Direct synthesis of CuIn(Sex, S1 − x)2 films from elemental In, Cu, Se, S particle precursor films and its dependence on energy

CuIn(Se_x, S_1 − x)₂ films were prepared by means of non-vacuum, instantaneous, direct synthesis from elemental In, Cu, Se and S particle precursor films by passing an electrical current with precise control through the metal substrate. For a constant reaction period of 1 s, unreacted elemental particles remained in the films for powers below 1 kV A, whereas the reaction to CuIn(Se_x, S_1 − x)₂ (x = 1) appeared to be complete at higher power. Chalcopyrite structure was observed in the range from 1.08 kV A to 1.24 kV A, the sphalerite structure appeared over 1.35 kV A. X-ray diffraction shows single (112) peaks of CuIn(Se_x, S_1 − x)₂ and the peak position agreed with the nominal composition of the precursors.     

Comparative study of sputtered and electrodeposited CI(S,Se) and CIGSe thin films

Copper indium disulphide CuInS₂ (CIS) and diselenide CuInSe₂ (CISe) and their alloys with gallium CuIn_1 − xGa_xSe₂ (CIGSe) thin films have been prepared using both high- and non-vacuum processes. The well known two-stage process consisting in a sequential sputtering of Cu and In thin layers and a subsequent sulfurisation has led to the formation of good quality CuInS₂ ternary compound. The films exhibit the well known chalcopyrite structure with a preferential orientation in the (112) plane suitable for the production of the efficient solar cells. The absorption coefficient of the films is higher than 10⁴ cm^− 1 and the band gap value is about 1.43 eV. A non-vacuum technique was also used. It consists on a one step electrodeposition of Cu, In and Se and in a second time Cu, In, Se and Ga. From the morphological and structural point of view, the films obtained are similar to those prepared by the first technique. The band gap value increases up from 1 eV for the CIS films to 1.26 eV for the CuIn_1 − xGa_xSe₂ with 0 < x < 0.23. The resistivity at room temperature of the films was adjusted to 10 Ωcm after annealing. The films exhibit an absorption coefficient more than 10⁵ cm^− 1. The most important conclusion of this study is the interesting potential of electrodeposition as a promising option in low-cost CISe and CIGSe thin film based solar cells processing.     

Influence of laser-irradiation on the optical constants Se75S25 − xCdx thin films

Amorphous thin films of glassy alloys of Se₇₅S_25 − xCd_x (x = 2, 4 and 6) were prepared by thermal evaporation onto chemically cleaned glass substrates. Optical absorption and reflection measurements were carried out on as-deposited and laser-irradiated thin films in the wavelength region of 500-1000 nm. Analysis of the optical absorption data shows that the rule of no-direct transitions predominates. The laser-irradiated Se₇₅S_25 − xCd_x films showed an increase in the optical band gap and absorption coefficient with increasing the time of laser-irradiation. The results are interpreted in terms of the change in concentration of localized states due to the shift in Fermi level. The value of refractive index increases decreases with increasing photon energy and also by increasing the time of laser-irradiation. With the large absorption coefficient and change in the optical band gap and refractive index by the influence of laser-irradiation, these materials may be suitable for optical disc application.     

Cost-effective solar cells containing copper indium chalcogenides prepared by SILAR method

CuInSe₂ (CISe) and CuIn(Se_1−x S _x )₂ (CISeS) thin films have been prepared using successive ionic layer adsorption and reaction method (SILAR) to produce CuSe and In₂Se₃, which were subsequently heated in appropriated conditions. Glass, TCO, and TCO/TiO₂ were used as substrates. A duplex layer of TiO₂ had been deposited by spray-pyrolysis and doctor blade. The thermal treatment leading to the formation of the chalcogenide was carried out in two different atmospheres. The first treatment was an annealing step under argon atmosphere, where CISe is obtained. The second consisted of a sulfurization process, where sulfur powder is melted and the vapor mixed into the argon flux. The morphology and composition of the annealed films were characterized by GXRD, micro-Raman spectroscopy, and SEM/EDS. Raman spectra and EDS showed an almost complete replacement of the Se atoms by S atoms, leading to the formation of CuIn(Se_1−x S _x )₂, with x = 0.8. Etching the films in KCN solution was necessary to dissolve residual secondary phases. Preliminary solar cell prototypes prepared with CISe and CISeS show a clear photo-response, although the photocurrent and open circuit potential are still low. This is a promising first result, showing that CuInSe₂ and CuIn(Se_1−x S _x )₂ prepared by SILAR can be used in solar cells.     

CuIn1 − xAlxSe2 thin films obtained by selenization of evaporated metallic precursor layers

CuIn_1 − xAl_xSe₂ (CIAS) thin films were grown by a two stage process. Cu, In and Al layers were sequentially evaporated and subsequently heated with elemental selenium in a quasi-closed graphite box. Different x values (0 ≤ x ≤ 0.6) were obtained by varying the Al and In precursor layers thicknesses. Selenization conditions such as Se amount provided during the selenization process were adjusted in order to optimize the film properties. Polycrystalline CuIn_1 − xAl_xSe₂ thin films with chalcopyrite structure were obtained. Referred to CuInSe₂ thin films the lattice parameters, the (112) orientation and the average crystallite size decreased and the band gap energy increased with increasing Al content. To optimize structural properties of the CIAS films a higher Se amount was required as the x value increased. The incorporation of Al changed the thin film morphology towards smaller grain sizes and less compact structures.     

Instantaneous preparation of CuIn(S1 − x, Sex)2 films by means of sparks using microwave irradiation

CuIn(S_1 − x,Se_x)₂ (CISSe) films aimed at flexible solar cells were directly prepared on Ti foils from elemental In, Cu, S, and Se particle precursor using microwave irradiation. The formation of the CISSe phase was deduced from X-ray diffraction (XRD) patterns. The (112) peaks of CISSe were well defined and the lattice constants increased in direct proportion to the S/(S + Se) ratio almost satisfying Vegard's law. In particular, CuInSe₂ was formed in the desired chalcopyrite lattice as indicated by the presence of (101), (103) and (211) peaks in the XRD pattern. Porous surfaces and formation of by-products were avoided by employing an evaporated In and Cu films instead of In and Cu particles.     

Band alignment of Cd(1 − x)ZnxS produced by spray pyrolysis method

Cd_(1 − x)Zn_xS thin films have been grown on glass substrates by the spray pyrolysis method using CdCl₂ (0.05 M), ZnCl₂ (0.05 M) and H₂NCSNH₂ (0.05 M) solutions and a substrate temperature of 260 °C. The energy band gap, which depends on the mole fraction × in the spray solution used for preparing the Cd_(1 − x)Zn_xS thin films, was determined. The energy band gaps of CdS and ZnS were determined from absorbance measurements in the visible range as 2.445 eV and 3.75 eV, respectively, using Tauc theory. On the other hand, the values calculated using Elliott-Toyozawa theory were 2.486 eV and 3.87 eV, respectively. The exciton binding energies of Cd_0.8Zn_0.2S and ZnS determined using Elliott-Toyozawa theory were 38 meV and 40 meV, respectively. X-ray diffraction results showed that the Cd_(1 − x)Zn_xS thin films formed were polycrystalline with hexagonal grain structure. Atomic force microscopy studies showed that the surface roughness of the Cd_(1 − x)Zn_xS thin films was about 50 nm. Grain sizes of the Cd_(1 − x)Zn_xS thin films varied between 100 and 760 nm.     

Antimony sulfide thin films in chemically deposited thin film photovoltaic cells

Antimony sulfide thin films of thickness ≈ 500 nm have been deposited on glass slides from chemical baths constituted with SbCl₃ and sodium thiosulfate. Smooth specularly reflective thin films are obtained at deposition temperatures from − 3 to 10 °C. The differences in the film thickness and improvement in the crystallinity and photoconductivity upon annealing the film in nitrogen are presented. These films can be partially converted into a solid solution of the type Sb₂S_xSe_3 − x, detected in X-ray diffraction, through heating them in contact with a chemically deposited selenium thin film. This would decrease the optical band gap of the film from ≈ 1.7 eV (Sb₂S₃) to ≈ 1.3 eV for the films heated at 300 °C. Similarly, heating at 300 °C of sequentially deposited thin film layers of Sb₂S₃-Ag₂Se, the latter also from a chemical bath at 10 °C results in the formation of AgSb(S/Se)₂ with an optical gap of ≈ 1.2 eV. All these thin films have been integrated into photovoltaic structures using a CdS window layer deposited on 3 mm glass sheets with a SnO₂:F coating (TEC-15, Pilkington). Characteristics obtained in these cells under an illumination of 850 W/m² (tungsten halogen) are as follows: SnO₂:F-CdS-Sb₂S₃-Ag(paint) with open circuit voltage (V_oc) 470 mV and short circuit current density (J_sc) 0.02 mA/cm²; SnO₂:F-CdS-Sb₂S₃-CuS-Ag(paint), V_oc ≈ 460 mV and J_sc ≈ 0.4 mA/cm²; SnO₂:F-CdS-Sb₂S_xSe_3 − x-Ag(paint), V_oc ≈ 670 mV and J_sc ≈ 0.05 mA/cm²; SnO₂:F-CdS-Sb₂S₃-AgSb(S/Se)₂-Ag(paint), V_oc ≈ 450 mV and J_sc ≈ 1.4 mA/cm². We consider that the materials and the deposition techniques reported here are promising toward developing ‘all-chemically deposited solar cell technologies.’     

Optical, electrical and thermal studies on (As2Se3)3−x(As2Te3)x glasses

Optical properties and conductivity of glassy (As₂Se₃)_3−x(As₂Te₃)_x were studied for 0 ≤ x ≤ 3. The films of the above mentioned compound were prepared by thermal evaporation with thickness of about 250 nm. The optical-absorption edge is described and calculated using the non-direct transition model and the optical band gap is found to be in the range of 0.92 to 1.84 eV. While, the width of the band gap tail exhibits opposite behaviour and is found to be in the range of 0.157 to 0.061 eV, this behaviour is believed to be associated with cohesive energy and average coordination number. The conductivity measurement on the thin films is reported in the temperature range from 280 to 190 K. The conduction that occurs in this low-temperature range is due to variable range hopping in the band tails of localized states, which is in reasonable agreement with Mott's condition of variable range hopping conduction. Some parameters such as coordination number, molar volume and theoretical glass transition temperature were calculated and discussed in the light of the topological bonding structure.     

Nickel diffusion in polycrystalline CuInSe2 thin films with a <112> fiber texture

Nickel diffusion in CuInSe₂ thin films was studied in the temperature range 430-520 °C. Thin films of copper indium diselenide (CuInSe₂) were prepared by selenization of CuInSe₂-Cu-In multilayered structure on glass substrate. A thin film of Nickel was deposited and annealed at different temperatures. Surface morphologies of the Ni diffused and undiffused CuInSe2 films were investigated using scanning electron microscope. The alteration of Nickel concentration in the CuInSe₂ thin film was measured by Energy Dispersive X-Ray Fluorescence (EDXRF) technique. These measurements were fitted to a complementary error function solution and the diffusion coefficients at four different temperatures were evaluated. The diffusion coefficients of Ni in CuInSe₂ films were estimated from concentration profiles at temperatures 430-520 °C as D = 1.86 × 10^− 7(cm²s^− 1)exp[− 0.68(eV)/kT].     

Tunable properties of wide-band gap p-type BaCu(Ch1 − xChx′)F (Ch = S, Se, Te) thin-film solid solutions

Thin-film solid solutions of BaCu(Ch_1 − xCh_x′)F (Ch, Ch′ = S, Se, or Te) wide-band gap p-type semiconductors are obtained by pulsed laser deposition at elevated substrate temperatures from alternating layers of BaCuChF and BaCuCh′F. Adjusting the thickness of the component layers varies the relative chalcogen content, which allows tunability of the film transparency and results in a conductivity change of more than three orders of magnitude. The tunability of the physical properties makes these chalcogen-based semiconductors potentially useful for optoelectronics applications. Lattice parameters of BaCuChF calculated using density functional theory agree with those previously reported for the powders. Deviations from Vegard's law are observed in BaCu(S_1 − xSe_x)F thin films with large sulfur content.     

Effect of tellurium addition on the optical and physical properties of germanium selenide glassy semiconductors

Thin films of the glasses Ge₁₀ Se_90−x Te_x (0 ≤ x ≥ 40) have been prepared by melt quenching technique; thin films were evaporated at a pressure of ≈10⁻⁴ Pa. The optical absorption behavior of these thin films was studied from the reflection and transmission spectrum in the spectral range 200-1200 nm. The optical constants i.e optical band gap (E_opt), absorption coefficient, refractive index (n) are calculated. The optical band gap has been estimated using Tauc extrapolation and found to decrease with Te content. The Dispersion of refractive index has been studied in terms of Wemple - Di Domenico model. The value of static refraction index has been found to increase with Te content. The distribution of the possible chemical bonds has been calculated. The obtained results were correlated with the character of the chemical bond for the prepared compositions through a study of parameters such as average heat of atomization (Hs), the cohesive energies of the bonds (CE), The mean bond energy <E> and average coordination number (m).     

AC plasma induced modifications in β-In2S3 thin films prepared by spray pyrolysis

β-In₂S₃ thin films, deposited by spray pyrolysis, were treated in N₂ and air plasmas at 240 and 400 Pa. X-ray diffraction, SEM, and EDS analysis, and optical and electrical studies have been used to characterize the as-prepared and plasma treated thin films. The post-deposition plasma treatments affect the morphology and the optoelectronic properties of the In₂S₃ thin films. The In₂S₃ thin films treated with N₂ plasma at 240 Pa showed an optical band gap, E_g, of 2.16 eV and an electrical conductivity of 2 × 10^− 2 (Ω cm)^− 1.     

Effects of different selenization conditions on the device parameters of CuIn(Se,S)2 solar cells

Parameter extraction can be used as a tool to determine the optimum chalcogenization condition of the absorber layer in manufacturing thin film solar cells. In this paper, CuIn(Se_1 − y,S_y)₂ solar cells fabricated at different selenization conditions using a two-step process were characterized. Device and performance parameters of the solar cells were determined from the current density-voltage (J-V) characteristics of the devices. The J-V curves were analyzed using the two-diode solar cell model. Devices selenized for 40-60 min exhibit relatively low series and shunt resistances, low fill factor and conversion efficiency. Better performance was observed for solar cells with absorber layer selenized for 10 to 20 min.     

Study of flash evaporated CuIn1 − xGaxTe2 (x = 0, 0.5 and 1) thin films

CuIn_1 − xGa_xTe₂ thin films with x = 0, 0.5 and 1, have been prepared by flash evaporation technique. These semiconducting layers present a chalcopyrite structure. The optical measurements have been carried out in the wavelength range 200-3000 nm. The linear dependence of the lattice parameters as a function of Ga content obeying Vegard's law was observed. The films have high absorption coefficients (4 · 10⁴ cm^− 1) and optical band gaps ranging from 1.06 eV for CuInTe₂ to 1.21 eV for CuGaTe₂. The fundamental transition energies of the CuIn_1 − xGa_xTe₂ thin films can be fitted by a parabolic equation namely E_g1(x) = 1.06 + 0.237x − 0.082x². The second transition energies of the CuInTe₂ and CuGaTe₂ films were estimated to be: E_g2 = 1.21 eV and E_g2 = 1.39 eV respectively. This variation of the energy gap with x has allowed the achievement of absorber layers with large gaps.     

Formation of CuIn1 − xAlxSe2 thin films studied by Raman scattering

CuIn_1 − xAl_xSe₂ (CIAS) thin films (x = 0.06, 0.18, 0.39, 0.64, 0.80 and 1) with thicknesses of approximately 1 μm were formed by the selenization of sputtered Cu―In―Al precursors and studied via X-ray diffraction, inductively coupled plasma mass spectrometry and micro-Raman spectroscopy at room temperature. Precursor films selenized at 300, 350, 400, 450, 500 and 550 °C were examined via Raman spectroscopy in the range 50-500 cm^− 1 with resolution of 0.3 cm^− 1. Sequential formation of In_xSe_y, Cu_2 − xSe, CuInSe₂ (CIS) and CIAS phases was observed as the selenization temperature was increased. Conversion of CIS to CIAS was initiated at 500 °C. For all CuIn_1 − xAl_xSe₂ products, the A₁ phonon frequency varied nonlinearly with respect to the aluminum composition parameter x in the range 172 cm^− 1 to 186 cm^− 1.     

Optical characterization of vacuum evaporated a-Se80Te20−xCux thin films

Thin films of a-Se₈₀Te_20−xCu_x (where x=2, 6, 8 and 10) were deposited on glass substrates by vacuum evaporation technique. The absorbance, reflectance and transmittance of as-deposited thin films were measured in the wavelength region 400-1000 nm. The optical band gap and optical constants of amorphous thin films have been studied as a function of photon energy. The optical band gap increases on incorporation of copper in Se₈₀Te_20−xCu_x system. The value of refractive index (n) decreases while the value of the extinction coefficient (k) increases with increasing photon energy. The results are interpreted in terms of concentration of localized states.     

Effect of substrate temperature on the optical parameters of thermally evaporated Ge-Se-Te thin films

Thin films of Ge₁₀Se_90 − xTe_x (x = 0, 10, 20, 30, 40, 50) glassy alloys were deposited at three substrate temperatures (303 K, 363 K and 423 K) using conventional thermal evaporation technique at base pressure of ~ 10^− 4 Pa. X-ray diffraction results show that films deposited at 303 K are of amorphous nature while films deposited at 363 K and 423 K are of polycrystalline nature. The optical parameters, refractive index and optical gap have been derived from the transmission spectra (using UV-Vis-NIR spectrophotometer) of the thin films in the spectral region 400-1500 nm. This has been observed that refractive index values remain almost constant while the optical gap is found to decrease considerably with the increase of substrate temperature. The decrease in optical gap is explained on the basis of change in nature of films, from amorphous to polycrystalline state, with the increase of substrate temperature. The optical gap has also been observed to decrease with the increase of Te content.     

Band offset calculations of ZnSxSe1−x/ZnSySe1−y heterostructures

In order to design devices based on II-VI materials, it is necessary to know the potential across the interface between two materials. Following our recent calculations which prove that the band gap energy of ZnS_xSe_1−x alloy has a nonlinear behaviour versus the sulphur composition x, it appears that an accurate knowledge of band offsets for ZnS_xSe_1−x/ZnS_ySe_1−y structures will be useful to model devices based on this heterostructure. On the basis of a model-solid theory, we report in this work the band offset calculations for zinc blende pseudomorphically strained ZnS_xSe_1−x/ZnS_ySe_1−y interface. From the results obtained, we have calculated the band gap energies of ZnS_xSe_1−x layers pseudomorphically strained on ZnS_ySe_1−y substrate as a function of compositions x and y in the whole range 0 ≤ x,y ≤ 1. Also, the band gaps of bulk ZnS_xSe_1−x deposed on ZnS_ySe_1−y for several values of y have been calculated versus the sulphur content x. Analytical formulas fitting these bands have been obtained. In view of the lack of theoretical calculations, our results seem likely to be useful especially in the design of ZnS_xSe_1−x structures for optoelectronic devices applications.     

Structural, electrical, and optical properties of copper indium diselenide thin film prepared by thermal evaporation method

Stoichiometric compound of copper indium diselenide (CuInSe₂) was synthesized by direct reaction of high-purity elemental copper, indium and selenium in an evacuated quartz ampoule. The phase structure and composition of the synthesized pulverized material analyzed by X-ray diffraction (XRD) and energy dispersive analysis of X-rays (EDAX) revealed the chalcopyrite structure and stoichiometry of elements. Thin films of CuInSe₂ were deposited onto organically cleaned soda lime glass substrates held at different temperatures (i.e. 300 K to 573 K) using thermal evaporation technique. CuInSe₂ thin films were then thermally annealed in a vacuum chamber at 573 K at a base pressure of 10^− 2 mbar for 1 h. The effect of substrate temperature (T_s) and thermal annealing (T_a) on structural, compositional, morphological, optical and electrical properties of films were investigated using XRD, transmission electron microscopy, EDAX, atomic force microscopy (AFM), optical transmission measurements and Hall effect techniques. XRD and EDAX studies of CuInSe₂ thin films revealed that the films deposited in the substrate temperature range of 423-573 K have preferred orientation of grains along the (112) plane and near stoichiometric composition. AFM analysis indicates that the grain size increases with increase of T_s and T_a. Optical and electrical characterizations of films suggest that CuInSe₂ thin films have high absorption coefficient (10⁴ cm^− 1) and resistivity value in the interval 10^− 2-10¹ Ω cm influenced by T_s and T_a.