Disorder-Enhanced Dimensionless Thermoelectric Figure of Merit Zt of Non-stoichiometric Organic Conductor (ttt)2i3+δ (δ ≤ 0.1)

Sample dependence of dimensionless thermoelectric figure of merit (zT) and power factor (PF) were determined for the non-stoichiometric organic conductor (TTT)(2)I3+delta (TTT = tetrathiatetracene, delta <= 0.1) with the simultaneous measurement of the electrical resistivity (rho), thermopower (S) and thermal conductivity on small single crystals. Both the zT and PF show large sample dependence between 10 and 310 K, even though all the samples have nearly stoichiometric composition of TTT : I-3(-) similar to 2 : 1 (delta similar to 0). It was found that both the electrical conductivity (sigma = 1/rho) and S increase at room temperature as disorder-that is phase mismatch among the iodine chains-becomes more pronounced. This behavior contrasts the usual tendency that the S decreases as the s increases in conventional conductors; and suggests a new strategy to improve the zT and PF by introducing an appropriate type of disorder.