J.C. Bernède

Properties of Ag2−xSe1+x/n-Si diodes

Abstract Ag 2− x Se 1+x thin films were prepared by vacuum evaporation onto n-Si substrates held at 173, 300 and 373 K. The electrical characteristics of Ag 2− x Se 1+ x n-Si diodes were studied by using current-voltage ( I-V ) and capacitance-voltage ( C-V ) measurements. The relationships between the preparation conditions of the samples and the parameters of the junctions were examined. The I-V characteristics were in good agreement with the standard thermionic emission theory while the relationship between C -2 and V was linear. This experimental investigation showed that a Schottky barrier was formed. Some aging effects were observed in diodes with chemically etched silicon wafers. The C-V data were frequency dependent. This was attributed to the occurrence of relaxation phenomena in a thin oxide layer at the interface. Diodes with a thick oxide layer on n-Si showed the highest barrier height and stability.

MoO3 surface passivation of the transparent anode in organic solar cells using ultrathin films

An original surface passivation technique of indium tin oxide (ITO) used as anode in organic solar cells is proposed. We demonstrate that a thin MoO3 film (3.5±1 nm) at the interface ITO/organic donor allows improving significantly the devices’ performances. The devices are based on the multiheterojunction structure copper phthalocyanine (CuPc)/fullerene (C60)/aluminum tris(8-hydroxyquinoline) (Alq3). The deposition of MoO3 onto ITO improves the charge transfer from CuPc to ITO. The enhancement in the hole collection efficiency in the presence of an oxide layer can be explained in terms of the reduction in the effective barrier against hole transfer from CuPc into the ITO anode. The contact ITO/MoO3/CuPc behaves like a metal-insulator-semiconductor (MIS) structure, which allows reducing the energy barrier due to the difference between the work function of ITO and the highest occupied molecular orbital of CuPc. It is shown that the optimum MoO3 thickness corresponds to a compromise between an optimum ITO c...

Polarized memory switching in mis thin films

Abstract The current-voltage ( I-V ) characteristics of metal/Se-point contact structures (M/Se-P) with electrodes of different metals (aluminium, gold, chromium, copper, silver and tungsten) were studied systematically. A stable polarized memory switching effect was obtained with the Al/Se-P Cu devices. The threshold voltage V s was independent of the selenium thickness. The memorized ON state disappeared progressively when the applied voltage was removed or when a reverse voltage was applied. The I-V characteristics were not disturbed by annealing (crystallization) of the selenium but they showed aging effects. The switching operations obtained with other electrode materials were not reproducible. In the case of the aluminium electrode this can be explained by the growth of an Al 2 O 3 layer between the aluminium and the selenium.

Improvement of organic solar cell performances using a zinc oxide anode coated by an ultrathin metallic layer

The authors have achieved an efficient organic solar cell based on copper phthalocyanine (CuPc) layer as donor and fullerene (C60) as acceptor. The aluminum doped zinc oxide (ZnO:Al) instead of indium tin oxide (ITO) is used as the anode. An ultrathin gold film is introduced among ZnO:Al, transparent conductor oxide, and the CuPc donor layer. We show that the power conversion efficiency of this cell is enhanced by one order of magnitude compared to that achieved with a ZnO anode without ultrathin gold film. Therefore, the power conversion efficiency of this cell is comparable to that with an ITO anode.

Preparation and characterization of pyrolytic spray deposited electrochromic tungsten trioxide films

Abstract Tungsten trioxide (WO 3 ) thin films were prepared by spray pyrolysis under various conditions using, for the first time, tungsten chloride (WCl 6 ) dissolved in 50% ethanol–50% water. All the as-deposited films were amorphous and dark. When annealed, they became crystallized and transparent. The structure and the composition of these films, before and after annealing, were studied. The X-ray diffraction diagrams revealed that all the annealed films consisted of the monoclinic WO 3 phase. The X-ray photoelectron spectroscopy studies of the W 4f doublet showed the W 6( oxidation state for all the films. The electron probe microanalysis revealed an over stoichiometry of oxygen for the unannealed films. The optical properties as well as the structure of the annealed, therefore transparent, films prepared at different spray rates were also determined. The optical gap was determined from reflection and transmission spectra. All annealed samples exhibited a reversible electrochromic effect in the presence of an acid electrolyte as demonstrated by cyclic voltammetry and optical transmission measurements.

Structural and Photoelectrical Properties of Sprayed β-In2S3 Thin Films

β-In 2 S 3 thin films have been prepared by the spray pyrolysis technique. X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) micrographs showed a good homogeneity of these layers. Microanalysis and XPS measurements have detected little oxygen in the films present in In 2 O 3 form. Photoconductivity measurements were carried out within the range of wavelengths in the visible spectrum at different modulation frequencies f and bias voltages V ranging from 5 to 300 Hz and from 3 to 25 V, respectively. The band gap energy Eg, deduced from these spectra, has approximately the same value as that obtained from spectrophotometric transmission and reflection measurements (Eg = 2.05 eV).

Structural, optical and electrical properties of β-In2S3-3xO3x thin films obtained by PVD

β-In2S3−3xO3x thin films have been synthesized following a dry physical process on glass substrates. The highest temperature used during the elaboration of the films is 473 K. The films have been structurally and optically characterized by X-ray diffraction, electronic microprobe analysis, X-ray photoelectron spectroscopy, scanning electronic microscopy and atomic force microscopy. They crystallize in the tetragonal β-In2S3 structure. Their optical band gap varies from 2.1 eV, when they are pure β-In2S3, to 2.9 eV when they contain 8.5 at.% of oxygen. The electrical properties of the thin films have also been determined, they have n-type electrical conductivity of approximately 10−3 S·cm−1. All these properties make β-In2S3−3xO3x thin films good candidates to substitute CBD-CdS as buffer layer in CIGS-based solar cells.

Electrical characterization of MSe structures (M = Ni, Bi)

Abstract I — V characteristics of Ni SeCr and Bi SeCr structures may be well described by the classical current-transport theories in Schottky junctions where the current density is given by : J ∼- exp q(V−R s l) nkT with n = 1.1 and 1.5 respectively. However the determination of the barrier height by C-V measurements confirms the presence of a thin oxide layer at the interface.

Silver movements in Ag2Te thin films: Switching and memory effects☆

Abstract A systematic study of the I–V characteristics of Ag2Te thin film samples with electrodes of different metals (silver, gold, chromium, copper, molybdenum, tantalum and tungsten) and of different shapes under forward, reverse or bipolar bias is presented. The results show that the switching effect arises from movements of the silver in the bulk. The threshold voltage Vs is roughly independent of the interelectrode distance but it depends weakly on the positive electrode metal; this is explained by a field-induced area of low conductivity at the interface owing to silver movements. The ease of inducing memory behaviour in silver electrode structures confirms this hypothesis.

Experimental evidence of the low-temperature formation of γ-In2Se3 thin films obtained by a solid-state reaction

Abstract In 2 Se 3 coatings were obtained by a solid-state reaction, induced by annealing, between the In and Se constituents sequentially deposited in thin film form. The films crystallized in the γ-In 2 Se 3 phase. To confirm and to characterize this phase, the films have been investigated by microprobe analysis and X-ray photoelectron spectroscopy, transmission electron microscopy and X-ray diffraction, Raman diffusion and optical absorption measurements. It is shown that selected area diffraction (SAD) patterns of the γ-In 2 Se 3 phase are obtained and that a very good correlation exists between the planes obtained from X-ray spectra and SAD patterns. Raman diffusion and optical absorption also lead to characteristic data of the γ-In 2 Se 3 phase, and are compared with the values of the other phases. Moreover it is shown that the occurrence of the γ-phase is related to the annealing under selenium atmosphere and to the initial Se/In atomic ratio.