Charge Transport Properties of Perylene–TCNQ Crystals: The Effect of Stoichiometry

In this work we have revisited the
charge-transfer crystal system
perylene–TCNQ and found that this complex can crystallize with
a 2:1 stoichiometric ratio in addition to the 1:1 and 3:1 stoichiometries
previously observed. The vibrational and electronic properties of
these perylene–TCNQ charge-transfer crystals have been investigated
by means of Raman scattering measurements and density functional theory
calculations. Electrical measurements were also performed by preparing
organic field-effect transistors (OFETs) from the crystals. The Raman
spectra in the low-frequency range (below 200 cm–1) are found to be unique to the specific crystal structure and can
therefore be used to determine the stoichiometry. The Raman data and
the X-ray diffraction measurements indicate that at room temperature
the amount of charge transferred to the TCNQ molecule is less than
0.2e for all three compounds, and is nearly the same
in P1T1 and P2T1 but is slightly larger in P3T1. The electronic structure
calculations suggest good intrinsic charge transport properties for
both holes and electrons in P1T1 and P2T1 and only for holes in P3T1.
Ambipolar charge transport characteristics were found for P2T1 and
hole and electron charge transport characteristics were found in P3T1
and P1T1, respectively.