Exciton Dynamics in Organic Semiconducting Materials

Abstract

World energy consumption is projected to rise dramatically over the next three decades. Presently, over 80% of worldwide energy consumption is derived from nonrenewable sources. In order to meet these demands, increased utilization of solar energy will be required. Organic semiconducting materials are an attractive alternative to the traditional crystalline silicon devices which at present dominate the photovoltaic market. They possess a number of material and physical processes which may ultimately prove for photogeneration of electricity at a cost competitive with conventional nonrenewable fuel sources. However, the power conversion efficiencies of these devices at present limits their widespread adoption. In order to improve their efficiencies, it is important to understand the relationship between chemical and physical characteristics of the molcules with the exhibited excited state photophysics. In this dissertation, the excited state properties of various organic semiconducting materials, such as polymers and small molecules, will be examined. First, in Chapter 3 a series of statistical donor/acceptor copolymers were generated where the monomer unit composition was tuned in order to adjust the absorption properties of the polymer. Ultrafast pump-probe spectroscopy was employed to characterize the effect that changing polymer composition has on the exciton lifetime within these materials. It was found that by tuning the composition of the monomer units, the lifetime could be extended nearly 30 times over that of either neat donor or neat acceptor monomer units. Moreover, the lifetime reached a maximum at a ratio of approximately 1:4 donor to acceptor monomer units, suggesting that fine-tuning the ratio of the two may provide enhancements in OPV performance. In Chapters 4 and 5, ultrafast pump-probe spectroscopy was used to characterize exciton transport in thin films of Subphthalocyanine and Subnaphthalocyanine. Adjusting the film composition was found to have a substantial influence on the exciton diffusion length in the films. Importantly, exciton-annihilation induced heating of the films resulted in the manifestation of thermal signatures in the transient spectra. These signatures were not previously well appreciated in literature as manifesting on a sub-nanosecond timescale, and have been potentially erroneously assigned as electronic signatures from excited state species. A method is proposed to isolate these thermal signatures from the excitonic signatures, yielding accurate exciton decay dynamics. Finally, the excited state dynamics of a series of novel boron dipyrromethane derivatives will be investigated in Chapter 6. Particular attention will be made as to whether there is a potential for these materials to spontaneously form a spontaneously self-assembled supramolecular complex with fullerene in solution. The formation of such complexes is of considerable interest, however the results herein suggest that many of the previously reported complexes may actually be misinterpretation of photoluminescence extinguishing due to inner filter effects as opposed to quenching arising from energy and charge transfer.