Understanding the role of Niobium, Molybdenum and Tungsten in realizing of the transparent n-type SnO2.

Based on the density functional theory, the defective band structures (DBSs), ionization energy and formation energy for Niobium (Nb), Molybdenum (Mo) and Tungsten (W)-doped SnO2 are calculated. The DBSs show Nb, Mo and W substituting Sn (labeled as NbSn, MoSn and WSn) could form the localized impurity states which are above the conduction band minimum (CBM). These characteristics can be attributed to the energy of dopants' d-orbitals are much higher than that of Sn-s and -d orbital as well as O-2p orbitals, and the dopants with their neighboring atoms would form the non-bonding impurity states. The DBSs confirm NbSn, MoSn and WSn are typical n-type defects in SnO2. The ionization energies ε(0/+) for NbSn, MoSn and WSn are higher than 0.22 eV above CBM, indicating these defects could be fully ionized. We find the NbO and MoO3 are promising dopant sources, as the thermodynamic equilibrium fabrication scheme is considered. Taking Nb-doped SnO2 as an example, we find a few NbSn could induce high conductivity (541 S cm-1). These results suggest that SnO2 containing NbSn, MoSn and WSn are promising n-type semiconductors. Our findings would provide a better understanding of the n-type properties in Nb, Mo and W-doped SnO2.