Tailoring the Electronic Structure of Y-doped NbBiO₄ using Oxygen-vacancy for All-oxide PV Applications: A Theoretical and Experimental Study

Recently, combined Density Functional Theory (DFT) and experimental studies 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.¹ 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 PV application.² With the aid of DFT calculations and experimental measurements, we have further studied the hybrid NbBiO₄ system and our calculations revealed that its valence and conduction bands 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 work, with the help of suitable doping as well as oxygen vacancy, we were able to reduce the energy band gap of NbBiO₄, from 3.50 eV to 2.20 eV without changing its electronic structure.

In the framework of DFT, we revealed that doping of Yttrium (Y) has a remarkable effect on NbBiO₄ for PV applications.³ Moreover, we find that Y doping at Bi-sites i.e. Y@Bi sites in NbBiO₄ with the presence of +2 oxygen charge defects change the electronic structure and reduces the band gap to 2.20 eV.⁴ The observed band gap is in the PV region and the transition between the CB and VB states are optically allowed. Thus, the combination of doping and defect has a dramatic effect on NbBiO₄ PV activity.

References:

[1] Chris E. Mohn, Svein Stølen, Stefan T. Norberg, and Stephen Hull. Phys. Rev. B, 2009, 80, 024205 - 024212

[2] Christoph Freysoldt, Blazej Grabowski, Tilmann Hickel, Jörg Neugebauer, Georg Kresse, Anderson Janotti, Chris G. Van de Walle. Rev. Mod. Phys. 2014, 86, 253 – 305.

[3] Nirmal Ganguli, Indra Dasgupta and Biplab Sanyal. J. Appl. Phys. 2010, 108, 123911 – 123918.

[4] Nimrod Yavo, Alaric D. Smith, Ori Yeheskel, Sidney R. Cohen, Roman Korobko,Ellen Wachtel, Peter R. Slater, and Igor Lubomirsky. Adv. Funct. Mater. 2016. 26, 1138–1142