First principles studies of point defects and impurities in semiconductors, insulators and metals have become an integral part of material research over the last few decades.¹ Point defects and impurities often have decisive effects on material properties.^{2, 3} Recently, using density functional theory (DFT) calculations we have revealed that Bi₂O₃ is a low band-gap material and the obtained energy band-gap, 2.30 eV, lies in the region of PV interest. The observed optical transitions between the valence band (VB) (O-2p states) and the conduction band (CB) (Bi-6p states) are not allowed due to violation of the dipole selection rule (Δl =+/-1). Therefore, Bi₂O₃ is not a potential absorber material for photovoltaic (PV) applications. With the aid of DFT calculations and experimental measurements, we have also studied the NbBiO₄ system extensively, and our results revealed that its valence band (VB) and conduction band (CB) consist of optically active states, but its large energy band gap, 3.50 eV, puts a constraint on its use as an absorber material. In the present talk, I shall discuss how, with the help of suitable doping as well as oxygen vacancy, we managed to reduce the energy band gap of NbBiO₄, from 3.50 eV to 2.20 eV without changing its electronic structure. The observed band-gap is in the region of PV interest and transitions between the CB and VB states are optically allowed. Using COHP (energy resolved visualization of chemical bonding) calculations, we revealed that Y doping in NbBiO₄ significantly reduces the covalent bonding between Bi and nearest neighbors oxygen ions from 1.00 eV/bond to 0.10 eV/bond. As a consequence, the computed oxygen defect formation energies for Y-doped NbBiO4 system is lower than the oxygen defect formation energies of the pure NbBiO₄ system, contributing to a large oxygen-defect concentration. Thus, the significance of oxygen vacancy in Y-doped NbBiO₄ (Y@Bi site only) was revealed and this study might serve to design better PV absorber materials in the future.

References

1. Freysoldt, C.; Grabowski, B.; Hickel, T.; Neugebauer, J.; Kresse, G.; Janotti, A.; Van de Walle, C. G., First-principles calculations for point defects in solids. Rev. Mod. Phys. 2014, 86, 253-305.

2. Ganguli, N.; Dasgupta, I.; Sanyal, B., Electronic structure and magnetism of transition metal doped Zn₁₂O₁₂ clusters: Role of defects. J. Appl. Phys. 2010, 108, 123911.

3. Yavo, N.; Smith, A. D.; Yeheskel, O.; Cohen, S.; Korobko, R.; Wachtel, E.; Slater, P. R.; Lubomirsky, I., Large Nonclassical Electrostriction in (Y, Nb)‐Stabilized δ‐Bi₂O₃. Adv. Funct. Mater. 2016, 26, 1138–1142.