This review summarizes current understanding of the band tailing problem in Cu2ZnSn(S, Se)(4), which is believed to be an important part of photovoltaic performance losses. Highlights of recent research are presented, which show that despite best efforts at process optimization, a minimum of 30% disorder is present on the Cu-Zn plane in the material, and significant band tailing is universally observed. Substitutions for Cu or Zn have been examined to increase the barrier to antisite formation and/or increase the static dielectric constant to better screen charged defects. These approaches have been met with some success, albeit new potential challenges. Although many elemental substitutions are possible, substitutions involving Cd and Ag have been investigated in most detail and have shown promise. Therefore, these are the focus of the present review. It is hoped that the review provided here will be applicable as new types of substitutional experiments are considered in the future.
