PROGRAM | Materials Science & Engineering

By: Tory Welsch Chair: Matthew Doty

ABSTRACT

Photon upconversion is an optical process by which two or more low-energy photons are absorbed in a material and one higher-energy photon is emitted. Photon upconverting materials are of interest for a broad range of optoelectronic, biomedical, sensing, and solar energy harvesting technologies. In the context of solar energy harvesting, upconverting materials can improve the efficiency of a single junction solar cell by converting photons with energy below the bandgap of the host solar cell into above-bandgap photons that can be absorbed, thus increasing the spectral range of incident solar photons that can be harvested. Upconverters based on colloidal semiconductor quantum dot (QD) nanostructures are advantageous for this purpose because they have wide absorption bandwidths and their optical properties are highly tunable with structure and composition. Previous work in our group demonstrated photon upconversion in CdSe(Te)/CdS/CdSe core/nanorod/emitter nanostructures, but the upconversion efficiency remained limited by nonradiative recombination due to defect states. The performance of these heterostructures can be significantly improved through higher quality material interfaces and finer control over their morphology and composition. However, reliable and predictive colloidal synthesis becomes increasingly difficult for complex multi-component nanostructures and presents a major challenge across the field. Detailed studies of synthesis-structure relationships are required to progress towards reliable synthesis of nanostructures with the specific morphologies and compositions engineered for efficient photon upconversion.

In the first part of this work I analyze the carrier separation and dynamics in the upconverting CdSe(Te)/CdS/CdSe nanorod heterostructures. Understanding the carrier separation and relaxation mechanisms is an important step toward rational engineering of structures and compositions that can improve the upconversion efficiency. We use a combination of computational simulations, time-resolved photoluminescence (TRPL), and photoluminescence quantum yield (PLQY) measurements to correlate structural changes to carrier separation and recombination behavior. This analysis of structure-property relationships reveals major loss pathways that limit the upconversion performance. I then use our findings to identify limitations of the nanorod upconversion platform and present a spherical CdTe/CdS/CdSe core/thick-shell/shell upconversion platform designed to overcome these limitations.

In the second part of this work I develop strategies for the reliable synthesis of the designed CdTe/CdS/CdSe core/thick-shell/shell upconversion nanostructures. I first focus on the hot-injection synthesis of CdTe/CdS core/thick-shell “giant” QDs, which introduces several synthetic challenges compared to conventional thin-shelled structures. We systematically analyze the separate and intersecting effects of three key variables (the reaction concentration, oleic acid ligand ratio, and Cd-S shell precursor:CdTe core ratio) on CdTe/CdS particle size and quality using TEM, PL, and PLQY. We analyze the results to better understand the balance between competing reaction processes and elucidate key insights into the thick shell growth mechanism. I then systematically explore the shelling of CdSe on CdTe/CdS core/thick-shell seeds. We perform shelling reactions via both successive ionic layer and adsorption reaction (SILAR) and hot-injection methods across four matrices of synthesis parameters to elucidate the effects of particle concentration, relative Cd-Se precursor concentration, reaction temperature, and Se-precursor reactivity on the reaction product. We also modify the CdTe/CdS seeds via ligand exchange to evaluate the impact of surface chemistry on the CdSe shelling reaction. The results demonstrate that independent nucleation of CdSe dominates and that ideal CdSe shell growth on “giant” seeds is likely favored in only a narrow cross-section of the many-dimensional parameter space. We conclude that the CdTe/CdS/CdSe core/thick-shell/shell upconversion nanostructure platform does not retain the benefits of relatively reliable synthesis common to Cd-based structures with thinner shells.

In the third part of this work I develop a PbS-based platform engineered for efficient upconversion of NIR photons for solar energy harvesting applications. We first use computational simulations to predict the band alignment and carrier separation behavior in PbS/CdS as a function of PbS core size and CdS shell thickness. We further examine how the extent of carrier separation changes upon the addition of a CdSe shell. We use the results to develop design criteria for PbS/CdS intermediate products predicted to enable efficient upconversion in full PbS/CdS/CdSe structures. Next, we develop cation exchange reaction methods for the predictive synthesis of high-quality PbS/CdS with the desired core/shell dimensions. We combine our computational and experimental findings to identify and understand a trade-off in design and synthetic factors required to realize PbS/CdS for improved upconversion performance. Finally, I investigate a gel impurity that forms in the synthesis of PbS/CdS due to supramolecular assembly of Cd oleate. We use NMR, FTIR, gelation tests, and rheological characterization to thoroughly examine the Cd oleate metallogel through the lens of QD synthesis, purification, and storage. We then assess the ability of an additional coordinating ligand, oleylamine, to inhibit the assembly of Cd oleate and enable robust purification of QD samples. We apply these findings to PbS/CdS synthesis to develop effective strategies to mitigate gelation in oleate-capped QDs. This work provides a framework for the reliable synthesis of PbS/CdS/CdSe heterostructures engineered for efficient NIR-to-visible photon upconversion.

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