Photophysics of Organic Semiconductors at the Nanoscale

Graduation date: 2017Organic semiconductors are used in a wide variety of applications including transistors,\ud solar cells, and light emitting diodes. These materials are solution-processable,\ud low cost, and tunable. Many successful organic optoelectronic materials utilize\ud blends of several types of molecules (such as donors and acceptors) in order to\ud promote charge generation. As blends are an inherently heterogeneous system,\ud nanoscale morphology plays a critical role to determine the optoelectronic properties\ud of the blend. The work presented in this dissertation aims to develop novel\ud methods of probing the local nanoenvironment in organic semiconductors as well\ud as establish the relations between the nanoscale environment of the molecules and\ud their photophysics.\ud \ud First, several experiments were performed via single molecule fluorescence microscopy\ud to study energy transfer (FRET) and photo-oxidation in blends containing\ud donor and acceptor molecules. Donor molecules were imaged with increasing\ud acceptor molecule concentration to determine the change in their photophysical\ud properties due to acceptor-modified morphology and donor-acceptor energy transfer.\ud As the concentration of acceptor molecules reaches a critical concentration such\ud that the average donor-acceptor distance is below the FRET radius, fluorescence\ud of donor molecules is quenched. This enables single-molecule-level microscopy at\ud relatively high donor concentrations, thus creating a new super-resolution tool to\ud image donor molecules in a modified local environment. As the concentration of acceptors\ud increased, the number of photons a donor emits over its lifetime decreased,\ud and fluorescence intermittency increased. These observations were quantified using\ud statistical analysis and complementary cumulative distribution functions. The\ud findings were attributed to the acceptor-modified morphology which reduced the\ud donor molecule protection from photo-oxidation reactions; however, the presence\ud of the acceptors also enhanced the reversibility of the photo-oxidation process by\ud quenching the highly reactive singlet oxygen. Such reversibility is important for\ud organic semiconductors as their photodegradation is one of the key drawbacks for\ud applications.\ud \ud Next, molecular packing and photostability changes are presented as a function\ud of different host matrices and different molecular side groups, again via single\ud molecule fluorescence spectroscopy. Molecules embedded in a crystalline organic\ud semiconductor host matrix exhibited higher photostability than a polymer matrix.\ud In addition, larger side groups lead to higher photostability, indicating the\ud larger side groups provide better protection from reactions with oxygen. Orientational\ud constraints for guest molecules in a crystalline host were also observed and\ud quantified.\ud \ud Lastly, a novel method to study photoinduced charge transfer between organic\ud semiconductors utilizing optical tweezers is presented. A silica microsphere\ud is coated with an organic semiconductor (e.g. donor) film and suspended in a\ud liquid with varying dielectric permittivity and containing other organic semiconductor\ud molecules (e.g. acceptors). The time-resolved surface charge of this sphere\ud is measured by trapping the sphere using optical tweezers and applying a sinusoidal\ud electric field across the sphere. (Dis)charging dynamics are measurable by\ud photoexciting the coating of the sphere and observing the dynamics of excitons\ud by monitoring the photoluminescence of the trapped sphere while simultaneously\ud measuring surface charge with optical tweezers