Preparation and characterization of Cu(In,Ga)Se2 thin film solar cell by printing technique with powders of quaternary alloy and NaF

This study prepared Cu(In,Ga)Se₂ (CIGS) thin films on bi-layer Mo coated soda-lime glass, by printing with mixed powders of CIGS quaternary alloy (average partial size of 680 nm) and NaF. A single-stage annealing process was performed to form a CIGS chalcopyrite phase. Experimental results show stoichiometric ratios of Cu/(In+Ga) = 0.94 and Ga/(In+Ga) = 0.26 in the CIGS film. The resistivity of the sample was 12.69 Ω cm, with a carrier concentration of 9.34 × 10¹⁶ cm^?3, and mobility of 5.27 cm² V^?1 s^?1. The CIGS film exhibited p-type conductivity. The incorporation of 1 wt% NaF in the CIGS powder produced a chalcopyrite structure with the best crystalinity. Raman analysis identified a number of phases, including CuInSe₂ and CuIn₃Se₅. The CIGS solar cells with AZO/i-ZnO/CdS/CIGS/Mo/soda-lime glass structure were fabricated. The CIGS thin film solar cells conversion efficiency of 1.23 % on 1 × 1.5 cm².     

Alternative sodium sources for Cu(In,Ga)Se2 thin-film solar cells on flexible substrates

Sodium (Na) is an important doping element for Cu(In,Ga)Se₂ (CIGS) solar cells. However, when using Na-free flexible substrates like steel foil or polyimide film, it is necessary to ensure an efficient supply of sodium to achieve high cell efficiencies. The common incorporation methods for Na on these Na-free substrates are either to deposit a Na-containing precursor layer (e.g. NaF) onto the molybdenum (Mo) back contact prior to CIGS growth or to coevaporate a Na compound during CIGS growth. Another way is to incorporate sodium after CIGS growth by a post-deposition treatment with NaF. In this work, we tested two alternative Na doping methods which are well suited for a production line due to their easy controllability. One approach is to dope the molybdenum target with Na. With Na-doped Mo layers (Mo:Na) as the back contact, we could achieve efficiencies of 13.1% both on titanium (Ti) and stainless Cr steel foil using a single-stage inline CIGS process. With a low-temperature single-stage CIGS process on polyimide (PI) we reached an efficiency of 11.2% using a Mo:Na back contact. Another doping method involves sol-gel-deposited silicon oxide layers which contain Na (SiO_x:Na). We have successfully deposited these sol-gel layers onto stainless steel foil by a roll-to-roll (R2R) method with short annealing times as needed in production. With these SiO_x:Na layers we could achieve efficiencies of 13.7% on stainless steel foil and 11.5% on mild steel sheet using a single-stage inline CIGS process.     

Pulvermetallurgisch hergestellte Sputtertargets

Sputtering Targets Made by Powder Metallurgy for CIGS Thin Film Photovoltaics Thin film deposition by magnetron sputtering is an important process step in the manufacturing of CIGS thin film solar cells. They are based on the semiconductor Cu(In,Ga)Se₂ and comprise a back contact of molybdenum. This article describes the manufacturing and application of sputtering targets made from molybdenum (Mo), MoNa and CuGa used in the CIGS production. The powder‐metallurgical manufacturing route for sputtering targets is introduced using Mo as example. Sputtering targets produced by pressing‐sintering‐deformation show the highest material purity and density. MoNa targets are a composite material made from a sodium (Na) compound and Mo powder. Sputtering a MoNa layer is an easy solution to dope the CIGS absorber with small amounts of Na. CuGa targets made by powder metallurgy show a much finer microstructure compared to casted targets, which results in a smooth sputtering surface and a more homogeneous lateral film composition.     

Growth and characterizations of dual ion beam sputtered CIGS thin films for photovoltaic applications

The growth of CIGS thin films on soda-lime glass substrates at different substrate temperatures by dual ion beam sputtering system in a single-step route from a single quaternary sputtering target with the composition of Cu (In_0.70 Ga_0.30) Se₂ was reported. The effects of the substrate temperature on structural, optical, morphological and electrical properties of CIGS films were investigated. Stoichiometry of one such film was investigated by X-ray photoelectron spectroscopy. All CIGS films had demonstrated a strong (112) orientation located at 2θ ~26.70^o, which indicated the chalcopyrite structure of films. The value of full-width at half-maximum of (112) peak was reduced from 0.58° to 0.19° and crystallite size was enlarged from 14.98 to 43.05 nm as growth temperature was increased from 100 to 400 °C. However, atomic force microscope results showed a smooth and uniform surface at lower growth temperature and the surface roughness was observed to increase with increasing growth temperature. Hall measurements exhibited the minimum film resistivity of 0.09 Ω cm with a hole concentration of 2.42 × 10¹⁸ cm^?3 and mobility of 28.60 cm² V^?1 s^?1 for CIGS film grown at 100 °C. Film absorption coefficient was found to enhance nominally from 1 × 10⁵ to 2.3 × 10⁵ cm^?1 with increasing growth temperature from 100 to 400 °C.     

Cu(In,Ga)Se2 thin films prepared from stacked precursors by post-selenization process

The copper–indium–gallium metallic precursors were fabricated corresponding to the sequence CuGa/CuIn/CuGa/soda-lime-glass by sequential direct current magnetron sputtering. The as-sputtered precursors were comprised of Cu₁₁In₉, Cu₁₆In₉, In, Cu₉Ga₄, and CuGa₂ phases, which may be closely correlated to the deposition sequences of multi-layered metallic precursors. Cu(In, Ga)Se₂ (CIGS) absorber films were prepared from the stacked precursors by post-selenization process with solid Se powder, and the morphological, structural, and compositional properties were investigated. The as-selenized CIGS film exhibited a smooth, compact, and densely packed morphology with well-defined and faceted crystal grains. The film was Cu-deficient and had a low Ga content. X-ray diffractometer results indicated the formation of single-chalcopyrite structure CIGS absorber layers. Depth-resolved Raman patterns showed the formation of a dominant CIGS phase in the as-selenized layer, and an ordered vacancy compound phase like Cu(In, Ga)₃Se₅ at the surface and inner region. With increasing the sputtering time, the full width at half maximum of the chalcopyrite A₁ Raman peak increased. Band broadening can be interpreted as a result of a higher density of defects within the chalcopyrite CIGS phase. The A₁ peak shifts with increasing sputtering depths were not apparent, which was related to a uniform distribution of Ga in the CIGS thin film.     

Fabrication of CIGS solar cells with a Na-doped Molayer on a Na-free substrate

The photovoltaic properties of CIGS cells on an alumina substrate were improved through the use of Na-doped Mo as the bottom layer of a Mo back contact. Na was supplied to the CIGS bulk region from an alumina/Na-doped Mo/Mo structure, similar to the Na diffusion from soda-lime glass. The diffusion of Na from the Na-doped Mo was controlled effectively compared to that from Soda-lime glass (SLG). The present results indicate that Na-doped Mo acts as a Na source material and that the Na amount can be controlled by adjustment of thickness of Na-doped Mo layer, without the use of an alkali barrier layer. The highest conversion efficiency of 13.34% (J_sc = 34.62 mA/cm², V_oc = 0.58 V and FF = 66%) for an active area of 0.45 cm² on an alumina substrate was obtained for 100 nm Na-doped Mo/1000 nm Mo.     

Non-toxic selenization using thermal annealing for CuGa/In bi-layer precursors deposited by sputtering

Cu(In_xGa_1?x)Se₂ (CIGS) thin films were produced using a two-step sputtering process consisting of precursor formation and selenization. In the first stage, we prepared Cu_0.75Ga_0.25/In bi-layer precursors by direct current sputtering on Mo/soda-lime glass substrates. In the second stage, the stacked precursors were selenized using non-toxic Se pellets in a graphite box in which the temperature was controlled at 475–680 °C during rapid thermal annealing. We investigated the effect of thermal annealing temperature on Ga distribution and the crystallinity of the Cu(In_xGa_1?x)Se₂ films. Thermal annealing significantly affected the distribution of Ga atoms. At low temperatures, segregation of Ga atoms into the CIGS/Mo interface and an absence of Ga content on the surface were observed. In addition, a phase-separated CuInSe₂/CuGaSe₂ structure and incomplete selenization phases were observed. At high temperatures, CIGS films were formed with the proper distribution of Ga content.     

Optimization of Zn(O,S)/(Zn,Mg)O buffer layer in Cu(In,Ga)Se<Subscript>2</Subscript> based photovoltaic cells

Device modeling of CIGS based thin film solar cell with Zn(O,S)/(Zn,Mg)O buffer layer was simulated in order to find the optimum ratios of magnesium in (Zn_1?x,Mg_x)O and oxygen in Zn(O_y,S_1?y) which led to the optimized values of x = 0.1?0.25 and y = 0.5?0.6. When the oxygen content of Zn(O,S) was lower than 30 %, the recombination at Zn(O,S)/CIGS interface became prominent and J_SC was severely limited. It was found that the V_OC is approximately independent of magnesium content in (Zn,Mg)O and oxygen content in Zn(O,S) layers, and the efficiency is highly affected by the fill factor. Also studied were the effect of thicknesses of (Zn,Mg)O and Zn(O,S) layers while the x and y were set at x = 0.2 and y = 0.6. Our simulations show that the optimum range for thickness of the (Zn,Mg)O layer is from 70 to 100 nm, while it is 20–30 nm for the Zn(O,S) layer.     

Selenization of CIS and CIGS layers deposited by chemical spray pyrolysis

Cu(In_1???xGa_x)Se₂ (CIGS) thin films with x?=?0 (CIS) and x?=?0.3 (CIGS) were prepared on Mo-coated glass substrate by using chemical spray pyrolysis at a substrate temperature of 350 °C, followed by selenization treatment at 550 °C in selenium environment under N₂ gas flow. X-ray diffraction patterns of as-deposited CIGS layers on Mo showed polycrystalline chalcopyrite phase with an intense (112) plane. Splitting of (204)/(220) and (116)/(312) planes for the film with x?=?0.3 reveals deviation of tetragonal nature. Field emission scanning electron microscopy cross-sectional images of selenized films showed clear re-crystallization of grains. During the selenization process of the CIGS absorber, a thin interface layer of MoSe₂ is formed. Line mapping of Mo/CIGS layer showed more gallium segregation at the interface of back contact resulting in band gap grading. Chemical composition and mapping of the as-deposited and selenized samples were determined by energy dispersive analysis of X-rays. This work leads to fabrication of low cost and large scale Mo/CIGS/CdS/ZnO/ZnO:Al device structure.     

The microstructural evolution in high sodium epitaxial sodium potassium niobate films deposited by low-temperature hydro-thermal method

Sodium potassium niobate [(K _x Na_1?x )NbO₃] films with high sodium composition (x = 0.06 and 0.24) were fabricated using a low-temperature (240 °C) hydro-thermal method on (001) niobium-doped strontium titanate (Nb-STO) substrate. The films were annealed subsequently at 600 °C. Thicknesses of the films were maintained in the range of ~800–1000 nm. Transmission electron microscopy-selected area electron diffraction studies revealed the appearance of super-spots, thereby confirming the tilting of the oxygen sub-lattice at both high Na compositions. The bright-field imaging and scanning transmission electron microscopy-energy dispersive spectroscopy elemental mapping revealed KNN film with K-rich interfacial regions and Na-rich top region of the film at both the high Na composition, whereas the potassium niobate (KNbO₃, x = 1) film showed no such oxygen sub-lattice tilting, and a sharp substrate-film interface was observed. The observed tilting of oxygen sub-lattice is a direct consequence of the reduced structural stability in high Na compositions in the KNN solid solutions.     

Effect of ZnO buffer layer on AZO film properties and photovoltaic applications

Aluminum-doped ZnO (AZO) transparent conducting films were deposited on glass substrates with and without intrinsic ZnO (i-ZnO) buffer layers by a home made and low cost radio-frequency (RF) magnetron sputtering system at room temperature in pure argon ambient and under a low vacuum level. The films were examined and characterized for electrical, optical, and structural properties for the application of CIGS solar cells. The influence of sputter power, deposition pressure, film thickness and residual pressure on electrical and optical properties of layered films of AZO, i-ZnO and AZO/i-ZnO was investigated. The optimization of coating process parameters (RF power, sputtering pressure, thickness) was carried out. The effects of i-ZnO buffer layer on AZO films were investigated. By inserting thin i-ZnO layers with a thickness not greater than 125 nm under the AZO layers, both the carrier concentration and Hall mobility were increased. The resistivity of these layered films was lower than that of single layered AZO films. The related mechanisms and plasma physics were discussed. Copper indium gallium selenide (CIGS) thin film solar cells were fabricated by incorporating bi-layer ZnO films on CdS/CIGS/Mo/glass substrates. Efficiencies of the order of 7–8% were achieved for the manufactured CIGS solar cells (4–5 cm² in size) without antireflective films. The results demonstrated that RF sputtered layered AZO/i-ZnO films are suitable for application in low cost CIGS solar cells as transparent conductive electrodes.     

Light emitting mechanisms dependent on stoichiometry of Si-rich-SiN<Subscript>x</Subscript> films grown by PECVD

Light emission and morphology of silicon-rich silicon nitride films grown by plasma-enhanced chemical vapor deposition were investigated versus film’s stoichiometry. The excess silicon content in the films was controlled varying the NH₃/SiH₄ gas flow ratio from 0.45 up to 1.0. High-temperature annealing was employed to form the silicon quantum dots (QDs) and to enhance the photoluminescence (PL) in visible spectral range. The PL spectrum was found to be complex. The competition of five PL bands leads to the non-monotonous variation of total PL peak position in the range of 1.55–2.95 eV when the Si excess content increases. The shape of PL spectra depends also on an excitation light wavelength. It is shown that for the films fabricated with R ≤ 0.56 and R ≥ 0.67 the dominant contribution into PL spectra is given by native SiN_x defects, whereas in the films obtained with R = 0.59–0.67 the Si-QDs form the main radiative channel. The highest PL intensity is detected in Si-rich SiN_x films grown at R = 0.59–0.67 as well. PL mechanisms are discussed in terms of the contribution of different radiative channels in the light emission process that can show the ways for the optimization of SiN_x light-emitting properties.     

CIGS thin films, solar cells, and submodules fabricated using a rf-plasma cracked Se-radical beam source

Coevaporated Cu(In,Ga)Se₂ (CIGS) film growth using a rf-plasma cracked Se-radical beam (R-Se) source leads to a significant reduction in the amount of raw Se source material wasted during growth and exhibits unique film properties such as highly dense, smooth surfaces and large grain size. R-Se grown CIGS solar cells also show concomitant unique properties different from conventional evaporative Se (E-Se) source grown CIGS cells. In the present work, the impact of modified surfaces, interfaces, and bulk crystal properties of R-Se grown CIGS films on the solar cell performance was studied. When a R-Se source was used, Na diffusion into CIGS layers was enhanced while a remarkable diffusion of elemental Ga and Se into Mo back contact layers was observed. Improvements in the bulk crystal quality as manifested by large grain size and increased Na concentration with the use of a R-Se source is expected to be effective to improve photovoltaic performance. Using a R-Se source for the growth of CIGS absorber layers at a relatively low growth temperature, we have successfully demonstrated a monolithically integrated submodule efficiency of 15.0% (17 cells, aperture area of 76.5 cm²) on 0.25-mm thick soda-lime glass substrates.     

A comparative study of structural,electrical and magnetic properties rare-earth (Dy and Nd)-modified BiFeO3

The polycrystalline samples of BiFeO₃ (BFO) and rare earth-modified bismuth iron oxide, Bi_0.95R_0.25FeO₃ (R = Nd, Dy) (BNFO, BDFO) are prepared by a standard high-temperature solid-state reaction technique. A preliminary x-ray structural analysis is carried out to examine the structural deformation and stability of rare earth-modified BFO. Room temperature surface morphologies and textures of the samples are recorded by a scanning electron microscope, which reveals the uniform distribution of the plate-and rod-shaped grains. Studies of dielectric and electric properties in a wide frequency (1 kHz–1 MHz) and temperature (30–400 °C) ranges using complex impedance spectroscopic method have provided many new results. The dielectric constant is found to be increases, and the tangent loss decreases as compared to BFO. The electrical polarizations (spontaneous and remnant) is found to be enhanced on rare-earth substitutions. Studies of ac conductivity suggest that the samples obey Jonscher’s universal power law. The enhancement of magnetization was observed in rare-earth doped samples compared to pure BFO.     

Enhancing the performance of electrodeposited CuInSe2 solar cells by suppressing secondary phases using sodium dodecyl sulfate

CuInSe₂ thin films were prepared on Mo-coated glass substrates using pulse electrodeposition with an aqueous solution containing sodium dodecyl sulfate (SDS) as an additive. The effect of SDS on the electrochemistry mechanism that inhibits secondary phases (Cu_xSe_y) was examined using cyclic voltammetry, which indicated that SDS can inhibit the reduction of Cu²⁺ and H₂SeO₃ and prohibit the formation of secondary phases. Scanning electron microscopy and atomic force microscopy revealed that the cracks and roughness of CuInSe₂ films decreased considerably after adding SDS into the electrolyte. The suppression of secondary phases was also observed using X-ray diffraction and Raman spectroscopy. The optical bandgap values of the CuInSe₂ films were measured using a UV–vis–NIR spectrophotometer; the bandgap values of the films deposited in the electrolyte with 0 and 1 mM SDS were approximately 0.96 and 1.05 eV, respectively. As expected, based on these differences, the CuInSe₂ solar cell with the Al/AZO/i-ZnO/CdS/CuInSe₂/Mo/glass structure derived from precursor film deposited in an electrolyte containing SDS demonstrated greater efficiency (η = 2.51 %) than that of the cell derived from precursor film deposited in an electrolyte without SDS (η = 0.63 %).     

One-step RF magnetron sputtering method for preparing Cu(In,Ga)Se<Subscript>2</Subscript> solar cells

Cu(In, Ga)Se₂ (CIGS) solar cell is one of the most promising thin film solar cells. However the marketization of the CIGS solar cells is hindered by the uncertainty of the element ratios. Traditional sputtering with post selenization is one of the most widespread methods to produce the CIGS solar cells. Nevertheless, the post selenization process is the most difficult part of this technique, which could lead to element mismatch and heterogeneous. To simplify the preparing process, Cu(In, Ga)Se₂ (CIGS) solar cells were prepared without post-selenization process by RF sputtering CIGS target with abundant Se element. We focus on the effect of working pressure, substrate temperature and sputtering power on the properties of CIGS solar cells. When CIGS thin film was deposited at 580 °C, 0.8 Pa working pressure and 160 W sputtering power, the solar cell showed the highest power conversion efficiency (PCE) of 5.77%, which is only 0.64% lower than that of the solar cell prepared by traditional sputtering with post selenization method, and the two kinds of solar cells have same structure without MgF₂ antireflection layer, but the one-step sputtering method could greatly simplify the manufacture process of the CIGS solar cells. Our work makes clear that element Se would run off almost half during the sputtering process. And the element atomic ratios and the photovoltaic properties could be controlled by changing the sputtering parameters.     

Structural, optical and non-linear optics properties of highly doped molybdenum indium oxide thin film

Molybdenum (Mo)-doped In₂O₃ thin film with 10 wt% was successfully prepared by evaporation method. After annealing at 600 °C the film changes it colour from very dark to a clear transparent film. SEM and AFM analysis reveal that the film is continuous with high metallic coverage >98 % and exhibits a granular structure with typical grain size of 50 nm. More interestingly, the film shows more than 90 % transparency from visible to near infrared region and with wide optical band gap of 4.26 eV. The widening of the band gap is due to the Burstein–Möss (BM) effect as Mo will occupy In sites within the structure of the film thus increasing the carrier concentration thus enhancing its electrical properties. The nonlinear optical properties of Mo-doped In₂O₃ film with glass substrate were investigated using z-scan technique. Under cw excitation the film exhibits large reverse saturation absorption and negative nonlinearities. The real and imaginary parts of third order susceptibility of the film were measured and found that the imaginary part which arises from the change in absorption is dominant.     

Cu(In,Ga)Se2 films prepared by sputtering with a chalcopyrite Cu(In,Ga)Se2 quaternary alloy and In targets

This study reports the successful preparation of Cu(In,Ga)Se₂ (CIGS) thin film solar cells by magnetron sputtering with a chalcopyrite CIGS quaternary alloy target. Bi-layer Mo films were deposited onto soda lime glass. A CIGS quaternary alloy target was used in combination with a stack indium target for compensating the loss of indium during annealing process. A one-stage annealing process was performed to form CIGS chalcopyrite phase. Experimental results show that the optimal adhesion strength, residual stress, and resistivity were obtained at a thickness ratio of 67% of bi-layer Mo films and a working pressure of 0.13 Pa. The CIGS precursor was layered through selenization at 798 K for 20 min. The stoichiometry ratios of the CIGS film were Cu/(In + Ga) = 0.91 and Ga/(In + Ga) = 0.23, which approached the device-quality stoichiometry ratio (Cu/(In + Ga) <0.95, and Ga/(In + Ga) <0.3). The resistivity of the sample was 11.8 Ωcm, with a carrier concentration of 3.6 × 10¹⁷ cm⁻³ and mobility of 1.45 cm²V⁻¹s⁻¹. The resulting film exhibited p-type conductivity.     

Surface modification of a Ti–7.5Mo alloy using NaOH treatment and Bioglass® coating

The objective of this study was to propose a surface modification for a low-modulus Ti–7.5Mo alloy to initiate the formation of hydroxyapatite (HA) during in vitro bioactivity tests in simulated body fluid (SBF). Specimens of commercially pure titanium (c.p. Ti) and Ti–7.5Mo were initially immersed in a 15 M NaOH solution at 60°C for 24 h, resulting in the formation of a porous network structure composed of sodium titanate (Na₂Ti₅O₁₁). Afterwards, bioactive Bioglass^® particles were deposited on the surface of NaOH-treated c.p. Ti and Ti–7.5Mo. The specimens were then immersed in SBF at 37°C for 1, 7 and 28 days, respectively. The apatite-forming ability of the NaOH-treated and Bioglass^®-coated Ti–7.5Mo was higher than that of the c.p. Ti under the same condition. The X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS) results indicated that the deposited amounts of calcium phosphate were much greater for the surface-treated Ti–7.5Mo than for the c.p. Ti, a finding attributable to or correlated with the higher pH value of the SBF containing surface-treated Ti–7.5Mo. Moreover, in the surface-treated Ti–7.5Mo, the pH value of the SBF approached a peak of 7.66 on the first day. A combination of NaOH treatment and subsequent Bioglass^® coating was successfully used to initiate in vitro HA formation in the surface of the Ti–7.5Mo alloy.     

Effect of W–Mo balance on long-term creep life of 9Cr steel

The effect of tungsten–molybdenum (W–Mo) balance on creep life has been investigated for five heats of martensitic 9Cr steel with 1.5 % Mo equivalent (= 1/2W + Mo) at 600, 650 and 700°C. The combination of W and Mo concentrations in the present steel is 3W–0Mo, 2.8W–0.1Mo, 2.4W–0.3Mo, 1.8W–0.6Mo and 0W–1.5Mo. The time to rupture t_r exhibits a monotonous increase with increasing the W–Mo balance parameter 1/2W/(1/2W + Mo), namely, with increasing W concentration and concomitantly with decreasing Mo. The increase in t_r with increasing 1/2W/(1/2W + Mo) becomes less significant at long times. The precipitation of Fe₂(W,Mo) Laves phase takes place preferentially at prior austenite grain boundaries during creep, which enhances the grain boundary (GB) precipitation hardening. The amount of Laves phase increases with increasing 1/2W/(1/2W + Mo). The coarsening of Laves phase takes place at long times during creep, which reduces the GB precipitation hardening.