Effect of Na on Cu(In,Ga)Se2 in-plane conductance and Seebeck coefficient

Despite numerous studies, there is still no definitive explanation on how Na improves Cu(In,Ga)Se₂ (CIGS) device efficiency. This paper investigates how the electrical transport properties of CIGS films are affected by the amount of Na at grain boundaries. The controlled removal of Na from CIGS grain boundaries is achieved by rinsing in a water bath at 70°C. Periodically during the rinse, the in-plane conductance and the Seebeck coefficient are measured, and both are found to decrease as Na is removed by the bath. Assuming carrier mobility remains unaffected, a simultaneous decrease in conductivity and Seebeck effect can only be caused by an increase in compensating donor-like defects.