Study On Structure Of Weak Epitaxy Growth Thin Films Based On Phenanthroimidazole Luminescence Material

Organic semiconductor thin films have the characteristics of low cost,good flexibility,and easy large-area processing,and are the core of preparing organic optoelectronic devices.The thin film morphology of organic semiconductors plays a decisive role in device performance.Organic crystals have been broadly applied in optoelectronics,particularly in organic light-emitting diodes(OLEDs),because of their highly ordered alignment and thermal stability,which help to improve carrier mobility and light outcoupling efficiency.Weak epitaxy growth(WEG)is a simple and convenient physical vapor deposition method,which can prepare low-defect,large-size continuous crystalline thin films,and provides technical conditions for the preparation of crystalline thin-film OLEDs(C-OLEDs).The phenanthroimidazole material 2FPPICz has the characteristics of wide bandgap,bi-polar transmission and deep-blue light emission.Through the WEG process,the C-OLED device prepared by using it as the main frame of the crystalline film has achieved efficient blue light emission.This is all attributed to the high light output and fast exciton recombination brought about by the high carrier mobility of 2FPPICz crystalline thin films.Therefore,2FPPICz is an important matrix for the development of C-OLEDs.It is of great theoretical and practical value to study the film morphology,orientation structure and growth behavior of 2FPPICz epitaxial systems.In our paper,the phenanthroimidazole material 2FPPICz was used as the object of study,and its single crystal structure was studied.As well as the morphology,molecular orientation and growth behavior of the 2FPPICz crystalline thin film prepared by WEG method were studied,which provide the film structure basis for the development of COLEDs.The specific content of this paper is as follows:1.Taking phenanthroimidazole material 2FPPICz as the research object,its single crystal structure is studied,which is the basis for the study of the thin film structure.Then the film morphology of the p-6P/2FPPICz epitaxial system was studied.Firstly,the morphology of 2FPPICz on different thicknesses p-6P films was studied.2FPPICz can only epitaxial growth form a continuous 2FPPICz crystalline film when the p-6P double-layer is completely covered.Then,the structure of the epitaxial film was tested and analyzed.There is a commensurate epitaxial relationship between 2FPPICz and p6P.Under the combined effect of channeling effect and lattice matching,the molecular long axes of 2FPPICz form an angle of 11° with the[110]and[110]channels of p-6P for orientation arrangement,respectively.2FPPICz molecules are aligned on the substrate in an edge-on manner,and the long axis of the molecules is parallel to the substrate,which is completely different from the vertical growth of epitaxial molecules in the past.On account of the transition dipole moment is parallel to the molecular long axis,the transition dipole moment of 2FPPICz molecules is parallel to the substrate,which is advantageous to improve the light outcoupling efficiency of the device.2.Taking 2FPPICz crystalline thin film as the research object,the hole transport layer BP IT used in C-OLEDs device was selected as the inducing layer,and the film morphology of the BP1T/2FPPICz epitaxial system was studied.Firstly,the morphological structure of BP1T thin film was studied.Then the morphology and structure of 2FPPICz epitaxial films on BP IT monolayer and double-layer films are compared.2FPPICz can epitaxial grow on both BP IT monolayer and double-layer films to form strip-like crystals.However,the film roughness on BP1T double-layer is significantly smaller than that on the monolayer,and the width of the 2FPPICz crystals is significantly larger than that on the monolayer.Through the test and analysis of the 2FPPICz epitaxial film structure,it is obtained that there is also a commensurate epitaxial relationship formed between 2FPPICz and BP 1T.Along the short axis direction of the strip-like crystal,the lattice mismatching on the BP 1T double-layer is significantly smaller than that on the monolayer.Therefore,in this direction,a larger number of 2FPPICz unit cells are allowed to stack on the BP 1T double-layer until reaching the upper limit of the lattice mismatching.So the width of the strip-like crystals on BP 1T double-layer is larger than that on monolayer,which is beneficial to reduce the film defects.Similarly.under the effect of channel effect and lattice matching,the molecular long axes of 2FPPICz form an angle of 8° with the[110]and[110]channels of BP 1T for orientation arrangement,respectively.and the molecular long axes are also parallel to the substrate.3.Taking 2FPPICz crystalline thin film as the research object,the growth behavior of epitaxial thin film was deeply studied.For the 2FPPICz films growing at different rates,when the growth rate is less than the transformation rate,the 2FPPICz films show obvious dewetting phenomenon,and the monolayer film will be transformed into multilayer films.For 2FPPICz films growing at different temperatures,metastable 2FPPICz films transition to a more stable crystalline phase with the increase of substrate temperature.When the crystalline phase of 2FPPICz appears,the continuity of the film becomes worse,and there will be gaps between the crystalline domains,leading to the defects of the film.Therefore,the film growth process is controlled by both dynamic and thermodynamic processes.When the dynamic process of film growth is dominant,continuous crystalline films can be obtained.On the surface with highly ordered geometric channels,the diffusion of molecules is anisotropic,which promotes the anisotropy of the nucleation and growth of epitaxial 2FPPICz molecules,forming striplike crystal nuclei with obvious orientation.As the thickness increases,multi-layer films grow at the same time,but the previous film always completes the growth before the subsequent film.The orientation does not change during the growth process,and finally forms a large-size continuous crystalline film with high orientation.The 2FPPICz epitaxial crystalline film can also be formed on the ITO substrate,whose film structure is consistent with that on Si/SiO2.And the molecules are aligned horizontally.