Electronic Structure and Charge-Transport Parameters of Functionalized Tetracene Crystals: Impact of Partial Fluorination and Alkyl or Alkoxy Derivatization

The charge-transport parameters of fluorine- and alkyl/alkoxy-substituted tetracene crystals have been investigated by means of density functional theory calculations. The intramolecular reorganization energy (vibronic coupling) is found to increase upon partial fluorination of tetracene and upon further alkoxy substitution, whereas alkyl substitution has a lesser impact. The calculated ionization energies are in agreement with electrochemical measurements and confirm that the electron injection barrier from conventional cathodes into partially fluorinated, alkyl/alkoxy-substituted tetracenes is expected to be smaller than into tetracene. Calculations of the intermolecular electronic couplings and of the crystal band structures have been performed to understand the role of packing on the charge-transport properties. A tight binding model with two sites per unit cell has been used to rationalize the results of the band-structure calculations. The largest electron mobility is predicted for the material where alkyl substitution of the partially fluorinated tetracene leads to a simple π-stacking motif; substitution resulting in dimerization along the π-stacks is found to significantly increase the charge-carrier effective mass and thus to adversely affect the carrier mobility.