Ultrafast dynamics of the surface photovoltage in potassium-doped black phosphorus

Black phosphorus is a quasi-two-dimensional layered semiconductor with narrow direct band gap of 0.3 eV. A giant surface Stark effect can be produced by the potassium doping black phosphorus, leading to semimetal phase transition originating from creation strong dipole and associated bending. By using time- angle-resolved photoemission spectroscopy, we report partial photoinduced screening this bending compensating photovoltage. We further resolve detailed dynamics at pertinent timescales related evolution structure near Fermi level. demonstrate that after fast rise time, photovoltage exhibits plateau over few tens picoseconds before decaying on nanosecond timescale. support our experimental results simulations based drift-diffusion equations.