A copper indium gallium selenide solar cell (CIGS cell, sometimes CI(G)S or CIS cell) is a type of thin-film solar cell. It is manufactured by depositing a thin layer of copper indium gallium selenide solid solution on glass or plastic backing, along with electrodes on the front and back to collect electric current. Because the material has a high absorption coefficient and strongly absorbs sunlight, a much thinner film is required than of other semiconductor materials.
CIGS is one of three mainstream thin-film photovoltaic (PV) technologies, the other two being cadmium telluride and amorphous silicon. Like these materials, CIGS layers are thin enough to be flexible, allowing them to be deposited on flexible substrates. However, as all of these technologies normally use high-temperature deposition techniques, the best performance normally comes from cells deposited on glass, even though advances in low-temperature deposition of CIGS cells have erased much of this performance difference. CIGS outperforms polysilicon at the cell level, however its module efficiency is still lower, due to a less mature upscaling.
Thin-film market share is stagnated at around 15 percent, leaving the rest of the PV market to conventional solar cells made of crystalline silicon. In 2013, the market share of CIGS alone was about 2 percent and all thin-film technologies combined fell below 10 percent. CIGS cells continue being developed, as they promise to reach silicon-like efficiencies, while maintaining their low costs, as is typical for thin-film technology. Prominent manufacturers of CIGS photovoltaics were the later bankrupted companies Nanosolar and Solyndra. The market leader is the Japanese company Solar Frontier, with Global Solar and Group Solar Hong Kong (GSHK Solar) also producing solar modules free of any heavy metals such as cadmium and/or lead. Many CIGS solar panel manufacturer companies have gone bankrupt.
Properties
CIGS is a I-III-VI2 compound semiconductor material composed of copper, indium, gallium, and selenium. The material is a solid solution of copper indium selenide (often abbreviated "CIS") and copper gallium selenide, with a chemical formula of , where the value of x can vary from 1 (pure copper indium selenide) to 0 (pure copper gallium selenide). It is a tetrahedrally bonded semiconductor, with the chalcopyrite crystal structure. The bandgap varies continuously with x from about 1.0 eV (for copper indium selenide) to about 1.7 eV (for copper gallium selenide).
CIGS has an exceptionally high absorption coefficient of more than 105/cm for 1.5 eV and higher energy photons. CIGS solar cells with efficiencies around 20% have been claimed by the National Renewable Energy Laboratory (NREL), the Swiss Federal Laboratories for Materials Science and Technology (Empa), and the German Zentrum für Sonnenenergie und Wasserstoff Forschung (ZSW) (translated: Center for Solar Energy and Hydrogen Research), which is the record to date for any thin film solar cell.ZSW: Press Releases. Zsw-bw.de. Retrieved on 2011-09-13. In 2024 a CIGS efficiency record of 23.64% was claimed.
All high performance CIGS absorbers in solar cells have similarities independent of production technique. First, they are polycrystalline α-phase which has the chalcopyrite crystal structure shown in Figure 3. The second property is an overall Cu deficiency. Cu deficiency increases the majority carrier (hole) concentration by increasing the number of (electron-accepting) Cu vacancies. When CIGS films are In rich (Cu deficient) the film's surface layer forms an ordered defect compound (ODC) with a stoichiometry of . The ODC is n-type, forming a p-n homojunction in the film at the interface between the α phase and the ODC. The recombination velocity at the CIGS/CdS interface is decreased by the homojunction's presence. The drop in interface recombination attributable to ODC formation is demonstrated by experiments which have shown that recombination in the bulk of the film is the main loss mechanism in Cu deficient films, while in Cu rich films the main loss is at the CIGS/CdS interface.
Sodium incorporation is necessary for optimal performance. Ideal Na concentration is considered to be approximately 0.1%. Na is commonly supplied by the soda-lime glass substrate, but in processes that do not use this substrate the Na must be deliberately added. Na's beneficial effects include increases in p-type conductivity, texture, and average grain size. Furthermore, Na incorporation allows for performance to be maintained over larger stoichiometric deviations. Simulations have predicted that Na on an In site creates a shallow acceptor level and that Na serves to remove In on Cu defects (donors), but reasons for these benefits are controversial. Na is also credited with catalyzing oxygen absorption. Oxygen passivates Se vacancies that act as compensating donors and recombination centers.
Alloying CIS (CuInSe2) with CGS (CuGaSe2) increases the bandgap. To reach the ideal bandgap for a single junction solar cell, 1.5 eV, a Ga/(In+Ga) ratio of roughly 0.7 is optimal. However, at ratios above ~0.3, device performance drops off. Industry currently targets the 0.3 Ga/(In+Ga) ratio, resulting in bandgaps between 1.1 and 1.2 eV. The decreasing performance has been postulated to be a result of CGS not forming the ODC, which is necessary for a good interface with CdS.
The highest efficiency devices show substantial texturing, or preferred crystallographic orientation. A (204) surface orientation is observed in the best quality devices. A smooth absorber surface is preferred to maximize the ratio of the illuminated area to the area of the interface. The area of the interface increases with roughness while illuminated area remains constant, decreasing open-circuit voltage (VOC). Studies have also linked an increase in defect density to decreased VOC. Recombination in CIGS has been suggested to be dominated by non-radiative processes. Theoretically, recombination can be controlled by engineering the film, and is extrinsic to the material.
Structure
The most common device structure for CIGS solar cells is shown in the diagram (see Figure 1: Structure of a CIGS device). Soda-lime glass of about of 1–3 millimetres thickness is commonly used as a substrate, because the glass sheets contains sodium, which has been shown to yield a substantial open-circuit voltage increase, notably through surface and grain boundary defects passivation. However, many companies are also looking at lighter and more flexible substrates such as polyimide or metal foils. A molybdenum (Mo) metal layer is deposited (commonly by sputtering) which serves as the back contact and reflects most unabsorbed light back into the CIGS absorber. Following molybdenum deposition a p-type CIGS absorber layer is grown by one of several unique methods. A thin n-type buffer layer is added on top of the absorber. The buffer is typically cadmium sulfide (CdS) deposited via chemical bath deposition. The buffer is overlaid with a thin, intrinsic zinc oxide layer (i-ZnO) which is capped by a thicker, aluminium (Al) doped ZnO layer. The i-ZnO layer is used to protect the CdS and the absorber layer from sputtering damage while depositing the ZnO:Al window layer, since the latter is usually deposited by DC sputtering, known as a damaging process. The Al doped ZnO serves as a transparent conducting oxide to collect and move electrons out of the cell while absorbing as little light as possible.
The CuInSe2-based materials that are of interest for photovoltaic applications include several elements from groups I, III and VI in the periodic table. These semiconductors are especially attractive for solar applications because of their high optical absorption coefficients and versatile optical and electrical characteristics, which can in principle be manipulated and tuned for a specific need in a given device.
Conversion efficiency
CIGS is mainly used in the form of polycrystalline thin films. The best efficiency achieved as of September 2014 was 21.7%. A team at the National Renewable Energy Laboratory achieved 19.9%, a record at the time, by modifying the CIGS surface and making it look like CIS. These examples were deposited on glass, which meant the products were not mechanically flexible. In 2013, scientists at the Swiss Federal Laboratories for Materials Science and Technology developed CIGS cells on flexible polymer foils with a new record efficiency of 20.4%. These display both the highest efficiency and greatest flexibility.
The U.S. National Renewable Energy Laboratory confirmed 13.8% module efficiency of a large-area (meter-square) production panel, and 13% total-area (and 14.2% aperture-area) efficiency with some production modules. In September 2012 the German Manz AG presented a CIGS solar module with an efficiency of 14.6% on total module surface and 15.9% on aperture, which was produced on a mass production facility.Top 10 World's Most Efficient CI(G)S Modules . Solarplaza.com. Retrieved on 2013-02-18. MiaSolé obtained a certified 15.7% aperture-area efficiency on a 1 m2 production module, and Solar Frontier claimed a 17.8% efficiency on a 900 cm2 module.
Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light. The use of gallium increases the optical band gap of the CIGS layer as compared to pure CIS, thus increasing the open-circuit voltage. Gallium's relative abundance, compared to indium, lowers costs.
Lab record CIGS efficiencies by substrate, ~0.9 cm2 Substrate Glass Steel Aluminium Polymer Efficiency 23.6% 17.7% 16.2% 20.4% Institute Uppsala University/Evolar Empa Empa Empa
Comparison
Conventional crystalline silicon
Unlike conventional crystalline silicon cells based on a homojunction, the structure of CIGS cells is a more complex heterojunction system. A direct bandgap material, CIGS has very strong light absorption and a layer of only 1–2 micrometers (μm) is enough to absorb most of the sunlight. By comparison, a much greater thickness of about 160–190 μm is required for crystalline silicon.
The active CIGS-layer can be deposited in a polycrystalline form directly onto molybdenum (Mo) coated on a variety of several different substrates such as glass sheets, steel bands and plastic foils made of polyimide. This uses less energy than smelting large amounts of quartz sand in electric furnaces and growing large crystals, necessary for conventional silicon cells, and thus reduces its energy payback time significantly. Also unlike crystalline silicon, these substrates can be flexible.
In the competitive PV industry, CIGS manufacturers faced increasing pressure, resulting in the bankruptcy of several companies as prices for conventional silicon cells declined. Despite this, CIGS solar cells have reached efficiencies comparable to multicrystalline silicon cells, the most common type. CIGS and CdTe-PV remain the only commercially successful thin-film technologies in the rapidly growing global PV market.
Other thin films