Highly Efficient Copper-Indium-Selenide Quantum Dot Solar Cells: Suppression of Carrier Recombination by Controlled ZnS Overlayers

Recent advances in photoelectrochemical hydrogen production using I-III-VI quantum dots

Photoelectrochemical (PEC) water splitting, recognized for its potential in producing solar hydrogen through clean and sustainable methods, has gained considerable interest, particularly with the utilization of semiconductor nanocrystal quantum dots (QDs). This minireview focuses on recent advances in PEC hydrogen production using I-III-VI semiconductor QDs. The outstanding optical and electrical properties of I-III-VI QDs, which can be readily tuned by modifying their size, composition, and shape, along with an inherent non-toxic nature, make them highly promising for PEC applications. The performance of PEC devices using these QDs can be enhanced by various strategies, including ligand modification, defect engineering, doping, alloying, and core/shell heterostructure engineering. These approaches have notably improved the photocurrent densities for hydrogen production, achieving levels comparable to those of conventional heavy-metal-based counterparts. Finally, this review concludes by addressing the present challenges and future prospects of these QDs, underlining crucial steps for their practical applications in solar hydrogen production.

Colloidal semiconductor nanocrystals: from bottom-up nanoarchitectonics to energy harvesting applications

Colloidal semiconductor nanocrystals (NCs) have been extensively investigated owing to their unique properties induced by the quantum confinement effect. The advent of colloidal synthesis routes led to the design of stable colloidal NCs with uniform size, shape, and composition. Metal oxides, phosphides, and chalcogenides (ZnE, CdE, PbE, where E = S, Se, or Te) are few of the most important monocomponent semiconductor NCs, which show excellent optoelectronic properties. The ability to build quantum confined heterostructures comprising two or more semiconductor NCs offer greater customization and tunability of properties compared to their monocomponent counterparts. More recently, the halide perovskite NCs showed exceptional optoelectronic properties for energy generation and harvesting applications. Numerous applications including photovoltaic, photodetectors, light emitting devices, catalysis, photochemical devices, and solar driven fuel cells have demonstrated using these NCs in the recent past. Overall, semiconductor NCs prepared via the colloidal synthesis route offer immense potential to become an alternative to the presently available device applications. This feature article will explore the progress of NCs syntheses with outstanding potential to control the shape and spatial dimensionality required for photovoltaic, light emitting diode, and photocatalytic applications. We also attempt to address the challenges associated with achieving high efficiency devices with the NCs and possible solutions including interface engineering, packing control, encapsulation chemistry, and device architecture engineering.

PbS Quantum Dots-Decorated BiVO4 Photoanodes for Highly Efficient Photoelectrochemical Hydrogen Production

While metal oxides such as TiO₂, Fe₂O₃, WO₃, and BiVO₄ have been previously studied for their potential as photoanodes in photoelectrochemical (PEC) hydrogen production, their relatively wide band-gap limits their photocurrent, making them unsuitable for the efficient utilization of incident visible light. To overcome this limitation, we propose a new approach for highly efficient PEC hydrogen production based on a novel photoanode composed of BiVO₄/PbS quantum dots (QDs). Crystallized monoclinic BiVO₄ films were prepared via a typical electrodeposition process, followed by the deposition of PbS QDs using a successive ionic layer adsorption and reaction (SILAR) method to form a p-n heterojunction. This is the first time that narrow band-gap QDs were applied to sensitize a BiVO₄ photoelectrode. The PbS QDs were uniformly coated on the surface of nanoporous BiVO₄, and their optical band-gap was reduced by increasing the number of SILAR cycles. However, this did not affect the crystal structure and optical properties of the BiVO₄. By decorating the surface of BiVO₄ with PbS QDs, the photocurrent was increased from 2.92 to 4.88 mA/cm² (at 1.23 V_RHE) for PEC hydrogen production, resulting from the enhanced light-harvesting capability arising from the narrow band-gap of the PbS QDs. Moreover, the introduction of a ZnS overlayer on the BiVO₄/PbS QDs further improved the photocurrent to 5.19 mA/cm², attributed to the reduction in interfacial charge recombination.

Quantum Dots, Passivation Layer and Cocatalysts for Enhanced Photoelectrochemical Hydrogen Production

Solar-driven photoelectrochemical (PEC) hydrogen production is one potential pathway to establish a carbon-neutral society. Nowadays, quantum dots (QDs)-sensitized semiconductors have emerged as promising materials for PEC hydrogen production due to their tunable bandgap by size or morphology control, displaying excellent optical and electrical properties. Nevertheless, they still suffer from anodic corrosion during long-term cycling, offering poor stability. This Review discussed advancements to improve long-term stability of QDs particularly in terms of cocatalysts and passivation layers. The working principle of PEC cells was reviewed, along with all important configurations adopted over recent years. The equations to assess PEC performance were also described. A greater emphasized was placed on QDs and incorporation of cocatalysts or passivation layers that could enhance the PEC performance by influencing the charge transfer and surface recombination processes.

Effects of Mono- and Bifunctional Surface Ligands of Cu-In-Se Quantum Dots on Photoelectrochemical Hydrogen Production

Semiconductor nanocrystal quantum dots (QDs) are promising materials for solar energy conversion because of their bandgap tunability, high absorption coefficient, and improved hot-carrier generation. CuInSe₂ (CISe)-based QDs have attracted attention because of their low toxicity and wide light-absorption range, spanning visible to near-infrared light. In this work, we study the effects of the surface ligands of colloidal CISe QDs on the photoelectrochemical characteristics of QD-photoanodes. Colloidal CISe QDs with mono- and bifunctional surface ligands are prepared and used in the fabrication of type-II heterojunction photoanodes by adsorbing QDs on mesoporous TiO₂. QDs with monofunctional ligands are directly attached on TiO₂ through partial ligand detachment, which is beneficial for electron transfer between QDs and TiO₂. In contrast, bifunctional ligands bridge QDs and TiO₂, increasing the amount of QD adsorption. Finally, photoanodes fabricated with oleylamine-passivated QDs show a current density of ~8.2 mA/cm², while those fabricated with mercaptopropionic-acid-passivated QDs demonstrate a current density of ~6.7 mA/cm² (at 0.6 V_RHE under one sun illumination). Our study provides important information for the preparation of QD photoelectrodes for efficient photoelectrochemical hydrogen generation.

Materials and design strategies for stretchable electroluminescent devices

Stretchable displays have recently received increasing attention as input and/or output interfaces for next-generation human-friendly electronic systems. Stretchable electroluminescent (EL) devices are a core component of stretchable displays, and they can be classified into two types, structurally stretchable EL devices and intrinsically stretchable EL devices, according to the mechanism for achieving their stretchability. We herein present recent advances in materials and design strategies for stretchable EL devices. First, stretchable devices based on ultrathin EL devices are introduced. Ultrathin EL devices are mechanically flexible like thin paper, and they can become stretchable through various structural engineering methods, such as inducing a buckled structure, employing interconnects with stretchable geometries, and applying origami/kirigami techniques. Secondly, intrinsically stretchable EL devices can be fabricated by using inherently stretchable electronic materials. For example, light-emitting electrochemical cells and EL devices with a simpler structure using alternating current have been developed. Furthermore, novel stretchable semiconductor materials have been presented for the development of intrinsically stretchable light-emitting diodes. After discussing these two types of stretchable EL devices, we briefly discuss applications of deformable EL devices and conclude the review.

In Situ Electron Microscopy Study of the Dynamics of Liquid Flow in Confined Cells

Confined liquid has attracted great attention due to its potential applications in nanofluidic devices. With the development of liquid-cell transmission electron microscopy (LC-TEM), investigating the behaviors of confined liquid can be realized in real time. However, the dynamics of the liquid layer in liquid cells have not been fully understood. Here, nanoparticles (NPs) adhered to the cell window membranes are used as reference objects to study the flow regime of the liquid layer, which causes cooperative motion of the membranes and the NPs. Two categories of motion behaviors are investigated. One is the contraction of NPs toward the interior viewing area which results from the spreading out of the liquid to the surrounding region, with the bending of the membranes increasing with the loss of liquid in the viewing area. The other motion behavior is the occasional movement of all the NPs in the same direction with the directional movement of the liquid layer. This work offers a new method to study the dynamics of liquids by LC-TEM, the discoveries of which are valuable for understanding the confined liquid dynamics.

Spectral-temporal luminescence properties of Colloidal CdSe/ZnS Quantum Dots in relevant polymer matrices for integration in low turn-on voltage AC-driven LEDs

This work employs spectral and spectral-temporal Photoluminescence (PL) spectroscopy techniques to study the radiative mechanisms in colloidal CdSe/ZnS Quantum Dot (QD) thin films without and with 1% PMMA polymer matrix embedding (QD_PMMA). The observed bimodal transient-spectral PL distributions reveal bandgap transitions and radiative recombinations after interdot electron transfer. The PMMA polymer embedding protects the QDs during the plasma-sputtering of inorganic layers electroluminescent (EL) devices, with minimal impact on the charge transfer properties. Further, a novel TiO₂-based, all-electron bandgap, AC-driven QLED architecture is fabricated, yielding a surprisingly low turn-on voltage, with PL-identical and narrow-band EL emission. The symmetric TiO₂ bilayer architecture is a promising test platform for alternative optical active materials.

Highly Efficient Photoelectrochemical Hydrogen Production Using Nontoxic CuIn1.5Se3 Quantum Dots with ZnS/SiO2 Double Overlayers

Quantum dots (QDs) are a promising material for photoelectrochemical (PEC) hydrogen (H₂) production because of their attractive optical properties including high optical absorption coefficient, band-gap tunability, and potential multiple exciton generation. To date, QDs containing toxic elements such as Cd or Pb have been mainly investigated for PEC H₂ production, which cannot be utilized in practice because of the environmental issue. Here, we demonstrate a highly efficient type II heterojunction photoanode of nontoxic CuIn_1.5Se₃ (CISe) QDs and a mesoporous TiO₂ film. In addition, ZnS/SiO₂ double overlayers are deposited on the photoanodes to passivate surface defect sites on the CISe QDs, leading to the enhancement of both photocurrent density and photostability. Due to a combination of a wide light absorption range of the CISe QDs and the reduced interfacial charge recombination by the overlayers, a remarkable photocurrent density of 8.5 mA cm^-2 (at 0.5 V_RHE) is obtained under 1 sun illumination, which is a record for the PEC sulfite oxidation based on nontoxic QD photoanodes.

Effect of One-Coordinated Atoms on the Electronic and Optical Properties of ZnSe Clusters

To understand the influence of one-coordinated Zn and Se atoms on the structures, electronic, and optical properties of ZnSe clusters, we investigate the Zn₃₇Se₂₀ clusters employing first-principles theoretical calculations. The Zn₃₇Se₂₀ cluster, constructed from the InP nanocrystal structure, possesses a Zn₂₁Se₂₀ core and 16 one-coordinated surface atoms. The effect of one-coordinated atoms is studied by adding or removing one-coordinated atoms of the Zn₃₇Se₂₀ cluster. The calculations show that the modifications of one-coordinated atoms change slightly the coordination states and bond lengths of the atoms on the cluster surface. The clusters with the same core structure and different amounts of one-coordinated atoms have similar optical spectra, suggesting the importance of the cluster core structure in their optical properties.

Multielement 2D layered material photodetectors

The pronounced quantum confinement effects, outstanding mechanical strength, strong light-matter interactions and reasonably high electric transport properties under atomically thin limit have conjointly established 2D layered materials (2DLMs) as compelling building blocks towards the next generation optoelectronic devices. By virtue of the diverse compositions and crystal structures which bring about abundant physical properties, multielement 2DLMs (ME2DLMs) have become a bran-new research focus of tremendous scientific enthusiasm. Herein, for the first time, this review provides a comprehensive overview on the latest evolution of ME2DLM photodetectors. The crystal structures, synthesis, and physical properties of various experimentally realized ME2DLMs as well as the development in metal-semiconductor-metal photodetectors are comprehensively summarized by dividing them into narrow-bandgap ME2DLMs (including Bi₂O₂X (X = S, Se, Te), EuMTe₃(M = Bi, Sb), Nb₂XTe₄(X = Si, Ge), Ta₂NiX₅(X = S, Se), M₂PdX₆(M = Ta, Nb; X = S, Se), PbSnS₂), moderate-bandgap ME2DLMs (including CuIn₇Se₁₁, CuTaS₃, GaGeTe, TlMX₂(M = Ga, In; X = S, Se)), wide-bandgap ME2DLMs (including BiOX (X = F, Cl, Br, I), MPX₃(M = Fe, Ni, Mn, Cd, Zn; X = S, Se), ABP₂X₆(A = Cu, Ag; B = In, Bi; X = S, Se), Ga₂In₄S₉), as well as topological ME2DLMs (MIrTe₄(M = Ta, Nb)). In the last section, the ongoing challenges standing in the way of further development are underscored and the potential strategies settling them are proposed, which is aimed at navigating the future advancement of this fascinating domain.

Ultralow Lattice Thermal Conductivity at Room Temperature in Cu4 TiSe4

Ultralow thermal conductivity draws great attention in a variety of fields of applications such as thermoelectrics and thermal barrier coatings. Herein, the crystal structure and transport properties of Cu₄ TiSe₄ are reported. Cu₄ TiSe₄ is a unique example of a non-toxic and low-cost material that exhibits a lattice ultra-low thermal conductivity of 0.19 Wm^-1  K^-1 at room temperature. The main contribution to the unusually low thermal conductivity is connected with the atomic lattice and its dynamics. This ultralow value of lattice thermal conductivity (k_L ) can be attributed to the presence of the localized modes of Cu, which partially hybridize with the Se atoms, which in turn leads to avoidance of crossing of acoustic phonon modes that reach the zone boundary with a reduced frequency. Like a phonon glass electron crystal, Cu₄ TiSe₄ could also open a route to efficient thermoelectric materials, even, with chalcogenides of relatively high electrical resistivity and a large band gap, provided that their structures offer a sublattice with lightly bound cations.

Improving the Efficiency of Quantum Dot Sensitized Solar Cells beyond 15% via Secondary Deposition

Low loading is one of the bottlenecks limiting the performance of quantum dot sensitized solar cells (QDSCs). Although previous QD secondary deposition relying on electrostatic interaction can improve QD loading, due to the introduction of new recombination centers, it is not capable of enhancing the photovoltage and fill factor. Herein, without the introduction of new recombination centers, a convenient QD secondary deposition approach is developed by creating new adsorption sites via the formation of a metal oxyhydroxide layer around QD presensitized photoanodes. MgCl₂ solution treated Zn-Cu-In-S-Se (ZCISSe) QD sensitized TiO₂ film electrodes have been chosen as a model device to investigate this secondary deposition approach. The experimental results demonstrate that additional 38% of the QDs are immobilized on the photoanode as a single layer. Due to the increased QD loading and concomitant enhanced light-harvesting capacity and reduced charge recombination, not only photocurrent but also photovoltage and fill factor have been remarkably enhanced. The average PCE of resulted ZCISSe QDSCs is boosted to 15.31% (J_sc = 26.52 mA cm^-2, V_oc = 0.802 V, FF = 0.720), from the original 13.54% (J_sc = 24.23 mA cm^-2, V_oc = 0.789 V, FF = 0.708). Furthermore, a new certified PCE record of 15.20% has been obtained for liquid-junction QDSCs.

Zn-Cu-In-S-Se Quinary "Green" Alloyed Quantum-Dot-Sensitized Solar Cells with a Certified Efficiency of 14.4

The photoelectronic properties of quantum dots (QDs) have a critical impact on the performance of quantum-dot-sensitized solar cells (QDSCs). Currently, I-III-VI group QDs have become the mainstream light-harvesting materials in high-performance QDSCs. However, it is still a great challenge to achieve satisfactory efficiency for light-harvesting, charge extraction, and charge collection simultaneously in QDSCs. We design and prepare Zn_0.4 Cu_0.7 In_1.0 S_x Se_2-x (ZCISSe) quinary alloyed QDs by cation/anion co-alloying strategy. The critical photoelectronic properties of target QDs, including band gap, conduction band energy level, and density of defect trap states, can be conveniently tailored. Experimental results demonstrate that the ZCISSe quinary alloyed QDs can achieve an ideal balance among light-harvesting, photogenerated electron extraction, and charge-collection efficiencies in QDSCs compared to its single anion or cation quaternary alloyed QD counterparts. Consequently, the quinary alloyed QDs boost the certified efficiency of QDSCs to 14.4 %, which is a new efficiency record for liquid-junction QD solar cells.

Developing Lattice Matched ZnMgSe Shells on InZnP Quantum Dots for Phosphor Applications

Indium phosphide quantum dots (QDs) have drawn attention as alternatives to cadmium- and lead-based QDs that are currently used as phosphors in lamps and displays. The main drawbacks of InP QDs are, in general, a lower photoluminescence quantum yield (PLQY), a decreased color purity, and poor chemical stability. In this research, we attempted to increase the PLQY and stability of indium phosphide QDs by developing lattice matched InP/MgSe core-shell nanoheterostructures. The choice of MgSe comes from the fact that, in theory, it has a near-perfect lattice match with InP, provided MgSe is grown in the zinc blende crystal structure, which can be achieved by alloying with zinc. To retain lattice matching, we used Zn in both the core and shell and we fabricated InZnP/Zn _x Mg_1-x Se core/shell QDs. To identify the most suitable conditions for the shell growth, we first developed a synthesis route to Zn _x Mg_1-x Se nanocrystals (NCs) wherein Mg is effectively incorporated. Our optimized procedure was employed for the successful growth of Zn _x Mg_1-x Se shells around In(Zn)P QDs. The corresponding core/shell systems exhibit PLQYs higher than those of the starting In(Zn)P QDs and, more importantly, a higher color purity upon increasing the Mg content. The results are discussed in the context of a reduced density of interface states upon using better lattice matched Zn _x Mg_1-x Se shells.

Adsorption and Cation-Exchange Behavior of Zinc Sulfide on Mesoporous TiO2 Film and Its Applications to Solar Cells

Zinc sulfide (ZnS) was deposited onto the surface of mesoporous TiO₂ film by a typical successive ionic layer adsorption and reaction (SILAR) process. By inducing a spontaneous cation exchange between ZnS and a target cation (Pb²⁺, Cu²⁺, Ag⁺, or Bi³⁺) dissolved in a chemical bath when they are in contact, it was demonstrated successfully that white translucent ZnS on the substrate could be changed to new brown-colored metal chalcogenides and the amount of ZnS deposited originally by different conditions could be compared in a qualitative way with the degree of color change. By utilizing this simple but effective process, the evolution of a well-known ZnS passivation layer prepared from different chemical baths in quantum dot (QD)-sensitized solar cells could be tracked visually by checking the degree of color change of TiO₂/ZnS electrodes after the induced specific cation exchange. When applied to representative CdS QD-sensitized solar cells, it was revealed clearly how the different degrees and rates of ZnS deposition could affect the overall power conversion efficiency while finding an optimized passivation layer over TiO₂/CdS electrode. An acetate anion-coupled Zn²⁺ source was observed to give a much faster deposition of a ZnS passivation layer than a nitrate anion one because of its higher pH-induced more-favorable adsorption of Zn²⁺ on the surface of TiO₂. As another useful application of the ZnS-based cation exchange, as-deposited ZnS was used as a template for preparing a more complex metal chalcogenide onto a mesoporous TiO₂ film. The ZnS-derived Sb₂S₃-sensitized electrode showed a promising initial result of over 1.0% overall power conversion efficiency with a very thin ZrO₂ passivation layer between TiO₂ and Sb₂S₃.

Spectroscopic and Magneto-Optical Signatures of Cu1+ and Cu2+ Defects in Copper Indium Sulfide Quantum Dots

Colloidal quantum dots (QDs) of I-III-VI ternary compounds such as copper indium sulfide (CIS) and copper indium selenide (CISe) have been under intense investigation due to both their unusual photophysical properties and considerable technological utility. These materials feature a toxic-element-free composition, a tunable bandgap that covers near-infrared and visible spectral energies, and a highly efficient photoluminescence (PL) whose spectrum is located in the reabsorption-free intragap region. These properties make them attractive for light-emission and light-harvesting applications including photovoltaics and luminescent solar concentrators. Despite a large body of literature on device-related studies of CISe(S) QDs, the understanding of their fundamental photophysical properties is surprisingly poor. Two particular subjects that are still heavily debated in the literature include the mechanism(s) for strong intragap emission and the reason(s) for a poorly defined (featureless) absorption edge, which often "tails" below the nominal bandgap. Here, we address these questions by conducting comprehensive spectroscopic studies of CIS QD samples with varied Cu-to-In ratios using resonant PL and PL excitation, femtosecond transient absorption, and magnetic circular dichroism measurements. These studies reveal a strong effect of stoichiometry on the concentration of Cu¹⁺ vs Cu²⁺ defects (occurring as Cu_In^″ and Cu_Cu^• species, respectively), and their effects on QD optical properties. In particular, we demonstrate that the increase in the relative amount of Cu²⁺ vs Cu¹⁺ centers suppresses intragap absorption associated with Cu¹⁺ states and sharpens band-edge absorption. In addition, we show that both Cu¹⁺ and Cu²⁺ centers are emissive but are characterized by distinct activation mechanisms and slightly different emission energies due to different crystal lattice environments. An important overall conclusion of this study is that the relative importance of the Cu²⁺ vs Cu¹⁺ emission/absorption channels can be controlled by tuning the Cu-to-In ratio, suggesting that the control of sample stoichiometry represents a powerful tool for achieving functionalities (e.g., strong intragap emission) that are not accessible with ideal, defect-free materials.

Influence of a bifunctional linker on the loading of Cu2AgInS4 QDs onto porous TiO2 NFs to use as an efficient photoanode to boost the photoconversion efficiency of QDSCs

Quaternary Cu₂AgInS₄ quantum dots anchored more onto porous TiO₂ NFs through a linker, 3-mercaptopropionic acid exhibits higher photoconversion efficiency of QDSC than that of the same anchored without a linker.

Strategies for extending charge separation in colloidal nanostructured quantum dot materials

Semiconductor colloidal metal chalcogenides (II-VI) in the form of quantum dots (QDs) and different heterostructures (core/shell, alloys, etc.) are of extensive interest in scientific research for both a fundamental understanding and technological applications because of their quantized size and different optical properties; however, due to their small size, the exciton (bound electron and hole) experiences a strong Coulombic attraction, which has a remarkable impact on the charge separation and photophysical properties of QDs. Thus, to achieve an efficient charge separation, numerous attempts have been made via the formation of different heterostructures, QD/molecular adsorbate (either organic or inorganic) assemblies, etc. These hybrid materials ameliorated the absorption of the incident light as well as charge separation. This article reviews the strategies for extending charge separation in these colloidal nanocrystals (NCs), which is one of the crucial steps to elevate the solar to electrical energy conversion efficiency in a quantum dot-sensitized solar cell (QDSC). The article summarizes the benefits of co-sensitization and experimental shreds of evidence for the multiple charge transfer processes involved in a QDSC. Studies have shown that in the co-sensitization process, prolonged charge separation occurs via the dual behavior of the molecular adsorbate, sensitization (electron injection) and capture of holes from photoexcited QDs. This perspective emphases band edge engineering and control of charge carrier dynamics in various core/shell structures. The impact of colloidal alloy NCs on charge separation and interesting photophysical properties was recapitulated via the steady-state and time-resolved photoluminescence (PL) and femtosecond transient absorption spectroscopic techniques. Finally, the prolonged lifetime and extent of charge separation for these hybrid NCs (or the composites) assisted in the development of a better light harvester as compared to the case of their pure counterparts.

ZnSxSe1-x Alloy Passivation Layer for High-Efficiency Quantum-Dot-Sensitized Solar Cells

Interface modification is an important means for improving the performance of almost all optoelectronic devices. In quantum-dot-sensitized solar cells (QDSCs), effective surface modification of photoanode also has a critical impact on photovoltaic performance. At present, ZnS and ZnSe wide band gap semiconductors are the mainstream materials used for photoanode/electrolyte interface passivation in QDSCs. However, the problem with these two materials is that the passivation effect and the lattice match with TiO₂/QD are difficult to be balanced. Although ZnS can form a larger energetic barrier due to the higher conduction band edge, its lattice mismatch with TiO₂ and QD (such as CdSe and CuInSe₂) is large, leading to the formation of additional defect states. On the contrary, ZnSe has a small lattice mismatch with TiO₂ and QD but a relatively lower conduction band edge. Herein, we propose a strategy to employ ZnS_xSe_1-x alloy materials as a passivation layer for the first time to solve the drawbacks of single-component passivation layers. The ZnS_xSe_1-x alloy passivation layer was deposited on the Zn-Cu-In-Se (ZCISe) QD-sensitized TiO₂ film electrode via successive ionic layer adsorption and reaction (SILAR) method. A stable polyselenosulfide/sulfide mixed anions were served as anion precursor for the formation of ZnS_xSe_1-x alloy passivation layer. Experimental results revealed that the alloy passivation layer is more favorable for the suppression of charge recombination at the photoanode/electrolyte interface. In addition, the ZnS_xSe_1-x alloy passivation layer can significantly improve the photogenerated electron extraction efficiency compared to the current classical ZnS passivation layer as confirmed by the transient absorption (TA) measurement. Consequently, the average efficiency of QDSCs was improved from 12.17 to 13.08% with the replacement of traditional ZnS passivation layer by ZnSSe-10 under AM 1.5G one full sun illumination.

Highly Efficient Zn-Cu-In-Se Quantum Dot-Sensitized Solar Cells through Surface Capping with Ascorbic Acid

The balance between band structure, composition, and defect is essential for improving the optoelectronic properties of ternary and quaternary quantum dots and the corresponding photovoltaic performance. In this work, ascorbic acid (AA) as capping ligand is introduced into the reaction system to prepare green Zn-Cu-In-Se (ZCISe) quantum dots. Results show that the addition of AA can increase the Zn content while decrease the In content, resulting in enlarged band gap, high conduction band energy level, and suppressed charge recombination. When AA/Cu ratio is 1, the quantum dots possess the largest band gap of 1.49 eV and the assembled quantum dot-sensitized solar cells exhibit superior photovoltaic performance with ∼17% increment mainly contributed by the dramatically increased current density. The new record efficiencies of 10.44 and 13.85% are obtained from the ZCISe cells assembled with brass and titanium mesh-based counter electrodes, respectively.

Low-Cost CuIn1−xGaxSe2 Ultra-Thin Hole-Transporting Material Layer for Perovskite/CIGSe Heterojunction Solar Cells

This paper presents a new type of solar cellwith enhanced optical-current characteristics using an ultra-thin CuIn1−xGaxSe2 hole-transporting material (HTM) layer (<400 nm). The HTM layer was between a bi-layer Mo metal-electrode and a CH3NH3PbI3 (MAPbI3) perovskite active absorbing material. It promoted carrier transportand led to an improved device with good ohmic-contacts. The solar cell was prepared as a bi-layer Mo/CuIn1−xGaxSe2/perovskite/C60/Ag multilayer of nano-structures on an FTO (fluorine-doped tin oxide) glass substrate. The ultra-thin CuIn1−xGaxSe2 HTM layers were annealed at various temperatures of 400, 500, and 600 °C. Scanning electron microscopy studies revealed that the nano-crystal grain size of CuIn1−xGaxSe2 increased with the annealing temperature. The solar cell results show an improved optical power conversion efficiency at ~14.2%. The application of the CuIn1−xGaxSe2 layer with the perovskite absorbing material could be used for designing solar cells with a reduced HTM thickness. The CuIn1−xGaxSe2 HTM has been evidenced to maintain a properopen circuit voltage, short-circuit current density and photovoltaic stability.

Enhancing Loading Amount and Performance of Quantum-Dot-Sensitized Solar Cells Based on Direct Adsorption of Quantum Dots from Bicomponent Solvents

Intrinsically weak interaction between oil-soluble quantum dots (QDs) and TiO₂ in a direct adsorption process limits QD loading and the performance of QD-sensitized solar cells (QDSCs). Herein, the underlying chemistry and mechanisms governing QD adsorption on TiO₂ were studied to improve QD loading and cell performance. Experimental results indicate that solvent polarity plays the crucial role in determining QD loading. Compared with single-component solvents, substantially greater QD loading can be realized at the critical point (CP) of bicomponent solvents, where QDs become metastable and start to precipitate. Through this strategy, average efficiency of 12.24% was obtained for ZCISe QDSCs, which is comparable to those based on the capping ligand induced self-assembly route. This report demonstrates the great potential of bicomponent solvents at the CP for high QD loading and excellent cell performance and presents a platform for assembling functional composites with the use of different nanocrystals and substrates.

Synthesis and characterisation of [Cu4In(PPh3)3SePh(μ-SePh)3(μ3-SePh)3], and its application as a precursor of a sensitizer for a photocatalyst

An active and robust photocatalyst for water splitting.

Charge-Transport Mechanisms in CuInSe xS2- x Quantum-Dot Films

Colloidal quantum dots (QDs) have attracted considerable attention as promising materials for solution-processable electronic and optoelectronic devices. Copper indium selenium sulfide (CuInSe _xS_{2- x} or CISeS) QDs are particularly attractive as an environmentally benign alternative to the much more extensively studied QDs containing toxic metals such as Cd and Pb. Carrier transport properties of CISeS-QD films, however, are still poorly understood. Here, we aim to elucidate the factors that control charge conductance in CISeS QD solids and, based on this knowledge, develop practical approaches for controlling the polarity of charge transport and carrier mobilities. To this end, we incorporate CISeS QDs into field-effect transistors (FETs) and perform detailed characterization of these devices as a function of the Se/(Se+S) ratio, surface treatment, thermal annealing, and the identity of source and drain electrodes. We observe that as-synthesized CuInSe _xS_{2- x} QDs exhibit degenerate p-type transport, likely due to metal vacancies and Cu_In^'' anti-site defects (Cu¹⁺ on an In³⁺ site) that act as acceptor states. Moderate-temperature annealing of the films in the presence of indium source and drain electrodes leads to switching of the transport polarity to nondegenerate n-type, which can be attributed to the formation of In-related defects such as In_Cu^•• (an In³⁺ cation on a Cu¹⁺ site) or In_i^••• (interstitial In³⁺) acting as donors. We observe that the carrier mobilities increase dramatically (by 3 orders of magnitude) with increasing Se/(Se+S) ratio in both n- and p-type devices. To explain this observation, we propose a two-state conductance model, which invokes a high-mobility intrinsic band-edge state and a low-mobility defect-related intragap state. These states are thermally coupled, and their relative occupancies depend on both QD composition and temperature. Our observations suggest that the increase in the relative fraction of Se moves conduction- and valence band edges closer to low-mobility intragap levels. This results in increased relative occupancy of the intrinsic band-edge states and a corresponding growth of the measured mobility. Further improvement in charge-transport characteristics of the CISeS QD samples as well as their stability is obtained by infilling the QD films with amorphous Al₂O₃ using atomic layer deposition.

Dynamic behavior of nanoscale liquids in graphene liquid cells revealed by in situ transmission electron microscopy

Recent advances in graphene liquid cells for in situ transmission electron microscopy (TEM) have opened many opportunities for the study of materials transformations and chemical reactions in liquids with high spatial resolution. However, the behavior of thin liquids encapsulated in a graphene liquid cell has not been fully understood. Here, we report real time TEM imaging of the nanoscale dynamic behavior of liquids in graphene nanocapillaries. Our observations reveal that the interfaces between liquid and gas bubble can fluctuate, leading to the generation of liquid nanodroplets near the interfaces. Liquid nanodroplets often show irregular shape with dynamic changes of their configuration under the electron beam. We consider that the dynamic motion of liquid-gas interfaces might be introduced by the electrostatic energy from transiently charged interfaces. We find that improving the wettability of graphene liquid cells by ultraviolet-ozone treatment can significantly modify the dynamic motion of the encapsulated liquids. Our study provides valuable information of the interactions between liquid and graphene under the electron beam, and it also offers key insights on the nanoscale fluid dynamics in confined spaces.

Enhanced light absorption and charge recombination control in quantum dot sensitized solar cells using tin doped cadmium sulfide quantum dots

The photovoltaic performance of quantum dot sensitized solar cells (QDSSCs) is limited due to charge recombination processes at the photoelectrode/electrolyte interfaces. We analyzed the effect of Sn⁴⁺ ion incorporation into CdS quantum dots (QDs) deposited onto TiO₂ substrates in terms of enhancing light absorption and retarding electron-hole recombination at the TiO₂/QDs/electrolyte interfaces. Sensitization involved depositing CdS QDs with different Sn⁴⁺ concentrations on the surface of TiO₂ using a facile and cost-effective successive ionic layer adsorption and reaction (SILAR) method. Optimized photovoltaic performance of Sn-CdS sensitized QDSSCs was explored using CuS counter electrodes (CEs) and a polysulfide electrolyte. Structural and optical studies of the photoanodes revealed that the gaps between CdS nanoparticles were partially filled by Sn⁴⁺ ions, which enhanced the light absorption of the solar cell device. Electrochemical impedance spectroscopy (EIS) and open circuit voltage decay (OCVD) tests suggested that Sn⁴⁺ ions can remarkably retard electron-hole recombination at the interfaces, stimulate electron injection into semiconductor QD layers, and provide long-term electron lifetime to the cells. We found that solar cells based on CdS photoanodes doped with 10% Sn⁴⁺ ions exhibited a superior power conversion efficiency (PCE) of 3.22%, open circuit voltage (Voc) of 0.593 V, fill factor (FF) of 0.561, and short-circuit current density (Jsc) of 9.68 mA cm^-2 under an air mass coefficient (AM) 1.5 G full sun illumination. These values were much higher than those of QDSSCs based on bare CdS photoanodes (PCE = 2.16%, Voc = 0.552 V, FF = 0.471, and Jsc = 8.31 mA cm^-2).

Colloidal Phase-Change Materials: Synthesis of Monodisperse GeTe Nanoparticles and Quantification of Their Size-Dependent Crystallization

Phase-change memory materials refer to a class of materials that can exist in amorphous and crystalline phases with distinctly different electrical or optical properties, as well as exhibit outstanding crystallization kinetics and optimal phase transition temperatures. This paper focuses on the potential of colloids as phase-change memory materials. We report a novel synthesis for amorphous GeTe nanoparticles based on an amide-promoted approach that enables accurate size control of GeTe nanoparticles between 4 and 9 nm, narrow size distributions down to 9-10%, and synthesis upscaling to reach multigram chemical yields per batch. We then quantify the crystallization phase transition for GeTe nanoparticles, employing high-temperature X-ray diffraction, differential scanning calorimetry, and transmission electron microscopy. We show that GeTe nanoparticles crystallize at higher temperatures than the bulk GeTe material and that crystallization temperature increases with decreasing size. We can explain this size-dependence using the entropy of crystallization model and classical nucleation theory. The size-dependences quantified here highlight possible benefits of nanoparticles for phase-change memory applications.

Solar light harvesting with multinary metal chalcogenide nanocrystals

The paper reviews the state of the art in the synthesis of multinary (ternary, quaternary and more complex) metal chalcogenide nanocrystals (NCs) and their applications as a light absorbing or an auxiliary component of light-harvesting systems. This includes solid-state and liquid-junction solar cells and photocatalytic/photoelectrochemical systems designed for the conversion of solar light into the electric current or the accumulation of solar energy in the form of products of various chemical reactions. The review discusses general aspects of the light absorption and photophysical properties of multinary metal chalcogenide NCs, the modern state of the synthetic strategies applied to produce the multinary metal chalcogenide NCs and related nanoheterostructures, and recent achievements in the metal chalcogenide NC-based solar cells and the photocatalytic/photoelectrochemical systems. The review is concluded by an outlook with a critical discussion of the most promising ways and challenging aspects of further progress in the metal chalcogenide NC-based solar photovoltaics and photochemistry.

LED based near infrared spectral acquisition for multiwavelength analytical ultracentrifugation: A case study with gold nanoparticles

A pulsed light emitting diode, coupled with a properly configured spectrometer is implemented in open source analytical ultracentrifugation detection hardware, allowing for spectral acquisition spanning the visible to near infrared. Details of the challenges arising with the new hardware are described, along with the necessary solutions to overcome them. The system is successfully tested with polystyrene and gold nanoparticles, demonstrating effectiveness for different material and particle geometries. Sedimentation coefficient distributions and analytically extracted spectra are presented for the multiwavelength datasets extending into the near infrared. Additionally, deconvolution algorithms are applied as a further demonstration of tools applicable to these datasets and the resolving power possible. The results of gold nanoparticle analysis presented here show effective near infrared spectral acquisition is now possible for multiwavelength analytical ultracentrifugation, opening the door for analysis of many important samples with spectral properties in this range, including rich classes of metal and semiconductor nanoparticles.

Towards Low-Toxic Colloidal Quantum Dots

Colloidal quantum dots (CQDs) are of enormous interest in the scientific and engineering fields. During the past few decades, significant efforts have been conducted in investigating Cd- and Pb-based CQDs, resulting in excellent photoluminescence (PL) properties and impressive performance in various applications. But the high toxicity of Cd and Pb elements pushed the scientific community to explore low-toxic CQDs excluding poisonous heavy metals. Several semiconductor materials with lower toxicity than Cd and Pb species have been proposed. This article presents a short overview of recent efforts involving low-toxic CQDs, focusing especially on IV–VI and III–V semiconductors which are active in the near- and short-wave-infrared (IR) regimes. Recent achievements pertinent to Sn- and In-based CQDs are highlighted as representative examples. Finally, limitations and future challenges are discussed in the review.

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.

Solar Paint from TiO2 Particles Supported Quantum Dots for Photoanodes in Quantum Dot-Sensitized Solar Cells

The preparation of quantum dot (QD)-sensitized photoanodes, especially the deposition of QDs on TiO₂ matrix, is usually a time-extensive and performance-determinant step in the construction of QD-sensitized solar cells (QDSCs). Herein, a transformative approach for immobilizing QD on the TiO₂ matrix was developed by simply mixing the as-prepared oil-soluble QDs with TiO₂ P25 particles suspension for a period as short as half a minute. The solar paint was prepared by adding the TiO₂/QD composite in a binder solution under ultrasonication. The QD-sensitized photoanodes were then obtained by simply brushing the solar paint on a fluorine-doped tin oxide substrate followed by a low-temperature annealing at ambient atmosphere. Sandwich-structured complete QDSCs were assembled with the use of Cu₂S/brass as counter electrode and polysulfide redox couple as an electrolyte. The photovoltaic performance of the resulting Zn-Cu-In-Se (ZCISe) QDSCs was evaluated after primary optimization of the QD/TiO₂ ratio as well as the thicknesses of photoanode films. In this proof of concept with a simple solar paint approach for photoanode films, an average power conversion efficiency of 4.13% (J _sc = 11.11 mA/cm², V _oc = 0.590 V, fill factor = 0.631) was obtained under standard irradiation condition. This facile solar paint approach offers a simple and convenient approach for QD-sensitized photoanodes in the construction of QDSCs.

A ZnS nanocrystal/reduced graphene oxide composite anode with enhanced electrochemical performances for lithium-ion batteries

A simple route for the preparation of ZnS nanocrystal/reduced graphene oxide (ZnS/RGO) by a hydrothermal synthesis process was achieved. The chemical composition, morphology, and structural characterization reveal that the ZnS/RGO composite is composed of sphalerite-phased ZnS nanocrystals uniformly dispersed on functional RGO sheets with a high specific surface area. The ZnS/RGO composite was utilized as an anode in the construction of a high-performance lithium-ion battery. The ZnS/RGO composite with appropriate RGO content exhibits a high reversible specific capacity (780 mA h g^-1), excellent cycle stability over 100 cycles (71.3% retention), and good rate performance at 2C (51.2% of its capacity when measured at a 0.1C rate). To further investigate this ZnS/RGO anode for practical use in full Li-ion cells, we tested the electrochemical performance of the ZnS/RGO anode at different cut-off voltages for the first time. The presence of RGO plays an important role in providing high conductivity as well as a substrate with a high surface area. This helps alleviate the typically problems associated with volume expansion and shrinkage during prolonged cycling. Additionally, the RGO provides multiple nucleation points that result in a uniformly dispersed film of nanosized ZnS that covers its surface. Thus, the high surface area RGO enables high electronic conductivity and fast charge transfer kinetics for ZnS lithiation/delithiation.

Quantum dot-sensitized solar cells

A comprehensive overview of the development of quantum dot-sensitized solar cells (QDSCs) is presented.

Origin of the effects of PEG additives in electrolytes on the performance of quantum dot sensitized solar cells

It has been well established that polymer additives in electrolyte can impede the charge recombination processes at the photoanode/electrolyte interface, and improve performance, especially V_oc, of the resulting sensitized solar cells. However, there are few reports about the effect of electrolyte additives on counter electrode (CE) performance. Herein, we systematically investigated the effect of polyethylene glycol (PEG) additives with various molecular weights (M_w from 300 to 20 000) in polysulfide electrolyte on the performance of two representative CdSe and Zn–Cu–In–Se (ZCISe) quantum dot sensitized solar cells (QDSCs), and explored the mechanism of the observed effects. Electrochemical impedance spectroscopy measurements indicate that all PEG additives can improve the charge recombination resistance at the photoanode/electrolyte interface, therefore suppressing the unwanted charge recombination process, and enhancing the V_oc of the resulting cell devices accordingly. On the CE side, with the increase of M_w of PEG additives, the initial effect of reducing the charge transfer resistance at the CE/electrolyte interface evolves into an increasing resistance; accordingly the initial positive effect on FF turns into negative one. Accordingly, low M_w PEG can improve efficiency for both CdSe (increasing from 6.81% to 7.60%) and ZCISe QDSCs (increasing from 9.26% to 10.20%). High M_w PEG is still effective for CdSe QDSCs with an efficiency of 7.38%, but falls flat on ZCISe QDSCs (with an efficiency of 9.11%).

The origin for the effect of PEG additives in polysulfide electrolyte on the performance of both photoanode and counter electrode was explored, and a facile and general route for remarkably improving photovoltaic performance of QDSCs was offered.