E. A. Konstantinova

Influence of oxygen and water related surface defects on the dye sensitized TiO2 solar cell

Oxygen- and water-related surface defects on porous TiO 2 (anatase) can be well controlled by the oxygen and water partial pressures and therefore such defects are of technological relevance for dye sensitized TiO 2 solar cells. We investigated the action of oxygen and water-related surface defects in situ by impedance spectroscopy, photoconductivity, photoluminescence, and optical transmission as well as by characterizing solar cells which were prepared under respective conditions. Oxygen loss from the TiO 2 surface leads to electrical doping by Ti3‘/oxygen donor states. Such defects create recombination paths for injected electrons back into the electrolyte. Pre-treatment of porous TiO 2 by chemisorption of water increases the open circuit voltage of the solar cells without altering the short circuit current. Water-related surface defects decrease the saturation current of the diode, probably by raising the barrier height at the TiO 2 /electrolyte interface. ( 1999 Elsevier Science B.V. All rights reserved.

Influence of molecule adsorption on porous silicon photoluminescence

Abstract As-anodized porous silicon (PS) was investigated in situ by photoluminescence (PL) during the adsorption of H 2 O, O 2 , C 2 H 5 OH and C 2 (CN) 4 molecules. The PL intensity was increased during H 2 O adsorption (to 130%) and decreased by adsorption of O 2 (to 30%), C 2 H 5 OH (to 22%) and C 2 (CN) 4 (to 5%). A strong blue shift was observed during the C 2 H 5 OH adsorption. The reversibility of the adsorption-induced PL changes was compared for the different atmospheres. Reversibility by the simple cycle high vacuum pumping (HV)-adsorption-HV could be reached only for C 2 H 5 OH. The PL changes induced by H 2 O and C 2 (CN) 4 adsorption were good (H 2 O) or partially (C 2 (CN) 4 ) reversible in combination with heat treatment at 420 K in HV. The O 2 -adsorption-induced PL changes are not reversible by simple or modified cycles.

Time-resolved photoluminescence of erbium centers in amorphous hydrogenated silicon

Abstract A time-resolved photoluminescence (PL) technique is employed to study the mechanisms of excitation and de-excitation of Er 3+ ions in amorphous hydrogenated silicon (a-Si:H) at temperatures 5–300 K. The PL at 1.5 μm is found to arise within times shorter than a 100 ns after irradiation with a nanosecond laser pulse, indicating a fast energy transfer from the electronic excitation of a-Si:H to the Er 3+ ions. The PL transients exhibit a stretched exponential decay with mean lifetime ranging from 20 to 35 μs when the temperature decreases from 300 K down to 5 K. The experimental results support the model of defect-related excitation and de-excitation of the Er 3+ ions.

Influence of Adsorbed Water and Oxygen on the Photoluminescence and EPR of Por-TiO2 (Anatase)

The influence of adsorbed water, oxygen, air and vacuum on the photoluminescence (PL) and electron paramagnetic resonance (EPR) has been investigated in situ and quasi in situ for por-TiO2 (anatase). The broad PL signal in the visible spectral region decreases with increasing partial pressure of oxygen and vanishes at pressures higher than 1 mbar. Adsorption of water leads (i) to a fast quenching and (ii) to a subsequent increase of the PL signal. The concentration of the O2−, O−, O3− anion-radicals depends sensitively on the surface conditioning and on the illumination of the por-TiO2. Ti3+ centers could be observed only in vacuum treated samples. The concentrations of the Ti3+ and oxygen anion radicals are in the range of 1015 and 1017 cm−3, respectively.

Facile preparation of nitrogen-doped nanostructured titania microspheres by a new method of Thermally Assisted Reactions in Aqueous Sprays

N-doped nanocrystalline titania microspheres with controlled visible light absorption were obtained by a new route utilizing a heterogeneous hydrolysis of TiCl4 vapor with reactive droplets of aqueous aerosols, followed by instant calcination of the products in a preheated flow reactor. Thus prepared separate microspheres with a diameter of 0.5–3 microns were found to consist of anatase or rutile nanocrystals, depending on the parameters of the synthesis. Doped titania samples with different nitrogen contents were synthesized using water solutions with various concentrations of urea in the hydrolyzing aerosols. N− species were found to be a major form of nitrogen impurity. A plausible mechanism for the incorporation of nitrogen into titanium dioxide was proposed. The photocatalytic activity of the obtained powders under UV-visible or visible illumination was found to have a complex dependence on the calcination temperature and the titania doping level.

Influence of photoluminescence and trapping on the photovoltage at the por-Si/p-Si structure

The influence of photoluminescence (PL) and trapping on photovoltage (PV) transients has been compared for as-anodized and for oxidized in air por-Si/p-Si structures. Measurements were carried out with time-resolved surface photovoltage and with contact potential difference techniques. Light sources with wavelengths from the infrared to the ultraviolet regions were used for excitation. It was shown that PV based on PL light can be used for the estimation of the PL efficiency. Electron trapping was found to modify the band bending at the por-Si/p-Si interface of the oxidized in air sample. Further it was shown that non-monotonous PV transient features took place on oxidized por-Si/p-Si structures at high levels of optical excitation.

Adsorption-induced modification of spin and recombination centers in porous silicon

Electron paramagnetic resonance (EPR) and photoluminescence (PL) techniques were employed to investigate the influence of molecules adsorption (H2O, O2, C2H5OH, C2(CN)4) on properties of porous silicon (por-Si). A photoinduced decrease (H2O atmosphere) and increase (O2 atmosphere) of the silicon dangling bond concentration NDB were detected while PL enhancement (H2O) and quenching (O2) took place. Decrease of NDB and quenching of the PL was observed under C2H5OH adsorptio. Adsorption of C2(CN)4 did not change the EPR signal of the DB. A new EPR signal of [C2(CN)4]− radicals appeared and the PL intensity decreased dramatically at the same time. The results are discussed on the basis of three main mechanisms of the influence of molecules on the por-Si PL properties: (i) chemical and photochemical modification of recombination centers (H2O and O2); (ii) dielectric exciton quenching (C2H5OH); and (iii) desctruction of excitons by strong Coulomb fields of the adsorbed molecules (C2(CN)4).

Modification of the properties of porous silicon on adsorption of iodine molecules

Infrared spectroscopy and electron spin resonance measurements are used to study the properties of porous silicon layers on adsorption of the I2 iodine molecules. The layers are formed on the p-an n-Si single-crystal wafers. It is established that, in the atmosphere of I2 molecules, the charge-carrier concentration in the layers produced on the p-type wafers can be noticeably increased: the concentration of holes can attain values on the order of ∼1018−1019 cm−3. In porous silicon layers formed on the n-type wafers, the adsorption-induced inversion of the type of charge carriers and the partial substitution of silicon-hydrogen bonds by silicon-iodine bonds are observed. A decrease in the concentration of surface paramagnetic defects, Pb centers, is observed in the samples with adsorbed iodine. The experimental data are interpreted in the context of the model in which it is assumed that both deep and shallow acceptor states are formed at the surface of silicon nanocrystals upon the adsorption of I2 molecules.

Ion implantation of porous gallium phosphide

The effect of irradiation by Ar ions and thermal annealing on the properties of porous gallium phosphide (por-GaP) obtained by electrolytic methods is investigated. It is shown on the basis of Raman scattering and photoluminescence data that, in contrast with porous silicon, por-GaP does not have high radiation hardness, and that thermal annealing of defects in layers amorphized by ion implantation is impeded by the absence of a good crystal base for solid-state epitaxial recrystallization processes. Data on radiation-induced defect formation and from probing of the material with a rare-earth “luminescence probe” are consistent with a mesoporous structure of the material.

Features of the structure and defect states in hydrogenated polymorphous silicon films

The structural and electronic properties of thin hydrogenated polymorphous silicon films obtained by plasma-enhanced chemical vapor deposition from hydrogen (H2) and monosilane (SiH4) gas mixture have been studied by means of transmission electron microscopy, electron paramagnetic resonance (EPR) spectroscopy, and Raman spectroscopy. It has been established that the studied films consist of the amorphous phase containing silicon nanocrystalline inclusions with the average size on the order of 4–5 nm and the volume fraction of 10%. A signal was observed in the hydrogenated polymorphous silicon films during the EPR investigation that is attributed to the electrons trapped in the conduction band tail of microcrystalline silicon. It has been shown that the introduction of a small fraction of nanocrystals into the amorphous silicon films nonadditively changes the electronic properties of the material.