Surface modification of Cu(In,Ga)Se2 thin films during aqueous oxidation etch

The control of the barrier height at the Cu(In,Ga)Se-2(CIGS)/CdS interface, via the chemical state of the CIGS surface, is a key to improve solar cell performances. In this paper chemical modifications have been achieved by the electrochemical method, by varying both the pH of the solution and the applied potential. It is shown that the potential for electrochemical oxidation depends on the pH, in good agreement with the thermodynamic predictions. The resulting surface composition of CIGS, analyzed by XPS, also depends on the pH. Contact Potential Differences (CPD) and Surface Photovoltage (SPV) measurements show that the electronic properties of the CIGS surface are modified. The important result is that the oxidation leads to the formation of permanent positive surface charges, which increase the band bending up to 0.20 V in a direction favorable for the solar cell performances.