Design and synthesis of colloidal quantum dots and their optoelectronic properties.

Zhang, Hui (2021). Design and synthesis of colloidal quantum dots and their optoelectronic properties. Thèse. Québec, Doctorat en sciences de l'énergie et des matériaux, Université du Québec, Institut national de la recherche scientifique, 181 p.

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Résumé

Colloidal quantum dots (QDs) are semiconductor nanocrystals (several to tens nm), whose size is smaller than the exciton Bohr radius of the bulk material. QDs exhibit discrete energy levels resulting from quantum confinement effect in all three spatial dimensions, rendering them unique size/composition/shape-dependent optoelectronic properties. Compared to bulk materials, QDs have unique properties such as tunable band gaps, broad absorption spectra, multiple exciton generation and high extinction coefficients. The controlled synthesis of colloidal semiconductor QDs has defined a new way to use them as potential building blocks in multiple emerging technologies such as biomedical labels, thermal sensors, light-emitting diodes, QDs sensitized solar cells (QDSCs), solar-driven photoelectrochemical (PEC) cells for hydrogen (H₂) generation and photodetectors. In this thesis, colloidal bare, doped and core/shell QDs are designed and synthesized with tunable size and narrow size distribution. Subsequently, their structural and optoelectronic properties are investigated systematically. A facile approach was designed for the synthesis of colloidal PbS QDs using thiourea and lead acetate as precursors for sulfur and lead, respectively. The sizes of the PbS QDs could be systematically controlled by simply adjusting the reaction parameters. Then, a Cd post-treatment on the bare PbS QDs via a cation exchange method was performed for increasing the stability of QDs, which is favorable for the fabrication of device under ambient environment. As a proof of concept, as-synthesized PbS QDs were employed as light harvesters for both (i) solar-driven PEC cells for H₂ generation and (ii) QDSCs. A saturated photocurrent density of ~2 mA/cm² under one sun illumination (AM 1.5G, 100 mW/cm²) is observed for PEC device based on PbS QDs synthesized using thiourea, which is comparable with that of PEC device based on PbS QDs synthesized using air-sensitive and unstable bis(trimethylsilyl) sulfide [(TMS)₂S]. For QDSCs, the devices fabricated with QDs synthesized from thiourea reveal a better power conversion efficiency (2.00%) compared to devices fabricated with QDs synthesized from traditional (TMS)₂S (1.40%) under one sun illumination (AM 1.5G, 100 mW/cm²). This work demonstrates that the developed synthetic method is a promising alternative to existing methodologies of PbS QDs and holds great potential for future solar technologies. Instead of adjusting the size of bare QDs to tune their optoelectronic of properties, doping of transition metal ions (such as Mn, Co, Cu, Ag, etc.) in semiconductor QDs is another promising approach for introducing unique properties. Although manganese (Mn) ions doped wide band gap (larger than the energy for Mn ⁴T₁₋⁶A₁ transition of 2.12 eV, such as ZnSe) QDs have been widely studied, the optical properties and emission mechanisms of Mn-doped narrow band gap (smaller than 2.12 eV, such as PbS) QDs are not well understood. The oil-soluble Mn-doped PbS QDs of identical size were designed and synthesized with different Mn contents. Compared with pure PbS QDs, the photoluminescence (PL) peak positions of Mn-doped PbS QDs are observed significantly red shifts, resulting in an enlarged Stokes shift. The large Stokes shift of Mn-doped QDs is attributed to a new emission mechanism (named as “Mn-PbS” emission), in which the photogenerated electron is transferred to the electronic states of Mn ions (⁴T₁) and then recombined with holes in the valence band (VB) of QDs host. The Mn-doped PbS QDs is also found to exhibit a faster temperature-dependent PL response compared to pure PbS QDs, demonstrating that the Mn-doped PbS QDs are promising alternatives for use in thermal sensing. Colloidal bare QDs suffer from high density of surface traps that acts as recombination center, resulting in undesirable charge recombination and limiting the efficiency and long-term stability for their application in optoelectronic device. The core/shell architecture is regarded as a promising approach to resolve these issues. The optoelectronic properties and band alignment in colloidal heterostructured CdS/CdSe core/shell QDs are engineered by tuning the shell thickness. The changes in structural and optical properties were investigated as a function of CdSe shell thickness (0.6-1.9 nm) overgrowth on a CdS core QD with a size of 3.0 nm in diameter. Results demonstrate that the optimization of the shell thickness can significantly broaden the light absorption range towards longer wavelengths and enhance the rate of photoelectron separation and transport in QDs/TiO₂ photoanode. Complementing by theoretical modeling, it is found that the band alignment in this heterostructured system could be engineered from a quasi type-II (electrons are localized over the whole QDs region but holes are delocalized mostly in the shell) to an inverted type-I (both electrons and holes are mostly delocalized in the shell) localization regime by changing the shell thickness. Optimizing the optoelectronic properties and band alignment in this colloidal core/shell QD system with a shell thickness of 1.6 nm together with carbon nanotubes (CNTs)-TiO₂ hybrid photoanode has enabled the demonstration of a PEC cell with a high photocurrent density of up to ~16.0 mA/cm² (at 0.9 V vs. reversible hydrogen electrode, RHE), which represents the highest value ever reported for PEC cells based on CdS/CdSe QDs. The PEC cells also reveal excellent long-term stability, remaining 83% of its initial value after 4 h under one continuous sun illumination (AM 1.5G, 100 mW/cm²). These results indicate that the design and optimization of the optoelectronics and band alignment of heterostructured core/shell QDs is a facile and efficient approach to fabricate highly efficient and stable QD-based PEC cells for H₂ generation and other optoelectronic devices.

Type de document:	Thèse Thèse
Directeur de mémoire/thèse:	Rosei, Federico
Co-directeurs de mémoire/thèse:	Sun, Shuhui
Mots-clés libres:	quantum dots; thiourea; manganese doped; photoluminescence mechanism; lead sulfide; temperature-dependent optical properties; engineered optoelectronic properties; Heterostructured core/shell quantum dots; band alignment tunable; theoretical calculation; quantum dots sensitized solar cells; photoelectrochemical cells
Centre:	Centre Énergie Matériaux Télécommunications
Date de dépôt:	10 juin 2021 15:29
Dernière modification:	10 juin 2021 15:29
URI:	https://espace.inrs.ca/id/eprint/11778

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