CuIn1-xGaxSe2 Research Articles

Improving the crystallinity of quaternary sputtered CIGS absorber layer properties via optimized deposition and annealing temperature

This study presents a method to improve Cu(In1−xGax)Se2 (CIGS) absorber layer performance through an annealing technique that enhances the quality of as-deposited CIGS films. CIGS films were deposited on soda-lime glass substrates using RF magnetron sputtering at 300 °C, 400 °C, and 500 °C, followed by annealing at 500 °C for 30 min. By annealing, the crystal structure of the CIGS films is aligned, and surface strain is minimized. It can significantly boost the films’ crystallinity, crystallite sizes, carrier concentration, mobility, and energy gap. The highest temperatures enhanced crystallinity due to larger crystallite size, resulting in lower resistivity. The properties improvements are crucial for optimizing CIGS absorber layer performance. Complete CIGS solar cell at 300 °C yield Voc = 253 mV, Jsc = 1.78 mA/cm2, and efficiency = 0.15 %.

A Review of CIGS Thin Film Semiconductor Deposition via Sputtering and Thermal Evaporation for Solar Cell Applications

Over the last two decades, thin film solar cell technology has made notable progress, presenting a competitive alternative to silicon-based solar counterparts. CIGS (CuIn1−xGaxSe2) solar cells, leveraging the tunable optoelectronic properties of the CIGS absorber layer, currently stand out with the highest power conversion efficiency among second-generation solar cells. Various deposition techniques, such as co-evaporation using Cu, In, Ga, and Se elemental sources, the sequential selenization/sulfidation of sputtered metallic precursors (Cu, In, and Ga), or non-vacuum methods involving the application of specialized inks onto a substrate followed by annealing, can be employed to form CIGS films as light absorbers. While co-evaporation demonstrates exceptional qualities in CIGS thin film production, challenges persist in controlling composition and scaling up the technology. On the other hand, magnetron sputtering techniques show promise in addressing these issues, with ongoing research emphasizing the adoption of simplified and safe manufacturing processes while maintaining high-quality CIGS film production. This review delves into the evolution of CIGS thin films for solar applications, specifically examining their development through physical vapor deposition methods including thermal evaporation and magnetron sputtering. The first section elucidates the structure and characteristics of CIGS-based solar cells, followed by an exploration of the challenges associated with employing solution-based deposition techniques for CIGS fabrication. The second part of this review focuses on the intricacies of controlling the properties of CIGS-absorbing materials deposited via various processes and the subsequent impact on energy conversion performance. This analysis extends to a detailed examination of the deposition processes involved in co-evaporation and magnetron sputtering, encompassing one-stage, two-stage, three-stage, one-step, and two-step methodologies. At the end, this review discusses the prospective next-generation strategies aimed at improving the performance of CIGS-based solar cells. This paper provides an overview of the present research state of CIGS solar cells, with an emphasis on deposition techniques, allowing for a better understanding of the relationship between CIGS thin film properties and solar cell efficiency. Thus, a roadmap for selecting the most appropriate deposition technique is created. By analyzing existing research, this review can assist researchers in this field in identifying gaps, which can then be used as inspiration for future research.

Enhancement of Cd‐Free All‐Dry‐Processed Cu(In1‐x,Gax)Se2 Thin‐Film Solar Cells by Simultaneous Adoption of an Enlarged Bandgap Absorber and Tunable Bandgap Zn1‐xMgxO Buffer

Attempts to remove environmentally harmful materials in mass production industries are always a major issue and draw attention if the substitution guarantees a chance to lower fabrication cost and to improve device performance, as in a wide bandgap Zn1‐xMgxO (ZMO) to replace the CdS buffer in Cu(In1‐x,Gax)Se2 (CIGSe) thin‐film solar cell structure. ZMO is one of the candidates for the buffer material in CIGSe thin‐film solar cells with a wide and controllable bandgap depending on the Mg content, which can be helpful in attaining a suitable conduction band offset. Hence, compared to the fixed and limited bandgap of a CdS buffer, a ZMO buffer may provide advantages in Voc and Jsc based on its controllable and wide bandgap, even with a relatively wider bandgap CIGSe thin‐film solar cell. In addition, to solve problems with the defect sites at the ZMO/CIGSe junction interface, a few‐nanometer ZnS layer is employed for heterojunction interface passivation, forming a ZMO/ZnS buffer structure by atomic layer deposition (ALD). Finally, a Cd‐free all‐dry‐processed CIGSe solar cell with a wider bandgap (1.25 eV) and ALD‐grown buffer structure exhibited the best power conversion efficiency of 19.1%, which exhibited a higher performance than the CdS counterpart.

Comparative simulation analysis of μc Si:H and SnS BSF layers in CIGS solar cells

In this paper, we presented Cu(In1-xGax)Se2 (CIGS) based solar cells in which hydrogenated microcrystalline silicon (μc Si:H) and tin sulfide (SnS) are used as back surface field (BSF) layers. In CIGS thin film solar cell technology, thickness serves a crucial factor. Optimizing absorber thickness reduces the usage of scarce and expensive materials such as indium (In) and gallium (Ga) in CIGS cells. Integrating a heavily doped layer between the absorber and the back surface, known as the back surface field (BSF) layer, in solar cell design is a successful strategy for minimizing the thickness of the absorber layer. The suggested configuration utilizes μc-Si:H and SnS as the back surface field (BSF) layer. The buffer layer in this configuration employs less harmful InP instead of the typically utilized CdS layer. All the simulation work is performed using SCAP-1D simulation tool. The utilization of μc Si:H and SnS BSF layers yields optimized efficiencies of 31.51% and 31.01%, respectively. This simulation work demonstrates that use of a proper BSF layer is very effective not only in performance enhancement but also in reducing absorber material thickness.

Improving the photovoltaic performance of CIGS solar cells with the modified 3-stage co-evaporation process

The 3-stage co-evaporation process has been adopted as one of the standard deposition techniques for developing high efficiency CuIn1-xGaxSe2 (CIGS) thin film solar cells. This method often results in a double-graded [Ga]/([Ga] + [In]) or x value, leading to a double-graded bandgap in the CIGS films. The main challenge is to increase open-circuit voltage (Voc) without losing short-circuit current density (Jsc) of the devices. To achieve this goal without extra elemental sources or processes during fabrication, the standard 3-stage co-evaporation process could be modified by adjusting the deposition rate of Ga in the 1st and 3rd stages to enhance the double-graded profile of x. In the 1st stage, x1 is varied in the range of 0.37–0.60, whereas x3 is varied in the range of 0.15–0.37 in the 3rd stage. This is to be compared with our reference 3-stage process using constant x1 = x3 = 0.37. The effects of the x values on the photovoltaic properties of the CIGS devices and the elemental depth profiles of the CIGS absorber have been examined. In this work, the champion device has a maximum efficiency of 17.61 %, a Voc of 698.4 mV, a Jsc of 33.81 mA/cm2 and a fill factor (FF) of 76.12 %. This was achieved by using x1 = 0.55 and x3 = 0.25. These parameters demonstrate a significant beneficial improvement over reference 3-stage process and result in the notch position (the minimum of the x value) being approximately 500–800 nm below the surface of the CIGS films.

High-energy sputtering for the deposition of a conductive and adherent single molybdenum layer for solar cell applications

Molybdenum (Mo) thin films are thought to be an excellent back contact for CuIn1-xGaxSe2 (CIGS) solar cells. Mo films require high conductivity and good adhesion to glass substrates; however, these characteristics cannot be combined in sputtered Mo under the same deposition conditions. Direct current Magnetron Sputtering (DcMS) and High-Power Impulse Magnetron Sputtering (HiPIMS) were used to deposit Mo films on glass substrates, and the effects of working pressure and substrate bias on their characteristics were investigated. To begin, single layers of Mo were produced at pressures ranging from 0.2 to 1.4 Pa. The Mo films deposited at the lowest working pressure of 0.2 Pa were then subjected to a negative substrate bias varying from -50 to -200 V. All of the HiPIMS films adhered well to the glass substrate, whereas the DcMS films deposited at various pressures showed poor adhesion. Furthermore, our findings demonstrated that applying a negative substrate bias reduced the resistivity of Mo films to the lowest values of 2.56 × 10−5 Ω.cm and 2.771 × 10−5 Ω.cm, respectively, for the HiPIMS and DcMS-deposited films, while improving the adhesion of the DcMS-samples due to the development of films with homogenous stress distribution.

A synergistic approach in designing InP/ZnS quantum dot based CIGS solar cell

This research work investigates the implementation of non-toxic InP/ZnS quantum dot (QDs) buffer layer in solar cell. Due to its nontoxic behaviour, it is a better replacement of commonly used CdS buffer layer. InP/ZnS quantum dots buffer layer enhances light absorption, leading to a broader spectral response and improved power conversion efficiency. Graded CIGS (CuIn1-xGaxSe2) is used as absorber layer in this study for better utilization of solar spectrum. The combined utilization of an InP/ZnS QD buffer layer and a graded absorber layer results in a synergistic enhancement of the cell's performance. Further optimization of CIGS layers is achieved by varying the gallium content (x) in the Cu(In1−xGax)Se2. For the gallium content (x) in the range of 0.20 to 0.15, maximum efficiency is achieved which is 31.20 %. Numerical modeling is employed in this study to analyze how variations in thickness, doping concentration, and defect density impact the solar cell performances.

Synergistic effect between molybdenum back contact and CIGS absorber in the degradation of solar cells

AbstractThe stability of molybdenum (Mo) back contact and Cu (InxGa(1‐x)Se2(CIGS) absorber layers interfaces relevant for CIGS‐based solar cells was investigated using accelerated aging test, considering humidity and temperature daily variations as well as atmospheric pollution. Different configurations of sputtered Mo and co‐evaporated CIGS layers deposited on soda lime glass with or without ALD‐Al2O3 encapsulation were investigated. They were exposed for 14 days to 24 h‐cycles of temperature and humidity (25°C at 85% RH and 80°C at 30% RH) with and without solution of the pollutant salts (NaCl, Na2SO4, and (NH4)2SO4) deposited as drops on the sample to mimic marine, industrial, and rural atmospheric conditions, respectively. ALD‐Al2O3 encapsulation failed to protect the samples against the pollutants regardless of configuration. The evolution of the films was characterized by Raman spectroscopy, grazing incidence X‐ray diffraction, X‐ray photoelectron spectroscopy, scanning electron microscopy, and energy‐dispersive X‐ray spectroscopy. Unencapsulated Mo degraded forming a mixture of oxides (MoO2, MoO3, and Mo8O23). Unencapsulated CIGS on glass substrates was not altered, whereas dark spots were visible at the surface of Mo/CIGS configurations. Further characterization evidenced that even though the Mo layer was buried, its corrosion products were formed on top of CIGS. Mo corrosion products and copper selenide, Cu2‐xSe, were identified in dark spots. Their formation and evolution were further investigated by in situ Raman spectroscopy. A speculative mechanism explaining the interplay of molybdenum and CIGS layers during aging is proposed. In place of Mo oxides, detected on the open surface of bare Mo, soluble molybdates are expected in confined environment where alkalinity locally increases. The molybdate ions may then react with sodium ions accumulated at the grain boundaries of CIGS, forming Na2MoO4. The latter could form Na2Mo2O7 during drying because of pH decrease by atmospheric CO2 adsorption. High pH in confined zone, combined with relatively high temperature, is also believed to lixiviate gallium into soluble tetragallates [Ga (OH)4]2−, which could precipitate into Ga2O3 with pH decrease leaving Ga depleted Cu2‐xSe.

Tuning Band Alignment at Grain Boundaries for Efficiency Enhancement in Cu2ZnSnS4 Solar Cells.

Conducting atomic force microscopy has been performed for a fundamental understanding of the mechanism responsible for the lower power conversion efficiency (PCE) of Cu2ZnSnS4 (CZTS) solar cells than that of CuIn1-xGaxSe2 (CIGS) solar cells. The difference in efficiency is partly attributed to the distinctly different band alignment between the grain boundaries (GBs) and grain interior (GI) for the two materials. While CIGS shows type-II band alignment, CZTS was discovered to demonstrate type-I band alignment with the conduction band shifting downward while the valence band shifting upward at the GBs. The type-I band alignment in CZTS leads to both electron and hole trapping, enhancing their recombination, and lowers the PEC. Band engineering was realized by moderate oxidative annealing of CZTS. The preferential GB oxidation changes the band alignment into inverse type-I (i.e., the conduction band upward bending and valence band downward bending at GBs). The blocking of carrier recombination at GBs leads to 30% enhancement in PCE. Our work reveals the critical role that band alignment between the grain boundary and interior plays in polycrystalline thin film solar cells and suggests band alignment engineering as a practical approach to enhance PCE. Furthermore, conducting AFM has been shown to be a powerful tool for qualitative and semiquantitative characterization of band alignment in polycrystalline films.

Aspects of effectiveness and significance: The use of machine learning methods to study CuIn1-xGaxSe2 solar cells

The goal of this work is to enhance the efficiency of CuIn1-xGaxSe2 (CIGS) thin film solar cells by investigating the critical factors affecting their device performance and the correlations between them. To achieve this goal, machine learning algorithms are employed to uncover the primary parameters and correlations affecting CIGS solar cell device performance. The experimental data is used to develop the data sets for machine learning analysis. The correlation studies allow for the investigation of the key factors governing device performance. The algorithms used in the study include linear regression (LR), random forest (RF), extreme gradient boosting (XG), decision tree (DT), support vector machine regressor (SVM), stochastic gradient descent regressor (SGD) and Bayesian ridge (Bayesian). The results showed that decision trees provide the most accurate predictions of CIGS solar cell efficiency, with a root mean square error of 0.11 and 1.83 and a Pearson coefficient of 0.9 and 0.88 for the training and test data sets, respectively. Additionally, this research provides important insight into the necessary components and ideal device dimensions, offering helpful guidelines for subsequent experimenting optimization endeavours.

Stability of Cu(InxGa1−x)Se2 Solar Cells Utilizing RbF Postdeposition Treatment under a Sulfur Atmosphere

Alkali halide postdeposition treatments (PDTs) have become a key tool to maximize efficiency in Cu(InxGa1−x)Se2 (CIGS) photovoltaics. RbF PDTs have emerged as an alternative to the more common Na‐ and K‐based techniques. This study utilizes temperature‐dependent current–voltage (JVT) measurements to study a unique RbF PDT performed in a S atmosphere. The samples are measured before and after 6 months in a desiccator to study device stability. Both samples contain Na and K which diffuse from the soda–lime glass substrate. A reference sample and a RbF + S PDT sample both show the development of a rear contact barrier after aging. The contact barrier is higher for the RbF + S PDT sample, leading to decreased current in forward bias. Series resistance is also higher in the RbF + S PDT device which leads to lower fill factor. However, after aging the reference sample has a larger decrease in open‐circuit voltage (VOC). Ideality factor measurements suggest Shockley–Read–Hall recombination dominates both samples. VOC versus temperature and a temperature‐dependent activation energy model are used to calculate diode activation energies for each sample condition. Both techniques produce similar values that indicate recombination primarily occurs within the bulk absorber.

Evaluation of Voltage-Matched 2T Multi-Junction Modules Based on Monte Carlo Ray Tracing

As Si single-junction technology is approaching its Shockley–Queisser theoretical limit, relevant efforts are being expended towards the development of multi-junction modules. In this work, we employ an optical model based on Monte Carlo ray tracing to compare four different multi-junction modules in a voltage-matched two-terminal (VM2T) configuration. In particular, we took into consideration the VM2T coupling of crystalline silicon cells with CuInxGa1-xSe2 (CIGS), CdTe, GaAs and perovskite (PVK) solar cells. We optimized the thicknesses of each layer in the top sub-module and determined the performance of VM2T modules in the Shockley–Queisser theoretical limit. We also considered the possibility of using modules in which the top Si surface is flat to determine the performance drop due to the absence of the texturization on the top Si surface. Moreover, we determined the optimal bandgap energy of PVK in a VM2T PVK/Si module as well as the highest efficiency achievable. Lastly, we show that when using state-of-the-art cells, the highest VM2T efficiency achievable for the considered materials is 34.2% under standard test conditions.

On the role of sodium and copper off‐stoichiometry in Cu (In,Ga)S2 for photovoltaic applications: Insights from the investigation of more than 500 samples

AbstractThe present article discusses the investigation of CuIn1−xGaxS2 (CIGS) thin films for photovoltaic applications. For decades, a Cu‐rich composition has been used to create solar cells with efficiencies of up to 13.5%; however, interest in chalcopyrite sulfide has recently been revived due to its high and adjustable bandgap, making it a serious candidate as a top cell in tandem configurations. Although chalcopyrite selenides share many properties with CIGS thin films, crucial differences have been reported. To further understand these materials, we studied more than 500 samples of absorbers and resulting solar cells. First, we found that the compositional window for obtaining single‐phase CIGS thin films with a three‐stage co‐evaporation process is very narrow. Second, we reported that a combination of low copper content and sodium addition during growth is required to maximize the photoluminescence intensity (i.e., to minimize the absorber‐related open‐circuit voltage losses). Finally, we showed that solar cell performance and stability depend not only on absorber quality but also on phenomena at interfaces (absorber/buffer and grain boundaries). Altogether, we formulate growth recommendations for the manufacture of stable CIGS/CdS solar cells with state‐of‐the‐art efficiency.

Characterization of the defect in CIGSe solar cell by admittance spectroscopy

In this work, we used admittance spectroscopy to accurately characterize the defect energy distribution of defects in the absorption layer of a CuIn1−xGaxSe2 (CIGSe) solar cell. We found that the dark current of the CIGSe cell decreased by an order of magnitude, and its ideality factor dropped from 2.16 to 1.85 after annealing. The capacitance of the CIGSe solar cell before annealing was higher than that after annealing under reverse bias. Moreover, a higher free carrier concentration was found in the absorption layer of the cell before annealing. The results of our tests show that the built-in electric voltages of the CIGSe cell before and after annealing were 0.52 and 0.64 V, respectively. After annealing, the activation energy of defects in the absorption layer decreased, and their concentration remained almost constant, as measured by admittance spectroscopy. The decrease of the defect activation energy indicated a corresponding decrease in the Shockley–Read–Hall recombination probability of defects in the present cell; hence, the increase of the open circuit voltage and parallel resistance of the CIGSe solar cells after annealing resulted in improved performance.

Influence of Cu-atomic ratio in the 3-stage deposition technique on the efficiency of CuIn1-xGaxSe2 solar cells

The 3-stage co-evaporation technique is one of the deposition processes used to fabricate photon absorber layer in high efficiency CuIn1-xGaxSe2 (CIGS) solar cells. For this technique, the [Cu]/[III] ratio (y), where [III] refers to group-III elements, evolves from Cu-poor (y < 1) to Cu-rich (y > 1) in the 2nd stage and finally ends with slightly Cu-poor (y ~ 0.9) in the 3rd stage of the 3-stage process. Here, the highest values of [Cu]/[III] in the 2nd stage are intentionally varied from 1.0 to 1.5 by setting the deposition time of the pre-calibrated Cu and Se fluxes in the 2nd stage. The [Ga]/[III] ratio (x) is set at 0.37 during the 1st and 3rd stages in all devices. The influences of the Cu-atomic ratio are examined for the crystal grain growth, elemental depth profiles of the CIGS absorbers as well as the photovoltaic parameters and external quantum efficiency (EQE) of the CIGS solar cells. The optimal value of y = 1.3 is found to provide the highest efficiency CIGS device. The double-grading depth profile in the [Ga]/[III] ratio has also been observed despite the constant fluxes of group-III elements set during the whole process. The performances of the CIGS solar cells are investigated under AM1.5 condition and found to have open-circuit voltage (V OC) of 670 mV, short-circuit current density (J SC) of 33.2 mA/cm2, fill factor (FF) of 75.5% and the power conversion efficiency of 16.8% for the best CIGS device. The J SC of the device with y = 1.3 is relatively higher than other devices due to the increase of photo-generated currents in the short wavelength region as seen in the EQE spectrum.

Fullerene-Liquid-Crystal-Induced Micrometer-Scale Charge-Carrier Diffusion in Organic Bulk Heterojunction.

The short charge-carrier diffusion length (LD ) (100-300nm) in organic bulk heterojunction (BHJ) impedes the further improvement in power conversion efficiency (PCE) of organic solar cells (OSCs), especially for thick-film (>400nm) devices matching with industrial solution processing. Here a facile method is developed to efficiently increase LD and then improve PCEs of OSCs via introducing a fullerene liquid crystal, F1, into the active layer. F1 combines the inherent high electron mobility of fullerene and strong self-assembly capacity of liquid crystal, providing a fast channel for charge-carrier transportand reducing energetic disorder and trap density in BHJ film via enhancing crystallization. Typically, in PM6:Y6:F1 BHJ, the enhanced charge-carrier mobility (>10-2 cm-2 V-1 s-1 ) and prolonged charge-carrier lifetime (55.3 µs) are acquired to realize the record LD of 1.6 or 2.4µm for electron or hole, respectively, which are much higher than those of the PM6:Y6 binary sample and comparable to or even better than those values reported for some inorganic/hybrid materials, such as CuInx Ga(1- x ) Se2 (CIGS) and perovskite thin films. Benefitting from the micrometer-scale LD , the PM6:Y6:F1 ternary OSCs sustain a remarkable PCE of 15.23% with the active layer thickness approaching 500nm.

Optical Properties Evaluation of CuIn1-xGaxSe2 Thin Films Using OPAL2 Calculator

OPAL2 calculator has proved to be an effective technique for simulating optical losses in various materials for various applications. This study demonstrates the use of this softwareto simulate the transmittance (T) and reflectance (R) and hence evaluate the optical properties of flash evaporated CuIn1-xGaxSe2(CIGS) thin films with a ratio x = 0.28 deposited on stainless-steel (STS) and glass substrates. The simulation results exhibit excellent accuracy of the modeled design presented in this work. The CIGS with a ratio of gallium x = 0.3 showed the best matching between simulated and experimental T and R patterns. Moreover, the simulation shows that an increase in the gallium concentration increases optical losses. Finally, the results demonstrate the application of the freeware program OPAL2 to practically simulate the optical proprieties of CIGS thin films.

Formation of CuIn(1-x)GaxS2 Thin Films through a Solution Approach: Nonlinear Variation of Fermi Energy and Band Gap Bowing.

We report the formation of CuIn(1-x)GaxS2 (CIGS) thin films through a solution approach, namely, successive ionic layer adsorption and reaction (SILAR) technique. The obtained films possessed a high degree of crystallinity indicating the efficacy of the deposition process in forming CIGS films. A series of alloys have evidenced band gap bowing, that is, the optical band gap does not follow a linear relationship with the composition; the band gap of an intermediate compound is higher than that is interpolated from a linear relationship or Vegard's law. The composition-dependent band gap followed a quadratic relationship evidencing reverse band gap bowing, manifesting an upward convex behavior. With scanning tunneling spectroscopy (STS) and thereby the density of states of the disordered semiconductors, we have observed a bowing behavior in the transport gap as well and identified the roles of the conduction and valence bands in yielding the bowing phenomenon. The results are explained in terms of the anionic and cationic orbitals involved in forming the two bands. The STS studies have been analyzed further to derive the Urbach energy of the disordered semiconductors. When STS studies are combined with Kelvin probe force microscopy, which in effect provides the Fermi energy of the alloyed semiconductors, we could derive the band edges and the Fermi energy of the whole series in an absolute energy scale during the manifestation of the bowing phenomenon.

Performance improvement of an ultra-thin film solar cell based on optimized CIGS (Cu(In1-x, Gax)Se2) using appropriate plasmonic nanoparticles

In this paper, the main aim is to improve the absorption and efficiency of an ultra-thin film Cu(InxGa1-x)Se2 (CIGS) solar cell using plasmonic nanoparticles on the back surface of the cell at an optimal molar concentration (X) of the active layer. For this purpose, the effect of changing the molar concentration as well as the key parameters of nanoparticles on the back surface of the cell such as shape, material, size, and period are analyzed. Based on characteristics parameters such as short-circuit current density, open-circuit voltage, fill factor, and power conversion efficiency, the optimal molar concentration for the non-plasmonic cell is 0.4. For the plasmonic case, cubic Ag nanoparticles, with a size of 55 nm and a period of 220 nm, show an improvement of 20.02% and 20.89% for the short-circuit current density and efficiency, respectively. This is due to the near field distribution effect and localized surface plasmon resonance that causes effective scattering of light into the active layer. In addition, for a more detailed analysis of the solar cell performance in the presence of metal nanoparticles, and to investigate the effect of metal oxidation, an Ag2O layer with different thicknesses was added to the optimized Ag nanoparticle.

The Effectiveness of Ga Percent on the Electrical Characteristics of Al/CuIn1-xGaxSe2/ITO Schottky Junctions

The electrodeposition method has been employed to deposit our quaternary semiconductor thin films CuIn1-xGaxSe2 (CIGS) which is deposited on Indium Tin Oxide (ITO) substrates with different Gallium ratios (x=0, 0.2, 0.4, 0.6, 0.8 and 1). The structural and optical characteristics variation of the films with altering Ga percent has been studied. The impact of changing the Ga/(In+Ga) atomic ratio on the electrical transport characteristics of electrodeposited CIGS thin films has been investigated using I-V and C-V measurements. All junctions have revealed the Schottky behavior. The energy gap of the studied compositions has increased with increasing Ga content. This lowered the values of charge mobility in the investigated films. Besides, the optimum Ga/(In+Ga) atomic proportion in the view of the obtained results is achieved by adding Ga with 20 %. Also, this finding approves the validation of our proposed criterion in expecting the most efficient photovoltaic junctions. The obtained results could improve our current knowledge of the quaternary photovoltaic solar cells.