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Copper indium gallium selenide

Copper indium gallium selenide (CIGS), with the CuInxGa1-xSe2, is a p-type material renowned for its role as an absorber layer in high-efficiency thin-film cells. This direct-bandgap compound exhibits a tunable bandgap ranging from 1.04 eV for CuInSe2 to 1.68 eV for CuGaSe2, achieved by varying the indium-to-gallium ratio, which optimizes light absorption across the spectrum. CIGS features a high absorption coefficient, enabling efficient conversion of to in layers as thin as 1-2 micrometers, and adopts a tetragonal . Its physical properties include a of approximately 5.7 g/cm³ and a between 990°C and 1070°C, while it remains insoluble in water. In photovoltaic applications, CIGS serves as the primary photoactive material in thin-film modules, offering advantages such as flexibility when deposited on metal foils or polymers, lightweight design, and superior performance under low-light or high-temperature conditions compared to . Laboratory-scale CIGS solar cells have achieved confirmed efficiencies exceeding 23%, with the record at 23.6% (as of July 2025) for small-area devices, though commercial modules typically range from 14% to 16% (as of 2025). Fabrication methods include co-evaporation, selenization of precursors, and multi-stage deposition processes, often incorporating elements like sodium to enhance efficiency by improving and reducing recombination. Beyond , CIGS's tunable optoelectronic properties position it for emerging uses in tandem solar cells, such as perovskite-CIGS hybrids, which have demonstrated efficiencies up to 26.3% (as of June 2025) in recent research, and potential applications in . Challenges include scaling production while maintaining uniformity and addressing material scarcity for and , driving ongoing innovations in cost-effective . Overall, CIGS remains a leading thin-film technology for , balancing high performance with adaptability to diverse substrates.

Composition and Structure

Chemical Composition

Copper indium gallium selenide (CIGS) is a quaternary I-III-VI₂ semiconductor with the general chemical formula Cu(InxGa1-x)Se₂, where the parameter x represents the indium fraction and varies from 0 (pure copper gallium selenide, CGS) to 1 (pure copper indium selenide, CIS). This tunable composition allows precise control over material properties by adjusting the In:Ga ratio. In this structure, (Cu) serves as the Group I element, contributing to p-type doping primarily through the formation of acceptor-like defects such as copper vacancies in non-stoichiometric conditions. (In) and (Ga), the Group III elements, enable bandgap engineering; increasing the Ga content widens the bandgap from approximately 1.0 eV in CIS to 1.7 eV in CGS, optimizing absorption for photovoltaic applications. (Se), the Group VI , forms the anionic framework, establishing the tetrahedral bonds essential to the chalcopyrite lattice. Stoichiometric deviations are common in high-performance CIGS, particularly Cu-poor compositions where the Cu/(In+Ga) ratio is less than 1, typically ranging from 0.8 to 0.9. These deviations introduce beneficial defects, such as ordered vacancy compounds, that reduce recombination losses and enhance collection without severely impacting . The of the Cu-In-Ga-Se system reveals stable composition ranges for the phase (α-CIGS) primarily in the Cu-poor region, coexisting with ordered defect phases (β-phases like CuIn₃Se₅) that support efficient synthesis by accommodating off-stoichiometry during growth. This two-phase domain ensures thermodynamic stability for absorber layers in devices, guiding deposition processes to avoid secondary phases that degrade performance.

Crystal Structure

Copper indium gallium selenide (CIGS), with the general formula Cu(In_{1-x}Ga_x)Se_2, adopts a tetragonal crystal structure, which represents a of the cubic zincblende found in binary III-V semiconductors. This arrangement features alternating copper and (In/Ga) atoms along the c-axis, resulting in a doubling of the edge compared to zincblende, while maintaining tetrahedral coordination for all atoms. The structure belongs to the I\bar{4}2d (No. 122), characterized by four units per and a high degree of symmetry that supports efficient charge transport in photovoltaic applications. The lattice parameters of CIGS exhibit a range of a \approx 5.6--$5.8[Å](/page/Å) andc \approx 11.2--$11.6 , with values decreasing as the gallium content (x) increases due to the smaller of Ga relative to In. For instance, in a typical high-efficiency CIGS sample, a = 5.73 and c = 11.41 have been reported. The ratio c/a remains close to 2, signaling minimal tetragonal distortion from the ideal chalcopyrite form, though slight deviations can occur depending on composition and processing conditions. These parameters directly stem from the compositional ratios, introducing lattice strain that accommodates the mixed occupancy of In and Ga sites. Within the unit cell, atoms are positioned at the 4a Wyckoff sites (0, 0, 0), while In and atoms randomly substitute at the 4b sites (0, 0, 1/2), forming a disordered cation sublattice. Selenium atoms occupy the 8d sites at coordinates approximately (1/4, 1/4, 1/4) and symmetry-equivalent positions, ensuring each Se is tetrahedrally bonded to two and two (In/Ga) atoms in the ideal structure. This configuration yields a body-centered tetragonal with strong directional bonding akin to diamond-like semiconductors. Intrinsic defects play a crucial role in the properties of CIGS, with copper vacancies (V_{\text{Cu}}) acting as dominant shallow acceptors that enhance p-type doping and carrier concentration. Antisite disorders, such as Cu substituting on In/Ga sites (Cu_{\text{In/Ga}}) or vice versa (In/Ga_{\text{Cu}}), introduce additional compensatory mechanisms that compensate for off-stoichiometry, particularly in Cu-poor growth conditions common for high-performance devices. These defects, often present at concentrations up to 10^{16}--10^{17} cm^{-3}, modulate the hole density without deeply trapping carriers, contributing to the material's tolerance for imperfections.

Physical Properties

Optical Properties

Copper indium gallium selenide (CIGS) is a direct bandgap , enabling efficient and emission processes critical for optoelectronic applications. The bandgap energy E_g of CIGS can be tuned by adjusting the indium-to- ratio, spanning from approximately 1.0 for pure CuInSe_2 (, where the gallium fraction x = 0) to 1.7 for pure CuGaSe_2 (CGS, x = 1). This tunability arises primarily from shifts in the conduction band edge, allowing customization for specific spectral responses. For photovoltaic applications, a bandgap of around 1.2 —corresponding to a gallium fraction of about 0.3—proves optimal, as it balances of the while minimizing thermalization losses. The material exhibits a high absorption coefficient exceeding $10^5 cm^{-1} across the (roughly 400–800 nm), which facilitates strong light harvesting in thin films as thin as 1–2 \mum. This property stems from the direct bandgap nature, promoting rapid electron-hole pair generation with minimal material thickness required. Such efficient absorption underpins the viability of CIGS in , flexible technologies. In the visible range, the n of CIGS typically ranges from 2.5 to 3.0, reflecting its dense structure and high . The k, indicative of strength, varies with wavelength and composition but generally falls between 0.1 and 0.5 in the 400–800 nm region, decreasing toward longer wavelengths near the bandgap edge. These optical constants, derived from spectroscopic , are essential for modeling light propagation and minimizing losses in device stacks. Photoluminescence (PL) spectra of CIGS reveal emissions primarily from defect-related recombination, providing insights into material quality and non-radiative pathways. Common features include broad bands at energies below the bandgap, such as ~0.96 eV and ~1.07 eV at low temperatures (10–160 K), attributed to selenium vacancies (V_{Se}) and indium-on-copper antisites (In_{Cu}) or their complexes. These defect states, often tunneling-assisted, broaden the spectra and influence carrier lifetimes, with higher-energy emissions (~1.13 eV) emerging at from band-to-band transitions. Such PL characteristics highlight the role of compositional grading in mitigating deep traps for improved optoelectronic performance.

Electrical Properties

Copper indium gallium selenide (CIGS) is a p-type , with its conductivity primarily arising from native defects such as copper vacancies (V_{\mathrm{Cu}}), which act as shallow acceptors. These defects introduce s into the valence band, resulting in typical concentrations ranging from $10^{16} to $10^{17} cm^{-3} in undoped or lightly doped films, depending on composition and growth conditions. The p-type nature is further influenced by copper antisites on group III sites (Cu_{III}), though compensation from donors like copper interstitials can modulate the net carrier density. Hole in CIGS, denoted as \mu_p, typically ranges from 10 to 100 cm^2/V\cdots in polycrystalline films, enabling reasonable charge transport despite the material's complex microstructure. This is significantly limited by grain boundaries and defects, which act as centers and recombination sites, reducing the effective length of minority carriers (electrons) to around 1-3 \mum in high-quality absorbers. Grain boundaries, inherent to the polycrystalline nature of CIGS, exhibit recombination velocities on the order of 100-200 cm/s, contributing to nonradiative losses that impact overall device performance. In configurations, the valence band offset at the CIGS/CdS buffer layer is approximately 1.0 , forming a positive barrier that facilitates extraction while the conduction band alignment creates a spike-like offset of approximately 0.1 to minimize back-transfer. This band alignment, determined via techniques like , ensures efficient carrier separation at the junction. Doping strategies, particularly sodium (Na) incorporation from substrates like soda-lime glass, enhance electrical properties by passivating defects such as undercoordinated Se atoms and Cu vacancies at grain boundaries and surfaces. Na doping increases hole concentration by up to an order of magnitude and reduces interface recombination, leading to improved open-circuit voltage through better band bending and suppressed leakage currents. Optimal Na levels, often achieved via diffusion during growth, balance passivation benefits against potential overcompensation that could introduce deep traps.

Synthesis and Fabrication

Vacuum-Based Methods

Vacuum-based methods for synthesizing copper indium gallium selenide (CIGS) absorbers rely on high-vacuum environments to deposit thin films with precise and minimal impurities, enabling the formation of high-quality structures essential for photovoltaic performance. These techniques, conducted under pressures typically below 10^{-6} , facilitate atomic-level control over layer composition and enable scalability from laboratory to industrial production. Co-evaporation stands as the primary vacuum-based approach for achieving record efficiencies in CIGS solar cells, involving the simultaneous thermal evaporation of elemental , In, , and sources from Knudsen cells onto a substrate heated to approximately 500°C. The process commonly employs a three-stage sequence to optimize phase formation and bandgap grading: in the first stage, In, , and are deposited at around 400°C to create an (In,Ga)_2Se_3 precursor layer; the second stage introduces and excess Se at 500–550°C, forming Cu-rich CIGS phases that enhance grain growth; and the third stage adds In, , and to adjust the composition to near-stoichiometry while incorporating a Ga gradient for improved carrier collection. This method, refined through research at the (NREL), has produced lab-scale cells with certified efficiencies exceeding 23%, including a record of 23.64% confirmed for a single-junction CIGS device by as of 2024. Sputtering represents another key vacuum-based technique, particularly suited for large-area deposition, where metallic , In, and precursors are sequentially or reactively sputtered onto substrates using magnetron sources under , followed by a post-deposition selenization step in a Se vapor atmosphere at 400–550°C to form the CIGS phase. This approach allows for uniform precursor layers with controlled thickness and , often integrated into inline systems for on flexible substrates. Industrial implementations of sputtering-based processes have demonstrated viability for continuous , yielding module efficiencies up to 17% while maintaining compositional uniformity over large areas. Critical process parameters in both methods include , which must be precisely controlled at 450–550°C to drive and avoid secondary phases, and Se flux or , typically set 4–5 times higher than metal rates to ensure complete selenidation and prevent Se deficiencies that degrade . These controls are monitored using quartz crystal microbalances or effusion cell temperatures to maintain the Cu/ (In+Ga) ratio near 0.9 and Ga/ (In+Ga) between 0.2–0.3 for optimal bandgap tuning around 1.1–1.2 eV. Vacuum-based methods offer advantages such as exceptional film purity—free from atmospheric contaminants—and high uniformity across large areas, which contribute to low defect densities and enhanced charge transport in finished devices. However, they incur high for vacuum chambers and heating systems, along with demands from elevated temperatures, limiting compared to ambient processes. Despite these drawbacks, ongoing innovations in inline co-evaporation and sputtering-selenization continue to bridge lab-to-fab transitions for viability.

Solution-Based Methods

Solution-based methods for fabricating copper indium gallium selenide (CIGS) thin films offer non-vacuum alternatives to traditional techniques, enabling cost-effective production through liquid-phase processing. These approaches typically involve the formation of metal precursor layers from aqueous or organic solutions, followed by thermal treatments to incorporate and crystallize the material into the structure. Key techniques include and ink-based printing, which leverage electrochemical or deposition processes to deposit uniform layers on substrates, potentially including flexible ones. Electrodeposition utilizes cathodic reduction to deposit , , and precursors from aqueous s onto a conductive , such as molybdenum-coated . The process employs solutions containing metal salts like CuCl₂, InCl₃, and GaCl₃, often with complexing agents such as to control deposition rates and improve Ga incorporation, which is challenging due to its higher . After precursor formation, the film undergoes selenization by annealing in a vapor or H₂Se atmosphere at temperatures around 500–600°C, converting the metallic layers into the polycrystalline CIGS phase. This method has been advanced through optimized compositions and multi-step annealing protocols to enhance film and minimize defects. Ink-based printing methods deposit CIGS precursors using nanoparticle inks or sol-gel solutions via techniques like spin coating, blade coating, or inkjet printing, allowing for scalable patterning on rigid or flexible substrates. Nanoparticle inks are prepared by synthesizing Cu-In-Ga-Se colloids through solvothermal or sonochemical routes in solvents such as oleylamine or ethanol, while sol-gel precursors dissolve metal salts in protic solvents like hydrazine for molecular-level uniformity. The printed layers are then annealed at 400–550°C to densify the film and promote crystallization, with selenium incorporation achieved through exposure to Se vapor during the annealing step. These processes enable roll-to-roll manufacturing, facilitating the production of large-area modules on polymers or metals. General process steps in solution-based CIGS fabrication begin with precursor layer formation via or , ensuring compositional control through and additive adjustments. Subsequent annealing at 400–550°C in an inert or reactive atmosphere crystallizes the material, often yielding the structure after brief reference to post-annealing phase evolution. Selenium integration occurs concurrently or sequentially via gas-phase selenization, avoiding high-vacuum requirements. These methods provide advantages such as lower equipment costs compared to systems and the potential for large-area through continuous , making them suitable for industrial scalability. However, challenges persist in achieving compositional uniformity across the film, particularly for Ga distribution, and controlling impurities from solvents or additives that can degrade electrical properties. Ongoing research focuses on eco-friendly solvents and precise deposition controls to mitigate these issues.

Applications

Photovoltaic Devices

Copper indium gallium selenide (CIGS) serves as the primary absorber material in thin-film photovoltaic devices due to its tunable bandgap and high absorption coefficient, enabling efficient conversion of to . The standard device architecture consists of a soda-lime glass substrate coated with a (Mo) back contact, followed by the p-type CIGS absorber layer (typically 1-2 μm thick), a thin (CdS) buffer layer, an intrinsic (i-ZnO) layer, and a transparent conductive (TCO) front contact such as aluminum-doped (AZO) or (ITO). This layered stack facilitates charge separation and collection, with the CIGS layer absorbing photons to generate electron-hole pairs while minimizing recombination at interfaces. Laboratory-scale CIGS solar cells have achieved certified power conversion efficiencies exceeding 23%, with a record of 23.64% reported for single-junction devices as of February 2024. For commercial modules, efficiencies typically range from 13% to 18% as of 2025, with a certified record of 19.64% for an encapsulated thin-film module as of 2023. Key performance factors include an (Voc) around 0.7 V and a fill factor (FF) greater than 80%, which contribute to overall device output by optimizing voltage and current extraction under standard test conditions. These metrics highlight CIGS's competitiveness with other thin-film technologies, though scaling to large-area modules remains a challenge due to uniformity issues. To surpass single-junction limits, CIGS has been integrated into tandem solar cell configurations, pairing its absorber with wide-bandgap materials like or to better match the solar spectrum and achieve higher efficiencies. Perovskite/CIGS tandems, often in four-terminal setups, have reached certified efficiencies of 26.3% as of June 2025. Silicon/CIGS tandems are also explored for mechanical flexibility and cost advantages, with potential efficiencies beyond 30% in monolithic designs, though interface engineering is critical for current matching. Despite strong performance, CIGS devices face stability challenges, including light-induced (LID) that reduces over time through mechanisms like defect formation or metastable states in the absorber. This , often observed under prolonged illumination and elevated temperatures, can lower and by up to 10-20% initially. doping, particularly with sodium (Na) or potassium (K) introduced during absorber growth, mitigates LID by passivating defects and enhancing passivation, leading to improved long-term with minimal loss after 1000 hours of exposure.

Emerging Uses

CIGS has emerged as a promising bottom material in architectures, particularly when paired with wide-bandgap , to surpass the limits of single-junction devices. In /CIGS , the tunable bandgap of CIGS (typically 1.0–1.3 eV) complements the top (1.6–1.8 eV), enabling better utilization of the solar spectrum and targeting exceeding 30%. Monolithic /CIGS have achieved certified power conversion of 26.3% as of June 2025 through optimized interface engineering and current matching. Four-terminal configurations have demonstrated up to 29.4% as of February 2025, highlighting the pathway to higher performance via decoupled optimization of sub-cells. While III-V/ hybrids dominate high- , CIGS integration in such systems remains exploratory, with potential for flexible, lightweight designs. The adaptability of CIGS to flexible substrates has opened avenues in wearable and integrated sensors, leveraging its high and robustness. Deposited on polymer foils like , CIGS thin films enable lightweight solar modules with efficiencies up to 20.4%, suitable for curved or conformable surfaces in wearables. Flexible /CIGS tandems have achieved a record efficiency of 23.64% as of April 2025. These flexible CIGS devices power portable electronics, such as health-monitoring sensors, by providing on-body without rigid components. In sensor applications, CIGS-based photodetectors on flexible substrates detect near-infrared light for biomedical or , benefiting from the material's low-temperature processing compatibility with polymers. Beyond , CIGS exhibits potential in optoelectronic devices due to its direct bandgap and tunable optoelectronic properties. CIGS thin films and nanostructures serve as photodetectors, particularly in the near-infrared range (up to 980 ), with solution-processed devices achieving high and detectivity through potassium-induced bandgap grading. CIGS photodetectors demonstrate enhanced speed and sensitivity, with detectivities exceeding those of traditional silicon-based detectors, making them suitable for and sensing applications. For light emission, CIGS quantum dots enable tunable from to (520–688 ), with quantum yields over 70% when capped with ZnS shells; these dots have been incorporated into quantum-dot LEDs (QD-LEDs) for displays, offering color-tunable emission via composition control of In/Ga ratios. Research prototypes have explored CIGS for thermoelectric generation, exploiting the Seebeck effect in its p-type to convert temperature gradients into . Measurements on CIGS reveal a peaking at 99 µV/K near 254 K, with electrical conductivity increasing with temperature, leading to a rising above 237 K that supports . The material's reaches 1.6 × 10^{-9} K^{-1} at , positioning CIGS as a candidate for hybrid photovoltaic-thermoelectric devices, though efficiencies remain modest compared to dedicated thermoelectrics.

History and Development

Early Research

The foundational research on copper gallium selenide (CIGS) originated with the exploration of its parent compound, copper (CuInSe₂ or CIS), in the 1970s. The first CuInSe₂-based photovoltaic devices were reported in 1975 by L.L. Kazmerski et al., achieving initial efficiencies around 5% using evaporated thin films paired with . This breakthrough highlighted the material's potential for photovoltaic applications due to its suitable bandgap and high absorption coefficient, sparking initial interest in semiconductors. By the early 1980s, focus shifted to polycrystalline thin-film to enable scalable fabrication. At , scientists developed evaporation-based deposition methods for thin-film , producing the first high-efficiency polycrystalline devices with 9.4% efficiency in 1981 through co-evaporation of , , and onto heated substrates. Similar advances occurred at Solar and the Institute of Energy Conversion, where two-stage processes involving metal precursor selenization yielded efficiencies exceeding 10% by 1984, demonstrating the viability of low-cost, non-single-crystal absorbers. These experiments emphasized as a key technique for achieving uniform films, though initial yields were limited by interface issues and material purity. To optimize light and device performance, was incorporated into in the mid-1980s, forming the CIGS (CuIn₁₋ₓGaₓSe₂) for . A seminal demonstration in 1987 by Devaney and Mickelsen at produced the first thin-film CIGS with 10% efficiency, using sequential to control Ga content and tune the bandgap from 1.0 (for x=0 in ) to higher values up to ~1.7 (for CuGaSe₂). This work established that increasing Ga fraction linearly widens the bandgap (E_g ≈ 1.0 + 0.6x ), enabling better matching to the solar spectrum and reducing thermalization losses, as confirmed in early optical studies linking composition to edges. Early CIGS films, however, presented significant challenges in stoichiometry control and defect management, particularly in polycrystalline forms deposited via evaporation. Deviations from ideal Cu/(In+Ga) and (In+Ga)/Se ratios led to phase impurities and deep-level defects, such as copper vacancies and antisite disorders, which pinned the and reduced carrier lifetimes, limiting open-circuit voltages to below 0.5 V despite promising short-circuit currents. Researchers at institutions like the investigated these issues through fundamental characterization, identifying the need for precise flux ratios during deposition to minimize recombination and improve film quality in initial lab-scale devices.

Commercial Milestones

The commercialization of copper indium gallium selenide (CIGS) technology began in the mid-2000s, with Global Solar Energy launching commercial production of flexible CIGS modules in 2006 at its facility, achieving module efficiencies around 12% at the time. This marked one of the earliest industrial-scale efforts, enabling applications and portable power systems due to the lightweight and flexible nature of the panels. By the early , Global Solar expanded production capacity to 40 MW annually, focusing on roll-to-roll manufacturing to reduce costs. Acquisitions and consolidations accelerated CIGS industrialization during the 2010s, particularly by China's Hanergy Holding Group, which acquired Global Solar in 2013 to integrate its flexible CIGS technology into a broader thin-film portfolio. Hanergy also purchased MiaSolé in 2013 and Solibro in 2012, creating a combined production capacity exceeding by mid-decade and enabling large-scale module fabrication for utility projects. These moves facilitated and , though Hanergy's later financial challenges in 2015 slowed some expansions. Key industry players drove further advancements, including Solar Frontier, a subsidiary of Showa Shell Sekiyu established in 2006, which scaled to over 900 MW annual production by 2016 through multiple factories in . Avancis, formed in 2006 from earlier CIS research dating to 1998, focused on high-durability modules for , reaching 120 MW capacity in before its 2014 acquisition by China's CNBM. , primarily known for CdTe modules, invested in CIGS R&D through partnerships, contributing to efficiency breakthroughs while leveraging its manufacturing expertise. Efficiency records progressed significantly, with the (NREL) certifying 23.35% for a CIGS cell by Solar Frontier in 2019, building on prior gains from compositional tuning. In 2024, achieved an NREL-certified record efficiency of 23.64% for a CIGS . Module production reached gigawatt levels in the , with companies like Solar Frontier achieving over 1 GW annual capacity by 2016, though production has since declined due to market shifts, with Solar Frontier ceasing operations in 2022. Market trends reflect CIGS's niche within thin-film photovoltaics, representing a niche within thin-film , with cumulative installations around 15 GW (less than 1% of global ) as of amid competition from . Cost reductions, driven by higher throughput and material , brought CIGS module prices below $0.50/W in high-volume scenarios, enhancing competitiveness for off-grid and flexible applications. However, selenium scarcity posed supply chain challenges, potentially increasing costs by 10-20% during price spikes and prompting initiatives to recover over 90% of from end-of-life modules. Following Hanergy's 2015 , several subsidiaries were restructured or sold, impacting global production. As of , emerging players continue to advance CIGS for niche applications.