Synthesis and characterization of near-infrared colloidal core/shell quantum dots for solar technologies.

Tong, Xin (2019). Synthesis and characterization of near-infrared colloidal core/shell quantum dots for solar technologies. Thèse. Québec, Doctorat en sciences de l'énergie et des matériaux, Université du Québec, Institut national de la recherche scientifique, 147 p.

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Résumé

Colloidal quantum dots (QDs) have attracted considerable attention owing to their outstanding optical properties including broad light absorption, size/composition-tunable optical emission, high photoluminescence quantum yield (PLQY) and multiple exciton generation. In this regard, QDs have been extensively used as building blocks in various solar technologies including luminescent solar concentrators (LSCs), solar cells and solar-driven photoelectrochemical (PEC) hydrogen generation. Specifically, QDs with near-infrared (NIR) optical properties exhibit great potential for various applications because their broadband NIR light absorption is favorable to improve the efficiency of photovoltaic devices and their tunable NIR photoluminescence (PL) emission matches well with the wavelength region of biomedical window (~750-1350 nm) and optical communication (~1350-1600 nm). As a result, NIR QDs have been employed to fabricate high performance optoelectronic devices such as solar cells, light-emitting diodes (LEDs) and photodetectors, and are used for biological imaging/detection and optical fibers as well. However, to date, a majority of NIR QDs contain highly toxic chemical elements (e.g. Pb and Hg), which are harmful to human health and the natural environment and are not favorable for their future commercialization towards real-life applications. NIR, environmentally friendly QDs (e.g. CuInS/Se QDs) have been developed in recent years, while their multicomponent nature leads to the easily formation of surface trap states and defects. These surface traps/ defects can deteriorate the PLQY and photo/chemical-stability of QDs, which leads to the non radiative recombination of QDs, thus hindering the development of high-efficiency and long term stable QDs-based optoelectronic devices. To address these problems, one approach is to develop heavy metal-free and NIR core/shell structured QDs with optimized optical properties and enhanced photo/chemical-stability, focusing on the comprehensively investigation of their growth mechanism, band structure engineering, photoexcited electron-hole dynamic to realize high performance solar energy conversion applications (e.g. solar-drive PEC cells). In the first part, we synthesized NIR, environmentally friendly CuInSexS2-x (CISeS, C represents Copper instead of Carbon)/ZnS core/shell QDs by employing cation exchange method. Morphological investigation of these QDssuggeststhatsuch cation exchange approach leads to the formation of a thin ZnS shell on the surface of CISeS core QDs. Optical characterizations show the NIR optical absorption (up to 1000 nm) and PL spectrum (emission center at~960 nm). By comparing the PL intensity and PL lifetime of bare CISeS QDs and CISeS/ZnS core/shell QDs, it is demonstrated that the ZnS shell enables the effective passivation of the surface defects/traps on the CISeS QDs. These NIR, heavy metal-free QDs were then used as photosensitizers to fabricate QDs-sensitized photoanode by using electrophoretic deposition (EPD) technique. As-fabricated CISeS/ZnS core/shell QDs-sensitized photoanode was applied as a working electrode in a PEC cell for solar-driven hydrogen generation. Under standard one sun illumination (AM 1.5G, 100 mW/cm²), the QDs-based PEC cell exhibits a saturated photocurrent density of ~5.3 mA/cm² , which is higher than the saturated photocurrent density (~2.57 mA/cm² ) of bare CISeS QDs-based PEC cell, indicating the effective surface passivation of CISeS QDs by the ZnS thin shell for suppressed non-radiative recombination, thus enhancing the device performance. Moreover, CISeS/ZnS core-shell QDs-based PEC cell exhibited improved device stability as compared to bare CISeS QDs-based devices. In the second part, we synthesized NIR, heavy metal-free CuInSe₂/CuInS₂ (CISe/CIS) “giant” core/shell QDs (g-QDs) via sequential cation exchange technique. Morphological investigation demonstrates the formation of a CuInS2 thick shell with wurtzite (WZ) phase. Optical characterization of these NIR, heavy metal-free g-QDs exhibit tunable NIR absorption and PL spectra (up to 1100 nm). These NIR, environment-friendly g-QDs show a red-shift of PL peaks and prolonged lifetime with increasing shell thickness, demonstrating their “quasi-type II” band structure, wherein the electrons can delocalize into the shell region while the holes are well confined in the core region. By choosing appropriate physical parameters and solving the Schrödinger equation of these g-QDs, the electron-hole wavefunction distribution as a function of shell thickness is presented. The simulation results further verify the “quasi-type II” band structure of as-synthesized g-QDs, which is in accordance with their optical characterization. As a proof of concept, a PEC cell based on such NIR and “green” g-QDs was fabricated, showing a saturated photocurrent density of ~3 mA/cm2 under standard one sun illumination (AM 1.5G, 100 mW/cm²). By comparing the stability of PEC device based on g-QDs with various shell thickness, it is demonstrated that g-QDs with thicker shell exhibit higher photo/chemical stability in solar-driven PEC hydrogen production. Apart from the heavy metal-free NIR core/shell QDs, heterostructured semiconductor nanocrystals with non-spherical shapes such as dot-in-rod, tetrapod and pyramid have emerged as potential candidates in solar technologies, which exhibit particularly small spatial electron hole wavefunction overlap for ultralong PL lifetime that are beneficial to the photogenerated charge carrier separation/transfer in boosting the efficiency of QDs-based photovoltaic applications. In the third part, we developed a new type of NIR-emitting, pyramid-shaped CISeS/CdSeS/CdS g-QDs, which were synthesized by using a facile two-step method. Assynthesized g-QDs exhibit a pyramidal shape and the CdSeS/CdS shell was demonstrated to possess a zinc blende (ZB) phase. Optical properties of g-QDs exhibit NIR PL emission (~830 nm) with considerable PLQY of 17%. This type of pyramidal-shaped g-QDs show unprecedentedly prolonged PL lifetime up to ~2 s with increasing shell thickness, indicating that the pyramidal shape of these g-QDs can induce efficient electron-hole separation and form a “quasi-type II” band structure. Theoretical simulation was used to calculate the spatial electron-hole wave function distribution of as-synthesized g-QDs with various shell thickness, demonstrating the direction-dependent electron-hole wave function distribution and "quasi type-II" band structure of such pyramidal g-QDs. We subsequently used these g-QDs to fabricate PEC cells, showing a high saturated photocurrent of ~5.5 mA/cm² and outstanding device stability under one sun illumination (AM 1.5 G, 100 mW/cm²). These results indicate that this type of pyramid-shaped g-QDs show efficient electron-hole separation and are promising to achieve high performance photovoltaic devices.

Type de document:	Thèse Thèse
Directeur de mémoire/thèse:	Rosei, Federico
Mots-clés libres:	énergie; matériaux
Centre:	Centre Énergie Matériaux Télécommunications
Date de dépôt:	16 mars 2022 12:44
Dernière modification:	16 mars 2022 12:44
URI:	https://espace.inrs.ca/id/eprint/12501

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