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 cm2/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 1015 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 cm2/Vs.
@article{arxiv.0707.0461,
title = {High charge-carrier mobility and low trap density in a rubrene derivative},
author = {S. Haas and A. F. Stassen and G. Schuck and K. P. Pernstich and D. J. Gundlach and B. Batlogg and U. Berens and H. -J. Kirner},
journal= {arXiv preprint arXiv:0707.0461},
year = {2007}
}