Influence of sodium compounds at the Cu(In,Ga)Se2/(PVD)In2S3 interface on solar cell properties

The present contribution deals with indium sulfide buffer layers grown by thermal co-evaporation of elemental indium and sulfur. It has been found necessary to deposit these buffer layers at low substrate temperatures in order to reach Voc values similar to those with (CBD)CdS. However, such deposition conditions lead to the formation of a highly recombinative Cu(In,Ga)Se2/indium sulfide interface. This behaviour may be associated to the presence of sodium carbonates/oxides at the interface even though the Cu(In,Ga)Se2 surface was cleaned in NH3 (1 M, room temperature) prior to the indium sulfide deposition. An explanation is that, despite the chemical etch, sodium carbonates/oxides remain in the air exposed Cu(In,Ga)Se2 grain boundaries and can migrate towards the surface when the Cu(In,Ga)Se2 is heated under vacuum. These polluted interface areas act as recombination zones and thus inferior devices. A possibility to improve the device performance (i.e. improve the interface quality) is to sulfurize the remaining sodium carbonates/oxides. The resulting Na2S can then leave the interface by formation of a solid solution with the indium sulfide. By adapting the buffer layer deposition process, 13.3% efficiency devices with co-evaporated indium sulfide are realized, performance which is close to that reached with (CBD)CdS.