Effects of Ga concentration on structural and electrical properties of screen printed-CIGS absorber layers on polyethylene terephthalate

Copper indium gallium diselenide (CIGS) films were deposited as an absorber layer on polyethylene terephthalate (PET) substrates by a screen printing technique using CIGS ink with a Ga content ranging from 0.3 to 0.6. The melting point of PET substrate is 254.9 °C; the average transmission in the visible (400 nm–800 nm) for PET substrates is greater than 85%. Effects of Ga content of the CIGS absorber layer on structural and electrical properties of the CIGS films were studied. The lattice parameters, a and c for all CIGS films were decreased with increasing Ga content. At room temperature, Hall mobility and charge-carrier concentration of the CIGS films varies from 97.2 to 2.69 cm² V⁻¹ s⁻¹ and 9.98×10¹⁶ to 3.23×10¹⁸ cm⁻³, respectively.     

Electrodeposition of CuIn1−xGaxSe2 solar cells with a periodically-textured surface for efficient light collection

The photoresponse of CuIn_1?xGa_xSe₂ (CIGS) solar cells is improved using a periodically-textured structure as an antireflection layer. The CIGS absorber layers were prepared by one-step electrodeposition from an aqueous solution containing 12 mM CuSO₄, 25 mM In₂(SO₄)₃, 28 mM Ga₂(SO₄)₃, and 25 mM SeO₂. The electrodeposited CIGS films exhibit the (112)-preferred orientation of the chalcopyrite structures and feature improved film stoichiometry after the selenization process. In addition, the lower bandgap value of 0.97 eV is caused by the discrepancy of the reduction potentials for each constituent, resulting in insufficient Ga content in the deposited films. Using self-assembled silica nanoparticles as the etching mask, periodically-textured structures can be easily formed on an indium tin oxide (ITO)-coated soda lime glass to achieve a low average reflection (<10.5%) in a wide wavelength and incident angle range. With the periodic textured structures suppressing light reflections from the front surface, the photogenerated current in the semi-transparent CIGS solar cells made with transparent conducting electrodes is 1.82 times higher than they otherwise would be.     

Growth of Zn doped Cu(In, Ga)Se2 thin films by RF sputtering for solar cell applications

Cu(In, Ga)Se₂ (CIGS) surface was modified with Zn doping using a magnetron sputtering method. CuInGa:Zn precursor films targeting a CuIn_0.7Ga_0.3Se₂ stoichiometry with increasing Zn content from 0 to 0.8 at% were prepared onto Mo-coated glass substrates via co-sputtering of Cu–Ga alloy, In and Zn targets. The CuInGa:Zn precursors were then selenized with solid Se pellets. The structures and morphologies of grown Zn doped CIGS films were found to depend on the Zn content. At zinc doping level ranging between 0.2 and 0.6 at%, the Zn doping improved the crystallinity and surface morphology of CIGS films. Compared with the performance of the non-doped CIGS cell, the fabricated CIGS solar cell displayed a relative efficiency enhancement of 9–22% and the maximum enhancement was obtained at a Zn content of 0.4 at%.     

Effects of deposition pressure on the properties of transparent conductive ZnO:Ga films prepared by DC reactive magnetron sputtering

Gallium (Ga)-doped zinc oxide (ZnO:Ga) transparent conductive films were deposited on glass substrates by DC reactive magnetron sputtering. Effects of deposition pressure on the structural, electrical and optical properties of ZnO:Ga films were investigated. X-ray diffraction (XRD) studies show that the films are highly oriented with their crystallographic c-axis perpendicular to the substrate almost independent of the deposition pressure. The morphology of the film is sensitive to the deposition pressure. The transmittance of the ZnO:Ga thin films is over 90% in the visible range and the lowest resistivity of ZnO:Ga films is 4.48×10⁻⁴ Ω cm.     

Effect of sodium doping on graded Cu(In1−xGax)Se2 thin films prepared by chemical spray pyrolysis

In the present work we have studied the effect of Na on the properties of graded Cu(In_1−xGa_x)Se₂ (CIGS) layer. Graded CIGS structures were prepared by chemical spray pyrolysis at a substrate temperature of 350 °C on soda lime glass. Sodium chloride is used as a dopant along with metal (Cu/In/Ga) chlorides and n, n-dimethyl selenourea precursors. The addition of Na exhibited better crystallinity with chalcopyrite phase and an improvement in preferential orientation along the (112) plane. Energy dispersive analysis of X-rays (line/point mapping) revealed a graded nature of the film and percentage incorporation of Na (0.86 at%). Raman studies showed that the film without sodium doping consists of mixed phase of chalcopyrite and CuAu ordering. Influence of sodium showed a remarkable decrease in electrical resistivity (0.49–0.087 Ω cm) as well as an increase in carrier concentration (3.0×10¹⁸–2.5×10¹⁹ cm⁻³) compared to the un-doped films. As carrier concentration increased after sodium doping, the band gap shifted from 1.32 eV to 1.20 eV. Activation energies for un-doped and Na doped films from modified Arrhenius plot were calculated to be 0.49 eV and 0.20 eV, respectively. Extremely short carrier lifetimes in the CIGS thin films were measured by a novel, non-destructive, noncontact method (transmission modulated photoconductive decay). Minority carrier lifetimes of graded CIGS layers without and with external Na doping are found to be 3.0 and 5.6 ns, respectively.     

Temperature effect on the physical properties of CuIn11S17 thin films for photovoltaic applications

CuIn₁₁S₁₇ compound was synthesized by horizontal Bridgman method using high-purity copper, indium and sulfur elements. CuIn₁₁S₁₇ thin films were prepared by high vacuum evaporation on glass substrates. The glass substrates were heated at 30, 100 and 200 °C. The structural properties of the powder and the films were investigated using X-ray diffraction (XRD). XRD analysis of thin films revealed that the sample deposited at a room temperature was amorphous in nature while those deposited on heated substrates were polycrystalline with a preferred orientation along the (311) plane of the spinel phase. Ultraviolet–visible (UV–vis) spectroscopy was used to study the optical properties of thin films. The results showed that CuIn₁₁S₁₇ thin films have high absorption coefficient α in the visible range (10⁵–10⁶ cm⁻¹). The band gap E_g of the films decrease from 2.30 to 1.98 eV with increasing the substrate temperature (T_s) from 30 to 200 °C. We exploited the models of Swanepoel, Wemple–DiDomenico and Spitzer–Fan for the analysis of the dispersion of the refractive index n and the determination of the optical constants of the films. Hot probe method showed that CuIn₁₁S₁₇ films deposited at T_s=30 °C and T_s=100 °C are p-type conductivity whereas the sample deposited at T_s=200 °C is highly compensated.     

The significant effect of film thickness on the properties of chalcopyrite thin absorbing films deposited by RF magnetron sputtering

In this paper, thickness dependent structural, surface morphological, optical and electrical properties of RF magnetron sputtered CuIn_0.8Ga_0.2Se₂ (CIGS) thin films were studied using X-ray diffraction (XRD), Transmission electron microscopy (TEM), Field emission scanning electron microscopy (FE-SEM), Atomic force microscopy (AFM), UV–vis–NIR spectrophotometer and Keithley electrical measurement unit. The peak intensity along (112) plane as well as crystallite size was found to increase with thickness. However, for higher film thickness >1.16 μm, crystallinity reduced due to higher % of Cu content. TEM analysis confirmed pollycrysallinity as well as chalcopyrite phase of deposited films. The band gap was found to decrease with increase in thickness yielding a minimum value of 1.12 eV for film thickness 1.70 μm. The I–V characteristics showed the ohmic behavior of metal semiconductor contact with higher conductivity for film thickness 1.16 μm.     

Residual strain and electrical resistivity dependence of molybdenum films on DC plasma magnetron sputtering conditions

Sputter deposited molybdenum (Mo) thin films are used as back contact layer for Cu(In_1−xGa_x)(Se_1−yS_y)₂ based thin film solar cells. Desirable properties of Mo films include chemical and mechanical inertness during the deposition process, high conductivity, appropriate thermal expansion coefficient with contact layers and a low contact resistance with the absorber layer. Mo films were deposited over soda-lime glass substrates using DC-plasma magnetron sputtering technique. A 2³ full factorial design was made to investigate the effect of applied power, chamber pressure, and substrate temperature on structural, morphological, and electrical properties of the films. All the films were of submicron thickness with growth rates in the range of 34–82 nm/min and either voided columnar or dense growth morphology. Atomic force microscope studies revealed very smooth surface topography with average surface roughness values of upto 17 nm. X-ray diffraction studies indicated, all the films to be monocrystalline with (001) orientation and crystallite size in the range of 4.6–21 nm. The films exhibited varying degrees of compressive or tensile residual stresses when produced at low or high chamber pressure. Low pressure synthesis resulted in film buckling and cracking due to poor interfacial strength as characterized by failure during the tape test. Measurement of electrical resistivity for all the films yielded a minimum _value of 42 μΩ cm for Mo films deposited at 200 W DC power.     

A study on phase transformation of SnOx thin films prepared by reactive magnetron sputtering

In the paper, SnO_x thin films were deposited by reactive magnetron sputtering from a tin target in O₂ containing working gas. The evolution from Sn-containing SnO to tetravalent SnO₂ films was investigated. The films could be classified into three groups according to their optical band gaps, which are E_g<2.5 eV, E_g=3.0–3.3 eV and E_g>3.7 eV. The electric measurements show that high conductivity can be obtained much easier in SnO₂ than in SnO films. A high electron mobility of 15.7 cm² V⁻¹ s⁻¹, a carrier concentration of 1.43×10²⁰ cm⁻³ and a resistivity of 2.8×10⁻³ Ω cm have been achieved in amorphous SnO₂ films. Films with the optical band gap of 3.0–3.3 eV remain amorphous though the substrate temperature is as high as 300 °C, which implies that °btaining high mobility in p-type SnO is more challenging in contrast to n-type SnO₂ films.     

Thickness-dependent optoelectronic properties of CuCr0.93Mg0.07O2 thin films deposited by reactive magnetron sputtering

CuCr_0.93Mg_0.07O₂ thin films were successfully deposited by DC reactive magnetron sputtering at 1123 K from metallic targets. The influence of film thickness on the structural and optoelectronic properties of the films was investigated. X-ray diffraction (XRD) results revealed that all the films had a delafossite structure with no other phases. The optical and electrical properties were investigated by UV–VIS spectrophotometer and Hall measurement, respectively. It was found that the optoelectronic properties exhibited a thickness-dependent behavior. The optical band gap and the average transmittance of the films showed a monotonous decrease with respect to the increase in thickness. The average transmittance in the visible region decreased from 67% to 47% as the thickness increased from ~70 nm to ~280 nm. Simultaneously, the conductivity of the films fell from 1.40 S∙cm⁻¹ to 0.27 S∙cm⁻¹. According to Haacke's figure of merit (FOM), a film with a maximum FOM value of about 1.72×10⁻⁷ Ω⁻¹ can be achieved when the thickness is about 70 nm (σ≈ 1.40 S·cm⁻¹ and T_av. ≈67%).     

Properties of fluorine-doped tin oxide films prepared by an improved sol-gel process

Fluorine-doped tin oxide (FTO) films were prepared by an improved sol-gel process, in which FTO films were deposited on glass substrates using evaporation method, with the precursors prepared by the conventional sol-gel method. The coating and sintering processes were combined in the evaporation method, with the advantage of reduced probability of films cracking and simplified preparation process. The effects of F-doping contents and structure of films on properties of films were analyzed. The results showed the performance index (Φ_TC=3.535×10⁻³ Ω⁻¹ cm) of the film was maximum with surface resistance (R_sh) of 14.7 Ω cm⁻¹, average transmittance (T) of 74.4% when F/Sn=14 mol%, the reaction temperature of the sol was 50 °C, and the evaporation temperature was 600 °C in muffle furnace, and the film has densification pyramid morphology and SnO_2−xF_x polycrystalline structure with tetragonal rutile phase. Compared with the commercial FTO films (Φ_TC=3.9×10⁻³ Ω⁻¹ cm, R_sh=27.4 Ω cm⁻¹, T=80%) produced by chemical vapor deposition (CVD) method, the Φ_TC value of FTO films prepared by an improved sol-gel process is close to them, the electrical properties are higher, and the optical properties are lower.     

Direct synthesis of quaternary Cd(Zn,S)Se thin films: Effects of composition

In the present communication, the binary CdSe and quaternary Cd_1-xZn_xSe_1-yS_y (0 ≤ x = y ≤ 0.35) thin films were synthesized using a chemical bath deposition. Thin film deposition was carried out at the optimized conditions (pH = 10 ± 0.1, deposition temperature = 70 ± 0.1 °C, deposition time = 100 min and substrate rotation speed = 65 ± 2 rpm). X-ray diffraction studies confirmed hexagonal-wurtzite crystal structure with the formation of quaternary Cd(Zn, S)Se phase along with binary CdSe, CdS, ZnS and ZnSe, phases of the as-grown Cd_1-xZn_xSe_1-yS_y thin films. Elemental analysis showed presence of Cd²⁺, Zn²⁺, S^2- and Se^2- in the deposited films. Fourier transform infrared spectroscopy shown the bands at 911.15 cm⁻¹ – 901.62 cm⁻¹ which are assigned to the stretching frequency of Cd–Se bond. Scanning electron microscopy show transformation of the microstructure from globular crystallites to a rhomboid flake like network. The electrical conductivity was typically ≈ 10⁻⁷ Ω⁻¹ cm⁻¹. At low temperatures, the conduction was by variable range hopping, and this changed to thermally activated grain boundary dominated conduction for T > 350 K.     

Effect of the film thickness on the crystal structure and ferroelectric properties of (Hf0.5Zr0.5)O2 thin films deposited on various substrates

In this work, the effect of the film thickness on the crystal structure and ferroelectric properties of (Hf_0.5Zr_0.5)O₂ thin films was investigated. The thin films were deposited on (111) Pt-coated SiO₂, Si, and CaF₂ substrates with thermal expansion coefficients of 0.47, 4.5, and 22×10⁻⁶/°C, respectively. From the X-ray diffraction measurements, it was found that the (Hf_0.5Zr_0.5)O₂ thin films deposited on the SiO₂ and CaF₂ substrates experienced in-plane tensile and compressive strains, respectively, in comparison with the films deposited on the Si substrates. For films deposited on all three substrates, the volume fraction of the monoclinic phase increased with increasing film thickness, with the SiO₂ substrate having the lowest monoclinic phase volume fraction at all film thicknesses tested. The grain size of the films, which is an important factor for the formation of the ferroelectric phase, remained almost constant at about 10 nm in diameter regardless of the film thickness and type of substrate utilized. Ferroelectricity was observed for the 17 nm-thick films deposited on SiO₂ and Si substrates, and the maximum remanent polarization (P_r) value of 9.3 µC/cm² was obtained for films deposited on the SiO₂ substrate. In contrast, ferroelectricity with P_r=4.4 µC/cm² was observed only for film on SiO₂ substrate in case of 55 nm-thick films. These results suggest that the films under in-plane tensile strain results in the larger ferroelectricity for 17 nm-thick films and have a ferroelectricity up to 55 nm-thick films.     

Influence of annealing temperature on the phase composition and optical properties of CuInS2 films

We studied the growth of CuInS₂ thin films by single-source evaporation of CuInS₂ powder in a high-vacuum system with a base pressure of 10^?3 Pa. After evaporation, the films were annealed in a sulfur atmosphere at temperatures from 200 to 500 °C for 1 h. XRD curves and Raman spectra of the films demonstrated that chalcopyrite CuInS₂ was the major crystalline phase. The morphology of Cu_xS exhibited a star-like structure, which we report for the first time. The phase composition and optical properties of our polycrystalline thin films were effectively modified by annealing in S. For films annealed at 200 and 350 °C, a secondary CuIn₁₁S₁₇ phase appeared, which may be related to solid-state reaction in the S atmosphere. This secondary CuIn₁₁S₁₇ phase has not been widely reported in previous studies. After annealing at 500 °C, only a chalcopyrite phase was detected, with bandgap energy of 1.46 eV, which is nearly identical to the optimal bandgap energy (1.5 eV) of single-crystal CuInS₂. This indicates that the composition of the CuInS₂ film annealed at 500 °C was nearly stoichiometric. The bandgap of the samples first increased and then decreased with increasing annealing temperature, which may be attributed to an increase in grain size, the secondary CuIn₁₁S₁₇ phase, and deviation from stoichiometry.     

Investigations on phosphorus doped amorphous/nanocrystalline silicon films deposited by a filtered cathodic vacuum arc technique in the presence of hydrogen gas

Phosphorus doped amorphous/nanocrystalline silicon (a-Si:H/nc-Si:H) thin films have been deposited by a filtered cathodic vacuum arc (FCVA) technique in the presence of hydrogen gas at different substrate temperatures (T_s) ranging from room temperature (RT) to 350 °C. The films have been characterized by using X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, dark conductivity (σ_D), activation energy (ΔE), optical band gap (E_g) and secondary ion mass spectroscopy. The XRD patterns show that RT grown film is amorphous in nature but high temperature (225 and 350 °C) deposited films exhibit nanocrystalline structure with (111) and (220) crystal orientations. The crystallite size of higher temperature grown silicon film evaluated was between 13 and 25 nm. Raman spectra reveal the amorphous nature of the film deposited at RT, whereas higher temperature deposited films show crystalline nature. The crystalline volume fraction of the silicon film deposited at higher temperatures (225 and 350 °C) was estimated to be 58 and 72%. With the increase of T_s, the bonding configuration changes from mono-hydride to di-hydride as revealed by the FTIR spectra. The values of σ_D, ΔE and E_g of silicon films deposited at different T_s were found to be in the range of 5.37×10⁻⁴–1.04 Ω⁻¹ cm⁻¹, 0.05–0.45 eV and 1.42–1.83 eV, respectively. Photoconduction of 3.5% has also been observed in n-type nc-Si:H films with the response and recovery times of 9 and 12 s, respectively. A n-type nc-Si:H/p-type c-Si heterojunction diode was fabricated which showed the diode quality factor between 1.6 and 1.8.     

Effects of ammonia concentration on property of Cd1−xZnxS films by chemical bath deposition

Cd_1−xZn_xS thin films were grown on soda–lime glass substrates by chemical-bath deposition (CBD) at 80 °C with stirring. All the samples were annealed at 200 °C for 60 min in the air. The crystal structure, surface morphology, thickness and optical properties of the films were studied with transmission electron microscopy (TEM), X-ray diffraction (XRD), scanning electron microscopy (SEM), step height measurement instrument and spectrophotometer respectively. The results revealed that Cd_1−xZn_xS thin films had cubic crystal structure and the intensity of the diffraction peak increased gradually as ammonia concentration rose and the grain size varied from 5.1 to 8.3 nm. All of Cd_1−xZn_xS thin films had a granular surface with some smaller pores and the average granule sizes increased from 92 to 163 nm with an increase in ammonia concentration. The Cd_1−xZn_xS thin films had the highest transmittance with ammonia concentration of 0.5 M L⁻¹, whose thickness was 50 nm and band gap was 2.62 eV.     

Control of the Ti diffusion in Pt/Ti bottom electrodes for the fabrication of PZT thin film transducers

For the achievement of microactuators based on piezoelectric thin films, a Pt/Ti/Si bottom electrode is widely used. This study presents the experimental results for Ti out-diffusion in Pt and Si for both sputtered Pt/Ti and Pt/TiO_x electrodes. These results have been compared before and after a rapid thermal annealing (RTA). The diffusion has been characterized by secondary ion mass spectroscopy (SIMS) analysis using Cs⁺ as a primary ion source. The Pt orientation has been observed by XRD measurements. Ti thin films (20 nm) have been sputtered in pure Ar whereas TiO_x films have been obtained by reactive sputtering in a mixture of Ar/O₂ (90/10). Finally, the Pt (100 nm) has been sputtered without vacuum breaking. After RTA (400°C, 30 s, in N₂), the Pt film exhibited a (1 1 1) orientation for both Ti and TiO_x adhesion films. The roughness of the Pt film measured by AFM with TiO_x underlayer was 80% less than that of the Pt/Ti bi-layer. The TiO_x film, as shown by SIMS analysis, has drastically reduced the diffusion of Ti in both Pt and Si. This phenomenon is accompanied by a very low Pt roughness. These results are analyzed in terms of diffusion and regrowth mechanisms inside the Pt film.     

Polymer films with multilayer low-E coatings

The paper presents the experimental results on depositing a multilayer low-emissivity (low-E) coating with oxide–metal–oxide structure on polyethylene terephthalate (PET) and polyethylene (PE) films by magnetron sputtering. The TiO₂/ZnO:Ga/Ag/ZnO:Ga/TiO₂ coating on the PET film with high water-resistance and capability to be used outside of sealed double-glazed panes was proposed. The optimal thickness of coating layers was experimentally determined. The coating with the optimal structure has 82% transmittance over the visible spectrum and 91% reflection over the infrared spectrum. The window with a PET film and low-E coating was investigated in terms of heat engineering. It was revealed that heat transfer resistance increased up to 0.73 m² °C W⁻¹ for the windows with a PET film and low-E coating. Heat transfer resistance of the windows without a polymer film was 0.38 m² °C W⁻¹. The water-resistant ZnO:Ga/Ag/ZnO:Ga/SiO₂ coating on a PE film with 77% transmittance and 91–92% reflection in the IR range was proposed to be used as greenhouse covering material. The possibility of using the PE film with a low-E coating to reduce heat loss in greenhouses and enhance yielding capacity was demonstrated.     

Study of structural,electrical and photoconductive properties of F and P co-doped SnO2 transparent semiconducting thin film deposited by spray pyrolysis

Transparent conducting phosphorus–fluorine co-doped tin oxide (SnO₂:(P, F)) thin films have been deposited onto preheated glass substrates using the spray pyrolysis technique by the various dopant quantity of spray solution. The [F/Sn] atomic concentration ratio (x) in the spray solution is kept at value of 0.7 and the [P/Sn] atomic ratio (y) varied at values of 0, 0.001, 0.005, 0.01, 0.02, 0.04, 0.06, and 0.10. The structural, morphological, X-ray diffraction, electrical, optical and photoconductive properties of these films have been studied. It is found that the films are polycrystalline in nature with a tetragonal crystal structure corresponding to SnO₂ phase having orientation along the (110) plane and polyhedrons like grains appear in the FE-SEM image. The average grain size increases with increasing P-dopant concentration. The compositional analysis of FTO:P thin films were studied using EDAX. The Hall effect measurements have shown n-type conductivity in all deposited films. The lowest sheet resistance and highest the carrier concentration about 6.4 Ω/□ and 7.4×10²², respectively, were obtained for the film deposited with y=[P/Sn]=0.01. The films deposited with y=0.04 phosphorus-doped SnO₂:F shows 68% optical transparency. From the photoconductive studies, the P-doped films exhibited sensitivity to incident light especially in y=0.04. The electrical resistivity and carrier concentration vary in rang 6.2×10⁻⁴ to 21.1×10⁻⁴ Ω cm and 7.4×10²² to 1.3×10²² cm⁻³, respectively.     

Sulfur stoichiometry driven chalcopyrite and pyrite structure of spray pyrolyzed Cu-alloyed FeS2 thin films

We report on fabrication of Cu_xFe_1−xS₂ (CFS) thin films using chemical spray pyrolysis followed by post-sulfurization. Post-sulfurized CFS films were grown with compact and good crystalline texture. The sulfur stoichiometry in CFS films was found to be crucial for determination of its crystal structure. The sulfur deficient CFS films were driven to chalcopyrite CFS (CH-CFS) structure whereas the sulfur cured CFS films were grown with Cu-incorporated pyrite CFS (P-CFS) structure which was confirmed by X-ray diffraction and Raman spectroscopy analysis along with UV–vis spectroscopy measurement. Electrical characterizations of both types of CFS films revealed p-type conductivity with carrier concentration in the range of 10¹⁸–10²⁰ cm⁻³ and mobility of 0.5–9 cm² V⁻¹ s⁻¹. The band gaps of CFS films of CH-CFS structure (0.885–0.949 eV) were found to be less than that of P-CFS structure (0.966–1.156 eV), which indicates its potential application for thermoelectric and photovoltaic devices.