Charge carriers in the divalent conductor (BEDT-TTF)Cu ₂Br ₄

An organic semiconductor (BEDT-TTF)Cu ₂Br ₄ exhibits rather high conductivity (10 ^-2 S/cm with little sample dependence) despite the fact that BEDT-TTF molecules have a divalent closed-shell state as evidenced by ESR and Raman measurements. In order to obtain insight into the properties of the charge carriers, the Hall coefficient for the semiconductor was measured in the temperature range of 120-300 K. Charge carriers are electronlike and their concentration is rather low [(4±2) × 10 ¹⁹/mol at room temperature]. The carrier concentration increases exponentially with temperature with an activation energy of 0.17 eV below 170 K and 0.12 eV above 170 K. The Hall mobility is as high as 2 cm ²/Vs, supporting the high conductivity of the material. Thermo-electric power measurements also support the sign and concentration of the charge carriers. The carrier concentration at room temperature is one order of magnitude lower than the concentration of Curie spin due to the presence of π electrons at low temperatures. The fine structure of the ESR signal from the Curie spin shows smearing above 100 K. This behavior is caused by exchange interaction mediated via conduction electrons. These results indicate that charge carriers localized at low temperatures are thermally activated to the conduction band at high temperatures. The possible origins of the charge carriers are discussed in terms of impurity sites of the different crystal phases of (BEDT-TTF) ₂[Cu ₄Br ₆(BEDT-TTF)] in the matrix.