High charge-carrier mobility and low trap density in a rubrene derivative
Authors: S. Haas, A. F. Stassen, G. Schuck, K. P. Pernstich, D. J. Gundlach, B. Batlogg, U. Berens, H.-J. KirnerJournal-ref: Phys. Rev. B 76 , 115203 (2007) , PDF
We have synthesized, crystallized and studied the structural and electric transport properties of organic molecular crystals based on a rubrene derivative with {\em t}-butyl sidegroups at the 5,11 positions. Two crystalline modifications are observed: one (A) distinct from that of rubrene with larger spacings between the naphtacene backbones, the other (B) with a in-plane structure presumably very similar compared to rubrene. The electric transport properties reflect the different structures: in the latter phase (B) the in-plane hole mobility of 12 cm$^2$/Vs measured on single crystal FETs is just as high as in rubrene crystals, while in the A phase no field-effect could be measured. The high crystal quality, studied in detail for B, reflects itself in the density of gap states: The deep-level trap density as low as $10^{15}$ cm$^{-3}$ eV$^{-1}$ has been measured, and an exponential band tail with a characteristic energy of 22 meV is observed. The bulk mobility perpendicular to the molecular planes is estimated to be of order of $10^{-3}$ — $10^{-1}$ cm$^2$/Vs.