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ISSN: 2348-9812 The advantages of Cu(In,Ga)Se2(CIGS), their direct band gap, high light absorption coefficient (1×10 5 cm-1), controllable band gap, etc., have made it one of the most attractive thin films for solar cells with a conversion efficiency of over 20% [1,2]. Nowadays, it has been demonstrated that the insufficient-copper CIGS thin films prepared by the three-stage deposition process can achieve a high conversion efficiency due to the lowering of the defect formation energy (less than 1eV) [3,4]. A composite defect structure ( 2 2 Cu Cu V In − + + ) that has a negative value of formation energy spontaneously appears as a consequence of order vacancy compounds (OVCs) at the surface, which can lead to Cd ion diffusion into the surface of the absorber layer to produce a buried homo-junction during the chemical bath deposition process [5]. Meanwhile, improving the open-circuit voltage and short-circuit current densities of solar devices during the three-stage deposition process depends upon the formation of the depth-profile gallium gradient [6]. The key for the development of high-efficiency devices is to effectively control the gallium gradient [7,8]. Introduction The objective of this work is to investigate the influence of sodium on the characteristics of Cu(In,Ga)Se2 (CIGS) thin films prepared by a three-stage deposition process with a MoNa back contact, in comparison to Mo/SLG. The XRD results show that all the CIGS thin films deposited onto Mo/SLG substrates have the (112) preferred orientation, rather than the (220/204) orientation of CIGS deposited onto Na-free substrates. As the thickness of the MoNa increases, the (220/204) orientation becomes more common in the CIGS thin films due to the surfactant effect of the sodium which can reduce the surface free energy during the growth process. A stronger Ga gradient and a decrease of grain size near the back contact are observed when the Na content increases. Raman analysis reveals that all the CIGS thin films exhibit a single peak without the occurrence of an OVC compound. The Raman peaks of the absorber layers shift from 174 cm-1 to 171 cm-1 as a result of the formation of a more stable NaInSe2 compound by the replacement of Cu vacancy sites by Na. The conversion efficiencies of CIGS solar cells deposited on SLG/Mo and Na-free glass/Mo/MoNa(200nm)/Mo are 13.71% and 14.24%, respectively.

The Influence of Sodium on the Characterization of Cu(In,Ga)Se2 Thin Films Prepared by Three-Stage Deposition Process