Exciton Processes In Organic Semiconductor Studied By In Situ Photoluminescence Measurements

There has been a great deal of interest and much activity in studying organic semiconductor in the past decades. Such interest has been motivated by a wide variety of applications such as organic light emitting diodes (OLEDs) for flat panel display technology. Although some applications have been demonstrated and began to emerge in market, some underlying elemental processes are still far from being well understood. A clear understanding of processes could benefit optimizing performances of these devices. In this thesis, using in situ organic photoluminescence measurements, we study some of these processes both experimentally and theoretically. The main work in this thesis consists of four parts:1．Exciton diffusion is one of fundamental physical processes for organic semiconductors. A simple and accurate method to study this process is of great interest. Here a method based on in situ PL measurements is presented to drive the most significant parameter describing this process---exciton diffusion length. In this method, two different ways were used: thickness-varying organic light emitting material film deposited on organic quenching film and thickness-varying organic quenching film deposited on organic light emitting material film. Both give the same exciton diffusion length values. A well fit to the value can be achieved by using transfer matrix method to model the interference and absorption effects on excitation and emission lights, and a rate equation to model the exciton processes.2．Metals films are mostly used in OLEDs as electrodes. However, metal-induced luminescence quenching of organic materials so as to restrict the OLEDs efficiency limitations have been widely reported. Previous studies on metallic-cathode-induced luminescence quench produce two different models. One is the contact model in which the quench was attributed to the diffusion of excitons generated in the organic film to the metal/organic interface and their dissociation there; the other is the noncontact model based on interactions between organic dipoles (excited organic molecules) and electron gas in metal, which would induce long-range nonradiative F(?)rster-like energy transfer from the organic to the metal. Here ours shows that the PL data in situ measured from a growing tris-(8-hydroxyquinoline) aluminum (Alq₃) film during its deposition onto an ultra-thin (2 nm) metal substrate can hardly be fitted by either of the models alone and that an appropriate model should take into account all the three exciton processes separately suggested in the two models: diffusion and interfacial dissociation of the excitons and nonradiative F(?)rster-like energy transfer from organic to metal. Using the model, the metal-induced electricluminescence in OLEDs is also calculated.3．Incorporating proper dyes into the organic host of emitter to tune color of emitting light is one of significant technologies of OLEDs. The excitation energy transfer from host (donor) to guest (acceptor) was often described within the dipole-dipole interaction mechanism (F(?)rster energy transfer theory). However, the exciton diffusion in the host probably also plays an important role in the process. Here, excitation energy transfer between Alq₃ and a red dye 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) has been investigated by means of in situ measurements of PL of bilayer Alq₃/DCM films with various Alq₃ thicknesses. It is found that exciton diffusion in Alq₃ and F(?)rster energy transfer from Alq₃ to DCM are contributive to the PL spectra measured. A rate equation model based on these two processes is derived and used to fit experimental results. The F(?)rster energy transfer radius obtained for the system is 4．0±0．5nm Energy transfer in the blend system is also analyzed based on the model. It is found that the exciton diffusion in the host largens the energy transfer range almost 2 times.4．About the metal-induced luminescence effect, besides those phenomoens induced by metal films as metioned above, previous studies have also shown that severe metal-induced PL quench could even occur in the presence a noncontinuous metal film as thin as less than 0．1 nm. The quenching phenomenon experimentally observed was thus attributed to excitons' diffusion to and dissociation at the defect sites where stuck the deposited metal atoms, and noncontact energy transfer was completely ignored. However whether there exist any long-range interactions between excited organic molecules and isolated metal atoms haven't considered by far. Here we present clear evidence for the existence of the interaction: Severe quenching of photoluminescence in dye-doped organic thin-films is observed after submonolayer metal (metal atoms) deposition. Insertion of a spacer layer between the organic film and metal atoms has minor effect on eliminating the quenching, indicating that it is a long-range noncontact interaction. The phenomenon is regarded as a result of F(?)rster energy transfer between organic molecules and metal atoms. The origin of the transfer is the couple between dipoles in excited organic molecules and electron transitions in isolated atoms. Atoms among most elements rather than metals are capable of being involved in such a noncontact energy transfer process.