No abstract

Cu-based I–III–VI2 materials have enjoyed much attention as candidate solar cell adsorbers. While the vast majority of studies has centered on materials with group 13 (In, Ga) as the trivalent metal, the scarcity and expense of In has motivated a research drive to discover alternative Cu-based absorber materials. In this study, we use screened hybrid density functional theory (DFT) to investigate the electronic structure and bonding in some novel I–III–VI2 materials, namely, CuMCh2 (M = Sb, Bi; Ch = S, Se). We demonstrate that these materials possess fundamental band gaps that are indirect in nature, which is at variance with previous experimental results. We analyze the crystal structures and rationalize the structural differences between these and typical chalcopyrite materials. The band structure features and bonding of these materials are then discussed in relation to their utility as solar cell absorbers.

Lead dioxide has been used for over a century in the lead-acid battery. Many fundamental questions concerning PbO2 remain unanswered, principally: (i) is the bulk material a metal or a semiconductor, and (ii) what is the source of the high levels of conductivity? We calculate the electronic structure and defect physics of PbO2, using a hybrid density functional, and show that it is an n-type semiconductor with a small indirect band gap of ∼0.2  eV. The origin of electron carriers in the undoped material is found to be oxygen vacancies, which forms a donor state resonant in the conduction band. A dipole-forbidden band gap combined with a large carrier induced Moss-Burstein shift results in a large effective optical band gap. The model is supported by neutron diffraction, which reveals that the oxygen sublattice is only 98.4% occupied, thus confirming oxygen substoichiometry as the electron source.

As the thin film photovoltaic sector continues to expand, there is an emerging need to base these technologies on abundant, low cost materials in place of the expensive, rare, or toxic elements such as Te, In, or Cd that currently constitute the industry standards. To this end, the geometric and electronic structure of four materials comprising low cost, earth abundant elements (Cu3SbS3, Cu3SbSe3, Cu3BiS3, and Cu3BiSe3) are investigated with the screened hybrid exchange-correlation functional HSE06 and their candidacy for use as absorber materials assessed. The materials are shown to exhibit low VBM effective masses, due partially to the presence of lone pairs that originate from the Sb and Bi states. Although all four materials possess indirect fundamental band gaps, calculated optical absorbance shows direct transitions close in energy. Optical band gaps within the visible-light spectrum are also predicted for three of the systems, (Cu3SbSe3, Cu3BiS3 and Cu3BiSe3) making them promising candidates for PV applications.

Nine sulvanite structured semiconductors, Cu3MCh4(M = V, Nb, Ta; Ch = S, Se, Te), have been examined computationally to comprehensively analyse their electronic structure and assess their potential for optoelectronic applications.