Research in organic electronics has included advances in materials, devices, and processes. Device architectures, increasingly complex circuitry, reliable fabrication methods, and new semiconductors are enabling the incorporation of organic electronic components in products including OLED displays and flexible electronic paper.

Pentacene-based thin-film integrated circuits patterned only with polymeric shadow masks and powered by near-field coupling at radio frequencies of 125 kHz and above 6 MHz have been demonstrated. Sufficient amplitude modulation of the rf field was obtained to externally detect a clock signal generated by the integrated circuit. The circuits operate without the use of a diode rectification stage. This demonstration provides the basis for more sophisticated low-cost rf transponder circuitry using organic semiconductors.

We show novel and selective means to modify the dielectric surfaces in organic TFTs. Modification schemes include alkylphosphonic acid monolayers that have a strong affinity for alumina surfaces. Monolayers form robust, extremely uniform thin films and are deposited through simple spin-coating with a dilute solution of the monolayer precursor in solvent. Adding monolayers to organic TFTs has resulted in polycrystalline devices with mobilities nearly equal to single-crystal values while maintaining acceptable values of other device parameters (for example, the threshold voltage, on/off ratio, and subthreshold slope) required for fully functional integrated circuits.

We report here methods of surface modification and device construction which consistently result in lab-scale pentacene-based TFTs with mobilities at or above 5 cm2/Vs. Surface modifications include polymeric ultrathin films presenting a passivated interface on which the semiconductor can grow. High performance TFTs have been fabricated on a variety of dielectric materials, both organic and inorganic, and are currently being implemented in manufacturable constructions. Our surface modifications have also proven useful for substituted pentacene materials and for a variety of other organic semiconductors. In addition, we report an all organic active layer, rf-powered integrated circuit. Further experiments and statistical analyses are underway to explain the elevated mobility in our samples, and efforts have been made to confirm these results through collaboration.

We present the results of two studies:  (1) a comparison of force−distance (F−D) profiles obtained by atomic force microscopy (AFM) and the surface forces apparatus (SFA) for a poly(2-vinylpyridine)−polystyrene (PVP−PS) brush in good solvent; (2) a series of F−D profiles for a poly(4-tert-butylstyrene)−sodium poly(styrene-4-sulfonate) (PtBS−NaPSS) brush as a function of aqueous NaCl concentration. The AFM force profiles of the neutral PVP−PS brush are less steep than the corresponding surface forces data in the regime of high brush compression, in agreement with a recent molecular simulation study that indicated the tip would splay polymer chains and penetrate the brush. We also observe a bimodal distribution of interaction distances for the AFM force profiles of the PVP−PS brush which we ascribe to the tip sampling regions of higher and lower chain density during consecutive force measurements. AFM F−D profiles of the PtBS−NaPSS brush show a strong dependence of interaction distance on NaCl concentration, and a plot of interaction distance vs salt concentration shows predicted power law behavior. Images of both the PVP−PS and PtBS−NaPSS brushes show that the chain density is not uniform which gives rise to variations in the interaction distances measured by AFM. By facilitating measurements of local force profiles, AFM complements SFA measurements of interfacial forces and allows measurement of brush heterogeneity.

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We report the structural and electrical characterization of two new p-channel organic semiconductors, 5,5'-bis(2-tetracenyl)-2,2'-bithiophene (1) and 5,5'-bis(2-anthracenyl)-2,2'-bithiophene (2). Both compounds exhibited a high degree of thermal stability with decomposition temperatures of 530 degrees C and 425 degrees C for 1 and 2, respectively. The thin-film structures of 1 and 2 were examined using wide-angle X-ray diffraction (XRD), grazing incidence X-ray diffraction (GIXD), and atomic force microscopy (AFM). Films of 1 and 2 pack in similar triclinic unit cells with the long axes of the molecules nearly perpendicular to the substrate. Thin-film transistors (TFTs) based on 1 and 2 exhibit contact-corrected linear regime hole mobility as high as 0.5 cm2/Vs and 0.1 cm2/Vs, respectively. The specific contact resistance at high gate voltages for gold top contacts was 2 x 10(4) Ohms cm and 3 x 10(4) Ohms cm for 35 nm thick films of 1 and 2, respectively. Long-term air stability tests revealed less degradation of the electrical properties of 1 and 2 in comparison to pentacene. Variable temperature measurements revealed activation energies as low as 22 and 27 meV for 1 and 2, respectively. The temperature and gate voltage dependence of the mobility are discussed in terms of a double exponential distribution of trap states and a model accounting for the layered structure of the organic films. The enhanced air and thermal stability over pentacene, combined with good electrical performance characteristics, make 2 a promising candidate for future organic TFT applications.

Conducting probe atomic force microscopy (CP-AFM) was used to examine electrical transport through an individual grain boundary (GB) in the organic semiconductor sexithiophene (6T, E gap ∼ 2.3 eV). The sample consisted of a pair of grains grown by vapor deposition onto an SiO 2 /Si substrate. A variable channel length transistor was constructed using a microfabricated Au electrode contacting one grain, a Au-coated AFM tip as a positionable electrode, and the doped Si substrate as a gate. The GB resistance was found to be gate voltage dependent and large, on the order of 10 9 -10 10 Ω for a 1 µm boundary length. Resistances across single 6T grains were an order of magnitude lower. The results indicate that GBs can be the principal bottleneck to charge transport in polycrystalline organic semiconductor films, particularly at low gate fields, consistent with a recent model that proposes potential barriers exist between grains. We estimate the GB barrier height to be on the order of 100 meV.We report the use of conducting probe atomic force microscopy (CP-AFM) 1 to measure resistances associated with individual grain boundaries in the organic semiconductor sexithiophene (6T, E gap ∼ 2.3 eV). 2 Grain boundaries (GBs) play an important role in the transport properties of field effect transistors (FETs) based on polycrystalline organic semiconductor films. 3 In particular, it is expected that GBs trap and scatter mobile charge carriers, increasing the overall film resistance. Understanding of GB effects and the extent to which they dominate transport in organic semiconductors can be improved by microscopic measurements on indiVidual grains and GBs. In the work described here, we have taken advantage of the submicron resolution of CP-AFM to show that GB resistances can be an order of magnitude higher than single grain resistances in sexithiophene. Furthermore, the GB resistances are gate voltage dependent, as predicted by a recently proposed transport model. 3b Our studies employ a commercial AFM 4 and the experimental configuration shown in Figure 1. We use a Au-coated AFM tip as a positionable electrical contact to 6T grains that are also contacted by a fixed microfabricated Au electrode. The doped Si substrate serves as a gate. Current-voltage (I-V) measurements are made in air as a function of the probe position and gate field by applying negative voltages to the microfabricated electrode while keeping the probe tip at ground. Effectively, the structure in Figure 1 constitutes a FET in which the source (hole injecting) contact is an AFM tip that can be positioned at arbitrary points on the grain. Figure 2A is an AFM topograph showing two 6T grains on SiO 2 /Si sharing a common boundary approximately 1 µm in length. Both grains display typical 2.3 nm thick terraces corresponding to single layers of 6T molecules. Near the GB, the total thickness of each crystal is 18 nm, or 8 6T layers stacked on top of one another. The well-defined facets on the grains allow identification of their crystallographic orientations; the...