Date of Award
Doctor of Philosophy
Chemical and Biological Engineering
Matthew G. Panthani
Hybrid organic-inorganic Pb halide perovskite semiconductors have shown excellent promise in a wide variety of optoelectronic applications; the impressive performance can be attributed to their excellent optoelectronic and charge transport properties. Unfortunately, hybrid organic-inorganic Pb halide perovskites suffer from intrinsic instabilities and contain a toxic Pb component. The focus of this PhD dissertation is in the development of alternative semiconductors that are predicted to share these excellent properties without the toxicity and stability concerns. Bi halide semiconductors could have the greatest potential as nontoxic and stable alternatives to hybrid organic-inorganic Pb halide perovskites due to the chemical similarity of Bi(III) and Pb(II). Of interest are BiI3 and A3Bi2I9 (A = FA, MA, Cs, Rb) compounds.
The present challenge facing BiI3 and A3Bi2I9 optoelectronic devices are the poor film morphology. Annealing BiI3 thin films in DMF vapor at relatively low temperatures (≤ 100 °C) resulted in increased grain size and crystallographic reorientation within the films. Non-optimized BiI3 solar cells achieved power conversion efficiencies of 1.0%, demonstrating the potential of BiI3 as a non-toxic and air-stable semiconductor for photovoltaic applications. Next, using BiI3 as a model system, we demonstrated that the film morphology and surface coverage are strongly dependent on the Gutmann donor number of Lewis base solvents. We demonstrate that coordinating BiI3 with a combination of solvents with high and low donor numbers results in conformal films that have been difficult to achieve using conventional solution-based deposition techniques.
To address the challenges with film morphology with A3Bi2I9 compounds an alternative deposition procedure was developed utilizing a two-step deposition procedure in which optimized BiI3 thin films were converted into A3Bi2I9. Thin films fabricated from the one-step deposition exhibited a preferred crystallographic orientation along the c-axis, while the two-step deposition decreased this preferred orientation. Films deposited from the two-step method exhibited increased homogeneity in the surface coverage and crystal grain sizes. After improving the film morphology, we attempt to tune the bandgap of A3Bi2I9 compounds. The bandgap is too wide for application in a single junction photovoltaic device. To tune the bandgap, we attempted to induce chemical pressure in the crystal structure through cation size mismatch. We determined that the bandgap is insensitive to A-site tuning because A3Bi2I9 compounds predominantly form a 0D structure that limits variations to the bandgap.
Umar Hussein Hamdeh
Hamdeh, Umar Hussein, "Solution-processed bismuth halide thin film semiconductors for photovoltaic application" (2019). Graduate Theses and Dissertations. 17198.