Near-surface defect distributions in Cu(In,Ga)Se2

The density and distribution of point defects in Cu(In,Ga)Se-2 (CIGS) layers used for solar cell applications is critical to the resulting device performance. These devices are generally thought to be limited by recombination in the space-charge region of the collecting heterojunction. The situation is complicated by the presumed presence of an n-type surface layer on the CIGS absorber. Both the surface inversion and space-charge recombination processes are intimately tied to near-surface point defects. Here, we overview recent results on surface chemistry, transient photocapacitance spectroscopy (TPC) and depth-resolved cathodoluminescence (CL) for polycrystalline device layers from two laboratories, and single crystal epitaxial layers of three orientations. The results are combined with device modeling to provide a picture of the near-surface defect structures in these materials. The TPC results show deep defect levels similar to0.7 and 0.9 eV above the valence band. CL shows evidence of subgap radiative recombination, which increases dramatically near the sample surfaces. The results point to a near-surface Cd-containing layer, which could be responsible for the surface carrier type inversion, a near-surface region containing an elevated defect density, possibly near the valence band edge, and deep hole traps near the conduction band. Implementation of the results in a device model provides reasonable fits to the device performances. (C) 2003 Elsevier Science B.V. All rights reserved.