CuIn1-xGaxSe2 Films Research Articles

Diffusion Coefficients of Selenium and Gallium during the Cu(In<sub>1-x</sub>Ga<sub>x</sub>)Se<sub>2</sub> Thin Films Preparation Process

Cu (In1-xGax)Se2 (CIGS) polycrystalline thin films with Ga-gradient structures were prepared by selenization of sputtered Cu-In-Ga precursors. The Ga contents of the as-selenized CIGS thin films were measured by EDS. With greater Ga content, the peaks in the diffraction pattern become broadened. Auger electron spectroscopy was used to measure the composition distribution of the Cu, In, Ga, and Se elements in the CIGS and CuInSe2 films. At 300°C, the diffusion coefficients DSe was approximately (6.7±1.0) x 10-16 m2s-1, and DGa was about (4.5±1.0) x 10-18 m2s-1. DSe are two orders of magnitude greater than DGa, which is also the reason why the selected CIGS film was almost completely selenized, but still able to keep certain Ga-grading profile. The temperature used in this work is within the low temperature range of the two-step selenization approach, which is more suitable for low-cost substrates like flexible substrates.

Influence of laser wavelength on the laser induced breakdown spectroscopy measurement of thin CuIn 1− xGaxSe2 solar cell films

Laser induced breakdown spectroscopy (LIBS) measurement of thin CuInxGa1−xSe2 (CIGS) films (1.2–1.9μm) with varying Ga to In ratios was carried out using the fundamental (1064nm) and second harmonic (532nm) wavelength Nd:YAG lasers (τ=5ns, spot diameter=150μm, top-hat profile) in air. The concentration ratios of Ga to In, xGa≡Ga/(Ga+In), of the CIGS samples ranged from 0.027 to 0.74 for which the band gap varied nearly proportionally to xGa from 0.96 to 1.42. It was found that the LIBS signal of 1064nm (1.17eV) wavelength laser was significantly influenced by xGa, whereas that of the 532nm (2.34eV) laser was consistent for all values of xGa. The observed dependency of the LIBS signal intensity on the laser wavelength was attributed to the large difference of photon energy of the two wavelengths that changed the absorption of incident laser energy by the film. The 532nm wavelength was found to be advantageous for multi-shot analysis that enabled depth profile analysis of the thin CIGS films and for improving measurement precision by averaging the multi-shot LIBS spectra.

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

The photoresponse of CuIn1−xGaxSe2 (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 12mM CuSO4, 25mM In2(SO4)3, 28mM Ga2(SO4)3, and 25mM 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 0.97eV 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.

Nd:YAG laser ablation characteristics of thin CIGS solar cell films

This work reports that the ablation characteristics of thin CuIn1−x Ga x Se2 (CIGS) solar cell film differ significantly with elemental composition and laser pulse energy. From in situ shadowgraphs measured during Nd:YAG laser (1,064 nm) irradiation of CIGS films and crater morphologies, it was found that strong surface evaporation is dominant for low Ga concentration films of which band gap is well below the photon energy. As the band gap of CIGS film becomes close to or over the laser photon energy due to increased Ga content, surface absorption diminishes and at low laser energy, laser heating of the film plays an important role. It is demonstrated that for the CIGS films with Ga/(Ga + In) ratio being approximately over 0.2, the laser irradiation leads to solid phase removal of the film due to thermomechanical fracture at low laser energy but to ablative evaporation at elevated energy.

Comparison of thin-film and bulk CuIn1 − xGaxSe2 samples by laser induced breakdown spectroscopy

The results for laser induced breakdown spectroscopy (LIBS) measurements of CuIn1−xGaxSe2 (CIGS) thin film samples deposited on Mo-coated soda–lime glass by co-evaporation technique and those of bulk form CIGS targets are reported. The LIBS intensity calibration results for the thin film and bulk CIGS samples were compared with respect to the concentration ratios. While the concentrations for thin film CIGS samples were measured by X-ray fluorescence, the compositions of bulk samples were provided by the manufacturer. It was found that the calibration results of Ga/(Ga+In) ratio obtained from the thin film and bulk form samples showed a good agreement at optimized measurement conditions. It is demonstrated that bulk samples can provide reliable reference composition data for the elemental analysis of thin CIGS films.

Formulation and Characterization of CuIn1-xGaxSe2 Ink for Gravure Offset Printing

CuIn1-xGaxSe2 (CIGS) paste for gravure offset printing was formulated by changing the amount of additives to determine the critical factor for gravure offset printing quality. With the addition of poly(vinyl pyrrolidone) (PVP), the viscosity and yield stress of CIGS ink decreased and shear thinning behavior disappeared. However, PVP effectively enhanced ink cohesion and elastic modulus, which critically affected the printability of the CIGS paste. Intense pulsed light (IPL) treatment in air made the microstructure of CIGS film denser without a phase change or oxidation. The gravure offset printing technique and IPL treatment were considered a good combination for a continuous and fast CIGS film growth process.

Polycrystalline Cu(In, Ga)Se2 Thin Films and PV Devices Sputtered From a Binary Target without Additional Selenization

This work presents a novel method to form polycrystalline CuIn1−xGaxSe2 (CIGS) thin film by co-sputtering of In–Se and Cu–Ga alloy targets without an additional selenization process. An attempt was also made to thoroughly elucidate the surface morphology, crystalline phases, physical properties, and chemical properties of the CIGS films by using material analysis methods. Experimental results indicate that CIGS thin films featured densely packed grains and chalcopyrite phase peaks of (112), (220), (204), (312), and (116). Raman spectroscopy analysis revealed chalcopyrite CIGS phase with Raman shift at 175cm−1, while no signal at 258cm−1 indicated the exclusion of Cu2-xSe phase. Devices built with these films exhibit efficiencies as high as 8.6%.

Effect of stacking type in precursors on composition, morphology and electrical properties of the CIGS films

The copper-indium-gallium (CIG) metallic precursors with different stacking type (A: CuGa/CuIn/CuGa/glass and B: CuInGa/CuIn/CuInGa/glass) were prepared onto glass substrates by magnetron sputtering method. In order to prepare Cu(In1−xGax)Se2 (CIGS) thin films, the CIG precursors were then selenized with solid Se powder using a three-step reaction temperature profile. The influence of stacking type in precursors on structure, composition, morphology and electrical properties of the CIGS films is investigated by X-ray diffraction, energy dispersive spectrometer, scanning electron microscope and Hall effect measurement. The results reveal that the stacking type of the precursor has a strong influence on composition, morphology and properties of the CIGS thin films. The atomic ratios of Cu/(In+Ga)/Se of the CIGS films A and B are 1.61:1:2.11 and 1.39:1:2.04, respectively. The better quality CIGS thin films can be obtained through selenization of metallic precursor of CuInGa/CuIn/CuInGa/glass. The CIGS films are p-type semiconductor material. The hole concentration, resistivity and hole mobility of the CIGS thin films is 2.51 × 1017 cm−3, 3.11 × 104 Ω cm and 19.8 cm2 V−1 s−1, respectively.

Impact of Se flux on the defect formation in polycrystalline Cu(In,Ga)Se2 thin films grown by three stage evaporation process

Cu(InxGa1−x)Se2 (CIGS) films, grown under various Se fluxes, have been investigated by the positron annihilation spectroscopy. The line-shape parameter (S) of the positron annihilation spectra was used to characterize the defects in the CIGS films. When Se flux was decreased, the S parameter at the surface and subsurface region of the films increased. This phenomenon was attributed to the increased concentration of the defect complex formed between Se and Cu vacancies. S parameter at the surface region was significantly higher than that in the bulk region of each film. It was explained with the model of compositional inhomogeneities along the depth of the film. Solar cell performance strongly correlated to the S parameter at the surface region of the CIGS films. Sufficient Se flux was found to be effective to reduce the S parameter, thereby, to suppress the defects in the films.

Selective removal of CuIn1−xGaxSe2 absorber layer with no edge melting using a nanosecond Nd : YAG laser

This paper reports that selective removal of a CuIn1−xGaxSe2 (CIGS) thin film on a Mo-coated glass substrate can be achieved with no edge melting or damage of the Mo layer using a nanosecond Nd : YAG laser with a wavelength of 1064 nm. It is shown that the two crucial parameters that determine the possibility of clean removal of only the CIGS layer are Ga concentration of the CIGS film and laser fluence. For CIGS films with Ga/(Ga+In) ratio greater than about 0.2 for which the band gap energy is close to or over the photon energy (1.17 eV), laser-induced thermal expansion proved to be the mechanism of film removal that drives an initial bulging of the film and then fracture into tens of micrometre sized fragments as observed in in situ shadowgraph images. The fracture-type removal of CIGS films was further verified by scanning electron micrographs of the craters showing that the original shapes of the CIGS polycrystals remain intact along the crater rim. A numerical simulation of film temperature under the irradiation conditions of selective removal was carried out to show that the magnitude of induced thermal stress within the film closely agreed to the yield strength of the CIGS thin film. The results confirmed that a nanosecond laser could be a better choice for P2 and P3 scribing of CIGS thin films if process conditions are properly determined.

Characterization of Cu(In,Ga)Se2 thin films prepared by RF magnetron sputtering using a single target without selenization

Cu(In1−xGax)Se2 (CIGS) thin films were prepared using a single quaternary target by RF magnetron sputtering. The effects of deposition parameters on the structural, compositional and electrical properties of the films were examined in order to develop the deposition process without post-deposition selenization. From X-ray diffraction analysis, as the substrate temperature and Ar pressure increased and RF power decreased, the crystallinity of the films improved. The scanning electron microscopy revealed that the grains became uniform and circular shape with columnar structure with increasing the substrate temperature and Ar pressure, and decreasing the RF power. The carrier concentration of CIGS films deposited at the substrate temperature of 500°C was 2.1×1017cm−3 and the resistivity was 27Ωcm. At the substrate temperature above 500°C, In and Se contents in CIGS films decreased due to the evaporation and it led to the deterioration of crystallinity. It was confirmed that CIGS thin films deposited at optimal condition had similar atomic ratio to the target value even without post-deposition selenization process.

Fabrication and centeracterization of ordered CuIn(1−x)Ga x Se2 nanopore films via template-based electrodeposition

Ordered CuIn(1−x)GaxSe2 (CIGS) nanopore films were prepared by one-step electrodeposition based on porous anodized aluminum oxide templates. The as-grown film shows a highly ordered morphology that reproduces the surface pattern of the substrate. Raman spectroscopy and X-ray diffraction pattern show that CIGS nanopore films had ideal chalcopyrite crystallization. Energy dispersive spectroscopy reveals the Cu-Se phases firstly formed in initial stage of growth. Then, indium and gallium were incorporated in the nanopore films in succession. Cu-Se phase is most likely to act as a growth promoter in the growth progress of CIGS nanopore films. Due to the high surface area and porous structure, this kind of CIGS films could have potential application in light-trapping CIGS solar cells and photoelectrochemical water splitting.

Reactive Sputtering Process for CuIn&lt;sub&gt;1-x&lt;/sub&gt;Ga&lt;sub&gt;x&lt;/sub&gt;Se&lt;sub&gt;2&lt;/sub&gt; Thin Film Solar Cells

CuIn1-xGaxSe2 (CIGS) thin films are grown on Mo/soda lime glass using a reactive sputtering process in which a Se cracker is used to deliver reactive Se molecules. The Cu and (In0.7Ga0.3)2Se3 targets are simultaneously sputtered under the delivery of reactive Se. The effects of Se flux on film composition are investigated. The Cu/(In+Ga) composition ratio increases as the Se flux increases at a plasma power of less than 30 W for the Cu target. The (112) crystal orientation becomes dominant, and crystal grain size is larger with Se flux. The power conversion efficiency of a solar cell fabricated using an 800-nm CIGS film is 8.5%.

Characterization of Cu(In,Ga)Se2 films deposited by single-step electron beam evaporation for solar cell applications

CuInxGa(1−x)Se2 (CIGS) thin films have been prepared by a single step of electron beam evaporation of a mixed alloy power made of high purity elemental copper, indium, gallium and selenium. The crystalline quality of CIGS films increases with increasing the substrate temperature, and stoichiometric CuIn0.7Ga0.3Se2 with chalcopyrite phase and (112) preferential orientation can be obtained at a substrate temperature of 650°C. X-ray diffractometer (XRD), scanning electron microscope (SEM) and absorption spectroscopy are used to characterize the obtained films. CuIn0.7Ga0.3Se2 films evaporated at 650°C is found to have a bandgap of about ∼1.11eV. In addition, two different acceleration voltages (5kV and 10kV) for the electron gun are also compared and a high acceleration voltage is found to be crucial in order to obtain stoichiometric CIGS films without Cu-deficiency. Our results suggest that the single-step electron beam evaporation at a high acceleration voltage could be potentially applied for fabrication of CIGS thin solar cells.

Ga-grading profiles formed by incorporation of gallium into Cu(In1−xGax)Se2 absorber thin films

Cu(In1−xGax)Se2 (CIGS) polycrystalline thin films were prepared by the two-stage method for two different precursors corresponding to the sequences CuGa/CuIn/CuGa and CuGa/In/Cu/CuGa, respectively. In order to obtain the required Ga-grading profile, co-evaporation of GaxSe for incorporation of gallium into CIGS was used. Experimental results show that CIGS films were prepared by the proposed process with a ‘notch’ Ga-graded distribution, and the surface morphologies of the films after gallium incorporation were significantly changed compared with that of the as-selenized films, and the process was found to have little effect on the film structure. No new impurity phases have been found. The main XRD peaks shows a slight shift to higher diffraction angles with increasing Ga content, which could be attributed to Ga atoms substituting for In atoms in the chalcopyrite structure. The result of the present work implied that the proposed process was feasible for forming a ‘notch’ Ga-graded structure in CIGS thin films.

The optical and structural properties of CuIn1−x Ga x Se2 thin films fabricated with various Ga contents by using the co-evaporation technique

The purpose of the present study is to investigate the optical and the structural properties of CuIn1−xGaxSe2 (CIGS) thin films with various Ga/[In+Ga] ratios grown by using the coevaporation technique. The Ga ratios were 0, 0.15, 0.29, 0.40, 0.56, 0.73, and 1. As the Ga/[In+Ga] ratio increased, the grain size of CIGS films decreased, and the X-ray diffraction (XRD) peaks of the CIGS films progressively shifted to higher diffraction angles. In the cross-sectional scanning electron microscopy (SEM) images, the grain size of the CIGS thin films near the molybdenum electrode was smaller than it was near the CIGS surface. These results indicate that the diffusion rates of elements were different. The photocurrent (PC) spectra also showed that the band gap energy of the CIGS films increased as the Ga/[In+Ga] ratio increased. Accordingly, the short-circuit current density (JSC) linearly decreased, and the open-circuit voltage (VOC) increased and saturated at high Ga/[In+Ga] ratios (x > 0.4) due to the defects on the CIGS films surface. A correlation between the Ga/[In+Ga] ratio and the number of surface defects for the CIGS films is discussed.

Two-step selenization of Cu-In-Ga precursors for CIGS thin-film solar cells by using a Se cracker

Cu(In1−xGax)Se2 (CIGS) thin-film solar cells have been fabricated using the CIGS compound prepared with a two-step selenization process at 400 °C followed by successive thermal annealing at 550 °C. The compositions of the Cu-In-Ga precursors were controlled by changing the thickness ratio of the Cu-In and the Cu-Ga films. After the selenization, the thickness of the CIGS film was approximately three times that of the precursor layer. X-ray diffraction peaks showed that the CIGS had the chalcopyrite structure with a preferential orientation in the (112) direction. The composition analysis using X-ray fluorescence revealed that the Cu/[III] and the Ga/[III] ratios of CIGS changed in ranges of 0.66 ∼ 0.80 and 0.11 ∼ 0.20, respectively, with variations in the precursor thickness. We obtained a power conversion efficiency of 3.91% from the fabricated CIGS solar cell.

Photoelectrochemical Characterization of CuInSe2 and Cu(In1−xGax)Se2 Thin Films for Solar Cells

Photoelectrochemical (PEC) studies of CuInSe2 (CIS) and Cu(In1−xGax)Se2 (CIGS) thin films prepared by electrodeposition and spray coating of nanocrystals were carried out. PEC reductions of methyl viologen (MV) in aqueous solution and ethyl viologen (EV) in acetonitrile were used to evaluate the thin-film performance by measuring the photocurrent. Deposition of a CdS layer on CIS and CIGS films strongly enhanced the photocurrent; for example, a nanocrystal-CIS film showed about 100 times higher photocurrent with a thin CdS layer. Capacitance measurements and Mott−Schottky plots were obtained to find the flat band potential. Incident photon to current conversion efficiencies (IPCE) and absorbed photon to current conversion efficiencies (APCE) obtained from PEC measurements were about 20% and 40−70%, respectively.

Effect of bath temperature on the properties of CuInxGa1−x Se2 thin films grown by the electrodeposition technique

Electrodeposition is a promising and low cost method to synthesize CuInx Ga1−x Se2 (CIGS)thin films as an absorber layer for solar cells. The effect of bath temperature on the properties of CIGS thin films was investigated in this paper. CIGS films of 1 μm thickness were electrodeposited potentiostatically from aqueous solution, containing trisodium citrate as a complexing agent, on Mo/glass substrate under a voltage of −0.75 V, and bath temperatures were varied from 20 to 60°C. The effects of bath temperature on the properties of CIGS thin films were characterized by X-ray diffraction (XRD) and scanning electron microscopy. XRD revealed the presence of the CuIni0.7Ga0.3 Se2 phase, the optimal phase for application in solar cells. The grain dimensions and crystallizability increase along with the increase of the bath temperature, and the films become stacked and homogeneous. There were few changes in surface morphology and the composition of the films.

Ps-laser scribing of CIGS films at different wavelengths

Continuous growth of the thin-film electronics market stimulates the development of versatile technologies for large-scale patterning of thin-film materials on rigid and flexible substrates, and laser technologies are a promising method to accomplish the scribing processes. Lasers with picosecond pulse duration were applied in scribing of complex multilayered CuInxGa(1−x)Se2 (CIGS) solar cells deposited on a polyimide substrate. The ablative properties of the films were examined as a function of the wavelength of laser radiation, pulse energy, and the irradiation dose. The selective removal of ITO and CIGS layers was achieved with 355 nm irradiation without any significant damage to the underlying layers in the ITO/CIGS/Mo/PI solar cell system. The 355 nm wavelength was also found to be favorable for scribing of absorber layer in a ZnO/CIGS/Mo/PI solar cell system. 266 nm radiation significantly modified the film structure due to high absorption. Extensive melt formation in the CIGS layer was found when 532 nm radiation was applied, though the trenches were smooth and crack-free.