F. Rueda

Structural characterization of CuIn2Se3.5, CuIn3Se5 and CuIn5Se8 compounds

Abstract Polycrystalline CuIn2Se3.5, CuIn3Se5 and CuIn5Se8 have been synthesized in vacuum under different cooling conditions resulting in different phases. X-ray diffraction and Rietveld refinement of the diagrams have been used to characterize the different structures obtained. The two former compounds were refined using two different types of structure: stannite (S.G.: I42m) and the proposed P–chalcopyrite (S.G.: P42c). The determined bond distances after refinements indicate a higher tetragonal distortion for stannite type structure than P–chalcopyrite structure. These results seem to indicate that the Cu–In–Se structure with these two stoichiometries belongs to the P42c S.G. On the other hand, for the CuIn5Se8 compound a far different XRD diagram was obtained where all the reflections have been assigned to two structures, named γ-phases, one trigonal (γ) and one hexagonal (γH).

COMPOSITION EFFECTS ON THE CRYSTAL STRUCTURE OF CUINSE2

By x‐ray powder diffraction and the Rietveld refinement method, the atomic positions in CuInSe2 were determined for compositions close to stoichiometry. The Se position, x(Se), was found to be correlated to the Cu content. According to a model proposed by Jaffe and Zunger [Phys. Rev. B 29, 1882 (1984)], changes in x(Se) induce a variation in the optical band gap, Eg. This could explain the spread in energy gap, Eg, found by many authors for this compound. The increase in the lattice parameter, a, correlated with the difference between the energy dispersive analysis of x rays and x‐ray diffraction determined Cu contents, suggests the presence of a fraction of Cu atoms as interstitials.

On the determination of the optical constants n(λ) and α(λ) of thin supported films

Abstract A general method was developed to determine the optical constants n(λ) and α(λ) for a thin semiconducting or metallic supported film from five or three measurements of the transmittance and the reflectance. Exact formulae were deduced for oblique incidence transmittance and reflectance considering multiple coherent reflections in the films and incoherent reflections in the non-absorbing substrate. A computer program was developed that, when used as a subroutine of a European Organization for Nuclear Research program called MINUIT, allowed the method to be tested for gold, silicon, CdS, Cu2S (chalcocite) and Cu1.96S (djurleite) polycrystalline films. A residual function shows good agreement between the calculated and measured values of the transmittance and the reflectance when a careful choice of the value of the thickness is made.

A comparative study of optical, electrical and structural properties of CuGaSe2 and CuGaTe2 thin films

Abstract The growth conditions, the composition and the structural, optical and electrical properties of thin films of CuGaSe2 and CuGaTe2 have been studied using “flash” and “slow” evaporation in vacuum. Single phase films, when analyzing the absorption coefficient, present several energy gaps. For CuGaSe2, they are 1.59, 1.66, 2.03 and 2.11 eV, for CuGaTe2 1.23 and 1.89 eV. Both the CuGaSe2 and CuGaTe2 evaporated films are p-type; the resistivities, carrier densities and mobilities are appropriate for thin film solar cells.

Differences in copper indium selenide films obtained by electron beam and flash evaporation

Abstract Two alternative crystal structures have been identified in evaporated films grown from synthesized CuInSe2. The first one, tetragonal CuInSe2 was obtained by ‘flash’ evaporation and the second one, the ordered vacancy compound CuIn2Se3.5 by e-beam evaporation. They had optical gaps of 1.0 for the first and 1.24 eV for the second. The conductivities were p- and n-type, respectively; the last one remained n-type even after annealing in selenium atmospheres. The combination of both films in the heterostructure Mo/CuInSe2/CuIn2Se3.5/CdS/ITO has been obtained and is at present being studied as a solar cell.

An approach to the energy gap determination from reflectance measurements

A method to obtain the band gap energy value from reflectance measurements has been developed for bulk materials with direct and indirect gaps. In an energy region around the gap value, the experimental reflectivity is fitted by two functions of the type R=A0+A1′(1/hν)2−A1′Eg×(1/hν)3 and R=A0+B[(hν−Eg)/(hν)]4, for direct and indirect transitions, respectively, in an energy range around the gap value. This implies that the absorption coefficient, α, shows a dependence with the energy α=(A/hν)(hν−Eg)m with m=1/2, 2 for direct and indirect gaps, respectively. The method has been checked in several materials of known energy gaps including Si and Ge.

Composition effects on structural and optical infrared properties of CuIn0.5Ga0.5Se2

The dependence of structural parameters and force constants for Cu–Se, Ga–Se, and In–Se bonds on compositional deviations in CuIn0.5Ga0.5Se2 have been studied. The composition gradient along the ingot was obtained by a single fusion at 1150 °C of the components and subsequent slow cooling in a still ampoule placed in a vertical furnace. All along the sample, a single chalcopyrite phase is present and its composition along its length was found by energy dispersive analysis of x-ray measurements on slices. Unit cell parameters, anion displacement, and Cu occupation fraction in its sublattice were analyzed by x-ray powder diffraction and Rietveld refinement methods. The anion displacement found is a function of the Cu defect in its sublattice. The existence of associated defects, i.e., two Cu vacancies and one Ga in Cu site, [2V(Cu)+GaCu], is proposed to explain the Cu defect in its sublattice and the changes in lattice parameters. This leads to the existence of BIII vacancies (BIII=In+Ga), and interstitial ...

Structural Properties of CuIn3Se5 and Influence of Growth Conditions

Polycrystalline samples of CuIn3Se5 have been synthesized by reaction of stoichiometric mixture of the constituents in evacuated quartz ampoules, in a vertical furnace and in an horizontal one. The composition has been studied by EDAX and Atomic Absorption Spectroscopy. Important deviations in composition have been measured. The structure has been analyzed by X-ray Powder Diffraction, existing mainly single phase. Two spatial groups (SG) have been attributed to CuIn3Se5 by the Rietveld refinement: I 2m (Stannitetype structure) and P 2c (P-chalcopyrite). However, only the refinement in the SG P 2c provides occupation numbers in the lattice that are coherent with the composition determined by EDAX. The discrepancy index, especially the goodness of fit, are smaller for the SG P 2c.

Thin Film Cu2S/CdS Junctions Produced by Evaporation and Sputtering: Effect of Thermal Treatments in Vacuum

Heterojunctions CuXS-CdS produced by evaporation and sputtering of chalcocite on evaporated CdS have been produced. The effect of vacuum annealing on the spectral quantum efficiency has been used as a means of deducing the chalcocite and djurleitepresence. A peak at 500nm is tentatively assigned to djurleite. Open circuit voltages are similar to those reported for junctions produced by the “Clevite” method.

Effect of Stoichiometric Deviations on the Structural Properties of CuInSe2, CuIn0.5Ga0.5Se2 and CuGaSe2.

Ingots of CuInSe2, CuIn0.5Ga0.5Se2 and CuGaSe2 were synthesized under similar fusion at 1150°C in silisa ampoule, cut into slices and determined their composition by energy dispersive analysis of X-Rays. The structural properties, including the occupation fraction of each atomic sublattice, were studied by X-ray powder diffraction and Rietveld refinement. The Cu occupation number, N(Cu), is lower than one, The complex of two Cu vacancies and a substitutional catio, 2V(Cu)+B3+Cu (B3+Cu= In, Ga+In and Ga in CIS, CIGS and CGS respectively in a Cu site) is assumed and would explain the presence of Cu vacancies and allow an estimate of the B3+ concentration in its sublattice. When this increases a decrease in concentration of both Cu in its sublattice and the complex 2V(Cu)+B3+Cu are found. The strucutrual parameters depend on Cu and B3+ sublattice occupancy fractions for all compounds..In CuGaSe2, the lattice parameters increase as does Ga3+(subl) while the opposite arrives in CuInSe2 for In3+(subl). The distance of Se atom to z axis depends on Cu(subl) for all the compounds increasing in the case of CuInSe2 with V(Cu) and InCu antisites, decreasing the InSe bond distance. The opposite effect is observed in CuIn0.5Ga0.5Se2, and CuGaSe2.