Designing the semiconductor nanocrystal to molecule interface for energy conversion
Nanoscale confinement extends the desirable properties of inorganic semiconductors for electronics and energy applications by adding synthetic tunability and solution processability. Controlling for size via simple reaction parameters can change the energy and properties in a way that bulk semiconductors cannot reach, and produces inks of nanocrystals, making processing large solids facile via printing methods. However, these synthetic routes use an insulating layer of organic ligands at the interface of each of these inorganic nanocrystals, diminishing conductivity in nanocrystal solids—one of the largest barriers to adoption of these technologies. To circumvent this problem, the insulating ligands are often removed or exchanged for other ligands which restore conductivity or provide new capabilities. This process of ligand exchange is heterogeneous, complex, and system- dependent. This dissertation focuses on the interface of semiconductor nanocrystals with functional organic ligands via ultrafast spectroscopy and electrochemical methods to understand how ligand exchange alters the electronic properties of the combined system. System parameters can be tuned synthetically and investigated with transient absorption and photocurrent measurements in order to pin down the electronic dynamics in each of these systems. Transient absorption shows the change in the light absorption of a material over short time periods and thus gives information about migration of energy or charge in these materials, while photocurrent measurements correlate microscopic parameters with device-like performance. The overarching goal of this dissertation is to understand the effect of ligand functionalization on the single nanocrystal and extend that to the nanocrystal solid. Specifically of interest are infrared absorbing nanocrystals that can be used to improve the efficiency of solar energy harvesting by capturing more of the incident solar spectrum.