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

摘要

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.