ORGANIC NANOSTRUCTURE DEVICES

Stephen Forrest
Princeton University
Advanced Technology Center for Photonics and Optoelectronic Materials
J301 POEM
Princeton, NJ 08544

Scientific Drivers

A primary motivation for the extensive research over the last several years concentrating on the growth and physics of organic thin film nanostructures is their very real potential for use in applications that are not accessible to more conventional, inorganic semiconductors. The recent demonstration of efficient electroluminescence from organic thin film devices promises to transform the flat panel display industry (Tang and VanSlyke 1987; Burrows and Forrest 1994; Burroughes et al. 1990), with the potential of replacing liquid crystal displays with an entirely new generation of efficient, emissive, full color flat panels based on light-emitting organic devices. In more recent developments, organic thin films are showing promise for use as thin film transistors (TFTs) (Dodabalapur et al. 1995; Garnier et al. 1990; Garnier et al. 1997), which might eventually replace amorphous or polysilicon TFTs currently used in the back planes of active matrix liquid crystal displays (AMLCD). These new developments also must be placed in the context of long efforts and progress that has been directed at employing organic thin films for solar energy conversion (Wohrle and Meissner 1991; Tang 1986) and in sensors of various kinds. Finally, vacuum-deposited OMCs have also been proposed as materials with large second or third order optical nonlinearities (Lam et al. 1991; Agranovich et al. 1995; Maruyama et al. 1995; Fang et al. 1993; Leegwater and Mukamel 1992; Wang and Mukamel 1994; Dubovsky and Mukamel 1992; Mukamel et al. 1994), or large ( (Burroughes et al. 1990). There are, in addition, many, somewhat less conventional uses of organic thin films deposited in vacuum, including waveguides and optical couplers (Zang et al. 1991; Zang and Forrest 1992; Taylor et al. 1997), organic/inorganic photodetectors (So and Forrest 1989; Forrest et al. 1982), and lasers.

The primary attraction of organic molecular nanostructures is their potential low cost and the extreme flexibility that the device engineer has in choosing a material whose properties have been specifically tailored to meet the needs of a particular application. The materials are easily integrated with conventional semiconductor devices, thereby providing additional functionality to existing photonic circuits and components.

The potentials of organic molecular nanostructures, however, must be balanced against the problems that have traditionally impeded acceptance of organics for use in active electronic or optoelectronic device applications, including unstable device characteristics; sensitivity to adverse environments (e.g., temperature, humidity, oxygen, etc.); non-ideal metal/organic contacts; and lack of reproducibility of material composition, purity, and fabrication conditions.

It is clear that the ultrahigh vacuum environment characteristic of organic thin film deposition processes can provide the necessary material purity and structural and chemical reproducibility necessary in modern, high performance optoelectronic device applications. While the costs and complexities associated with ultrahigh vacuum (UHV) deposition processes may offset the attractive (but possibly misleading) "simplicity" often attributed to organic-based devices, it is clear that the performance advantages of such structures outweigh these apparent disadvantages. Furthermore, while purity and structural precision are key to the ultimate success of all optoelectronic device technologies, it is not clear how sophisticated the deposition system must be to achieve acceptable device performance. At this point, vacuum deposition serves as the most powerful tool for investigating the detailed growth and physical characteristics of organic nanostructure devices, and hence will ultimately be able to address questions regarding the need for UHV in the production of practical display, transistor, NLO, or other molecular organic thin film device applications.

Recent Progress in Organic Nanostructure Devices

As a background for understanding the current status of organic nanostructures R&D, below is a compilation of some of the best results for organic thin film devices reported to date:

1. Solar Cells (Tang 1986; Forrest and So 1988): Highest power conversion efficiency for bilayer organic cells: 1-2%. Challenges that remain include insufficient efficiency to be practical. Also, stability of the devices is inadequate for power generation applications.
2. Organic Light-Emitting Devices (OLEDs) (Tang and Van Slyke 1987; Burrows and Forrest 1994; Burrows et al. 1996; Dodabalapur et al. 1994; Tang et al. 1989): These devices are positioning themselves to replace LCDs in several display applications. Some performance characteristics include high brightness (1,000 times brighter than CRT elements) and high efficiency (1-3%). Full color has been demonstrated, but saturated color is not obtainable at all corners of the color palate (R, G, and B), and stability of the devices exceeds 10,000 hrs at video brightness. Several challenges yet remain before widespread acceptance of OLEDs will occur. These include the ability to make fully color tunable devices at low cost and to manufacture very thin structures with an acceptable yield. While the operational lifetime of some devices (particularly green emitters) is quite good, other colors (e.g., red) do not have the same high performance and stability attributes. However, there are numerous groups worldwide pursuing OLED technology, and Pioneer intends to bring out a display based on OLEDs in early 1998.
3. Organic Thin Film Transistors (Dodabalapur et al. 1995; Garnier et al. 1990; de Leeuw et al. 1997; Dodabalapur, Torsi, and Katz 1995; Lin et al. 1997, 143): Remarkable progress in TFTs based on organic materials has been made in the last couple of years. In particular, pentacene-based organic transistors have resulted in channel mobilities equal to that of amorphous Si (~1 cm²/V-s), threshold voltages V_T~0V, and on/off current ratios ~ 10⁶, albeit not necessarily all on the same device. In addition, circuits consisting of 10 to 20 polymer-based TFTs have also recently been reported (de Leeuw et al. 1997). These results represent major breakthroughs in organic electronics, although many challenges remain. Nevertheless, significant economic drivers exist for ultralow-cost electronics for identification cards, low density memories, display backplanes, etc., which may provide the driving force necessary to bring these devices into widespread use.

These are only three examples of where organic nanostructures are finding application due to the advanced state of their development. As in the case of all immature technologies, there are still significant barriers to their adoption in the commercial world. However, for the first time, active organic nanostructures appear to be on the verge of transforming a large number of optical and electronic applications.

References

Agranovich, V.M., G.C. LaRocca, and F. Bassani. 1995. Chem. Phys. Lett. 247: 355.

Burroughes, J.H., D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, and A.B. Holmes. 1990. Nature 347: 539.

Burrows, P.E., and S.R. Forrest. 1994. Appl. Phys. Lett. 64: 2285.

Burrows, P.E., S.R. Forrest, S.P. Sibley, and M.E. Thompson. 1996. Appl. Phys. Lett. 69: 2959.

de Leeuw, D.M., A.R. Brown, M. Matters, K. Chmil, and C.M. Hart. 1997. Paper H3.5, p. 144. Field-effect transistors constructed from precursor-route conjugated polymers, San Francisco.

Dodabalapur, A., H.E. Katz, L. Torsi, and R.C. Haddon. 1995. Science 269: 1560.

Dodabalapur, A., L.J. Rothberg, and T.M. Miller. 1994. Appl. Phys. Lett. 65: 2308.

Dodabalapur, A., L. Torsi, and H.E. Katz. 1995. Science 268: 270.

Dubovsky, O., and S. Mukamel. 1992. J. Chem. Phys. 15: 417.

Fang, S., K. Kohama, H. Hoshi, and Y. Maruyama. 1993. Japan. J. Appl. Phys. 32: L1418.

Forrest, S.R., M.L. Kaplan, P.H. Schmidt, W.L. Feldmann, and E. Yanowski. 1982. Appl. Phys. Lett. 90.

Forrest, S.R., and F.F. So. 1988. J. Appl. Phys. 64: 399.

Garnier, F., G. Horowitz, D. Fichou, and A. Yassar. 1997. Supramolec. Sci. 4: 155.

Garnier, F., G. Horowitz, X. Peng, and D. Fichou. 1990. Adv. Mater. 2.

Lam, J.F., S.R. Forrest, and G.L. Tangonan. 1991. Phys. Rev. Lett. 66: 1614.

Leegwater, J.A., and S. Mukamel. 1992. Phys. Rev. A 46: 452.

Lin, Y.-Y., D.J. Gundlach, and T.N. Jackson. 1997. MRS Spring Mtg., San Francisco, CA.

Maruyama, Y., H. Hoshi, S.L. Fang, and K. Kohama. 1995. Synthetic Metals 71: 1653.

Mukamel, S., A. Takahashi, H.X. Huang, and G. Chen. 1994. Science 266: 250.

So, F.F., and S.R. Forrest. 1989. IEEE Trans. Electron. Dev. 36: 66.

Tang, C. W. 1986. Appl. Phys. Lett. 48: 183.

Tang, C.W., and S. A. VanSlyke. 1987. Appl. Phys. Lett. 51: 913.

Tang, C.W., S.A. VanSlyke, and C.H. Chen. 1989. J. Appl. Phys. 65: 3610.

Taylor, R.B., P.E. Burrows, and S.R. Forrest. 1997. IEEE Photonics Technology Lett. 9: 365.

Wang, N., and S. Mukamel. 1994. Chem. Phys. Lett. 231: 373.

Wohrle, D., and D. Meissner. 1991. Adv. Mater. 3: 129.

Zang, D.Y., and S.R. Forrest. 1992. IEEE Photon. Technol. Lett. 4: 365.

Zang, D.Y., Y.Q. Shi, F.F. So, S.R. Forrest, and W.H. Steier. 1991. Appl. Phys. Lett. 58: 562-564.