Meyer-Neldel temperature on carrier transport of C70 molecular solid

Recently, the Meyer-Neldel rule was also observed on charge carrier mobility of fullerene materials such as C60 and C60/C70 mixture. At the Meyer-Neldel temperature, the charge carrier mobility becomes independent of the... [ view full abstract ]

Recently, the Meyer-Neldel rule was also observed on charge carrier mobility of fullerene materials such as C60 and C60/C70 mixture. At the Meyer-Neldel temperature, the charge carrier mobility becomes independent of the activation energy for carrier transport and temperature. Usually, the activation energy is a function of electric field and charge carrier concentration. Also, the Meyer-Neldel temperature depends only on structural properties of the disordered materials and can be used as an important material characterizing parameter. However, despite being one of the most abundant fullerenes, the Meyer-Neldel behaviors of the C70 material have not yet been studied in detail. In this paper, we report carrier transport properties of the C70 molecular solid at various temperature and electric field strength.
To make a solid sample for measurement of the electrical properties, the C70 powder was pressed into a pellet at room temperature at pressure of 125 MPa. In measurements of the current-voltage characteristics of the C70 sample, the current passing through the sample was obtained using a digital electrometer (ADVANTEST R8252) with a current resolution of 1.0 fA at various d.c. bias voltages from 30 V to 200 V. The current measurements were carried out in the course of heating up and cooling down process between temperatures of 15 K and 450 K. The rate of heating up and cooling down process was 0.14 K/min with a stepwise increment of 1.0 K.
The current-voltage (i-v) characteristics can be described conjecturally as a cubic polynomial of the voltage, i=av^3+bv^2+cv+d as shown in the figure. Moreover, the Meyer-Neldel temperature of the C70 solid was confirmed to be 310 K, at which a linear relationship between the current and voltage was observed. Also, at temperatures below the Meyer-Neldel temperature, the current increases with increasing voltage. On the other hand, at temperatures above the Meyer-Neldel temperature a negative differential conductivity effect was observed at high voltage side. The negative differential conductivity was related to the electric field and temperature effects on the mobility of charge carrier, which involve two variations in the carrier concentration and the activation energy for carrier hopping transport.