Spatially Resolving Dynamics and Nanoscale Migration of Excitons in Organic Semiconductors Using Transient Absorption Imaging and STED Microscopy

This thesis focuses on the development and application of new methods for spatially and temporally resolving measurements of exciton dynamics and migration in organic semiconducting thin films, which are commonly used in organic field effect transistors (OFETs), organic light emitting diode (OLEDs), and organic photovoltaics (OPVs). Three methods of probing exciton dynamics and migration on length scales better matched to the structural heterogeneity in organic semiconductors are presented. First, the benefits of spatially resolving ultrafast dynamics are explored, by employing transient absorption microscopy on single domains of polycrystalline films of 6,13-bis-(triisopropylsilylethynyl)-pentacene (TIPS-Pn), revealing a polarization dependence that significantly aids the assignment of the excited state dynamics. A full kinetic model of population dynamics, as a function of both polarization and time, is developed and fit to the experimental data, where the polarization dependence provides a several fold increase in the number of constraints for the fitting routine. The global fitting analysis successfully reproduces the experimental data, and the observed dynamics are determined to include ultrafast thermalization of the initially hot exciton in ∼ 50 fs, followed by singlet fission in the first few picoseconds, and then internal conversion over several hundred picoseconds. The success of the kinetic model and the assignment of the dynamics are direct results of the polarization dependence, which is only revealed at the single domain level./nSecond, stimulated emission depletion (STED) fluorescence microscopy, originally developed for super-resolution fluorescence imaging of isolated, robust, fluorescent dye labels in biological imaging applications, is adapted to image conjugated polymer solids using their endogenous, densely packed, non-ideal chromophores. Notably, the challenge posed by the strong two photon absorption of the so called “STED pulse”, which depletes the initially diffraction-limited excited state population to yield a sub-diffraction resolution excitation volume, in conjugated polymers is successfully mitigated through careful control of the STED pulse parameters in combination with the pile-up correction and excitation modulation. This technique is demonstrated on nanoparticles of the conjugated polymer poly(2,5-di(hexyloxy)cyanoterephthalylidene) (CN-PPV), where an imaging resolution of better than 90 nm is achieved./nFinally, a new method to measure exciton migration on its native nanometer and picosecond scales is presented, based on a further adaptation of STED microscopy, which provides ultrafast time resolution of spatial migration dynamics. This technique of time-resolved ultrafast stimulated emission depletion (TRUSTED) is achieved by adding a second STED pulse, with a controlled time delay, to define an optical quenching boundary that preferentially quenches excitation that has migrated beyond a critical radius. The theoretical and experimental sensitivity of this technique to migration processes is demonstrated, through kinetic simulations and experimental studies. The application of TRUSTED to CN-PPV thin films, in combination with a custom fitting routine, reveals the exciton migration length to be Ld = 16 ± 2 nm. Additionally, Monte Carlo simulations of incoherent exciton hopping are performed for a variety of possible spatioenergetic landscapes, revealing the migration process in CN-PPV to be approximately diffusive in nature, where the 5 ns lifetime capitalizes on the diffusive motion, resulting in the relatively long observed migration length. The simulations also reveal more generally how the energetic and spectral parameters of a material combine to determine the extent and nature of exciton migration./nThe results presented here, and the future experiments enabled by this work, will reveal the importance of matching the scale of the experimental resolution to the natural scale of the process or heterogeneity of the material. The insight that stands to be gained through the continued pursuit of these research goals will elucidate the nature of the structure/function relationship in organic semiconductors, informing the rational design of the next generation of semiconducting materials for applications in displays, computing, lighting, and light harvesting.

Advisor: Ginsberg, Naomi S