Chalcopyrite solar cells achieve efficiencies above 23%. The latest improvements are due to post‐deposition treatments (PDT) with heavy alkalis. This study provides a comprehensive description of the effect of PDT on the chemical and electronic structure of surface and bulk of Cu(In,Ga)Se2. Chemical changes at the surface appear similar, independent of absorber or alkali. However, the effect on the surface electronic structure differs with absorber or type of treatment, although the improvement of the solar cell efficiency is the same. Thus, changes at the surface cannot be the only effect of the PDT treatment. The main effect of PDT with heavy alkalis concerns bulk recombination. The reduction in bulk recombination goes along with a reduced density of electronic tail states. Improvements in open‐circuit voltage appear together with reduced band bending at grain boundaries. Heavy alkalis accumulate at grain boundaries and are not detected in the grains. This behavior is understood by the energetics of the formation of single‐phase Cu‐alkali compounds. Thus, the efficiency improvement with heavy alkali PDT can be attributed to reduced band bending at grain boundaries, which reduces tail states and nonradiative recombination and is caused by accumulation of heavy alkalis at grain boundaries.

No abstract

We present dynamic force-microscopy experiments and first-principles simulations that contribute to clarify the origin of atomic-scale contrast in Kelvin-probe force-microscopy (KPFM) images of semiconductor surfaces. By combining KPFM and bias-spectroscopy imaging with force and bias-distance spectroscopy, we show a significant drop of the local contact potential difference (LCPD) that correlates with the development of the tip-surface interatomic forces over distinct atomic positions. We suggest that variations of this drop in the LCPD over the different atomic sites are responsible for the atomic contrast in both KPFM and bias-spectroscopy imaging. Our simulations point towards a relation of this drop in the LCPD to variations of the surface local electronic structure due to a charge polarization induced by the tip-surface interatomic interaction.

We present a comprehensive three-dimensional analysis of Kelvin probe force microscopy of semiconductors. It is shown that high-resolution electronic defect imaging is strongly affected by free carrier electrostatic screening, and the finite size of the measuring tip. In measurements conducted under ambient conditions, defects that are not more then 2 nanometers below the surface, and are at least 50 nanometers apart can be imaged only if the tip-sample distance is not larger then 10 nanometers. Under ultrahigh vacuum conditions, when the tip-sample distance can be as small as 1 nanometer, it is shown that the tip-induced band bending is only around a few millivolts, and can be neglected for most practical purposes. Our model is compared to ultrahigh vacuum Kelvin probe force microscopy measurements of surface steps on GaP, and it is shown that it can be used to obtain local surface charge densities.

No abstract