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

Abstract

The photovoltaic performance of solar cells, based on a Cu(In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-X</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</sub> )Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (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.