Surface passivation and point defect control in Cu(In,Ga)Se2 films with a Na2S post deposition treatment for higher than 19% CIGS cell performance

Abstract

Na₂S as a new alkaline source was employed to passivate the surface of a CIGS film. Na₂S has less formation enthalpy compared with alkali fluoride compounds, leading to more effective dissolution at the surface. An efficiency of 19.2% was obtained by post deposition treatment (PDT) with Na₂S as a source for cells with anti-reflective coating and an active area of 0.43 cm². The remarkable performance enhancement compared to the efficiency of 16.64% of the cell without the PDT was due to the increase in V_oc and FF. With the Na₂S PDT, Na distribution in the CIGS film was much shallower and the valence band lowering was smaller compared to those with NaF PDT. Emission intensity of low-temperature photoluminescence at 1.04 eV, which corresponds to electron transition from In-in-Cu (In_Cu) antisite to the valence band maximum, was greatly reduced by Na₂S PDT compared to NaF PDT. These results suggest that in addition to lowering the Cu vacancy concentration, the concentration of In_Cu defects significantly decreased at the CIGS surface using Na₂S PDT compared with NaF PDT. Our results indicated that the surface passivation by Na₂S PDT was more effective than by NaF PDT.