High surface recombination velocity limits Quasi-Fermi level splitting in kesterite absorbers

Kelvin Probe Force Microscopy, Photoluminescence imaging and numerical simulations are used to study the surfaces of Cu₂ZnSnSe₄ absorber layers. In particular, the effect of NH₄OH and annealing under ambient conditions is investigated. We observe drastic changes in the measured quasi Fermi-level splitting (QFLs) after chemical cleaning of the absorber surface with NH₄OH, which is traced back to a removal of the surface inversion. Air annealing recovers surface inversion, which reduces the recombination current at the surface. Annealing above 200 °C leads to a permanent change in the work function which cannot be modified by NH₄OH etching anymore. This modification makes the QFLs insensitive to surface cleaning and explains why air annealing in Cu₂ZnSnSe₄ is important. From numerical simulations we find that a large surface recombination velocity needs to be present in order to describe the experimental observations.