Cu(In,Ga)Se2 Films Research Articles

Cu(In,Ga)Se<sub>2</sub> Solar Cells With Amorphous Oxide Semiconducting Buffer Layers

A transparent amorphous oxide semiconductor (TAOS) layer for the suppression of interface recombination and enhancement of open-circuit voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> ) in Cu(In,Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (CIGS) solar cells is demonstrated. A 60-nm-thick n-type a-In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-2x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (x = 0.6, 0.7, 0.8, 0.9, 1)or a-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-2y</sub> Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2y</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (y = 0.1, 0.2) layer was introduced between the CIGS film and the ZnO transparent front contact. The solar cell performance systematically varied as a function of x and y, and CIGS solar cells with a-In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-2x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (x = 0.9, 1) buffer layers showed V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> values comparable with those of a reference cell with standard i-ZnO/CdS buffer layers. Current density-voltage (J-V) curve behavior can be explained by conduction band discontinuity at the TAOS/CIGS interface and the carrier density of the TAOS layer.

Photosplitting of Water from Wide-Gap Cu(In,Ga)S2Thin Films Modified with a CdS Layer and Pt Nanoparticles for a High-Onset-Potential Photocathode

Photoelectrochemical water splitting from photocathodes based on wide-gap Cu(In,Ga)S2 (CIGS) thin films modified with a CdS layer and Pt nanoparticles was investigated. CIGS films with various amounts of Ga were fabricated using spray pyrolysis followed by sulfurization. As analyzed using 0.1 M Na2SO4 (pH 9) as an electrolyte under illumination of simulated sunlight (AM 1.5G), both photocurrent densities and onset potentials of the photocathodes were gradually improved by an increase in the amount of Ga in the CIGS films up to a Ga/(In + Ga) ratio of 0.25 (Pt-CdS/CIGS(25)). Further inclusion of Ga in the CIGS film was detrimental for both photocurrent density and onset potential. The maximum photocurrent density of 6.8 mA cm–2 (at 0 V vs RHE) and the highest photocurrent onset potential of 0.89 V vs RHE were obtained by using the Pt-CdS/CIGS(25) photocathode. Achievements of a relatively wide interface band gap of the CIGS/CdS heterointerface and formation of relatively large grains in the Pt-CdS/CIGS(25)...

Deep absorption band in Cu(In,Ga)Se2 thin films and solar cells observed by transparent piezoelectric photothermal spectroscopy

AbstractThe photo‐acoustic spectroscopy (PAS) using a transparent piezoelectric photo‐thermal (Tr‐PPT) method was carried out on Cu(In,Ga)Se2 (CIGS) thin films (both CIGS/Mo/SLG and CdS/CIGS/Mo/SLG) and solar cells (ZnO/CdS/CIGS/Mo/SLG). Using the Tr‐PPT method, the high background absorption in the below gap region observed in both a microphone and a conventional transducer PAS spectra was strongly reduced. This high background absorption came from the CIGS/Mo interface. This result proves that the Tr‐PPT PAS is the surface sensitive method. In the below‐band region, a bell‐shape deep absorption band has been observed at 0.76 eV, in which a full‐width at the half‐maximum value was 70‐120 meV. This deep absorption band was observed for both CdS/CIGS/Mo/SLG and ZnO/CdS/CIGS/Mo/SLG structures. The peak energy of the absorption band was independent of the alloy composition for 0.25≤Ga/III≤0.58. Intensity of the PA signal was negatively correlated to the Na concentration at the CIGS film surface. The origin of the 0.76 eV peak is discussed with relation to native defects such as a Cu‐vacancy‐related defect (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Na‐induced efficiency boost for Se‐deficient Cu(In,Ga)Se2 solar cells

AbstractAmong different process routes for Cu(In,Ga)Se2 (CIGS) solar cells, sufficient Se supply is commonly required to obtain high‐quality CIGS films. However, supplying extra Se increases the cost and the complexity. In this work, we demonstrate that extra Na incorporation can substantially increase efficiency of Se‐deficient CIGS solar cells, fabricated by sputtering from a quaternary CIGS target without extra Se supply, from 1.5% to 11.0%. The Se‐deficient CIGS device without extra NaF reveals a roll‐over I–V curve at room temperature as well as significantly reduced Jsc and fill factor at low temperatures. The electrical characteristics of Se‐deficient CIGS films are well explained and modeled by the low p‐type doping due to high density of compensating donors and the presence of deep defects possibly originating from the anti‐bonding levels of Se vacancies. The significant improvement after extra Na incorporation is attributable to the Na‐induced passivation of Se vacancies and the increased p‐type doping. Our result suggests that extra Na addition can effectively compensate the Se deficiency in CIGS films, which provides a valuable tuning knob for compositional tolerance of absorbers, especially for the Se‐deficient CIGS films. We believe that our findings can shine light on the development of novel CIGS processes, distinct from previous ones fabricated in Se‐rich atmosphere. Copyright © 2015 John Wiley &amp; Sons, Ltd.

CCQM pilot study CCQM-P140: quantitative surface analysis of multi-element alloy films

A pilot study for a quantitative surface analysis of multi-element alloy films has been performed by the Surface Analysis Working Group (SAWG) of the Consultative Committee for Amount of Substance (CCQM). The aim of this pilot study is to evaluate a protocol for a key comparison to demonstrate the equivalence of measures by National Metrology Institutes (NMIs) and Designated Institutes (DI) for the mole fractions of multi-element alloy films.A Cu(In,Ga)Se2 (CIGS) film with non-uniform depth distribution was chosen as a representative multi-element alloy film. The mole fractions of the reference and the test CIGS films were certified by isotope dilution—inductively coupled plasma/mass spectrometry. A total number counting (TNC) method was used as a method to determine the signal intensities of the constituent elements acquired in SIMS, XPS and AES depth profiling. TNC method is comparable with the certification process because the certified mole fractions are the average values of the films.The mole fractions of the CIGS films were measured by Secondary Ion Mass Spectrometry (SIMS), Auger Electron Spectroscopy (AES), X-ray Photoelectron Spectroscopy (XPS), X-Ray Fluorescence (XRF) Analysis and Electron Probe Micro Analysis (EPMA) with Energy Dispersive X-ray Spectrometry (EDX). Fifteen laboratories from eight NMIs, one DI, and six non-NMIs participated in this pilot study.The average mole fractions of the reported data showed relative standard deviations from 5.5 % to 6.8 % and average relative expanded uncertainties in the range from 4.52 % to 4.86 % for the four test CIGS specimens. These values are smaller than those in the key comparison CCQM-K67 for the measurement of mole fractions of Fe-Ni alloy films. As one result it can be stated that SIMS, XPS and AES protocols relying on the quantification of CIGS films using the TNC method are mature to be used in a CCQM key comparison. Main text.To reach the main text of this paper, click on Final Report. The final report has been peer-reviewed and approved for publication by CCQM.

Joule heating-assisted growth of Cu(In,Ga)Se2 solar cells

We report on the development of an unconventional method for heating a Mo-coated substrate during the deposition of a Cu(In,Ga)Se2 (CIGS) layer by the pulsed electron deposition technique, to be used as absorber in thin film solar cells. This method is based on the application of a DC electrical power directly through the Mo back contact of the cell, converting electrical energy into heat by Joule effect. Since the current flows only on the superficial metal-coated region of the substrate, a localized heating of the surface can be achieved, thus limiting the heat losses. Due to the very efficient heat transfer to the thin Mo layer, a very little electrical power density (few W/cm2) is enough to achieve the required deposition temperature on the Mo surface, much lower compared to the traditional resistor- or lamp-based external heaters. The morphological and electrical properties of Joule-heated samples have been compared to those of CIGS films heated by a conventional external heater. As far as the structure concerns, a remarkable difference is revealed by Scanning Electron Microscopy analysis, indicating a significant enlargement of the CIGS grains size on Joule-heated samples. On the contrary, Capacitance-Voltage and Current-Voltage measurements evidence similar electrical features: both types of heated samples have a net free carrier concentration ≈5 × 1015 cm−3, resulting in a similar photovoltaic conversion efficiency (≈15%). The main recombination path, deduced from the dependence of VOC on the temperature, results to be the Shockley-Read-Hall mechanism in both types of the absorber layer. These results indicate that the Joule effect could be adopted as a feasible, low cost alternative heating method for growing high quality CIGS layers.

Theoretical and experimental investigation of the recombination reduction at surface and grain boundaries in Cu(In,Ga)Se2 solar cells by valence band control

ABSTRACTWe carried out theoretical calculation for Cu(In,Ga)Se2 (CIGS) solar cells with energy bandgap of 1.4 eV assuming formation of a Cu-poor layer on the surface of CIGS films. This calculation result revealed that formation of a thinner Cu-poor layer such as a few nanometers leads to improvement of the solar cells performance. This is because interfacial recombination was suppressed due to repelling holes from the interface by valence band offset (ΔEV). Next, we investigated composition distribution in the cross section of CIGS solar cells with Ga contents of 30% and 70% by transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX). It was revealed that the Cu-poor layer was formed on the surface and at the grain boundary (GB) in the case of conversion efficiency (η) of 17.3%, although it was not formed in the case of lower η of 13.8% for a Ga content of 30%. These results indicate that formation of the Cu-poor layer contributed to improvement of cell performance by suppression of carrier recombination. Moreover, it was also confirmed that although the Cu-poor layer was observed on the surface, it was not observed at the GB in the case of CIGS solar cells with a Ga content of 70% which had η of 12.7%. It is thought that the effect of repelling holes by ΔEV is not obtained at the GB and the solar cell performance in the Ga content of 70% is lower than that in the Ga content of 30%. Thus, we suggest importance of the Cu-poor layer at the GB for high efficiency of CIGS solar cells with high Ga contents.

Damp heat and thermal cycling-induced degradation mechanism of AZO and CIGS films in Cu(In,Ga)Se2 photovoltaic modules

This paper characterizes and compares the degradation mechanisms of Cu(In,Ga)Se2 (CIGS)-based thin film photovoltaic (PV) modules during exposure to damp heat (85 °C/85% RH) for 1000 h and thermal cycling (−40 °C/85 °C) for 1000 cycles. After damp heat (DH) exposure, the efficiency of the PV modules was reduced from the initial value of ∼12.5 ± 0.1% to 10.5 ± 0.2% due to increase of the resistivity in the AZO and CIGS layers. The optical band gap was also decreased from the initial value of 3.60 eV–3.54 eV after 1000-h DH exposure. This behavior was associated with oxygen adsorption and the generation of hydroxides in the AZO layer. The efficiency of the PV modules after subjection to thermal cycling (TC) was decreased to 11.4 ± 0.2% due to increase of the resistivity of the AZO and CIGS layers. The increase in the resistivity was interpreted as being due to oxygen adsorption, as well as the formation of micro-cracks in the AZO films.

Estimation of open-circuit voltage of Cu(In,Ga)Se2 solar cells before cell fabrication

Cu(In,Ga)Se2 (CIGS) absorbers with several Ga/III, Ga/(In + Ga), profiles were fabricated by the so-called “multi layer precursor method” under the control of Ga flux rate during film's deposition. Open-circuit voltage (VOC) of CIGS solar cell is principally dependent on average band-gap energy (Eg) in space-change region (SCR) of below 1.3 eV. This average Eg in SCR, principally controlled by average Ga/III ratio in SCR, is estimated by the peak position of (220/204)-oriented CIGS films on soda-lime grass (SLG) substrates investigated by grazing incidence X-ray diffraction (GIXRD) with 0.1° incident angle, whereas the average Eg in SCR is predicted by Raman or photoluminescence (PL) peak positions of CIGS absorbers on both SLG and stainless steel (SUS) substrates. Ultimately, with the average Eg in SCR of below 1.3 eV, the (220/204) peak position (GIXRD) can be well used as a predictor of the VOC on rigid SLG substrates. On the other hand, Raman peak and PL peak positions can be well utilized as indicators of the VOC on not only rigid SLG but also flexible SUS substrates before solar cell fabrication. These are fast and non-destructive methods to evaluate the Ga content, Eg near CIGS surface, and corresponding VOC for high cell performance.

Effects of Substrate Temperature on the Properties of Cu(In,Ga)Se&lt;sub&gt;2&lt;/sub&gt; Thin Films Prepared by Sputtering from a Quaternary Target

Cu (In,Ga)Se2 (CIGS) thin films were prepared by direct magnetron sputtering CIGS quaternary target at the substrate temperature varying from room temperature (RT) to 300 °C. The effects of substrate temperature on the structural and electrical properties of CIGS films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and Hall effect measurement. The CIGS thin films with a chalcopyrite structure were obtained between 100 and 300 °C and the crystallinity of films were enhanced with the increase of the substrate temperature from 100 to 300 °C. The film compositions were consisted with the target when the substrate temperatures were between RT and 200 °C, however, it deviated from the stoichiometry of the target when the substrate temperature was 300 °C. The CIGS films deposited at 200 °C had the higher carrier mobility of 3.522 cm2/Vs.

Electrical characterization of Cu(In,Ga)Se2 thin films peeled off from Mo-coated soda-lime glass substrate by AC Hall measurement

We have developed a new evaluation method for electrical properties of Cu(In,Ga)Se2 (CIGS) grown on a Mo-coated soda-lime glass (SLG). The method consists of the peel-off process and the AC Hall measurement, which enables us to evaluate CIGS films grown on the Mo electrode. It was found, from the measurement, that the hole concentration of CIGS grown on a Mo-coated SLG was approximately two orders of magnitude higher than that on a SLG, suggesting the Na-doping effect. Furthermore, the hole mobility of 0.47 cm2/(V·s) was simultaneously measured, even though the film was deposited on the Mo electrode.

Fabrication and Crystal Structure Analysis of a CIGS Solar Cell Absorber by Thermal Annealing of Sputtered CIG Precursors Deposited with a Se Layer

Various Cu(In,Ga)Se2 (CIGS) films were prepared by using in-line sputter for CIG precursor and evaporator for selenization. Deposition parameters are substrate temperature at sputtering process, post annealing temperature (400∼500°C) and annealing method (thermal furnace and high-temperature in-situ XRD). Temperature dependent XRD patterns of the samples were measured with high temperature option and this pattern shows that post annealing at 400∼500°C is necessary for CIGS absorber deposition with no second phase by in-line sputtering. The CIGS film which is annealed at 500°C shows good crystallographic property and there is no second phase like Cu2−xSe.

Surface modification of CIGS film by annealing and its effect on the band structure and photovoltaic properties of CIGS solar cells

The composition of Cu(In,Ga)Se2 (CIGS) films employed in CIGS solar cells is Cu deficient. There can be point defects, including Cu vacancies, Se vacancies, and metal anti-site defects. The surface composition and defects are not well controlled right after CIGS film fabrication with a three-stage co-evaporation process. This fabrication technique can result in a large variation in cell efficiency. In order to control the CIGS film in a reproducible way, we annealed the CIGS film in air, S, or Se. With this annealing procedure, the Cu content of the CIGS surface was significantly reduced and Ga content was strongly increased. An intrinsic CIGS layer with a lower valence-band maximum and a wider ban gap was formed at the surface. By annealing the CIGS film, the open-circuit voltage and fill factor were significantly improved, which indicates that the surface intrinsic layer acts as a hole-blocking layer so that the surface recombination rate is suppressed. In addition to CIGS film annealing, with subsequent annealing of the completed devices using rapid thermal annealing, the efficiency and reproducibility of CIGS solar cells were markedly improved.

Influence of electrode position in the electrolytic cell configuration for the electrodeposition of Cu(In,Ga)Se2 thin films

Cu(In,Ga)Se2 (CIGS) is a semiconductor that has an absorption coefficient (more than 105) suitable for building high efficiency thin film solar cells. There are a lot of reports that mention different techniques to CIGS synthesis. These techniques can involve physical or chemical processes with technical and economical advantages and disadvantages. Probably, one of the major obstacles to CIGS solar cell commercialization is that there are various synthesis techniques for this type of solar cell and the most economical and feasible techniques do not produce an adequate absorber formation. Among these processes, electrodeposition is a versatile technique due to the possibility of making CIGS films in a large area. However, one of the major problems during the CIGS growth by electrodeposition is the microcracks formation stemming from some experimental conditions such as bath composition, type of substrate, electrode distance, etc. In this paper, the electrolytic cell for CIGS co-electrodeposition was analyzed in order to know the film composition and morphology as a function of electrode position in the electrolytic cell. We found that the CIGS film composition was not homogeneous and the morphology was not uniform in an electrolytic cell with electrodes placed vertically. As a result of the previous experimentation, we have designed an electrolytic cell with horizontal electrodes where the composition was more homogeneous and the morphology was more uniform.

Investigation of Cu(In,Ga)Se2 absorber by time-resolved photoluminescence for improvement of its photovoltaic performance

Time-resolved photoluminescence (TRPL) measurement has been performed on Cu(In,Ga)Se2 (CIGS) absorbers. In this contribution, CIGS films on both rigid soda-lime glass and flexible stainless steel (SUS) substrates are fabricated by the so-called “multi-layer precursor method” consisting of Ga–Se/In–Se/Cu–Se stacks. The TRPL lifetime, demonstrating a positive relationship with PL intensity, exhibits a close correlation with all photovoltaic parameters. According to TRPL and sensitive capacitance measurements, the average band-gap energy (Eg) should be in a range of 1.25–1.30eV, giving rise to sufficiently long TRPL lifetimes. Ultimately, CIGS absorber is fabricated with double graded Eg profile with a proper average Eg of 1.27eV and back-surface field of 0.22V/μm, defined as a ratio of the change in Eg divided by the change in a depth range from 1 to 2μm from CIGS surface. This graded Eg profile results in the improvement of the efficiency of the CIGS solar cell on a flexible SUS substrate up to 16.22% without an anti-reflective layer.

Analysis of photovoltaic cell parameters of non-vacuum solution processed Cu(In, Ga)Se2 thin film based solar cells

The losses in Cu(In, Ga)Se2 (CIGS) solar cells due to photovoltaic (PV) cell parameters, namely the shunt resistance Rsh, series resistance Rs, diode ideality factor n, and reverse saturation current density J0, were analyzed in this study. The PV cell parameters of the solar cells were analytically determined for various doping concentrations of Cu and In in CIGS films. The CIGS films were deposited using a low cost non-toxic solvent (deionized) by a non-vacuum process (spray pyrolysis technique). They were subsequently characterized using XRD, FE-SEM, I–V and UV–Vis techniques to correlate the structural, electrical, and optical properties of the films with solar cell performance. Maximum short circuit current density of 0.0218A/cm2 is achieved for Cu/Se and In/Se molar ratios of 0.131 and 0.318, respectively. However, the maximum obtained Voc value is 0.431V for Cu/Se and In/Se molar ratios of 0.105 and 0.191, respectively. The maximum achieved efficiency was ∼4.38% for Cu/Se and In/Se molar ratios of 0.105 and 0.191, respectively. Analytically predicted values of Rsh, Rs, n, and J0 were 116.82Ωcm2, 4.64Ωcm2, 1.8016, and 1.4952×10−6A/cm2, respectively.

Na effect on flexible Cu(In,Ga)Se2 photovoltaic cell depending on diffusion barriers (SiOx, i-ZnO) on stainless steel

Cu(In,Ga)Se2 (CIGS) based-photovoltaic (PV) cells with different diffusion barriers of SiOx and i-ZnO were fabricated on stainless steel (STS) substrate and their electrical characteristics were investigated by measuring J–V curves under illuminated and dark conditions. The physical properties of the CIGS film depending on type of diffusion barrier were also analyzed using X-ray diffraction and secondary ion mass spectroscopy. The efficiency of the CIGS-PV cell with i-ZnO barrier was approximately 2% higher than that with the SiOx barrier. Through the analysis of dark J–V curves, we discovered that distinctive defects were formed in the band gap of CIGS based on which diffusion barrier contacted the STS. The diffraction pattern showed a slightly different tendency of the peak intensity ratio of (220/204)/(112) in the PV cell with the i-ZnO barrier, which was slightly higher than that in the PV cell with SiOx barrier. In elemental depth profile, a deficient Ga profile was observed near the surface of the CIGS film with the SiOx barrier, and an abundant Na profile within the CIGS film with the i-ZnO barrier was detected. This is attributed to a difference in thermal conduction through the diffusion barriers during CIGS film growth, originating from the larger thermal conductivity of ZnO compared with SiOx.

Developments of High-Efficiency Flexible Cu(In,Ga)Se2 Thin Film Solar Cells on a Polyimide Sheet by Sodium Incorporation

We present the fabrication of flexible Cu(In,Ga)Se2 (CIGS) solar cells on a polyimide (PI) sheet with and without Na incorporation. A sodium element is incorporated into the CIGS absorber by using a NaF precursor after Mo back contact deposition. X-ray diffraction patterns show that the (112) preferred orientation of the as-grown CIGS films is decreased by Na incorporation. The secondary phase of (Inx,Ga1−x)2Se3 is observed for the CIGS films with Na. There is no significant difference in the grain size with and without Na incorporation from surface and cross-sectional SEM images. Additionally, the increase of carrier concentration and decrease of resistivity of CIGS absorber are induced by Na doping. Finally, the flexible CIGS solar cells on PI sheets with efficiency close to 11%, containing Na, are achieved. The improvement of cell efficiency can be attributed to the modified electrical properties of the CIGS film by Na incorporation.

The Advantages of Ga-Graded Obtained by Growth Profile Modification and Na Incorporation on Cu(In,Ga)Se&lt;sub&gt;2&lt;/sub&gt; Solar Cells

Cu (In,Ga)Se2 (CIGS) compound is a p-type semiconductor that has been used as light absorber layer in high efficiency thin film solar cell. The CIGS compound can be adjusted the band gap energy by varying the ratio of [Ga]/([In]+[Ga]) ratio (x). From theoretical and simulation, it was found that band gap grading in CIGS thin films showed the advantages to increase the efficiency of solar cells. Generally, the band gap grading can be done by the growth of non homogeneous x-ratio in depth of CIGS thin films. In this work, we develop two approaches to create band gap grading in CIGS thin films; (1) modifying the growth profile and (2) using Na incorporation in the growth process. The effects of Ga-graded would be revealed and compared with homogeneous CIGS thin films. CIGS thin films were grown on soda-lime glass and Al2O3 coated soda-lime glass substrates by molecular beam deposition method. The growth process was based on 2-stage and 3-stage growth profiles. The as grown films were characterized for their structural property, chemical composition and optical transmission as well as solar cell performance. The Auger electron spectroscopy in depth profiles revealed the variation of x-ratio increasing from the surface toward the back contact in CIGS films with our modified growth profile and Na incorporation. This result indicated Ga-graded in CIGS thin films. The structural property of Ga-graded CIGS films showed the (112) preferred orientation of the chalcopyrite structure with a broad asymmetric spectrum related to the inhomogeneous structure. The optical transmission measurements of the Ga-graded CIGS film showed the broad transition near the absorption edge indicating the effect of the band gap grading as a result of the variation in depth of the Ga-content. From I-V measurements, the solar cell efficiencies significantly increase due to the advantages of Ga-graded constitute.

Cu(In,Ga)Se&lt;sub&gt;2&lt;/sub&gt; Films Deposited by Sputtering with Single Cu(In,Ga)Se&lt;sub&gt;2&lt;/sub&gt; and Cu-Ga-In&lt;sub&gt;2&lt;/sub&gt;Se&lt;sub&gt;3&lt;/sub&gt; Targets and a Subsequent Selenization Procedure

Cu(In,Ga)Se2 (CIGSe) thin films were prepared by sputtering with single CIGSe or Cu-Ga-In2Se3 target and subsequent selenization at 550-700°C. The one- and two-step selenization procedures and the ceramic and cermet targets were used for process comparisons. Microstructure, film growth and film composition were used to evaluate the growth performance. CIGSe films sputtered from the CIGSe target had a low Cu content. CIGSe films prepared with single Cu-Ga-In2Se3 target had shown different performance after the one- and two-step selenization procedures. The two-step process did not grow the dense films due to the vaporization of Se-containing species from the incomplete reaction. The high-temperature requirement is the major disadvantage for the post-selenization approach.