Crystal Growth Of Organic-Inorganic Lead Halide Perovskites: Impact Of Kinetic Parameters On Morphology, Structure And Properties
In this work I present both a phenomenological and fundamental study of the crystallization of the organic-inorganic halide perovskites; this includes determining the overall structural evolution, extraction of kinetic parameters for the primary crystallization step, and establishing several processing-structure relationships. In addition, I will show examples of the application of the primary results to control crystal growth and enhance the material's performance in working devices. The organic-inorganic halide perovskites have shown great promise as a potential next generation photovoltaic (PV) material with device efficiencies exceeding 17%. For the last 2-3 years most research efforts have been dedicated to understanding the general operating principles of perovskite PV devices and engineering better device architectures. Although these efforts have resulted in an unprecedented increase in efficiencies over a very short period of time, many reports point to film or crystal morphology as a limiting factor. In order to control film and crystal growth a better understanding of this systems fundamental crystallization behavior is needed. Employing in-situ wide angle X-ray scattering allowed for the determination of the general evolution of thin films cast from solution. This initial work revealed an Ostwald Step Rule path in which the final perovskite is preceded by a solid-state precursor structure. Understanding the general evolution allowed for monitoring of the precursor-perovskite transition to establish processing-structure relationships. One such relationship is the temperature profile used in the annealing step, optimization of this profile results in better film coverage and better crystal texture, both of which increase device performance. The second major study was to elucidate the disposition of the constituent salts during crystallization and confirmed that all reagent salts are completely dissociated during film formation. This result led to the realization that the crystallization was insensitive to the lead source allowing for manipulation of the system kinetics by using alternate lead compounds. Finally, a study of the kinetics of the precursor-perovskite transition provided for the extraction of the activation energy. Combined with the previous work, the kinetics study gave the insights needed to determine a general formula for the precursor structure as well as a general pathway for the solid-state transformation. The implication of the work presented here is better control of perovskite thin film growth through manipulation of the processing and chemistry employed. Several recent reports, as well as my own experimental results, are beginning to show the value of this understanding through better films, made at lower temperatures and faster times, and providing improved performance in working PV devices.
perovskite; crystallization; kinetics
Marohn, John A.; Estroff, Lara A.; Wiesner, Ulrich B.
Materials Science and Engineering
Ph. D., Materials Science and Engineering
Doctor of Philosophy
dissertation or thesis