Semiconductor Quantum Dot Sensitized Solar Cells Based on Ferricyanide/Ferrocyanide Redox Electrolyte Reaching an Open Circuit Photovoltage of 0.8 V

A Low Driving-Voltage Hybrid-Electrolyte Electrochromic Window with Only Ferreous Redox Couples

Even after decades of development, the widespread application of electrochromic windows (ECW) is still seriously restricted by their high price and inadequate performance associated with structural/fabrication complexity and electrochemical instability. Herein, a simple hybrid electrochromic system based on PFSA (perfluorosulfonic acid)-coated Prussian blue (PB, Fe4III [FeII(CN)6]3) film and Ferricyanide-Ferrocyanide ([Fe(CN)6]4-/[Fe(CN)6]3-)-containing hybrid electrolyte is reported. The PB film and the [Fe(CN)6]4-/[Fe(CN)6]3- couple show near redox potentials well inside the electrochemical window of water, resulting in a low driven voltage (0.4 V for coloring and -0.6 V for bleaching) and a relatively long lifespan (300 cycles with 76.9% transmittance contrast retained). The PFSA layer, as a cation-exchange structure, significantly improves the transmittance modulation amplitude (ΔT: 23.3% vs. 71.9% at a wavelength of 633 nm) and optical memory abilities (ΔT retention: 10.1% vs. 67.0% after 300 s open-circuit rest increases) of the device, by means of preventing the direct contact and charge transfer between the PB film and the [Fe(CN)6]4-/[Fe(CN)6]3- couple. This "hybrid electrolyte + electron barrier layer" design provides an effective way for the construction of simple structured electrochromic devices.

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

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

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

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

SILAR controlled CdSe nanoparticles sensitized ZnO nanorods photoanode for solar cell application: Electrolyte effect

Controlled growth of different sizes of cadmium selenide (CdSe) nanoparticles over well aligned ZnO nanorods have been performed using successive ionic layer adsorption and reaction (SILAR) technique at room temperature (27 °C) in order to form nano heterostructure solar cells. Deposition of compact layer of zinc oxide (ZnO) by SILAR technique on fluorine doped tin oxide (FTO) coated glass substrate followed by growth of vertically aligned ZnO nanorods array using chemical bath deposition (CBD) at low temperature (<100 °C). Different characterization techniques viz. X-ray diffractometer, UV-Vis spectrophotometer, field emission scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy have been used to know the structural, optical, morphological and compositional properties of synthesized nano heterostructure. The photovoltaic performance of the cells with variation in SILAR cycles for CdSe and with use of different electrolytes have been recorded as J-V characteristics and the maximum conversion efficiency of 0.63% have been attained with ferro/ferri cyanide electrolyte for 12 cycles CdSe coating over 1-D ZnO nanorods.

Photoluminescence of CdSe/ZnS quantum dots in nematic liquid crystals in electric fields

We have experimentally investigated the effect of the reorientation of a nematic liquid crystal (LC) in an electric field on the photoluminescence (PL) of CdSe/ZnS semiconductor quantum dots (QDs). To the LC with positive dielectric anisotropy, 1 wt % QDs with a core diameter of 5 nm was added. We compared the change of PL intensity and decay times of QDs in LC cells with initially planar or vertically orientated molecules, i.e., in active or passive LC matrices. The PL intensity of the QDs increases four-fold in the active LC matrix and only 1.6-fold in the passive LC matrix without reorientation of the LC molecules. With increasing electric field strength, the quenching of QDs luminescence occurred in the active LC matrix, while the PL intensity did not change in the passive LC matrix. The change in the decay time with increasing electric field strength was similar to the behavior of the PL intensity. The observed buildup in the QDs luminescence can be associated with the transfer of energy from LC molecules to QDs. In a confocal microscope, we observed the increase of particle size and the redistribution of particles in the active LC matrix with the change of the electric field strength. At the same time, no significant changes occurred in the passive LC matrix. With the reorientation of LC molecules from the planar in vertical position in the LC active matrix, quenching of QD luminescence and an increase of the ion current took place simultaneously. The obtained results are interesting for controlling the PL intensity of semiconductor QDs in liquid crystals by the application of electric fields.

Quantum dot-sensitized solar cells

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

Connecting quantum dots with enzymes: mediator-based approaches for the light-directed read-out of glucose and fructose oxidation

The combination of the biocatalytic features of enzymes with the unique physical properties of nanoparticles in a biohybrid system provides a promising approach for the development of advanced bioelectrocatalytic devices. This study describes the construction of photoelectrochemical signal chains based on CdSe/ZnS quantum dot (QD) modified gold electrodes as light switchable elements, and low molecular weight redox molecules for the combination with different biocatalysts. Photoelectrochemical and photoluminescence experiments verify that electron transfer can be achieved between the redox molecules hexacyanoferrate and ferrocene, and the QDs under illumination. Since for both redox mediators a concentration dependent photocurrent change has been found, light switchable enzymatic signal chains are built up with fructose dehydrogenase (FDH) and pyrroloquinoline quinone-dependent glucose dehydrogenase ((PQQ)GDH) for the detection of sugars. After immobilization of the enzymes at the QD electrode the biocatalytic oxidation of the substrates can be followed by conversion of the redox mediator in solution and subsequent detection at the QD electrode. Furthermore, (PQQ)GDH has been assembled together with ferrocenecarboxylic acid on top of the QD electrode for the construction of a funtional biohybrid architecture, showing that electron transfer can be realized from the enzyme over the redox mediator to the QDs and subsequently to the electrode in a completely immobilized fashion. The results obtained here do not only provide the basis for light-switchable biosensing and bioelectrocatalytic applications, but may also open the way for self-driven point-of-care systems by combination with solar cell approaches (power generation at the QD electrode by enzymatic substrate consumption).

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.

Recent advances in counter electrodes of quantum dot-sensitized solar cells

The recent progress in the development of counter electrodes (CEs) is reviewed, and the key issues for the materials, structures and performance evaluation of CEs are also addressed.