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

The density and distribution of point defects in Cu(In,Ga)Se₂ (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 ∼0.7 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.