Hybrid Lead Halide Perovskites for Light Energy Conversion: Excited State Properties and Photovoltaic Applications

The burgeoning class of metal halide perovskites constitutes a paradigm shift in the study and application of solution-processed semiconductors. Advancements in thin film processing and our understanding of the underlying structural, photophysical, and electronic properties of these materials over the past several years have led to development of perovskite solar cells with power conversion efficiencies that rival much more mature first- and second-generation technologies. Characterizing the behavior of photogenerated charges in metal halide perovskites is integral for understanding the operating principles and fundamental limitations of perovskite optoelectronics. The majority of studies outlined in this dissertation involve fundamental study of the prototypical organic-inorganic compound methylammonium lead iodide (CH3NH3PbI3). Time-resolved pump-probe spectroscopy serves as a principle tool in these investigations to analyze excited state decay kinetics and optical nonlinearities in perovskite thin films.

It is demonstrated herein that nonresonant photoexcitation of CH3NH3PbI3 yields a large fraction of free carriers on a sub-picosecond time scale. In practical optoelectronic applications, these photogenerated carriers must travel large distances across the thickness of the material to realize large external quantum efficiencies and efficient charge extraction. Using a powerful technique known as transient absorption microscopy, long-range carrier diffusion in a CH3NH3PbI3 thin film is directly imaged. Charges are unambiguously shown to travel 220 nm over the course of 2 ns after photoexcitation, with an extrapolated diffusion length greater than one micrometer over the full excited state lifetime. Furthermore, through structural and steady-state and time-resolved absorption studies, the important link between the excited state properties of perovskite precursor components, composed of solvated and solid-state halometallate complexes, and CH3NH3PbI3 is evinced. This connection provides insight into optical nonlinearities and electronic properties of the perovskite phase.

In the final chapter, the operation of CH3NH3PbI3 solar cells in a tandem architecture is presented. Utilizing an all solution-processed tandem water splitting assembly composed of a BiVO4 photoanode and a single-junction CH3NH3PbI3 hybrid perovskite solar cell, hydrogen is generated from pure water at neutral pH at an efficiency of 2.5% without external bias. The design of low-cost tandem water splitting assemblies employing single-junction hybrid perovskite materials establishes a potentially promising new frontier for solar water splitting research.