High charge-carrier mobility and low trap density in a rubrene derivative

We have synthesized, crystallized, and studied the structural and electric transport properties of organic molecular crystals based on a rubrene derivative with 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 an 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∕V s measured on single-crystal field-effect transistors 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 of 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 the order of 10⁻³–10⁻¹ cm²∕V s.