CIGS

CIGS (copper, indium, gallium, selenium) has been labeled by experts to be the solar technology of the future. CIS (copper, indium, selenium) technology allows us to achieve efficiency values comparable to conventional multi-crystalline silicon technology. Because the material has a high absorption coefficient and strongly absorbs sunlight, a much thinner film is required than of other semiconductor materials. CIGS solar cells have 5 main competitive advantages over other solar cell types, which are:

1. Best light absorption 2. Short energy payback time3. High stability regarding photoelectric generation 4. High photovoltaic conversion efficiency and 5. Low cost 

Figure 2: CIGS Solar Cell

CIGS Properties

CIGS is a I-III-VI 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 CuInxGa(1-x)Se2, 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). CdS is used optionally and some CIGS cells contain no cadmium at all. CIGS has an exceptionally high absorption coefficient of more than 105/cm for 1.5 eV and higher energy photons.

CIGS Technical Advantages1

1)     The spectrum of light spans from the ultraviolet, visible to the near-infrared zone. The spectrum absorbed by mono/poly/amorphous silicon cells ranges from 400-700 nm, while the absorption range for CIGS cells is from 300-1300 nm. This means that the effective time for generating power is much longer each day for CIGS than other technologies. The electron power generating process keeps working from morning to sunset.

2)     Different materials exhibit varying energy-emitting qualities, following exposure to sunlight. Both crystalline silicon and amorphous silicon PV exhibit degradation following long-term exposure to strong light, which reduces their ability to generate electricity. A study conducted at Arizona State University and TUV Rheinland PTL showed that modules made with crystalline silicon decrease by 0.5 percent per Celsius degree as temperature increases. CIGS modules do not degrade in efficiency over time like amorphous silicon, but in fact CIGS efficiency increases in the first few days of “sun soaking”. They generate consistent (kWh) output regardless of long periods of exposure to sunlight.

3)     According to the National Renewable Energy Labs (NREL), CIGS PV can, currently attain up to a 19.9% efficiency rate. However, the highest rate recorded in the industry has been up to 16%, with an average rate of 12%. It is worth noting that when crystalline silicon modules (efficiency of 16%) and CIGS modules are tested outdoors, CIGS generates 1.2 times more energy than crystalline silicon modules. CIGS modules produce consistent superior output (kWh/kWp) when tested in the same natural environment.

4)     In assessing whether a particular type of renewable energy is genuinely sustainable, one needs to look not only at its efficiency but also at how long it takes for the renewable energy produced to offset the resources needed to produce that facility. This is known as “energy payback time.” According to the US Department of Energy, assuming a 30 year-life of the solar installation, crystalline silicon PV payback time is estimated to be 2-4 years. CIGS PV payback time is estimated to be 1-2 years.

5)     Silicon based modules have a degradation factor of 1-2% in (kWh) output per year. CIGS modules provide consistent (kWh) output each year with little or no degradation.

6)     CIGS modules have excellent performance during low light, cloudy or hazy conditions. CIGS panels are activated by the presence of a low level of photons. They are less dependent on direct sun radiation or the position of the sun in order to function. For this reason, they are the most diverse and effective panel available and they operate in most light conditions.

7)     CIGS modules are the most light-sensitive type panel in the market. The choice is basically between panels with a slightly higher efficiency rating generating power for a shorter period of time each day or panels that generate electricity from morning to sunset and in low-light conditions. Research shows that CIGS panels produce more electricity per day operating in the same natural environment. They deliver high efficiency and high overall electrical output. Even

1 http://www.uspvmc.org/technology_PVMC.html

though the current in use CIGS PV’s efficiency may be lower than that of silicon-based modules, CIGS panels absorb more sunlight per day and so they produce more energy in total each day.

8)     Material-saving production without silicon.

9)     Copper, indium, selenium (CIS) forms an extremely thin layer that has a high absorption quality that allows 99% of the available light to be absorbed in the first micron of material . It is all applied to energy production. Adding a small amount of gallium forming (CIGS) boosts the light absorbing band gap, which makes it more closely match the solar spectrum thereby improving the voltage and efficiency of the PV cells.

10)  Esthetics: Most agree that the uniform solid black color of the panels has a more appealing appearance.

Device Structure2

The most common device structure for CIGS solar cells is shown in Figure 2. Soda lime glass is commonly used as a substrate, because it 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 layer is deposited (commonly by sputtering) which serves as the back contact and reflects most unabsorbed light back into the 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, aluminum (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.

2 https://en.wikipedia.org/wiki/Copper_indium_gallium_selenide_solar_cells

Figure 2: CIGS Device Structure

The CuInSe2-based materials that are of interest for photovoltaic applications include several elements from group 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.

CIGS Solar Cell Production3 4

i) Film production

The most common vacuum-based process is to co-evaporate or co-sputter copper, gallium, and indium onto a substrate at room temperature, then anneal the resulting film with a selenide vapor. Another process is to co-evaporate copper, gallium, indium and selenium onto a heated substrate.

ii) Selenization

For Selenization, Se is supplied in the gas phase (for example as H2Se or elemental Se) at high temperatures; the Se becomes incorporated into the film by absorption and subsequent diffusion. During this step, called chalcogenization, complex interactions occur to form a chalcogenide. These interactions include formation of Cu-In-Ga intermetallic alloys, formation of intermediate metal-selenide binary compounds and phase separation of various stoichiometric CIGS compounds. Because of the variety and complexity of the reactions, the properties of the CIGS film are difficult to control.

3 http://www.solarpowerworldonline.com/2014/01/cigs-solar-cells-simplified/4 https://en.wikipedia.org/wiki/Copper_indium_gallium_selenide_solar_cells

iii) Sputtering of Metallic Layers

In this method a metal film of Cu, In and Ga is sputtered at or near room temperature and reacted in a Se atmosphere at high temperature. Sputtering a stacked multilayer of metal –Cu/In/Ga/Cu/In/Ga structure, produces a smoother surface and better crystallinity in the absorber compared to a simple bilayer (Cu-Ga alloy/In) or trilayer (Cu/In/Ga) sputtering. These attributes result in higher efficiency devices, but forming the multilayer is a more complicated deposition process.

iv) Chalcogenization of Particulate Precursor Layers

Metal or metal-oxide nanoparticles are used as the precursors for CIGS growth in Chalcogenization process. These nanoparticles are generally suspended in a water based solution and then applied to large areas by various methods, such as printing. The film is then dehydrated and, if the precursors are metal-oxides, reduced in a H2/N2 atmosphere. Following dehydration, the remaining porous film is sintered and selenized at temperatures greater than 400 °C.

v) Electrodeposition

Precursors can be deposited by electrodeposition. Two methodologies exist: deposition of elemental layered structures and simultaneous deposition of all elements (including Se). Both methods require thermal treatment in a Se atmosphere to make device quality films. Because electrodeposition requires conductive electrodes, metal foils are a logical substrate.

Figure 3: Electrodeposition

Simultaneous deposition employs a working electrode (cathode), a counter electrode (anode), and a reference electrode as in Figure 3. A metal foil substrate is used as the working electrode in industrial processes. An inert material provides the counter electrode and the reference electrode measures and controls the potential. The reference electrode allows the process to be performed potentiostatically, allowing control of the substrate's potential. The resulting films have small grains, are Cu-rich, and generally contain Cu2-xSex phases along with impurities from the

solution. Annealing is required to improve crystallinity. Efficiencies higher than 7%, a stoichiometry correction are required. The correction was originally done via high temperature physical vapor deposition.

vi) Precursor Combination by Wafer-Bonding Inspired Technique

In this process, two different precursor films are deposited separately on a substrate and a superstrate. The films are pressed together and heated to release the film from the reusable superstrate, leaving a CIGS absorber on the substrate, which is known as FASST process.

vii) CoevaporationCoevaporation, or codeposition, is the most prevalent CIGS fabrication technique. Coevaporation process deposits bilayers of CIGS with different stoichiometries onto a heated substrate and allows them to intermix.

viii) Chemical Vapor Deposition (CVD)

Chemical vapor deposition (CVD) has been implemented in multiple ways for the deposition of CIGS. Processes include atmosphere pressure metal organic CVD (AP-MOCVD), plasma-enhanced CVD (PECVD), low-pressure MOCVD (LP-MOCVD), and aerosol assisted MOCVD (AA-MOCVD).

ix) Electrospray Deposition

CIS films can be produced by electrospray deposition. The technique involves the electric field assisted spraying of ink containing CIS nano-particles onto the substrate directly and then sintering in an inert environment.[36] The main advantage of this technique is that the process takes place at room temperature and it is possible to attach this process with some continuous or mass production system like roll-to-roll production mechanism.

Conclusion

CIGS (copper, indium, gallium, selenium) has been labeled by experts to be the solar technology of the future. CIS (copper, indium, selenium) technology allows us to achieve efficiency values comparable to conventional multi-crystalline silicon technology. The advancement and development of this technology, however has been very slow. Laboratory success has been quite common but very few companies have been able to produce results that satisfy larger commercial mass production requirements. Intensive research should be conducted to establish this technology to the utilization of the energy generation of the common usage.