  Recently, a new quantum phase, the topological insulator, has been vividly
investigated in a variety of materials. Its unique bandstructure allows for
optical generation and control of spin-polarized currents based on the circular
photogalvanic effect. In this paper, we generate and distinguish the different
photocurrent contributions via the the polarization of the driving light wave.
We discuss the helicity-dependent spin-polarized current and the polarization
independent thermoelectric current as spatially resolved maps, focusing on the
influence of the topological insulator/metallic contact interface. We observe
for both current contributions a significant enhancement of the current values
at the topological insulator/metallic contact interface and moreover a
dipole-like distribution of the spin-polarized current close to the contacts.
We discuss the general behavior of the thermovoltage as a three-material
Seebeck effect and explain the enhanced values by the acceleration of the
photoelectrons generated in the space charge region of the topological
insulator/metallic contact interface. Furthermore, we interpret the temperature
gradient together with the spin Nernst effect as a possible origin for the
enhancement and dipole-like distribution of the spin-polarized current.
