The Study Of The Thermal Stability And Molecular Orientation In Small Molecular Organic Semiconductor Thin Films

In recent years,organic semiconductor-based optoelectronic devices have developed rapidly and have gradually appeared in our lives.The most representative one is the organic light-emitting diodes(OLEDs)technology.It has achieved commercial success in the display market.The most basic units in an organic semiconductor device are organic amorphous thin films.Therefore,the fundamental physical properties of these organic amorphous thin films have a crucial influence on the device performance.Generally,there are two methods fabricating these organic amorphous films into a device:solution method and the physical vapor deposition method.At present,it has been proved that physical vapor deposition is a better choice to ensure the quality of thin-film devices.In addition,compared with solution methods,organic small molecular films prepared by physical vapor deposition have special properties that are not available in any other preparation method:physical ultra-stability and molecular orientation.This particularity determines that organic small molecular films have a unique influence on the performance of organic semiconductor devices,so the basic properties of organic small molecular films and their effects on devices are still a noting topic in the field of organic semiconductors.This thesis will explore two issues:the glass transition temperature in organic small molecule films and the thickness effect of its molecular orientation.First,the glass transition temperature has been proved to have a significant impact on device stability such as device thermal stability,device life,and the inter-diffusion between films.However,when discussing the glass transition temperature,every work is based on the result obtained by the differential thermal analysis method in the bulk state.The deviation between the glass transition temperature in bulk or thin-film state has not been adequately discussed.In addition,the glass transition in the organic small molecular composite film is still blank in the literature.In this thesis,we have extended the ellipsometry technique from measuring the glass transition temperature of polymer films to the organic small molecular films.We found that organic small molecular films exhibit a disruptive glass transition which is similar to the crystal phase transition,and the intermediate temperature of this disrupt region is defined as this special glass transition temperature,and the validity of this definition have been also discussed.This method then is applied to study two-component composite film systems:non-interacting organic/organic small molecular composite films and interacting organic/C₆₀ composite films.The corresponding equation for predicting the glass transition temperature in each system is proposed.An OLED device with better thermal stability was prepared by using the organic/C₆₀ small molecular composite film as a hole transport layer,and corresponding empirical theories were proposed to describe the glass transition temperature in composite thin-films and the relationship of device failure temperature.In addition,since organic small molecule films have been considered as completely disordered amorphous films for a long time,until recently,people have found that they have a preferred orientation,but currently,due to the complexity of amorphous films and the limitation of testing methods,the molecular orientation is assumed to be uniform throughout the film thickness.However,we suggest that the molecular orientation is varied throughout a thin film,and this inconsistency will be reflected in that the molecular orientation of the thin film has a certain dependence on the film thickness.Therefore,in this thesis,the dependence of the molecular orientation and glass transition temperature on the thickness of small organic molecular thin films is studied by using the variable angle spectral ellipsometry,and a theoretical model of a tri-layer structure is proposed based on the original theory of molecular orientation.The corresponding discussion and empirical equation are also presented.We believe that the research in this thesis can provide experimental results or theoretical flames for the design,application and performance improvement of organic semiconductor devices fabricated by physical vapor deposition.