  Monolayer black and blue phosphorenes possess electronic and optical
properties that result in unique features when the two materials are stacked.
We devise a low-strain van-der-Waals double layer and investigate its
properties with ab initio many-body perturbation theory techniques. A type-II
band alignment and optical absorption in the visible range are found. The study
demonstrates that spatially indirect excitons with full charge separation can
be obtained between two layers with the same elemental composition but
different crystalline structure, proving the system interesting for further
studies where dipolar excitons are important and for future opto-electronic
applications.
