  Topological materials are of high interest due to the promise to obtain low
power and fast memory devices based on efficient spin-orbit torque switching or
spin-orbit magnetic state read-out. In particular, sputtered polycrystalline
Bi$_x$Se$_{1-x}$ is one of the materials with more potential for this purpose
since it is relatively easy to fabricate and has been reported to have a very
high spin Hall angle. We study the spin-to-charge conversion in
Bi$_x$Se$_{1-x}$ using the spin pumping technique coming from the ferromagnetic
resonance in a contiguous permalloy thin film. We put a special emphasis on the
interfacial properties of the system. Our results show that the spin Hall angle
of Bi$_x$Se$_{1-x}$ has an opposite sign to the one of Pt. The charge current
arising from the spin-to-charge conversion is, in contrast, lower than Pt by
more than one order of magnitude. We ascribe this to the interdiffusion of
Bi$_x$Se$_{1-x}$ and permalloy and the changes in chemical composition produced
by this effect, which is an intrinsic characteristic of the system and is not
considered in many other studies.
