Evolution of Structure and Properties in Optical Materials During Solution Processing

Abstract

Solution processing is a common first step in optoelectronic device fabrication, finding applications in everything from displays to solar cells, as it is inexpensive and flexible. Despite this widespread use, the method is often treated as a black box—materials are suspended in complex mixtures of solvents and additives to alter their properties, but these properties are generally evaluated after drying without knowledge of how they arose. While it has been observed that precursor inks often contain nanostructures that are affected by ink chemistry and in turn affect properties of the deposited film, our understanding of these effects is limited by the difficulty of studying materials in a solvated state. This dissertation addresses this challenge by adapting cryo-electron microscopy (cryo-EM)—a life sciences technique for studying hydrated biomolecules—to study solution-processed optical materials. A sample preparation protocol is developed for inks based in organic solvents, and an image processing technique known as single particle analysis is adapted to study the detailed structure of solvated particles in two and three dimensions.

These techniques are first applied to study solution-processed chalcogenide glasses (ChGs)—photonic materials of interest for their unique photoresponsive behaviors and high optical nonlinearities. By pairing cryo-EM with traditional microscopy and spectroscopy, the structure of arsenic (III) sulfide in amine solvents is determined, confirming two decades-old theories for the first time. The dependence of the shape and size of the ChG nanostructure on solvent choice and ink concentration is explored and explained in the context of a new dissolution model. Finally, these results are connected to material properties by employing nonlinear optical spectroscopy and an effective mass theory to show how solvent-dependent changes in local atomic structure influence two-photon absorption in films.

Later, this methodology is extended to study another class of promising optical materials, metal halide perovskites. Cryo-EM is used to show that previously unidentified colloids in prototypical inks consist of a crystalline precursor phase and explore structural variations with ink chemistry. These results sheds new light on perovskite processing and demonstrate the value of these experimental and analytical techniques as generalized tools to study any solution-processed system.