  It is well known that the DX center is a kind of defect that limits the
n-type doping in some tetrahedral coordinated semiconductors. It is a deep
negatively charged defect complex converted from a nominal shallow donor
defect, which can serve as a trap center of electrons, thus is detrimental to
the performance of optoelectronic devices. Similar to the DX center, we find
that a donor-yielded complex center (DY center) also exists in six-fold
coordinated semiconducting materials. For example, Bi is commonly expected as a
shallow n-type dopant in perovskite APbX3. However, our first-principles
calculations show that the DY center is formed in Bi-doped MAPbBr3 when the
Fermi level is high in the gap, but, interestingly, it does not form in MAPbI3.
The reason that the DY center is formed in MAPbBr3 instead of MAPbI3 is
attributed to the high conduction band minimum (CBM) of MAPbBr3. Our results
are able to explain recent puzzling experiment observations and the thorough
discussions of the formation and the properties of the DY center in perovskites
provide enlightening insights to the defect study in six-fold coordinated
semiconductors.
