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In the present study, a morpho-structural analysis of heat treated TiO2-WO3-Au aerogel composites is presented. Simultaneously, the influence of the loaded Au nanoparticles on the morphological and structural characteristics of the TiO2-WO3 aerogel is also analyzed. X-ray diffraction data (XRD) and Raman spectroscopy measurements indicated that TiO2 crystallizes mainly in anatase phase, while the WO3 structure remains amorphous. The morphological features of the samples, i.e. crystallites mean size, particle size distribution, and specific surface area of the pores, are investigated by using XRD, transmission electron microscopy (TEM), and N2 adsorption/desorption, respectively. The results indicate a decrease in size of the titania crystallites as the tungsten oxide content increases. The diffuse reflectance spectra modified by applying a Kubelka-Munk transformation reveal that the presence of gold nanoparticles improves the material's response in the visible domain. [ABSTRACT FROM AUTHOR]

Abstract: Stable and efficient organic solar cells with Mg (20 at. %): Ag alloy cathodes and bulk-heterojunction absorber layers from metal (e.g., Cu, Zn)-phthalocyanine and C60 small molecules are demonstrated. Device efficiencies of 4.0% under an illumination of 100 mW/cm2 at 25̊C were achieved as a result of the fine adjustment of the cathode work function as well as of the absorber design. By combining low and high work function materials, the work function in both Mg/Ag bilayer and Mg:Ag alloy layer cathodes was adjusted for optimum photovoltaic parameters. The electric and photovoltaic properties of the devices are discussed with respect to the cathode layer structure. The formation of the absorber/cathode interface was investigated by x-ray photoelectron spectroscopy measurements (XPS). The work function of the absorber and cathode layers were determined from the XPS high binding energy cutoff (HBEC) spectra. For optimized devices, the work function of the cathode at the side adjacent to the absorber layer equals 4.0…4.1 eV. While devices with Mg/Ag bilayer contacts exhibit a 65% efficiency drop in the first month after the preparation, devices with Mg:Ag alloy contacts demonstrate stable photovoltaic parameters within the time of the study of ∼ 1 year. [Copyright &y& Elsevier]

Abstract: The transport properties of thin-film solar cells based on wide-gap CuGaSe2 absorbers have been investigated as a function of the bulk [Ga]/[Cu] ratio ranging from 1.01 to 1.33. We find that (i) the recombination processes in devices prepared from absorbers with a composition close to stoichiometry ([Ga]/[Cu]=1.01) are strongly tunnelling assisted resulting in low recombination activation energies (Ea) of approx. 0.95eV in the dark and 1.36eV under illumination. (ii) With an increasing [Ga]/[Cu] ratio, the transport mechanism changes to be dominated by thermally activated Shockley-Read-Hall recombination with similar Ea values of approx. 1.52–1.57eV for bulk [Ga]/[Cu] ratios of 1.12–1.33. The dominant recombination processes take place at the interface between CdS buffer and CuGaSe2 absorber independently from the absorber composition. The increase of Ea with the [Ga]/[Cu] ratio correlates with the open circuit voltage and explains the better performance of corresponding solar cells. [Copyright &y& Elsevier]

Abstract: Device-grade ternary Cu–Ga–Se chalcopyrite thin films used for photovoltaic energy conversion have been prepared by a novel chemical close-spaced vapor transport (CCSVT) technique developed for a deposition on areas of up to 10×10cm2. A two-step process has been developed which allows the fine tuning of the film composition and the electronic properties. The extension of deposition times in the two-step process led to final film compositions with [Ga]/[Cu] ratios ranging from 0.9 to 5.7, allowing the study of the structural phase transitions. In this paper the main focus of interest is related to the material properties of the device-grade thin films prepared by CCSVT technique. We present our recent studies on (i) the growth, compositional, structural and electronic structural properties, (ii) the degradation under ambient conditions and (iii) the feasibility of n-type doping this p-type semiconducting material by germanium. Thin films were grown with chalcopyrite (1:1:2) and CuGaSe2-related defect compound structures (DC) with stoichiometries of CuGa3Se5 and CuGa5Se8. In order to derive the DC structure, X-ray and neutron powder diffraction investigations have been carried out on powders of these CuGaSe2-related compounds grown by elemental synthesis (powder) and CCSVT (thin films), respectively. We found no hints for an ordering of defects, as proposed in the past and giving name to the so-called Ordered Defect Compounds (ODC) in this and related structures. From our results a growth model is presented for CuGa3Se5 formation in gallium-rich CCSVT-grown CuGa x Se y films. The chemical and electronic surface and interface structure of CuGaSe2 thin films with bulk [Ga]/[Cu] ratios between 0.94 and 1.39 is investigated by X-ray and UV-excited photoelectron spectroscopy (XPS and UPS, respectively). A transition of the Cu:Ga:Se surface composition from 1:1:2 for the Cu-rich bulk sample to 1:3:5 for the sample with the highest bulk [Ga]/[Cu] ratio is observed. Simultaneously, a downward shift of the valence band maximum position with respect to the Fermi energy is found. The comparison of the estimated conduction band minimum with that of CdS reveals the formation of a pronounced “cliff-like” conduction band offset at the respective interface. Furthermore, the CuGaSe2 thin film degradation under ambient as well as under thermal conditions of CuGaSe2 thin films has been studied by XPS. During thermal oxidation, the formation of predominantly Ga2O3 and some amount of SeO2 were observed, but no copper oxides could be detected in the near-surface region of the thin films. The same oxides are found after native oxidation in air under ambient conditions. An additional sodium oxide compound formed at the thin film surface, Na x O and Na2CO3 after thermal and native oxidation, respectively. Germanium ion implantation technique of the near-surface region of CuGaSe2 thin films has been used in order to prove the feasibility of n-type doping. In photoluminescence (PL) studies, the occurrence of a new emission line is identified as Ge related and explained as a donor–acceptor-pair (DAP) recombination. The precise role the Ge is playing in this doping of CuGaSe2 is revealed by X-ray absorption spectroscopy (XANES and EXAFS) and ab initio calculations based on the density functional theory. The studies indicate that the incorporated Ge atoms preferentially occupy Ga sites when relaxation around the dopant is taken into account. Additionally, our corresponding theoretical band structure model predicts the existence of additional localized electronic acceptor and donor defect bands within the band gap of CuGaSe2 originating from a strong covalent interaction between Ge 4s and Se 4p states for Ge atoms tetrahedrally surrounded by the Se nearest-neighbor atoms. A theoretically predicted anti-bonding Ge–Se4sp3 defect band appearing well above the Fermi level for the Ge1+ Ga point defect system can be directly linked to a Ge-dopant-related donor–acceptor-pair transition as observed in our photoluminescence spectra. [Copyright &y& Elsevier]

Abstract: Two polymers:fullerene bulk heterojunction solar cells: [(I) poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene (MDMO-PPV): methanofullerene phenyl-C61-butyric-acid-methyl-ester ([60]PCBM) and (II) poly(3-hexylthiophene-2,5-diyl), highly regioregular (P3HT): [60]PCBM] were studied. Indium tin oxide (ITO) and/or poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) were/was used as transparent electrode, and lithium fluoride (LiF) was used as buffer layer for aluminium cathode. The role of ITO and PEDOT:PSS on the current–tension characteristics, on open circuit voltage V oc, and on density of short-circuit current J sc was investigated for the two polymers:PCBM blend solar cells as deposited and after 40 days ageing. [Copyright &y& Elsevier]

Abstract: ZnO films were prepared by post deposition thermal oxidation in the ambient atmosphere of metallic Zn films (d =100–170nm) vacuum evaporated onto unheated indium tin oxide (ITO)-coated glass substrates. To study the effect of the substrate position during the Zn film deposition on the microstructure and optical properties (transmittance, reflectance and absorbance) of as obtained ZnO films, two set of Zn samples simultaneously deposited onto horizontally and obliquely arranged substrates were prepared. The as obtained ZnO films had a polycrystalline wurtzite structure, those obtained from normally deposited Zn films having a higher c-axis preferred orientation and a lower optical transmittance in the visible wavelength range. The optical band-gap was found to be of 3.14eV for oxidized normally deposited virgin Zn films and of 3.16eV for those obliquely deposited. [Copyright &y& Elsevier]

Abstract: Organic vapour-phase deposition (OVPD®) is used for the growth of the organic solar cell component materials such as the donor copper phthalocyanine (CuPc), the acceptor fullerene C60, and electron-conducting buffer layers of bathocuproine (BCP) on Si〈100〉 wafers and indium tin oxide (ITO) substrates on areas as large as 15×15cm2. By means of X-ray diffraction (XRD) analysis we show that under continuous operating conditions the source materials possess long-term stability. The CuPc, C60 and BCP thin film morphology and structure are characterised using scanning electron microscopy and XRD analysis. We demonstrate CuPc thin films with a highly folded surface morphology suitable for the preparation of solar cells with an interpenetrating donor–acceptor interface. The XRD diffraction patterns of the CuPc and C60 layers deposited under conditions appropriate for the preparation of organic solar cells show spectra typical for these materials. Mixed CuPc:C60 layers with controlled constituent ratios and layer thickness are deposited for the preparation of organic solar cells. First ITO/CuPc:C60/Al organic photovoltaic devices are prepared with an efficiency of ∼1% (conditions AM1.5). [Copyright &y& Elsevier]

Abstract: CdZnTe thin films of thickness 450–1400nm have been evaporated under vacuum onto unheated glass substrates, using a multilayer method. During film deposition, the two evaporation sources, separated by two glass cylinders, were maintained at temperatures of 720K for Zn and at 925–1200K for CdTe, respectively. After deposition, the samples were annealed in air up to 775K. The structural and optical properties of both as-deposited and heat-treated samples were investigated. Depending on the preparation conditions and the annealing temperature, the value of the optical band gap, E g, of respective films varied between 1.16 and 1.63eV. The obtained results are discussed in correlation with the structure of the films and the role of Zn atoms in CdTe films. [Copyright &y& Elsevier]

Abstract: The microstructure of the CuGaSe2 (CGSe) thin films deposited by a novel chemical close-spaced vapour transport (CCSVT) technique on clean and Mo-coated soda lime glass (SLG) substrates has been investigated by transmission electron microscopy (TEM). The CGSe bulk and the interface between the CGSe and Mo films have been investigated. The as-grown CGSe films possess high bulk crystalline quality. At the CGSe/Mo interface, a MoSe2 interfacial layer (∼20–40 nm) has been observed and also an excess of Ga. Additionally, composition measurements and depth profiling of the elements were performed by elastic recoil detection analysis (ERDA). It has been found that the CGSe constituent elements are homogeneously distributed in the bulk, whereas the surface composition is influenced by the [Ga]/[Cu] ratio in the film. With the [Ga]/[Cu] ratio increase, the CGSe surface composition changes from Ga- and Cu-poor, and Se-rich to Cu-poor, and Ga-and Se-rich. Photoluminescence (PL) spectroscopy has been used as a complementary technique to study the defect profiles at the CGSe front and rear sides as a function of the [Ga]/[Cu] ratio. The PL data support the results of structural investigations, pointing out higher Ga concentration at the films rear side. [Copyright &y& Elsevier]

Abstract: For solar cells based on CuGaSe2 (CGSe) polycrystalline thin films, a novel efficient chemical close-spaced vapor transport (CCSVT) technique is used to deposit the CGSe absorber. Clean and Mo-coated soda lime glass (SLG) substrates are used for the CGSe deposition. The CGSe thickness ranges from 1.6 to 1.9 μm and the corresponding [Ga]/[Cu] ratio of the thin films is adjusted within the range of 0.9–1.3. The high bulk homogeneity of the as-grown CGSe films is shown. Transmittance and reflectance measurements were performed to monitor the changes in the CGSe band gap as a function of composition. The optical spectra reveal a shift of the absorption edge towards longer wavelengths with increasing Ga content, as well as a broadening of the distinct structure corresponding to three band-to-band transitions characteristic for CGSe. These results are used for a discussion of the behaviour of the ZnO/CdS/CuGaSe2 solar cell photovoltaic parameters as well of the current transport. The transport mechanism is analysed on an our best solar cell device with an active area efficiency of 8.7%. For the first time, a thermally activated Shockley–Read–Hall recombination mechanism is observed for the CGSe-based solar cells in a large temperature region. [Copyright &y& Elsevier]

A novel chemical close-spaced vapor transport (CCSVT) technique has been developed for the growth of the CuGaSe2 (CGSe) thin films on areas as large as 10×10 cm2. Cu precursors deposited on clean and Mo-coated soda lime glass substrates are thermally and chemically treated under gaseous GaClx/H2Se atmosphere in the CCSVT cell. The Ga2Se3 employed as source material is stoichiometrically volatilised at 550 °C by a controlled amount of HCl/H2 agent. Single phase CGSe thin films are prepared with a growth rate of 230–240 nm/min by using a single stage process. A two-stage process is applied for the fine tuning of the CGSe composition and electronic properties appropriate for the subsequent solar cell preparation. Film characterisation including X-ray diffraction measurements, scanning electron microscopy observations, X-ray fluorescence analysis and elastic recoil detection analysis has been carried out. An 8.7% active area efficient ZnO/CdS/CuCaSe2 solar cell under AM1.5 conditions has been achieved. [Copyright &y& Elsevier]

Temperature-dependent current-voltage measurements are used to determine the dominant recombination mechanism in thin-film heterojunction solar cells based on Cu(In,Ga)(S,Se)2 absorbers with chemical bath deposited ZnS buffer layer. The measurements are carried out in the dark and under illumination in the temperature range 200–320 K. The activation energy of the recombination under illumination follows the absorber band gap energy Eg=1.07 eV of bulk Cu(In,Ga)(S,Se)2. The thermal dependence of the diode ideality factor is described by classical Shockley–Read–Hall (SRH) recombination via an exponential distribution of trap states in the bulk of the absorber. In the dark, the current flow is dominated by tunnelling enhanced bulk recombination with a tunnelling energy E00=18 meV. Two activation energies higher than Eg, namely 1.21 and 1.40 eV, have been found. These results may be explained by dominant recombination in a region close to the surface of the Cu(In,Ga)(S,Se)2 absorber with an enlarged band gap. Thus, the electronic loss in the ZnO/Zn(S,OH)/Cu(In,Ga)(S,Se)2 solar cell takes place mainly in the absorber and is determined by tunnelling enhanced bulk recombination with a tunnelling energy E00 influenced by illumination. [Copyright &y& Elsevier]

We present an optimisation of our recipe for the CdS chemical bath deposition process as applied to solar cells based on polycrystalline CuGaSe2 (CGSe) absorber layers prepared in two stages by physical vapour deposition. We investigate the influence of the ammonia (NH3) and the thiourea (H2NCSNH2) concentration, both being constituents of the chemical bath deposition (CBD) solution, at a deposition temperature of 80 °C on the microstructural and optical properties of CdS layers and on ZnO/CdS/CuGaSe2/Mo device parameters. The composition of the CdS layers and their thickness were determined using X-ray Fluorescence Analysis. Transmission and reflection measurements performed at 300 K were used for the calculation of absorption and optical band gap energy (Eg). The Eg values of the films varied from 2.41 to 2.46 eV depending on deposition conditions. Cubic phase of the as-grown layers was identified by X-ray diffraction analysis. An improvement in the investigated solar cells efficiency was achieved when the ammonia concentration was increased and the thiourea concentration was reduced, compared to the previously used standard HMI recipe. The influence of the CBD CdS preparation recipe on the ZnO/CdS/CuGaSe2/Mo electrical and photoelectrical properties is discussed. [Copyright &y& Elsevier]

Presents a study in which the ZnSe/CuGaSe[sub2] heterojunctions were fabricated by a flash evaporation technique. Thickness of the intermediate layer of the heterojunctions; Investigation of the photoelectrical properties of the heterojunctions; Transport mechanism in the heterojunctions.

ZnSe/CuGaSe2 heterojunctions were prepared by growing CuGaSe2 absorber layers onto the (110) surface of ZnSe single crystals by flash evaporation, physical vapor deposition, chemical vapor deposition and metal–organic vapor phase epitaxy. The morphology of the absorber layers was studied by scanning electron microscopy. Elemental distribution in the thin films investigated and along the heterojunction cross-section was characterized using energy-dispersive X-ray analysis. All heterojunctions prepared show high open-circuit voltages of 0.85–1.12 V, which slightly depend on the preparation method. Currents through the ZnSe/CuGaSe2 heterojunctions are limited by the presence of a high-resistivity (ρ∼108–109 Ω cm) i-ZnSe layer at the interface, which is highly compensated by Cu impurities. [Copyright &y& Elsevier]

Abstract: The studied polymers, poly(azomethine sulfone)s, were prepared by the reaction of bis(4-chlorophenyl)sulfone with a mixture of bisphenols: 2,2-bis(p-hydroxyphenyl)propane (bisphenol A) and 4,4′-bis(4-hydroxybenzylideneiminophenoxy)biphenyl in various molar ratios. The temperature dependences of the electrical conductivity and Seebeck coefficient of polymers were investigated using thin-film samples deposited from chloroform solutions (spin coating method) onto glass substrates. It was found that the respective polymers show typical semiconducting properties. Some correlations between these properties and the chemical structures of the polymers were established. The mechanism of electronic transport in the films studied is discussed. The study of optical absorption (in spectral range, 300–1400nm) evidenced direct bandgaps ranged between 1.30 and 1.80eV. [Copyright &y& Elsevier]

This work focuses on the fabrication of stoichiometric CuInSe 2 nanostructures with controllable physical parameters of the nanocrystals suitable for hybrid organic/inorganic photovoltaics. CuInSe 2 nanostructures were prepared by the chemical close-spaced vapor transport (CCSVT) method onto Mo/barrier/glass substrates by using an In 2 Se 3 source material and Cu precursors. The In 2 Se 3 source material was volatilized in H 2 ambience with the addition of HCl vapors at 550 °C. Three different types of Cu precursors were used: (i) Cu thin films (6–250 nm thick) deposited by e-beam, (ii) Cu nanoparticles prepared by spray pyrolysis and (iii) Cu nanostructures formed by applying the nanosphere lithography (using a monolayer of 450 nm nanospheres). The CCSVT process parameters were varied to reveal the optimum conditions for the preparation of secondary phases free CuInSe 2 nanostructures. The structural characterization by x-ray diffraction in both grazing incidence and Θ-2Θ configurations revealed the formation of CuInSe 2 chalcopyrite phase independently on the applied precursor type. The elemental composition of the as-prepared CuInSe 2 nanostructures was analyzed by laser ablation-inductively coupled plasma mass-spectrometry. In non-optimised processes, an excess of Se compared to stoichiometric composition was detected and attributed to the formation of molybdenum selenide and indium selenide phases. The formation of the latter secondary phases was suppressed by tuning the CCSVT deposition parameters. [ABSTRACT FROM AUTHOR]

The superstrate configuration of Cu(In,Ga)Se 2 (CIGS) photovoltaic devices exhibits the potential for improved light harvesting compared to the standard substrate configuration. Due to the limited power conversion efficiency of superstrate devices caused by the poor hetero-interface between the p -type CIGS and the n -type component, these advantages have not been fully considered so far. This work shows that the superstrate configuration can significantly improve the light management within CIGS devices. Experimentally, it is shown that the electric and optical properties of ZnO window layer strongly improve during the CIGS deposition, allowing the use of structured ZnO layers for efficient light-trapping. A highly reflective and scattering back contact is realized by using the high work function material MoO 3− x for the hole-transport layer in combination with a low work function Ag metal layer. Combined with the unique Ga gradient achievable in the superstrate configuration, simulation results show that a strong reduction of the CIGS layer thickness is possible while increasing the power conversion efficiency. This proof of concept highlights the appeal of the superstrate configuration for the future of cost-effective CIGS solar cell production. [ABSTRACT FROM AUTHOR]

We determine the optical constants, that is, refractive index n, extinction coefficient k, and absorption coefficient α, of diindenoperylene (DIP) films deposited by organic vapor phase deposition (OVPD®) on quartz glass at different substrate temperatures between 60°C and 140°C and chamber pressures between 0.08 and 1.10kPa. The corresponding optical properties of the OVPD-films are compared with those of DIP layers deposited by physical vapor deposition (PVD) in high vacuum (~10−5 kPa) at substrate temperatures of 20°C–140°C. Measured transmission/reflection spectra are evaluated numerically by a dielectric function of a sum of Lorentz oscillators to reproduce the optical properties of the DIP films. The optical results are analyzed in relation to the morphological and structural properties of the DIP films investigated by scanning electron microscopy. The optical and structural properties of the PVD-DIP films show a weak dependence on substrate temperature. A similar temperature behavior is observed for OVPD-DIP films for the chamber pressure range of 0.08–0.18kPa. For these conditions, we found from the optical calculations in the spectral range 250–660nm that (i) n varies between 1.4 and 1.9, (ii) k increases from 0.02 to 0.6, and (iii) α varies between 3.0×105 cm−1 and 5×104 cm−1. These data allow satisfactory determinations of thin film thicknesses by optical methods. At OVPD chamber pressures higher than 0.18kPa, the growth of spongelike film structures is observed which induce large deviations from computed film thicknesses. Nevertheless, the high absorption coefficients in the visible spectral region make all the investigated DIP films suitable for application as absorber in organic solar cells. [ABSTRACT FROM AUTHOR]

Abstract: The sandwich-type K10[(VO)2Sb2W20O70]·20H2O heteropolyoxotungstate was investigated by means of elemental analyses, thermogravimetric and spectroscopic (FT-IR, UV–vis and EPR) methods. The stretching vibration of the terminal W=Ot bonds is shifted with 8cm−1 towards higher wave numbers in the FT-IR spectrum of the complex, which indicates the involvement of the terminal oxygen atoms in the coordination to the vanadium ions. The UV electronic spectra of the ligand and complex spectrum presents two bands assigned to ligand to metal charge transfer pπ→dπ transitions in the W=Ot bonds and the electron transition dπ→pπ→dπ between the energetic levels of the tricentric bonds W–Ob–W. The powder EPR spectrum obtained at room temperature is typical for mononuclear oxovanadium species in an axial environment. The spectrum exhibits eight components, both in perpendicular and parallel bands due to the hyperfine coupling of the spin of one unpaired electron with the nuclear spin of the 51V isotope (g ⊥ =1.908, g II =1.974, A ⊥ =69G, A II =201G). [Copyright &y& Elsevier]

Abstract: The system CuGaSe2–CuGa3Se5 in thin films has been investigated. Layer synthesis was carried out by chemical close-spaced vapour transport (CCSVT) using Cu precursors on Mo/soda-lime glass substrates. The extension of deposition times in a two-step process led to final film compositions with [Ga]/[Cu] ratios ranging from 1 to 3, allowing the study of the phase transition mentioned above. Films showing chalcopyrite (1:1:2), OVC (1:3:5) and both phases were grown. X-ray emission spectroscopy and X-ray diffraction (XRD) techniques have been combined for a compositional and structural study of this material system probing both bulk and near surface properties of the films. This analysis was also extended to the rear-surface investigation of selected two-phase thin films and complemented with surface sensitive photoelectron spectroscopy (PES). From these results a growth model is presented for CuGa3Se5 formation in gallium rich, CCSVT-grown CuGa x Se y -films. [Copyright &y& Elsevier]

Abstract: The surface of transparent indium-tin oxide (ITO) electrode is electronically modified by a chemically bonded monolayer of organic molecules or a single layer of adsorbed acid ions. The electrode surface was examined with Kelvin probe force microscopy in vacuum. Treatments with phosphoric acid and periodic acid both lead to an increase in work function of about 0.7 eV with a good homogeneity. For both chemical monolayer and acid treatments the substrates show a uniform work function different from untreated ITO as expected for an uninterrupted film. We show that these treatments lead to an improvement of the phthalocyanine/C60 solar cell performance compared to cells on untreated ITO. Monochromatic external quantum efficiencies of up to 36% and power conversion efficiencies of up to 1.7% were obtained. [Copyright &y& Elsevier]