  Using the first-principles GW-Bethe-Salpeter equation method, here we study
the excited-state properties, including quasi-particle band structures and
optical spectra, of phosphorene, a two-dimensional (2D) atomic layer of black
phosphorus. The quasi-particle band gap of monolayer phosphorene is 2.15 eV and
its optical gap is 1.6 eV, which is suitable for excitonic thin film solar cell
applications. Next, this potential application is analysed by considering
type-II heterostructures with single layered phosphorene and transition metal
dichalcogenides (TMDs). These heterojunctions have a potential maximum power
conversion efficiency of up to 12\%, which can be further enhanced to 20\% by
strain engineering. Our results show that phosphorene is not only a promising
new material for use in nanoscale electronics, but also in optoelectronics.
