Evolution of the electrical characteristics of Cu(In,Ga)Se2 devices with sodium content

The impact of sodium on the electrical properties of Cu(In,Ga)Se-2 (CIGS) thin films and corresponding solar cells was investigated by preparing nearly alkali-free CIGS layers and doping them with different Na amounts via NaF post-deposition treatment (PDT) at temperatures between 110 and 400 degrees C. The mean Na concentrations in the CIGS layers ranged from 0.1 to 400 ppm. Sodium was found also in the grain interior even for the lowest PDT temperature. All samples were subjected to extensive electrical characterization: current-voltage, capacitance profiling, conductivity, steady-state, and transient capacitance spectroscopy. A continuous increase in open-circuit voltage V-OC and fill factor FF, an accompanying increase in hole density and mobility, and a decrease in secondary barriers responsible for the distortion of current-voltage characteristics were observed with increasing sodium content. An abrupt change in defect spectra and a dominant transport mechanism was found for PDT temperatures T(PDT) of >= 150 degrees C. We attribute a further improvement in V-OC observed above 150 degrees C PDT temperature to the reduced concentration of recombination centers with increased sodium content. An explanation of both gradual evolution and the abrupt change is proposed based on passivation of grain boundaries and interfaces by sodium.