  Black phosphorous (BP) is a layered semiconductor with high carrier mobility,
anisotropic optical response and wide bandgap tunability. In view of its
application in optoelectronic devices, understanding transient photo-induced
effects is crucial. Here, we investigate by time- and angle-resolved
photoemission spectroscopy BP in its pristine state and in the presence of
Stark splitting, chemically induced by Cs ad-sorption. We show that
photo-injected carriers trigger bandgap renormalization and a concurrent
valence band attening caused by Pauli blocking. In the biased sample,
photoexcitation leads to a long-lived (ns) surface photovoltage of few hundreds
mV that counterbalances the Cs-induced surface band bending. This allows us to
disentangle bulk from surface electronic states and to clarify the mechanism
underlying the band inversion observed in bulk samples.
