First Principles Studies Of Defects In Colloidal Nanocrystals

Solar energy is one of the few renewable, low-carbon sources with both the maturity and accessibility to meet the ever-increasing global demand for energy. There are also accounting for an increasing percentage of our energy output due to increased adoption in both industrial and residential areas. Wafer based silicon photovolatics (PV) technology has dominated the solar market, whereby its price has increased significantly over the last decades. In order to fully capture the solar energy from the sun and extend the flexibility of PV technology, there is a need for constant innovation for new materials. Currently, there is a class pf emerging PV technologies that offer the potential of increased scalability, flexibility and lower prices. They include hybrid organic-inorganic lead halide perovskite PV, organic PV and colloidal quantum dot (CQD) PV. Colloidal quantum dots are semiconducting nanocrystals that exhibit size tunable electronic and optical properties. Owing to their versatility and facile synthesis, they have seen wide application photovoltaics, light emitting diodes, solar concentrators and bio-imaging. In particular, their PV power conversion efficiency has grown rapidly over the last 9 years from 3% to 16.6%. Despite the rapid progress, the search for better PV materials has been carried out almost exclusively through tremendous numbers of trial and error experiments. This is due to the fact that many fundamental aspects of the materials has not been fully understood, especially the role of defects and trap states. Due to the nature of wet chemistry synthesis, vacancies, intersitial and other extended defects inevitably form. These defects often cause in gap states within the semiconductor bandgap, which sensitively impact the performance of the PV devices. In addition, defects are difficult to measure directly using experimental techniques, and we often rely on spectroscopic and imaging to probe their properties indirectly. The core of the work described in this thesis deals with the theoretical understanding of nanocrystals with the goal of achieving a deeper and more fundamental understanding of the material's properties at the atomic scale, focusing on the roles of defects. To this end, we employ a technique of computational electronic structure calculation methods, namely density functional theory (DFT) calculations. In this thesis we will use DFT to investigate and find the role that defects play at controlling the 1) Stokes shift and 2) trap states in PbS quantum dot, as well as the 3) luminescent properties of CuAlS2 nanocrystals. While we show that points defects can cause excessive Stokes shift in single PbS CQDs, and dimer defects are a source of detrimental trap states in PbS CQD solids, the presence of point defects are the source of high luminescence in CuAl2 nanocrystals. We have also provided insights and design guidelines to control defects to design ever more efficient PV devices at an atomic level. This thesis document is organized as follows: Chapter 1 introduce CQD and their applications in PV and other optoelectronic devices. Chapter 2 summarizes the computational techniques employed in this thesis work. Chapter 3 focuses on the origins of the Stokes shift in PbS nanocrystal. Chapter 4 focuses on the PbS superlattice solids, and highlight the origin of trap states in these solids as due to the presence of dimers. Chapter 5 studies the defect physics of CuAlS2, and identifies the defect states responsible for the high photoluminescene.