Improvement Of Solid State Cathodoluminescence Intensity And Research On Interfaces Structure

The main researches focus on the several methods to improve the intensity of solid state cathodoluminescence. The interfaces between small molecular and polymer were studied by using ultraviolet photoemission spectroscopy (UPS). Investigations by atomic force microscopy (AFM) revealed morphology of organic materials on different substrates. The detailed researches on improvement of intensity and efficiency of solid state cathodoluminescence can be divided into three steps:The first step includes two research topics: the source of primary electrons and materials for accelerating electrons. From the transient spectra, we analyzed the source of the primary electrons and electrons acceleration ability of ZnS and SiO₂. These experimental results show the fact that primary electrons come from electrode, the electrons acceleration ability of SiO₂ is stronger than that of ZnS.The second step starts from the research on intrinsic factors which influence the intensity and efficiency of luminescence. We find out the applicable condition of molecular theory and band model for electroluminescence from solid state cathodoluminescence. The demarcation is the appearance of field ionization of excitons. The excitons emission obeys molecular theory and the recombination emission obeys band model. After consideration of luminescence lifetime of organic materials is on ns scale, we chose inorganic material with longer luminescence lifetime to study the effect of lifetime on intensity and efficiency under different driving frequency. Absorption is the underlying reason for the change of luminescent traits. And the lifetime influences on the absorption in turn. So the lifetime should influence the luminescent intensity and efficiency.The third work originates from our one idea about mixed exciation on organic materials. Firstly, we demonstrate that the electroluminescence mechanism of our studied rare earth complex is charge carriers captured by the ligand and then energy transferring from the ligand to rare earth ions. We introduce the ZnS into organic electroluminescence devices as electrons functional layer and hole blocking layer. The luminescence intensity and efficiency was improved by using the first and second electric properties of ZnS. For the inorganic-organic combined devices, there are two kinds of excitation mechanisms: injected recombination and impacted excitation. Due to the 5d electrons of rare earth ions shielded by 4f electrons, it is very difficult to absorb energy from environment for 5d electrons. The intramolecular energy transfer from the ligand to rare earth ions is sole way to excite rare earth ions. However, if the energy transfer from the ligand to rare earth ions is not very efficient, the electron on the excited state of ligand may transfer to the ground state of host molecular. In the case of electroluminescence of Eu(o-BBA)3(phen), there is a new emission peak at 494nm. After detailed analysis, this emission should be attributed to electrons transition from the excited state of ligand to the ground state of host materials. This emission influences the color purity of emission from rare earth ions. In order to restrain this emission and obtain high intensity of red electroluminescence, a high efficient red dye (DCJTB) and Eu(o-BBA)3(phen) were co-doped into PVK. In the co-doping system, we obtained high intensity of red emission without PVK emission and 494nm emission. The EL intensityarrived to 650cd/m² when driving voltage is 20V.Interface energy alignment is one basic and key problem for organicelectroluminescence. It directly influences the injection and transporting of charge carriers. We found that the hole injection barrier for C₆₀ was drastically larger on P3HT pre-coated PEDOT:PSS (1.7 eV) compared to pristine PEDOT:PSS (0.3 eV). This was facilitated by a large dipole (-1.0 eV) formed at the P3HT/PEDOT:PSS interface underneath the C₆₀ layer. This effect of deposition-sequence-dependent energy level alignment needs to be considered when using multi-heterolayers in organic functional devices. We attribute this to the fact that pentacene shows virtually the same interfacial dipole when deposited on PEDOT:PSS as does P3HT, and to a similar growth of PEN on both polymeric substrates. However, the valence band onset of P3HT was closer to EF on PEDOT:PSS (0.2 eV) compared to PEN pre-covered PEDOT:PSS (0.55 eV). The ionization energy of P3HT on PEN/PEDOT:PSS was increased by almost the same amount. This was explained by different P3HT inter-molecular packing on the two substrates, which was corroborated by the observation by AFM of a disordered P3HT film on PEDOT:PSS and nanoscopic P3HT-related crystallites on PEN/PEDOT:PSS.Long axis of pentacene molecular is vertical with the surface of Si wafer. When the thickness of pentacene thin film goes beyond 10nm, the morphology keeps constant.