  The \emph{p}-type Bi$_{2}$Se$_{3}$ is much desirable as a promising
thermoelectric material and topological insulator, while the naturally grown
Bi$_{2}$Se$_{3}$ is always \emph{n}-type doped by native point defects. Here we
use first-principles calculations to identify the origin of the \emph{n}-type
tendency in bulk Bi$_{2}$Se$_{3}$: The Se vacancies (V$_\text{Se1}$ and
V$_\text{Se2}$) and Se$_{\text{Bi}}$ antisite dominate the donorlike doping
with low formation energy, while the predisposed Bi$_{\text{Se1}}$ defect
results in the pair of V$_\text{Se1}$ and Bi interstitial, which is also a
donor rather than an acceptor. Moreover, for Bi$_{2}$Se$_{3}$(111) surface, we
find that the band structures modulated by the defects explicitly account for
the existing experimental observations of \emph{n}-type preference.
