Direct evidence for grain boundary passivation in Cu(In,Ga)Se2 solar cells through alkali-fluoride post-deposition treatments

Abstract

The properties and performance of polycrystalline materials depend critically on the properties of their grain boundaries. Polycrystalline photovoltaic materials – e.g. hybrid halide perovskites, copper indium gallium diselenide (CIGSe) and cadmium telluride – have already demonstrated high efficiencies and promise cost-effective electricity supply. For CIGSe-based solar cells, an efficiency above 23% has recently been achieved using an alkali-fluoride post-deposition treatment; however, its full impact and functional principle are not yet fully understood. Here, we show direct evidence for the passivation of grain boundaries in CIGSe treated with three different alkali-fluorides through a detailed study of the nanoscale optoelectronic properties. We determine a correlation of the surface potential change at grain boundaries with the open-circuit voltage, which is supported by numerical simulations. Our results suggest that heavier alkali elements might lead to better passivation by reducing the density of charged defects and increasing the formation of secondary phases at grain boundaries.

Heavy Alkali Treatment of Cu(In,Ga)Se2Solar Cells: Surface versus Bulk Effects

“…This observation indicates that successful alkali PDTs which lead to an improved open‐circuit voltage also reduce the band bending (or at least its variation) at grain boundaries . This observation confirms several numerical studies that show that increased band bending at grain boundaries reduces solar cell efficiency, particularly by reducing the open‐circuit voltage …”

“…In fact, for KF‐treated Cu(In,Ga)Se 2 , a reduced variation of band bending at grain boundaries was previously also observed, and it was shown by numerical simulations that smaller downward band bending at grain boundaries leads to higher open‐circuit voltage . This observation indicates that successful alkali PDTs which lead to an improved open‐circuit voltage also reduce the band bending (or at least its variation) at grain boundaries . This observation confirms several numerical studies that show that increased band bending at grain boundaries reduces solar cell efficiency, particularly by reducing the open‐circuit voltage …”

Passivation of the CuInSe2 surface via cadmium pre-electrolyte treatment

“…In that respect, scanning probe techniques are ideal tools since they allow us to probe the electronic properties of semiconductors on the nanometer scale. In particular, Kelvin probe force microscopy (KPFM) was used frequently to study band bending at the grain boundaries [21][22][23]. Moreover, the effect of CPE treatment was also investigated on Cu-poor CIGSe with KPFM and it was shown that the band bending at the grain boundaries after CPE treatment was reduced [24].…”

Metastable Behavior on Cesium Fluoride‐Treated Cu(In1–x,Gax)Se2 Solar Cells

“…In addition, at the grain boundaries in the surface region, the enhanced Na and K passivate the charged defects to form better contact potential differences (Figure b) . Nicoara et al.…”

“…Nicoara et al. suggested that charged defects remained at the grain boundaries even after an alkali PDT and the degree of successful passivation of these defects is a key factor for an efficiency improvement . Therefore, under HLS treatment on CsF‐treated CIGS solar cells, the donor defect at grain boundaries might have been successfully passivated by alkali metals (Na and K) migration.…”