Abstract:
High efficiency Cu(In,Ga)Se2 (CIGS) thin film solar cells are usually deposited on Mo-coated soda-lime glass (SLG) substrates by the three-stage co-evaporation process. The formation of (In,Ga)2Se3 precursor layer is strongly affected by the Se flux supplied during the 1st stage. The Se source temperature of 300oC is sufficient for the entire deposition in this work. The influence of the substrate temperatures during 1st stage (T1) and the 2nd and 3rd stages (T2) is investigated for the optimum values of T1 and T2. The values of T1= 370oC and T2=520oC result in the device’s maximum efficiency of 13.7%. However, this technique consumes a lot of material, e.g. Cu, In, Ga and Se, as well as the deposition time. The depositions of CIS/CGS bilayer and CGS/CIS/CGS trilayer are employed in order to enhance the efficiency of the devices when compared with those fabricated by the 3-stage deposition process. The front and back Ga-grading are obtained from the depositions of the bilayer and trilayer absorbers, respectively. The bilayer absorbers with back Ga-grading show the increasing trend of the average value of the short-circuit current density (Jsc) due to the assisting back surface field, but the average open-circuit voltage (Voc) is significantly low due to the reduction of Ga content at the front surface. On the other hand, the CGS/CIS/CGS trilayer absorbers show double Ga-grading resulting in the increase of the Voc when compared with the CIS/CGS bilayers. The highest efficiencies of the devices fabricated from the 1.8 µm thick CIS/CGS bilayer and CGS/CIS/CGS trilayer absorber, show the maximum value of 12.5% and 15.5%, respectively. The external quantum efficiency (EQE) of the 1.8 µm thick bilayer and trilayer absorbers shows the enhancement in the long wavelengths. It was found that the 0.8 and 1.2 µm thick trilayer absorbers can maintain the same level of efficiency to that of the bilayer absorber of 1.8 µm thick.
