  Optimizing the interfaces in perovskite solar cells (PSCs) is essential for
enhancing their performance, improving their stability, and making them
commercially viable for large-scale deployment in solar energy harvesting
applications. Point defects, like vacancies, have a dual role, as they can
inherently provide a proper doping, but they can also reduce the collected
current by trap-assisted recombination. Moreover, they can play an active role
in ion migration. Using {\it ab initio} density functional theory (DFT)
calculations we investigate the changes in the band alignment induced by
interfacial vacancy defects in a TiO$_2$/MAPI/Cu$_2$O based PSC. Depending on
the type of the vacancy (Ti, Cu, O, Pb, I) in the oxide and perovskite
materials, additional doping is superimposed on the already existing
background. Their effect on the performance of the PSCs becomes visible, as
shown by SCAPS simulations. The most significant impact is observed for $p$
type doping of TiO$_2$ and $n$ type doping of Cu$_2$O, while the effective
doping of the perovskite layer affects one of the two interfaces. We discuss
these results based on modification of the band structure near the active
interfaces and provide further insights concerning the optimization of electron
and hole collection.
