Influence of the Cu Content in Cu_2ZnSn(S,Se)_4 solar cell absorbers on order-disorder related band gap changes

摘要

We investigate the electronic structure and the radiative recombination in wet-chemically fabricated Cu_2ZnSn(S,Se)_4 solar cell absorbers utilizing photoluminescence (PL) and photolumines-cence excitation (PLE) spectroscopy, focusing especially on the effects of varying Cu content. This includes the impact of the latter on the band gap energy and the change in band gap energy related to the order-disorder transition. Characteristic PL and PLE parameters like the energetic position of the PL maximum and the PL yield as a function of the excitation power as well as the PLE tailing parameter do not depend on composition indicating that the nature of the radiative transition is not altered by the Cu content. However, the band gap energy E_g significantly increases as a function of decreasing Cu content. This increase is more pronounced in the disordered than in the ordered atomic arrangement of Cu and Zn atoms in the Cu-Zn planes of the kesterite crystal structure.