Influence of Na on grain boundary and properties of Cu(In,Ga)Se2 solar cells

Whether added from external sources or supplied via substrate, incorporation of Na has always improved the conversion efficiency of polycrystalline thin-film Cu(In,Ga)Se-2 (CIGS) solar cells. Segregation of Na along grain boundary (GB) is thought to be one of the main reasons for better efficiency, and changing its concentration levels may tune the device properties accordingly. Our previous works have shown that changes in GB chemistry over various Ga/In concentrations are the main reason for poor performance of Ga-rich (18%) CIGS, besides its optimum band gap (1.4eV) for energy conversion. It was suspected that changes in GB chemistry or device performance or both could be driven by variation in Na concentration at GB. In this research, atomic level fluctuations of Na atoms at GB for various CuIn1-xGaxSe2 thin films were measured accurately with atom probe tomography technique. Fluctuations observed in Na concentration at GB are restricted to the range 0.5-6.5 atoms per nm(2) and is irrespective of x value (GGI) and GB surface density. Results also show that changes in GB chemistry for similar CIGS compositions are not necessary due to changes in Na concentration levels at GB. However, an increased density of GBs per unit volume with GGI may indicate an overall rise in Na in thin films and could explain the drop in CIGS performance. Copyright (C) 2017 John Wiley & Sons, Ltd.