The development of superresolution spectroscopic techniques and characterization of microscale exciton diffusion in organic semiconducting polymers

Abstract

Disordered semiconducting materials offer cost effective, solution processable alternatives to highly crystalline semiconducting materials for utilization in a variety of optoelectronic devices. However, characterization of these complex materials systems using bulk spectroscopic methods is heavily influenced by chemical and morphological heterogeneity inherent to the material. The experiments described in this thesis are designed to improve the fundamental understanding of the photophysical processes in disordered solution processed semiconducting materials by developing and utilizing high spatial resolution spectroscopic methods. Chapters 2-4 will outline the experimental and theoretical development of two superresolution spectroscopic techniques. First (chapters 2 & 3), structured pump-probe microscopy (SPPM) utilizes a structured excitation profile along with a diffraction limited probe pulse to achieve ~100 nm spatial resolution. Using SPPM it is also possible to collect time resolved spectroscopic data from a sub-diffraction limited volume. Second (chapter 4), label-free saturated structured excitation microscopy (LF-SSEM) is theoretically developed. LF-SSEM is experimentally similar to SPPM but exploits the saturation of the absorption process to achieve even greater resolution enhancement. Here, simulated LF-SSEM is shown to achieve ~33 nm spatial resolution. Chapter 5 demonstrates the utilization of PPM to investigate exciton transport in the organic semiconducting polymer (OSP), poly [N-9''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT). Although OSPs have shown great promise for use in a variety of optoelectronic applications, much remains un-known about their excited state dynamics. The data reported here represents a significant contribution to the rapidly growing wealth of knowledge pertaining to OSP systems. Specifically, the microscale exciton diffusivity observed in PCDTBT thin films using PPM is found to reach 3.2 cm 2/s. Chapter 6 examines a technique in the early stages of development and optimization that is able to detect excited state carrier diffusion with increased sensitivity and accuracy compared to PPM. Differential detection pump-probe microscopy (DDPPM) uses two probe pulses to selectively eliminate the signal of carriers that have not diffused beyond the boundaries of the initial excitation. The experiments described within this dissertation are diverse, yet the common goal is to increase and improve the knowledge of photophysical properties in disordered semi-conducting materials. This goal takes two forms in the development of novel spectroscopic methodology and the characterization of complex materials using PPM. The singular result is the advancement of basic science pertaining to complex semiconducting materials systems.