Ahmed Ennaoui

Iron disulfide for solar energy conversion

Abstract Pyrite ( E g =0.95 eV) is being developed as a solar energy material due to its environmental compatibility and its very high light absorption coefficient. A compilationof material, electronic and interfacial chemical properties is presented, which is considered relevant for quantum energy conversion. In spite of intricate problems existing within material chemistry, high quantum efficiencies for photocurrent generation (>90%) and high photovoltages (≈500 mV) have been observed with single crystal electrodes and thin layers respectively. The most interesting aspect of this study is the use of pyrite as an ultrathin (10–20 nm) layer sandwiched between large gap p-type and n-type materials in a p-i-n like structure. Such a system, in which the pyrite layer only acts as photon absorber and mediates injection of excited electrons can be defined as sensitization solar cell. The peculiar electron transfer properties of pyrite interfaces, facilitating interfacial coordination chemical pathways, may turn out to be very helpful. Significant research challenges are discussed in the hope of attracting interest in the development of solar cells from this abundant material.

Chemical bath deposition of indium sulphide thin films : preparation and characterization

Abstract Indium sulphide (In2S3) thin films have been successfully deposited on different substrates under varying deposition conditions using chemical bath deposition technique. The deposition mechanism of In2S3 thin films from thioacetamide deposition bath has been proposed. Films have been characterized with respect to their crystalline structure, composition, optical and electrical properties by means of X-ray diffraction, TEM, EDAX, optical absorption, TRMC (time resolved microwave conductivity) and RBS. Films on glass substrates were amorphous and on FTO (flourine doped tin oxide coated) glass substrates were polycrystalline (∈ phase). The optical band gap of In2S3 thin film was estimated to be 2.75 eV. The as-deposited films were photoactive as evidenced by TRMC studies. The presence of oxygen in the film was detected by RBS analysis.

Photoactive Synthetic Polycrystalline Pyrite ( FeS2 )

Polycrystalline layers of As‐doped pyrite have been produced in bromine atmosphere with the aim of developing this sulfide material for solar energy applications. Its photoelectrochemical behavior in contact with an aqueous electrolyte was investigated. It operated as a photoelectrochemical solar cell and showed reasonably stable behavior under illumination. Optical measurements performed on show that visible light is absorbed in an extremely thin layer of 160A in spite of the apparently indirect gap of this semiconductor . This would make this photosensitive material an interesting candidate for thin‐layer solar cells. Scanning electron micrographs of the samples reveal well‐developed crystallites of about 5–10 μm with distinct boundaries. Scanning laser spot analysis over macroscopic areas (5 mm) showed homogeneous as well as inhomogeneous regions. The photoelectric properties of these first polycrystalline pyrite samples studied are poor, but there is presently no reason to assume that they cannot be developed.

Process and characterisation of chemical bath deposited manganese sulphide (MnS) thin films

Manganese sulphide (MnS) thin films have been deposited by a simple and inexpensive chemical bath deposition (CBD) method using thioacetamide as a sulphide ion source from an aqueous medium. The effect of preparative parameters on the film growth and quality has been studied. The MnS films have been characterised by XRD, TEM, SEM, EDAX, RBS, optical absorption and (time resolved microwave conductivity) TRMC techniques for their structural, compositional, and optical properties. The as-deposited MnS film on glass substrate consists of nanocrystalline grains. The film consists of mixed (cubic and hexagonal) phases. The optical band gap of the film is estimated to be 3.02 eV.

Photoelectrochemical Energy Conversion Obtained with Ultrathin Organo‐Metallic‐Chemical‐Vapor‐Deposition Layer of FeS2 (Pyrite) on TiO2

Ultrathin (10 to 20 nm thick), polycrystalline films of FeS[sub 2] (pyrite) were grown on TiO[sub 2] (anatase) by chemical vapor deposition. The FeS[sub 2] films were characterized using optical absorption and high-resolution electron microscopy. Photoelectrochemical solar cells, using TiO[sub 2] (anatase) coated with FeS[sub 2] ultrathin films, generated high open-circuit photo-voltages, of up to 600 mV, compared with a single crystal of pyrite electrode (200 mV). The photoelectrochemical behavior shows a strong dependence of photovoltage and photocurrent on the pH of the solution. This paper reports that it is explained by electron injection from the conduction band of FeS[sub 2] to the conduction band of TiO[sub 2]. Regeneration of holes is taking place by electron transfer from the redox system in the electrolyte.

Chemical bath ZnSe thin films: deposition and characterisation

Abstract The zinc selenide (ZnSe) thin films have been deposited by a simple and inexpensive chemical bath deposition (CBD) method. The selenourea was used as a selenide ion source. The ZnSe films have been characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDAX), Rutherford back scattering (RBS), and optical absorption. The as-deposited ZnSe films on various substrates are found to be amorphous and contain O2 and N2 in addition to Zn and Se. The optical band gap of the film is estimated to be 2.9 eV. The films are photoactive as evidenced by time resolved microwave conductivity (TRMC).

Highly textured films of layered metal disulfide 2H-WS2: Preparation and optoelectronic properties

Highly textured films of 2H-WS 2 can be obtained by sulfurization of up to 4 μm thick WO 3 layers in the presence of hydrogen using amorphous (quartz glass, glassy carbon) or crystalline (sapphire, muscovite, highly oriented pyrolytic graphite) substrates. Best conditions have been found employing (00.1) oriented sapphire substrates with a 5 nm thick nickel layer interposed between substrate and oxide film (0.5 to 4 μm thickness) and a reaction temperature ranging from 973 to 1173 K. Depending on time the crystallites, oriented with their hexagonal basal planes parallel to the substrate, exhibited a lateral extension of up to 20 μm and a thickness of ≤300 nm. Conductivity measurements of the films showed a p-type conductivity in the range from 0.1 to 3 Ω -1 cm -1 and a lateral mobility as high as 105 cm 2 V -1 s -1 at room temperature. The conductivity type has been confirmed by ultraviolet photoelectron and x-ray photoelectron spectroscopy which were compared with n-type single crystals. A freestanding film pealed off from a Pt coated quartz substrate and mounted on a brass holder was investigated photoelectrochemically. Using a 0.2 M Fe 2+ /Fe 3+ redox electrolyte in 0.5 M H 2 SO 4 an open-circuit voltage of 100 mV and a short-circuit current of 5 mA/cm 2 has been detected for the first time.

Highly textured films of layered metal disulfide 2H WS2: Preparation conditions and optoelectronic properties

Highly textured films of 2H-WS 2 can be obtained by sulfurization of up to 4 μm thick WO 3 layers in the presence of hydrogen using amorphous (quartz glass, glassy carbon) or crystalline (sapphire, muscovite, highly oriented pyrolytic graphite) substrates. Best conditions have been found employing (00.1) oriented sapphire substrates with a 5 nm thick nickel layer interposed between substrate and oxide film (0.5 to 4 μm thickness) and a reaction temperature ranging from 973 to 1173 K. Depending on time the crystallites, oriented with their hexagonal basal planes parallel to the substrate, exhibited a lateral extension of up to 20 μm and a thickness of ≤300 nm. Conductivity measurements of the films showed a p-type conductivity in the range from 0.1 to 3 Ω -1 cm -1 and a lateral mobility as high as 105 cm 2 V -1 s -1 at room temperature. The conductivity type has been confirmed by ultraviolet photoelectron and x-ray photoelectron spectroscopy which were compared with n-type single crystals. A freestanding film pealed off from a Pt coated quartz substrate and mounted on a brass holder was investigated photoelectrochemically. Using a 0.2 M Fe 2+ /Fe 3+ redox electrolyte in 0.5 M H 2 SO 4 an open-circuit voltage of 100 mV and a short-circuit current of 5 mA/cm 2 has been detected for the first time.

Preparation of textured and photoactive 2H-WS2 thin films by sulfurization of WO3

Abstract Photoactive thin films of tungsten disulfide have been prepared by sulfurization of WO3 layers. Tungsten trioxide films were deposited on heated quartz and glassy carbon substrates by gaseous reaction of W(CO)6 with oxygen. Subsequently, the oxide films were treated at 700 °C in a gaseous sulfur atmosphere either in a flowing system or in evacuated and sealed quartz ampoules. The films were investigated by X-ray diffractometry, scanning electron and scanning tunneling microscopy. The photoactivity was measured using time-resolved microwave conductivity. Films consisting of the 2H-WS2 platelets, highly textured and oriented with their c axis perpendicular to the substrate, were found. In addition, the phase relations in the phase triangle W-O-S were studied by thermochemical equilibrium calculations using the Gibbs free energy minimization technique. The inferred predominance area diagram log Ps2 vs. log pso2 confirms the high stability of WO2,9 and WS2 as observed under related experimental conditions. The effect of the nickel on the orientation of the 2H-WS2 crystallites obtained after sulfurization of the WO3 films can be explained by a flux on the basis of the binary Ni-S phase diagram or surfactant mediated growth effect.

Fabrication of CuInS2 films from electrodeposited Cu/In bilayers: effects of preheat treatment on their structural, photoelectrochemical and solar cell properties

Polycrystalline CuInS(2) films were fabricated by sulfurization of electrodeposited Cu and In metallic precursor films in a Cu-rich composition at 520 °C in H(2)S (5% in Ar). Structural analyses revealed that the adherence of the thus-formed CuInS(2) film to the Mo substrate was strongly dependent on heating profiles of the Cu/In bilayer film: a CuInS(2) film with poor adherence having many crevices was formed when the Cu/In bilayer film was heated monotonously from room temperature to 520 °C in Ar within 25 min followed by sulfurization, whereas CuInS(2) films with good adherence were obtained when the Cu/In films were pretreated at 110 °C in Ar for 10-60 min just before increasing the temperature up to 520 °C for sulfurization. It was also clarified that the CuInS(2) film obtained without 110 °C pretreatment had pinholes inside the film, whereas the CuInS(2) films formed after 110 °C pretreatment showed no notable pinholes. Photoelectrochemical responses of these CuInS(2) films in an electrolyte solution containing Eu(III) indicated that the CuInS(2) films obtained after 110 °C pretreatment had higher external quantum efficiency (EQE) values than those of films obtained without 110 °C pretreatment, mainly due to better adherence of 110 °C pretreated CuInS(2) films to the Mo substrate than the CuInS(2) film obtained without 110 °C pretreatment. The performance of solar cells with an Al:ZnO/Zn(S,O)/CdS/CuInS(2)/Mo structure also depended on the structural characteristics of the CuInS(2) films, i.e., preliminary conversion efficiencies of ca. 5% were obtained for devices based on the CuInS(2) films obtained after 110 °C pretreatment, whereas the device prepared by the CuInS(2) film without 110 °C pretreatment showed the conversion efficiency less than 1.5%.