CIGSS- Copper indium gallium diselenide
Cu(In,Ga)Se2 (CIGS) solar cells are a highly promising alternative to silicon in terms of efficiency record (22.3%)  and stability. CIGS is a complex material - a quaternary compound of Cu, In, Ga and Se with a chalcopyrite lattice structure - but it posseses various advantages for PV application: it is a p-type direct band gap semiconductor and the band gap energy can be modulated from 1.0 to 1.7 eV with the Ga content. In high-performance CIGS solar cells, the Ga/(In+Ga) ratio typically is about 0.3, corresponding to a band gap energy of approximately 1.15 eV.
High quality CIGS absorbers can be fabricated by four source co-evaporation using the so-called “3-stage process”. An alternative method of CIGSe production is the sequential process, where a Cu-In-Ga precursor film is selenized in H2Se or in Se atmosphere via rapid thermal processing at 520–550 °C . Several other approaches exist, which however usually result in lower efficiency. Non-vacuum methods for CIGSe deposition have also been developed in the last 10 years [4,5]. Yet, despite their potential of providing low-cost deposition techniques, the resulting efficiencies still stay behind compared to the conventional approaches.
The standard CIGSe solar cell structure is a superstrate configuration with the absorber layer grown on top of Molybdenum-coated soda lime glass (SLG). The pn-junction and front contact are formed by a chemical-bath deposited CdS buffer, a sputtered intrinsic/Al-doped ZnO bilayer and Ni/Al grids. MgF2 can be added as anti-reflection layer and Cd-free buffers like In2S3 or Zn(O,S) have proven similar efficiencies . Na, diffusing from the SLG into the absorber has early been identified to have significant impact on absorber quality and in recent times a well-controlled Na- and K-doping has contributed to significant gain in efficiency .
Lighter and flexible substrates, like plastics or metal foils, have been recently considered for various applications like building or product-integrated PV (BIPV- PIPV), and 20.4% of efficiency have already been obtained on flexible substrates . Furthermore, CIGS solar cells are suitable for BIPV applications not only for the feasibility of using different substrates, but also for high shading tolerance and good low light performances.
CIGS solar cells are a mature technology, with conventional modules that have reached efficiency of 14 % in industrial process . At present, the total world-wide CIGS production capacity is about 2 GWp/a with the biggest factories located in Germany and Japan. Due to its proven high efficiency it can also be used for novel high efficiency concepts. For example, CIGS can be employed as bottom cell in combination with suitable wide bandgap absorbers like perosvskite in tandem devices, enabling efficiency values well beyond 30% . Innovative approaches like CIGS micro concentrator solar cells combining material saving and efficiency enhancement are also being investigated (CHEEATAH WP9]
Summarizing, CIGS solar cell production technology is one of the best sustainable technologies, as it has a low carbon footprint and low energy payback time. Furthermore a complete recycling of end-of-life modules is technically feasible. Given the already low cost level of CIGS today, there is an enormous potential for cost reduction .
Published by Simona Binetti, UNIMIB and Martina Schmid, HZB
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