UC San Diego

Organic-inorganic hybrid lead halide perovskites are promising for next-generation solar cells and light-emitting diodes, and it is of high demand to solve their critical issues and to understand their working mechanisms. In this dissertation, using first-principles calculations,

we focus on design of stable and non-toxic alternatives to this class of materials, as well as to understand and optimize their structural, energetic, electronic, and ferroelectric properties for optoelectronic applications.

In the first project, we designed novel optoelectronic materials based on 24 perovskite-related prototype structures by high-throughput computing and data mining. Out of 4507 hybrid halide compounds calculated, we selected 29 compounds adopting five prototype structures for light-emitting diodes and solar energy conversion. All these candidates show appropriate electronic properties and robust stability. The approach of exploring a large variety of prototype structures is transformative to computational design of other functional materials.

In the second project, we further investigated stability diagrams, defect tolerance, and optical absorption of the 29 hybrid halide compounds by high-throughput first-principles calculations. We calculated 2160 neutral and about 5000 charged defect structures to determine defect formation energies and transition energy levels for all possible point defects. Out of the 29 compounds, 15 candidates show high defect tolerance. This work provides detailed guidance on experimental investigation of these novel lead-free optoelectronic materials.

In the third project, we studied ferroelectric dipole ordering in hybrid perovskites. We found that organic cations’ rotational energy barrier is dependent on the cell aspect ratio, and that spontaneous ferroelectric dipole ordering exists with small energy advantage. More importantly, we found that by increasing the cell aspect ratio, strain and doping can enhance the dipole ordering, which could boost electron-hole separations for photovoltaic applications.

In the fourth project, we studied strained epitaxial growth of halide perovskites. Our calculations demonstrate the epitaxial stabilization by calculating detailed thermodynamic terms in the epitaxial nucleation process. We also show that strains control the crystal structure, the bandgap, and the hole effective mass.