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The current work presents a novel structure of epitaxially grown CuInGaSe2/SiO2 on n-Si substrate using the liquid phase epitaxy (LPE) technique. The electrical and dielectric properties of quaternary alloy CuInGaSe2/SiO2/n-Si were investigated as a Schottky barrier device. The effect of temperature, voltage, and frequency on the electric and dielectric parameters such as dielectric constant ɛ′, dielectric loss ɛ″, dielectric loss tangent tanδ, the real and imaginary part of modulus M′, M″, ac conductivity σac and series resistance Rs were studied by measuring the capacitance–voltage within the temperature range of (303–393 K) and DC voltage range (± 5 V). Both capacitance and conductance were highly affected by varying voltage, frequency, and temperature. Remarkably, capacitance has positive values at high frequencies while it takes negative values at low frequencies. The negative capacitance (NC) was observed for all temperatures. The thickness of the oxide layer was 84 × 10–9 nm. The density of states Nss, Ideality factor, series resistance, shunt resistance, and barrier height were assessed. The impedance spectrum investigation was tailored by suitable equal circuits comprising the contributions of grain and grain boundaries in the conduction mechanism. [ABSTRACT FROM AUTHOR]

CuInGaSe2 (CIGS) based compound semiconductors are among the leading materials for photovoltaic applications. Pulsed current electrochemical deposition is employed in this study to fabricate CIGS films wherein deposition is carried out from an electrolyte consisting chlorides of copper, indium, gallium and selenous acid at a pH around 2.2. The process also uses trisodium citrate which plays the role of complexing agent and aids in narrowing the reduction potentials of elements thereby easing the optimization to obtain desired composition. The precursor films are annealed in Ar at 550 °C and are characterized comprehensively. Results unveil that CIGS films possess compact agglomerated particle like morphology with an average particle size of ≈500 nm, copper-poor stoichiometric composition, and tetragonal chalcopyrite structure devoid of any unwanted secondary phases. Optical studies yield the bandgap to be ≈1.2 eV. Photoelectrochemical performance ascertains the photoactivity of CIGS under illumination. The low-cost pulse electrodeposition for obtaining pure quality CIGS films, which is also scalable and can be a great choice for fabrication of economic solar cells.

Cu(In,Ga)Se2 (CIGS) absorbers are prepared by direct current electrodeposition process followed by selenization of precursors. Selenization of electrodeposited layers is performed in a tubular furnace at 550 °C in elemental selenium atmosphere using Ar as carrier gas. The effect of evacuation of tube prior to the selenization on the formation of CIGS absorbers is studied. Characterization of CIGS absorbers reveals that the samples selenized without prior evacuation found to have excess MoSe2 formation at the CIGS/Mo interface leading to bulk cracks in underlying Mo back contact compared to their counterparts. Although the fabricated solar cells using the absorbers, prepared with and without evacuation, are observed to be photoactive, the cells from vacuum-based selenization showed improvement in performance compared to the cells from non-vacuum selenization. The process is further being improved to enhance the efficiency, which can pave way towards environmentally friendly low-cost solar cells. [ABSTRACT FROM AUTHOR]

Thin films of CuGaSe were deposited on n-Si (1 0 0) by rf magnetron sputtering from a stoichiometric CuGaSe2 target. The objective of this study was to characterize the thin film/Si heterojunction for potential photovoltaic applications, evaluate possible candidates for metal contacts and to establish whether heteroepitaxial growth could be achieved, particularly as the mismatch of lattice parameters corresponding to the base of the copper gallium selenide (CGS) tetragonal cell is quite close to that of Si, with a 2.9% mismatch. For this study, Si substrates were prepared by the standard Radio Corporation of America (RCA) cleaning procedure immediately followed by the deposition of CGS by sputtering at a substrate temperature of 600°C. The deposited thin-film stoichiometry and morphology were characterized by Rutherford backscattering spectroscopy (RBS) and transmission electron microscopy (TEM). Rutherford back scattering (RBS) analysis indicated a thin-film composition of Cu1Ga1Se1 indicating that the films were Se deficient, although channeling was not observed. The polycrystalline nature of the deposited thin film was established by cross-sectional TEM. An estimated 1.5-nm thick layer likely to be SiO x was observed at the CGS–Si interface. It is believed that this interfacial layer prevented heteroepitaxy CGS on Si. Additionally, circular metal contacts were deposited on the thin films and characterized by capacitance and current–voltage measurements. It was observed that Al and Ag contacts were rectifying, from which the thin-film carrier density was estimated to be ~5 × 1015 and ~7.68 × 1015 cm−3 with Al and Ag contacts, respectively. Au, Pt, W and Cr were ohmic, and Mo and Ni provided semi-ohmic contacts to CGS films.

Cu(In,Ga)Se2 (CIGS) solar cells in superstrate configuration promise improved light management and higher stability compared to substrate devices, but they have yet to deliver comparable power conversion efficiencies (PCEs). Chemical reactions between the CIGS layer and the front contact were shown in the past to deteriorate the p-n junction in superstrate devices, which led to lower efficiencies compared to the substrate-type devices. This work aims to solve this problem by identifying a buffer layer between the CIGS layer and the front contact, acting as the electron transport layer, with an optimized electron affinity, doping density and chemical stability. Using combinatorial material exploration we identified amorphous gallium oxide (a-GaO x) as a potentially suitable buffer layer material. The best results were obtained for a-GaO x with an electron affinity that was found to be comparable to that of CIGS. Based on the results of device simulations, it is assumed that detrimental interfacial acceptor states are present at the interface between CIGS and a-GaO x. However, these initial experiments indicate the potential of a-GaO x in this application, and how to reach performance parity with substrate devices, by further increase of its n-type doping density. [ABSTRACT FROM AUTHOR]

In this work, we report the fabrication method of sputtering from a CIGS quaternary target to obtain high-efficiency CIGS thin-film solar-cell devices. The as-deposited CIGS absorbers are prepared by sputtering from a CIGS target and a subsequent annealing treatment under the Ar + H2Se (5% H2Se) atmosphere is applied on the absorbers. A high conversion efficiency of 16.7% is achieved with an average conversion efficiency of 15.9% over 10 cm × 10 cm on soda-lime glass. This result shows sputtering directly from a quaternary CIGS target is a very promising fabrication method to obtain high-performance solar-cell devices and is suitable for the further fabrication of large-area devices and modules. J- V characteristic curve of the CIGS solar cell with the best efficiency achieved by sputtering from a quaternary target. [ABSTRACT FROM AUTHOR]

An optimized test structure to study rear surface passivation in Cu(In,Ga)Se2 (CIGS) solar cells by means of photoluminescence (PL) is developed and tested. The structure - illustrated in the abstract figure - is examined from the rear side. To enable such rear PL assessment, a semi-transparent ultra-thin Mo layer has been developed and integrated in place of the normal rear contact. The main advantages of this approach are (i) a simplified representation of a rear surface passivated CIGS solar cell is possible, (ii) it is possible to assess PL responses originating close to the probed rear surface, and (iii) a stable PL response as a function of air exposure time is obtained. In this work, PL measurements of such structures with and without rear surface passivation layers have been compared, and the measured improvement in PL intensity for the passivated structures is associated with enhanced CIGS rear interface properties. [ABSTRACT FROM AUTHOR]

The benefits of gallium (Ga) grading on Cu(In,Ga)Se2 (CIGS) solar cell performance are demonstrated by comparing with ungraded CIGS cells. Using drive-level capacitance profiling (DLCP) and admittance spectroscopy (AS) analyses, we show the influence of Ga grading on the spatial variation of deep defects, free-carrier densities in the CIGS absorber, and their impact on the cell's open-circuit voltage Voc. The parameter most constraining the cell's Voc is found to be the deep-defect density close to the space charge region (SCR). In ungraded devices, high deep-defect concentrations (4.2 × 1016cm-3) were observed near the SCR, offering a source for Shockley-Read-Hall recombination, reducing the cell's Voc. In graded devices, the deep-defect densities near the SCR decreased by one order of magnitude (2.5 × 1015 cm-3) for back surface graded devices, and almost two orders of magnitude (8.6 × 1014 cm-3) for double surface graded devices, enhancing the cell's Voc. In compositionally graded devices, the free-carrier density in the absorber's bulk decreased in tandem with the ratio of gallium to gallium plus indium ratio GGI = Ga/(Ga + In), increasing the activation energy, hindering the ionization of the defect states at room temperature and enhancing their role as recombination centers within the energy band. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

ZnO nanorod arrays placed within the architecture of the Cu(In,Ga)Se2 solar cell present promising concepts to maximize photoconversion by optical and electrical enhancement. Three different Cu(In,Ga)Se2 solar cell architectures utilizing ZnO nanorods are introduced. New technological insights are given for electrodeposited ZnO nanorod anti-reflection coatings, examining the reflectance of devices before and after encapsulation. For the first time, device results from superstrate and bifacial CIGSe solar cells incorporating ZnO nanorods are presented. The potential benefits as well as technological aspects of fabricating these new architectures are discussed. Scanning electron microscope picture of the cross-section of an industrially produced CIGSe solar cell with a ZnO nanorod anti-reflection coating. [ABSTRACT FROM AUTHOR]

The increasing importance of the Cu(In,Ga)Se2 based thin films photovoltaic industry claims for the development of new assessment and monitoring tools to answer the needs existing in the improvement of the control of the processes involved in the production of solar cells modules. In this frame, a strong interest has been given to the development methodologies for the assessment of the CIGS absorber, nevertheless advanced optical tools for the characterization of the other layers in the solar cells are still missing. In this work, we report a non-destructive optical methodology based on resonant Raman concepts that has been developed for the characterization of Al doped ZnO layers (AZO) that are used as window layer in Cu(In,Ga)Se2 solar cells. Doping the ZnO layer with Al leads to the presence of a characteristic defect induced band at 510 cm−1 spectral region. The correlation of the relative intensity of this band with the resistivity of the layers provides a fast and reliable tool for their electrical monitoring. Analysis of solar cells fabricated with layers of different conductivities has allowed demonstration at cell level of the proposed methodology for the determination of efficiency losses related to degradation of the resistivity of the AZO layers. [ABSTRACT FROM AUTHOR]

In this article, an environmentally friendly and non-vacuum electrostatic spray assisted vapor deposition (ESAVD) process has been developed as an alternative and low cost method to deposit CIGS absorber layers. ESAVD is a non-vacuum chemical vapor deposition based process whereby a mixture of chemical precursors is atomized to form aerosol. The aerosol is charged and directed towards a heated substrate where it would undergo decomposition and chemical reaction to deposit a stable solid film onto the substrate. A sol containing copper, indium, and gallium salts, as well as thiourea was formulated into a homogeneous chemical precursor mixture for the deposition of CIGS films. After selenization, both XRD and Raman results show the presence of the characteristic peaks of CIGSSe in the fabricated thin films. From SEM images and XRF results, it can be seen that the deposited absorbers are promising for good performance solar cells. The fabricated solar cell with a typical structure of glass/Mo/CIGSSe/CdS/i-ZnO/ITO shows efficiency of 2.82% under 100 mW cm−2 AM1.5 illumination. [ABSTRACT FROM AUTHOR]

Ultra-thin Cu(In,Ga)Se2 (CIGS) solar cells with an Al2O3 rear surface passivation layer between the rear contact and absorber layer frequently show a 'roll-over' effect in the J-V curve, lowering the open circuit voltage ( VOC), short circuit current ( JSC) and fill factor (FF), similar to what is observed for Na-deficient devices. Since Al2O3 is a well-known barrier for Na, this behaviour can indeed be interpreted as due to lack of Na in the CIGS absorber layer. In this work, applying an electric field between the backside of the soda lime glass (SLG) substrate and the SLG/rear-contact interface is investi-gated as potential treatment for such Na-deficient rear surface passivated CIGS solar cells. First, an electrical field of +50 V is applied at 85 °C, which increases the Na concentration in the CIGS absorber layer and the CdS buffer layer as measured by glow discharge optical emission spectroscopy (GDOES). Subsequently, the field polarity is reversed and part of the previously added Na is removed. This way, the J - V curve roll-over related to Na deficiency disappears and the VOC (+25 mV), JSC(+2.3 mA/cm2) and FF (+13.5% absolute) of the rear surface passivated CIGS solar cells are optimized. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Abstract: Utilizing the energy in the sun to efficiently split water into hydrogen and oxygen can have a huge beneficial impact on a future post-carbon energy system. There is still, however, some way to go before this concept will be fully competitive. At the heart of the problem is finding and designing materials that can drive the photoreaction in an efficient and stable way. In this work we demonstrate how CIGS (CuIn x Ga1−x Se2), can be used for photo reduction of water into hydrogen. CIGS, which is a proven good solar cell material, does not in itself have the appropriate energetics to drive the reaction to any larger extent. Here we show that by utilizing a solid state pn-junction for charge separation and a catalyst deposited on the surface, the efficiency is significantly improved and photocurrents of 6 mA/cm2 are demonstrated for the reduction reaction in the configuration of a photo-electrochemical cell. The stability of CIGS in water under illumination turns out to be a problem. In our present set-up, we demonstrate that separation between the charge carrier generation, which takes place in the solar cell, from the catalysis, which takes place in the electrolyte leads to improved stability, while keeping the essential functions of the processes. By incorporating appropriate charge separation layers and optimizing the catalytic conditions at the surface of the electrodes, photocurrents in excess of 20 mA/cm2 are reached for the reduction half reaction, demonstrating how essentially the full potential of CIGS as an efficient absorber material can be utilized in photocatalytic reduction of water into hydrogen. [Copyright &y& Elsevier]

Abstract: The growth of grain size of CuInGaSe2 and the Ga distribution in the thin film CuInGaSe2 solar cell devices fabricated using a sputtering CuInGa ternary target have been studied. It was observed, adding a thin CuGa layer on top of the surface of CuInGa ternary precursor would increase the Ga concentration, and thus the energy gap in the space-charge region after selenization. As a result, the open circuit voltage (V oc) of the device was increased by 15%. The SEM and XRD studies further show that the addition of a CuGa layer enhanced the growth of grain size of CuInGaSe2 during selenization and increased the conversion efficiency of the solar cell devices by 27% (from 6.3% to 8%). [Copyright &y& Elsevier]

Abstract: This study examined the effects of Ga content in the CIGS absorber layer on the properties of the corresponding thin films and solar cells fabricated using a co-evaporation technique. The grain size of CIGS films decreased with increasing Ga content presumably because Ga diffusion during the 2nd stage of the co-evaporation process is more difficult than In diffusion. The main XRD peaks showed a noticeable shift to higher diffraction angles with increasing Ga content, which was attributed to Ga atoms substituting for In atoms in the chalcopyrite structure. Band gap energy and the net carrier concentration of CIGS films increased with Ga/(In+Ga) ratios. Regarding the solar cell parameters, the short circuit current density (J SC) decreased linearly with Ga/(In+Ga) ratios due to the lack of absorption in the long-wavelength portion of the spectrum, while the open circuit voltage (V OC) increase with those. However, V OC values at high Ga/(In+Ga) regions (>0.35) was far below than those extrapolated from the low Ga contents regions, finally resulting in an optimum Ga/(In+Ga) ratio of 0.28 where the solar cell showed the highest efficiency of 15.56% with V OC, J SC and FF of 0.625V, 35.03 mAcm−2 and 0.71, respectively. [Copyright &y& Elsevier]

Abstract: We have developed a chemical process for incorporating copper into indium gallium selenide layers with the goal of creating a precursor structure for the formation of copper indium gallium diselenide (CIGS) photovoltaic absorbers. Stylus profilometry, EDX, Raman spectroscopy, XRD and SIMS measurements show that when indium gallium selenide layers are immersed in a hot copper chloride solution, copper is incorporated as copper selenide with no increase in the thickness of the layers. Further measurements show that annealing this precursor structure in the presence of selenium results in the formation of CIGS and that the supply of selenium during the annealing process has a strong effect on the morphology and preferred orientation of these layers. When the supply of Se during annealing begins only once the substrate temperature reaches ≈400 °C, the resulting CIGS layers are smoother and have more pronounced preferred orientation than when Se is supplied throughout the entire annealing process. [Copyright &y& Elsevier]

Abstract: A simple model for the generation of carriers by photons incident on a (linearly) decreasing band-gap material, such as has been described in recent CIGS solar cells, is developed. The model can be generalized for different cases such as increasing band-gap grading or for having a more complex band-gap profile. The model developed for direct band semiconductors such as CIGS or AlGaAs allows us to define an effective absorption coefficient, so that the ideal photocurrent density can be calculated in a similar manner as for solar cells with non-graded band-gap materials. We show that this model gives completely different results as those expected from intuitive approaches for calculating this ideal photocurrent density. We also show that grading of the band-gap of the absorbing material in solar cells makes the photocurrent less sensitive to the total band-gap change, in such a way that the design of the band-gap variation can be more flexible in order to have other advantages such as higher built-in voltage or higher back surface field in the device structure. [Copyright &y& Elsevier]

Abstract: CuInSe2 and CuIn0.95Ga0.05Se2 single crystals, grown by vertical Bridgman, were annealed in air at temperatures from 100 to 160 °C. Both CuInSe2 and CuIn0.95Ga0.05Se2 samples were studied using photoluminescence (PL). The CuInSe2 samples were also studied using Rutherford Backscattering Channeling (RBS/C) and nuclear reaction analysis (NRA). Before annealing the samples showed low dechanneling parameters suggesting high quality lattice structure. The PL spectra of both CuInSe2 and CuIn0.95Ga0.05Se2 revealed bands associated with band-tail recombination mechanism. Annealing at 120 °C and higher temperatures considerably modified the RBS/C and PL spectra. NRA indicated a gradual increase in the oxygen content. PL analysis suggested that annealing increases both the mean-square amplitude of potential fluctuations and the level of compensation. Changes in the RBS/C spectra demonstrated that annealing at up to 160 °C does not result in the growth of In2O3 surface layer but modifies primarily the Se- and Cu-related RBS/C yield. These modifications are consistent with an increase in the concentration of defect complexes. [Copyright &y& Elsevier]

Abstract: Electron irradiation effects in CuInSe2 and CuInGaSe2 were investigated using electron spin resonance (ESR) method. ESR signal caused by electron irradiation was found in irradiated samples. The ESR spectra of irradiated samples were well reproduced by a computer simulation of powder spectrum assuming Cu2+ ion. Electron irradiation effects in CuInSe2 and CuInGaSe2 are discussed based on present ESR study together with previous deep level transient spectroscopy (DLTS) and Hall effect measurements. [Copyright &y& Elsevier]

CIGS nanoparticles were synthesized by a low temperature colloidal route for the absorber layer of photovoltaic devices. The CIGS nanoparticles were prepared by reacting CuI, InI3, GaI3 in pyridine with Na2Se in methanol at 0°C under inert atmosphere. The reaction products of dark red and yellow colors were turned out to be NaI and CIGS nanoparticles, respectively, by ICP-AES and SEM-EDS analyses. Chalcopyrite structure of the CIGS nanoparticles was confirmed by XRD and TEM diffraction patterns. As compared to the particles from Cu0.9In0.8 Ga0.3Se2 ratio, more uniform and smaller nanoparticles were obtained from Cu1.1In0.68Ga0.23Se1.91 stoichiometric ratio. The CIGS nanoparticles were measured to be in the ranges of 5-20 nm. However, tube like CIGS particles with length of several γn and width in the range of 100-300 nm were obtained from Cu0.9In0.8Ga0.3Se2, and Cu0.9In0.7Ga0.4Se2. The morphological change of the CIGS particles seems to be closely related to the ratio of Cu/(In+Ga). [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we present recent results on the growth and characterization of CuInGaSe2 (CIGS) thin film solar cells by the three-stage process. A conversion efficiency of 19.3% and 18.4% has been achieved for solar cells made from absorbers with band gap values of 1.15 and 1.21 eV, respectively. High open circuit voltages and fill factors are obtained. We attempt to relate these improvements to material and device properties. The results suggest that it might be possible to produce a 20% efficient solar cell by further optimization of the current collection. [Copyright &y& Elsevier]

Abstract: Common buffer materials used with CuInGaSe2 (CIGS) absorbers produce conduction-band barriers that may significantly distort the current–voltage (J–V) curves, especially when short-wavelength photons are excluded from the illumination spectrum. Earlier work documented this effect for CuInSe2 (CIS) absorbers (band gap near 1.0 eV) with CdS buffers. Higher band-gap (∼1.15 eV) CIGS absorbers show little or no distortion with CdS buffer layers. However, wider band gap (lower electron affinity) ZnS(O,OH) or InS(O,OH) buffers, prepared by chemical-bath deposition (CBD), clearly show the J–V distortion. The distortions have a turn-on time constant the order of a minute and turn-off time constant the order of a day, and they correlate with major variations in apparent quantum efficiency (QE) measured with varying intensity and spectral content of bias light. The results are consistent with a conduction-band spike barrier that increases with buffer band gap and is larger when the electron concentration in the buffer is small. [Copyright &y& Elsevier]

Abstract: Chalcopyrite CuInGaSe2 (CIGS) nanoparticles for solar cell were successfully synthesized by using a relatively simple and convenient elemental solvothermal route. From the reactions of elemental Cu, In, Se and Ga powders in an autoclave with ethylenediamine as a solvent, spherical CIGS nanoparticles with diameter in the range of 30–80 nm were obtained at temperatures in the range of 180–280 °C, whereas plate-like particles were obtained at 140 °C. The addition of gallium to the elemental solvothermal route for CuInSe2 particles lowered the reaction temperature for the formation of the CIGS nanoparticles, which was studied with the Solution–Liquid–Solid (SLS) mechanism. [Copyright &y& Elsevier]

Abstract: Copper indium gallium diselenide (CuInGaSe2) layers with p+, p, i, n, and n+-type electrical conduction, as pre-determined, have been electrodeposited from aqueous solutions in the same bath. The photoelectrochemical cell (PEC) has been used as the key analytical tool to determine the electrical conduction type, and the corresponding stoichiometry of the layers was determined using X-ray fluorescence (XRF). A four-layer n-n-i-p solar cell structure was fabricated and a corresponding energy band diagram for the device was constructed. Current–voltage (I–V) and capacitance–voltage (C–V) measurements were carried out to assess the devices and these indicate encouraging characteristics enabling further development of multilayer thin film solar cells based on CuInGaSe2. [Copyright &y& Elsevier]

Recently, we have developed an improved three-stage growth method by simultaneously using a pyrometer and a thermocouple, to accurately control thickness and composition during growth of CuInGaSe2 (CIGS). As a result, we have obtained solar cells that show preliminary efficiencies up to 16.4% without anti-reflection coating. Using the technique, we have investigated possible mechanism behind non-optimal cell efficiency, focusing on the effects of the molybdenum contact layer. Some additional advantages of our pyrometer monitoring technique are described. Optimizing the properties of Mo layer leads to suppression of Na segregation and distinctive damage to the CIGS surface, improving both cell performance and reproducibility. [Copyright &y& Elsevier]

The temperature effect on a CuInGaSe2 (CIGS) solar cell was investigated in the temperature range −10 to 80°C using direct current (DC) and alternating current (AC) characteristic analysis of CIGS solar cells. The change rates of short‐circuit current density (Jsc), open‐circuit voltage (Voc), fill factor (FF) and efficiency (η) with respect to temperature were investigated. In addition, the variation of impedance, total capacitance and dynamic resistance due to the temperature change implied that the width of the depletion region in a p–n junction shrank, and the effective minority carrier lifetime (τeff) decreased. Consequently, Jsc changed slightly due to the balance between the variation of the energy bandgap (Eg) and τeff. However, Voc linearly decreased due to the relationship of Eg when the temperature increased. From these results, it has been concluded that Voc plays an important role in the characteristics of the CIGS solar cell with change of temperature. [ABSTRACT FROM AUTHOR]

We have carried out a study of the electronic states of the quaternary chalcopyrite CuIn1-xGaxSe2 using the Tight-binding method (TB) and the Virtual Crystal Approximation (VCA). We have used the TB parameters determined by Rodriguez et. al.[1]. For the application of the VCA we used a quadratic relation with a bowing parameter which was calculated by means of the procedure of Olguín [2], plus a correction that improve the results. Such correction is proposed in this work. The values of the energy gap as a function of x, calculated with that bowing parameter, showed a good agreement with the experimental values. After that, the bowing parameter was used in order to calculate the electronic bands for x = 0.6. We have found that the bands are strongly affected by x. [ABSTRACT FROM AUTHOR]

In order to analyze why the CIGS (CuInGaSe2) - based solar cells efficiency decrease for wide band gap (high Ga content), we have performed a series of samples by PVD using the three stage process. The average compositions of our solar cells range from x = 0 to x = 0.88, as measured by X-ray diffraction. An important feature of this process is to create a double Ga gradient into the absorber, which contributes to improve efficiencies, and this has a major impact on the determination of the sample composition from x-ray diffraction data. We have developed a model in order to assess this impact and question the validity of the compositions extracted from x-ray data. This model allows to get some information about gradient shapes. Using our model, we have obtained some insights on the evolution of the gallium gradient in samples with increasing Ga content, and we determine that this gradient is less pronounced when increasing the amount of gallium. It is a well known fact that this gradient assists the extraction of photocreated carriers, and the modifications of the gallium profile that we have determined may explain, for some part, the degradation of solar cells efficiency for high gallium compositions. • Explain how average composition determination by x-ray might be flawed. • Demonstrate that group III elements interdiffusion decreases at high Ga content. • Propose a model for analyzing the CIGS (112) x-ray peak versus composition profile. • Evidence the need to grow wide bandgap CIGS at higher temperature than usual. [ABSTRACT FROM AUTHOR]

Cu(In,Ga)Se 2 (CIGS) absorbers are prepared by direct current electrodeposition process followed by selenization of precursors. Selenization of electrodeposited layers is performed in a tubular furnace at 550 °C in elemental selenium atmosphere using Ar as carrier gas. The effect of evacuation of tube prior to the selenization on the formation of CIGS absorbers is studied. Characterization of CIGS absorbers reveals that the samples selenized without prior evacuation found to have excess MoSe 2 formation at the CIGS/Mo interface leading to bulk cracks in underlying Mo back contact compared to their counterparts. Although the fabricated solar cells using the absorbers, prepared with and without evacuation, are observed to be photoactive, the cells from vacuum-based selenization showed improvement in performance compared to the cells from non-vacuum selenization. The process is further being improved to enhance the efficiency, which can pave way towards environmentally friendly low-cost solar cells.

Thin films of CuGaSe were deposited on n-Si (1 0 0) by rf magnetron sputtering from a stoichiometric CuGaSe2 target. The objective of this study was to characterize the thin film/Si heterojunction for potential photovoltaic applications, evaluate possible candidates for metal contacts and to establish whether heteroepitaxial growth could be achieved, particularly as the mismatch of lattice parameters corresponding to the base of the copper gallium selenide (CGS) tetragonal cell is quite close to that of Si, with a 2.9% mismatch. For this study, Si substrates were prepared by the standard Radio Corporation of America (RCA) cleaning procedure immediately followed by the deposition of CGS by sputtering at a substrate temperature of 600°C. The deposited thin-film stoichiometry and morphology were characterized by Rutherford backscattering spectroscopy (RBS) and transmission electron microscopy (TEM). Rutherford back scattering (RBS) analysis indicated a thin-film composition of Cu1Ga1Se1 indicating that the films were Se deficient, although channeling was not observed. The polycrystalline nature of the deposited thin film was established by cross-sectional TEM. An estimated 1.5-nm thick layer likely to be SiOx was observed at the CGS-Si interface. It is believed that this interfacial layer prevented heteroepitaxy CGS on Si. Additionally, circular metal contacts were deposited on the thin films and characterized by capacitance and current-voltage measurements. It was observed that Al and Ag contacts were rectifying, from which the thin-film carrier density was estimated to be ~5 × 1015 and ~7.68 × 1015 cm−3 with Al and Ag contacts, respectively. Au, Pt, W and Cr were ohmic, and Mo and Ni provided semi-ohmic contacts to CGS films. [ABSTRACT FROM AUTHOR]