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The recent investigation of kesterite type quaternary compounds showed that one of the main detrimental problems, which limits the efficiency of the solar cells based on these materials, is related to the high defect concentrations, mainly Cu-Zn disorder. To overcome this problem partial replacement of Cu or Zn cations in the classical Cu2ZnSn(S,Se)4 compounds was proposed. One of the promising cations was found to be Ag, which partially replaces Cu and the solid solution of (AgxCu1-x)2ZnSn(S,Se)4 (ACZTSSe) is formed. In the present study, ACZTSSe thin films with different Ag concentrations were deposited by the spray pyrolysis method, with subsequent annealing in S + Se atmosphere. From the analysis of the chemical composition of the thin films a good correlation of measured silver concentration with initial concentration (dissolved in the main solution for deposition) for the samples with up to 15 % Ag were found. These results were also confirmed by the structural studies and analysis of the Raman scattering spectra. On the other hand, thin film with higher Ag concentration (20 %) has shown strong compositional inhomogeneity and presence of secondary phases was detected. Analysis of the modulated surface photo-voltage yielded the optical transition at ~ 1.5 eV, correlated with the band gap energy, and a transition at lower energy. From the temperature dependence of resistivity investigation no significant change in the resistivity for the samples with low silver concentration and strong increase for the samples with 20 % of Ag were found. more...

The measurement of surface photovoltage (SPV) transients over 12 orders of magnitude in time was recently demonstrated [Rev. Sci. Instrum. 88, 053904 (2017)]. In dedicated experiments, however, a high-impedance buffer shall be placed outside the measurement chamber, which has consequences for SPV measurements at very short times. By varying the LCR circuit of a measurement configuration, applying a multi-parameter fit and simulating the corresponding SPV transients, we show, on the examples of highly doped silicon and a CdS thin film, that the source function of SPV transients can be reconstructed with a resolution time better than 1 ns. more...

Cold gas spraying (CGS) is presented as an innovative approach to deposit semiconductor particles onto substrates in order to produce photoelectrodes for electrochemical applications, e.g. the oxygen evolution reaction (OER). The spraying technique is characterized by high velocity particles which impact and deposit on a surface at relatively low temperature. Compared to established wet-chemical techniques, an increased photoelectrochemical activity is observed due to an enhanced particle-to-substrate bonding. For closer investigation of the influence of the process parameters on the photoelectrochemical activity, TiO2 electrodes (P25-20 by Evonik Industries) sprayed with different carrier gases (nitrogen, argon, helium) are analyzed. Due to different acceleration conditions of the particles in the de Laval nozzle, these carrier gases allow to investigate the influence of the impact energy of the particles on the binding mechanism and thus the resulting photocurrent density in the OER. Photoelectrochemical activities, structural properties as well as the electrode structure are correlated in order to discuss the history of the semiconductor and its photoelectrochemical properties evoked in the CGS process. Surface photovoltage measurements are considered to analyze the charge carrier dynamics in the porous TiO2 film. For the gas carrier nitrogen, beneficial conditions for the particle-to-particle and particle-to-substrate coupling are provided due to the sufficient temperature and velocity of the particles. more...

In2S3 thin layers can be prepared by Spray-ILGAR® (Ion Layer Gas Reaction) from indium trichloride or from indium triacetylacetonate solutions. In the first case, the film contains a controllable amount of Cl. The concentration of H2O or Cl was varied systematically in the precursor solutions that were used for the deposition of In2S3 buffer layers on commercially available Cu(Ga,In)(S,Se)2 absorbers. The layers were characterized by ultraviolet photo electron spectroscopy, optical spectroscopy, modulated surface photovoltage spectroscopy and X-ray photoelectron spectroscopy, combined with a sequential etching procedure. The latter allowed to estimate an element-specific depth profile. Experimental findings were correlated with the performance of the corresponding solar cells. The presence of H2O or Cl in the precursor solutions has a strong influence on the diffusion of Cu+ and Na+ cations from the absorber into the buffer layer. The open circuit voltage of solar cells with In2S3 buffer layers prepared from Cl-free, H2O-containing precursor solutions was higher by up to 100 mV than those from Cl-containing, but H2O-free solutions. For this reason, the average solar energy conversion efficiency of cells of the first buffer type was higher (14.7%) than for solar cells of the second type (12.1%). more...

Molecules with donor (diphenylamine) and acceptor moieties (dicyano or cyanoacrylic acid moieties) were linked by fluorene or spirobisfluorene cores and the chain length has been changed by introducing a thiophene group between fluorene and diphenylamine. Four different kinds of fluorene and spirobisfluorene compounds were adsorbed from highly diluted solutions at ultra-thin nanoporous TiO2 (np-TiO2), Au and ITO surfaces. Charge separation has been investigated by surface photovoltage spectroscopy in the fixed capacitor and Kelvin probe arrangements in vacuum. Striking differences between the interaction of linking (dicyano or cyanoacrylic moieties) and different substrates were observed. Intra-molecular charge separation and electron injection have been distinguished and the directed adsorption of spiro compounds was deduced. more...

Cu(In,Ga)Se 2 absorbers were investigated by surface photovoltage (SPV) in the Kelvin probe and fixed capacitor arrangements before and after deposition of CdS or In 2 S 3 buffer layers as well as before and after deposition of ZnO window layers. Effects such as passivation of surface states, partial electron transfer from ZnO into In 2 S 3 , decrease of the ideality factor after deposition of ZnO and slow electron transfer through In 2 S 3 were demonstrated. The results show that SPV measurements open opportunities for dedicated studies of charge separation at hetero-junctions between ordered and disordered semiconductors. more...

An off-stoichiometric but single phase Cu1.95Zn1.1Sn0.96Se4 kesterite powder was investigated by temperature dependent transient surface photovoltage (SPV) spectroscopy. SPV signals excited at different wavelengths were transformed into SPV spectra that depended on the response time of measurement. Shallow electronic states and states with transition energies at 0.83 eV or 0.78… 0.9 eV were distinguished. The temperature dependence of the band gap of Cu1.95Zn1.1Sn0.96Se4 was obtained. Results were discussed on the basis of defects in Cu-poor and Zn-rich kesterite. more...

The present investigation has studied the impact of Nb surface segregation on charge carrier separation in polycrystalline Nb-doped (0.65 at % Nb) TiO2. By use of secondary ion mass spectrometry, the effect of applied processing conditions (temperature and oxygen activity) on the segregation of Nb has been obtained and correlated against surface photovoltage data obtained using surface photovoltage spectroscopy (SPS). It has been observed that within the range of the applied processing conditions, the segregation of Nb toward or away from the surface can be achieved. The corresponding SPS results demonstrate that, when Nb is removed from the surface and near-surface regions of Nb-TiO2, charge separation increases, as reflected by an increase in the surface photovoltage. After consideration of the impact of oxygen activity on the defect disorder of Nb-TiO2, it is concluded that the application of very low oxygen activities (p(O2) in the vicinity of 10–15 Pa) at temperatures of 1473 K and above leads to the... more...

Rutile IrO(2) is known as being among the best electrocatalysts for water oxidation. Here we report on the unexpected photocatalytic water oxidation activity of 1.98 nm ± 0.11 nm succinic acid-stabilized IrO(2) nanocrystals. From aqueous persulfate and silver nitrate solution the nonsensitized particles evolve oxygen with initial rates up to 0.96 μmol min(-1), and with a quantum efficiency of at least 0.19% (measured at 530 nm). The catalytic process is driven by visible excitations from the Ir-d(t(2g)) to the Ir-d(e(g)) band (1.5-2.75 eV) and by ultraviolet excitations from the O-p band to the Ir-d(e(g)) (3.0 eV) band. The formation of the photogenerated charge carriers can be directly observed with surface photovoltage spectroscopy. The results shed new light on the role of IrO(2) in dye- and semiconductor-sensitized water splitting systems. more...

The influence of molecular side groups on the electron backtransfer into ruthenium-dye molecules adsorbed on ultra-thin nanoporous TiO 2 layers has been investigated by modulated surface photovoltage spectroscopy in argon atmosphere. All ruthenium-dye molecules contained one bipyridine (bpy) ligand with two carboxylic groups for anchoring at the TiO 2 surface. Ruthenium (II) complexes with a second bpy ligand with branching and non-branching side groups containing C and H only and ruthenium (II) complexes with a second pyridine (py) ligand with side groups containing different numbers of –C–O–C– sequences were synthesized. The electron backtransfer was practically not affected by branching or non-branching side groups in the bpy–bpy complexes. In contrast, electron backtransfer was much less for bpy–py complexes in comparison to bpy–bpy complexes and increased strongly with increasing number of –C–O–C– sequences in the side groups. more...

Charge separation has been investigated at Pd−porphyrin (Pd−P)/TiO2 interfaces by surface photovoltage spectroscopy in the Kelvin-probe arrangement. Ultrathin nanoporous TiO2 layers were covered with Pd−P molecules from highly diluted organic solution starting from the submonolayer range. Mechanisms of charge separation including charge separation in TiO2 nanoparticles, an ionic transport component, and injection of electrons into TiO2 from states of solitary Pd−P molecules not interacting with each other on the surface and from Pd−P molecules interacting with each other and with TiO2 were identified. A developed model for the simulation of surface photovoltage spectra considered mechanisms of charge separation and respective recombination paths. Characteristic charge separation lengths and recombination coefficients were obtained for the different components of separated charge. more...

Solar cells with an extremely thin sulfidic absorber have been prepared by spray ion layer gas reaction (ILGAR) of In 2 S 3 on ZnO-nanorod arrays. As transparent hole conductor, CuSCN was deposited on the coated ZnO nanorods by impregnation. Surface photovoltage spectroscopy was applied to characterize states contributing to excess carrier generation and charge separation. The charge-selective contact is formed at the In 2 S 3 /CuSCN interface region the states of which also contribute significantly to the photocurrent. The influence of annealing temperature and annealing time of the In 2 S 3 /CuSCN contact region on the open-circuit potential ( V OC ), short-circuit current ( I SC ) and fill factor (FF) was studied in detail. For solar cells based on ZnO-nanorod arrays (rod length 1.5 μm), efficiency of 2.8% is obtained at AM1.5. more...

Ultra-thin MeS x H y (Me = In, Cu, Pb) heterojunctions were sandwiched between compact TiO 2 and PEDOT:PSS layers. The MeS x H y layers were prepared by SILAR (successive ion layer adsorption and reaction). Intensity and temperature dependent current-voltage characteristics were investigated for small area solar cell structures. Highest values of open circuit voltage (0.9 V) were obtained for the In 2 S 3 /PbIn x S y H z system annealed at moderate temperatures (150-200 °C) while PbIn x S y H z did not act as an absorber. Largest values of short circuit current were reached for the In 2 S 3 /CUIn x S y H z system in which both layers act as an absorber. The role of Pb and hydrosulfide for interface formation and defect passivation is discussed. more...

DC reactively sputtered TiO 2 layers on SnO 2 :F substrates were investigated by Raman and surface photovoltage spectroscopy. The stoichiometry and layer thicknesses were investigated by elastic recoil detection analysis. The deposition temperature, the O 2 /(O 2 + Ar) ratio and the deposition time were varied systematically. With increasing temperature, the layers become crystalline with the rutile modification dominating. Rutile phase preferentially forms on vertical facets of SnO 2 crystallites. Anatase phase starts to form during prolonged deposition and at lower O 2 /(O 2 + Ar) ratios. The energy of the exponential absorption tails below the band gap, a measure of the defect density of the films, is determined by the deposition temperature and not by other parameters if the deposition temperature is relatively high, irrespective of the content of crystalline phases or the value of the band gap. Charge separation takes place at length scales significantly shorter than the layer thicknesses (diffusion length less than 6 nm). TiO 2 films sputtered at 380 °C show rectifying behaviour with a carbon contact. more...

The porosity of electrophoretically prepared nanoporous ZnO and TiO2 films was systematically decreased by postpressing at different pressures. The nanoporous structure of the films was fixed by sintering after the postpressing procedure. The postpressing-induced change of the internal surface area of the nanoporous films was monitored using the dye-removal technique. The effective electron diffusion coefficient (Deff) of the unpressed nanoporous films depended on the thickness according to Fick’s second law. When pressed, the diffusion coefficient of the films increases significantly. In nanoporous TiO2, the increase of Deff follows the percolation theory where transport rate depends on the particle-coordination number. In contrast to the TiO2 films, the value of Deff of pressed nanoporous ZnO films changed with the porosity much stronger than one would expect from the percolation theory with hard spheres. This property has been attributed to the strong increase of necking between ZnO nanoparticles with increasing pressure as indicated by a strong decrease of the internal surface area. more...

Illumination induced charge separation processes at tetraphenyl-porphyrin (H2TPP)/InP interfaces are characterized. The results indicate that upon illumination an electronic transition takes place between the organic highest occupied molecular orbital (HOMO) and the InP conduction band. A 70 meV blue shift in the characteristic modulated surface photovoltage spectrum of a 50 nm H2TPP film was observed when n-InP substrate was used. Such blue shift was not observed when Au, SnO2:F or p-InP substrates were used. The shift may suggest a presence of an ordered interfacial sub-layer in an almost lying orientation, due to chemical or physical interactions at the interface. The results indicate that the Fermi level is unpinned at the interface. An exciton diffusion barrier in close proximity to the H2TPP/n-InP interface has been demonstrated. more...

Injection of photoelectrons from colloidal CdSe quantum dots (QDs) capped by thioglycolic acid into compact ultrathin TiO2 layers was investigated by direct detection of modulated charge separation with surface photovoltage spectroscopy. A size-dependent peak related to light absorption in the CdSe QDs and charge transfer to TiO2 was observed for QDs of different sizes. Correlation between CdSe QDs absorption and photoelectron injection into TiO2 has been demonstrated. Our results show that surface photovoltage spectroscopy is a valuable tool to determine the effectiveness of carrier injection from colloidal semiconductor QDs to electron transport materials in advanced solar cell configurations. more...

Nanocrystalline titanium oxinitride (TiO2−2xNx) thin films (0⩽x⩽1) were prepared by reactive dc magnetron sputtering from a titanium target in an argon-oxygen-nitrogen atmosphere. By increasing the reactive gas component nitrogen the phases changed from TiO2 to TiO2−2xNx and finally to TiN. The reactively sputtered films were characterized by elastic recoil detection analysis, Raman spectroscopy, and resistivity measurements. The crystallographic structures of TiO2 were maintained up to a nitrogen concentration of about 20at.%; however, the crystallite size decreased significantly and a transition from the low-temperature anatase to the high-temperature rutile phase of TiO2 occurred. Optical transmission measurements revealed that the indirect energy band gap can be reduced from 3.2to2.6eV for nitrogen concentrations up to 20at.%. The film properties for photocatalytic water oxidation were investigated by differential electrochemical mass spectroscopy. While the photoactivity in the visible increases, the... more...

Ultra-thin In(OH)xSy/Pb(OH)xSy heterostructures were formed by the wet chemical SILAR (successive ion layer adsorption and reaction) technique. ERDA (elastic recoil detection analysis) was used for stoichiometry analysis. The heterocontacts were conditioned by joint annealing of the two layers at different low temperatures in air. The charge selectivity was demonstrated with various small area solar cell structures. The results are discussed on the base of formation of bonds between sulphide clusters and passivation of defects with hydrogen containing species in hydroxy-sulphides. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) more...

Photoelectrical properties of uncoated In(OH)xSy and PbS(O) layers deposited by SILAR (successive ion layer adsorption reaction) and of TiO2/In(OH)xSy/PbS(O)/PEDOT:PSS solar cell structures were investigated by spectral surface photovoltage, Kelvin-probe, current–voltage and quantum efficiency analysis. By changing the annealing temperature of In(OH)xSy in air between 50 and 350 °C, the band gap of In(OH)xSy was tuned between 2.6 and 2 eV while the band gap of PbS(O) remained unchanged at about 0.7 eV. The open circuit voltage of the solar cell structures correlated well with the band gap and the workfunction of the In(OH)xSy. Surprisingly, excess charge carriers generated in the PbS(O) layer do not contribute significantly to the short circuit current. A short circuit current of more than 10 mA cm−2 was reached by modifying TiO2 with Nb. Possible ways of optimization are discussed. more...

A system of highly structured TiO 2 /In(OH) x S y /PbS/PEDOT:PSS has been developed and investigated by photovoltage spectroscopy, X-ray photo- and Auger electron spectroscopies, electron microscopy, and photovoltaic response. TiO 2 , In(OH) x S y , PbS, and PEDOT:PSS serve as electron conductor, buffer layer, absorber, and hole conductor, respectively. Both buffer and absorber layers were prepared by chemical bath deposition. The band gap of as-prepared In(OH) x S y varied between 2.4 and 3.5 eV depending on the pH-value of the solution. In addition, the band gap of the PbS could be widened to about 0.85 eV making the application as absorber for solar cells feasible. At present, corresponding solar cell devices reach short-circuit current densities of about 8 mA/cm 2 and open-circuit voltages of about 0.3 V. more...

CuI has been synthesized at room temperature on Cu-tape/n-CuInS 2 by using the SILAR technique (successive ionic layer adsorption and reaction). The influence of wet chemical iodine treatment on the CuI has been investigated in more detail. The films were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), elastic recoil detection analysis (ERDA) and surface photovoltage (SPV) techniques. The CuI films contain the γ-phase of the Zinkblende structure. The crystallites are preferentially oriented in the (111) direction. After wet chemical iodine treatment, the fibrous surface morphology changed to a more dense CuI film with larger crystallites. Oxides could not be detected on the CuI surface. The density of surface states of CIS decreased after the CuI deposition. The importance of the wet chemical iodine treatment for the performance of Cu-tape/n-CuInS 2 /p-CuI solar cells has been demonstrated. more...

Nanopores and macropores can be formed electrochemically on p-type silicon depending on the silicon resistivity and composition of the electrolyte. In order to understand if the structural dimensionality of the porous p-type Si, either nanopore or macropore formation, depends on the electrochemical process at the Si/electrolyte interface, in situ pulsed surface photovoltage (SPV) and photoluminescence (PL) measurements have been undertaken. The SPV and PL studies have been made as a function of the applied current density as well as the electrolyte composition (aqueous or in presence of organic solvent) and the silicon doping density. Main results show that the Si surface is well passivated with preferential formation of ionic species and the Si band bending is around 100 mV, during porous Si formation. It varies slightly with the doping density, but is not affected by the composition of the electrolyte (HF/water and HF/organic solvent). This demonstrates that the chemistry of the electrolyte plays a major role in the formation of macroporous and nanoporous Si, but has still to be determined. more...

Ultra thin organic layers of benzene-type molecules are able to passivate Si surfaces. The organic layers were electrochemically deposited on Si surfaces from aqueous solution of diazonium compounds and show a blocking of the charge transfer from Si into the electrolyte after the deposition process. Electron microscopic images reveal a compact and homogeneous organic layer of 4-bromobenzene on the Si. The surface recombination increases only slightly with respect to a well H-passivated Si surface, so that the interface state density is about 10(11) cm(2) or slightly below. Organic layer modified Si surfaces are much longer stable in ambient air than the H-terminated surface as observed by a slower decay of the integrated photoluminescence intensity with time. Thermal desorption measurements show that the organic layer is stable up to about 200 degrees C. more...

Giant nonlinear optical response has been observed for nanoparticle TiO2 (anatase modification) layers within sub-gap photon energy excitation in picosecond range. The obtained effective cubic optical nonlinearity value χ(3)eff ∼10–5 esu is six orders of magnitude higher than the bulk response in transparency range. The effect can be explained with resonant excitation of oxygen vacancies and related defect states , which also act as photocatalytic active centers at the nanoparticle surface. The magnitude of the nonlinear optical response correlates with the photocatalytic activity and depends on the samples preparation conditions. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) more...

Thick and homogeneous layers of porous TiO2 (anatase and amorphous phases) have been prepared by anodization of Ti foils in fluoride solutions. The porous TiO2 layers have been investigated by electron microscopy, Raman spectroscopy and photovoltage spectroscopy. The nanostructure of the porous TiO2 layers depends sensitively on the pH and on the concentration of the NH4F solution. The photovoltage spectra of as-prepared samples are dominated by defects (electron traps) below the bandgap of anatase. The defects are removed after annealing in air. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) more...

Semiconductor nanowires of CdS and CU x S were embedded into porous anodic alumina (PAA) by sulfurization of the metal precursors. Pores of PAA were filled with Cd and Cu by ac electrochemical preparation while the PAA layers remained on the Al-substrate. Deposited metal and semiconductor wires were characterized by scanning electron microscopy and X-ray diffraction. Photovoltage spectroscopy was applied to demonstrate semiconductor behaviour of CdS nanowires manufactured by the proposed technique. more...

The passivation of sol‐gel TiO2 by ultra-thin layers of Al-oxide has been investigated using transient and spectral photovoltage (PV) techniques. The ultra-thin layers of Al-oxide were prepared by the ion-layer gas reaction (ILGAR) technique and modified by thermal treatments in air, vacuum or Ar/H2S atmosphere. The samples where characterized by elastic recoil detection analysis (ERDA), X-ray photoelectron spectroscopy (XPS), and contact potential difference (CPD) technique. Without an Al-oxide surface layer, electronic states in the forbidden gap of TiO2 are formed during thermal treatments in vacuum and Ar/H2S. The trap density is strongly reduced at the TiO2/Al-oxide interface. The formation of electronic defects is prevented by a closed ultra-thin layer of Al-oxide. more...

A giant nonlinear optical response has been observed for nanoporous layers of titanium dioxide (anatase) under picosecond laser excitation with photon energy below the gap. At excitation intensity of 10 MW/cm2 the nonlinear refractive index variation at the wavelength of 1064 nm corresponds to χ(3)=2×10-5 esu which is six orders of magnitude higher than the respective value of bulk TiO2. This effect is explained by resonant excitation of electronic states of defects at the developed surface of anatase nanoparticles. more...

A powder containing the CuAlO 2 phase has been prepared from α-LiAlO 2 by ion exchange with CuCl at temperatures significantly below 500 °C. The diameter of the particles in the powder was in the order of 1 μm. Spectral photovoltage (PV) measurements have been carried out on layers of CuAlO 2 particles deposited on glass substrates covered with conductive SnO 2 :F. The sign of the PV corresponds to a p-type semiconductor. The optical band gap and the parameter of the exponential absorption tail of the prepared CuAlO 2 amount to 3.54 eV and 40 meV, respectively. more...

Anodic oxidation of Ti has been used to produce layers of TiO2 consisting of anatase or rutile or a mixture of both. While the anatase phase of TiO2 is formed at lower anodization potentials, the formation of the rutile phase starts at potentials above 100 V due to electrical breakdowns. Defects with broad optical absorption in the infrared range are generated only in anodic TiO2 layers with coexisting anatase and rutile phases under ultraviolet illumination in water containing atmosphere. These defects partially disappeared by annealing at about 400 °C for 30 min. The reversible defect reaction takes place in the surface regions between the anatase and rutile phases. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) more...

The photoinduced change of electrical conductivity and dielectric function were investigated in porous silicon and porous TiO 2 by means of Complex Transient Microwave Photoconductivity Method. In porous TiO 2 , the photoexcitation leads to the increase of the dielectric function. The amplitude of this increase depends strongly on the size of the nanoparticles. The sign of the photoinduced change of the dielectric function is negative for mesoporous silicon and positive for nanoporous silicon. The photoinduced increase of the dielectric function in porous TiO 2 is explained by the spatial separation of photoexcited charge carriers inside one nanoparticle while the interparticle transitions have very small probability during one microwave half period. In porous silicon, the confinement of charge carriers depends on the porosity. Therefore, the positive photoinduced change of the dielectric function is observed only in nanoporous silicon where most of photoexcited carriers are strongly localized. more...

Spectral and transient photovoltage (PV) phenomena are investigated in hydrogenated and oxidised free standing mesoporous silicon (meso-PS) layers. The size and the surface conditioning were changed systematically. The absorption tails of hydrogenated and oxidised meso-PS have been related to disorder in the bulk nanoparticle (similar to p + -Si) and to a disordered surface region. The limits of surface and diffusion PV are discussed. For as prepared meso-PS, a fast PV response has been observed. The fast component disappeared and the PV transients became more and more retarded in time with ongoing decrease of the mean dimensions of the Si nanoparticles. The retarded component in the PV transients has been assigned to interparticle transport in ondulated quantum wires and correlates with the dielectric relaxation time. more...

The photoassisted electrical transport is investigated in porous TiO 2 (anatase) layers by impedance spectroscopy (IS) and time-of-flight (TOF) at different temperatures and pressures of oxygen. The experiments were carried out under the conditions of zero bias potential (no electron injection at the contacts takes place) and of relatively high bias potential when electrons are injected from the contact into the porous TiO 2 layer. Illumination with photons having energies below the band gap leads to a strong increase of the conductivity while the conductivity is practically unchanged for illumination with ultra violet light independent of the applied bias potential. Besides the light induced excess carrier generation, TOF measurements show an increase of the drift mobility under illumination with blue light. The results were explained by the lowering of the barriers between sintered TiO 2 nanocrystals due to separation of photoexcited charge carriers. more...

Electrical properties of two kinds of sandwich structures (SnO 2 :F/por-TiO 2 /Ti and Si/por-TiO 2 /Pd) are investigated by the methods of I-V, C-V characteristics and DLTS in vapors of water and alcohol. It has been demonstrated that besides the change of the conductivity and of the capacitance of the por-TiO 2 layers one can also use for gas sensing the change of the high frequency capacitance of the MOS type structure, measurement of the current response at relatively low frequencies and the modification of the charge transfer between the Si bulk and states near the por-TiO 2 /Si interface. more...

The surface recombination velocity at clean and modified Si(1 0 0)2×1, Si(1 1 1)-1×1 and Si(1 1 1)-7×7 surfaces was studied with the pulsed photoluminescence technique. High resolution electron energy loss spectroscopy and low energy electron diffraction were employed to correlate the surface recombination velocity to the chemical bonds at the surface and the surface morphology. Passivation of surface dangling bonds by the chemisorption of water and maleic anhydride shows that the surface dangling bonds are not recombination active. In contrast, we observe that the introduction of oxygen or carbon atoms into Si backbonds causes the generation of non-radiative recombination centres. more...

A comparative study of the transient space-charge-limited current in porous silicon and porous anatase (TiO2) has been carried out. The room-temperature drift mobilities of electrons in porous silicon and porous anatase, determined from the transit times, are, respectively, 10−2 and 5×10−6 cm2 V−1 s−1. The specific features of space-charge-limited currents in porous anatase are discussed. more...

The surface dipole and non-radiative recombination at p-Si(111) surfaces were investigated by in-situ photovoltage and photoluminescence techniques during the electrochemical formation of a Si ∣ organic layer interface by reduction of different diazonium salts. The reaction rate of the radical with the negatively charged p-Si(111) surface depends on the strength and the direction of the dipole moment of the radical. The formation of the surface dipole and of non-radiative surface defects, i.e. Si dangling bonds, correlates with the reaction rate. Well passivated Si ∣ organic layer interfaces can be prepared and the strong dependence of the surface band bending on the effective dipole moment of the grafted organic molecules has been demonstrated. more...

A contactless method of microwave photoconductivity was used to study the relaxation of photoexcited charge carriers in nanocrystalline porous titanium oxide. A decrease in the amplitude and a change in the microwave photoconductivity relaxation time is observed with reduction in the nanocrystal size, which is explained by localization of carriers due to dielectric confinement. Capture of holes by the traps on the TiO2 surface increases the amplitude and prolongs the relaxation time of the microwave photoconductivity. more...

The defect transformation under growth of submonolayer oxides on silicon surfaces is studied in situ on reconstructed Si(100)-2×1 and Si(111)-7×7 surfaces and on hydrogenated Si surfaces of Si nanocrystallites. The methods of the quenching of the photoluminescence of bulk c-Si due to surface recombination and of the infrared free carriers absorption in mesoporous Si are used. It is shown that dangling bonds at Si–Si dimers are not recombination active. Non-radiative surface defects are generated during the earliest stages of oxidation. The generation of hole traps is tightly related to an increase of physisorbed H 2 O at hydrogenated Si surfaces during the native oxidation. more...

The time dependent electroluminescence (EL) of c-Si (at 1.1 eV) is investigated at room temperature for a p-i-n structure under excitation with forward biased current pulses. The EL intensity increases by square law at shorter times ( 10 μs). The parabolic dependence of the EL intensity on the current density at the shorter times points to the bimolecular recombination mechanism. The EL response time has been decreased to less than 200 ns for the given p-i-n structure by application of a reverse bias potential. The maximal EL quantum efficiency is of the order of 0.01% for the investigated p-i-n structure and possible ways to increase this value are discussed. more...

Electron injection into porous TiO 2 is investigated by current-voltage, dark current transients, impedance spectroscopy and photocurrent transients. Strong electron injection takes place and the existence of positive compensating charge has been shown. The compensating charge is originated by polarization of the TiO 2 nanoparticles. A scenario has been proposed for the processes of injection and formation of conducting channels in porous TiO 2 . more...

The positive oxide charge (Qox) and the concentration of nonradiative recombination defects (Nit) at a thin anodic oxide/p-Si interface are probed in situ by pulsed photovoltage and photoluminescence techniques during electron injection. Qox and Nit decreased strongly due to electron injection. The observed effect is suggested to be inverse to the negative-bias-temperature instability. Defect reactions at the anodic oxide/p-Si interface are discussed. more...

Titanium dioxide (TiO 2 ) is widely considered to be a strongly n-type semiconductor due to its tendency for oxygen deficiency. In this work, however, room temperature p-type semiconductivity has been observed in rutile TiO 2 , as determined using surface photovoltage spectroscopy and cyclic voltammetry subsequent to controlled processing at elevated temperatures. In particular, room temperature p-type semiconductivity has been imposed via strong oxidation [p(O 2 ) = 75 kPa] at elevated temperatures (1273 K), followed by rapid cooling in the same gas phase. It is reasoned that under such conditions, the observed p-type behaviour is due to the formation of titanium vacancies (acceptor-type ionic defects) at the TiO 2 surface. It is also concluded that the extremely slow diffusion kinetics of these defects towards the bulk is what limits the p-type behaviour to the surface and near surface regions. Nevertheless, this could be overcome by applying appropriately lengthy annealing times. The reported observation of p-type semiconductivity in TiO 2 is expected to have far reaching consequences for this intensely researched material. more...

The formation of electronic states in the fundamental gap at the maleic anhydride/Si(1 0 0)-2×1 interface has been studied with photoluminescence (PL) measurements and high resolution electron energy loss spectroscopy (HREELS). We observe that the room temperature adsorption of maleic anhydride on Si(1 0 0)-2×1, where the organic molecules are mainly in a di-σ coordination, results in a gap state density D it =1.8(2)×10 12 eV −1 cm −2 which is slightly lower than determined for the clean Si(1 0 0)-2×1 surface. After heating maleic anhydride/Si(1 0 0)-2×1 to 1115 K, the gap state density has increased to 6.4(6)×10 12 eV −1 cm −2 which we relate to the formation of silicon carbide structures on the silicon surface. more...