Exploring the Potential of Quantum Dot-Sensitized Solar Cells: Innovation and Insights

Photovoltaic technologies have garnered significant attention towards generating renewable and clean energy from solar power. Quantum-dot-sensitized solar cells represent a promising third-generation photovoltaic technology that offers alternatives to conventional silicon-based solar cells due to their unique properties, their favourable optoelectronic properties for photovoltaic applications including simplified manufacturing, lower processing temperatures, enhanced flexibility, semi-transparent design, and a theoretical efficiency up to 44 %. The unique characteristic of tailoring the size and composition of quantum dots makes them valuable absorber materials capable of efficiently harnessing a broader range of the solar spectrum. The potential of quantum dot-sensitized solar cells to revolutionize the field of photovoltaic technology is a cause for optimism. However, the major limitation of the overall power conversion efficiency lies in their inability to absorb ultraviolet and near-infrared. Therefore, a photovoltaic technology that can effectively harness the entire solar spectrum becomes imperative. This review discusses the synthesis and light conversion mechanisms of these solar cells. Additionally, it offers an overview of the various advancements made in quantum dot-sensitized solar cells for enhancement in the efficiency of energy conversion. It focuses on the light-absorbing materials used, their efficiency, and the advantages and drawbacks of quantum dot solar cell technology.

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.

Electrochemical Conversion of CO2 into Formate Boosted by In Situ Reconstruction of Bi-MOF to Bi2O2CO3 Ultrathin Nanosheets

Substantial emissions of CO2 have presented formidable challenges for global climate dynamics. Electrochemical reduction of CO2 to produce formic acid (HCOOH) is considered to be a promising approach for achieving carbon neutrality. Nevertheless, the development of a catalyst exhibiting both high catalytic activity and selectivity toward desired products remains an arduous task. Herein, we report the synthesis of a unique porous bismuth-based MOF (Bi-BTC) through microwave-assisted agitation. The Bi-BTC MOF has a good catalytic performance in electrochemical CO2RR to formate products. At -0.9 V (vs RHE) potential, the Faradaic efficiency of formate can reach 96%, and the current density of the CO2RR is 25 mA/cm2. Bi-BTC also exhibits good electrochemical stability. FEformate and current density were maintained for 24 h with almost no attenuation. It was found that Bi-BTC was reconstructed in the CO2RR process. The shape of nanocolumn before electrolysis is transformed into an ultrathin nanosheet. The soft and hard acid-base theory (HSAB) proves that the reason for the reconfiguration is that the hard base ions (HCO3-) and the intermediate acid (Bi3+) break in the Bi-O bond in Bi-MOF, resulting in the transition of the original column structure of Bi-BTC to Bi2O2CO3 ultrathin nanosheeets. The DFT calculation shows that the restructured Bi2O2CO3 nanosheet exposes a crystal surface structure, which is conducive to lower the activation energy barrier of the electrochemical CO2RR intermediate *OCHO and stabilizing the reaction intermediate. Therefore, it is more beneficial to improve the selectivity of the electrochemical CO2RR to formate formation. This result proves that irreversible reconfiguration of catalyst is beneficial to electrochemical CO2RR. In addition, coupling a Bi-BTC cathode with a stable anode (IrO2) enables battery-driven high-activity CO2RR and an OER with good activity and efficiency.

Anisotropic Heavy-Metal-Free Semiconductor Nanocrystals: Synthesis, Properties, and Applications

Heavy-metal (Cd, Hg, and Pb)-containing semiconductor nanocrystals (NCs) have been explored widely due to their unique optical and electrical properties. However, the toxicity risks of heavy metals can be a drawback of heavy-metal-containing NCs in some applications. Anisotropic heavy-metal-free semiconductor NCs are desirable replacements and can be realized following the establishment of anisotropic growth mechanisms. These anisotropic heavy-metal-free semiconductor NCs can possess lower toxicity risks, while still exhibiting unique optical and electrical properties originating from both the morphological and compositional anisotropy. As a result, they are promising light-emitting materials in use various applications. In this review, we provide an overview on the syntheses, properties, and applications of anisotropic heavy-metal-free semiconductor NCs. In the first section, we discuss hazards of heavy metals and introduce the typical heavy-metal-containing and heavy-metal-free NCs. In the next section, we discuss anisotropic growth mechanisms, including solution-liquid-solid (SLS), oriented attachment, ripening, templated-assisted growth, and others. We discuss mechanisms leading both to morphological anisotropy and to compositional anisotropy. Examples of morphological anisotropy include growth of nanorods (NRs)/nanowires (NWs), nanotubes, nanoplatelets (NPLs)/nanosheets, nanocubes, and branched structures. Examples of compositional anisotropy, including heterostructures and core/shell structures, are summarized. Third, we provide insights into the properties of anisotropic heavy-metal-free NCs including optical polarization, fast electron transfer, localized surface plasmon resonances (LSPR), and so on, which originate from the NCs' anisotropic morphologies and compositions. Finally, we summarize some applications of anisotropic heavy-metal-free NCs including catalysis, solar cells, photodetectors, lighting-emitting diodes (LEDs), and biological applications. Despite the huge progress on the syntheses and applications of anisotropic heavy-metal-free NCs, some issues still exist in the novel anisotropic heavy-metal-free NCs and the corresponding energy conversion applications. Therefore, we also discuss the challenges of this field and provide possible solutions to tackle these challenges in the future.

Theoretical prediction and design for chalcogenide-quantum-dot/TiO2 heterojunctions for solar cell applications

Quantum dot sensitized solar cells have attracted much attention due to their high efficiency of photoelectric conversion and low manufacturing cost. In this study, a series of heterojunction structures with cubic (MA)4 chalcogenide quantum dots adsorbing on the (001) surface of TiO2 were investigated, in order to explore new quantum dot sensitizers for solar cell applications. Our study revealed that sulfide and selenide quantum dots are more suitable for solar energy harvesting, compared to their oxide counterparts, due to their smaller ionization potentials and smaller HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) gaps, but in general exhibit weaker adsorption on TiO2. M4A3B and M4A2B2 quantum dots were designed in combination with the advantage of higher adsorption stability and photoelectric conversion capability. Our theoretical predictions for the structurally precise chalcogenide systems suggest a possible direction for the design of quantum-dot sensitized solar cells.

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 VRHE under one sun illumination). Our study provides important information for the preparation of QD photoelectrodes for efficient photoelectrochemical hydrogen generation.

Aqueous Synthesis, Doping, and Processing of n-Type Ag2Se for High Thermoelectric Performance at Near-Room-Temperature

Herein, we have successfully synthesized binary Ag₂Se, composite Ag⁰:Ag₂Se, and ternary Cu⁺:Ag₂Se through an ambient aqueous-solution-based approach in a one-pot reaction at room temperature and atmospheric pressure without involving high-temperature heating, multiple-processes treatment, and organic solvents/surfactants. Effective controllability over phases and compositions/components are demonstrated with feasibility for large-scale production through an exquisite alteration in reaction parameters especially pH for enhancing and understanding thermoelectric properties. Thermoelectric ZT reaches 0.8-1.1 at near-room-temperature for n-type Ag₂Se and Cu⁺ doping further improves to 0.9-1.2 over a temperature range of 300-393 K, which is the largest compared to that reported by wet chemistry methods. This improvement is related to the enhanced electrical conductivity and the suppressed thermal conductivity due to the incorporation of Cu⁺ into the lattice of Ag₂Se at very low concentrations (x%Cu⁺:Ag₂Se, x = 1.0, 1.5, and 2.0).

Doping MAPbBr3 hybrid perovskites with CdSe/CdZnS quantum dots: from emissive thin films to hybrid single-photon sources

We report the first doping of crystalline methyl-ammonium lead bromide perovskite (MAPbBr₃) films with CdSe/CdZnS core/shell quantum dots (QDs), using a soft-chemistry approach that preserves their high quantum yield and other remarkable luminescence properties. Our approach produces MAPbBr₃ films of around 100 nm thickness, doped at volume ratios between 0.01 and 1% with colloidal CdSe/CdZnS QDs whose organic ligands were exchanged with halide ions to allow for close contact between the QDs and the perovskite matrix. Ensemble photoluminescence (PL) measurements demonstrate the retained emission of the QDs after incorporation into the MAPbBr₃ matrix. Photoluminescence excitation (PLE) spectra exhibit signatures of wavelength-dependent coupling between the CdSe/CdZnS QDs and the MAPbBr₃ matrix, i.e., a transfer of charges from matrix to QD, which increases the QD luminescence by up to 150%, or from QD to matrix. Spatially-resolved PL experiments reveal a strong correlation between the positions of QDs and an enhancement of the PL signal of the matrix. Lifetime imaging of the doped films furthermore shows that the emission lifetime of MAPbBr₃ is slower in the vicinity of QDs, which, in combination with the increased PL signal of the matrix, suggests that QDs can act as local nucleation seeds that improve the crystallinity of MAPbBr₃, thus boosting its emission quantum yield. Luminescence antibunching measurements provide clear evidence of single-photon emission from individual QDs in perovskite. Finally, the analysis of blinking statistics indicates an improvement of the photostability of individual QDs in perovskite as compared to bare CdSe/CdZnS QDs. At high CdSe/CdZnS QD doping levels, this work thus opens a route to hybrid solar concentrators for visible-light harvesting and hybrid-based LEDs, while a low degree of doping could yield hybrid single-photon sources than can be embedded in field-effect devices for single-charge control, which would allow the construction of nanophotonic devices via low-cost solution-processing techniques as an alternative to solid-state quantum dots.

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.

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.

Quantum Dot Cellular Uptake and Toxicity in the Developing Brain: Implications for Use as Imaging Probes

Nanometer-sized luminescent semiconductor quantum dots (QDs) have been utilized as imaging and therapeutic agents in a variety of disease settings, including diseases of the central nervous system. QDs have several advantages over traditional fluorescent probes including their small size (5-10 nm), tunable excitation and emission spectra, tailorable surface functionality, efficient photoluminescence, and robust photostability, which are ideal characteristics for in vivo imaging. Although QDs are promising imaging agents in brain-related applications, no systematic evaluation of QD behavior in brain-relevant conditions has yet been done. Therefore, we sought to investigate QD colloidal stability, cellular uptake, and toxicity in vitro, ex vivo, and in vivo in the brain environment. We found that QD behavior is highly dependent on surface functionality and that treatment of cultured organotypic whole hemisphere (OWH) slices with QDs results in dose-dependent toxicity and metallothionein increase, but no subsequent mRNA expression level changes in inflammatory cytokines or other oxidative stress. QDs coated with poly(ethylene glycol) (PEG) were protected from aggregation in neurophysiologically relevant fluids and in tissue, allowing for greater penetration. Importantly, QD behavior differed in cultured slices as compared to monolayer cell cultures, and behavior in cultured slices aligned more closely with that seen in vivo. Irrespective of surface chemistry and brain-relevant platform, non-aggregated QDs were primarily internalized by microglia in a region-dependent manner both in slices and in vivo upon systemic administration. This knowledge will help guide further engineering of candidate QD-based imaging probes for neurological application.

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.

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.

Gold Rod-Polyethylene Glycol-Carbon Dot Nanohybrids as Phototheranostic Probes

Emphasis using phototheranostics has been placed on the construction of multifunctional nanoplatforms for simultaneous tumor diagnosis and therapy. Herein, we put forth a novel nanosized luminescent material using the incorporation of red emissive carbon dots on gold nanorods through polyethylene glycol as a covalent linkage for dual-modal imaging and photothermal therapy. The novel nanohybrids, not only retain the optical properties of the gold nanorod and carbon dots, but also possess superior imaging performance in both confocal laser scanning microscopy and fluorescence lifetime imaging microscopy. The nanohybrids also exhibit excellent photothermal performance as phototheranostic nanohybrid probes for in vitro assays. This study promises a new multifunctional nanoplatform for cancer diagnostics and therapeutics.

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.

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.

An ultrathin SiO2 blocking layer to suppress interfacial recombination for efficient Sb2S3-sensitized solar cells

An SiO₂ thin layer efficiently suppresses the recombination at the TiO₂/Sb₂S₃ interface and enhances the photovoltaic performance of Sb₂S₃ sensitized solar cells.

Quantum dot-sensitized solar cells

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

Comparative advantages of Zn–Cu–In–S alloy QDs in the construction of quantum dot-sensitized solar cells

Alloyed structures of quantum dot light-harvesting materials favor the suppression of unwanted charge recombination as well as acceleration of the charge extraction and therefore the improvement of photovoltaic performance of the resulting solar cell devices. Herein, the advantages of Zn–Cu–In–S (ZCIS) alloy QD serving as light-harvesting sensitizer materials in the construction of quantum dot-sensitized solar cells (QDSCs) were compared with core/shell structured CIS/ZnS, as well as pristine CIS QDs. The built QDSCs with alloyed Zn–Cu–In–S QDs as photosensitizer achieved an average power conversion efficiency (PCE) of 8.47% (V_oc = 0.613 V, J_sc = 22.62 mA cm⁻², FF = 0.610) under AM 1.5G one sun irradiation, which was enhanced by 21%, and 82% in comparison to those of CIS/ZnS, and CIS based solar cells, respectively. In comparison to cell device assembled by the plain CIS and core/shell structured CIS/ZnS, the enhanced photovoltaic performance in ZCIS QDSCs is mainly ascribed to the faster photon generated electron injection rate from QD into TiO₂ substrate, and the effective restraint of charge recombination, as confirmed by incident photon-to-current conversion efficiency (IPCE), open-circuit voltage decay (OCVD), as well as electrochemical impedance spectroscopy (EIS) measurements.

Benefiting from the accelerative electron injection and retarded charge recombination, Zn–Cu–In–S alloy QD based QDSC achieved a PCE of 8.55%, which is 21%, and 82% higher than those of CIS/ZnS, and pristine CIS QDs based solar cells, respectively.

Size dependence of efficiency of PbS quantum dots in NiO-based dye sensitised solar cells and mechanistic charge transfer investigation

Quantum dots (QDs) are very attractive materials for solar cells due to their high absorption coefficients, size dependence and easy tunability of their optical and electronic properties due to quantum confinement. Particularly interesting are PbS QDs owing to their broad spectral absorption until long wavelengths, their easy processability and low cost. Here, we used control of the PbS QD size to understand charge transfer processes at the interfaces of a NiO semiconductor and explain the optimal QD size in photovoltaic devices. Towards this goal, we have synthesized a series of PbS QDs with different diameters (2.8 nm to 4 nm) and investigated charge transfer dynamics by time resolved spectroscopy and their ability to act as sensitizers in nanocrystalline NiO based solar cells using the cobalt tris(4,4'-ditert-butyl-2,2'-bipyridine) complex as a redox mediator. We found that PbS QDs with an average diameter of 3.0 nm show the highest performance in terms of efficient charge transfer and light harvesting efficiency. Our study showed that hole injection from the PbS QDs to the NiO valence band (VB) is an efficient process even with low injection driving force (-0.3 eV) and occurs in 6-10 ns. Furthermore we found that direct electrolyte reduction (photoinduced electron transfer to the cobalt redox mediator) also occurs in parallel to the hole injection with a rate constant of similar magnitude (10-20 ns). In spite of its large driving force, the rate constant of the oxidative quenching of PbS by Co(iii) diminishes more steeply than hole injection on NiO when the diameter of PbS increases. This is understood as the consequence of increasing the trap states that limit electron shift. We believe that our detailed findings will advance the future design of QD sensitized photocathodes.

Fluorescence-based CdTe nanosensor for sensitive detection of cytochrome C

Cytochrome c (Cyt c) is commonly used as intrinsic biomarker for several characteristics of the cell such as respiration, energy level and apoptosis. In the present study a simple colorimetric sensor should be developed and tested for the real-time detection of Cyt c in living cells. We synthesized cadmium telluride quantum dots (CdTe QDs) capped with thioglycolic acid (TGA) as a fluorometric Cyt c nanosensor. The synthesized TGA/CdTe QDs nanosensor was characterized by Fourier transform infrared spectroscopy, transmission electron microscopy, and absorption as well as fluorescence spectrophotometry. We investigated the developed TGA/CdTe QDs sensor with regard to its applicability in the fluorometric detection of Cyt c. Results showed that the TGA/CdTe QDs could be used as a sensitive fluorescence probe for the quantification of different concentrations of Cyt c ranging from 0.5 - 2.5μM. Increased binding of QDs to Cyt c results in decreasing fluorescence. The fluorescence of the QDs is inversely correlated to the Cyt c concentration. Based on these data, a standard curve up to 2.5μM Cyt c was established. Moreover, the developed nanosensor was applied in different concentrations on primary human dermal fibroblasts. Results showed that TGA/CdTe QDs were taken up by cells and could be visualized by fluorescence microscopy. Quantification of Cyt c within living cells via QDs is, however, influenced by various factors such as cell damage, QD aggregation or the level of reactive oxygen species, which have to be taken into account.

Inorganic Ligand Thiosulfate-Capped Quantum Dots for Efficient Quantum Dot Sensitized Solar Cells

The insulating nature of organic ligands containing long hydrocarbon tails brings forward serious limitations for presynthesized quantum dots (QDs) in photovoltaic applications. Replacing the initial organic hydrocarbon chain ligands with simple, cheap, and small inorganic ligands is regarded as an efficient strategy for improving the performance of the resulting photovoltaic devices. Herein, thiosulfate (S₂O₃^2-), and sulfide (S^2-) were employed as ligand-exchange reagents to get access to the inorganic ligand S₂O₃^2-- and S^2--capped CdSe QDs. The obtained inorganic ligand-capped QDs, together with the initial oleylamine-capped QDs, were used as light-absorbing materials in the construction of quantum dot sensitized solar cells (QDSCs). Photovoltaic results indicate that thiosulfate-capped QDs give excellent power conversion efficiency (PCE) of 6.11% under the illumination of full one sun, which is remarkably higher than those of sulfide- (3.36%) and OAm-capped QDs (0.84%) and is comparable to the state-of-the-art value based on mercaptocarboxylic acid capped QDs. Photoluminescence (PL) decay characterization demonstrates that thiosulfate-based QDSCs have a much-faster electron injection rate from QD to TiO₂ substrate in comparison with those of sulfide- and OAm-based QDSCs. Electrochemical impedance spectroscopy (EIS) results indicate that higher charge-recombination resistance between potoanode and eletrolyte interfaces were observed in the thiosulfate-based cells. To the best of our knowledge, this is the first application of thiosulfate-capped QDs in the fabrication of efficient QDSCs. This will lend a new perspective to boosting the performance of QDSCs furthermore.

Alloying Strategy in Cu-In-Ga-Se Quantum Dots for High Efficiency Quantum Dot Sensitized Solar Cells

I-III-VI₂ group "green" quantum dots (QDs) are attracting increasing attention in photoelectronic conversion applications. Herein, on the basis of the "simultaneous nucleation and growth" approach, Cu-In-Ga-Se (CIGSe) QDs with light harvesting range of about 1000 nm were synthesized and used as sensitizer to construct quantum dot sensitized solar cells (QDSCs). Inductively coupled plasma atomic emission spectrometry (ICP-AES), wild-angle X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analyses demonstrate that the Ga element was alloyed in the Cu-In-Se (CISe) host. Ultraviolet photoelectron spectroscopy (UPS) and femtosecond (fs) resolution transient absorption (TA) measurement results indicate that the alloying strategy could optimize the electronic structure in the obtained CIGSe QD material, thus matching well with TiO₂ substrate and favoring the photogenerated electron extraction. Open circuit voltage decay (OCVD) and impedance spectroscopy (IS) tests indicate that the intrinsic recombination in CIGSe QDSCs was well suppressed relative to that in CISe QDSCs. As a result, CIGSe based QDSCs with use of titanium mesh supported mesoporous carbon counter electrode exhibited a champion efficiency of 11.49% (J_sc = 25.01 mA/cm², V_oc = 0.740 V, FF = 0.621) under the irradiation of full one sun in comparison with 9.46% for CISe QDSCs.

Sensitizers for Aqueous-Based Solar Cells

Aqueous dye-sensitized solar cells (DSSCs) are attractive due to their sustainability, the use of water as a safe solvent for the redox mediators, and their possible applications in photoelectrochemical water splitting. However, the higher tendency of dye leaching by water and the lower wettability of dye molecules are two major obstacles that need to be tackled for future applications of aqueous DSSCs. Sensitizers designed for aqueous DSSCs are discussed based on their functions, such as modification of the molecular skeleton and the anchoring group for better stability against dye leaching by water, and the incorporation of hydrophilic entities into the dye molecule or the addition of a surfactant to the system to increase the wettability of the dye for more facile dye regeneration. Surface treatment of the photoanode to deter dye leaching or improve the wettability of the dye molecule is also discussed. Redox mediators designed for aqueous DSSCs are also discussed. The review also includes quantum-dot-sensitized solar cells, with a focus on improvements in QD loading and suppression of interfacial charge recombination at the photoanode.

Mn doped CdS passivated CuInSe2 quantum dot sensitized solar cells with remarkably enhanced photovoltaic efficiency

This paper explores that novel architecture of CuInSe₂/Mn-CdS exhibits remarkable enhancement in photovoltaic performance of the QDSSCs, which presents an excellent power conversion efficiency of 3.96%.

Assembly of CdS nanoparticles on boron and fluoride co-doped TiO2 nanofilm for solar energy conversion applications

Boron and fluoride co-doped TiO₂ nanomaterial is successfully synthetized using a facile process, followed by chemical bath deposition in an organic solution to ensure high wettability and superior penetration ability of the B/F co-doped TiO₂ films.

High performance of TiO2/CdS quantum dot sensitized solar cells with a Cu–ZnS passivation layer

A Cu–ZnS passivation layer effectively suppresses the charge recombination and increases the light harvesting in QDSSCs.

Zinc dopant inspired enhancement of electron injection for CuInS2quantum dot-sensitized solar cells

The PCE of doped CuInS₂QDSCs increased from 5.21% to 5.90%, due to broadened optoelectronic response range and accelerated electron injection.

Carbon Counter-Electrode-Based Quantum-Dot-Sensitized Solar Cells with Certified Efficiency Exceeding 11

The mean power conversion efficiency (PCE) of quantum-dot-sensitized solar cells (QDSCs) is mainly limited by the low photovoltage and fill factor (FF), which are derived from the high redox potential of polysulfide electrolyte and the poor catalytic activity of the counter electrode (CE), respectively. Herein, we report that this problem is overcome by adopting Ti mesh supported mesoporous carbon (MC/Ti) CE. The confined area in Ti mesh substrate not only offers robust carbon film with submillimeter thickness to ensure high catalytic capacity, but also provides an efficient three-dimension electrical tunnel with better conductivity than state-of-art Cu2S/FTO CE. More importantly, the MC/Ti CE can down shift the redox potential of polysulfide electrolyte to promote high photovoltage. In all, MC/Ti CEs boost PCE of CdSe0.65Te0.35 QDSCs to a certified record of 11.16% (Jsc = 20.68 mA/cm(2), Voc = 0.798 V, FF = 0.677), an improvement of 24% related to previous record. This work thus paves a way for further improvement of performance of QDSCs.