10.1117/2.1201108.003815 High-performance devices using organic semiconductors Andrew G. Rinzler, Mitchell A. McCarthy, and Bo Liu New device architectures overcome the limitations of organic semicon- ductors and could accelerate development of large organic LEDs. Synthetic control over the molecular constituents of organic semiconductors allows unprecedented control over their aggre- gate 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 semi- conductors where atoms fully concede their individuality to collective quantum states, resulting in charge-carrier mobilities measuring in the hundreds to over a thousand square cen- timeters per volt second (cm 2 /Vs), the molecular individuality retained in organic semiconductors leads to localization and mo- bilities typically amounting to less than 3cm 2 /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 conven- tional, 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 source- drain 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 transis- tor (not contributing to light generation). Second, the organic semiconductor channel width (C W / 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 chan- nel length (C L / short. But that requires high-resolution pattern- ing, 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 chan- nel length and width are labeled C L and C W , respectively. Note the direction of the current flow. Figure 2. The carbon nanotube-enabled vertical field effect transistor (CN-VFET) architecture. The single-wall carbon nanotubes making up the source electrode are the interconnected network of lines lying across the gate dielectric surface. Here, the channel length, C L , is the thickness of the channel layer and is naturally much thinner than the channel length in the lateral-channel TFT (drawings not to scale). Current flow is vertical from the nanotubes to the top drain electrode. their commercial relevance in TFTs for OLEDs (or other similar high-current drive applications). In 2008, we reported a new architecture transistor that circumvents this mobility limitation (see Figure 2). 1 The Continued on next page