Copper indium gallium selenide solar cells (CIGS)

1. Copperindiumgalliumselenide solar cells (CIGS) ESRA DANIŞAN KAZIM YALÇIN

2. TOPICS What is CIGS? Manufacturing? Positions in the world? Efficiency Advantages Disadvantages

3. WHAT IS CIGS? Copper indium gallium selenide (CuIn1-xGaxSe2 or CIGS) is a direct bandgap semiconductor useful for the manufacture of solar cells. The CIGS absorber is deposited on a glass or plastic backing, along with electrodes on the front and back to collect current. Because the material has a high absorption coefficient and strongly absorbs sunlight, a much thinner film is required than of other semiconductor materials. Devices made with CIGS belong to the thin-film category of photovoltaics.

4. STRUCTUREOF A CIGS THIN-FILM SOLAR CELL The basic structure of a Cu(In,Ga)Se2 thin-film solar cell is depicted in the image of Figure 2 to the right. The most common substrate is soda-lime glass of 1–3 mm thickness. This is coated on one side with molybdenum (Mo) that serves as metal back contact. The heterojunction is formed between the semiconductors CIGS and ZnO, separated by a thin layer of CdS and a layer of intrinsic ZnO. The CIGS is doped p-type from intrinsic defects, while the ZnO is doped n-type to a much larger extent through the incorporation of aluminum (Al).

5. This asymmetric doping causes the space-charge region to extend much further into the CIGS than into the ZnO. Matched to this are the layer thicknesses and the bandgaps of the materials: the wide CIGS layer serves as absorber with a bandgap between 1.02 eV (CuInSe2) and 1.65 eV (CuGaSe2). Absorption is minimized in the upper layers, called window, by the choice of larger bandgaps: Eg,ZnO=3.2 eV and Eg,CdS=2.4 eV. The doped ZnO also serves as front contact for current collection. Laboratory scale devices, typically 0.5 cm2 in size, are provided with a Ni/Al-grid deposited onto the front side to contact the ZnO.

6. MANUFACTURING MANUFACTURING : The most common vacuum-based process co-evaporates or co-sputters copper, gallium, and indium, then anneals the resulting film with a selenide vapor to form the final CIGS structure. An alternative is to directly co-evaporate copper, gallium, indium and selenium onto a heated substrate.Most CIGS solar cells are produced using a co-evaporation technique that involves vacuums and can be costly and time-consuming. The elements are heated and deposited on a surface in a vacuum.

7. EFFICIENCY • CIGS is mainly used in the form of polycrystalline thin films. The best efficiency achieved as of December 2005 was 19.5%. In 2013, scientists at Empa, the Swiss Federal Laboratories for Materials Science and Technology, developed CIGS cells on flexible polymer foils with a new record efficiency of 20.4% for converting sunlight into electricity. These are both the highest efficiency examples as well as flexible. • 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. In another point of view, gallium is added to replace as much indium as possible due to gallium’s relative availability to indium.

8. Flexible CIGS solar cells Chirila et al. Nature materials, 10 (2011), 857 Record efficiency: 18.7% Process: substrate configuration. Co-evaporation of CIGS on polyimide covered by molybdenum Other parameters: VOC = 712 mV, JSC = 34.8 mA/cm2, FF = 75.7%, area = 0.582 cm2

9. PV module production in 2011 2011 Total Production : 37 GW

10. PRODUCTİON OF CIGS CIGS module production is expanding: it reached 0.9 GWpof production in 2011 and it will increase to about 2 GWp in 2013. CIGS module efficiencies are approaching those of c-Si modules. If the predicted cost reduction will be achieved, CIGS could be a strong competitor.

11. Direct gap thin film competitor: CIGS & CdTe Small area solar cells: record efficiency evolution Eff=20.3% Eff=17.3%

12. ADVANTAGES OF CIGS 1. The active layer (CIGS) can be deposited in a polycrystalline form directly onto molybdenum coated glass sheets or steel bands. This uses less energy than growing large crystals, which is a necessary step in the manufacture of crystalline silicon solar cells. Also unlike crystalline silicon, these substrates can be flexible. 2. One environmental advantage of CIGS solar cell technologies have over Cadmium Telluride solar cell panels is that it uses a much lower level of cadmium, in the form of cadmium sulfide. In some designs, sometimes zinc is used instead of cadmium sulfide all together. 3. Like Cadmium Telluride panels, CIGS solar cell panels show a better resistance to heat than silicon based solar panels.

13. DISADVANTAGES OF CIGS 1. Like all thin film solar panels, CIGS panels are not as efficient as crystalline silicon solar cells, for which the record efficiency lies at 24.7%. They are however, the most efficient of the thin film technologies. 2. So far being able to produce solar panels at prices that can compete with polycrystalline or cadmium telluride panels has not been possible. There is growing concern by some parties, that the cost of fabricating the product makes it difficult to be competitive with current grid prices. It may take several more years to solve the manufacturing problems and bring the production costs in line with the other leading producers of solar panels.

14. CIGS & SILICON Unlike the silicon cells based on a homojunction, the structure of CIGS cells is a more complex heterojunction system. CIGS solar cells are not as efficient as crystalline silicon solar cells, for which the record efficiency lies at 24.7%.Though some argue that CIGS will be substantially cheaper due to much lower material costs and potentially lower fabrication costs, in the second quarter of 2013, Veeco, Inc., a company with expertise in thin film deposition technology, closed a major CIGS development project due in part to the continued declining costs of the competing silicon materials and technology. A direct bandgap material, CIGS has very strong light absorption and only 1–2 micrometers of CIGS is enough to absorb most of the sunlight. A much greater thickness of crystalline silicon is required for the same absorption. The active layer (CIGS) can be deposited in a polycrystalline form directly onto molybdenum coated glass sheets or steel bands. This uses less energy than growing large crystals, which is a necessary step in the manufacture of crystalline silicon solar cells. Also unlike crystalline silicon, these substrates can be flexible.

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