Arie Zaban

Quantum-Dot-Sensitized Solar Cells

Quantum-dot-sensitized solar cells (QDSCs) are a promising low-cost alternative to existing photovoltaic technologies such as crystalline silicon and thin inorganic films. The absorption spectrum of quantum dots (QDs) can be tailored by controlling their size, and QDs can be produced by low-cost methods. Nanostructures such as mesoporous films, nanorods, nanowires, nanotubes and nanosheets with high microscopic surface area, redox electrolytes and solid-state hole conductors are borrowed from standard dye-sensitized solar cells (DSCs) to fabricate electron conductor/QD monolayer/hole conductor junctions with high optical absorbance. Herein we focus on recent developments in the field of mono- and polydisperse QDSCs. Stability issues are adressed, coating methods are presented, performance is reviewed and special emphasis is given to the importance of energy-level alignment to increase the light to electric power conversion efficiency.

The Study of Electrolyte Solutions Based on Ethylene and Diethyl Carbonates for Rechargeable Li Batteries II . Graphite Electrodes

The electrochemical behavior of Li-graphite intercalation anodes in ethylene and diethyl carbonates (EC-DEC) solutions was studied using surface sensitive Fourier transform infrared spectroscopy (FTIR) and impedance spectroscopy in conjunction with standard electrochemical techniques. Three different solvent combinations, four different salts: LiBF{sub 4}, LiPF{sub 6}, LiClO{sub 4}, and LiAsF{sub 6}, and the influence of the presence of CO{sub 2} were investigated. Graphite electrodes could be cycled hundreds of times obtaining a reasonable reversible capacity. The best electrolyte was found to be LiAsF{sub 6} and the presence of CO{sub 2} in solutions considerably increased the reversible capacity upon cycling. This improved performance is due to precipitation of the ethylene carbonate reduction product, (CH{sub 2}OCO{sub 2}Li){sub 2}, which is an excellent passivating agent, on the electrode surface. Aging processes of these surface films and their influence on the electrode properties are discussed.

Recent studies on the correlation between surface chemistry, morphology, three-dimensional structures and performance of Li and Li-C intercalation anodes in several important electrolyte systems

Abstract This paper reviews some advances in the comparative study of lithium and graphite electrodes in a large matrix of solvents, salts and additives. The major purpose of this work was to support RD (ii) successful and useful application of AFM and EQCM in order to study the surface film formation and Li-deposition processes; (iii) understanding the correlation between the reversibility and stability of graphite electrodes in Li-intercalation processes and their surface chemistry, and (iv) finding an interesting correlation between the three-dimensional structure of graphite electrodes, the diffusion coefficient of Li + and their voltammetric behaviour in Li-intercalation processes.

The Study of Electrolyte Solutions Based on Ethylene and Diethyl Carbonates for Rechargeable Li Batteries I . Li Metal Anodes

The behavior of Li electrodes was studied in ethylene and diethyl carbonates (EC-DEC) solutions of LiAsF{sub 6}, LiClO{sub 4}, LiBF{sub 4}, and LiPF{sub 6}. The correlation of the surface chemistry to the interfacial properties, morphology, and Li cycling efficiency was investigated using surface sensitive Fourier transform infrared spectroscopy and impedance spectroscopy, scanning electron microscopy, X-ray energy dispersive microanalysis, and standard electrochemical techniques. The Li surface chemistry is initially dominated by EC reduction to an insoluble species, probably (CH{sub 2}OCO{sub 2}Li){sub 2}. Upon storage, several aging processes may take place, depending on the salt used. Their mechanisms are discussed. Although EC-DEC solutions were found to be adequate for Li ion rechargeable batteries, this work indicates that they are not suitable as electrolyte solutions for batteries with Li metal electrodes. This is mostly because Li electrodes cannot be considered stable in these systems and Li deposition is highly dendritic.

Extremely Slow Photoconductivity Response of CH3NH3PbI3 Perovskites Suggesting Structural Changes under Working Conditions

Photoconductivity measurements of CH3NH3PbI3 deposited between two dielectric-protected Au electrodes show extremely slow response. The CH3NH3PbI3, bridging a gap of ∼2000 nm, was subjected to a DC bias and cycles of 5 min illumination and varying dark duration. The approach to steady -state photocurrent lasted tens of seconds with a strong dependence on the dark duration preceding the illumination. On the basis of DFT calculations, we propose that under light + bias the methylammonium ions are freed to rotate and align along the electric field, thus modifying the structure of the inorganic scaffold. While ions alignment is expected to be fast, the adjustment of the inorganic scaffold seems to last seconds as reflected in the extremely slow photoconductivity response. We propose that under working conditions a modified, photostable, perovskite structure is formed, depending on the bias and illumination parameters. Our findings seem to clarify the origin of the well-known hysteresis in perovskite solar cells.

Quantum Dot Sensitized Solar Cells with Improved Efficiency Prepared Using Electrophoretic Deposition

Quantum dot sensitized solar cells (QDSSC) may benefit from the ability to tune the quantum dot optical properties and band gap through the manipulation of their size and composition. Moreover, the inorganic nanocrystals may provide increased stability compared to organic sensitizers. We report the facile fabrication of QDSSC by electrophoretic deposition of CdSe QDs onto conducting electrodes coated with mesoporous TiO(2). Unlike prior chemical linker-based methods, no pretreatment of the TiO(2) was needed, and deposition times as short as 2 h were sufficient for effective coating. Cross-sectional chemical analysis shows that the Cd content is nearly constant across the entire TiO(2) layer. The dependence of the deposition on size was studied and successfully applied to CdSe dots with diameters between 2.5 and 5.5 nm as well as larger CdSe quantum rods. The photovoltaic characteristics of the devices are greatly improved compared with those achieved for cells prepared with a linker approach, reaching efficiencies as high as 1.7%, under 1 sun illumination conditions, after treating the coated electrodes with ZnS. Notably, the absorbed photon to electron conversion efficiencies did not show a clear size-dependence indicating efficient electron injection even for the larger QD sizes. The electrophoretic deposition method can be easily expanded and applied for preparations of QDSSCs using diverse colloidal quantum dot and quantum rod materials for sensitization.

Correlation between surface chemistry, morphology, cycling efficiency and interfacial properties of Li electrodes in solutions containing different Li salts

The influence of the Li salt used on the behaviour of Li electrodes in tetrahydrofurane (THF) and propylene carbonate (PC) solutions was investigated. The salts studied included Li halides (LiBr, LiI), LiBF4, LiPF6, LiSO3CF3 and LiN(SO2CF3)2. The correlation between the electrochemical properties, surface chemistry and morphology of Li electrodes in the above systems was studied using impedance spectroscopy, surface sensitive in situ and ex situ FTIR, X-ray microanalysis, electron microscopy and standard experiments of charge discharge cycling. It was found that all the salt anions explored have strong effects on all the above aspects, eg they strongly affect Li surface chemistry in solutions and participate in the build-up of surface films. The electrical properties of the Li-solution interphase formed in the different salt solutions are remarkably dependent on the salt anion. Consequently, the morphology and Li utility in repeated charge-discharge cycling are also strongly influenced by the salt used. Except for the Li halides, all the salts studied seem to be more reactive to lithium than LiClO4 and LiAsF6. They are worse than the commonly used LiAsF6 for rechargeable Li battery systems because their strong involvement in the Li surface chemistry adversely affects Li cycling efficiency.

Built-in Quantum Dot Antennas in Dye-Sensitized Solar Cells

A new design of dye-sensitized solar cells involves colloidal semiconductor quantum dots that serve as antennas, funneling absorbed light to the charge separating dye molecules via nonradiative energy transfer. The colloidal quantum dot donors are incorporated into the solid titania electrode resulting in high energy transfer efficiency and significant improvement of the cell stability. This design practically separates the processes of light absorption and charge carrier injection, enabling us to optimize each of these separately. Incident photon-to-current efficiency measurements show a full coverage of the visible spectrum despite the use of a red absorbing dye, limited only by the efficiency of charge injection from the dye to the titania electrode. Time resolved luminescence measurements clearly relate this to Forster resonance energy transfer from the quantum dots to the dye. The presented design introduces new degrees of freedom in the utilization of quantum dot sensitizers for photovoltaic cells. In particular, it opens the way toward the utilization of new materials whose band offsets do not allow direct charge injection.

Energy Level Alignment in CdS Quantum Dot Sensitized Solar Cells Using Molecular Dipoles

The energy levels of CdS quantum dots (QDs) can be shifted in a systematic fashion with respect to the TiO(2) bands using molecular dipoles. Dipole moments pointing toward the QD surface shift the energy levels toward the vacuum level (a), thus enabling electron injection from excited QD states into the TiO(2) conduction band at lower photon energies compared to QDs with adsorbed molecular dipoles which are pointing away from the QD surface (b). In CdS QD sensitized solar cells this leads to a dipole dependent shift of the photovoltage onset and the photocurrent.

Mott-Schottky Analysis of Nanoporous Semiconductor Electrodes in Dielectric State Deposited on SnO2 ( F ) Conducting Substrates

This paper analyzes the dark capacitance of nanostructured electrodes in the dielectric state, with particular emphasis on TiO 2 electrodes deposited over a transparent conducting substrate of SnO 2(F). It is shown that at those potentials where the TiO 2 nanostructure is in the dielectric state, the capacitance is controlled by the contact SnO 2(F)/~electrolyte, TiO2). The partial or total covering of the substrate by a dielectric medium causes a modification of the Mott-Schottky plot of the bare substrate. We provide a mapping of the various Mott-Schottky curves that will appear depending on the film characteristics. If the dielectric nanoparticles completely block part of the substrate surface, the slope of the Mott-Schottky plot increases ~with the same apparent flatband potential! as an effect of area reduction. The covering of a significant fraction of the surface by a thin dielectric layer shifts the apparent flatband negatively. Measurements on several TiO 2 nanostructured electrodes show that the capacitance contribution of the semiconductor network in the dielectric state is very low, indicating that the field lines penetrate little into the TiO2 network, not much further than the first particle. The different surface covering observed for rutile-anatase and pure anatase colloids is explained by lattice matching rules with the substrate. By comparing different electrodes, the Helmholtz capacitance at the SnO2(F)/solution interface was calculated and the apparent flatband potential was corrected for the effect of band unpinning.