The thermoelectric power of band-filling controlled organic conductors, β′-(BEDT-TTF) 3 (CoCl 4 ) 2−x (GaCl 4 ) x

The carrier-number dependence of the thermoelectric power is investigated in the band-filling controlled organic conductors, β′-(BEDT-TTF) 3 (CoCl 4 ) 2−x (GaCl 4 ) x (BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene). Although the BEDT-TTF molecules are positively charged, the thermopower is negative in the entire temperature range and produces a broad peak around 150 K. The nontrivial temperature and carrier-number dependence is quantitatively analyzed by calculating the thermopower from the band structure. The thermopower becomes gradually smaller with increasing x , where electron doping occurs at the upper band of the strongly dimerized system. At the same time, the bandwidth shrinks due to the negative chemical pressure, and the conductivity decreases in spite of the increasing electron number. Since the parent compound at x = 0 has a semimetallic band structure, the thermoelectric power factor monotonously decreases upon doping, in sharp contrast to that in organic polymers.