  This study is carried out using first principles density functional theory
calculations within gpaw code. Atomic size effect is analyzed and investigated
by doping either Li, Cs or both on Barium doped BiFeO$_3$ (BFO) which belongs
to monoclinic $P2_1/m$ space group. The calculated results reveal that Cs doped
BFO had significantly improved photocurrent density seemingly due to broadened
absorption peaks and biplasmons generation. Co-doping two atoms with large size
difference have a significant effect on plasmon width and peak than doping with
a single and small sized atom. At higher photon energy realms of the order
10eV, the index of refraction reduces to $n(\omega) \to 1$ implying that light
wave can tunnel through the pristine and doped BFO without any phase change,
thus indicating its potential as an efficient candidate for a photonic
application. In addition, doped BFO shows abundant photocurrent generation
properties which would be important in solar cell and photovoltaic
applications.
