Controlling the Morphology and Conductivity of Organic Electronic Devices through Sequential Processing: Use of Solvent Additives as Polymer Swelling Agents and Evaporated F4TCNQ as a Polymer Dopant
This thesis focuses on adding small-molecule acceptor materials to semiconducting polymer films. Semiconducting polymers are widely studied because they share the low-cost processing of plastics, but also have many electronic applications. Semiconducting polymers, however, require nanoscale mixing with an electron acceptor to be useful for these device applications. Based on the energy-level offset of the two materials, two classes of devices are present: mixing the polymer with an electron acceptor for the excited state yields a solar cell while mixing the polymer with a ground-state electron acceptor chemically dopes the polymer and significantly increases the polymer conductivity. My work focuses on investigating the fundamental mechanism of adding acceptor molecules to polymers and how this affects the device properties and structure.
Chapter 1 introduces the field of organic electronics, providing background for organic materials and their device physics in solar cell and doping applications. Chapter 2 is a published book chapter that describes sequential processing (SqP), which is an alternative fabrication technique for intercalating small-molecule acceptors into polymer films. This chapter discusses and connects the various work on SqP to-date. Chapter 3 investigates the widespread use of solvent additives for use in organic photovoltaics. In this chapter I detail how fabricating solar cells via SqP allows the mechanism of low-vapor pressure solvent additives such as 1,8-diiodooctane (DIO) to be understood: low-vapor-pressure solvent additives act as polymer swelling agents that give extra time and enhanced mobility for the electron acceptor to effectively distribute throughout the polymer network. Since almost all high-performing solar cells require the use of additives such as DIO, this published work helps explain a key component in the solar cell fabrication process. Chapter 4 studies the photophysics of a perylenediimide dimer molecule (di-PDI) used as a solar cell acceptor material. This chapter details that a solution of the di-PDI acts a blend of two molecules, each with very different spectroscopy. The different spectroscopy for each molecule results from the presence of two stable di-PDI conformers: one conformer adopts a `closed' geometry, with significant pi-stacking between the PDI monomers, while the other conformer adopts an `open' geometry with little interaction between the monomers. With two conformers present, this leads to important considerations for device applications with regards to extra light absorption and the possibility of traps. Chapter 5 investigates the addition of ground-state acceptor dopant molecules to P3HT by both evaporation and solution sequential processing. In this chapter I describe how both methods effectively dope films of P3HT ranging from 25 nm to 2 $\mu$m. The 2 $\mu$m thick films are the thickest films doped by single-step fabrication for the materials combination studied. Thermoelectric devices were made via both methods and the thermoelectric performance is quite similar, indicating that one can select from two very different processing techniques to fabricate thermoelectrics. In summary, sequential processing is a reproducible technique that allows fundamental investigations into a broad range of organic electronics in addition to device application studies.