PROGRAM | Materials Science & Engineering

By: Jill Cleveland Chair: Matthew Doty

ABSTRACT

Photon upconversion is a unique optical process in which multiple low-energy photons are absorbed to generate higher-energy photon emission. There has been increasing interest in developing materials for upconversion due to its potential application across a range of technologies including optoelectronic devices, biotechnology, and solar energy harvesting. In the context of solar energy harvesting, upconversion could be used to increase the efficiency of photovoltaics (PV) by converting low-energy photons that are not absorbed by the PV device to higher-energy photons that can be absorbed. In order for upconversion-backed PVs to deliver a meaningful performance increase, the upconverter material needs to be able to absorb a wide spectral range of low-energy photons and upconvert them with relatively high efficiency. Traditional upconversion materials, such as lanthanide-doped particles and triplet-triplet annihilation systems, have demonstrated very good upconversion efficiency but are not ideal for solar energy harvesting due to their narrow absorption bandwidths. An alternative approach developed in recent years is to use semiconductor quantum dot heterostructures to design new upconversion platforms. Semiconductor nanostructures have inherently wide absorption bandwidths and tunable absorption and emission spectra, making them appealing for upconversion. However, most semiconductor-based upconversion materials demonstrated to date suffer from very low upconversion efficiency.

In the first part of this work, I introduce an upconverting heterostructure designed by our group consisting of an alloyed CdSe(Te) absorber “core” QD and a CdSe emitter QD that are connected by a CdS nanorod. This heterostructure is designed to undergo upconversion via an excited state absorption (ESA) pathway that involves absorption of two different low-energy photon bands, making it ideal for solar energy harvesting applications. By utilizing photoluminescence spectroscopy techniques, we confirm this structure upconverts under solar-relevant conditions via an ESA pathway. Additionally, we show that upconversion performance can be improved through heterostructure engineering, specifically improving the homogeneity of the absorber QD and introducing alloying in the CdS nanorod. Even with these improvements, however, the upconversion efficiency in these structures is quite low. To better understand the loss pathways, we used a combination of time-resolved photoluminescence, photoluminescence quantum yield, and computational modeling to study carrier dynamics. This analysis identified two major sources of losses in these heterostructures: (1) trap states in the absorber QD and CdS nanorod, likely due to unreliable synthesis methods and (2) low PLQY of the CdSe emitter QD due to its exposed, unpassivated surface.

In the second part of this work I propose an alternative heterostructure designed to address the major limitations discovered during the characterization of our first generation upconverting structure. The new structure is an inversion of the original one, a seemingly simply change that provides several important advantages both synthetically and for the expected upconversion efficiency. However, to realize this new heterostructure design, new synthesis protocols were needed. I present work on two synthesis studies leading to the complete synthesis of the proposed structure. In the first, I demonstrated the first seeded-growth synthesis of alloyed CdSSe nanorods from CdSe seed quantum dots and performed a comprehensive study on the effect of synthesis conditions on particle morphology and alloy composition. In the second, I explore the SILAR addition of an absorber QD to complete the upconverting heterostructure and analyze its performance using photoluminesence techniques. The results show that while the inverted structure is synthetically more robust and reliable than our first generation heterostructure, more work is needed to optimize the synthesis and structure to improve upconversion performance.

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