence device reached 3.3%.In a word,through investigating the growth mechanisms and photoelectronic properties of PBNA and BPCNDSB materials,we put forward the conjecture that organic molecules might had formed the stable clusters structures in vapor phase and then moved towards into the crystal phase by the clusters structure.We also discovered the ASE phenomenon under the optical pump and luminescence phenomena in the electric devices of the two materials.At present,the mechanism of luminescence phenomenon under the condition of optical pump,electric injection and differences between them are not well understood.Since the green phase crystals of PBNA and crystals of BPCNDSB had both the ASE phenomenon under the light pump and luminescence phenomenon in the electric devices,they can be regarded as the structural models for the further mechanism-studies between the two processes.