Mitchell A. McCarthy

Low-Voltage, Low-Power, Organic Light-Emitting Transistors for Active Matrix Displays

Efficient organic light-emitting transistors use carbon nanotubes as the source electrode. Intrinsic nonuniformity in the polycrystalline-silicon backplane transistors of active matrix organic light-emitting diode displays severely limits display size. Organic semiconductors might provide an alternative, but their mobility remains too low to be useful in the conventional thin-film transistor design. Here we demonstrate an organic channel light-emitting transistor operating at low voltage, with low power dissipation, and high aperture ratio, in the three primary colors. The high level of performance is enabled by a single-wall carbon nanotube network source electrode that permits integration of the drive transistor and the light emitter into an efficient single stacked device. The performance demonstrated is comparable to that of polycrystalline-silicon backplane transistor-driven display pixels.

Electronic Junction Control in a Nanotube-Semiconductor Schottky Junction Solar Cell

We exploit the low density of electronic states in single wall carbon nanotubes to demonstrate active, electronic modulation of their Fermi level offset relative to n-type silicon in a nanotube-Si (metal-semiconductor) Schottky junction solar cell. Electronic modulation of the Fermi level offset, the junction interface dipole and a field developed across the depletion layer modifies the built-in potential in the device and its power generation characteristics. As produced (before modulation) devices exhibit ∼8.5% power conversion efficiency (PCE). With active modulation the PCE is continuously and reversibly changed from 4 to 11%.

Electric field gating with ionic liquids

The authors show that ionic liquids are well suited to specialized electric field gating applications in which large surface charge densities can be induced on the surfaces of low-carrier density thin-film metals. Using either coplanar or overlay gate configurations, they demonstrate field-induced resistance changes on the order of a factor of 104 for thin conducting InOx films. The areal capacitances and field effect mobilities noticeably exceed those that can be achieved using AlOx dielectrics. In addition, the charge state can be frozen in by reducing the temperature, thus providing an opportunity for electric field tuning of metal-insulator transitions in a variety of thin-film systems.

Improved Transfer of Graphene for Gated Schottky-Junction, Vertical, Organic, Field-Effect Transistors

An improved process for graphene transfer was used to demonstrate high performance graphene enabled vertical organic field effect transistors (G-VFETs). The process reduces disorder and eliminates the polymeric residue that typically plagues transferred films. The method also allows for purposely creating pores in the graphene of a controlled areal density. Transconductance observed in G-VFETs fabricated with a continuous (pore-free) graphene source electrode is attributed to modulation of the contact barrier height between the graphene and organic semiconductor due to a gate field induced Fermi level shift in the low density of electronic-states graphene electrode. Pores introduced in the graphene source electrode are shown to boost the G-VFET performance, which scales with the areal pore density taking advantage of both barrier height lowering and tunnel barrier thinning. Devices with areal pore densities of 20% exhibit on/off ratios and output current densities exceeding 10(6) and 200 mA/cm(2), respectively, at drain voltages below 5 V.

High Current, Low Voltage Carbon Nanotube Enabled Vertical Organic Field Effect Transistors

State-of-the-art performance is demonstrated from a carbon nanotube enabled vertical field effect transistor using an organic channel material. The device exhibits an on/off current ratio >10(5) for a gate voltage range of 4 V with a current density output exceeding 50 mA/cm(2). The architecture enables submicrometer channel lengths while avoiding high-resolution patterning. The ability to drive high currents and inexpensive fabrication may provide the solution for the so-called OLED backplane problem.

Current transport across the pentacene/CVD-grown graphene interface for diode applications

We investigate the electronic transport properties across the pentacene/graphene interface. Current transport across the pentacene/graphene interface is found to be strikingly different from transport across pentacene/HOPG and pentacene/Cu interfaces. At low voltages, diodes using graphene as a bottom electrode display Poole–Frenkel emission, while diodes with HOPG and Cu electrodes are dominated by thermionic emission. At high voltages conduction is dominated by Poole–Frenkel emission for all three junctions. We propose that current across these interfaces can be accurately modeled by a combination of thermionic and Poole–Frenkel emission. Results presented not only suggest that graphene provides low resistive contacts to pentacene where a flat-laying orientation of pentacene and transparent metal electrodes are desired but also provides further understanding of the physics at the organic semiconductor/graphene interface.

Reorientation of the High Mobility Plane in Pentacene-Based Carbon Nanotube Enabled Vertical Field Effect Transistors

The large current densities attained by carbon nanotube enabled vertical field effect transistors using crystalline organic channel materials are somewhat unexpected given the known large anisotropy in the mobility of crystalline organics and their conventional ordering on dielectric surfaces which tends to orient their high mobility axes parallel to the surface. This seeming contradiction is resolved by the finding that the nanotubes induce a molecular ordering that reorients the high mobility axes to favor current flow in a direction perpendicular to the substrate surface.

N-channel carbon nanotube enabled vertical field effect transistors with solution deposited ZnO nanoparticle based channel layers

N-channel carbon nanotube enabled vertical field effect transistors (CN-VFETs) exploiting a solution deposited ZnO nanoparticle thin film as the channel material are demonstrated. Transistor performance benefits from a thermal anneal followed by an oxygen plasma treatment. The devices exhibit on/off ratios approaching 104 with output current densities exceeding 60 mA/cm2. Combined with p-channel organic CN-VFETs, the solution based processing could allow for the development of low-cost complementary circuits.

High-performance devices using organic semiconductors

Synthetic control over the molecular constituents of organic semiconductors allows unprecedented control over their aggregate solid-state properties. Band-gap-like and band-edge-like properties can be tuned, seemingly at will (through the sweat and toil of brilliant synthetic chemists). This power comes, however, with a Faustian bargain. In contrast to inorganic semiconductors where atoms fully concede their individuality to collective quantum states, resulting in charge-carrier mobilities measuring in the hundreds to over a thousand square centimeters per volt second (cm2/V s), the molecular individuality retained in organic semiconductors leads to localization and mobilities typically amounting to less than 3cm2/V s. That creates a problem for applications requiring appreciable currents such as, for example, organic LEDs (OLEDs). OLED brightness is directly tied to the current fed to it by its drive transistor. For an organic semiconductor in a conventional, lateral-channel thin-film transistor (TFT, see Figure 1), the current needed for high brightness can be achieved in any of three possible ways. First, the voltage across the TFT sourcedrain electrodes can be made large, but, since the same current flows to the OLED from the TFT, a large voltage drop across the latter means high power dissipation in the transistor (not contributing to light generation). Second, the organic semiconductor channel width (CW/ can be increased, but each pixel is only allocated so much space, and room taken by the drive transistor is room not available to the OLED. Smaller OLEDs, for equal brightness, require higher current density, which degrades OLED lifetime. Finally, the source and drain electrodes can be brought close together, making the channel length (CL/ short. But that requires high-resolution patterning, and the other great lure of organic semiconductors is the low expense of vapor and/or solution processing (think ‘printing’) techniques for their fabrication. Hence, the comparatively low mobility that bedevils organic semiconductors is problematic for Figure 1. Layout of the conventional, lateral-channel TFT. The channel length and width are labeled CL and CW , respectively. Note the direction of the current flow.