Colloidal Quantum Dots for Solar Technologies

Advanced Interface Engineering in Gradient Core/Shell Quantum Dots Enables Efficient Photoelectrochemical Hydrogen Evolution

Semiconductor core/shell quantum dots (QDs) are considered promising building blocks to fabricate photoelectrochemical (PEC) cells for the direct conversion of solar energy into hydrogen (H2). However, the lattice mismatch between core and shell in such QDs results in undesirable defects and severe carrier recombination, limiting photo-induced carrier separation/transfer and solar-to-fuel conversion efficiency. Here, an interface engineering approach is explored to minimize the core-shell lattice mismatch in CdS/CdSexS1-x (x = 0.09-1) core/shell QDs (g-CSG). As a proof-of-concept, PEC cells based on g-CSG QDs yield a remarkable photocurrent density of 13.1 mA cm-2 under AM 1.5 G one-sun illumination (100 mW cm-2), which is ≈54.1% and ≈33.7% higher compared to that in CdS/CdSe0.5S0.5 (g-CSA) and CdS/CdSe QDs (g-CS), respectively. Theoretical calculations and carrier dynamics confirm more efficient carrier separation and charge transfer rate in g-CSG QDs with respect to g-CSA and g-CS QDs. These results are attributed to the minimization of the core-shell lattice mismatch by the cascade gradient shell in g-CSG QDs, which modifies carrier confinement potential and reduces interfacial defects. This work provides fundamental insights into the interface engineering of core/shell QDs and may open up new avenues to boost the performance of PEC cells for H2 evolution and other QDs-based optoelectronic devices.

Efficient Photoelectrochemical Hydrogen Generation Based on Core Size Effect of Heterostructured Quantum Dots

Colloidal quantum dots (QDs) are shown to be effective as light-harvesting sensitizers of metal oxide semiconductor (MOS) photoelectrodes for photoelectrochemical (PEC) hydrogen (H2) generation. The CdSe/CdS core/shell architecture is widely studied due to their tunable absorption range and band alignment via engineering the size of each composition, leading to efficient carrier separation/transfer with proper core/shell band types. However, until now the effect of core size on the PEC performance along with tailoring the core/shell band alignment is not well understood. Here, by regulating four types of CdSe/CdS core/shell QDs with different core sizes (diameter of 2.8, 3.1, 3.5, and 4.8 nm) while the thickness of CdS shell remains the same (thickness of 2.0 ± 0.1 nm), the Type II, Quasi-Type II, and Type I core/shell architecture are successfully formed. Among these, the optimized CdSe/CdS/TiO2 photoelectrode with core size of 3.5 nm can achieve the saturated photocurrent density (Jph) of 17.4 mA cm-2 under standard one sun irradiation. When such cores are further optimized by capping alloyed shells, the Jph can reach values of 22 mA cm2 which is among the best-performed electrodes based on colloidal QDs.

Efficient Photoelectrochemical Hydrogen Generation Using Eco-Friendly "Giant" InP/ZnSe Core/Shell Quantum Dots

InP quantum dots (QDs) are promising building blocks for use in solar technologies because of their low intrinsic toxicity, narrow bandgap, large absorption coefficient, and low-cost solution synthesis. However, the high surface trap density of InP QDs reduces their energy conversion efficiency and degrades their long-term stability. Encapsulating InP QDs into a wider bandgap shell is desirable to eliminate surface traps and improve optoelectronic properties. Here, we report the synthesis of "giant" InP/ZnSe core/shell QDs with tunable ZnSe shell thickness to investigate the effect of the shell thickness on the optoelectronic properties and the photoelectrochemical (PEC) performance for hydrogen generation. The optical results demonstrate that ZnSe shell growth (0.9-2.8 nm) facilitates the delocalization of electrons and holes into the shell region. The ZnSe shell simultaneously acts as a passivation layer to protect the surface of InP QDs and as a spatial tunneling barrier to extract photoexcited electrons and holes. Thus, engineering the ZnSe shell thickness is crucial for the photoexcited electrons and hole transfer dynamics to tune the optoelectronic properties of "giant" InP/ZnSe core/shell QDs. We obtained an outstanding photocurrent density of 6.2 mA cm^-1 for an optimal ZnSe shell thickness of 1.6 nm, which is 288% higher than the values achieved from bare InP QD-based PEC cells. Understanding the effect of shell thickness on surface passivation and carrier dynamics offers fundamental insights into the suitable design and realization of eco-friendly InP-based "giant" core/shell QDs toward improving device performance.

Surface Stoichiometry Control of Colloidal Heterostructured Quantum Dots for High-Performance Photoelectrochemical Hydrogen Generation

Manipulating the separation and transfer behaviors of charges has long been pursued for promoting the photoelectrochemical (PEC) hydrogen generation based on II-VI quantum dot (QDs), but remains challenging due to the lack of effective strategies. Herein, a facile strategy is reported to regulate the recombination and transfer of interfacial charges through tuning the surface stoichiometry of heterostructured QDs. Using this method, it is demonstrated that the PEC cells based on CdSe-(Se_x S_{1- x} )₄ -(CdS)₂ core/shell QDs with a proper S_surface /Cd_surface ratio exhibits a remarkably improved photocurrent density (≈18.4 mA cm^-2 under one sun illumination), superior to the PEC cells based on QDs with Cd-rich or excessive S-rich surface. In-depth electrochemical and spectroscopic characterizations reveal the critical role (hole traps) of surface S atoms in suppressing the recombination of photogenerated charges, and further attribute the inferior performance of excessive S-rich QDs to the impeded charge transfer from QDs to TiO₂ and electrolyte. This work puts forward a simple surface engineering strategy for improving the performance of QDs PEC cells, providing an efficient method to guide the surface design of QDs for their applications in other optoelectronic devices.

Ligand-Engineered Quantum Dots Decorated Heterojunction Photoelectrodes for Self-Biased Solar Water Splitting

A cost-effective and high-efficiency photoelectrochemical (PEC) water splitting system based on colloidal quantum dots (QDs) represents a potential solar-to-hydrogen (STH) conversion technology to achieve future carbon neutrality. Herein, a self-biased PEC cell consisting of BiVO₄ photoanode and Cu₂ O photocathode both decorated with Zn-doped CuInS₂ (ZCIS) QDs is successfully fabricated. The intrinsic charge dynamics of the photoelectrodes are efficiently optimized via rational engineering of the surface ligands capped on QDs with controllable chain lengths and binding affinities to the metal oxide electrodes. It is demonstrated that the short-chain monodentate 1-dodecanethiol ligands are beneficial to ZCIS QDs for suppressing charge recombination, which enables the construction of tight heterojunction with coupled metal oxide electrodes, leading to effective photo-induced charge transfer/injection for enhanced PEC performance. The QD decorated BiVO₄ and Cu₂ O photoelectrodes in pairs demonstrate a self-biased PEC water splitting process, delivering an STH efficiency of 0.65% with excellent stability under AM 1.5 G one-sun illumination. The results highlight the significance of synergistic ligand and heterojunction engineering to build highly efficient and robust QDs-based PEC devices for self-biased solar water splitting.

Nonadiabatic Molecular Dynamics with Extended Density Functional Tight-Binding: Application to Nanocrystals and Periodic Solids

In this work, we report a new methodology for nonadiabatic molecular dynamics calculations within the extended tight-binding (xTB) framework. We demonstrate the applicability of the developed approach to finite and periodic systems with thousands of atoms by modeling "hot" electron relaxation dynamics in silicon nanocrystals and electron-hole recombination in both a graphitic carbon nitride monolayer and a titanium-based metal-organic framework (MOF). This work reports the nonadiabatic dynamic simulations in the largest Si nanocrystals studied so far by the xTB framework, with diameters up to 3.5 nm. For silicon nanocrystals, we find a non-monotonic dependence of "hot" electron relaxation rates on the nanocrystal size, in agreement with available experimental reports. We rationalize this relationship by a combination of decreasing nonadiabatic couplings related to system size and the increase of available coherent transfer pathways in systems with higher densities of states. We emphasize the importance of proper treatment of coherences for obtaining such non-monotonic dependences. We characterize the electron-hole recombination dynamics in the graphitic carbon nitride monolayer and the Ti-containing MOF. We demonstrate the importance of spin-adaptation and proper sampling of surface hopping trajectories in modeling such processes. We also assess several trajectory surface hopping schemes and highlight their distinct qualitative behavior in modeling the excited-state dynamics in superexchange-like models depending on how they handle coherences between nearly parallel states.

Engineered Environment-Friendly Colloidal Core/Shell Quantum Dots for High-Efficiency Solar-Driven Photoelectrochemical Hydrogen Evolution

"Green" colloidal quantum dots (QDs)-based photoelectrochemical (PEC) cells are promising solar energy conversion systems possessing environmental friendliness, cost-effectiveness, and highly efficient solar-to-hydrogen conversion. In this work, eco-friendly AgInSe (AISe)/ZnSe core/shell QDs with wurtzite (WZ) phase were synthesized for solar hydrogen production. It was demonstrated that appropriately engineering the ZnSe shell thickness resulted in effective surface defects passivation of the AISe core for suppressed charge recombination in the consequent core/shell AISe/ZnSe QDs. The fabricated environmentally friendly core/shell QDs-based PEC device exhibited improved photo-excited electrons extraction efficiency under optimized conditions and delivered a maximum photocurrent density as high as 7.5 mA cm^-2 and long-term durability under standard AM 1.5G illumination (100 mW cm^-2 ). These findings suggest that AISe/ZnSe core/shell QDs with tailored optoelectronic properties are potential light sensitizers for eco-friendly, cost-effective, and highly efficient solar energy conversion applications.

Band Structure Engineering and Defect Passivation of Cu x Ag1-x InS2/ZnS Quantum Dots to Enhance Photoelectrochemical Hydrogen Evolution

The AgInS₂ colloidal quantum dot (CQD) is a promising photoanode material with a relatively wide band gap for photoelectrochemical (PEC) solar-driven hydrogen (H₂) evolution. However, the unsuitable energy band structure still forms undesired energy barriers and leads to serious charge carrier recombination with low solar to hydrogen conversion efficiency. Here, we propose to use the ZnS shell for defect passivation and Cu ion doping for band structure engineering to design and synthesize a series of Cu _x Ag_1-x InS₂/ZnS CQDs. ZnS shell-assisted defect passivation suppresses charge carrier recombination because of the formation of the core/shell heterojunction interface, enhancing the performance of PEC devices with better charge separation and stability. More importantly, the tunable Cu doping concentration in AgInS₂ CQDs leads to the shift of the quantum dot band alignment, which greatly promotes the interfacial charge separation and transfer. As a result, Cu _x Ag_1-x InS₂/ZnS CQD photoanodes for PEC cells exhibit an enhanced photocurrent of 5.8 mA cm^-2 at 0.8 V versus the RHE, showing excellent photoelectrocatalytic activity for H₂ production with greater chemical-/photostability.

Recent advances of near infrared inorganic fluorescent probes for biomedical applications

Near infrared (NIR)-excitable and NIR-emitting probes have fuelled advances in biomedical applications owing to their power in enabling deep tissue imaging, offering high image contrast and reducing phototoxicity. There are essentially three NIR biological windows, i.e., 700-950 nm (NIR I), 1000-1350 nm (NIR II) and 1550-1870 nm (NIR III). Recently emerging optical probes that can be excited by an 800 nm laser and emit in the NIR II or III windows, denoted as NIR I-to-NIR II/III, are particularly attractive. That is because the longer wavelengths in the NIR II and NIR III windows offer deeper penetration and higher signal to noise ratio than those in the NIR I window. NIR imaging has indeed become a quickly evolving field and, simultaneously, stimulated the further development of new classes of NIR I-to-NIR II/III inorganic fluorescent probes, which include PbS, Ag₂S-based quantum dots (QDs) and rare earth (RE) doped NPs (RENPs) that possess quite diverse optical properties and follow different emission mechanisms. This review summarizes the recent progress on material merits, synthetic routes, the rational choice of excitation in the NIR I window, NIR II/III emission optimization, and surface modification of aforementioned fluorescent probes. We also introduce the latest notable accomplishments enabled by these probes in fluorescence imaging, lifetime-based multiplexed imaging and photothermal therapy (PTT), together with a critical discussion of forthcoming challenges and perspectives for clinic use.

Manipulating the Optoelectronic Properties of Quasi-type II CuInS2/CdS Core/Shell Quantum Dots for Photoelectrochemical Cell Applications

Colloidal core/shell heterostructured quantum dots (QDs) possessing quasi-type II band structure have demonstrated effective surface passivation and prolonged exciton lifetime, leading to enhanced charge separation/transfer efficiencies that are promising for photovoltaic device applications. Herein, we synthesized CuInS₂ (CIS)/CdS core/shell heterostructured QDs and manipulated the optoelectronic properties via controlling the CdS shell thickness. The shell-thickness-dependent optical properties indicate the existence of a quasi-type II band structure in such core/shell QDs, which was verified by ultrafast spectroscopy and theoretical simulations. These quasi-type II core/shell QDs having various shell thicknesses are used as light absorbers for the fabrication of solar-driven QDs-based photoelectrochemical (PEC) devices, exhibiting an optimized photocurrent density of ∼6.0 mA/cm² and excellent stability under simulated AM 1.5G solar illumination. The results demonstrate that quasi-type II CIS/CdS core/shell heterostructured QDs with tailored optoelectronic properties are promising to realize high-performance QDs-based solar energy conversion devices for the production of solar fuels.

Low-Cost, Air-Processed Quantum Dot Solar Cells via Diffusion-Controlled Synthesis

Despite significant advances in the development of high-efficiency and stable quantum dot (QD) solar cells (QDSCs), recent synthetic and fabrication routes still require improvements to render QDSCs commercially feasible. Here, we describe a low-cost, industrially viable fabrication method of QDSCs under an ambient atmosphere (humid air and room temperature) using stable, high-quality, and small-sized PbS QDs prepared with low-cost, greener precursors [i.e., thioacetamide (TAA)] compared to the widely used bis(trimethylsilyl)sulfide [(TMS)₂S], at low temperatures without requiring any stringent conditions. The low reaction temperature, medium reactivity of TAA, and diffusion-controlled particle growth adopted in this approach provide an opportunity to synthesize ultrasmall (emission peak ∼700 nm) to larger PbS QDs (emission peak ∼1050 nm). This also enables well-controlled large-scale (multigram) synthesis with a rough estimated production cost of PbS of 8.11 $ per gram (based on materials cost), which is the lowest among the available PbS QDs produced using wet chemistry routes. QDSCs fabricated using 3.25 nm PbS QDs (bandgap 1.29 eV) under ambient conditions yield a high circuit current density (J_sc) of 32.4 mA/cm² (one of the highest values of J_sc ever reported) with a power conversion efficiency of 7.8% under 1 sun simulated sunlight at AM 1.5 G (100 mW/cm²). These devices exhibit better photovoltaic performance compared to devices fabricated with more traditional PbS QDs synthesized with (TMS)₂S under an ambient atmosphere, confirming the quality of PbS QDs produced with our method. The diffusion-controlled TAA-based synthetic route developed herein is found to be very promising for synthesizing size-tunable PbS QDs for photovoltaic and other optoelectronic applications.

Tailoring the Heterostructure of Colloidal Quantum Dots for Ratiometric Optical Nanothermometry

Colloidal quantum dots (QDs) are a fascinating class of semiconducting nanocrystals, thanks to their optical properties tunable through size and composition, and simple synthesis methods. Recently, colloidal double-emission QDs have been successfully applied as competitive optical temperature sensors, since they exhibit structure-tunable double emission, temperature-dependent photoluminescence, high quantum yield, and excellent photostability. Until now, QDs have been used as nanothermometers for in vivo biological thermal imaging, and thermal mapping in complex environments at the sub-microscale to nanoscale range. In this Review, recent progress for QD-based nanothermometers is highlighted and perspectives for future work are described.

Experimental study of the size-dependent photoluminescence emission of CBD-grown PbSe nanocrystals on glass

In this work, we study the size-dependent properties of Photoluminescence (PL) emissions of PbSe Nanocrystals (NCs) grown by Chemical Bath Deposition (CBD) method. In previous studies, PL emissions have been tuned by CBD-grown PbSe, and the growth mechanism was dependent on crystalized substrates such as GaAs. In this research, however, PL emissions are controlled over the midinfrared (MIR) range, through PbSe NCs, which are deposited on glass as an amorphous material. This study proposes an alternative approach to control PL emissions, which provides us with more freedom to fabricate low-cost MIR light sources as crucial components in remote sensing and gas analysis. Moreover, in this study, the advantage of the post-thermal method to control the NCs size, compared to the growth temperature, is shown.

Photoexcited carrier dynamics in colloidal quantum dot solar cells: insights into individual quantum dots, quantum dot solid films and devices

The certified power conversion efficiency (PCE) record of colloidal quantum dot solar cells (QDSCs) has considerably improved from below 4% to 16.6% in the last few years. However, the record PCE value of QDSCs is still substantially lower than the theoretical efficiency. So far, there have been several reviews on recent and significant achievements in QDSCs, but reviews on photoexcited carrier dynamics in QDSCs are scarce. The photovoltaic performances of QDSCs are still limited by the photovoltage, photocurrent and fill factor that are mainly determined by the photoexcited carrier dynamics, including carrier (or exciton) generation, carrier extraction or transfer, and the carrier recombination process, in the devices. In this review, the photoexcited carrier dynamics in the whole QDSCs, originating from individual quantum dots (QDs) to the entire device as well as the characterization methods used for analyzing the photoexcited carrier dynamics are summarized and discussed. The recent research including photoexcited multiple exciton generation (MEG), hot electron extraction, and carrier transfer between adjacent QDs, as well as carrier injection and recombination at each interface of QDSCs are discussed in detail herein. The influence of photoexcited carrier dynamics on the physiochemical properties of QDs and photovoltaic performances of QDSC devices is also discussed.

Semiconductor nanocrystals for small molecule activation via artificial photosynthesis

The protocol of artificial photosynthesis using semiconductor nanocrystals shines light on green, facile and low-cost small molecule activation to produce solar fuels and value-added chemicals.

Epitaxial growth and defect repair of heterostructured CuInSexS2-x/CdSeS/CdS quantum dots

Heterostructured quantum dots (hetero-QDs) have outstanding optical properties and chemical/photostability, which make them promising building blocks for use in various optoelectronic devices. Here, CuInSe_xS_2-x/CdSeS/CdS hetero-QDs were synthesized through a facile two-step method. Their particle size, three-dimensional (3D) shapes and the epitaxial relationship between the CuInSe_xS_2-x/CdSeS core and CdS shell were investigated by high-resolution transmission electron microscopy (HRTEM). Our investigation proves that the as-synthesized hetero-QDs have a regular tetrahedron 3D shape with four {111} crystal facets. The epitaxial relationship between the CuInSe_xS_2-x/CdSeS core and CdS shell is determined to be [110]_core//[110]_shell, {112}_core//{111}_shell. In situ HRTEM observations show that the screw dislocation inside the hetero-QDs can be efficiently repaired using e-beam irradiation. These results may help in designing hetero-QDs with high-quality interfaces and identifying the strategies for synthesizing defect-free hetero-QDs.

Simultaneous Existence of Confined and Delocalized Vibrational Modes in Colloidal Quantum Dots

Coupling to phonon modes is a primary mechanism of excitonic dephasing and energy loss in semiconductors. However, low-energy phonons in colloidal quantum dots and their coupling to excitons are poorly understood because their experimental signatures are weak and usually obscured by the unavoidable inhomogeneous broadening of colloidal dot ensembles. We use multidimensional coherent spectroscopy at cryogenic temperatures to extract the homogeneous nonlinear optical response of excitons in a CdSe/CdZnS core/shell colloidal quantum dot ensemble. A comparison to the simulation provides evidence that the observed lineshapes arise from the coexistence of confined and delocalized vibrational modes, both of which couple strongly to excitons in CdSe/CdZnS colloidal quantum dots.

Semiconductor Quantum Dots: An Emerging Candidate for CO2 Photoreduction

As one of the most critical approaches to resolve the energy crisis and environmental concerns, carbon dioxide (CO₂ ) photoreduction into value-added chemicals and solar fuels (for example, CO, HCOOH, CH₃ OH, CH₄ ) has attracted more and more attention. In nature, photosynthetic organisms effectively convert CO₂ and H₂ O to carbohydrates and oxygen (O₂ ) using sunlight, which has inspired the development of low-cost, stable, and effective artificial photocatalysts for CO₂ photoreduction. Due to their low cost, facile synthesis, excellent light harvesting, multiple exciton generation, feasible charge-carrier regulation, and abundant surface sites, semiconductor quantum dots (QDs) have recently been identified as one of the most promising materials for establishing highly efficient artificial photosystems. Recent advances in CO₂ photoreduction using semiconductor QDs are highlighted. First, the unique photophysical and structural properties of semiconductor QDs, which enable their versatile applications in solar energy conversion, are analyzed. Recent applications of QDs in photocatalytic CO₂ reduction are then introduced in three categories: binary II-VI semiconductor QDs (e.g., CdSe, CdS, and ZnSe), ternary I-III-VI semiconductor QDs (e.g., CuInS₂ and CuAlS₂ ), and perovskite-type QDs (e.g., CsPbBr₃ , CH₃ NH₃ PbBr₃ , and Cs₂ AgBiBr₆ ). Finally, the challenges and prospects in solar CO₂ reduction with QDs in the future are discussed.

Photoinduced electron transfer in novel CdSe-Cu2Se type II core-shell quantum dots

Herein we report the synthesis, characterisation and electron transfer studies of CdSe–Cu₂Se QDs, a novel type II core–shell system. The synthesis was achieved by a high temperature organometallic method with oleylamine as ligand. Structural and optical properties of the nanostructures were investigated using X-ray diffraction, high resolution transmission electron microscopy, selected area electron diffraction, energy dispersive X-ray spectroscopy, inductive coupled plasma optical emission spectroscopy, cyclic voltammetry, X-ray photoelectron spectroscopy and absorption spectroscopy. The electron transfer dynamics were investigated by observing the variations in steady state and time resolved emission spectra in the presence of an electron acceptor-methyl viologen. Localization of electrons in the shells was evident from the studies performed indicating efficient charge separation.

Herein we report the synthesis, characterisation and electron transfer studies of CdSe–Cu₂Se QDs, a novel type II core–shell system.

A colloidal heterostructured quantum dot sensitized carbon nanotube-TiO2 hybrid photoanode for high efficiency hydrogen generation

Solar-driven photoelectrochemical (PEC) hydrogen (H₂) generation is a promising approach to harvest solar energy for the production of a clean chemical fuel. However, the low photon-to-fuel conversion efficiency and long-term stability of PEC devices are major challenges to be addressed to enable large-scale commercialization. Here we report a simple, fast and cost-effective approach to fabricate high efficiency and stable PEC devices for H₂ generation, by fabricating a hybrid photoanode obtained by incorporating small amounts of multiwall carbon nanotubes (MWCNTs) into a TiO₂ mesoporous film and sensitizing with colloidal heterostructured CdSe/(CdSe_xS_1-x)₅/(CdS)₂ quantum dots (QDs). The latter were specially designed to accelerate the exciton separation through a band engineering approach. The PEC devices based on the TiO₂/QD-MWCNT (T/Q-M) hybrid photoanode with an optimized amount of MWCNTs (0.015 wt%) yield a saturated photocurrent density of 15.90 mA cm^-2 (at 1.0 V_RHE) under one sun illumination (AM 1.5G, 100 mW cm^-2), which is 40% higher than that of the reference device based on TiO₂/QD (T/Q) photoanodes. This is attributed to a synergistic effect of the promising optoelectronic properties of the colloidal heterostructured QDs and improved electron transport (reduced charge transfer resistance) within the TiO₂-MWCNT hybrid anodes enabled by the directional path of MWCNTs for the photo-injected electrons towards FTO. Furthermore, the PEC device based on the T/Q-M hybrid photoanode is more stable (∼19% loss of its initial photocurrent density) when compared with the T/Q photoanode (∼35% loss) after two hours of continuous one sun illumination. Our results provide fundamental insights and a different approach to improve the efficiency and long-term stability of PEC devices and represent an essential step towards the commercialization of this emerging solar-to-fuel conversion technology.

Tailoring the interfacial structure of colloidal "giant" quantum dots for optoelectronic applications

Colloidal semiconductor quantum dots (QDs) are promising building blocks for the realization of future optoelectronic technologies, thanks to their size-tunable electronic and optical properties. Among various types of QDs, colloidal "giant" QDs (g-QDs, core/thick-shell) have been widely used in different applications, such as solar cells, light emitting devices, luminescent solar concentrators and photoelectrochemical (PEC) hydrogen production. However, g-QDs have a thick-shell which serves as a physical barrier for electron and hole transfer, leading to a slow charge transfer rate. In this work, we synthesized CdSe/CdSexS1-x/CdS core/shell/shell g-QDs with an intermediate CdSexS1-x alloyed layer. The presence of this interfacial layer largely improves the absorption of CdSe/CdS QDs, particularly in the 300-650 nm range. By engineering the interfacial layer, the holes can leak more into the CdS shell region compared to that of CdSe/CdS QDs. PEC devices based on alloyed g-QDs exhibit a 20% higher saturated photocurrent density (11 ± 0.5 mA cm-2) compared to CdSe/CdS QDs. In addition, after one-hour illumination (100 mW cm-2), the PEC cell based on alloyed g-QDs still exhibits a photocurrent density of 7.5 mA cm-2, maintaining 70% of its initial value. Such alloyed g-QDs are very promising for several emerging optoelectronic applications, where charge separation, transfer and transport play a critical role for the realization of high performance devices.

Optoelectronic Properties in Near-Infrared Colloidal Heterostructured Pyramidal "Giant" Core/Shell Quantum Dots

Colloidal heterostructured quantum dots (QDs) are promising candidates for next-generation optoelectronic devices. In particular, "giant" core/shell QDs (g-QDs) can be engineered to exhibit outstanding optical properties and high chemical/photostability for the fabrication of high-performance optoelectronic devices. Here, the synthesis of heterostructured CuInSe x S2-x (CISeS)/CdSeS/CdS g-QDs with pyramidal shape by using a facile two-step method is reported. The CdSeS/CdS shell is demonstrated to have a pure zinc blend phase other than typical wurtzite phase. The as-obtained heterostructured g-QDs exhibit near-infrared photoluminescence (PL) emission (≈830 nm) and very long PL lifetime (in the microsecond range). The pyramidal g-QDs exhibit a quasi-type II band structure with spatial separation of electron-hole wave function, suggesting an efficient exciton extraction and transport, which is consistent with theoretical calculations. These heterostructured g-QDs are used as light harvesters to fabricate a photoelectrochemical cell, exhibiting a saturated photocurrent density as high as ≈5.5 mA cm-2 and good stability under 1 sun illumination (AM 1.5 G, 100 mW cm-2). These results are an important step toward using heterostructured pyramidal g-QDs for prospective applications in solar technologies.

Cosensitized Quantum Dot Solar Cells with Conversion Efficiency over 12

The improvement of sunlight utilization is a fundamental approach for the construction of high-efficiency quantum-dot-based solar cells (QDSCs). To boost light harvesting, cosensitized photoanodes are fabricated in this work by a sequential deposition of presynthesized Zn-Cu-In-Se (ZCISe) and CdSe quantum dots (QDs) on mesoporous TiO₂ films via the control of the interactions between QDs and TiO₂ films using 3-mercaptopropionic acid bifunctional linkers. By the synergistic effect of ZCISe-alloyed QDs with a wide light absorption range and CdSe QDs with a high extinction coefficient, the incident photon-to-electron conversion efficiency is significantly improved over single QD-based QDSCs. It is found that the performance of cosensitized photoanodes can be optimized by adjusting the size of CdSe QDs introduced. In combination with titanium mesh supported mesoporous carbon as a counterelectrode and a modified polysulfide solution as an electrolyte, a champion power conversion efficiency up to 12.75% (V_oc = 0.752 V, J_sc = 27.39 mA cm^-2 , FF = 0.619) is achieved, which is, as far as it is known, the highest efficiency for liquid-junction QD-based solar cells reported.

Controlled synthesis of near-infrared quantum dots for optoelectronic devices

We designed a facile approach for the synthesis of PbS quantum dots (QDs) using thiourea and lead acetate as sources of sulfur and lead, respectively. The sizes of the PbS QDs could be systematically controlled by simply adjusting the reaction parameters. Cd post-treatment via a cation exchange method was performed to increase the stability of QDs. As a proof of concept, colloidal PbS QDs synthesized by using air-stable thiourea were employed as light harvesters for both (i) solar driven photoelectrochemical (PEC) hydrogen generation and (ii) QDs sensitized solar cells (QDSSCs). For PEC hydrogen generation, similar saturated photocurrent densities are observed by using thiourea compared to bis(trimethylsilyl) sulfide, which is air-sensitive and unstable. For QDSSCs, the devices fabricated with QDs synthesized from thiourea reveal a better performance compared to devices fabricated with QDs synthesized from traditional bis(trimethylsilyl) sulfide. Our work demonstrates that this synthetic method is a promising alternative to the existing methodologies of PbS QDs and holds great potential for future solar technologies.