Cu(In,Ga)Se2 Absorber Research Articles

Silver-assisted optimization of band gap gradient structure of Cu(In,Ga)Se2 solar cells via SCAPS

Recently, silver (Ag) substitution for copper (Cu) into Cu(In,Ga)Se2 (CIGS) films has attracted widespread attention due to the lower synthesis temperature and the enlarge band gap. In this paper, SCAPS simulation is employed to investigate the influence of Ag doped CIGS absorber layer on the lattice defect and band gap gradient. First, the impact of the [Ga]/([In] + [Ga]) (GGI) and [Ag]/([Ag] + [Cu]) (AAC) on the defect concentrations is calculated systematically. It is demonstrate that the compression (GGI > 0.25) is more likely to cause the tetragonal distortion compared with the tension (GGI < 0.25) on the unit cell. And appropriate substitution of Cu with Ag can reduce the defect concentrations. Further investigations on the variation of the lattice structure indicate the ideal band gap can be enlarged to the range of about 1.20–1.29 eV by silver. With the ideal lattice structure, the efficiency of solar cells has a relative increase of about 48 % compared with that of the reference solar cell. This study offers a new sight for the incorporation of Ag into CIGS films from the perspective of lattice structure, which should be further improve device performance of CIGS solar cells at low temperature deposition process.

Numerical study of a high‑performance thin film CIGS solar cell with a-Si and MoTe2 hole transport layer

Ultrathin Cu(InGa)Se2 (CIGS) material has become one of the emerging material in the research of solar cell fields due to its excellent optical, electrical and tunable bandgap properties. In this study, we have described a numerical simulation of ultrathin CIGS solar cell using SCAPS-1D program tool based on WS2 as a buffer layer, CIGS as an absorber layer with a-Si/MoTe2 as a hole transport layer (HTL). The proposed study also reveals that a-Si is better choice for HTL as compared to MoTe2 for CIGS solar cell. The cell parameters such as thickness, defect density and acceptor concentration of CIGS absorber layer have been optimized in proposed work. Based on optimization, the maximum power conversion efficiency (PCE) of 30.17% (Voc =1.093Volts,Jsc=32.41 mA/cm2 and FF=85.12%) has been reached for CIGS solar cell with a-Si HTL. The simulation further analyzed at various operating temperature and series resistance for better understanding of the solar device. All these simulation provide noteworthy manifestation for the fabrication of high efficient ultrathin energy harvesting application.

Optoelectronic Inactivity of Dislocations in Cu(In,Ga)Se2 Thin Films

High‐efficiency Cu(In,Ga)Se2 (CIGS) thin‐film solar cells are based on polycrystalline CIGS absorber layers, which contain grain boundaries, stacking faults, and dislocations. While planar defects in CIGS layers have been investigated extensively, little is still known about the impact of dislocations on optoelectronic properties of CIGS absorbers. Herein, evidence for an optoelectronic inactivity of dislocations in these thin films is given, in contrast to the situation at grain boundaries. This unique behavior is explained by the extensive elemental redistribution detected around dislocation cores, which is connected with the dislocation strain field, probably leading to a shift of defect states toward the band edges.

Effects of Sputtered InxSy Buffer on CIGS with RbF Post-Deposition Treatment

Indium sulfide (InxSy) is an attractive candidate to be used as a buffer layer for Cu(In,Ga)Se2 (CIGS) thin-film solar cells and modules. InxSy sputtering would be the preferred deposition method of choice as it allows for much higher growth rates compared to all other current deposition methods. A key feature of CIGS absorber processing, established during the last years, is the use of a post-deposition treatment (PDT) with alkali compounds. In this work, we apply rf-magnetron sputtered InxSy buffers from ceramic targets, optionally doped with NaF, on industrially relevant CIGS absorbers with or without RbF-PDT prepared in an in-line coater. We analyze the influence of wet chemical treatments of the CIGS surface after RbF-PDT and alkali accumulation at the InxSy/CIGS interface on the solar cell performance. Good efficiencies could be achieved with sputtered InxSy on CIGS with RbF in an all-dry process. An InxSy deposition temperature around 200 °C and/or a post-annealing step of the complete cell in air emerged as a key feature for decent efficiencies. This is also due for sputtering from a NaF-doped In2S3 target. Our best cell with sputtered InxSy on CIGS with RbF-PDT shows an efficiency of 18.0% with a high open-circuit voltage of 701 mV.

Application of Quantum Dot Down-Conversion Layer in Thin-Film Solar Cells to Increase Short-Wavelength Spectral Response

The short-wavelength optical loss in the Cu(In,Ga)Se2 (CIGS) thin-film solar cells is inevitable owing to the substantial light absorption in the front layers such as the buffer layer and transparent conducting oxide (TCO) layer. Quantum dots (QDs) with CdSe/ZnS core–shell structure is utilized to increase the short-wavelength spectral response of the CIGS thin-film solar cells. The QDs absorbs photons in the short-wavelength region (<540 nm) and re-emits the photons at approximately 540 nm; these photons penetrate the front layers and reach the CIGS absorber layer. The thickness of the QD layer was varied via drop coating with different QD concentrations, thereby facilitating the application of the optimized QD layer as a down-conversion layer in the CIGS thin-film solar cells. The photoelectric parameters of the CIGS thin-film solar cells were dependent on the QD thickness, and they were characterized using quantum efficiency measurements, spectrophotometric analysis, and current–voltage measurements. The CIGS thin-film solar cells with a 0.7 μm-thick QD layer exhibited the highest increase of 1.86 mA cm−2 and 0.75% in the short-circuit current density and efficiency, respectively.

Interface Engineering of Cu(In,Ga)Se2 Solar Cells by Optimizing Cd- and Zn-Chalcogenide Alloys as the Buffer Layer.

The optimization of band alignment at the buffer/absorber interface is realized by tuning compositions of Cd and Zn chalcogenides as the buffer layer toward high-efficiency Cu(In,Ga)Se2 (CIGS) solar cells. Using the special quasi-random structure (SQS) approach, we construct randomly disordered ZnxCd1-xSySe1-y alloys and ZnSxO1-x alloys as alternatives to the traditional CdS buffer layer. The compositional dependence of formation energies, lattice parameters, band-gap energies, and band alignments of ZnxCd1-xSySe1-y and ZnSxO1-x alloys is investigated by first-principles density functional theory calculations. For quaternary ZnxCd1-xSySe1-y alloys, we find that the miscibility temperatures and the bandgap bowing coefficients are proportional to the lattice mismatch between the mixing elements. The linear dependence of lattice parameters, trinomial dependence of band-gap energies and band-edge positions on the alloy-composition of ZnxCd1-xSySe1-y alloys are established. For ZnSxO1-x alloys, we find the lattice parameters also exhibit a linear dependence on its composition. Because of the large lattice mismatch and the chemical disparity between ZnO and ZnS, the bowing coefficient for the bandgap energies of ZnSxO1-x alloys is composition dependent, and is larger for dilute ZnSxO1-x alloys. With the optimization criteria of moderate spike-like conduction band offset, large valance band offset, sufficiently wide bandgap, and lattice match with respect to the CIGS absorber, we illustrate the optimal composition range of both ZnxCd1-xSySe1-y alloys and ZnSxO1-x alloys as the buffer layer of the CIGS solar cells. Our work demonstrates that ZnxCd1-xSySe1-y alloys and ZnSxO1-x alloys are promising buffer layers for high-efficiency CIGS solar cells.

Influence of Ga back grading on voltage loss in low‐temperature co‐evaporated Cu(In,Ga)Se2 thin film solar cells

AbstractThe performance of Cu(In,Ga)Se2 (CIGS) solar cells is limited by the presence of the highly recombinative CIGS/Mo interface. The recombination at the CIGS/Mo interface is influential for the open circuit voltage (VOC) in high quality CIGS absorbers with increased charge carriers diffusion length. A quantitative understanding of the role of the Ga back grading height (ΔGGI) in suppressing back interface recombination is needed. In this work, we take advantage of a low temperature process to modify the ΔGGI while keeping the composition in the notch and front regions almost unchanged. Improvement in both VOC deficit and time‐resolved photoluminescence lifetime are observed with increasing ΔGGI. With a combination of back surface modification experiments and numerical simulations, we quantify a voltage loss in ungraded devices of approximately 100 mV solely from the back interface recombination. Nice agreement between simulation and experimental data is reached while constraining the values of possible diffusion lengths. Our results suggest that a ΔGGI of about 0.50 is required to effectively suppress the back interface recombination, highlighting the importance of grading control in high‐performance CIGS solar cells and devices.

Effect of Cd diffusion on the electrical properties of the Cu(In,Ga)Se2 thin-film solar cell

Cu(In,Ga)Se2 (CIGSe)-based solar cells are promising candidates for efficient sunlight harvesting. However, their complex composition and microstructure can change under operation conditions, for instance heating from sun light illumination can lead to a degradation in performance. Here, we investigate the thermally-induced degradation processes in a set of CIGSe-based solar cells that were annealed at temperatures between 150 °C and 300 °C. Using correlative atom probe tomography (APT)/transmission electron microscope (TEM), we found that the buffer/absorber interface is not sharp but consists of an interfacial zone (2–6.5 nm wide) where a gradient of constituent elements belonging to the CdS buffer and CIGSe absorber appears. An enhanced short-range Cd in-diffusion inside the CIGSe was observed whenever a low Ga/(Ga + In) ratio (≤ 0.15) occurred at the interface. This might indicate the presence of Ga vacancies as a channeling defect for Cd in-diffusion inside the CIGSe layer leading to a buried p/n-homojunction. We evidence that a considerable amount of Cd is found inside the CIGSe layer at annealing temperatures higher than 150 °C. Further investigations of the elemental redistribution inside the CIGSe layer combined with C–V measurements support the formation of CdCu+ donor like defects deep inside the p-type CIGSe which lead to a strong compensation of the CIGSe layer and hence to strong deterioration of cell efficiency at annealing temperatures higher than 200 °C. Hence, understanding the degradation processes in Cu(In,Ga)Se2 (CIGSe)-based solar cells opens new opportunities for further improvement of the long-term device performance.

Semitransparent and bifacial ultrathin Cu(In,Ga)Se2 solar cells via a single-stage process and light-management strategy

Semitransparent and bifacial photovoltaic (PV) technology has received considerable attention recently owing to its potential for energy-harvesting window applications. Ultrathin Cu(In,Ga)Se2 (CIGS) solar cells with transparent conducting front and rear electrodes, which enable partial visible light transmission, are considered promising candidates for this purpose. However, to realize improved performance, a variety of challenges, including fabrication processes, material selection, and device structure design, remain to be addressed. Here, the material properties and performance characteristics of semitransparent ultrathin CIGS solar cells prepared on two types of transparent conducting oxide (TCO) rear contacts, namely, textured SnO2:F and flat In2O3:Sn films, using a single-stage co-evaporation method, are reported. Comparative performance evaluations of devices with the different TCO rear contacts reveal that the single-stage process is a very effective method for fabricating high-performance semitransparent CIGS solar devices, and that light management plays an important role in devices with ultrathin CIGS absorbers. A textured polydimethylsiloxane (PDMS) layer with light-scattering and antireflection properties enhances the light absorption properties of the devices. The device (absorber thickness: 322 nm) prepared on the flat In2O3:Sn rear contact with the textured PDMS layer exhibits superior performance as compared to other state-of-the-art solar devices, with a conversion efficiency of 10.5% and an average visible transmittance of 12.3%.

Understanding deposition temperature dependent photovoltaic characteristics of Cu(In,Ga)Se2 solar cells: A study with thermally stable alkali aluminosilicate glass substrates

During 3-stage thermal co-evaporation of Cu(In,Ga)Se2 (CIGSe), alkali aluminosilicate (AAS) glass has proven to be stable at process temperatures Tmax of up to 650 °C. This is considerably higher than the maximum endurable Tmax for soda-lime glass (SLG), which is in the range of 530–550 °C. As the Na-content of the AAS glass used here is comparable to that of SLG, growing CIGSe at elevated Tmax is expected to promote Na diffusion from the glass substrate through the Mo back contact (BC) into the absorber layer. An increased Na concentration in the CIGSe absorber would be expected to result in an improved device performance as it is mostly associated with an increased open-circuit voltage (Voc) and fill factor (FF). This paper discusses the influence of varying Tmax from 530 to 650 °C during the 2nd and the 3rd stage of the 3-stage process on the morphological, electrical, and photovoltaic performance of CIGSe solar cells. Upon raising Tmax from 530 to 650 °C, the short-circuit current density (Jsc) decreased due to bandgap (Eg) widening. Glow discharge optical emission spectrometry analysis reveals that the Na concentration in the absorber is gradually decreased when elevating Tmax. As will be seen, we attribute this in part to a densification of the Mo BC during the high temperature process. A maximum conversion efficiency (η) was realized at Tmax = 600 °C. An increased Voc at Tmax = 600 °C is due to the wider Eg and to an increased carrier concentration despite the fact that the Na concentration in the CIGSe thin film was low compared to lower Tmax. Admittance spectroscopy analysis is performed to access information on defect energy level and density in the finished devices. In the light of present findings, ways to further improve η at elevated Tmax are suggested.

Control over MoSe2 formation with vacuum-assisted selenization of one-step electrodeposited Cu-In-Ga-Se precursor layers.

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.

Superstrate-type flexible and bifacial Cu(In,Ga)Se2 thin-film solar cells with In2O3:SnO2 back contact

Bifacial and flexible Cu(In,Ga)Se2 (CIGS) thin-film solar cells with superstrate-type structure were fabricated. For this fabrication, the substrate-type solar cells were first transferred from Mo/Glass substrates to 50-µm-thick flexible fluorinated ethylene propylene (FEP) copolymer films as superstrates using a lift-off process. The 300-nm-thick In2O3:SnO2 (ITO) thin-films were subsequently deposited on the rear sides of the CIGS absorber layers as back contacts (BC) of the finished bifacial devices with the structure of FEP/Epoxy/Al/Ni/Zn0.9Mg0.1O:Al/Zn0.8Mg0.2O/CdS/CIGS/ITO/Ni/Al. Particular attention was paid to maintain the high optical and electrical transport properties of ITO-BC as well as to improve the Ohmic-like contact behavior at the CIGS/ITO interface to improve cell performances. Under the optimized ITO preparation, the resulting ITO thin film possesses the average optical transparency of ITO (>77%) in the wavelength range of 400–1100 nm, and the resistivity of 6.1 × 10−4 Ωcm with corresponding high carrier mobility of 42.8 cm2/Vs, as well as Ohmic-like characteristics at the CIGS/ITO interface can be realized with shunt resistance as low as 5.5 Ωcm2. Consequently, the flexible and bifacial CIGS thin-film solar cell was fabricated with efficiencies of 9.1% and 2.4% for frontside and backside illuminations, respectively. In this study, highly transparent and conductive ITO thin films have been successfully used as BC in flexible and bifacial CIGS solar cells.

Real-Time Optimization of Anti-Reflective Coatings for CIGS Solar Cells.

A new method combining in-situ real-time spectroscopic ellipsometry and optical modeling to optimize the thickness of an anti-reflective (AR) coating for Cu(In,Ga)Se2 (CIGS) solar cells is described and applied directly to fabricate devices. The model is based on transfer matrix theory with input from the accurate measurement of complex dielectric function spectra and thickness of each layer in the solar cell by spectroscopic ellipsometry. The AR coating thickness is optimized in real time to optically enhance device performance with varying thickness and properties of the constituent layers. Among the parameters studied, we notably demonstrate how changes in thickness of the CIGS absorber layer, buffer layers, and transparent contact layer of higher performance solar cells affect the optimized AR coating thickness. An increase in the device performance of up to 6% with the optimized AR layer is demonstrated, emphasizing the importance of designing the AR coating based on the properties of the device structure.

Cu(In,Ga)Se2 thin film solar cells produced by atmospheric selenization of spray casted nanocrystalline layers

Cu(In,Ga)Se2 (CIGS) is suitable absorber material for thin film photovoltaic owing to its excellent thermo-chemical stability and demonstration of high power conversion efficiency of 23.35% for lab-scale devices. The manufacturing of CIGS thin film solar cells need to be economical for commercially viability. Non-vacuum approach for the synthesis of the CIGS thin film absorber is desired for its cost-effective benefits over vacuum based processes. We report development of solution-processed CIGS absorber layer involving spraying of sonochemically prepared nanocrystal ink to obtain pristine CIGS nanocrystal layer, followed by single-step atmospheric pressure selenization utilizing Se pallets as a Se vapor source. Influence of selenization conditions and NaF doping on grain growth of the CIGS thin film absorber is studied in detail by structural, morphological, compositional and electrical characterization. Solar cells prepared under optimized conditions yielded a maximum efficiency of 6.7% without employing any thermal pre-treatment and toxic Potassium Cyanide etching.

Bifacial photovoltaic performance of semitransparent ultrathin Cu(In,Ga)Se2 solar cells with front and rear transparent conducting oxide contacts

We report the bifacial photovoltaic performance of semitransparent ultrathin Cu(In,Ga)Se2 (CIGS) solar cells with Sn-doped In2O3 front and F-doped SnO2 rear contacts prepared using a single-stage co-evaporation process under front- and rear-illumination conditions. The power conversion efficiencies (PCEs) of the solar cells measured at 100 mW/cm2 increased with increasing ultrathin CIGS absorber thickness (200–400 nm), from 6.89 to 9.75% when front illuminated, and from 4.91 to 6.46% when rear illuminated, while the corresponding average visible transmission values (420–720 nm) gradually decreased from 18.53% to 5.06%. The bifacial photovoltaic performance of semitransparent solar cells with 200-nm- and 300-nm-thick CIGS absorber layers was also investigated under low-light conditions (100–10 mW/cm2). The PCE of the solar cell with a 300-nm-thick CIGS absorber layer remained almost constant with decreasing light intensity (100–10 mW/cm2), at ~8% (front illumination) and ~6% (rear illumination), while that of the solar cell with 200-nm-thick layer decreased from 6.84% to 3.43% (front illumination) and from 4.91% to 2.41% (rear illumination). The enhanced bifacial performance of the solar cell with the 300-nm-thick CIGS absorber layer is attributed to high shunt resistance in the solar cell owing to the improved microstructural qualities.

RF/DC Magnetron Sputtering Deposition of Thin Layers for Solar Cell Fabrication

Thin film Cu(In,Ga)Se2 (CIGS)-based solar cells with relatively high efficiency and low material usage might become a promising alternative for crystalline silicon technology. The most challenging task nowadays is to decrease the PV module fabrication costs by application of easily scalable industrial process. One of the possible solutions is the usage of magnetron sputtering system for deposition of all structures applied in CIGS-based photovoltaic device. The main object of these studies was fabrication and characterization of thin films deposited by sputtering technique. Structural and electrical properties of the sputtered films were analyzed using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray Powder Diffraction (XRD), and four-point probe resistivity measurements. The presented findings revealed technological parameters for which sheet resistance of molybdenum (Mo) back contact decreased up to 0.3 Ω/□ and to even 0.08 Ω/□ in case of aluminum layer. EDS analysis provided evidence for the appropriate stoichiometry of CIGS absorber (with CGI and GGI equal to 0.96 and 0.2, respectively). XRD characterization confirmed high-quality chalcopyrite polycrystalline structure of Cu(In,Ga)Se2 film fabricated at relatively low substrate temperature of 400 °C. Characteristic XRD peaks of hexagonal-oriented structures of sputtered CdS and i-ZnO layers were noticed.

Production of CIGS solar cell with an appropriate atomic ratio using magnetron sputtering

A Cu (In, Ga)Se2 (CIGS) chalcopyrite-type structure solar cell is produced using magnetron sputtering with Cu0.75Ga0.25 and In targets, followed by a three-stage selenization process. A metallic precursor CuGa (maintained at ∼300 nm)/In (various indium film thickness) is used to study the process for the fabrication of a CIGS film. The sample that is selenized at 550 °C has a tetragonal chalcopyrite phase structure with main diffraction peaks (112) at 2θ ∼ 26.6° and other prominent peaks (220/204) and (312)/(116) crystal planes. A favorable atomic ratio for the CIGS films is achieved for respective ratios of [Ga/(In+Ga)] and [Cu/(In+Ga)] in the CIGS absorber layers of 0.28 and 0.99. These ratios are almost equal to the ratios for the respective precursor layer. The near-stoichiometric films are uniform and crack-free and have better crystalline quality and large surface grains. A CIGS heterojunction solar cell is synthesized and the photo-conversion efficiency is calculated.

Electrostatic potential fluctuations and light‐soaking effects in Cu(In,Ga)Se2 solar cells

AbstractIn Cu(In,Ga)Se2 (CIGS) thin‐film solar cells, laterally inhomogeneous distributions of point defects may induce electrostatic potential fluctuations and thus reduce the open‐circuit voltage (Voc). In the present work, we investigate possible origins of fluctuating potentials and estimate the amplitude of fluctuations and Voc losses in solar cells with various [Ga] in the CIGS absorber, with different buffer layers and with different durations of an RbF postdeposition treatment (PDT). Electron‐beam‐induced current measurements were employed to study the local difference in the width of the space‐charge region (wSCR). It is shown that the amplitude of fluctuations in the wSCR depends significantly on the choice of buffer system and on the duration of the RbF PDT. In addition, energy‐dispersive X‐ray spectroscopy and cathodoluminescence measurements reveal that band‐gap fluctuations do not have substantial impact on the device performance. Finally, some of the investigated cells were exposed to light soaking, which was found to be a means to reduce the detected electrostatic potential fluctuations and also to increase the effective electron diffusion length in the CIGS absorber for a part of the investigated cells.

Reactively sputtered Zn(O,S) buffer layers for controlling band alignment of Cu(In,Ga)Se2 thin-film solar cell interface

Abstract Buffer layers in Cu(In,Ga)Se2 (CIGS) solar cells are highly preferable to be vacuum-compatible and use non-toxic materials not only for the film but also for the manufacturing process. Moreover, high controllability for the energy levels of buffer layers is greatly desirable for achieving higher solar cell efficiency because the buffer layer is formed directly onto the absorber layer which generates carriers. In this study, the S/(O + S) composition ratio of the Zn(O,S) thin films was finely controlled by varying O2/(Ar + O2) gas flow ratio using a reactive sputtering method including an Ar/O2 mixture gas and a single ZnS sputter target. The structural, electrical, and photovoltaic performance properties of the Zn(O,S) thin films and their CIGS solar cells were investigated at varied S/(O + S) composition ratios. As the S/(O + S) composition ratio of the Zn(O,S) films increased, bandgap bowing occurred with the increase of the conduction band and valence band energy levels. The photovoltaic performance was greatly influenced by the difference of the conduction band energies between the Zn(O,S) buffer and the CIGS absorber. When the conduction band energy of the Zn(O,S) was too high or too low compared to that of the CIGS, it increased the carrier recombinations or series resistance, respectively, which induced losses of the open-circuit voltage and the fill factor. The controlling and optimizing the energy levels at the Zn(O,S)/CIGS interface were crucial for improved solar cell performances.

Engineering a Ga-Gradient by One-Step Sputtering to Achieve Over 15% Efficiency of Cu(In,Ga)Se2 Flexible Solar Cells without Post-selenization.

A one-step sputtering process using a quaternary target has been demonstrated to be a simple route to form Cu(In,Ga)Se2 (CIGSe) absorber without post-selenization; however, the lack of a Ga-grading structure in the CIGSe absorber confines its efficiency. Here, we demonstrate a one-step cosputtering process to control the Ga profile in the CIGSe absorber on flexible stainless steel substrates. Special attention was paid to the formation of second phases and their effects on the cell performance. Although the normal Ga-grading and efficiency enhancement could be achieved by cosputtering of CIGSe and Ga2Se3 targets, high-energy ion bombardment during the sputtering process might cause the decomposition of the Ga2Se3 target, leading to the formation of Ga2O3 in the CIGSe absorber, which gradually degraded the device performance. We replaced the Ga2Se3 target with a stoichiometric CuGaSe2 target for cosputtering, which can further enhance the cell efficiency due to the elimination of Ga2O3. However, when the Ga content at the back side of CIGSe is further increased by raising the deposition power of the CuGaSe2 target, the phase separation of CuGaSe2 may take place, resulting in the formation of Cu2-XSe and CuGaSe2 at the back side of the CIGSe absorber; therefore, the recombination at the back side is increased. By cosputtering a CIGSe target with a Cu-deficient CuGaSe2 target, we can suppress the formation of second phases and achieve designable normal grading, leading to the highest efficiency of 15.63% without post-selenization on flexible substrates.