Electronic processes in organic optoelectronics : insights gained through modeling and magnetic field effects

Abstract

Organic photovoltaics (OPVs) and organic light-emitting diodes (LEDs) are organic optoelectronics offering a number of unique benefits that may play an important role in the future of clean energy generation and efficient energy consumption. In this thesis, we explore key electronic processes in OPVs and OLEDs, with a major focus on quantum-mechanical kinetic modeling of magnetic field effects (MFEs) that probe underlying subprocesses. Certain organics are capable of dividing excited states in a process termed singlet fission, which can increase the maximum theoretical efficiency of an OPV by a factor of nearly 1/3. The MFEs on photocurrent measurements from our collaborators are combined with theoretical models to determine optimal device architectures for singlet fission OPVs, allowing us to exceed the conventional limit of one electron per photon. We also use MFEs to determine the spin of charge transfer states most efficient at generating photocurrent and demonstrate microscopic insight into the mechanism of their diffusion, offering new design principles for the engineering of donor-acceptor interfaces in OPVs. Thermally activated delayed fluorescence (TADF) is becoming an increasingly important OLED technology that extracts light from non-emissive triplet states via reverse intersystem crossing (RISC) to the bright singlet state. We use MFEs to prove a rather surprising finding that in TADF materials composed of donor-acceptor bends, the electron-hole distance fluctuates as a function of time, resulting in spontaneous cycling between states that are advantageous to fluorescence at one moment and then advantageous to RISC at another. Combined with additional topics in the fields of metal organic frameworks and reaction pathfinding methods, the work in this thesis provides insight into how to achieve optimal performance in OPV and OLED devices, which may serve an important role in the future of our energy landscape.