Third generation photovoltaics are inexpensive modules that promise power conversion efficiencies exceeding the thermodynamic Shockley–Queisser limit, perhaps by using up or down-converters, intermediate band solar cells, tandem cells, hot carrier devices, or multiexciton generation. Photon upconversion has attracted enormous attention due to its wide range of applications in biological imaging, photocatalysis, and especially photovoltaics. Here, in the first chapter, the effect of quantum confinement on the efficiency of Dexter energy transfer from PbS and PbSe nanocrystals (NCs) to a rubrene acceptor is studied. A series of experiments exploring the relationship between NC size and the upconversion quantum yield (QY) in this hybrid platform show that energy transfer occurs in the Marcus normal regime. By decreasing the NC diameter from 3.5 to 2.9 nm for PbS and from 3.2 to 2.5 nm for PbSe, the relative upconversion QY is enhanced about 700 and 250‐fold respectively. In addition, the dynamic Stern–Volmer constant (KSV) for the quenching of PbSe NCs by rubrene increases approximately fivefold with a decrease in NC diameter from 3.2 to 2.5 nm to a value of 200 M−1. This work shows that high quality, well‐passivated, small NCs are critical for efficient triplet energy transfer to molecular acceptors. In the second chapter, we report an efficient upconversion of infrared to visible light. Colloidally synthesized core–shell lead sulfide– cadmium sulfide nanocrystals in combination with tetracene derivatives absorb near infrared light and emit visible light at 560 nm with an upconversion quantum yield (QY) of 8.4 ± 1.0%, which is a factor of 4 lower than the maximum upconversion QY possible. The molecular and nanocrystal engineering here paves the way toward utilizing this hybrid upconversion platform in photovoltaics, photodetectors and photocatalysis. In the third chapter, we study the effect of shell composition and thickness on triplet energy transfer (TET) from PbS QDs to rubrene in a hybrid organic–inorganic photon upconverison system and show that defect states introduced by surface adsorbed Zn and Cd result in up to 700 and 325 fold enhancements in the photon upconverison quantum yield (QY). Time-correlated single-photon counting, photoluminescence (PL), and photon upconversion QY measurements on PbS QD light absorbers with submonolayer shells of 0.37–1.60 Å of CdS and 0.04–0.22 Å of ZnS reveal these core–shell QDs have reduced radiative and nonradiative rates relative to the core. Their broadened absorption and PL line widths suggest TET may occur from thermally accessible surface defects, in addition to the dark excitonic states at the band-edge. Finally by study the photophysics and electronic properties of differently sized PbS and PbSe QDs we study the effect of composition in the photon upconveersion process and we demonstrate that appropriate QDs synthesis precursors are essential for gaining high quality QDs that results in efficient upconversion QY of ~12%.

Colloidal semiconductor nanocrystals (NCs) are a novel set of materials with unique properties. Due to their intrinsic high surface area to volume ratio, surfaces play a dominant role in many physical and chemical processes. Surface ligands — molecules that bind to the surface — is the essential component of nano-material synthesis, processing and application. My dissertation here is mainly focusing on two parts: (a) the characterization of surface ligand binding using fluorescence-based techniques and (b) the rational design and engineering of surface ligands for our targeted application: photon upconversion.

We start from developing a FRET based model sensing the initial few ligand binding events on the oleic-acid capped CdS NCs. The binding behavior can be described as an interaction between a ligand with single binding group and a substrate with multiple, identical binding sites. A modified Langmuir isotherm can be applied to obtain the average number of newly introduced ligand and its binding constant. With this model, we also demonstrate that inorganic NCs have distinct binding sites for different chemical species. This is evidenced by the results that only resonance energy transfer (RET) is observed for the carboxylic acid ligand, while both RET and charge transfer are observed for the amine ligand.

Then, the length and binding geometries of surface ligand are prudently selected to promote triplet energy transfer, which is the key step in the photon upconversion process. First, a series of anthracene transmitter ligands with variable-length aromatic oligo-p-phenylene and aliphatic linkage were employed. The triplet sensitization of anthracene transmitter molecules by CdSe NCs shows a strong distance dependence. Anthracene transmitter bound closest to the NC surface gave the highest quantum yield of 14.3% for the conversion of green to violet light, the current record for a hybrid platform. Later, a series of bidentate bis-pyridine anthracene isomers that differ in binding geometries were designed to find the best complementary fit to the NC surface. Among them, only the ligand with an intramolecular N-N distance of 8.2 Å had the best match to the surface of CdSe NCs, yielding a upconversion QY as high as 12.1±1.3%.

This work lays the foundation of incorporating the hybrid complexes as active materials in optoelectronic devices, such as solar cell, transistors and photodetectors, and also relate the performance of those devices to the structure at their atomic level.

The sequential absorption of two low energy photons, followed by the emission of one higher energy photon, is known as photon upconversion. This two-photon process has potential applications in biological imaging, solar energy conversion and photocatalysis etc. Two existing strategies to realize upconversion are an inorganic system based on lanthanide doping, and an organic system based on triplet-triplet annihilation. This dissertation introduces a novel hybrid molecule-nanocrystal system that upconverts photons across visible to near-infrared. It combines the advantages of high photostability and low excitation intensity, and overcomes the limits in the previous systems. Here, the absorption of low energy photons by the semiconductor nanocrystals (NCs) is followed by the energy transfer to molecular triplet states. Triplet-triplet annihilation then occurs to create high energy singlet states that emit the upconverted light. I show the introduction of functionalized acene molecules on NC surfaces greatly enhances the upconversion quantum yields (QYs) by up to three orders of magnitidue in different systems. As one of the efficiency limiting steps in upconversion, TET is systematically studied by modifying both NC donor and molecular acceptor. TET is characterized by steady state upconversion, time-resolved transient absorption and photoluminescence spectroscopies. TET is dependent on the anchoring groups of the transmitter molecules due to different binding affinities, docking geometries, sterics, intra-molecular spin-orbit couplings and molecule-NC triplet-triplet couplings. Transmitters with phosphonic acid, carboxylic acid and imidazole as binding groups show the best upconversion performance. For the NC donors, small sizes lead to high upconversion QYs due to the large driving force. While the growth of inorganic shells passivates surface traps, TET from core-shell NCs to transmitters is dependent on the shell composition and thickness. TET is improved when submonolayer shells suppress charge transfer and decrease exciton-phonon coupling.

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.