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

Abstract

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.