Sylvain Marsillac

High-efficiency solar cells based on Cu(InAl)Se2 thin films

A Cu(InAl)Se2 solar cell with 16.9% efficiency is demonstrated using a Cu(InAl)Se2 thin film deposited by four-source elemental evaporation and a device structure of glass/Mo/Cu(InAl)Se2/CdS/ZnO/indium tin oxide/(Ni/Algrid)/MgF2. A key to high efficiency is improved adhesion between the Cu(InAl)Se2 and the Mo back contact layer, provided by a 5-nm-thick Ga interlayer, which enabled the Cu(InAl)Se2 to be deposited at a 530 °C substrate temperature. Film and device properties are compared to Cu(InGa)Se2 with the same band gap of 1.16 eV. The solar cells have similar behavior, with performance limited by recombination through trap states in the space charge region in the Cu(InAl)Se2 or Cu(InGa)Se2 layer.

CuIn1−xAlxSe2 thin films and solar cells

CuIn1−xAlxSe2 thin films are investigated for their application as the absorber layer material for high efficiency solar cells. Single-phase CuIn1−xAlxSe2 films were deposited by four source elemental evaporation with a composition range of 0⩽x⩽0.6. All these films demonstrate a normalized subband gap transmission >85% with 2 μm film thickness. Band gaps obtained from spectroscopic ellipsometry show an increase with the Al content in the CuIn1−xAlxSe2 film with a bowing parameter of 0.62. The structural properties investigated using x-ray diffraction measurements show a decrease in lattice spacing as the Al content increases. Devices with efficiencies greater than 10% are fabricated on CuIn1−xAlxSe2 material over a wide range of Al composition. The best device demonstrated 11% efficiency, and the open circuit voltage increases to 0.73 V.

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.

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.

Physico-chemical characterization of spray-deposited CuInS2 thin films

Abstract CuInS 2 thin films have been deposited by spray pyrolysis. X-Ray diffraction spectra show that a substrate temperature T s ≥590 K with a ratio of the concentration in the pulverized solution of [Cu]/[In]=1.1 permits well crystallized thin films with a preferential orientation along the (112) direction to be obtained. Scanning electron micrographs of the cross-sectional and top views of the surface show evidence of the good homogeneity of the films. Microprobe analysis as well as X-ray photoelectron spectroscopy show that an almost stoichiometric composition is obtained. In the best films only 3–8 at.% of oxygen is present, which is very promising for such a simple growth technique. Moreover, the oxygen present in the films segregates at the grain boundaries since there is not any significant variation of the interplanar spacing and the optical band gap with the presence of oxygen. The optical band gap values (1.45 eV) increases slightly with the crystalline state.

Comprehensive comparison of various techniques for the analysis of elemental distributions in thin films

In a recent publication by Abou-Ras et al., various techniques for the analysis of elemental distribution in thin films were compared, using the example of a 2-µm thick Cu(In,Ga)Se2 thin film applied as an absorber material in a solar cell. The authors of this work found that similar relative Ga distributions perpendicular to the substrate across the Cu(In,Ga)Se2 thin film were determined by 18 different techniques, applied on samples from the same identical deposition run. Their spatial and depth resolutions, their measuring speeds, their availabilities, as well as their detection limits were discussed. The present work adds two further techniques to this comparison: laser-induced breakdown spectroscopy and grazing-incidence X-ray fluorescence analysis.The present work shows results on elemental distribution analyses in Cu(In,Ga)Se2 thin films for solar cells performed by use of wavelength-dispersive and energy-dispersive X-ray spectrometry (EDX) in a scanning electron microscope, EDX in a transmission electron microscope, X-ray photoelectron, angle-dependent soft X-ray emission, secondary ion-mass (SIMS), time-of-flight SIMS, sputtered neutral mass, glow-discharge optical emission and glow-discharge mass, Auger electron, and Rutherford backscattering spectrometry, by use of scanning Auger electron microscopy, Raman depth profiling, and Raman mapping, as well as by use of elastic recoil detection analysis, grazing-incidence X-ray and electron backscatter diffraction, and grazing-incidence X-ray fluorescence analysis. The Cu(In,Ga)Se2 thin films used for the present comparison were produced during the same identical deposition run and exhibit thicknesses of about 2 μm. The analysis techniques were compared with respect to their spatial and depth resolutions, measuring speeds, availabilities, and detection limits.

Preparation of highly oriented α-In2Se3 thin films by a simple technique

Abstract α-In 2 Se 3 thin films were prepared by sequential thermal evaporation of indium and selenium layers followed by annealing in flowing argon. The structure and the phase of the films were confirmed by X-ray diffraction (XRD), scanning electron microscope (SEM), microprobe analysis, optical absorption, Raman measurements and room temperature conductivity measurements. The influence of the preparation on the formation of different In 2 Se 3 -modifications is discussed.

The effect of deposition parameters on radiofrequency sputtered molybdenum thin films

Employing r.f. (radiofrequency) magnetron sputtering, molybdenum thin films were fabricated on soda-lime glass substrates for use in Cu(In,Ga)Se2 based solar cells. The physical, electrical and structural properties of the films were studied as a function of two deposition parameters: argon pressure and r.f. power. The strain reversal from tensile to compressive occurs at high pressure and is found to decrease with increasing applied r.f. power. The grain sizes of films deduced from x-ray diffraction measurements (full width at half-maximum), and consistent with atomic force microscope images, increase with increasing argon pressure and power. The resistivity of the films was found to increase with increasing argon pressure and decrease with increasing r.f. power.

New Cd-free buffer layer deposited by PVD: In2S3 containing Na compounds

Abstract In this paper it is first shown that In2S3 containing sodium (BINS) thin films can be grown on glass substrates heated at 200 °C. The films have a n-type electrical conductivity and their optical band gap can be managed between 2.15 and 2.90 eV by controlling their sodium content. The wide band gap BINS films (Eg>2.5 eV) have interesting properties to be used as buffer layer in CIGS-based solar cells. In the aim of realizing efficient solar cells, the physico-chemical and electrical properties of Mo/CIGS/BINS structures have first been studied. Then solar cells are studied. For this first study, the thickness of the BINS buffer has been chosen as parameter, whereas the optical band gap of the BINS was constant at 2.8 eV. An efficiency of 8.2% has been reached with a 100 nm thick BINS buffer, while the efficiency of the cell with chemical bath deposited-CdS buffer was 10.2%.

Study of the new β-In2S3 containing Na thin films Part I: Synthesis and structural characterization of the material

Thin films of In2S3 containing different quantity of sodium have been synthesized by annealing at 400°C of structures composed of thin indium, sulphur and indium fluoride layers sequentially evaporated on sodium-free glass substrates. The thin films obtained have been studied by X-ray diffraction (XRD), electronic microprobe analysis and X-ray photoelectron spectroscopy. They exhibit a β-In2S3-like structure and the sodium atoms are homogeneously distributed in the whole films. In order to determine the structure of the material and particularly the position of the sodium atoms in the β-In2S3 spinel matrix, single crystal of the same compound has been synthesized and then studied by XRD and transmission electron microscopy. These studies have shown that the materials obtained can be described by the general formulation [In16]Oh[In5.33−xNa3x□2.66−2x]TdS32 (0⩽x⩽1.33), where Oh and Td, respectively, represent the octahedral and tetrahedral sites of the spinel structure. In thin films form, the maximum of sodium which can be introduced in the crystalline matrix has been shown to correspond to x=0.9, which corresponds to the formula [In16]Oh[In4.4Na2.7□0.9]TdS32. This new material family is called BINS.