UC Riverside

Photon upconversion is the process where two low energy photons combine to form one higher energy photon. This two-photon process has many diverse applications in solar energy conversion, catalysis, and bioimaging. Molecular triplet-triplet annihilation (TTA) based upconversion benefits from using inorganic nanocrystal (NC) sensitizers because this hybrid system combines the stability and absorption properties of inorganic semiconductors with the high efficiencies of TTA in conjugated organic materials. In TTA based photon upconversion, controlling triplet energy transfer (TET) through the system is key to unlocking higher efficiencies. This dissertation focuses on better understanding this TET process and how this knowledge can be used to optimize inorganic-organic hybrid upconversion systems as we transition to solid state thin film applications. Primary amines are used to better understand how the large surface area of nanocrystals impacts TET. The introduction of primary amines increases the upconversion quantum yield approximately 5-fold with further additions of amine not improving photon upconversion, as CdSe NC photoluminescence (PL) increases at the expense of triplet energy transfer. Transient absorption measurements show that the amines enhance NC PL by decreasing the nonradiative decay rate, increasing the rate of triplet energy transfer, and enable the broad trap state in these CdSe NCs to participate in triplet photosensitization. Further understanding of TET is found by size dependence studies of the CdSe nanocrystal sensitizer. As CdSe NC size increases the photon upconversion quantum yield (QY) decreases due to the decrease in the driving force for TET from CdSe to the surface bound transmitter ligand, as expected for the Marcus description of energy transfer from the transmitter to the NC. Long microsecond transmitter lifetimes are critical to high photon upconversion QYs. Ultimately, these enhancements in efficiencies pave the way for a transition to high efficiency thin film upconverting materials, which already show promise by exhibiting high efficiencies, increased stability, and reduction in reliance on organic solvents over solution-based systems. The future of useful upconverting materials relies on taking this understanding of energy transfer in nanocrystals and applying it to biocompatible upconverting thin films.