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Synthesis and Characterization of Semiconductor Nanocrystals for Solar Technologies.

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Zhou, Yufeng (2018). Synthesis and Characterization of Semiconductor Nanocrystals for Solar Technologies. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 153 p.

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Semiconductor nanocrystals (NCs) have been widely investigated in recent decades due to their unique optical, electrical properties and various applications in solar technologies, such as solar cells, luminescent solar concentrators (LSCs) and solar-driven water splitting. Among these semiconductor NCs, colloidal semiconducting quantum dots (QDs), are regarded as promising candidates for LSCs applications, with size/composition- tunable optical properties (including high photoluminescence quantum yield (PLQY), broad absorption, high photo/chemical stability, etc. However, it is still challenging to synthesize colloidal QDs with large Stokes shift, high PLQY and broad absorption spectrum, which are the criteria for large-area high efficient LSCs fabrications. In addition, ultra-thin semiconducting nanoplatelets (NPLs) with high carrier mobility, narrow emission spectrum and high PLQY, have been investigated and used for light emitting diodes, solar-driven hydrogen generation applications. To broaden the absorption range of ultra-thin NPLs for solar technologies, it is demanding to synthesize near infrared (NIR) NPLs with high QY and colloidal photo/chemical stability. I firstly synthesized mixed-halide perovskite CsPb(BrxI1−x)3 QDs with different ratios of Br/I precursors via hot-injection method, which show tunable absorption and emission spectra. The mixed-halide perovskite CsPb(Br0.2I0.8)3 QDs exhibit high PLQY of ~60%, small overlap of absorption and emission, broad absorption ranging from 300-650 nm, which is a promising material as PL emitters in LSCs applications. The optical performance of LSCs based on perovskite CsPb(Br0.2I0.8)3 QDs was investigated and an external optical efficiency of 2% with a geometrical (G) gain factor of 45 has been achieved. In addition, the LSCs exhibit long-term stability without any noticeable variations in PL emissions and lifetimes under ultraviolet (UV, 4W) illumination for over four hours. To extend the light absorption spectrum of QDs, near infrared (NIR) PbS/CdS core/shell QDs have been synthesized with various core sizes and shell thicknesses. Specifically, the bare PbS QDs was prepared via hot-injection method with different capping ligands. An engineered Stokes shift in NIR core/shell PbS/CdS QDs was achieved via a cation exchange approach by varying the core size and shell thickness through the refined reaction parameters, such as reaction time, temperature, precursor molar ratio, etc. The as-synthesized core/shell QDs with high PLQY and excellent chemical/photo- stability exhibit a large Stokes shift with respect to the bare PbS QDs due to the strong core-to-shell electrons leakage. The large-area planar LSC based on core/shell QDs exhibits the higher value (6.1% with a G factor of 10) for optical efficiency than that of the bare NIR QD-based LSCs and other reported NIR QD-based LSCs. The suppression of reabsorption loss and the broad absorption of PbS/CdS QDs offer a promising pathway to integrate LSCs and photovoltaic devices with well spectral matching. Compared to conventional inorganic QDs which act as PL emitters in LSCs, carbon dots (C-dots) have superior advantages of non-toxicity, environmental friendliness, low-cost and simple preparation using abundant carbon based feedstock. We synthesized colloidal C-dots via solvothermal method using different solvents, surfactants and carbon sources. We further demonstrated large-area LSCs (up to 100 cm2) using colloidal C-dots. Two types of LSCs were fabricated by either incorporating oil-soluble oleylamine-treated C-dots into photo-polymerized poly(laurylmethacrylate) (PLMA) or spin-coating the water-soluble C-dots/polyvinylpyrrolidone (PVP) mixture on the glass substrate. The LSCs based on C-dots/PLMA exhibit an internal quantum efficiency of 4% (G factor of 38), and an optical efficiency of 1.1% (100 cm2, G factor of 12.5) of tandem thin-film LSCs based on C-dots/PVP was achieved under one sun illumination. The optical performance is comparable to those of LSCs based on inorganic QDs with similar G factor. The LSCs based on C-dots are highly air-stable without any noticeable variations in PL emissions under UV illumination (1.3 W/cm2) for over 12 h. These large-area LSCs based on C-dots exhibit a highly transparency (over 90% for wavelengths longer than 500 nm) with low reabsorption losses, excellent optical performance including high optical efficiency and very good photostability. Aside from the fabrication of QDs based LSCs, we developed a cation exchange route to synthesize ultrasmall lead chalcogenides NIR PbSe1–xSx NPLs. Basically, the as-prepared PbSe1–xSx NPLs with small lateral dimensions, controlled thickness of ~2 nm, and different compositions were obtained via a cation exchange process on template CdSe or alloyed CdSe1–xSx NPLs using various Pb precursors. The NIR NPLs exhibit continuously tunable PL emission ranging from 1180 to 1380 nm thanks to the variation of the S/Se ratio, which cannot be achieved in binary NPLs, and a high PLQY up to ~60%. Theoretical simulations of the bandgap as a function of thickness, geometry, and lateral size show that ultrasmall NPLs exhibit strong 3D quantum confinement, compared to 1D confinement in larger sized NPLs with similar thickness. As a proof-of-concept, we used the NPLs as photosensitizers for solar-driven hydrogen generation. After surface treatment with Cd, a saturated photocurrent density of ~5 mA/cm2 at 1.0 V vs RHE was obtained using NIR NPLs based photoanode under one sun illumination.

Type de document: Thèse Thèse
Directeur de mémoire/thèse: Rosei, Federico
Co-directeurs de mémoire/thèse: Ma, Dongling
Mots-clés libres: énergie et matériaux
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 10 avr. 2019 15:18
Dernière modification: 10 avr. 2019 15:18
URI: http://espace.inrs.ca/id/eprint/8030

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