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Organic Electronics and Functional Inorganic Electronic Devices

Program: Electronics, Photonics and Sensors
Researcher Name: Vitaly Podzorov
Department: Physics and Astronomy
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Figure 1: The record high mobility free-standing OFETs fabricated in our group using the technologies developed at Rutgers that involve the growth of high-purity molecular crystals, deposition of low-resistance contacts, and fabrication of defect-free interface with a gate dielectric [1].



Figure 2: (top) Commercial full-color OLED display by SONY, which is only 2 mm thick [2]; (bottom) Flexible electronic paper produced by Philips (left) and Lucent (right).



Figure 3: A series of organic-based photovoltaic approaches have been demonstrated and are now under intense investigation [3].

Organic Electronics is a new research area directed toward development of a new generation of electronic devices, such as organic field-effect transistors (OFETs) [1] (Fig. 1), light emitting displays (OLEDs) [2] (Fig. 2) and photo-voltaic cells (OPV) [3] (Fig. 3). These devices are based on conjugated small molecules and polymers – the organic semiconducting materials that can help us address a number of important global issues, such as clean renewable energy sources, low-power flexible and wearable electronics (e.g., rollable computer and TV screens, electronic paper and solar cells), energy efficient solid state lighting, photo-detectors and green chemistry. These kinds of technologies are becoming increasingly important in modern world, but yet they are difficult (or even impossible) to tackle using conventional inorganic semiconductors, such as silicon. In addition, enormous possibilities of synthetic chemistry in tailoring electronic properties of organic molecules and availability of cheap solution-based and printing deposition techniques make organic semiconductors an extremely viable novel technology of the future.

Although applied organic electronics is rapidly developing, with the first commercial products already available on the market (see, e.g., the first SONY full color 2-mm thick OLED display and Philips electronic paper at Fig. 2), many fundamental aspects related to charge (polaron) and energy (exciton) generation and transport remain poorly understood in organic semiconductors. Our group’s interests are in the physics of basic electronic processes that determine operation of organic transistors and solar cells. Recently developed single-crystal OFETs (see, e.g. [4,5]) allow studies of charge transport and optical properties of organic semiconductors that are not limited by disorder. Owing to a great reproducibility and a very low density of defects (traps) in these devices, experimental observation of a band-like polaronic transport [6,7], Hall effect [8] and novel photo-induced phenomena [9,10,11] became possible in OFETs for the first time. Availability of well characterized single-crystal devices opens new exciting opportunities for research on charge carrier transport, photo-physics and sensing applications of OFETs. In addition, novel nanoscale surface functionalization techniques using self-assembled monolayers are emerging for organic semiconductors [12].

Some particular projects related to Organic Electronics that we are working on in our lab are: 1. High-performance single-crystal OFETs. Studies of charge transport and photo-physical properties of these devices to understand the fundamentals of the charge carrier mobility. 2. Conjugated polymer OFETs. Studies of the physics of charge transport. 3. Memory devices based on organic semiconductors. 4. Excitons in highly crystalline organic semiconductors. 5. Physics of photovoltaic effect in highly crystalline organic (and hybrid) solar cells. 6. Molecular self-assembly at the surface of organic and inorganic semiconductors and its effect on electronic and optical properties of these materials.

Functional devices based on novel inorganic semiconductors (e.g., field-effect transistors based on novel oxides) is another very young and important area, in which we are involved. These materials frequently exhibit a variety of interesting electronic phases and strongly correlated effects, such as spin and charge density waves or superconductivity. By fabricating a high quality field-effect devices at the surface of these materials, one can vary the carrier concentration in these systems by applying a gate voltage, instead of introducing chemical dopants, and achieve a controllable and tunable functional devices, in which the physical properties could in principle be varied across phase transitions. Our group was the first to demonstrate FET devices based on layered inorganic semiconductors from the WSe2 (dichalcogenides) family [13] (Fig. 4).

 



    REFERENCES:

  1. Book: “Organic Field-Effect Transistors”, Ed. Z. Bao, (Taylor & Francis, 2007)
  2. S. R. Forrest, “The path to ubiquitous and low-cost organic electronic appliances on plastic”, Nature, 428, 911 (2004)
  3. “Organic based photovoltaics”, MRS Bulletin, Vol. 30, (2005)
  4. R. W. I. de Boer, A. F. Morpurgo, M. E. Gershenson, V. Podzorov, Phys. Stat. Sol. (a) 201, 1302 (2004);
  5. M. E. Gershenson, V. Podzorov, and A. F. Morpurgo, Rev. Mod. Phys. 78, 973 (2006)
  6. V. Podzorov et al., Phys. Rev. Lett. 93, 086602 (2004)
  7. V. C. Sundar et al., Science 303, 1644 (2004)
  8. V. Podzorov et al., Phys. Rev. Lett. 95, 226601 (2005)
  9. V. Podzorov et al., Phys. Rev. Lett. 95, 016602 (2005)
  10. H. Najafov et al., Phys. Rev. Lett. 96, 056604 (2006)
  11. M. F. Calhoun, C. Hsieh and V. Podzorov, Phys. Rev. Lett. 98, 096402 (2007)
  12. M. F. Calhoun, J. Sanchez, D. Olaya, M. E. Gershenson and V. Podzorov, Nature Mat. 7, 84 (2008)
  13. Podzorov et al., “High-mobility field-effect transistors based on transition metal dichalcogenides”, Appl. Phys. Lett. 84 3301 (2004)

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