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 show that it is possible to reach one of the ultimate goals of organic electronics: producing organic field-effect transistors with trap densities as low as in the bulk of single crystals. We studied the spectral density of localized states in the band gap (trap DOS) of small molecule organic semiconductors as derived from electrical characteristics of organic field-effect transistors or from space-charge-limited-current measurements. This was done by comparing data from a large number of samples including thin-film transistors (TFT's), single crystal field-effect transistors (SC-FET's) and bulk samples. The compilation of all data strongly suggests that structural defects associated with grain boundaries are the main cause of "fast" hole traps in TFT's made with vacuum-evaporated pentacene. For high-performance transistors made with small molecule semiconductors such as rubrene it is essential to reduce the dipolar disorder caused by water adsorbed on the gate dielectric surface. In samples with very low trap densities, we sometimes observe a steep increase of the trap DOS very close (< 0.15 eV) to the mobility edge with a characteristic slope of 10 − 20 meV. It is discussed to what degree band broadening due to the thermal fluctuation of the intermolecular transfer integral is reflected in this steep increase of the trap DOS. Moreover, we show that the trap DOS in TFT's with small molecule semiconductors is very similar to the trap DOS in hydrogenated amorphous silicon even though polycrystalline films of small molecules with van der Waals-type interaction on the one hand are compared with covalently bound amorphous silicon on the other hand. Although important conclusions can already be drawn from the existing data, more experiments are needed to complete the understanding of the trap DOS near the band edge in small molecule organic semiconductors.

We have fabricated single crystal, thermally evaporated, and spin-coated thin-film transistors (TFTs) from the same organic semiconductor N,N′-1H,1H-perfluorobutyl dicyanoperylene carboxydiimide (PDIF-CN2) using various combinations of deposition methods and gate dielectrics to investigate how the charge transport properties vary with the degree of crystalline order. Never before has a semiconductor been studied in such a wide variety of processing methods, allowing cross-comparison of the microscopic factors influencing the charge transport, and in particular the trap density of states (DOS). Excellent transistor performance was achieved for PDIF-CN2 single crystals in combination with Cytop as a dielectric layer resulting in a mobility of up to 6 cm2/Vs, an on/off-ratio exceeding 108, and a subthreshold swing of 0.45 V/dec. Furthermore, gate-bias-stress effects are not present in these transistors and we observed low stress effects in the evaporated TFTs with Cytop as the gate dielectric. These findings are reflected in the trap DOS. The single crystal field-effect transistor with Cytop has a low trap DOS, whereas in evaporated TFTs, the trap DOS is higher by 2–3 orders of magnitude. Surprisingly, the trap DOS of the spin-coated TFT is similar to that in evaporated TFTs, except for additional discrete trap states centered around 0.24 eV below the conduction band.

We present results on small-molecule p- and n-type organic semiconductors in combination with the highly water repellent fluoropolymer Cytop™ as the gate dielectric. Using pentacene and N,N′-ditridecylperylene-3,4,9,10-tetracarboxylicdiimide (PTCDI-C13), we fabricated complementary inverters of high electrical quality and stability that are almost unaffected by repeated gate bias stress. The combined p- and n-type field-effect transistors show nearly ideal characteristics, very small hysteresis, and similar saturation mobility (∼0.2 cm2/V s). Particularly PTCDI-C13 thin-film transistors exhibit a remarkable performance in the subthreshold regime if chromium is used as contact material for electron injection: a near zero onset and a subthreshold swing as low as 0.6 V/decade.

We report on the simple fabrication of hysteresis-free and electrically stable organic field-effect transistors (OFETs) and inverters operating at voltages <1-2 V, enabled by the almost trap-free interface between the organic semiconductor and an ultra-thin (<20 nm) and highly insulating single-layer fluoropolymer gate dielectric (Cytop). OFETs with PTCDI-C13 (N,N'-ditridecylperylene-3,4,9,10tetracarboxylicdiimide) as semiconductor exhibit outstanding transistor characteristics: very low threshold voltage (0.2 V), onset at 0 V, steep subthreshold swing (0.1-0.2 V/decade), no hysteresis and excellent stability against gate bias stress. It is gratifying to notice that such small OFET operating voltages can be achieved with the relatively simple processing techniques employed in this study. * Electronic address: walserma@phys.ethz.ch