Albert Goossens

Visible light sensitisation of titanium dioxide using a phenylene vinylene polymer

Abstract The photophysical properties of thin films of poly[2-methoxy-5-(2′-ethyl-hexyloxy), para -phenylene vinylene] (MEH-PPV) on TiO 2 substrates have been investigated. Evidence for dissociation of excitons at the MEH-PPV/TiO 2 interface and electron injection into the conduction band of TiO 2 has been found from current/voltage characteristics. The open circuit voltage is to 0.92 V for these cells and an energy conversion efficiency of 0.15% is found for white light. Capacitance measurements follow the Mott–Schottky relationship and reveal an acceptor density of N A ≈9×10 17 cm −3 for the polymer layers. From analysis of the luminescence as a function of the film thickness, an exciton diffusion length of 20±3 nm for MEH-PPV is derived.

Mott‐Schottky Analysis of Nanometer‐Scale Thin‐Film Anatase TiO2

Recently, much attention has been focused on the use of nanostructured metal oxide semiconductors for various applications, such as electrochromic windows, photocatalytic devices, lithium-ion batteries, dielectrics in integrated circuits, and dye-sensitized TiO{sub 2} solar cells. Smooth nanometer-scale films of anatase TiO{sub 2} on indium-tin oxide substrates (ITO) are obtained by electron-beam evaporation of reduced TiO{sub 2} powder. Mott-Schottky analysis shows an abrupt change in slope when the depletion layer reaches the TiO{sub 2}/ITO interface. An electrostatic model is derived, which gives a quantitative description of the observed change in slope. From the potential at which the slope changes, the dielectric constant of anatase could be accurately determined. A value of 55 is found, which is significantly lower than those reported for anatase TiO{sub 2}.

Photoelectrochemical study of thin anatase TiO 2 films prepared by metallorganic chemical vapor deposition

Thin films of TiO{sub 2} have found application as antireflection coatings, transparent conductors, dielectrics, electrochromic films, and photoelectrodes. Titanium dioxide is also used as a self-cleaning coating on windows and tiles. Thin films of TiO{sub 2} have been prepared by metallorganic chemical vapor deposition. The resulting anatase films are highly resistive. By electrochemical doping, the donor density can easily be raised by two orders of magnitude, resulting in a conducting film. From optical and photocurrent measurements, an indirect bandgap of 3.26 eV is derived for polycrystalline anatase films. Electrolyte electroreflection reveals a direct optical transition of anatase at 3.8 eV. The anatase electrodes can be sensitized by zinc carboxyphenyl porphyrin.

Laser-induced chemical vapor deposition of nanostructured silicon carbonitride thin films

Laser-induced chemical vapor deposition of silicon carbonitride thin films has been investigated using a continuous wave CO2 laser in parallel configuration with the substrate. The reactant gases in this process, hexamethyl disilazane and ammonia, are rapidly heated by CO2 laser radiation due to their absorption of the laser energy. Polymerlike silicon carbonitride films or agglomerated nanosized particles are formed depending on process conditions. Dense, smooth films or nanostructured deposits have been synthesized at low substrate temperatures (Ts<300 °C) on quartz, copper, and silicon and can be obtained with controlled microstructures. Surface morphology, composition, and type of chemical bonding have been studied with electron microscopy and spectroscopic analysis and are correlated to the most important laser process parameters. X-ray photoelectron spectroscopy and reflectance Fourier transform infrared spectroscopy show that the deposits consist of Si–N, Si–C, and Si–O bonds, linked together in a ...

Study of electronic defects in CdSe quantum dots and their involvement in quantum dot solar cells.

To enhance efficiencies of quantum dot CdSe/TiO(2) based solar cells, understanding of the space charge at the CdSe/TiO(2) interface is crucial. In this paper, the presence of a shallow acceptor in the CdSe quantum dots is found by means of a detailed impedance and Mott-Schottky (C(-2)-phi) study. Furthermore, it is clearly shown that this acceptor density decreases strongly with increasing quantum dot size. The presence of these defect states may give rise to Auger recombination in small quantum dots and therewith decrease the efficiency of quantum-dot-sensitized solar cells.

Nanoporous TiO2/Cu1.8S heterojunctions for solar energy conversion

Thin films of p-type Cu 1.8 S have been deposited onto smooth and nanoporous n-type TiO 2 with Atomic Layer Chemical Vapor Deposition (AL-CVD). As precursors, Cu(thd) 2 and H 2 S have been used and self-limited deposition takes place between 125 and 240 °C. The crystalline phase and growth rate strongly depend on the process conditions; below 175 °C CuS and above this temperature, Cu 1.8 S is formed. Photospectroscopy shows that Cu 1.8 S has a bandgap of 1.75 eV and an absorption coefficient of 2.3 X 10 4 cm -1 at 500 nm. A 35-nm-thick Cu 1.8 S film on flat TiO 2 substrates shows a short circuit photocurrent of 30 μA/cm 2 and an open circuit photovoltage of 200 mV at broad-band irradiation of 2.8 kW/m 2 . The internal quantum efficiency is 6% at 500 nm.

Supercrystals of CdSe Quantum Dots with High Charge Mobility and Efficient Electron Transfer to TiO2

Thermal annealing of thin films of CdSe/CdS core/shell quantum dots induces superordering of the nanocrystals and a significant reduction of the interparticle spacing. This results in a drastic enhancement of the quantum yield for charge carrier photogeneration and the charge carrier mobility. The mobile electrons have a mobility as high as 0.1 cm(2)/(V x s), which represents an increase of 4 orders of magnitude over non-annealed QD films and exceeds existing literature data on the electron mobility in CdSe quantum dot films. The lifetime of mobile electrons is longer than that of the exciton. A fraction of the mobile electrons gets trapped at levels below the conduction band of the CdSe nanocrystals. These electrons slowly diffuse over 50-300 nm on longer times up to 20 micros and undergo transfer to a TiO2 substrate. The yield for electron injection in TiO2 from both mobile and trapped electrons is found to be >16%.

Photo- and electroreflectance of cubic boron phosphide

In order to reveal the band structure of cubic boron phosphide a photo- and electroreflectance study has been undertaken. Towards this end, epitaxial thin films of this III–V semiconductor are grown on silicon (100) by chemical vapor deposition. Both modulated reflectance techniques reveal a critical point at 4.25 eV. The value of this first direct transition is more than 1 eV lower than previous reports, which are based on transmission spectra.

Gas-Phase Synthesis of Nanostructured Anatase TiO2

Nanostructured anatase titanium dioxide (TiO2) has been obtained using chemical vapor deposition (CVD). In order to arrive at a fractal-type morphology, two different titanium precursors were injected simultaneously into a cold-wall CVD system. With this method, a wide variety of morphologies can be obtained. For solar light harvesting, the structures of trees, bushes, and grasses appear to be good choices. Small variations in deposition parameters have a dramatic effect on the structure of the deposits, which allows adequate optimization of nanostructures by gas-phase synthesis techniques.

Electrical and optical properties of TiO2 in accumulation and of lithium titanate Li0.5TiO2

Changes in the optical absorption and electrical conductivity of dense and mesoporous anatase TiO2 films were measured in situ as a function of electrode potential during electrochemical lithium intercalation. A special two-electrode geometry was used for the conductivity measurements, in which the contacts were separated by a small gap bridged by the TiO2. When electrons are injected, an accumulation layer is formed and the conductivity increases several orders of magnitude. A monotonic increase of the optical absorption with wavelength confirms the presence of (partially) delocalized electrons. Insertion of lithium ions results in the formation of the Li0.5TiO2 phase and a decrease of the overall conductance. The specific conductivity of the Li0.5TiO2 phase is (9.1±0.2) S/cm, significantly lower than that of Li-doped anatase TiO2. This is corroborated by the absorption spectrum of Li0.5TiO2, which shows two pronounced peaks around 440 and 725 nm and no characteristic free-electron features. At potential...