Giant Seebeck Coefficient in V-TCNE thin films*

The disordered structure of organic conductors results in a naturally low thermal conductivity ($\kappa )$ but their ZT is known to be low because of their low thermopower (S) and electrical conductivity ($\sigma )$. Here we report an exception, with results obtained from 220 to 320K for the thermopower of V-TCNE$_{x}$ (V-(C$_{2}$(CN)$_{4})_{x})$ thin films deposited on a Si wafer (111). At room temperature S=+21.8 mV/K and increases with decreasing temperature. Those values are matched only by very pure semiconductors such as Si at low temperature, Bi nanowires, or strongly correlated electron systems like FeSb$_{2}$. The valence band of V-TCNE has a very high density of states over a very narrow energy range, ascribed mostly to vanadium 3d(t$_{2g})$ orbitals,\footnote{Y-J Yoo et al., Nat. Mat. \textbf{9} 638 2010} which is consistent with the exceptionally large value of S. The dependence of S and $\sigma $ upon illumination will also be shown, alongside preliminary estimates for the ZT.