Characterization of Local field effect in organic film using pressure technique

Abstract

This thesis proposes and demonstrates a pressure probing technique for studying the effects of local dielectric changes on the excitonic energy levels in amorphous organic thin films for optoelectronic device applications. Compression of organic films causes a decrease in intermolecular spacing and, through solvation effects, lowers the exciton transition energy. A series of steady-state photoluminescence (PL) measurements performed on doped organic thin films demonstrated the applicability of pressure probing in measuring solvation effects, and fitted to solvation theory. Since a pressure probing technique eliminates composition differences and sample-to-sample variability, in comparison with doping methods, it may be a simpler method of observing energy shifts in solvation effects. Further investigation into spectral diffusion for films under compression indicates a change in spectral diffusion rate due to change in molecular packing density. Comparisons were made between spectral diffusion rates for films under pressure and films of different doping concentrations. Initial measurements of pressure effects on exciplex charge-transfer states in bulk heterojunction films are performed to show change in emission lifetimes. This work could provide a better understanding of the singlet-triplet exciton coupling rates and have a significant impact on device optimization for organic light-emitting diodes (OLEDs) and solar cell applications.