| Popis: |
The conductivity and Hall mobility in single crystals of the organic semiconductor, copper phthalocyanine, were investigated over the temperature range 300-425\ifmmode^\circ\else\textdegree\fi{}K, using a new differential electrometer scheme. A two-carrier Hall mobility of the order of - 75 ${\mathrm{cm}}^{2}$/V-sec was found at 300\ifmmode^\circ\else\textdegree\fi{}K which decreased with increasing temperature. Conductivity studies as a function of various gaseous ambients indicated that oxygen was an electrically active impurity. It was impossible to determine whether it gave rise to a donor level 0.4 eV above the valence band or an acceptor level 1.6 eV above the valence band. In one sample the Hall mobility changed sign from negative to positive for temperatures above 100\ifmmode^\circ\else\textdegree\fi{}C. This was approximately the same temperature at which the activation energy for conductivity changed from 1.6 to 2 eV. From these data assuming a donor model for the oxygen impurity the ratio of electron to hole mobility was found to be 0.67 at 373\ifmmode^\circ\else\textdegree\fi{}K. Assuming that the temperature dependences of the hole and electron mobility were the same, and that the effective densities of states for electrons and holes were equal, the individual hole and electron mobilities were calculated as a function of temperature. The applicability of a band model to this material was suggested by the fact that the carrier mobility decreased with increasing temperature. The greater mobility of carriers in copper phthalocyanine crystals as compared to the 0.1 ${\mathrm{cm}}^{2}$/V-sec mobility in metalfree phthalocyanine crystals can be attributed to the mixing of the copper $3d$ orbital with the $\ensuremath{\pi}$ orbital system of the phthalocynine molecule. The copper $3d$ orbital also provides an out-of-plane interaction with an antibonding orbital of a nitrogen atom in molecules above and below it. Since this interaction takes place at a distance of nearest approach of adjacent molecules, charge transport is facilitated. |
