Effects of Bismuth Incorporation on the Electronic Properties of 2D Hybrid Perovskites

Student’s name: Grace Dono Home Institution: University of Rochester NNCI Site: RTNN @ Duke REU Principal Investigator: Dr. Volker Blum, Thomas Lord Department of Mechanical Engineering and Materials Science, Duke University; Department of Chemistry, Duke University REU Mentor: Dr. Rayan Chakraborty, Thomas Lord Department of Mechanical Engineering and Materials Science, Duke University Abstract: 2D hybrid perovskites are a class of semiconducting materials that combine layers of inorganic metal-halides and organic cations to access a wide range of electronic properties. While lead-based perovskites have become notable for their potential within photovoltaic applications, the toxicity of their composition creates a push for lead-free perovskites. As an alternative, this project investigates the structural stability of bismuth-containing and bismuth-based perovskites, using first principles methodology and density functional theory to predict the geometry of the structures and to assess ordering preferences associated with compensating vacancies. From this, the electronic band structure of these materials can be calculated and analyzed to better understand the electronic properties. The project first explores different vacancy arrangements of layered (AE2T)Bi2VacI4 compounds, consisting of metal-halide layers with ratios of Bi2 to one site vacancy (Vac). Structure optimization of five different vacancy patterns (each leading to large structure models including 928 atoms) reveals that an arrangement of single next-nearest neighbor vacancy rows on B sites, alternating with two Bi rows, shows the lowest energy among the arrangements probed (FHI-aims code, “light” settings) and also displays an energy band gap consistent with experimental observations. In contrast, nearest-neighbor vacancy rows and zig-zag arrangements of second nearest neighbor vacancies are higher in energy and, during relaxation calculations, remain metallic in character and present slow, computationally expensive convergence during structure optimization. We next studied a set of (AE2T)PbI4 structures with partial Pb substitution by Bi and vacancies, in which the metal-halide layer was modified to contain Pb5Bi2 and one site vacancy. Band structure calculations for these vacancy arrangements are in progress at the time of writing and are expected to reveal the character of Bi derived energy levels alloyed into a traditional lead halide based layered perovskite.