  Recent research on perovskite solar cells has caught much attention for the
application in renewable energy materials. However, the effect of external
dopants on the performance of perovskite solar cell is yet to be understood
properly. Oxygen atom or molecule is important dopant to influence the
stability of structural, electronic and optical properties as well as the
performance of perovskite solar cells. RbPbX3-type perovskites have fantastic
chemical stability and good power conversion efficiency. Here for the first
time, we have studied the effect of interstitial oxygen atom (O1) and molecule
(O2) on the structural properties, and hence the electronic structure of RbPbI3
from first principles. A significant reduction of the band gap from ~2.6 eV to
~ 1.0 eV, which is close to the optimal band gap, has been predicted when
incorporating oxygen. This could in turn be applied to improve the optical
properties for harvesting light if we can control the oxygen level
appropriately. In addition, an exotic metallic state has been found in our
calculations for interstitial oxygen molecule when there are strong O-O, O-Pb,
and O-I bonds, indicating the complex nature of oxygen-doped perovskite solar
cells. The comparison between oxygen atom and molecules is consistent with the
previous report about oxygen-molecule passivation of perovskite solar cells.
This indicated oxygen incorporation can not only improve efficiency and
stability but also facilitate the optimal band-gap engineering. Our work has
therefore provided an important and timely theoretical insight to the effect of
oxygen dopants in perovskite solar cells. Moreover, these results also provide
theoretical foundation for further simulations such as molecular dynamics.
