  Phosphorene, a new elemental two dimensional (2D) material recently isolated
by mechanical exfoliation, holds the feature of a direct band gap of around 2.0
eV, overcoming graphene's weaknesses (zero band gap) to realize the potential
application in optoelectronic devices. Constructing van der Waals
heterostructures is an efficient approach to modulate the band structure, to
advance the charge separation efficiency, and thus to optimize the
optoelectronic properties. Here, we theoretically investigated three type-II
heterostructures based on perfect phosphorene and its doped monolayers
interfaced with TiO$_2$(110) surface. Doping in phosphorene has a tunability on
built-in potential, charge transfer, light absorbance, as well as electron
dynamics, which helps to optimize the light absorption efficiency. Three
excitonic solar cells (XSCs) based on the phosphorene$-$TiO$_2$ heterojunctions
have been proposed, which exhibit high power conversion efficiencies dozens of
times higher than conventional solar cells, comparable to MoS$_2$/WS$_2$ XSC.
The nonadiabatic molecular dynamics within the time-dependent density
functional theory framework shows ultrafast electron transfer time of
6.1$-$10.8 fs, and slow electron$-$hole recombination of 0.58$-$1.08 ps,
yielding $>98\%$ quantum efficiency for charge separation, further guaranteeing
the practical power conversion efficiencies in XSC.
