2026-09-20T09:06:46Zhttps://dspace.mit.edu/server/oai/requestoai:dspace.mit.edu:1721.1/1228722026-06-16T18:54:37Zcom_1721.1_7582com_1721.1_7581col_1721.1_131022
Marc A. Baldo and Vladimir Bulović.
Wu, Mengfei,Ph.D.Massachusetts Institute of Technology.
Massachusetts Institute of Technology. Department of Electrical Engineering and Computer Science.
Massachusetts Institute of Technology. Department of Electrical Engineering and Computer Science
2019-11-12T17:40:38Z
2019-11-12T17:40:38Z
2018
2018
https://hdl.handle.net/1721.1/122872
1126650718
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2018
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 152-163).
Photon upconversion is a process where two or more low-energy photons are converted into a single higher-energy photon. Upconversion that turns infrared photons into visible ones is particularly useful, having potential applications in photovoltaics, infrared sensing, and biological imaging. In this thesis, I present a solid-state thin-film device that converts infrared photons with wavelength up to 1.1 [mu]m into visible wavelengths around [lambda] = 610 nm. The device consists of a monolayer of lead sulfide colloidal nanocrystals (NCs) and a thin film of rubrene mixed with emissive DBP molecules. Upconversion is realized via triplet-triplet annihilation (TTA) in rubrene sensitized by the NCs. We demonstrate that compared to the previous all-molecular upconverting systems, the use of inorganic NCs helps extend the excitation wavelength into the infrared and offers simple wavelength tunability.
However, a monolayer of NCs has low infrared absorption, severely limiting the upconversion efficiency and necessitating a high excitation intensity. Here, by adding a silver back reflector with an optical spacer to the device structure, we achieve a five-fold increase in the NC absorption due to optical interference effects and an eleven-fold enhancement in the up-converted output. To extend the idea, we further introduce a distributed Bragg reflector at the front of the device. A resonant microcavity is formed with the NCs placed at the peak of a drastically enhanced optical field. The upconversion efficiency is improved by another order of magnitude, with threshold excitation intensity falling to 13 mW/cm² , which is below the available solar flux. At resonance, the device converts (0.06±0.01)% of incident photons at [lambda] = 980 nm into emitted higher-energy photons. In addition, we improve the upconversion efficiency by shortening the surface ligands on NCs.
With faster triplet transfer, the upconverting device attains higher intrinsic efficiency, converting (7±l)% of the absorbed photons at [lambda] = 808 nm into higher-energy emissive excitons in rubrene. This thesis demonstrates the feasibility of NC-sensitized infrared-to-visible upconversion in solid thin films under low excitation intensities comparable to the solar flux, and paves the way toward the practical utilization of TTA-based upconversion in photovoltaics, imaging, and sensing technologies.
by Mengfei Wu.
Ph.D.
Ph.D. Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science
163 pages
eng
Massachusetts Institute of Technology
MIT theses are protected by copyright. They may be viewed, downloaded, or printed from this source but further reproduction or distribution in any format is prohibited without written permission.
http://dspace.mit.edu/handle/1721.1/7582
Electrical Engineering and Computer Science.
Infrared-to-visible upconversion in hybrid thin films of colloidal nanocrystals and organic molecules
Thesis
application/pdf
2019-11-12T17:40:37Z
Publication
Doctoral
EECS
unknown
unknown
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<Abstract>Photon upconversion is a process where two or more low-energy photons are converted into a single higher-energy photon. Upconversion that turns infrared photons into visible ones is particularly useful, having potential applications in photovoltaics, infrared sensing, and biological imaging. In this thesis, I present a solid-state thin-film device that converts infrared photons with wavelength up to 1.1 [mu]m into visible wavelengths around [lambda] = 610 nm. The device consists of a monolayer of lead sulfide colloidal nanocrystals (NCs) and a thin film of rubrene mixed with emissive DBP molecules. Upconversion is realized via triplet-triplet annihilation (TTA) in rubrene sensitized by the NCs. We demonstrate that compared to the previous all-molecular upconverting systems, the use of inorganic NCs helps extend the excitation wavelength into the infrared and offers simple wavelength tunability.</Abstract>
<Abstract>However, a monolayer of NCs has low infrared absorption, severely limiting the upconversion efficiency and necessitating a high excitation intensity. Here, by adding a silver back reflector with an optical spacer to the device structure, we achieve a five-fold increase in the NC absorption due to optical interference effects and an eleven-fold enhancement in the up-converted output. To extend the idea, we further introduce a distributed Bragg reflector at the front of the device. A resonant microcavity is formed with the NCs placed at the peak of a drastically enhanced optical field. The upconversion efficiency is improved by another order of magnitude, with threshold excitation intensity falling to 13 mW/cm² , which is below the available solar flux. At resonance, the device converts (0.06±0.01)% of incident photons at [lambda] = 980 nm into emitted higher-energy photons. In addition, we improve the upconversion efficiency by shortening the surface ligands on NCs.</Abstract>
<Abstract>With faster triplet transfer, the upconverting device attains higher intrinsic efficiency, converting (7±l)% of the absorbed photons at [lambda] = 808 nm into higher-energy emissive excitons in rubrene. This thesis demonstrates the feasibility of NC-sensitized infrared-to-visible upconversion in solid thin films under low excitation intensities comparable to the solar flux, and paves the way toward the practical utilization of TTA-based upconversion in photovoltaics, imaging, and sensing technologies.</Abstract>
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