Highly efficient inverted type-I CdS/CdSe core/shell structure QD-sensitized solar cells

Tailored Environment-Friendly Reverse Type-I Colloidal Quantum Dots for a Near-Infrared Optical Synapse and Artificial Vision System

Colloidal quantum dots (QDs) are emerging as potential candidates for constructing near-infrared (NIR) photodetectors (PDs) and artificial optoelectronic synapses due to solution processability and a tunable bandgap. However, most of the current NIR QDs-optoelectronic devices are still fabricated using QDs with incorporated harmful heavy metals of lead (Pb) and mercury (Hg), showing potential health and environment risks. In this work, we tailored eco-friendly reverse type-I ZnSe/InP QDs by copper (Cu) doping and extended the photoresponse from the visible to NIR region. Transient absorption spectroscopy analysis revealed the presence of Cu dopant states in ZnSe/InP:Cu QDs that facilitated the extraction of photogenerated charge carriers, leading to an enhanced photodetection performance. Specifically, under 400 nm illumination, the Cu-doped ZnSe/InP QDs-based PDs presented a broadband photodetection ranging from ultraviolet (UV) to NIR, with a responsivity of 70.5 A W-1 and detectivity of 2.8 × 1011 Jones, surpassing those of the undoped ZnSe/InP QDs-based PDs (49.4 A W-1 and 1.9 × 1011 Jones, respectively). More importantly, the ZnSe/InP:Cu QDs-PDs demonstrated various synapse-like characteristics of short-term plasticity (STP), long-term plasticity (LTP), and learning-forging-relearning under NIR light illumination, which were further used to construct PD array devices for simulating the artificial visual system that is available in prospective optical neuromorphic applications.

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.

Flexible and accurate total variation and cascaded denoisers-based image reconstruction algorithm for hyperspectrally compressed ultrafast photography

Hyperspectrally compressed ultrafast photography (HCUP) based on compressed sensing and time- and spectrum-to-space mappings can simultaneously realize the temporal and spectral imaging of non-repeatable or difficult-to-repeat transient events with a passive manner in single exposure. HCUP possesses an incredibly high frame rate of tens of trillions of frames per second and a sequence depth of several hundred, and therefore plays a revolutionary role in single-shot ultrafast optical imaging. However, due to ultra-high data compression ratios induced by the extremely large sequence depth, as well as limited fidelities of traditional algorithms over the image reconstruction process, HCUP suffers from a poor image reconstruction quality and fails to capture fine structures in complex transient scenes. To overcome these restrictions, we report a flexible image reconstruction algorithm based on a total variation (TV) and cascaded denoisers (CD) for HCUP, named the TV-CD algorithm. The TV-CD algorithm applies the TV denoising model cascaded with several advanced deep learning-based denoising models in the iterative plug-and-play alternating direction method of multipliers framework, which not only preserves the image smoothness with TV, but also obtains more priori with CD. Therefore, it solves the common sparsity representation problem in local similarity and motion compensation. Both the simulation and experimental results show that the proposed TV-CD algorithm can effectively improve the image reconstruction accuracy and quality of HCUP, and may further promote the practical applications of HCUP in capturing high-dimensional complex physical, chemical and biological ultrafast dynamic scenes.

Towards a promising systematic approach to the synthesis of CZTS solar cells

This study aims to enhance the CZTS device's overall efficiency, the key research area has been identified in this study is to explore the effects of a novel, low-cost, and simplified, deposition method to improve the optoelectronic properties of the buffer layer in the fabrication of CZTS thin film solar cells. Herein, an effective way of addressing this challenge is through adjusting the absorbers' structure by the concept of doping, sensitized CdS thin film by the bi-functional linker, and an environmentally friendly catalytic green agent. The Linker Assisted and Chemical Bath Deposition (LA-CBD) method was introduced as an innovative and effective hybrid sensitization approach. In the one-step synthesis process, Salvia dye, Ag, and 3-Mercaptopropionic acid (MPA) were used. Generally, the results for all samples displayed varying bandgap as achieved between (2.21-2.46) eV, hexagonal structure with considerably decreased strain level, broader grain size, and dramatically enhanced crystalline property. Hence, the rudimentary CdS/CZTS solar cell devices were fabricated for the application of these novel CdS films. Preliminary CZTS thin film solar cell fabrication results in the highest conversion efficiency of 0.266% obtained CdS + Salvia dye, indicating the potential use of the CdS films as a buffer layer for CZTS photovoltaic devices.

Dual-function perovskite light-emitting/sensing devices for optical interactive display

Interactive display devices integrating multiple functions have become a development trend of display technology. The excellent luminescence properties of perovskite quantum dots (PQDs) make it an ideal luminescent material for the next generation of wide-color gamut displays. Here we design and fabricate dual-function light-sensing/displaying light-emitting devices based on PQDs. The devices can display information as an output port, and simultaneously sense outside light signals as an input port and modulate the display information in a non-contact mode. The dual functions were attributed to the device designs: (1) the hole transport layer in the devices also acts as the light-sensing layer to absorb outside light signals; (2) the introduced hole trapping layer interface can trap holes originating from the light-sensing layer, and thus tune the charge transport properties and the light-emitting intensities. The sensing and display behavior of the device can be further modulated by light signals with different time and space information. This fusion of sensing and display functions has broad prospects in non-contact interactive screens and communication ports.

An in-depth analysis of nucleation and growth mechanism of CdS thin film synthesized by chemical bath deposition (CBD) technique

The aim of this study is to acquire a deeper understanding of the response mechanism that is associated with the formation of CdS thin films. We presented an effective and new hybrid sensitisation technique, which involved the 1-step linker between the related chemical bath deposition (CBD) process and the traditional doping method during CBD for synthesising high-quality, CdS thin films. The mechanism for the combined synthesis of the films is also describes. CdS films were electrostatically bonded to soda-lime glass, causing the formation of the intermediate complexes [Cd(NH₃)₄]²⁺, which aided in the collision of these complexes with a soda-lime glass slide. In the one-step fabrication technique, 3-Mercaptopropionic Acid (MPA) was employed as a second source of sulphur ions and a linker molecule. Optical studies showed that the bandgap ranged between (2.26–2.52) eV. CdS + MPA films exhibited a uniform distribution of spherical molecules based on their morphological properties. After annealing, this approach significantly altered the electrical characteristics of CdS films. The CdS + MPA films displayed the highest carrier concentration whereas the CdS + Ag + MPA films exhibited the lowest resistivity, with a jump of 3 orders of magnitude.

New systematic study approach of green synthesis CdS thin film via Salvia dye

In this study, we aimed to increase the knowledge regarding the response mechanisms which were associated with the formation of CdS thin films. CdS thin film remains the most appealing alternative for many researchers, as it has been a capable buffer material for effect in film based polycrystalline solar cells (CdTe, CIGSe, CZTS). The Linker Assisted and Chemical Bath Deposition (LA-CBD) technique, which combines the Linker Assisted (LA) technique and the chemical bath deposition (CBD) method for forming high quality CdS thin film, was presented as an efficient and novel hybrid sensitization technique. CdS films were bound to soda lime with the help of electrostatic forces, which led to the formation of the intermediate complexes [Cd (NH3)4]2+ that helped in the collision of these complexes with a soda lime slide. Salvia dye and as a linker molecule 3-Mercaptopropionic acid (MPA) was used in the one step fabrication technique. Optical results showed that the bandgap varied in the range of (2.50 to 2.17) eV. Morphological properties showed a homogeneous distribution of the particles that aspherical in shape in the CdS + MPA + Salvia dye films. This technique significantly affected on the electrical characterizations of CdS films after the annealing process. The CdS + Ag + MPA + Salvia dye films showed the maximum carrier concentration and minimum resistivity, as 5.64 × 10 18 cm-3 and 0.83 Ω cm respectively.

Interfacial carrier transport properties of a gallium nitride epilayer/quantum dot hybrid structure

Electron transport layers (ETLs) play a key role in the electron transport properties and photovoltaic performance of solar cells. Although the existing ETLs such as TiO₂, ZnO and SnO₂ have been widely used to fabricate high performance solar cells, they still suffer from several inherent drawbacks such as low electron mobility and poor chemical stability. Therefore, exploring other novel and effective electron transport materials is of great importance. Gallium nitride (GaN) as an emerging candidate with excellent optoelectronic properties attracts our attention, in particular its significantly higher electron mobility and similar conduction band position to TiO₂. Here, we mainly focus on the investigation of interfacial carrier transport properties of a GaN epilayer/quantum dot hybrid structure. Benefiting from the quantum effects of QDs, suitable energy level arrangements have formed between the GaN and CdSe QDs. It is revealed that the GaN epilayer exhibits better electron extraction ability and faster interfacial electron transfer than the rutile TiO₂ single crystal. Moreover, the corresponding electron transfer rates of 4.44 × 10⁸ s⁻¹ and 8.98 × 10⁸ s⁻¹ have been calculated, respectively. This work preliminarily shows the potential application of GaN in quantum dot solar cells (QDSCs). Carefully tailoring the structure and optoelectronic properties of GaN, in particular realizing the low-temperature deposition of high-quality GaN on various substrates, will significantly promote the construction of highly efficient GaN-ETL based QDSCs.

A suitable energy level arrangement is formed between GaN and CdSe QDs, and the GaN epilayer exhibits better electron extraction ability and faster interfacial electron transfer than the rutile TiO₂ single crystal.

Quantized doping of CdS quantum dots with twelve gold atoms

Through a bottom-up strategy, CdS quantum dots (QDs) doped with 12 gold atoms in each nanocrystal (NC) were prepared by cation exchange reactions. The (Au12) dopants inside the CdS matrix were directly observed using Cs-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and quantitatively confirmed using the inductively coupled plasma atomic emission spectroscopy (ICP-AES) data. With a photoluminescence quantum yield (PLQY) of 37.5%, the as-prepared (Au12)@CdS QDs emitted light at 635 nm. Due to the injection of excited electrons from the lowest unoccupied molecular orbital (LUMO) of dopants to the conduction band (CB) of CdS, multiple fine peaks were observed in the photoluminescence excitation (PLE) spectra. By using clusters as starting materials, we demonstrate a universal approach for the precise tailoring of dopants and provide a pathway for band energy engineering of doped QDs.

Room-Temperature Diffusion-Induced Extraction for Perovskite Nanocrystals with High Luminescence and Stability

Metal halide perovskite nanocrystals (NCs) serve as a kind of ideal semiconductor for luminescence and display applications. However, the optoelectronic performance and stability of perovskite NCs are mainly subjected to current ligand strategies since these ligands exhibit a highly dynamic binding state, which complicates NC purification and storage. Herein, a method named diffusion-induced extraction is developed for crystallization (DEC) at room temperature, in which silicone oil serves as a medium to separate the solvent from perovskite precursors and diethyl ether promotes the nucleation, leading to highly emissive perovskite NCs. The formation mechanism of NCs using this approach is elucidated, and their optoelectronic properties are fully characterized. The resultant NCs ink exhibits a high photoluminescence quantum yield (PLQY) over 90% with a narrow full width at half maximum of 17 nm. The DEC method strengthens the interaction between ligand and NCs via the hydrophobic silicone oil. Therefore, the NCs maintain almost 95% of their initial PLQYs after aging more than seven months in air. The findings will be of great significance for the continued advancement of high PLQY perovskite NCs through a better understanding of formation dynamics. The DEC strategy presents a major step forward for advancing the field of perovskite semiconductor nanomaterials.

Controllable chiral behavior of type-II core/shell quantum dots adjusted by shell thickness and coordinated ligands

Chiral semiconductor nanomaterials induced by capped chiral ligands are of great interest for both theoretical studies and advanced applications. In this study, CdTe/CdSe quantum dots (QDs), defined as type-II core/shell nanostructure, with the advantage of a good separation of holes and electrons are imparted chirality with L/D-cysteine and L/D-penicillamine molecules. Circular dichroism (CD) at exciton transitions from cysteine- and penicillamine-capped QDs is different in shape and intensity. CD intensities decrease with increasing shell thickness from three monolayers to six monolayers, indicating a decreased hybridization degree between the holes in CdTe core and the electrons in chiral ligands. Elevated cysteine concentration leads to decreased g-factor, probably due to an altered binding mode from tridentate to bidentate. Our observations provide further insights into the understanding of chiral phenomenon as well as optimized design and applications of chiral nanostructures.

Sensibilization of p-NiO with ZnSe/CdS and CdS/ZnSe quantum dots for photoelectrochemical water reduction

Core/shell quantum dots (QDs) paired with semiconductor photocathodes for water reduction have rarely been implemented so far. We demonstrate the integration of ZnSe/CdS and CdS/ZnSe QDs with porous p-type NiO photocathodes for water reduction. The QDs demonstrate appreciable enhancement in water-reduction efficiency, as compared with the bare NiO. Despite their different structure, both QDs generate comparable photocurrent enhancement, yielding a 3.8- and 3.2-fold improvement for the ZnSe/CdS@NiO and CdS/ZnSe@NiO system, respectively. Unraveling the carrier kinetics at the interface of these hybrid photocathodes is therefore critical for the development of efficient photoelectrochemical (PEC) proton reduction. In addition to examining the carrier dynamics by the Mott-Schottky technique and electrochemical impedance spectroscopy (EIS), we performed theoretical modelling for the distribution density of the carriers with respect to electron and hole wave functions. The electrons are found to be delocalized through the whole shell and can directly actuate the PEC-related process in the ZnSe/CdS QDs. The holes as the more localized carriers in the core have to tunnel through the shell before injecting into the hole transport layer (NiO). Our results emphasize the role of interfacial effects in core/shell QDs-based multi-heterojunction photocathodes.

Photosensitive Thin Films Based on Drop Cast and Langmuir-Blodgett Hydrophilic and Hydrophobic CdS Nanoparticles

Comparative photoelectrochemical studies of cadmium sulfide (CdS) nanoparticles with either hydrophilic or hydrophobic surface properties are presented. Oleylamine organic shells provided CdS nanoparticles with hydrophobic behavior, affecting the photoelectrochemical properties of such nanostructured semiconductor. Hydrophilic CdS nanoparticles were drop-cast on the electrode, whereas the hydrophobic ones were transferred in a controlled manner with Langmuir-Blodgett technique. The substantial hindrance of photopotential and photocurrent was observed for L-B CdS films as compared to the hydrophilic, uncoated nanoparticles that were drop-cast directly on the electrode surface. The electron lifetime in both hydrophilic and hydrophobic nanocrystalline CdS was determined, revealing longer carrier lifetime for oleylamine coated CdS nanoparticles, ascribed to the trapping of charge at the interface of the organic shell/CdS nanoparticle and to the dominant influence of the resistance of the organic shell against the flux of charges. The “on” transients of the photocurrent responses, observed only for the oleylamine-coated nanoparticles, were resolved, yielding the potential-dependent rate constants of the redox processes occurring at the interface.

Stereoselective C-C Oxidative Coupling Reactions Photocatalyzed by Zwitterionic Ligand Capped CsPbBr3 Perovskite Quantum Dots

Semiconductor quantum dots (QDs) have attracted tremendous attention in the field of photocatalysis, owing to their superior optoelectronic properties for photocatalytic reactions, including high absorption coefficients and long photogenerated carrier lifetimes. Herein, by choosing 2-(3,4-dimethoxyphenyl)-3-oxobutanenitrile as a model substrate, we demonstrate that the stereoselective (>99 %) C-C oxidative coupling reaction can be realized with a high product yield (99 %) using zwitterionic ligand capped CsPbBr₃ perovskite QDs under visible light illumination. The reaction can be generalized to different starting materials with various substituents on the phenyl ring and varied functional moieties, producing stereoselective dl-isomers. A radical mediated reaction pathway has been proposed. Our study provides a new way of stereoselective C-C oxidative coupling via a photocatalytic means using specially designed perovskite QDs.

Wavefunction engineering for efficient photoinduced-electron transfer in CuInS2 quantum dot-sensitized solar cells

The high efficiency of quantum dot-sensitized solar cells (QDSSCs) is a benefit of the highly efficient photoinduced-electron transfer (PET) to external electrodes. Here, we investigated how the surface defects and conduction-band (CB) offsets between core and shell materials affect the PET from CuInS₂ quantum dots (QDs) by means of time-resolved femtosecond transient absorption and nanosecond photoluminescence spectroscopy. The transfer of 1S excited electrons from CuInS₂ QDs to TiO₂ films is demonstrated and we find that the surface-electron trapping can significantly reduce the efficiency of the PET. Though the electron trapping can be suppressed after ZnS surface passivation, the PET decreases significantly to a low efficiency of ∼33% from the type I CuInS₂/ZnS core/shell QDs because of their low electron density at the surface of the QDs. The surface-electron density is increased with the strategy of wavefunction engineering by reducing the CB offset, which allows us to achieve a quasi-type II carrier confinement in CuInS₂/CdS core/shell QDs. The PET efficiency appears to be as high as ∼95% from the CuInS₂/CdS core/shell QDs, which is ascribed to synergistic effects of the surface passivation and enhanced delocalization of the electron wavefunction from the CuInS₂ core to the CdS shell. Finally, we demonstrate that these new mechanistic understandings of the PET processes are crucial to improving the efficiency of CuInS₂ QDSSCs.

Effect of co-sensitization of InSb quantum dots on enhancing the photoconversion efficiency of CdS based quantum dot sensitized solar cells

The effect of co-sensitization of CdS and InSb Quantum Dots (QDs) on the enhancement of efficiency of Quantum Dots Sensitized Solar Cells (QDSSCs) has been investigated. InSb is synthesized by a facile solvothermal method using indium metal particles and antimony trichloride as precursors. From TEM images the average particle size of InSb was found to be less than 25 nm. The I-V data showed photoconversion efficiency (PCE) of 0.8% using InSb QDs as a sensitizer layer for QDSSC. However, co-sensitization of InSb QDs and CdS QDs on the TiO2 photoanode in QDSSCs showed an enhanced PCE of 4.94% compared to that of CdS sensitized solar cells (3.52%). The InSb QD layer broadens the light absorption range with reduced spectral overlap causing an improvement in light harvesting along with suppression of surface defects which reduced the recombination losses. As a result, co-sensitized TiO2/CdS/InSb QDSSC exhibits a greatly improved PCE of 4.94%, which is 40% higher than that of TiO2/CdS (3.52%) based QDSSCs due to improved light absorption with low recombination losses.

Electrophoretic Deposition of Quantum Dots and Characterisation of Composites

Electrophoretic deposition (EPD) is an emerging technique in nanomaterial-based device fabrication. Here, we report an in-depth study of this approach as a means to deposit colloidal quantum dots (CQDs), in a range of solvents. For the first time, we report the significant improvement of EPD performance via the use of dichloromethane (DCM) for deposition of CQDs, producing a corresponding CQD-TiO₂ composite with a near 10-fold increase in quantum dot loading relative to more commonly used solvents such as chloroform or toluene. We propose this effect is due to the higher dielectric constant of the solvent relative to more commonly used and therefore the stronger effect of EPD in this medium, though there remains the possibility that changes in zeta potential may also play an important role. In addition, this solvent choice enables the true universality of QD EPD to be demonstrated, via the sensitization of porous TiO₂ electrodes with a range of ligand capped CdSe QDs and a range of group II-VI CQDs including CdS, CdSe/CdS, CdS/CdSe and CdTe/CdSe, and group IV-VI PbS QDs.

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.

Quantum Dot Based Solar Cells: Role of Nanoarchitectures, Perovskite Quantum Dots, and Charge-Transporting Layers

Quantum dot solar cells (QDSCs) are attractive technology for commercialization, owing to various advantages, such as cost effectiveness, and require relatively simple device fabrication processes. The properties of semiconductor quantum dots (QDs), such as band gap energy, optical absorption, and carrier transport, can be effectively tuned by modulating their size and shape. Two types of architectures of QDSCs have been developed: 1) photoelectric cells (PECs) fabricated from QDs sensitized on nanostructured TiO₂ , and 2) photovoltaic cells fabricated from a Schottky junction and heterojunction. Different types of semiconductor QDs, such as a secondary, ternary, quaternary, and perovskite semiconductors, are used for the advancement of QDSCs. The major challenge in QDSCs is the presence of defects in QDs, which lead to recombination reactions and thereby limit the overall performance of the device. To tackle this problem, several strategies, such as the implementation of a passivation layer over the QD layer and the preparation of core-shell structures, have been developed. This review covers aspects of QDSCs that are essential to understand for further improvement in this field and their commercialization.

Ligand induced switching of the band alignment in aqueous synthesized CdTe/CdS core/shell nanocrystals

CdTe/CdS core/shell quantum dots (QDs) are formed in aqueous synthesis via the partial decomposition of hydrophilic thiols, used as surface ligands. In this work, we investigate the influence of the chemical nature (functional group and chain length) of the used surface ligands on the shell formation. Four different surface ligands are compared: 3-mercaptopropionic acid, MPA, thioglycolic acid, TGA, sodium 3-mercaptopropanesulfonate, MPS, and sodium 2-mercaptoethanesulfonate, MES. The QD growth rate increases when the ligand aliphatic chain length decreases due to steric reasons. At the same time, the QDs stabilized with carboxylate ligands grow faster and achieve higher photoluminescence quantum yields compared to those containing sulfonate ligands. The average PL lifetime of TGA and MPA capped QDs is similar (≈20 ns) while in the case of MPS shorter (≈15 ns) and for MES significantly longer (≈30 ns) values are measured. A detailed structural analysis combining powder X-ray diffraction, and X-ray photoelectron spectroscopy (XPS) indicates the existence of two novel regimes of band alignment: in the case of the mercaptocarboxylate ligands the classic type I band alignment between the core and shell materials is predominant, while the mercaptosulfonate ligands induce a quasi-type II alignment (MES) or an inverted type I alignment (MPS). Finally, the effect of the pH value on the optical properties was evaluated: using a ligand excess in solution allows achieving better stability of the QDs while maintaining high photoluminescence intensity at low pH.

Efficiency Enhancement of Solid-State CuInS2 Quantum Dot-Sensitized Solar Cells by Improving the Charge Recombination

Copper indium sulfide quantum dots (CuInS₂ QDs) were incorporated into a nanocrystalline TiO₂ film by using spin coating-assisted successive ionic layer adsorption and reaction process to fabricate CuInS₂ QD-sensitized TiO₂ photoelectrodes for the solid-state quantum dot-sensitized solar cell (QDSSC) applications. The result shows that the photovoltaic performance of solar cell is extremely dependent on the number of cycles, which has an appreciable impact on the coverage ratio of CuInS₂ on the surface of TiO₂ and the density of surface defect states. In the following high-temperature annealing process, it is found that annealing TiO₂/CuInS₂ photoelectrode at a suitable temperature would be beneficial for decreasing the charge recombination and accelerating the charge transport. After annealing at 400 °C, a significantly enhanced photovoltaic properties of solid-state CuInS₂ QDSSCs are obtained, achieving the power conversion efficiency (PCE) of 3.13%, along with an open-circuit voltage (V_OC) of 0.68 V, a short-circuit photocurrent density (J_SC) of 11.33 mA cm^-2, and a fill factor (FF) of 0.41. The enhancement in the performance of solar cells is mainly ascribed to the suppression of charge recombination and the promotion of the electron transfer after annealing.

Efficient Type-II Heterojunction Nanorod Sensitized Solar Cells Realized by Controlled Synthesis of Core/Patchy-Shell Structure and CdS Cosensitization

Here, we report the successful application of core/patchy-shell CdSe/CdSe _xTe_{1- x} type-II heterojunction nanorods (HNRs) to realize efficient sensitized solar cells. The core/patchy-shell structure designed to have a large type-II heterointerface without completely shielding the CdSe core significantly improves photovoltaic performance compared to other HNRs with minimal or full-coverage shells. In addition, cosensitization with CdS grown by successive ionic layer adsorption and reaction further improves the power conversion efficiency. One-diode model analysis reveals that the HNRs having exposed CdSe cores and suitably grown CdS result in significant reduction of series resistance. Investigation of the intercorrelation between diode quality parameters, diode saturation current density ( J₀) and recombination order (β = (ideality factor)^-1) reveals that HNRs with open CdSe cores exhibit reduced recombination. These results confirm that the superior performance of core/patchy-shell HNRs results from their fine-tuned structure: photocurrent is increased by the large type-II heterointerface and recombination is effectively suppressed due to the open CdSe core enabling facile electron extraction. An optimized power conversion efficiency of 5.47% (5.89% with modified electrode configuration) is reported, which is unmatched among photovoltaics utilizing anisotropic colloidal heterostructures as light-harvesting materials.

Chiroptical Activity of Type II Core/Shell Cu2S/CdSe Nanocrystals

Ligand-induced chirality in core/shell nanocrystals (NCs) has attracted extensive attention because of many valuable potential applications. However, the cause of chirality especially in semiconductor nanomaterials is still under debate despite the creation of chiral type I core/shell structures. Herein, we synthesized a kind of new Cu₂S/CdSe core/shell nanostructure to study the underlying reason. Four samples of Cu₂S/CdSe were synthesized utilizing successive ion layer adsorption and reaction to vary the thickness of the CdSe shell upon a Cu₂S core with 5 nm diameter. The chirality of type II Cu₂S/CdSe NCs is imparted by l-/d-cysteine and penicillamine, which could be modulated with an increasing thickness of the CdSe shell. To the best of our knowledge, this is the first report of chiral type II core/shell semiconductor NCs. The hybridization theory can explain the variation trend of g factors with every increase in shell thickness from four monolayers (4 ML) to 7 ML. The results indicate that the chiroptical activity of semiconductor NCs is mainly due to hybridization between the holes in the valence band of NCs and the highest occupied molecular orbitals of the chiral ligands. In addition, Cu₂S/CdSe NCs show a better chiroptical intensity in comparison with the type I structure according to previous work. The first design of chiral type II Cu₂S/CdSe core/shell NCs and a detailed investigation of chiral variation trend help to give a better understanding of the chiral interaction between ligands and core/shell semiconductor nanostructures.

Review of Core/Shell Quantum Dots Technology Integrated into Building’s Glazing

Skylights and windows are building openings that enhance human comfort and well-being in various ways. Recently, a massive drive is witnessed to replace traditional openings with building integrated photovoltaic (BIPV) systems to generate power in a bid to reduce buildings’ energy. The problem with most of the BIPV glazing lies in the obstruction of occupants’ vision of the outdoor view. In order to resolve this problem, new technology has emerged that utilizes quantum dots semiconductors (QDs) in glazing systems. QDs can absorb and re-emit the incoming radiation in the desired direction with the tunable spectrum, which renders them favorable for building integration. By redirecting the radiation towards edges of the glazing, they can be categorized as luminescent solar concentrators (QD-LSCs) that can help to generate electricity while maintaining transparency in the glazing. The aim of this paper is to review the different properties of core/shell quantum dots and their potential applications in buildings. Literature from various disciplines was reviewed to establish correlations between the optical and electrical properties of different types, sizes, thicknesses, and concentration ratios of QDs when used in transparent glazing. The current article will help building designers and system integrators assess the merits of integrating QDs on windows/skylights with regards to energy production and potential impact on admitted daylighting and visual comfort.

Nanoporous Semiconductor Electrode Captures the Quantum Dots: Toward Ultrasensitive Signal-On Liposomal Photoelectrochemical Immunoassay

Liposomal photoelectrochemical (PEC) bioanalysis has recently emerged and exhibited great potential in sensitive biomolecular detection. Exploration of the facile and efficient route for advanced liposomal PEC bioanalysis is highly appealing. In this work, we report the split-type liposomal PEC immunoassay system consisting of sandwich immunorecognition, CdS quantum dots (QDs)-loaded liposomes (QDLL), and the release and subsequent capture of the QDs by a separated TiO₂ nanotubes (NTs) electrode. The system elegantly operated upon the protein binding and lysis treatment of CdS QDLL labels within the 96-well plate, and then the CdS QDs-enabled sensitization of TiO₂ NTs electrode. Exemplified by cardiac markers troponin I (cTnI) as target, the proposed system achieved efficient activation of TiO₂ NTs electrode and thus the signal generation toward the split-type PEC immunoassay. This work features the first use of QDs for liposomal PEC bioanalysis and is expected to inspire more interests in the design and implementation of numerous QDs-involved liposomal PEC bioanalysis.

Performance Enhancement of CdS/CdSe Quantum Dot-Sensitized Solar Cells with (001)-Oriented Anatase TiO2 Nanosheets Photoanode

CdS/CdSe quantum dot-sensitized solar cells (QDSSCs) were fabricated on two types of TiO₂ photoanodes, namely nanosheets (NSs) and nanoparticles. The TiO₂ NSs with high (001)-exposed facets were prepared via a hydrothermal method, while the TiO₂ nanoparticles used the commercial Degussa P-25. It was found that the pore size, specific surface area, porosity, and electron transport properties of TiO₂ NSs were generally superior to those of P-25. As a result, the TiO₂ NS-based CdS/CdSe QDSSC has exhibited a power conversion efficiency of 4.42%, which corresponds to a 54% improvement in comparison with the P-25-based reference cell. This study provides an effective photoanode design using nanostructure approach to improve the performance of TiO₂-based QDSSCs.

Interface Engineering in Quantum-Dot-Sensitized Solar Cells

The unique properties of II-VI semiconductor nanocrystals such as superior light absorption, size-dependent optoelectronic properties, solution processability, and interesting photophysics prompted quantum-dot-sensitized solar cells (QDSSCs) as promising candidates for next-generation photovoltaic (PV) technology. QDSSCs have advantages such as low-cost device fabrication, multiple exciton generation, and the possibility to push over the theoretical power conversion efficiency (PCE) limit of 32%. In spite of dedicated research efforts to enhance the PCE, optimize individual solar cell components, and better understand the underlying science, QDSSCs have unfortunately not lived up to their potential due to shortcomings in the fabrication process and with the QDs themselves. In this feature article, we briefly discuss the QDSSC concepts and mechanisms of the charge carrier recombination pathways that occur at multiple interfaces, viz., (i) metal oxide (MO)/QDs, (ii) MO/QDs/electrolyte, and (iii) counter electrode (CE)/electrolyte. The rational strategies that have been developed to minimize/block these charge recombination pathways are elaborated. The article concludes with a discussion of the present challenges in fabricating efficient devices and future prospects for QDSSCs.

Multifunctional CdSNPs@ZIF-8: Potential Antibacterial Agent against GFP-Expressing Escherichia coli and Staphylococcus aureus and Efficient Photocatalyst for Degradation of Methylene Blue

Multifunctional novel core-shell composites, CdSNPs@ZIF-8, have been synthesized by in situ encapsulation of different amounts of CdSNPs (150, 300, and 500 μL suspension of CdSNPs in methanol) in ZIF-8 at room temperature. These composites have been characterized by powder X-ray diffraction, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), and diffuse reflectance spectroscopy techniques and Brunauer-Emmett-Teller surface analysis. XPS and HRTEM indicate the encapsulation of CdSNPs within ZIF-8 crystal without disturbing the crystal order of ZIF-8. The average size of embedded CdSNPs (determined by the particle size distribution from HRTEM) is found to be 16.34 nm. CdSNPs@ZIF-8 showed potential to be used as an antibacterial agent against the broad spectrum of bacterial strains such as Gram-positive Staphylococcus aureus and Gram-negative green fluorescent protein-expressing Escherichia coli in aqueous medium, as evident by various biophysical experiments, viz., 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, optical density and fluorescence spectroscopic studies, fluorescence and optical microscopic image analysis, disk diffusion assay, field emission scanning electron microscopy, and flow cytometry for reactive oxygen species induction assay. Further, the composite has been used as an efficient photocatalyst for the degradation of organic pollutants, such as methylene blue dye, in aqueous medium and found that the core-shell composite, CdSNPs@ZIF-8 (150 μL) (abbreviated as NC-1) (5 mg), exhibited higher photocatalytic activity (≈1.8 times) than CdSNPs for degradation of 90% of methylene blue (10 mL of 10 ppm) at pH ≥ 7 due to the synergetic effect. Therefore, in situ encapsulation of CdSNPs in ZIF-8 provides an easy executable measure for purification of wastewater effluents for the effective photocatalytic degradation of organic pollutants as well as to remove the bacterial contamination under sunlight.

Incorporation of Mn2+ into CdSe quantum dots by chemical bath co-deposition method for photovoltaic enhancement of quantum dot-sensitized solar cells

A photoelectric conversion efficiency (PCE) of 4.9% was obtained under 100 mW cm^-2 illumination by quantum-dot-sensitized solar cells (QDSSCs) using a CdS/Mn : CdSe sensitizer. CdS quantum dots (QDs) were deposited on a TiO₂ mesoporous oxide film by successive ionic layer absorption and reaction. Mn²⁺ doping into CdSe QDs is an innovative and simple method-chemical bath co-deposition, that is, mixing the Mn ion source with CdSe precursor solution for Mn : CdSe QD deposition. Compared with the CdS/CdSe sensitizer without Mn²⁺ incorporation, the PCE was increased from 3.4% to 4.9%. The effects of Mn²⁺ doping on the chemical, physical and photovoltaic properties of the QDSSCs were investigated by energy dispersive spectrometry, absorption spectroscopy, photocurrent density-voltage characteristics and electrochemical impedance spectroscopy. Mn-doped CdSe QDs in QDSSCs can obtain superior light absorption, faster electron transport and slower charge recombination than CdSe QDs.

Incorporation of Mn2+ into CdSe quantum dots by chemical bath co-deposition method for photovoltaic enhancement of quantum dot-sensitized solar cells

A photoelectric conversion efficiency (PCE) of 4.9% was obtained under 100 mW cm^-2 illumination by quantum-dot-sensitized solar cells (QDSSCs) using a CdS/Mn : CdSe sensitizer. CdS quantum dots (QDs) were deposited on a TiO₂ mesoporous oxide film by successive ionic layer absorption and reaction. Mn²⁺ doping into CdSe QDs is an innovative and simple method-chemical bath co-deposition, that is, mixing the Mn ion source with CdSe precursor solution for Mn : CdSe QD deposition. Compared with the CdS/CdSe sensitizer without Mn²⁺ incorporation, the PCE was increased from 3.4% to 4.9%. The effects of Mn²⁺ doping on the chemical, physical and photovoltaic properties of the QDSSCs were investigated by energy dispersive spectrometry, absorption spectroscopy, photocurrent density-voltage characteristics and electrochemical impedance spectroscopy. Mn-doped CdSe QDs in QDSSCs can obtain superior light absorption, faster electron transport and slower charge recombination than CdSe QDs.

Quantum dot-sensitized solar cells

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

Charge recombination control for high efficiency CdS/CdSe quantum dot co-sensitized solar cells with multi-ZnS layers

ZnS as an inorganic passivation agent has been proven to be effective in suppressing charge recombination and enhancing power conversion efficiency (PCE) in quantum dot-sensitized solar cells (QDSCs).

Photodegradation of Rhodamine B over Biomass-Derived Activated Carbon Supported CdS Nanomaterials under Visible Irradiation

A family of new composite materials was successfully prepared through the deposition of as-synthesized CdS nanomaterials on lotus-seedpod-derived activated carbon (SAC). The SAC supports derived at different activation temperatures exhibited considerably large surface areas and various microstructures that were of great importance in enhancing photocatalytic performance of CdS@SAC composite materials toward the photodegradation of rhodamine B (RhB) under visible irradiation. The best-performing CdS@SAC-800 showed excellent photocatalytic activity with a rate constant of ca. 2.40 × 10^-2 min^-1, which was approximately 13 times higher than that of the CdS nanomaterials. Moreover, the estimated band gap energy of CdS@SAC-800 was significantly lowered down to 1.99 eV compared to that of the CdS precursor (2.22 eV), which suggested considerable strength of interface contact between the CdS and SAC support, as well as efficient light harvesting capacity of the composite material. Further photocatalytic study indicated that the SAC supports enhanced the separation of photogenerated electrons and holes in this system. Improved photocatalytic activity of the composite materials was largely due to the increased generation of catalytically active species such as h⁺, OH•, [Formula: see text] etc. This work provided a facile and low-cost pathway to fabricate photocatalysts for viable degradation of organic dye molecules.