Abstract In this work, the sodium effect on Cu(In,Ga)Se2 thin films deposited by sputtering from a single quaternary target without any further post-selenization was investigated. Inductively-coupled plasma-mass spectrum (ICP-MS) shows the selenium concentration of CIGS films is 49.07%, which is selenium-poor. Current-voltage measurement (IV) shows a 8.2% efficiency enhancement from Na-free sample to Na-extra sample. In addition, the current blocking effect in Na-free sample cannot be seen in Na-extra sample. Subsequently, capacitance-voltage measurement (CV), admittance spectroscopy (AS), and drive-level capacitance profile (DLCP) were carried out to investigate sodium effects in our system. CV presents that Na incorporation can elevate free carrier density. AS combined with DLCP indicates that in Na-free sample there should be a large amount of selenium vacancies, which would be annihilated by Na incorporation. Photoluminescence measurement (PL) shows that Na incorporation introduces an additional peak, whose origin might be OSe. Based on the above information, it is plausible to take on the argument that sodium behaves as a catalyzer for oxygen atoms to neutralize selenium vacancies. *corresponding author, Chih-Huang Lai chlai@mx.nthu.edu.tw