  We report efficient spin to charge conversion (SCC) in the topological
insulator (TI) based heterostructure ($BiSbTe_{1.5}Se_{1.5}/Cu/Ni_{80}Fe_{20}$)
by using spin-pumping technique where $BiSbTe_{1.5}Se_{1.5}$ is the TI and
$Ni_{80}Fe_{20}$ is the ferromagnetic layer. The SCC, characterized by inverse
Edelstein effect length ($\lambda_{IEE}$) in the TI material gets altered with
an intervening Copper (Cu) layer and it depends on the interlayer thickness.
The introduction of Cu layer at the interface of TI and ferromagnetic metal
(FM) provides a new degree of freedom for tuning the SCC efficiency of the
topological surface states. The significant enhancement of the measured
spin-pumping voltage and the linewidth of ferromagnetic resonance (FMR)
absorption spectra due to the insertion of Cu layer at the interface indicates
a reduction in spin memory loss at the interface that resulted from the
presence of exchange coupling between the surface states of TI and the local
moments of ferromagnetic metal. The temperature dependence (from 8K to 300K) of
the evaluated $\lambda_{IEE}$ data for all the trilayer systems, TI/Cu/FM with
different Cu thickness confirms the effect of exchange coupling between the TI
and FM layer on the spin to charge conversion efficiency of the topological
surface state.
