Molecular dynamics and sensing with low-cost organic-inorganic nanostructures

Abstract

The organic-inorganic methylammonium lead iodide (MAPI) perovskite has become a prominent research topic as a low-cost solution for solar cell technol- ogy. However, this material has yet to be commercialized due to its limiting factor of a high degradation rate. One major cause of this degradation is ionic migration. In the first part of this thesis, the migration process is simulated using molecular dynamics. We find that the dominant diffusion corresponds to the migration of io- dide vacancies. Our simulations indicate two ways to reduce the degradation rate of the MAPI perovskite due to this migration. The first is adding a compressive strain, which we prove causes the diffusion coefficient to decrease significantly. The second is adding a hydroxyl group (OH-) into the crystal structure, which re- places the iodide vacancy and stops the migration. Each mediating factor is tested on a perfect crystal structure and a grain boundary-infused structure. Combining the MAPI material with silicon nanowires (SiNWs) prepared at our Nanophysics Center at Reykjavik University is an attempt to stabilize the MAPI material and boost the photovoltaic effect experimentally. This thesis demon- strates an excellent physical matching of such a hybrid structure. However, during this research, we found that the behavior of the silicon nanowires is more complex than expected. The second part of the thesis is dedicated to that, specifically to the detection of organic molecules with silicon nanowires. SiNWs contain unique and versatile capabilities mainly due to their large surface- to-volume ratio and high sensitivity of current-voltage characteristics. These wires can enhance performance and miniaturization in various fields, ranging from only a few tens of nanometers in diameter. Using a form of wet chemistry, so-called metal-assisted chemical etching (MACE) process, allows for low-cost and highly repeatable fabrication of SiNWs. In the utilization of this structure, we were able to create an ultra-sensitive NO2 gas sensor. Unlike many commercially available gas sensors, our SiNW-based sensors can operate under high relative humidity con- centrations and can experimentally detect 20 parts per billion of NO2 gas through chemisorption.