| Excitonic processes dominate the electric and optical properties of organic materials. From delocalized charge-transfer (CT) excitons in very closely packed organic molecular crystals (OMCs) to localized Frenkel excitons in loosely packed amorphous organic solids, they determine material characteristics such as absorption, photo-conduction and luminescence.; Recently, organic light emitting displays (OLEDs) and organic thin film transistors whose functionality partially depends on fundamental excitations have attracted substantial interest due to their unique properties unattainable with conventional semiconductors. To optimize the device performance such as shifting absorption peak wavelength, enhancing current injection, controlling channel conduction, maximizing electroluminescence (EL) efficiency, and obtaining saturated red, green and blue emission colors, it is essential to understand how excitonic processes is modified in organic nanostructures. In this work, we examine CT and Frenkel excitons in OMCs and amorphous organic materials.; A quantum mechanical model is developed to study electrooptical properties of delocalized CT excitons in closely packed OMC nanostructures. Based on this model, we analyze the electroabsorption (EA) spectrum in bulk PTCDA and the absorption spectral shifts in PTCDA/NTCDA multilayers, and obtain consistent values of effective masses and exciton radii along difference crystalline axes. The same treatment is extended to fit the GaAs EA spectrum, suggesting a common physical origin for both CT and Wannier excitons.; We also examine the Frenkel excitons in more decoupled amorphous organic solids used for OLEDs. Assuming an exponentially distributed trap states in the lowest unoccupied molecular orbital and the highest occupied molecular orbital gap, we are able to explain current-voltage characteristics and EL efficiencies observed under various temperature and charge injection conditions. We infer that the traps are due to molecular polarons, which also determine the energy distribution of excitons, and hence the EL emission spectrum.; To utilize some unique properties of OMCs, we demonstrate a vertically stacked, three color OLED which allows for independent tuning of color, gray scale and intensity. The radiative recombination of Frenkel excitons is influenced by the heterogeneous multilayer structure via microcavity interference. Controlling the recombination environment by varying the layer thickness, and emissive layer positions, color saturation and EL efficiency can be optimized. |
