Crystal Engineering and Structure Property Relationships in Hybrid Organic-Inorganic Lead Halide Perovskites

In the last 10 years perovskite semiconductors have gone from being unknown to the hottest topic in solar energy research. Perovskite thin films grown from solution can enable low cost solar cell production from roll to roll manufacturing techniques not amendable to other materials. However, efficiencies in the lab have advanced more rapidly than the understanding of thin film nucleation and growth mechanisms, and insights into material properties are obscured by the different crystal sizes and orientations of perovskites fabricated from varying compositions. For perovskites to escape the lab, both thin film deposition and the relationships between material composition and properties must be understood and engineered at more a fundamental level.
This work combines research directions on rational design of the precursor solution for control of nucleation and growth with fundamental studies on the impact of crystallographic orientation and composition on electronic properties. Additives which lower the free energy of the perovskite precursor were used to stabilize the solution, allowing control over nucleation density and crystallographic orientation. In situ X-ray diffraction and density function theory calculations were used to determine the nucleation mechanism and cause of orientation. It was found that crystallographic disorder caused electronic disorder, and it was demonstrated that polycrystalline thin films with perfect crystallographic orientation are electronically uniform.
The structure and molecular motion of the polar organic cation within the lattice of the perovskite was determined from neutron scattering data. A systematic comparison of a crystal structure where the cation is stationary to one in which it is rotating concluded that the rotation is responsible for the favorable electronic properties of the material. Further experiments revealed that the nature of this organic cation strongly impacted the strength of the interaction between photoexcited charge carriers and trap sites.

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