AgBiS₂ as a low-cost and eco-friendly all-inorganic photovoltaic material : nanoscale morphology-property relationship

Solar cells made of low-cost solution-processed all-inorganic materials are a promising alternative to conventional solar cells made of high-temperature processed inorganic materials, especially because many high-temperature processed inorganic materials contain toxic element(s) such as lead or cadmium (e.g., CsPbI₃ perovskite, PbS, CdTe and CdS(Se)). AgBiS₂ nanocrystals, consisting of earth-abundant elements but without lead and cadmium, have already emerged as a promising candidate in high-performance solar cells. However, the nanoscale morphology-optoelectronic property relationship for AgBiS₂ nanocrystals is still largely unknown. Herein, we investigate the electronic properties of various AgBiS₂ nanocrystals by using first-principles computation. We show that the optoelectronic properties of bulk AgBiS₂ are highly dependent on the M-S-M-S- (M: Ag or Bi) orderings. Moreover, because Ag-S-Ag-S- and Bi-S-Bi-S- in AgBiS₂ bulk crystals contribute respectively to the valence band maximum and conduction band minimum, these unique chemical orderings actually benefit easy separation of mobile electrons and holes for photovoltaic application. More importantly, we find that AgBiS₂ nanocrystals (NCs) can exhibit markedly different optoelectronic properties, depending on their stoichiometry. NCs with minor off-stoichiometry give rise to mid-gap states, whereas NCs with substantial off-stoichiometry give rise to many deep defect states in the band gap, and some NCs even show metallic-like electronic behavior. We also find that the deep-defect states can be removed through ligand passivation with optimal coverage. The new insights into the nanoscale morphology-optoelectronic property relationship offer a rational design strategy to engineer the band alignment of AgBiS₂ NC layers while addressing some known challenging issues inherent in all-inorganic photovoltaic materials.