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.     

High-frequency magneto-electrical properties of Zn1 − x − yAlxCoyO thin films

The Al doping effects on high-frequency magneto-electric properties of Zn_{1 − x − y}Al_xCo_yO (x = 0-10.65 at.%) thin films were systematically studied. In the current work, the Zn_{1 − x − y}Al_xCo_yO thin films were deposited by magnetron co-sputtering onto quartz substrates. The magneto-impedance spectra of the thin films were measured by an impedance analyzer. Among all the doped films studied, the thin film with 6.03 at.% Al-doping showed the highest ac conductivity and relaxation frequency. To characterize the relaxation mechanism underlying the magneto-electric properties, a Cole-Cole impedance model was applied to analyze the impedance spectra. The analyzed result showed that the magneto-impedance of the Zn_{1 − x − y}Al_xCo_yO is contributed by multiple processes of magnetization dynamics and dielectric relaxation. The results imply that Zn_{1 − x − y}Al_xCo_yO may be applicable for high-frequency magneto-electric devices.     

Preparation and characterization of CuIn1 − xGaxSe2 − ySy thin film solar cells by rapid thermal processing

This paper describes the synthesis and characterization of CuIn_1 − xGa_xSe_2 − yS_y (CIGSeS) thin-film solar cells prepared by rapid thermal processing (RTP). An efficiency of 12.78% has been achieved on ~ 2 µm thick absorber. Materials characterization of these films was done by SEM, EDS, XRD, and AES. J-V curves were obtained at different temperatures. It was found that the open circuit voltage increases as temperature decreases while the short circuit current stays constant. Dependence of the open circuit voltage and fill factor on temperature has been estimated. Bandgap value calculated from the intercept of the linear extrapolation was 1.1-1.2 eV. Capacitance-voltage analysis gave a carrier density of 4.0 × 10¹⁵ cm^− 3.     

Growth of single-phase Cu(In,Al)Se2 photoabsorbing films by selenization using diethylselenide

Selenization growth of purely single-phase, polycrystalline CuIn_1 − xAl_xSe₂ (0 ≤ x ≤ 0.26) alloy films was demonstrated using a less-hazardous metal-organic selenide, diethylselenide [(C₂H₅)₂Se: DESe], by simple thermal annealing of metal precursor. Approximately 2.0 µm thick alloy films exhibited X-ray diffraction peaks originating exclusively from the chalcopyrite structure. Transitions seen in the low temperature photoluminescence spectra were attributed to characteristic donor-acceptor pair emissions of the state-of-the-art CuIn_1 − xAl_xSe₂ photoabsorbing layers.     

Raman spectroscopy of CuIn1 − xGaxSe2 for in-situ monitoring of the composition ratio

Metal organic vapor deposition (MOCVD) is a well known method for preparing high quality and large area CuIn_1 − xGa_xSe₂ (CIGS) absorber layers. Some in-situ non-contact monitoring systems are needed when CIGS absorber layers are manufactured in industry. In this study, CuInSe₂ (CIS) and CIGS thin films with different composition ratios, [Cu]/[In + Ga], were prepared by MOCVD using [Me₂In(μ-SeMe)]₂, hexafluoroacetylacetonate Cu(I) (3,3-dimethyl-1-butene), trimethyl gallium and dimethyle diselenide as the In-Se single source, Cu, Ga and Se precursors, respectively. The Raman shift spectra of the films with various composition ratios were analyzed to produce a basic algorithm that can determine the composition ratios of CIS and CIGS thin films indirectly.     

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.     

Steady state and transient photoconductivity measurements in a-Se90 − xSb10Inx thin films

Amorphous thin films of Se_90 − xSb₁₀In_x (0 ≤ x ≤ 15) have been prepared by electron beam evaporation method. The steady state and transient photoconductivity measurements on the thin films of Se_90 − xSb₁₀In_x (0 ≤ x ≤ 15) were carried out at different levels of light intensities (500 lx-5000 lx) at room temperature (301 K). The plot of photocurrent (I_ph) versus light intensity (F) follows a power law I_ph ∝ F^γ. The value of exponent γ lies between 0.5 and 1.0, which indicates there exists a continuous distribution of localized states in the mobility gap of Se_90 − xSb₁₀In_x (0 ≤ x ≤ 15) thin films. For transient photoconductivity, when the samples were illuminated with light, the photocurrent reaches the maximum value during the first 5 s of exposure time and thereafter, it starts decreasing and becomes stable after 15 min of exposure. This kind of phenomenon is termed as photo-degradation of photocurrent. The results have been explained on the basis of charged defect model and the intercluster interaction model. The magnitude of photocurrent of the system a-Se₇₅Sb₁₀In₁₅ is higher than the parent system a-Se₉₀Sb₁₀. The photosensitivity shows a minimum value at 5 atomic percentage of indium (In) concentration, which is explained based on chemically ordered network model and the topological model.     

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.     

Electrical properties of lead-free thick film NTC thermistors based on perovskite-type BaCoxCo2xBi1 − 3xO3

Lead-free thick film negative temperature coefficient (NTC) thermistors based on perovskite-type BaCo^II_xCo^III_2xBi_1 − 3xO₃ (x ≤ 0.1) were prepared by mature screen-printing technology. The microstructures of the thick films sintered at 720 °C were examined by X-ray diffraction and scanning electron microscopy. The electrical properties were analyzed by measuring the resistance-temperature characteristics. For the BaBiO₃ thick films, the room-temperature resistivity is 0.22 MΩ cm, while the room-temperature resistivity is sharply decreased to about 3 Ω cm by replacing of Bi with a small amount of Co. For compositions 0.02 ≤ x ≤ 0.1, the values of room-temperature resistivity (ρ₂₃), thermistor constant (B_25/85) and activation energy are in the range of 1.995-2.975 Ω cm, 1140-1234 K and 0.102-0.111 eV, respectively.     

Characterization of Zn1 − xMgxO transparent conducting thin films fabricated by multi-cathode RF-magnetron sputtering

Transparent conducting thin films of Al-doped and Ga-doped Zn_1 − xMg_xO with arbitrary Mg content x were deposited on glass substrates by simultaneous RF-magnetron sputtering of doped ZnO and MgO targets, and their fundamental properties were characterized. MgO phase separation in Zn_1 − xMg_xO films was not detected by X-ray diffraction. The Zn_1 − xMg_xO films show high optical transparency in the visible region. Although the carrier density of the Zn_1 − xMg_xO films decreased with increasing x, the Zn_1 − xMg_xO films showed good electrical conductivity; electrical resistivity as low as 8 × 10^− 4 Ω ·cm was achieved for the Zn_0.9Mg_0.1O:Ga thin film.     

CuIn1 − xGaxSe2 photovoltaic devices for tandem solar cell application

CuIn_1 − xGa_xSe₂ (CIGS) solar cells show a good spectral response in a wide range of the solar spectrum and the bandgap of CIGS can be adjusted from 1.0 eV to 1.7 eV by increasing the gallium-to-indium ratio of the absorber. While the bandgaps of Ga-rich CIGS or CGS devices make them suitable for top or intermediate cells, the In rich CIGS or CIS devices are well suited to be used as bottom cells in tandem solar cells. The photocurrent can be adapted to the desired value for current matching in tandem cells by changing the composition of CIGS which influences the absorption characteristics. Therefore, CIGS layers with different [Ga]/[In + Ga] ratios were grown on Mo and ZnO:Al coated glass substrates. The grain size, composition of the layers, and morphology strongly depend on the Ga content. Layers with Ga rich composition exhibit smaller grain size and poor photovoltaic performance. The current densities of CIGS solar cells on ZnO:Al/glass varied from 29 mA cm^− 2 to 13 mA cm^− 2 depending on the Ga content, and 13.5% efficient cells were achieved using a low temperature process (450 °C). However, Ga-rich solar cells exhibit lower transmission than dye sensitized solar cells (DSC). Prospects of tandem solar cells combining a DSC with CIGS are presented.     

Study of the Al-grading effect in the crystallisation of chalcopyrite CuIn1−xAlxSe2 thin films

Chalcopyrite CuIn_1−xAl_xSe₂ (CIAS) thin films with an atomic ratio of Al/(In + Al) = 0.4 were grown by a two-stage process onto soda-lime glass substrates. The selenisation was carried out at different temperatures, ranging from 400 °C to 550 °C, for metallic precursors layers evaporated with two different sequences. The first sequence, C1, was evaporated with the Al as the last layer, while in the second one, C2, the In was the last evaporated element. The optical, structural and morphological characterisations led to the conclusion that the precursors sequence determines the crystallisation pathway, resulting in C1 the best option due to the homogeneity of the depth distribution of the elements. The influence of the selenisation temperature was also studied, finding 540 °C as the optimum one, since it allows to achieve the highest band gap value for the C1 sequence and for the given composition.     

Structural and optical properties of rock-salt Cd1 − xZnxO thin films prepared by thermal decomposition of electrodeposited Cd1 − xZnxO2

Cd_1 − xZn_xO nanocrystalline thin films with rock-salt structure were obtained through thermal decomposition of Cd_1 − xZn_xO₂ (x = 0, 0.37, 0.57, 1) thin films which were electrodeposited from aqueous solution at room temperature. X-ray diffraction results showed that the Zn ions were incorporated into rock salt-structure of CdO and the crystal lattice parameters decreased with the increase of Zn contents. The bandgaps of the Cd_1 − xZn_xO thin films were obtained from optical transmission and were 2.40, 2.51, 2.63 and 3.25 eV, respectively.     

The properties of transparent semiconductor Zn1 − xTixO thin films prepared by the sol-gel method

Transparent semiconductor thin films of Zn_1 − xTi_xO (0 ≦ x ≦ 0.12) were deposited on alkali-free glass substrates by the sol-gel method. The effects of Ti addition on the crystallization, microstructure, optical properties and resistivity of ZnO thin films were investigated. The as-coated films were preheated at 300 °C, and then annealed at 500 °C in air ambiance. X-ray diffraction results showed all polycrystalline Zn_1 − xTi_xO thin films with preferred orientation along the (002) plane. Ti incorporated within the ZnO thin films not only decreased surface roughness but also increased optical transmittance and electrical resistivity. In the present study, the Zn_0.88Ti_0.12O film exhibited the best properties, namely an average transmittance of 91.0% (an increase of ~ 12% over the pure ZnO film) and an RMS roughness value of 1.04 nm.     

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.     

Structural, electrical and optical properties of transparent Zn1 − xMgxO nanocomposite thin films

Zn_1 − xMg_xO thin films of various Mg compositions were deposited on quartz substrates using inexpensive ultrasonic spray pyrolysis technique. The influence of varying Mg composition and substrate temperature on structural, electrical and optical properties of Zn_1 − xMg_xO films were systematically investigated. The structural transition from hexagonal to cubic phase has been observed for Mg content greater than 70 mol%. AFM images of the Zn_1 − xMg_xO films (x = 0.3) deposited at optimized substrate temperature clearly reveals the formation of nanorods of hexagonal Zn_1 − xMg_xO. The variation of the cation-anion bond length to Mg content shows that the lattice constant of the hexagonal Zn_1 − xMg_xO decreases with corresponding increase in Mg content, which result in structure gradually deviating from wurtzite structure. The tuning of the band gap was obtained from 3.58 to 6.16 eV with corresponding increase in Mg content. The photoluminescence results also revealed the shift in ultraviolet peak position towards the higher energy side.     

Synthesis and magnetic properties of layered MnPSxSe3−x (0 < x < 3) and corresponding intercalation compounds of 2,2′-bipyridine

In this work, we synthesize a series of new MnPS_xSe_3−x (0 < x < 3) compounds by high temperature solid-state reaction and also obtain the corresponding intercalation compounds (Mn_1−yPS_xSe_3−x(bipy)_4y, x = 1.2, 1.8 and 2.4) via the intercalation of 2,2′-bipyridine with MnPS_xSe_3−x. XRD results confirm that MnPS_xSe_3−x compounds show the layered structure and can be regarded as the solid solution of MnPS₃ and MnPSe₃. Magnetic measurements indicate that MnPS_xSe_3−x compounds exhibit paramagnetism with negative Weiss constant in the paramagnetic temperature region, and an antiferromagnetic phase transition occurs at the Neel temperature. It is found that the magnetic properties of MnPS_xSe_3−x slab are dramatically changed after the intercalation of 2,2′-bipyridine, which is close related to the relative ratio of S and Se atom as well as the intralayered Mn²⁺ vacancies of MnPS_xSe_3−x slab.     

Electrical transport and structural study of CuCr1 − xMgxO2 delafossite thin films grown by pulsed laser deposition

The growth and properties of delafossites CuCr_1 − xMg_xO₂ thin films are examined. These films are grown by pulsed laser deposition. As a class of materials delafossites have received recent interest since some members show p-type behavior. While not considered true wide-bandgap materials due to a narrow indirect bandgap that fails to adsorb light due to a forbidden same parity transition, optical transparencies greater than 40% in the visible can be observed. In order to be useful for transparent device applications, CuCr_1 − xMg_xO₂ films are needed with low resistivity and high optical transparency. Epitaxial films of CuCr_1 − xMg_xO₂ were grown on c-sapphire, examining the effects of oxygen pressure and growth temperature on film properties. Films were realized with resistivity of ~ 0.02 Ω-cm and optical transparency of 40% in the visible. The formation of a problematic secondary minority spinel phase of (Cu,Mg)Cr₂O₄ is discussed. While conductivity increases substantially with Mg doping, the incidence of the spinel phase increases as well.     

Origin of defects in CuIn1 − xGaxSe2 solar cells with varied Ga content

A series of CuIn_1 − xGa_xSe₂ solar cells with varied Ga content (0 ≤ x ≤ 1) was prepared using a three-stage co-evaporation process. The grain sizes of these devices vary with gallium content, exhibiting a maximum for approximately x = 0.2, which does not coincide with the maximum of the solar conversion efficiency observed between 0.34 < x < 0.37 for these devices.Admittance spectroscopy and drive-level capacitance profiling measurements were performed yielding a defect level with an activation energy of E_a = 0.1 eV which is independent of the amount of Ga and the grain size respectively. This defect closely resembles the N1 defect level reported in the literature. Only for relatively high Ga contents (x > 0.7) an additional defect appears. An equivalent circuit model describing a parallel connection of bulk and grain boundary capacitors allows us to conclude that the detected shallow defect is not predominantly located at the grain boundaries.     

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.