  We investigate the current-induced spin-orbit torque in thin topological
insulator (TI) films in the presence of hybridization between the top and
bottom surface states. We formulate the relation between spin torque and TI
thickness, from which we derived the optimal value of the thickness to maximize
the torque. We show numerically that in typical TI thin films made of
$\mathrm{Bi_2Se_3}$, the optimal thickness is about 3-5 nm.
