Marcus Theory and Tunneling Method for the Electron Transfer Rate Analysis in Quantum Dot Sensitized Solar Cells in the Presence of Blocking Layer

In this research study, the effects of different parameters on the electron transfer rate from three quantum dots (QDs), CdSe, CdS, and CdTe, on three metal oxides (MOs), TiO2, SnO2, and SnO2, in quantum-dot-sensitized solar cells (QDSSCs) with porous structures in the presence of four types of blocking layers, ZnS, ZnO, TiO2, and Al2O3, are modeled and simulated using the Marcus theory and tunneling between two spheres for the first time. Here, the studied parameters include the change in the type and thickness of the blocking layer, the diameter of the QD, and the temperature effect. To model the effect of the blocking layer on the QD, the effective sphere method is used, and by applying it into the Marcus theory equation and the tunneling method, the electron transfer rate is calculated and analyzed. The obtained results in a wide range of temperatures of 250-400 °K demonstrate that, based on the composition of the MO-QD, the increase in the temperature could reduce or increase the electron transfer rate, and the change in the QD diameter could exacerbate the effects of the temperature. In addition, the results show which type and thickness of the blocking layer can achieve the highest electron transfer rate. In order to test the accuracy of the simulation method, we calculate the electron transfer rate in the presence of a blocking layer for a reported sample of a QDSSC manufacturing work, which was obtained with an error of ~3%. The results can be used to better interpret the experimental observations and to assist with the design and selection of the appropriate combination of MO-QD in the presence of a blocking layer effect.

Surface State-Assisted Delayed Photocurrent Response of Au Nanocluster/TiO2 Photoelectrodes

Gold nanoclusters (NCs) can be used as sensitizers to extend the absorption capabilities of TiO₂ as photoelectrodes. However, the adsorption of NCs also creates additional surface states on the TiO₂ surface, which gives rise to intricacies in the understanding of various interfacial phenomena occurring in NC-sensitized TiO₂. One of the complexities that have recently been discovered is the size-dependent hole-transfer mechanism. In this work, we reveal another anomalous behavior in the hole-transfer process that the hole scavenging ability of the electrolyte also plays a role in determining the hole-transfer mechanism in the NC-TiO₂ system, which is unprecedented in other photoelectrode systems. In the presence of an efficient hole scavenger (Na₂SO₃), the hole transfer in Au₁₈-TiO₂ occurs directly through the highest occupied molecular orbital (HOMO) of Au₁₈ NCs. However, in the presence of a less efficient hole scavenger (ethylenediaminetetraacetic acid), hole transfer in Au₁₈-TiO₂ does not occur through the HOMO and shifts to surface state-assisted hole transfer. Due to surface state charging, this surface state-assisted hole-transfer mechanism results in delayed photocurrent response in Au₁₈-TiO₂. Evidence for this exotic hole-transfer mechanism shift is provided by photoelectrochemical electrochemical impedance spectroscopy, and its implications are discussed.

Quantum-dot sensitized hierarchical NiO p–n heterojunction for effective photocatalytic performance

A facile and low-cost pseudo successive ionic layer adsorption and reaction technique was used to deposit cadmium sulfide quantum dots (CdS QDs) on hierarchical nanoflower NiO to form effective and intimate NiO/CdS, p–n heterojunctions.

SILAR Deposition of Metal Oxide Nanostructured Films

Methods for the fabrication of thin films with well controlled structure and properties are of great importance for the development of functional devices for a large range of applications. SILAR, the acronym for Successive Ionic Layer Adsorption and Reaction, is an evolution and combination of two other deposition methods, the Atomic Layer Deposition and Chemical Bath Deposition. Due to a relative simplicity and low cost, this method has gained increasing interest in the scientific community. There are, however, several aspects related to the influence of the many parameters involved, which deserve further deepening. In this review article, the basis of the method, its application to the fabrication of thin films, the importance of experimental parameters, and some recent advances in the application of oxide films are reviewed. At first the fundamental theoretical bases and experimental concepts of SILAR are discussed. Then, the fabrication of chalcogenides and metal oxides is reviewed, with special emphasis to metal oxides, trying to extract general information on the effect of experimental parameters on structural, morphological and functional properties. Finally, recent advances in the application of oxide films prepared by SILAR are described, focusing on supercapacitors, transparent electrodes, solar cells, and photoelectrochemical devices.

Interfacial Engineering at Quantum Dot-Sensitized TiO2 Photoelectrodes for Ultrahigh Photocurrent Generation

Metal oxide semiconductor/chalcogenide quantum dot (QD) heterostructured photoanodes show photocurrent densities >30 mA/cm² with ZnO, approaching the theoretical limits in photovoltaic (PV) cells. However, comparative performance has not been achieved with TiO₂. Here, we applied a TiO₂(B) surface passivation layer (SPL) on TiO₂/QD (PbS and CdS) and achieved a photocurrent density of 34.59 mA/cm² under AM 1.5G illumination for PV cells, the highest recorded to date. The SPL improves electron conductivity by increasing the density of surface states, facilitating multiple trapping/detrapping transport, and increasing the coordination number of TiO₂ nanoparticles. This, along with impeded electron recombination, led to enhanced collection efficiency, which is a major factor for performance. Furthermore, SPL-treated TiO₂/QD photoanodes were successfully exploited in photoelectrochemical water splitting cells, showing an excellent photocurrent density of 14.43 mA/cm² at 0.82 V versus the Reversible Hydrogen Electrode (RHE). These results suggest a new promising strategy for the development of high-performance photoelectrochemical devices.

Role of Regeneration of Nanoclusters in Dictating the Power Conversion Efficiency of Metal-Nanocluster-Sensitized Solar Cells

Metal nanoclusters (NCs) have emerged as feasible alternatives to dyes and quantum dots in light energy conversion applications. Despite the remarkable enhancement in power conversion efficiency (PCE) in recent years and the increase in the number of NCs available as sensitizers, a comprehensive understanding of the various interfacial charge-transfer, transport, and recombination events in NCs is still lacking. This understanding is vital to the establishment of design principles for an efficient photoelectrode that uses NCs. In this work, we carefully design a comparison study of two representative NCs, Au and Ag, based on transient absorption spectroscopy and electrochemical impedance spectroscopy, methods that shed light on the true benefits and limitations of NC sensitizers. Low NC regeneration efficiency is the most critical factor that limits the performance of metal-nanocluster-sensitized solar cells (MCSSCs). The slow regeneration that results from sluggish hole transfer kinetics not only limits photocurrent generation efficiency but also has a profound effect on the stability of MCSSCs. This finding calls for urgent attention to the development of an efficient redox couple that has a great hole-extraction ability and no corrosive nature. This work also reveals different interfacial behaviors of Au and Ag NCs in photoelectrodes, suggesting that utilizing the benefits of both types of NCs simultaneously by cosensitization or using AuAg alloy NCs may be one avenue for further PCE improvement in MCSSCs.

Electrochemical Performance of Photovoltaic Cells using HDA Capped-SnS Nanocrystal from bis (N-1,4-Phenyl-N-Morpho-Dithiocarbamato) Sn(II) Complexes

Great consideration is placed on the choice of capping agents' base on the proposed application, in order to cater to the particular surface, size, geometry, and functional group. Change in any of the above can influence the characteristics properties of the nanomaterials. The adoption of hexadecylamine (HDA) as a capping agent in single source precursor approach offers better quantum dots (QDs) sensitizer materials with good quantum efficiency photoluminescence and desirable particles size. Structural, morphological, and electrochemical instruments were used to evaluate the characterization and efficiency of the sensitizers. The cyclic voltammetry (CV) results display both reduction and oxidation peaks for both materials. XRD for SnS/HDA and SnS photosensitizers displays eleven peaks within the values of 27.02° to 66.05° for SnS/HDA and 26.03° to 66.04° for SnS in correlation to the orthorhombic structure. Current density-voltage (I-V) results for SnS/HDA exhibited a better performance compared to SnS sensitizers. Bode plot results indicate electrons lifetime (τ) for SnS/HDA photosensitizer have superiority to the SnS photosensitizer. The results connote that SnS/HDA exhibited a better performance compared to SnS sensitizers due to the presence of HDA capping agent.

The effect of ZnO/ZnSe core/shell nanorod arrays photoelectrodes on PbS quantum dot sensitized solar cell performance

ZnO nanorod (NR) based inorganic quantum dot sensitized solar cells have gained tremendous attention for use in next generation solar cells. ZnO/ZnSe-core/shell NR arrays (NRAs) with various densities were grown on an Au@ZnO seed layer (Au = 0.0, 4.0, 8.0 and 16.0 nm) on glass supported fluorine-doped tin oxide (FTO) substrates using low cost hydrothermal and ion-exchange approaches. PbS quantum dots (QDs) were loaded into the ZnO/ZnSe core/shell NRAs via a successive ionic layer adsorption and reaction (SILAR) method. The morphology, structural and optical properties of the core/shell NRAs were investigated using field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and UV-vis spectroscopy measurements. It was observed that the density of the ZnO/ZnSe NRAs decreases with increasing Au buffer layer thickness. The absorption decreases along with a decrease in the ZnO/ZnSe NRA density. The ZnO NRs/PbS QD photoelectrode performs poorly; however, after introducing a ZnSe shell on the core-ZnO, the solar cells parameters changed according to the ZnO/ZnSe NRA density. Values of η = ∼0.88%, J SC = 14.60 mA cm-2, and V OC = 190 mV, and η = ∼0.25%, J SC = 6.77 mA cm-2, and V OC = 115 mV were obtained for the highest and lowest NRA densities, respectively. Although the photovoltaic performance of these photoelectrodes is still inferior, further improvement of the device would be possible by suppressing surface defects, and through quality optimization of the ZnO/ZnSe NRAs, PbS QDs, counter electrode and electrolyte.

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.

Ag(I)-Thiolate-Protected Silver Nanoclusters for Solar Cells: Electrochemical and Spectroscopic Look into the Photoelectrode/Electrolyte Interface

Intrinsic low stability and short excited lifetimes associated with Ag nanoclusters (NCs) are major hurdles that have prevented the full utilization of the many advantages of Ag NCs over their longtime contender, Au NCs, in light energy conversion systems. In this report, we diagnosed the problems of conventional thiolated Ag NCs used for solar cell applications and developed a new synthesis route to form aggregation-induced emission (AIE)-type Ag NCs that can significantly overcome these limitations. A series of Ag(0)/Ag(I)-thiolate core/shell-structured NCs with different core sizes were explored for photoelectrodes, and the nature of the two important interfacial events occurring in Ag NC-sensitized solar cells (photoinduced electron transfer and charge recombination) were unveiled by in-depth spectroscopic and electrochemical analyses. This work reveals that the subtle interplay between the light absorbing capability, charge separation dynamics, and charge recombination kinetics in the photoelectrode dictates the solar cell performance. In addition, we demonstrate significant improvement in the photocurrent stability and light conversion efficiency that have not been achieved previously. Our comprehensive understanding of the critical parameters that limit the light conversion efficiency lays a foundation on which new principles for designing Ag NCs for efficient light energy conversion can be built.

Sulfur in Hyper-cross-linked Porous Polymer as Cathode in Lithium-Sulfur Batteries with Enhanced Electrochemical Properties

Sulfur was impregnated into hyper-cross-linked porous polymer (HCP) with a high specific area and unique porous structure. Compared to its inorganic or carbon counterparts, the HCP has a relatively high specific surface area of 1980 m² g^-1 with a total pore volume of 2.61 cm³ g^-1, resulting in sulfur content in HCP/S of as high as 80 wt %. As a benefit of the unique HCP structure, the HCP/S composite exhibits a high initial discharge specific capacity (1333 mA h g^-1 at 0.2 C), high-rate property, and good cycling stability (658 mA h g^-1 after 120 cycles at 0.5 C and 604 mA h g^-1 after 80 cycles at 1 C). Furthermore, the capacity of cells loses less than 1% after the first 20 charge/discharge cycles, while the HCP/S cathode can be cycled with an excellent Coulombic efficiency of above 94% after 120 cycles. Compared with pristine sulfur, the superior electrochemical performance of HCP/S composite is related to the cross-linked porous framework. Such structure could provide short ionic/electronic conduction pathways and suppress the polysulfide shuttle during the discharge process.

Control of morphology and defect density in zinc oxide for improved dye-sensitized solar cells

While zinc oxide (ZnO) with a mesoporous network has long been explored for adsorption of dyes and as an electron-transporting medium in dye-sensitized solar cells (DSSCs), the performance of ZnO-based DSSCs remains unsatisfactory. Despite the importance of understanding the surface characteristics of ZnO in DSSC applications, most of the studies relevant to ZnO-based DSSCs are focused on the synthesis of unique nanostructures of ZnO. In this study, we not only introduce a novel disk-shaped ZnO nanostructure, but also provide insight into the distinctive surface properties of ZnO and its influence on DSSC performance. When utilized in DSSCs, the mesoporous ZnO nanodisk yields 60% better power conversion efficiency (PCE) compared to commercial ZnO nanoparticles, which is attributed to less surface and bulk trap densities as concluded by an in-depth open-circuit voltage decay (OCVD) analysis and electrochemical impedance spectroscopy (EIS). Another aspect that contributes to the higher PCE is the better connectivity of primary particles that join together to form secondary disk-shaped particles. Furthermore, a 40% improvement in the PCE was observed by coating the mesoporous ZnO nanodisk with TiO₂, which results from the passivation of the surface defects that aid in suppressing the interfacial charge recombination.

Iron Pyrite/Titanium Dioxide Photoanode for Extended Near Infrared Light Harvesting in a Photoelectrochemical Cell

The design of active and stable semiconducting composites with enhanced photoresponse from visible light to near infrared (NIR) is a key to improve solar energy harvesting for photolysis of water in photoelectrochemical cell. In this study, we prepared earth abundant semiconducting composites consisting of iron pyrite and Titanium oxide as a photoanode (FeS₂/TiO₂ photoanode) for photoelectrochemical applications. The detailed structure and atomic compositions of FeS₂/TiO₂ photoanode was characterized by high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (XRD), inductively coupled plasma with atomic emission spectroscopy (ICPAES) and Raman spectroscopy. Through the proper sulfurization treatment, the FeS₂/TiO₂ photoanode exhibited high photoresponse from visible light extended to near infrared range (900 nm) as well as stable durability test for 4 hours. We found that the critical factors to enhance the photoresponse are on the elimination of surface defect of FeS₂ and on the enhancement of interface charge transfer between FeS₂ and TiO₂. Our overall results open a route for the design of sulfur-based binary compounds for photoelectrochemical applications.

Photoelectrochemical properties of PbS quantum dot sensitized TiO2 nanorods photoelectrodes

Cross-sectional view FESEM images of (a) TiO₂ NRs, (b) PbS(4)/TiO₂ NRs (the insets show the top-view of the corresponding cross-sectional view FESEM images). 3D schematic diagrams of (c) TiO₂ NRs, (d) PbS(4)/TiO₂ NRs.