The Study On Aggregate Structure Of Organic Optoelectronic Materials By Raman Spectroscopy

In recent years,the molecular microstructure of organic semiconductor amorphous films has been considered as a key parameter to determine the performance of organic optoelectronic devices.Macrocosm is the appearance of microcosm and its nature is determined by microcosm.Organic semiconductors have been widely used,and further improvement of performance requires the development of molecular to mesoscopic scale characterization methods,in which Raman spectroscopy is a simple and effective tool to detect the molecular microstructure.Raman spectroscopy is particularly suitable for environment-sensitive organic semiconductors（π-electron delocalization systems）due to changes in molecular polarizability.The polarization dependence of Raman scattering provides a method to detect molecular orientation.Raman spectroscopy has non-destructive characteristics and can be used to detect organic optoelectronic devices in working state and obtain in-situ device information.In this paper,the conformational transition process（second order phase transition）and the orientation of molecules in crystal and thin film are studied based on Raman spectrum scattering technique.Through multispectral imaging test,the relationship between microscopic changes and Raman spectra in organic light-emitting diodes was established,and the relationship between microscopic changes and macroscopic properties was explored.The work of this paper includes the following three aspects:1.Two p-phenylenethane molecules,BPCNDSB[（2Z,2’Z）-3,3’-（[1,1’-biphenyl]-4,4’-diyl）bis（2-phenylacrylonitrile）]and PBNA[（2Z,2’Z）-3,3’-（1,4-phenylene）bis（2-（naphthalen-2-yl）acrylonitrile）],were detected by in-situ thermometric Raman spectroscopy.The microstructure of single crystal was studied by in-situ detection.During the cooling process,the entire molecule of BPCNDSB single crystal gradually becomes more twisted from the tendency plane structure,and its phase transition temperature is 130 K observed through the temperature-dependent Raman peak.The conformation at the center of the single crystal is more stable,and the crystal quality is better than that at the edge.The quantitative relationship between Raman scattering intensity and intramolecular torsion angle and intermolecular hydrogen bond distance was established by Raman spectroscopy for PBNA single crystal with similar molecular structure.2.Based on the uniaxial crystal and uniaxial orientation model,the dependence of Raman scattering intensity and molecular orientation angle was studied by using polarization Raman spectroscopy,and the relationship between Raman intensity and molecular deflection angle and orientation angle was established（I_s∝K₁cos⁴βsi n⁴θ+K₂cos²βsin² 2θ）.The relationship between Raman peak strength and molecular orientation angle was obtained by correlation calculation（I_f=3/8A₁sin⁴（θ+A ₄）+1/2A₂sin² 2（θ+A ₄）+A₃）,and the average orientation angle and its distribution of the long axis of molecules in the film were obtained through the relationship.With the increase of BPCNDSB film thickness,molecular orientation angle increases.3.A confocal micro Raman spectrometer was used to conduct multispectral imaging tests on the same microregion of organic light-emitting diodes under working conditions,including Raman spectrum,PL（Photoluminescence）spectrum and EL（Electroluminescence）spectrum.According to the three spectral analyses,the causes of dark regions in EL spectrum may be different.The luminance of the device is affected by the thickness of the active layer and the degree of molecular aggregation.Large thickness of the active layer or high degree of molecular aggregation will lead to the decrease of EL luminance.Both of them have spatial distribution in the device,resulting in spatial inhomogeneity of EL image.The area with large thickness and high degree of aggregation has large resistance,and the current passing through this area is small when the device is working,so it is not easy to form degradation.The area with loose structure and small thickness has small resistance,and the current passing through this area is large in the process of device operation,which is easy to form local overheating and increase the possibility of chemical bond fracture.