  We have measured the electrical resistivity and the thermopower of the
electron-doped perovskite cobaltites LaCo$_{1-y}$Te$_y$O$_3$. In contrast to
the hole-doped systems such as metallic ferromagnets La$_{1-x}M_x$CoO$_3$ ($M$
= Ca, Sr, Ba), the electron-doped samples show an insulating behavior even in a
heavily doped range due to a spin-state blockade mechanism that an electron
hopping from high-spin Co$^{2+}$ to low-spin Co$^{3+}$ site is energetically
suppressed. We find that, despite the electron doping, the thermopower shows
relatively large positive values above $y=0.05$, strikingly distinct from the
hole-doped case where it comes close to zero with doping. This prominent
electron-hole asymmetry seen in the thermopower originates from a bipolar
conduction which consists of a slight amount of mobile holes and the main
immobile electrons, demonstrating an impact of spin-state blockade on
thermoelectric transport.
