Ga Composition Dictates Macroscopic Photovoltaic and Nanoscopic Electrical Characteristics of Cu(In $_{1-X}$Ga$_X$)Se $_2$ Thin Films via Grain-Boundary-Type Inversion

The photovoltaic performance of solar cells, based on a Cu(In
_1-X
Ga
_X
)Se
₂
(CIGS) absorber layer, is directly correlated with Ga composition. We have used scanning capacitance microscopy and conducting probe atomic force microscopy (CP-AFM) to provide microscopic electrical characterization of CIGS films with different Ga content. We found p- to n-type inversion at grain boundaries of the polycrystalline CIGS film, especially for Ga-poor compositions. The fraction of grain boundaries undergoing inversion dramatically decreased for Ga compositions above x = 0.32, the composition corresponding to a sharp efficiency drop of the complete cells. CP-AFM measurements showed a marked current drop at grain boundaries as the Ga composition rose above x = 0.32.