CIGS Solar Cells

These are essentially thin film solar cells. So, thin film of CIGS is used as primary absorber. These are essentially called chalcopyrite based solar cells. CuInSe2 ( Copper Indium Selenide ), CuInS2 ( Copper Indium Sulphide ) and CuGaSe2 ( Copper Gallium Selenide ) with band gap of 1 eV, 1.5 eV and 1.7 eV are the major chalcopyrite compounds that are being used for solar cell applications. With  a band gap of 1 eV CuInSe2 ( Copper Indium Selenide ) provides the heighest efficiency and its efficiency can be further increased by addition of Gallium, thus making it Cu(InGa)Se2 (Copper Indium Gallium Selenide) with a band gap of 1.15 eV.

Note :

Band gap of 1.45 eV  is most appropriate for solar cell applications because wavelength corresponding to 1.45 eV comes out to be 855 nm (λ = hc/E = 1240/1.45 = 855 nm). Thus, whole visible region will be absorbed and wavelength above 855 nm will not be absorbed.

Absorption coefficient of chalcopyrites is quite high; therefore, a few micron thick active layer is required for the absorption of visible region of sunlight. CIGS is an interesting material as by varying the ratio of Indium to Gallium, one can change the bandgap. As we can vary this ratio with large tolerance CIGS band gap is tuned efficiently. Commercially bandgap of CIGS is between 1.01 eV to 1 eV. CIGS is used as P-type semiconductor and CdS is used as N-type semiconductor leading to the formation of junction and depletion region. Band gap of CIGS and CdS is 1.1 eV and 2.42 eV. As the major portion of the solar spectrum is absorbed by CIGS, CdS is mainly used as partner layer to form the cell and is referred as "Buffer Layer".

   CIGS Solar Cell Parameters

Efficiency (η) : 20 %

Open Circuit voltage (V_OC) : 0..69 V

Short Circuit Current Density (J_sc) : 35.5 mA/cm²

Fill Factor (FF) : 81 %

Mobility of Electrons (μ_e) : 90-900 cm² / Vs

Mobility of Holes (μ_h) : 5-50 cm² / Vs

Melting Temperature : 1259 K
Density : 5.75 g / cm³

Fabrication

For substrate, Mo coated glass or Soda Lime glass is used because sodium helps in surface passivation by controlling dangling bond density. Moreover coefficient of thermal expansion of soda lime glass matches with that of CIGS leading to lower thermal stress and prevents cracking in thin film. CIGS is the heart of this solar cell. CIGS is commonly deposited with co-evaporation technique. In this process temperature of around 550 degree celsius is required and all the elements ( Copper, Indium, Gallium, Arsenide ) are evaporated together. CdS is deposited using Chemical Bath Deposition (CBD) method. In CIGS solar cell substrate configuration is used instead of superstrate configuration as in CdTe solar cell. Since CIGS deposition requires high temperature conditions so, CIGS is deposited before CdS.