Cosensitized Quantum Dot Solar Cells with Conversion Efficiency over 12

Perovskite Colloidal Nanocrystal Solar Cells: Current Advances, Challenges, and Future Perspectives

The power conversion efficiencies (PCEs) of polycrystalline perovskite (PVK) solar cells (SCs) (PC-PeSCs) have rapidly increased. However, PC-PeSCs are intrinsically unstable without encapsulation, and their efficiency drops during large-scale production; these problems hinder the commercial viability of PeSCs. Stability can be increased by using colloidal PVK nanocrystals (c-PeNCs), which have high surface strains, low defect density, and exceptional crystal quality. The use of c-PeNCs separates the crystallization process from the film formation process, which is preponderant in large-scale fabrication. Consequently, the use of c-PeNCs has substantial potential to overcome challenges encountered when fabricating PC-PeSCs. Research on colloidal nanocrystal-based PVK SCs (NC-PeSCs) has increased their PCEs to a level greater than those of other quantum-dot SCs, but has not reached the PCEs of PC-PeSCs; this inferiority significantly impedes widespread application of NC-PeSCs. This review first introduces the distinctive properties of c-PeNCs, then the strategies that have been used to achieve high-efficiency NC-PeSCs. Then it discusses in detail the persisting challenges in this domain. Specifically, the major challenges and solutions for NC-PeSCs related to low short-circuit current density Jsc are covered. Last, the article presents a perspective on future research directions and potential applications in the realm of NC-PeSCs.

Heavy-Metal-Free Colloidal Quantum Dots: Progress and Opportunities in Solar Technologies

Colloidal quantum dots (QDs) hold great promise as building blocks in solar technologies owing to their remarkable photostability and adjustable properties through the rationale involving size, atomic composition of core and shell, shapes, and surface states. However, most high-performing QDs in solar conversion contain hazardous metal elements, including Cd and Pb, posing significant environmental risks. Here, a comprehensive review of heavy-metal-free colloidal QDs for solar technologies, including photovoltaic (PV) devices, solar-to-chemical fuel conversion, and luminescent solar concentrators (LSCs), is presented. Emerging synthetic strategies to optimize the optical properties by tuning the energy band structure and manipulating charge dynamics within the QDs and at the QDs/charge acceptors interfaces, are analyzed. A comparative analysis of different synthetic methods is provided, structure-property relationships in these materials are discussed, and they are correlated with the performance of solar devices. This work is concluded with an outlook on challenges and opportunities for future work, including machine learning-based design, sustainable synthesis, and new surface/interface engineering.

Improving the efficiency of quantum dot-sensitized solar cells by increasing the QD loading amount

In quantum dot-sensitized solar cells (QDSCs), optimized quantum dot (QD) loading mode and high QD loading amount are prerequisites for great device performance. Capping ligand-induced self-assembly (CLIS) mode represents the mainstream QD loading strategy in the fabrication of high-efficiency QDSCs. However, there remain limitations in CLIS that constrain further enhancement of QD loading levels. This review illustrates the development of various QD loading methods in QDSCs, with an emphasis on the outstanding merits and bottlenecks of CLIS. Subsequently, thermodynamic and kinetic factors dominating QD loading behaviors in CLIS are analyzed theoretically. Upon understanding driving forces, resistances, and energy effects in a QD assembly process, various novel strategies for improving the QD loading amount in CLIS are summarized, and the related functional mechanism is established. Finally, the article concludes and outlooks some remaining academic issues to be solved, so that higher QD loading amount and efficiencies of QDSCs can be anticipated in the future.

Reduced Surface Trap States of PbS Quantum Dots by Acetonitrile Treatment for Efficient SnO2-Based PbS Quantum Dot Solar Cells

The solution-phase ligand-exchange strategy offers a simple pathway to prepare PbS quantum dots (QDs) and their corresponding solar cells. However, the production of high-quality PbS QDs with reduced surface trap state density for efficient PbS QD solar cells (QDSCs) still faces challenges. As the hydroxyl group (-OH) has been demonstrated to be the primary source of the surface trap states on PbS QDs in the general oleic acid method, here, we present an effective and facile strategy for reducing the surface -OH content of PbS QDs by using acetonitrile (ACN) as precipitant to wash the surface of QDs, which significantly decreases the trap state density and enables the preparation of superior PbS QDs. The resulting solar cell with an ITO/SnO2/n-PbS/p-PbS/Au structure obtained an improved photoelectric conversion efficiency (PCE) from 8.53 to 10.49% with an enhanced air storage stability, realizing a high PCE for SnO2-based PbS QDSCs.

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.

Recent Progress in Colloidal Quantum Dot Thermoelectrics

Semiconducting colloidal quantum dots (CQDs) represent an emerging class of thermoelectric materials for use in a wide range of future applications. CQDs combine solution processability at low temperatures with the potential for upscalable manufacturing via printing techniques. Moreover, due to their low dimensionality, CQDs exhibit quantum confinement and a high density of grain boundaries, which can be independently exploited to tune the Seebeck coefficient and thermal conductivity, respectively. This unique combination of attractive attributes makes CQDs very promising for application in emerging thermoelectric generator (TEG) technologies operating near room temperature. Herein, recent progress in CQDs for application in emerging thin-film thermoelectrics is reviewed. First, the fundamental concepts of thermoelectricity in nanostructured materials are outlined, followed by an overview of the popular synthetic methods used to produce CQDs with controllable sizes and shapes. Recent strides in CQD-based thermoelectrics are then discussed with emphasis on their application in thin-film TEGs. Finally, the current challenges and future perspectives for further enhancing the performance of CQD-based thermoelectric materials for future applications are discussed.

A Review on Multiple I-III-VI Quantum Dots: Preparation and Enhanced Luminescence Properties

I-III-VI type QDs have unique optoelectronic properties such as low toxicity, tunable bandgaps, large Stokes shifts and a long photoluminescence lifetime, and their emission range can be continuously tuned in the visible to near-infrared light region by changing their chemical composition. Moreover, they can avoid the use of heavy metal elements such as Cd, Hg and Pb and highly toxic anions, i.e., Se, Te, P and As. These advantages make them promising candidates to replace traditional binary QDs in applications such as light-emitting diodes, solar cells, photodetectors, bioimaging fields, etc. Compared with binary QDs, multiple QDs contain many different types of metal ions. Therefore, the problem of different reaction rates between the metal ions arises, causing more defects inside the crystal and poor fluorescence properties of QDs, which can be effectively improved by doping metal ions (Zn²⁺, Mn²⁺ and Cu⁺) or surface coating. In this review, the luminous mechanism of I-III-VI type QDs based on their structure and composition is introduced. Meanwhile, we focus on the various synthesis methods and improvement strategies like metal ion doping and surface coating from recent years. The primary applications in the field of optoelectronics are also summarized. Finally, a perspective on the challenges and future perspectives of I-III-VI type QDs is proposed as well.

A Review of Transition Metal Sulfides as Counter Electrodes for Dye-Sensitized and Quantum Dot-Sensitized Solar Cells

Third-generation solar cells, including dye-sensitized solar cells (DSSCs) and quantum dot-sensitized solar cells (QDSSCs), have been associated with low-cost material requirements, simple fabrication processes, and mechanical robustness. Hence, counter electrodes (CEs) are a critical component for the functionality of these solar cells. Although platinum (Pt)-based CEs have been dominant in CE fabrication, they are costly and have limited market availability. Therefore, it is important to find alternative materials to overcome these issues. Transition metal chalcogenides (TMCs) and transition metal dichalcogenides (TMDs) have demonstrated capabilities as a more cost-effective alternative to Pt materials. This advantage has been attributed to their strong electrocatalytic activity, excellent thermal stability, tunability of bandgap energies, and variable crystalline morphologies. In this study, a comprehensive review of the major components and working principles of the DSSC and QDSSC are presented. In developing CEs for DSSCs and QDSSCs, various TMS materials synthesized through several techniques are thoroughly reviewed. The performance efficiencies of DSSCs and QDSSCs resulting from TMS-based CEs are subjected to in-depth comparative analysis with Pt-based CEs. Thus, the power conversion efficiency (PCE), fill factor (FF), short circuit current density (J_sc) and open circuit voltage (V_oc) are investigated. Based on this review, the PCEs for DSSCs and QDSSCs are found to range from 5.37 to 9.80% (I^-/I₃^- redox couple electrolyte) and 1.62 to 6.70% (S^-2/S_x^- electrolyte). This review seeks to navigate the future direction of TMS-based CEs towards the performance efficiency improvement of DSSCs and QDSSCs in the most cost-effective and environmentally friendly manner.

Tunable 0D/2D/2D Nanocomposite Based on Green Zn-Doped CuInS2 Quantum Dots and MoS2/rGO as Photoelectrodes for Solar Hydrogen Production

Charge separation, transmission, and light absorption properties are critical to determining the performance of photoelectrochemical (PEC) devices. An important strategy to control such properties is based on using heterostructured materials. Herein, a tunable zero-dimensional (0D)/two-dimensional (2D) heterostructure is designed based on quantum dots (QDs) and 2D nanosheets (NSs). Specifically, eco-friendly Zn-doped CuInS₂ QDs prepared by hot injection were anchored on hierarchical (2D/2D) MoS₂/rGO (MG) NSs through a facile sonication-assisted method to develop a 0D/2D/2D heterojunction-based photoelectrode for solar hydrogen production. The interfacial structure and band alignment between the proposed 0D QDs and 2D/2D MG NSs were engineered by modulating the Zn molar ratio during the QD synthesis. As proof of concept, the optimized 0D/2D/2D photoanode exhibits almost five times higher PEC activity than MG/CuInS₂ and MoS₂/Zn-CuInS₂ NSs due to the enhanced light absorption, efficient charge separation, and transmission. Zn doping and the presence of graphene are essential in enhancing performance in the proposed heterostructure, reducing recombination of charge carriers, and improving sunlight absorption. This work shows how optimal band alignment control and carbon addition can facilitate charge transfer, enabling the development of highly efficient PEC devices based on 0D/2D/2D heterostructure nanocomposites.

Structural evolution from the CdSSe alloy to the CdS/CdSe core/shell in Cd(S and Se) composite quantum dots and its impact on the performance of sensitized solar cells

CdSSe alloy and CdS/CdSe core/shell quantum dots (QDs) are widely studied in quantum dot solar cells (QDSSCs). However, to date, there have been no detailed comparative investigations into the cell performance between CdSSe alloy and CdS/CdSe core/shell structures prepared with the same preparation process. In this work, the performances of CdSSe alloy and CdS/CdSe core/shell QDSSCs, which are prepared with the same SILAR (successive ionic layer adsorption and reactions) process, are investigated in detail. By simply tuning the layer numbers and arrangement sequence of the CdS and CdSe layers, a series of QDs, including CdSSe alloy structures, CdS/CdSe multilayer structures, and CdS/CdSe core/shell structures, are successfully prepared with a layer-by-layer technique, while maintaining a similar morphology. Based on these QD sensitized TiO₂ photoanodes, QDSSCs are assembled. The CdS/CdSe core/shell QDSSCs yield a maximum power conversion efficiency of 5.08% under AM 1.5 illumination of 100 mW cm^-2, which is increased by 77% in comparison with that of CdSSe alloy QDSSCs (2.87%). The significantly enhanced photovoltaic performance of QDSSCs with core/shell architectures is mainly attributed to their high short-circuit current density, which arises from the enhanced absorption intensity. In addition, the CdS/CdSe core-shell contributes to the attenuation of the interfacial charge recombination rate and prolongs the electron lifetime, resulting in more efficient charge collection in QDSSCs.

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.

Hybrid Triboelectric Nanogenerators: From Energy Complementation to Integration

Energy collection ways using solar energy, wave, wind, or mechanical energy have attracted widespread attention for small self-powered electronic devices with low power consumption, such as sensors, wearable devices, electronic skin, and implantable devices. Among them, triboelectric nanogenerator (TENG) operated by coupling effect of triboelectrification and electrostatic induction has gradually gained prominence due to its advantages such as low cost, lightweight, high degree of freedom in material selection, large power, and high applicability. The device with a single energy exchange mechanism is limited by its conversion efficiency and work environment and cannot achieve the maximum conversion of energy. Thus, this article reviews the research status of different types of hybrid generators based on TENG in recent years. Hybrid energy generators will improve the output performance though the integration of different energy exchange methods, which have an excellent application prospect. From the perspective of energy complementation, it can be divided into harvesting mechanical energy by various principles, combining with harvesters of other clean energy, and converting mechanical energy or various energy sources into hydrogen energy. For integrating multitype energy harvesters, mechanism of single device and structural design of integrated units for different application scenarios are summarized. The expanding energy harvesting efficiency of the hybrid TENG makes the scheme of self-charging unit to power intelligent mobile electronic feasible and has practical significance for the development of self-powered sensor network.

Recent research on the luminous mechanism, synthetic strategies, and applications of CuInS2 quantum dots

We discuss the synthesis and luminescence mechanisms of CuInS₂ QDs, the strategies to improve their luminous performance and their potential application in light-emitting devices, solar energy conversion, and the biomedical field.

Heat-treatment-induced development of the crystalline structure and chemical stoichiometry of a CuxS counter electrode, and the influence on performance of quantum-dot-sensitized solar cells

Recently, various phases of Cu_xS (1 ≤ x ≤ 2) were extensively explored as superb counter electrode (CE) materials for quantum dot-sensitized solar cells (QDSSCs). Herein, hexagonal covellite CuS (HC-CuS) with hierarchical nanostructure was grown on porous Ti substrates by chemical bath deposition, and then heat treated in the temperature range of 150-450 °C under N₂ atmosphere. The reaction process and the evolution of morphology, composition and crystalline structure of Cu_xS with the variation of heat treatment temperature were studied by XRD, SEM, EDX, TEM and XPS. The photovoltaic properties of TiO₂/CdS/CdSe QDSSCs based on Cu_xS CEs showed an obvious dependence on the element stoichiometry and crystalline structure of the Cu_xS. With HC-Cu_1.28S heat-treated at 230 °C as CEs, QDSSCs achieved a power conversion efficiency of 3.88% under one sun illumination (100 mW cm^-2, AM 1.5 G), which was higher than the counterparts with other compositions. Electrochemical impedance spectroscopy, Tafel polarization and cyclic voltammetry measurement showed that the electrocatalytic activity of HC-Cu_1.28S CE was much higher than that of other Cu_xS CEs, which supported the results of the enhanced short-circuit current density, open circuit voltage and filling factor.

Boosting Power Conversion Efficiency of Quantum Dot-Sensitized Solar Cells by Integrating Concentrating Photovoltaic Concept with Double Photoanodes

Despite great efforts dedicated to enhance power conversion efficiency (PCE) of quantum dot-sensitized solar cells (QDSSCs) in the past two decades, the efficiency of QDSSCs is still far behind its theoretical value. The present approaches for improving PCE are mainly focused on tailoring the bandgap of QDs to broadening light-harvesting and optimizing interfaces of component parts. Herein, a new solar cell architecture is proposed by integrating concentrating solar cell (CPV) concept into QDSSCs with double photoanode design. The Cu2S mesh is used as a counter electrode and sandwiched between two photoanodes. This designed battery structure can increase the PCE by 260% compared with a single photoanode. With the most extensively used CdS/CdSe QD sensitizers, a champion PCE of 8.28% (Voc = 0.629 V, Jsc = 32.247 mA cm-2) was achieved. This is mainly due to the increase in Jsc due to the double photoanode design and adoption of the CPV concept. In addition, another reason is that concentrated sunshine illumination induced a photothermal effect, accelerating the preceding chemical reactions associated with the conversion of polysulfide species. The cell fabrication and design reported here provides a new insight for further development of QDSSCs.

n-type polyaniline hole-blocking layer for high-efficiency QDSC by one-pot electropolymerization and selective aprotic cation ([EMIM]+) doping

In the field of clean solar-to-current devices, the photoelectron transfer process is essential for photovoltaic conversion in the typical n-i-p solar-cell structure. With regard to the oriented injection and ejection of photoelectrons, the development of hole-blocking layer (HBL) materials with a high electron transfer capability are exceedingly desirable. Profiting from the distortion of the p-π electron cloud attracted by a doped aprotic cation, a modified n-type polyaniline (PANI) as the HBL of a photoanode has been successfully fabricated through a facial one-pot square-wave potentiostatic electropolymerization method. In terms of flat-band potential, charge-carrier concentration and device impedance, the synthesized n-type polyaniline layer doped by aprotic ionic liquid (AIL; [EMIM] [EtSO₄]) (AIL-PA layer) for quantum dot solar cells (QDSCs) directly facilitates the high electron carrying capacity as well as the electron transfer driving force. Furthermore, the n-type polyaniline layer doped by AIL ([EMIM] [EtSO₄]) (AIL-PA layer) has a widely matching band gap for electron exportation and improved photovoltaic performance of CdS_xSe_1-x QDSCs: the power conversion efficiency is 10.5% and the J _sc is 21.59 mA cm^-2 for the device with an AIL-PA HBL. The electron diffusion length L _D is 8.07 μm for the photoanode with AIL-PA I and 7.58 μm for the photoanode with AIL-PA II.

Synthesis and optical applications of low dimensional metal-halide perovskites

Metal halide perovskites have received substantial attention in research communities due to their outstanding efficiency achievements in the field of photovoltaics, optoelectronics and electronics, exhibiting extraordinary optical, electrical and mechanical properties. The exceptional structural tunability enables perovskite material to possess low-dimensional form at the atomic level and extends their applications into optoelectronic and photonic fields. This review discusses the recent progress of synthetic routes and fundamental optoelectronic properties of low-dimensional metal halide perovskites. In addition, the focus is to highlight the potential applications of perovskites in various devices including solar cells, light-emitting diodes, lasers, waveguides and memory devices. Finally, outlooks and the challenges that face the development of the perovskite materials in the near future are also presented.

Quantification of the Photon Absorption, Scattering, and On-Resonance Emission Properties of CdSe/CdS Core/Shell Quantum Dots: Effect of Shell Geometry and Volumes

Reliable quantification of the optical properties of fluorescent quantum dots (QDs) is critical for their photochemical, -physical, and -biological applications. Presented herein is the experimental quantification of photon scattering, absorption, and on-resonance-fluorescence (ORF) activities of CdSe/CdS core/shell fluorescent QDs as a function of the shell sizes and geometries. Four spherical QDs (SQDs) with different diameters and four rod-like QDs (RQDs) with different aspect ratios (ARs) have been analyzed using UV-vis, fluorescence, and the recent polarized resonance synchronous spectroscopic (PRS2) methods. All quantum dots are simultaneous absorbers and scatterers in the UV-vis wavelength region, and they all exhibit strong ORF emission in the wavelength regions where the QDs both absorb and emit. The absorption and scattering cross-sections of the CdS shell are linearly and quadratically, respectively, proportional to the shell volume for both the SQDs and RQDs. However, the effects of CdS shell coating on the core optical properties are different between SQDs and RQDs. For RQDs, increasing the CdS shell volume through the length elongation has no effect on either the peak wavelength or intensity of the CdSe core UV-vis absorption and ORF, but it reduces the QD fluorescence depolarization. In contrast, increasing CdS shell volume in the SQDs induces red-shift in the CdSe core peak UV-vis absorption and ORF wavelengths, and increases their peak cross-sections, but it has no effect on the SQD fluorescence depolarization. The RQD ORF cross-sections and quantum yields are significantly higher than their respective counterparts for the SQDs with similar particle sizes (volumes). While these new insights should be significant for the QD design, characterization, and applications, the methodology presented in this work is directly applicable for quantifying the optical activities of optically complex materials where the common UV-vis spectrometry and fluorescence spectroscopy are inadequate.

A Review on Quantum Dots Modified g-C3N4-Based Photocatalysts with Improved Photocatalytic Activity

In the 21st century, the development of sustainable energy and advanced technologies to cope with energy shortages and environmental pollution has become vital. Semiconductor photocatalysis is a promising technology that can directly convert solar energy to chemical energy and is extensively used for its environmentally-friendly properties. In the field of photocatalysis, graphitic carbon nitride (g-C3N4) has obtained increasing interest due to its unique physicochemical properties. Therefore, numerous researchers have attempted to integrate quantum dots (QDs) with g-C3N4 to optimize the photocatalytic activity. In this review, recent progress in combining g-C3N4 with QDs for synthesizing new photocatalysts was introduced. The methods of QDs/g-C3N4-based photocatalysts synthesis are summarized. Recent studies assessing the application of photocatalytic performance and mechanism of modification of g-C3N4 with carbon quantum dots (CQDs), graphene quantum dots (GQDs), and g-C3N4 QDs are herein discussed. Lastly, challenges and future perspectives of QDs modified g-C3N4-based photocatalysts in photocatalytic applications are discussed. We hope that this review will provide a valuable overview and insight for the promotion of applications of QDs modified g-C3N4 based-photocatalysts.

Highly Efficient and Thermally Stable QD-LEDs Based on Quantum Dots-SiO2-BN Nanoplate Assemblies

Silica encapsulation effectively elevates the resistance of quantum dots (QDs) against water and oxygen. However, QDs-SiO₂ composites present low thermal conductivity and strong thermal accumulation, leading to considerable fluorescence quenching of QDs in optoelectronic devices at high power. Here, a sandwich structural QDs-SiO₂-BN nanoplate assembly material (QDs-SiO₂-BNAs) is developed to reduce the thermal quenching and enhance the stability of QDs in LEDs. The QDs-SiO₂-BNAs is fabricated by embedding QDs-SiO₂ into the interlayer of layer-by-layer assembled BN nanoplates, and the BN nanoplates are pretreated by SiO₂ encapsulation to strengthen the interaction with QDs-SiO₂. This assembly structure endows the QDs with fast heat dissipation and double surface protection against air. The medium power QDs-converted LEDs (QD-LEDs) fabricated by direct on-chip packaging of the QDs-SiO₂-BNAs gain 44.2 °C temperature reduction at 0.5 W in comparison with conventional QD-LEDs. After aging, the resulting QD-LEDs present degradation of only 1.2% under sustained driving for 250 h. The QD-LEDs also pass the 1 week reliability test at 85 °C/85% RH with <±0.01 shift of the color coordinates, demonstrating the profound potential of the QDs-SiO₂-BNAs in LED lighting and display applications.

Enhanced performance of all solid-state quantum dot-sensitized solar cells via synchronous deposition of PbS and CdS quantum dots

The synchronous deposition of PbS and CdS affords band-structure tailoring and surface recombination passivation for efficient and stable solid-state QDSCs.

Anchoring CuS nanoparticles on accordion-like Ti3C2 as high electrocatalytic activity counter electrodes for QDSSCs

A Ti₃C₂/CuS composite has been fabricated as a counter electrode for quantum dot-sensitized solar cells by anchoring CuS nanoparticles on Ti₃C₂via a facile ion-exchange method at room temperature.

Al/Zn co-incorporated Cu–In–Se quantum dots for high efficiency quantum dot sensitized solar cells

A facile method for synthesizing high-quality Cu–In–Se quantum dots (QDs) was developed by Al/Zn co-incorporation. Benefiting from the reduction of trap-state defects in QDs, the efficiency of solar cells basing prepared QDs is obviously improved.

H3PW12O40/Co3O4–Cu2S as a low-cost counter electrode catalyst for quantum dot-sensitized solar cells

The PW₁₂/Co₃O₄–Cu₂S composite film shows excellent electrocatalytic performance and achieves a high photoelectric conversion efficiency of 4.67%, which is 46%, 55.6%, and 72%, respectively, higher than those of Cu₂S, PW₁₂/Co₃O₄ and Co₃O₄ CEs.

Boosting the Performance of Environmentally Friendly Quantum Dot-Sensitized Solar Cells over 13% Efficiency by Dual Sensitizers with Cascade Energy Structure

Generally, high light-harvesting efficiency, electron-injection efficiency, and charge-collection efficiency are the prerequisites for high-efficiency quantum-dot-sensitized solar cells (QDSCs). However, it is fairly difficult for a single QD sensitizer to meet these three requirements simultaneously. It is demonstrated that these parameters can be felicitously balanced by a cosensitization strategy through the adoption of environmental-friendly Zn-Cu-In-Se and Zn-Cu-In-S dual QD sensitizers with cascade energy structure. Experimental results indicate that: i) the combination of the dual QDs can improve the light-harvesting capability of the cells, especially in the visible light window; ii) the cosensitization approach can facilitate electron injection, benefitting from the cascade energy structure of the two QD sensitizers employed; iii) the charge-collection efficiency can be remarkably enhanced by the suppressed charge-recombination process due to the improved QD coverage on TiO₂ . Consequently, this cosensitization strategy delivers a new certified efficiency record of 12.98% for liquid-junction QDSCs under AM 1.5G 1 sun irradiation. Moreover, the constructed cells exhibit good stability in a high-humidity environment.

Improved light-harvesting and suppressed charge recombination by introduction of a nanograss-like SnO2 interlayer for efficient CdS quantum dot sensitized solar cells

Quantum dot sensitized solar cell (QDSSC) performance is primarily limited by the recombination of charges at the interfaces of TiO₂/quantum dot (QD) sensitizer/electrolyte. Hence, blocking or suppressing the charge recombination is an essential requirement to elevate the QDSSC performance to the next level. To retard the charge recombination, herein, we propose the introduction of a SnO₂ nanograss (NG) interlayer on the surface of TiO₂ using the facile chemical bath deposition method. The SnO₂ NG interlayer not only inhibits the interfacial recombination processes in QDSSCs but also enhances the light-harvesting capability in generating more excitons. Hence, the TiO₂/SnO₂ NG/CdS QDSSCs can achieve the power conversion efficiency of 3.15%, which is superior to that of a TiO₂/CdS device (2.16%). Electrochemical impedance spectroscopy, open-circuit voltage decay and dark current analyses confirm that the recombination of charges at the photoanode/electrolyte interface is suppressed and the life time is improved by introducing the SnO₂ NG interlayer between the TiO₂ and CdS QD sensitizer.

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.

Fiber-Spinning-Chemistry Method toward In Situ Generation of Highly Stable Halide Perovskite Nanocrystals

All-inorganic halide perovskite nanocrystals (PNCs) have drawn increasing attention owing to their splendid optical properties. However, such nanomaterials suffer from intrinsic instability, greatly limiting their practical application. Meanwhile, environmental regulation has restricted the emissions of volatile organic compounds (VOCs), initiating a search for alternative approaches to PNC synthesis and film forming. Herein, fiber-spinning chemistry (FSC) is proposed for easy-to-perform synthesis of highly stable PNC fibrous films. The FSC process utilizes spinning fibers as reactors, reducing the generation of VOCs. This method enables the fabrication of CsPbX3 (X = Cl, Br, I) PNCs/poly(methyl methacrylate)/thermoplastic polyurethanes fibrous films at room temperature in one step, exhibiting tunable emission between 450 and 660 nm. Significantly, the in situ generation of PNCs in hydrophobic core-shell nanofibers results in highly improved fluorescence stability. PNCs/polymer fibrous films keep constant in photoluminescence (PL) after storage at atmosphere for 90 d and retain 82% PL after water immersion for 120 h (vs fluorescence quenching in 10 d in air or 5 h in water for pristine PNCs). The PNCs/polymer fibrous films endowed with superior optical stability and great flexibility show promising potentials in flexible optoelectronic applications. This work paves a facile way toward high-performance nanoparticles/polymer fibrous films.

Se-Assisted Performance Enhancement of Cu2ZnSn(S,Se)4 Quantum-Dot Sensitized Solar Cells via a Simple Yet Versatile Synthesis

The earth-abundant Cu₂ZnSnS₄ (CZTS) quantum dots (QDs) have emerged as one potential substitute to toxic cadmium or rare indium QDs, but their application in quantum dot-sensitized solar cells (QDSSCs) is still limited by the improper particle size and the rigorous synthesis and ligand exchange conditions. Herein, we developed a one-pot hot injection method by using Tri-n-octylphosphine oxide (TOPO) as the solvent and oleylamine as the capping agent to synthesize Cu₂ZnSn(S,Se)₄ (CZTSSe) QDs with adjustable size and narrow size distribution. The key feature of this approach is that we can take advantage of the high-temperature nucleation, low-temperature growth, and strong reducibility of NaHB₄ to prepare small-sized CZTSSe QDs without using 1-dodecanethiol (DDT) and to extend the light harvesting range through Se incorporation. After Se incorporation, it turns out that the conduction band (CB) level of CZTSSe QDs decreases, implying that the injection driving force of the electron to the CB of TiO₂ films becomes weaker and a larger recombination would be induced at the TiO₂/QDs/electrolyte interface. Benefiting from the broadened optoelectronic response range, the induced higher J_sc (16.80 vs 14.13 mA/cm²) finally leads to the increase of the conversion efficiency of CZTSSe QDSSC from 3.17% to 3.54% without further modification. Despite the fact that the efficiency is still far behind those of literature reported values through use of other chalcogenide sensitizers, this DDT-free approach solves the main hindrance for the application of CZTSSe QDs in QDSSCs and holds a more convenient way for ligand exchange, light absorption improvement, and particle size control.

MOF-Derived Co,N Codoped Carbon/Ti Mesh Counter Electrode for High-Efficiency Quantum Dot Sensitized Solar Cells

Carbon supported on titanium mesh electrodes has been recognized as the best performing counter electrodes (CEs) in quantum dot sensitized solar cells (QDSCs). Herein, layered double hydroxides (LDHs) are applied as a scaffold template for the growth of cobalt-zeolite-imidazole framework (ZIF-67) crystals, and micrometer-sized Co,N codoped porous carbon materials (Co,N-C) are obtained through a carbonization process. The as-prepared Co,N-C exhibits favorable features for electrocatalytic reduction of polysulfide, including a high surface area of 491.36 m²/g, highly effective active sites, and a hierarchical micro/mesoporous structure. Due to the large particle size, the obtained Co,N-C can couple with a Ti mesh substrate for the fabrication of high-performance Co,N-C/Ti CEs for QDSCs. As a result, the corresponding QDSCs exhibit an average efficiency of 13.55% (J_sc = 25.93 mA/cm², V_oc = 0.778 V, FF = 0.672), which is a 10.5% enhancement compared to the previous best result from the N-doped mesoporous carbon counterpart.

High efficiency solar cells tailored using biomass-converted graded carbon quantum dots

The solar-to-electric conversion efficiency of mesoscopic solar cells is highly dependent on electron extraction under solar irradiation and determines the charge recombination processes within devices. Boosting charge transfer via building energy level alignment has been proved to be a promising method to enhance the output power of solar cells. In the current work, we present the successful fabrication of functional biomass-converted carbon quantum dots with graded energy levels by doping nitrogen or sulphur atoms from lotus root powder. When employed as light absorbers for mesoscopic solar cells, light harvesting and electron extraction can be markedly increased arising from the band alignment of graded carbon quantum dots, yielding maximal power conversion efficiencies of 0.158% and 0.208% for bi- and tri-cascaded photovoltaics, respectively. The primary results demonstrate that the employment of an energy-graded architecture is a promising strategy to optimize the device output. Following this line of thought, we further fabricate a co-sensitized device by integrating graded carbon quantum dots with N719 dyes to enhance the electron extraction capability. The final device yields an efficiency as high as 9.04%, showing the potential application of carbon quantum dots in high-performance solar cells.

Enhanced photoelectrochemical performance of TiO2 nanorod array films based on TiO2 compact layers synthesized by a two-step method

An innovative two-step method perfectly prepared TCLs with different thicknesses, and then the TNA films based on TCLs were successfully prepared. The effects of different thicknesses of TCLs on the morphology and photoelectrochemical performance of TNA films were investigated. The results indicated that TCLs with appropriate thickness could effectively improve the morphology and photoelectrochemical performance of TNA films. Compared with the TNA films based on TCL₅, TCL₁₀ and TCL₃₀, the TNA film based on TCL₂₀ exhibited more ideal and comprehensive photoelectrochemical performance. Moreover, dye-sensitized solar cells (DSSCs) based on this TNA film achieved the highest J_sc (10.2054 mA cm⁻²), V_oc (0.5737 V), PCE (3.3%) and P_out (3.31 mW cm⁻²).

An innovative two-step method perfectly prepared TCLs with different thicknesses, and then the TNA films based on TCLs were successfully prepared.

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.

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.

Synthesis of ZnxCd1-xSe@ZnO Hollow Spheres in Different Sizes for Quantum Dots Sensitized Solar Cells Application

ZnxCd1-xSe@ZnO hollow spheres (HS) were successfully fabricated for application in quantum dot sensitized solar cells (QDSSCs) based on ZnO HS through the ion-exchange process. The sizes of the ZnxCd1-xSe@ZnO HS could be tuned from ~300 nm to ~800 nm using ZnO HS pre-synthesized by different sizes of carbonaceous spheres as templates. The photovoltaic performance of QDSSCs, especially the short-circuit current density (Jsc), experienced an obvious change when different sizes of ZnxCd1-xSe@ZnO HS are employed. The ZnxCd1-xSe@ZnO HS with an average size distribution of ~500 nm presented a better performance than the QDSSCs based on other sizes of ZnxCd1-xSe@ZnO HS. When using the mixture of ZnxCd1-xSe@ZnO HS with different sizes, the power conversion efficiency can be further improved. The size effect of the hollow spheres, light scattering, and composition gradient structure ZnxCd1-xSe@ZnO HS are responsible for the enhancement of the photovoltaic performance.