Photoluminescence Properties of Core/Shell CdSe/ZnS Quantum Dots Encapsulated with Transparent layers for Third Generati
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Photoluminescence Properties of Core/Shell CdSe/ZnS Quantum Dots Encapsulated with Transparent layers for Third Generation Photovoltaics Bahareh Sadeghimakki1, 2, Navid Mohammad Sadeghi Jahed1, 2 and Siva Sivoththaman1, 2 Center for Advanced Photovoltaic Devices and Systems (CAPDS), Waterloo, ON N2L3G1 Canada 2 Electrical and Computer Engineering Department, University of Waterloo, Waterloo, ON N2L3G1 Canada 1
ABSTRACT In this work hydrophobicaly ligated cadmium selenide/zinc sulfide CdSe/ZnS quantum dots (QDs) were incorporated in transparent matrices by formation of CdSe/ZnS/SiO2 core/shell/shell structure using microemolsion synthesis method. The optical properties of the QDs encapsulated with a chemically grown oxide layers were studied. Intense luminescence properties of the QD/silica nanoparticles (NPs) were observed using steady state photoluminescence (PL) measurements. Confocal microscopy demonstrates fluorescence of the single core/shell/shell nanoparticles. The obtained results along with the Secondary Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) images provide information on the geometry of the QDs. The excitonic emission of nanoparticles was also mapped using a liquid nitrogen cryostat in the 77K - 300K range. The temperature dependent PL spectra of the film demonstrate the temperature-dependent band gap shrinkage of the QDs. PL lifetime measurements were performed on the ensemble of NPs. Experimental data was fitted to the numerical model with lifetime constants in nanoseconds range. We demonstrate that the main nonradiative processes that limit the quantum yield (QY) of the QDs at room temperature are the carrier trapping at the interface of QD/silica and the exciton-phonon coupling. These studies give us insight to exploit the QD layers for photon down shifting and multiple exciton generation for application in photovoltaics. INTRODUCTION Colloidal semiconductor quantum dots with a diameter of a few nanometers have generated remarkable technological interest as the active material either in optoelectronic devices such as light-emitting diodes, lasers and solar cells or for biological applications [1]. Their high photoluminescence QY, tunable emission wavelength, photostability and multiplexing capabilities are dramatically different than the bulk material due to electron confinement in three special dimensions. The luminescence properties of QDs are sensitive to surface interactions, and the degree of surface passivation has been shown to be a crucial parameter in determining the QY. In order to use colloidal quantum dots for new generation of photovoltaic devices thin films of QDs have to be realized [2]. Close-packed nanocrystal (NC) superlatices ordered over a large area produce a media with a great potential to deploy as down shifter layers on the solar cells for efficiency enhancement. The larger are the nancrystals the easier to form self-assembled layers. In this work, an oxide layer was grown on the CdSe/ZnS QDs using microemolsion synthesis method in order to increas
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