Beyond the Silicon Band-Gap: Near-Infrared Upconversion for Solar Energy Capture Restricted; Files Only
Ho, Eric (Summer 2026)
Abstract
Climate change induced by the burning of fossil fuels to fulfill humanity’s energy needs is one of the greatest challenges facing society today. This dissertation aims to address one very small part of a monumental task by developing a technology complementary to solar energy harvesting. The oft-cited Shockley-Queisser limit prescribes a 33% ceiling for the power conversion efficiency of single-junction solar cells – a hard sell for replacing fossil fuels. But this limit considers the sun’s light to be fixed and immutable. Upconversion via triplet-triplet annihilation repackages the energy from two low energy photons into a single high-energy photon. By upconverting NIR-II photons which lie below the band-gap of silicon photovoltaics, we could harvest up to an additional 25% of the sun’s irradiance. Chapter 1 introduces the spectroscopic equipment and facilities constructed in pursuit of this dissertation and for the future use of the research group. Chapter 2 presents a novel pseudo-solid-state polymer material which makes compatible liquid nanocrystal-sensitized NIR upconversion solutions with durable solid-state photovoltaics. We achieve efficient NIR upconversion and effectively preserve that efficiency in a pseudo-solid state, and report the first measured (pseudo-)solid state NIR-II upconversion efficiency. Chapter 3 investigates binary annihilator mixtures for alleviating the bottleneck of limited annihilator molecules for NIR-II upconversion. Chapter 4 describes ongoing work studying sterically hindered QD ligands for increasing sensitization efficiency in the energetically limited NIR-II regime with broad applicability to all nanocrystal-sensitized upconversion systems. The sum total of this Dissertation makes a meaningful advance in the field of infrared upconversion in the hopes of a brighter and greener future.
Table of Contents
Introduction. 1
Motivation. 1
Triplet-Triplet Annihilation Upconversion. 5
Chapter 1: Construction of Spectroscopic Instrumentation. 10
Introduction. 10
Steady-State Photoluminescence (SSPL) 10
Time-Resolved Photoluminescence (TRPL) 14
Transient Absorption (TA) 17
Adaptations for Microsecond TA.. 23
Microscopy, Cryostat, and Magneto-Optics 24
Chapter 2: Pseudo-Solid-State Polymer Materials for QD-Sensitized NIR-I and NIR-II Upconversion Beyond the Silicon Bandgap. 26
Introduction. 26
Methods. 28
Synthesis and Functionalization of PbS QDs 28
Preparation of Droplet-Containing Polymers 29
Photoluminescence Quantum Yields (PLQY) and Normalized UC Emission Efficiencies 29
Time-Resolved Photoluminescence 30
Transient Absorption. 30
Results & Discussion. 31
Solution-Phase Optimization of NIR-I UC.. 32
Pseudo-Solid-State NIR-I UC.. 36
NIR-II UC in Solution and Pseudo-Solid States 44
Conclusion. 46
Appendix A: Supporting Information for Chapter 2. 47
General Considerations 47
Synthesis and Functionalization of PbS Quantum Dots (QDs) 48
Photoluminescence Quantum Yields (PLQY) and Normalized UC Emission Efficiencies 51
Full Results of NIR-I (785 nm) Solution-Phase UC.. 54
Preparation and Imaging of Nanodroplet-Containing Polymers 55
Linear Optical Measurements 58
Picosecond Transient Absorption (TA) Spectroscopy 62
Nanosecond TA Spectroscopy and Calculation of TET Efficiency 66
Comparison of Normalized Upconversion Emission Efficiencies 70
Chapter 3: Binary Annihilator Systems for NIR-II Upconversion: Synergistic FRET and Hetero-TTA Enable Enhanced Upconversion Quantum Yield. 71
Introduction. 71
Methods. 75
Synthesis of PbS QDs 75
Functionalization with 5-CT.. 75
Steady-State Photoluminescence & Upconversion. 75
Time-Correlated Single Photon Counting (TCSPC) 76
Results & Discussion. 76
Conclusion. 90
Appendix B: Supporting Information for Chapter 3. 91
Calculation of 5-CT on the PbS QD Surface 91
Sensitization of 5-CT.. 92
Calculation of the Upconversion Quantum Yield. 94
Calculation of the Optical Outcoupling 95
Fitting for the Concentration Dependence of Fluorescence QY.. 104
TCSPC Fitting and Calculation of FRET Efficiency 105
Calculation of the V79 Emission Resulting from TES-ADT Annihilation. 106
Chapter 4: Disrupted Ligand Packing Allows Close Approach and Enhanced Sensitization of Annihilators for TTA-UC.. 108
Introduction. 108
Methods. 110
Synthesis and Purification of PbS QDs 110
Ligand Exchange with Branching Ligands 111
Characterization of Ligand Exchange by 1H-NMR.. 111
Preparation of Upconversion Solutions 112
Results & Discussion. 112
Outlook & Future Directions. 124
Appendix C: Supporting Information for Chapter 4. 126
General Considerations 126
Ligand Exchange with Aromatic Ligands 126
Subtraction of Oleic Acid Signals in 1H-NMR.. 128
Molecular Dynamics Simulations 131
Appendix D: 1H-NMR Spectra of the Studied Ligands. 134
Group A Ligands 134
Group B Ligands 141
Group C Ligands 150
References. 158
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