Graduate Thesis Or Dissertation
 

Photophysics of organic semiconductors at the nanoscale

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  • Organic semiconductors are used in a wide variety of applications including transistors,solar cells, and light emitting diodes. These materials are solution-processable,low cost, and tunable. Many successful organic optoelectronic materials utilizeblends of several types of molecules (such as donors and acceptors) in order topromote charge generation. As blends are an inherently heterogeneous system,nanoscale morphology plays a critical role to determine the optoelectronic propertiesof the blend. The work presented in this dissertation aims to develop novelmethods of probing the local nanoenvironment in organic semiconductors as wellas establish the relations between the nanoscale environment of the molecules andtheir photophysics.First, several experiments were performed via single molecule fluorescence microscopyto study energy transfer (FRET) and photo-oxidation in blends containingdonor and acceptor molecules. Donor molecules were imaged with increasingacceptor molecule concentration to determine the change in their photophysicalproperties due to acceptor-modified morphology and donor-acceptor energy transfer.As the concentration of acceptor molecules reaches a critical concentration suchthat the average donor-acceptor distance is below the FRET radius, fluorescenceof donor molecules is quenched. This enables single-molecule-level microscopy atrelatively high donor concentrations, thus creating a new super-resolution tool toimage donor molecules in a modified local environment. As the concentration of acceptorsincreased, the number of photons a donor emits over its lifetime decreased,and fluorescence intermittency increased. These observations were quantified usingstatistical analysis and complementary cumulative distribution functions. Thefindings were attributed to the acceptor-modified morphology which reduced thedonor molecule protection from photo-oxidation reactions; however, the presenceof the acceptors also enhanced the reversibility of the photo-oxidation process byquenching the highly reactive singlet oxygen. Such reversibility is important fororganic semiconductors as their photodegradation is one of the key drawbacks forapplications.Next, molecular packing and photostability changes are presented as a functionof different host matrices and different molecular side groups, again via singlemolecule fluorescence spectroscopy. Molecules embedded in a crystalline organicsemiconductor host matrix exhibited higher photostability than a polymer matrix.In addition, larger side groups lead to higher photostability, indicating thelarger side groups provide better protection from reactions with oxygen. Orientationalconstraints for guest molecules in a crystalline host were also observed andquantified.Lastly, a novel method to study photoinduced charge transfer between organicsemiconductors utilizing optical tweezers is presented. A silica microsphereis coated with an organic semiconductor (e.g. donor) film and suspended in aliquid with varying dielectric permittivity and containing other organic semiconductormolecules (e.g. acceptors). The time-resolved surface charge of this sphereis measured by trapping the sphere using optical tweezers and applying a sinusoidalelectric field across the sphere. (Dis)charging dynamics are measurable byphotoexciting the coating of the sphere and observing the dynamics of excitonsby monitoring the photoluminescence of the trapped sphere while simultaneouslymeasuring surface charge with optical tweezers.
  • Keywords: effective charge, organic semiconductors, optical tweezers, FRET, charge transfer, single molecule fluorescence spectroscopy, photobleaching, fluorescence intermittency
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