We have performed a comparative study of rubrene single-crystal field-effect transistors fabricated using different materials as gate insulator. For all materials, highly reproducible device characteristics are obtained. The achieved reproducibility permits one to observe that the mobility of the charge carriers systematically decreases with increasing the dielectric constant of the gate insulator, the decrease being proportional to −1 . This finding demonstrates that the mobility of carriers in organic single-crystal field-effect transistors is an intrinsic property of the crystal/ dielectric interface and that it does not only depend on the specific molecule used. The quality of organic single-crystal field-effect transistors (FETs) opens new opportunities for investigations of both fundamental and applied character. In particular, the use of single-crystalline devices permits one to study the intrinsic-not limited by disorder-transport properties of organic semiconductors as a function of carrier density, as recently demonstrated by the observation of an anisotropic mobility in rubrene FETs exhibiting a "metallic-like" temperature dependence. 7,8 In addition, the reproducibility of single-crystal FETs permits one to investigate in detail how different aspects of the devices influence transistor operation, which is necessary to individuate the ultimate performance limits of organic transistors.In this letter, we report a comparative experimental study of the electrical characteristics of rubrene single-crystal FETs fabricated using Ta 2 O 5 , Al 2 O 3 , SiO 2 , and Parylene C as gate insulator. For the different dielectrics, field-effect transistors exhibiting stable and hysteresis-free electrical behavior can be reproducibly realized. In all cases, the hole mobility extracted from room-temperature measurement of the transistor characteristics is remarkably gate-voltage independent. From these measurements, we find that the mobility decreases from 10 cm 2 /V s (Parylene C, = 3.15) to 1.5 cm 2 /V s ͑Ta 2 O 5 , =25͒ with increasing the relative dielectric constant. By comparing our data to those recently reported for transistors fabricated using Parylene N ͑ =15 cm 2 /V s; = 2.65͒ 2,9 and polydimethylsiloxane (PDMS) air-gap stamps ͑ =20 cm 2 /V s; =1͒, 7 we conclude that a decrease in mobility with increasing the dielectric constant of the gate dielectric occurs systematically in rubrene singlecrystal FETs. This result demonstrates that the mobility measured in organic transistors is not only a property of the specific organic molecule used, but that it intrinsically depends on the organic/dielectric interface.The devices used in our investigations have been fabricated by means of two different, recently developed techniques. Transistors based on Parylene C have been built following the processing described in Ref. 1, using aqueous colloidal graphite or silver epoxy for the source, drain, and gate electrodes (with colloidal graphite resulting in better performances as compared to epoxy). For these devices, rathe...

We report on organic field-effect transistors with unprecedented resistance against gate bias stress. The single crystal and thin-film transistors employ the organic gate dielectric Cytop TM . This fluoropolymer is highly water repellent and shows a remarkable electrical breakdown strength. The single crystal transistors are consistently of very high electrical quality: near zero onset, very steep subthreshold swing (average: 1.3 nF V/(dec cm 2 )) and negligible current hysteresis. Furthermore, extended gate bias stress only leads to marginal changes in the transfer characteristics. It appears that there is no conceptual limitation for the stability of organic semiconductors in contrast to hydrogenated amorphous silicon.

We report the observation of ambipolar transport in field-effect transistors fabricated on single crystals of copper-and iron-phthalocyanine, using gold as a high work-function metal for the fabrication of source and drain electrodes. In these devices, the room-temperature mobility of holes reaches 0.3 cm 2 / V s in both materials. The highest mobility for electrons is observed for iron-phthalocyanines and is approximately one order of magnitude lower. Our measurements indicate that these values are limited by extrinsic contact effects due to the transistor fabrication and suggest that considerably higher values for the electron and hole mobility can be achieved in these materials.

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 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.

Two new isostructural compounds, dichlorobis(1-(2-chloroethyl)tetrazole)copper(II) (1) and dibromobis(1-(2-chloroethyl)tetrazole)copper(II) (2), have been prepared. The synthesis, characterization, and spectral and magnetic properties as well as the crystal and molecular structures of 1 and 2 have been studied. Both complexes form two-dimensional, distorted square grid planes of copper and halides, distinctly separated by layers of tetrazole ligands. The differential (ac) magnetic susceptibility, chi = (deltaM/deltaH)(T), and magnetization M(H) of both complexes have been studied as a function of temperature and field. The compounds possess a ferromagnetic interaction within the isolated copper-halide layers (J/k(B) = 8.0 K, J/k(B) = 10.2 K, respectively, for the chloride and the bromide, and T(c) = 4.75 K, T(c) = 8.01 K). The magnetic coupling J'/k(B) between the different layers is found to be very weak (|J'/J|