Show simple item record

dc.contributor.advisorMarc A. Baldo and Vladimir Bulović.en_US
dc.contributor.authorWu, Mengfei,Ph.D.Massachusetts Institute of Technology.en_US
dc.contributor.otherMassachusetts Institute of Technology. Department of Electrical Engineering and Computer Science.en_US
dc.date.accessioned2019-11-12T17:40:38Z
dc.date.available2019-11-12T17:40:38Z
dc.date.copyright2018en_US
dc.date.issued2018en_US
dc.identifier.urihttps://hdl.handle.net/1721.1/122872
dc.descriptionThesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2018en_US
dc.descriptionCataloged from PDF version of thesis.en_US
dc.descriptionIncludes bibliographical references (pages 152-163).en_US
dc.description.abstractPhoton 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.en_US
dc.description.abstractHowever, 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.en_US
dc.description.abstractWith 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.en_US
dc.description.statementofresponsibilityby Mengfei Wu.en_US
dc.format.extent163 pagesen_US
dc.language.isoengen_US
dc.publisherMassachusetts Institute of Technologyen_US
dc.rightsMIT 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.en_US
dc.rights.urihttp://dspace.mit.edu/handle/1721.1/7582en_US
dc.subjectElectrical Engineering and Computer Science.en_US
dc.titleInfrared-to-visible upconversion in hybrid thin films of colloidal nanocrystals and organic moleculesen_US
dc.typeThesisen_US
dc.description.degreePh. D.en_US
dc.contributor.departmentMassachusetts Institute of Technology. Department of Electrical Engineering and Computer Scienceen_US
dc.identifier.oclc1126650718en_US
dc.description.collectionPh.D. Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Scienceen_US
dspace.imported2019-11-12T17:40:37Zen_US
mit.thesis.degreeDoctoralen_US
mit.thesis.departmentEECSen_US


Files in this item

Thumbnail

This item appears in the following Collection(s)

Show simple item record