Experimental and theoretical EBIC analysis for grain boundary and CdS/Cu (In, Ga)Se2 heterointerface in Cu (In, Ga)Se2 solar cells

Abstract

The behavior of carriers for an electron-beam-induced current (EBIC) evaluation is experimentally and theoretically analyzed for the polycrystalline Cu (In, Ga)Se2 (CIGS) thin-film solar cells. The experimental EBIC signal in-depth profiles of the CIGS layers show four features: peaks at grain boundaries (GBs), narrowed peaks near the surface, shifted peaks near the surface, and double peaks at the GBs and surface. Operating principles for CIGS solar cell devices under electron–hole pair excitation are systematically revealed utilizing a two-dimensional theoretical EBIC simulation: (i) The EBIC peaks at the GBs are caused by a high hole barrier, including a valence band offset (ΔEV) and a band bending at the GBs; (ii) narrowing of EBIC peaks near the surface is caused by high defect densities at the GBs, grading of a conduction band minimum, and grading of acceptor concentration (Na) along with the CIGS depth direction; (iii) a shift in EBIC peaks near the surface is caused by high donor-like defect densities at the surface; (iv) double peaks at the GBs and surface are caused by a thick surface layer (SL) with the ΔEV and the hole barrier at GBs. These findings of the EBIC analysis will help to comprehensively understand the complex polycrystalline CIGS carrier transport mechanism in realizing the highest-efficiency CIGS solar cells.