>ECE 423 1

Organic Semiconductor

Gaojie Lu

provide potential solutions. Those advantages are resulted Abstract—Organic and inorganic semiconductors are from the great characteristics of organic materials: easy compared, in terms of materials, structures and electronic shaping and manufacturing, infinite variety and tunable properties. Organic semiconductor devices, light-emitting diodes properties by changing the chemical structure. and organic thin film transistors, are introduced. Fabrication challenges of organic semiconductor devices are discussed. 1.1 Material Organic materials are based on conjugated organic small Keywords—organic transistor, organic semiconductor, OTFT molecules and polymers. In the last decade, organic materials were used to produce plenty of products of devices, because they are large-area, low-cost, plastic substrates. A great of I. INTRODUCTION progress have been made in a lot of fields, like optoelectronic HE field of organic electronic is an active emerging devices, Organic Light-Emitting Diodes (OLEDs) and Organic T technology with immense promise for innovative, Field Effect Transistors (OFETs) for switching functions. convenient and high-performance electronics [1-5]. In the late Organic semiconductors offer several advantages because 1970s three researchers found types of plastics capable of of their easy processing, good compatibility with a wide being modified to enable them as conducting metal. variety of substrates including flexible plastics and Conventional plastics are electrical insulators, but the opportunities of modifying the structure of organic discovery found they also can conduct electricity. It has semiconductor. Also thin films of organic semiconductors are opened a new era of plastics science and technology to be mechanically robust and flexible and possible to get flexible employed in organic semiconductors. electronics. Inorganic materials are rigid crystalline, which must be The main advantage of organic semiconductor is assembled extremely pure, and require very precise processing under to a fully flexible insulating film without any substrate. It is highly demanding conditions. possible to expose the gate side of the film to an external Organic materials combine novel semiconducting electronic medium to apply to any kind of substrate. Compared to silicon properties with the scope for easy shaping and manufacture of structures, organic materials have several advantages: Low plastics. There is a nearly infinite variety of these organic cost of the technology and possible to achieve fully flexible materials and their properties can be tuned by changing their structures, relatively low voltages, comparable with the chemical structure, making them very versatile [6-8]. performance for solution monitoring and some innovative Because of the remarkable properties of these organic applications, which are not possible for silicon based devices materials, they can be used to make a wide range of nowadays, for example, system embedded in textiles and semiconducting electronic devices, such as transistors, smart food packages. light-emitting diodes, solar cells and even lasers. Organic semiconductor can provide us flat and flexible electronics, and light –emission from these materials is particularly promising- when a voltage is applied to a thin film of a semiconducting polymer to give out light. And it is possible to deposit semiconducting polymers to greatly simplify the manufacturing process, increase flexibility, and reduce cost [9-16].

II. CHARACTERISTICS OF ORGANIC SEMICONDUCTORS Inorganic semiconductor has been widely applied; however, it requires extremely high purity, and needs very precise Figure 1. Structure of the Organic semiconductor transistor processing under highly demanding conditions. To overcome those challenges, organic semiconductors are proposed to 1.2 Structure and properties The structure of organic semiconductor devices is Manuscript received December 8, 2006. manufactured by an organic semiconductor uniform and high Q. Yu is with University of Rochester, Rochester, NY 14627 USA (corresponding author to provide phone: (585)275-1769; fax: (585)273-2073; carrier transport property over a relatively large area. Modern e-mail: [email protected]). >ECE 423 2 chemistry enables the synthesis of complex molecules, such as (OLED) has been fabricated, which is made by pentacene carbon, hydrogen, oxygen, and nitrogen atoms. They can be films grown on flexible plastic structures [13]. A similar linked together with strong covalent bonds where two structure can be used also for detecting mechanical neighboring atoms each as addiction. F or non-covalent deformations on flexible surfaces, the flexibility of the interactions, they can be built to complex supermolecular substrate and the low cost of the employed technology, the structures. They can approach nanotechnology; their production of flexible chemical and strain gauge sensors can interactions allow the materials to be tunable and mechanical be employed in a variety of innovative applications such as properties. The structure of the Organic semiconductor wearable electronics [14-16]. transistor is shown in Figure 1. Over the last decade, blue Organic Light Emitting Diodes Carbon can form four bonds with neighboring carbon atoms (blue OLEDs) with high efficiencies and advantages have or other atoms. Such as methane (CH4), four valence electrons attracted considerable attention for their potential applications occupy four sp3 in and can form four covalent bonds. When to the full color ultra-thin flat panel display. Moreover, blue two adjacent carbon atoms come together, a band is formed light can be converted into green or red with the use of proper between two electrons. We call it conjugated; atoms are linked dyes giving the possibility to generate all colors from the blue by alternative double bonds. emitter. This latter property leads to an important The contribution of change has been observed in the crystal simplification in the design of the OLEDs displays. Efficient structure with temperature to the variation of carrier mobility blue OLEDs are doped transport layers in a p-i-n type with temperature. With the temperature increasing, there will structure. Figure 2 shows the molecular structure of the be slight increase in electron mobility along the crystal organic compounds used in this work, while Figure 3 details direction. Since the changes in crystal lattice dimensions of the layer structures of the two types of devices [15-17]. anthraquinone at various temperatures are small, the electronic band structure of organic compounds may be highly structure-sensitive.

1.3 Classification Organic semiconductors can be widely classified into two groups on the basis of their molecular weight: conjugated polycyclic compounds of molecular weight less than 1000, and heterocyclic polymers with molecular weight greater than 1000. Due to the ease with which they form thin films with large surface area, polymers are very useful materials for semiconductors. Small-molecule organic semiconductors may further be classified as linear, two-dimensional fused ring compounds, and heterocyclic oligomers. It is more facile control of charge transport by modifying various molecular parameters. Figure 2. Molecular Structures of the organic materials used for the Blue-Oled.

III. ORGANIC SEMICONDUCTOR DEVICES

2.1 Organic Light-emitting diodes Light-emitting diode (LED) is a semiconductor device that light emits from a solid material, caused by an electrical power source. Its effect is a form of electroluminescence. The composition and condition of the semiconducting material used decide the color of the emitted light also can be infrared, such as visible or near-ultraviolet, infrared emission from gallium arsenide (GaAs) and other semiconductor alloys.

A phenomenon termed electroluminescence is introduced Figure 3. OLED structure with the detail of the thickness and the organic here. The first light-emitting diode (LED) had been born. At material used in each layers; (a) the emissive layer is DEC or PMC, (b) the that time, the properties of materials were poorly controlled, emissive layer is DPVBi-doped with DEC or with PMC. and the emission process was not well understood. The 2.2 Organic thin film transistors light-emitting active material was SiC crystallites, GaAs and Organic thin-film transistors (OTFTs) using organic AlGaAs as used for sandpaper abrasive. semiconductor on organic semiconducting compounds in The first example of totally flexible field effect device for electronic components, notably computer displays, sensors, chemical detection based on an organic Light-emitting diode smart cards, and radio-frequency identification (RFID) tags. >ECE 423 3

Because OTFTs can be fabricated with a low-temperature extensively used in xerography. It can add certain process, they are more compatible with polymeric substrate combinations of amorphous organic semiconductors replaced than conventional silicon-based transistors. amorphous Si and selenium as the active material in this Organic thin-film transistors (OTFTs) inherit the design application, which might be considered as the first large scale architecture from its inorganic counterpart MOSFET, and it commercial application of an organic semiconductor. Small consists of source, drain, and gate electrodes, a dielectric layer, molecule and conjugated polymer light-emitting diodes are on and the active semiconductor layer, as shown in Figure.4 (a). the verge of commercialization. In order to improve the As illustrated in Figure. 4 (b), the OTFT can achieve the efficiency and stability of OTFTs devices, we have got typical I-V curves similar to that of MOSFET. The electrical impressive strides. characteristics of OTFTs were measured using two Organic semiconductor based thin-film transistors have independent parameters: picoammeter/dc voltage sources. been proposed for number of applications, for example, The OTFTs use a great deal of organic semiconductors in displays and RFID tags, lower cost and simpler packaging the field of lightweight, low-cost, large-area and flexible with flexible substrates. They also appear in a wide range of electronic products, and they become more compatible with photo-excited laser action, digital circuit, electronic noses, and polymeric substrate than conventional silicon-based transistors. so on. Now it is successful to make it possible excellent electron mobility, such as n-type OTFT and excellent hole mobility V. CONCLUSIONS [18]. There are ambipolar OTFTs and complementary Organic semiconductors pose a range of molecular weights inverters reported recently [19]. The performance of OTFTs and include small molecules. Because of chemical tenability, has been steadily developed to fabricate those devices [20-21]. organic semiconductors become attractive materials for fabrication of FETs. Organic semiconductors remain the best performance, due to very well ordered structures. It has been proved that organic transistors fabrication is easier and cheaper than silicon transistors fabrication, although the carrier mobility is not sufficient fast. In addition, challenges exist in large-scale manufacturing.

REFERENCES [1] H. E. Katz, Z. Bao, and S. J. Gilat, J. Acc. Chem. Res., vol.34, pp.359, 2001. [2] Nandita Madhavan, “Small-molecule organic semiconductors,” April 01, 2002. [3] A. Dodabalapur, “The future of organic semiconductor devices,” Device Research Conference 2000, Conference Digest, pp. 11-14, June 2000. [4] http://assets.cambridge.org/052182/3307/excerpt/0521823307_excerpt.p df (a) [5] Shi J M and Tang C W, Appl. Phys. Lett. pp.80, 2002 [6] http://www.aist.go.jp/aist_e/latest_research/2004/20041118/20041118.ht ml [7] C. D. Dimitrakopoulos and D. J. Mascaro, “Organic thin-film transistors: A review of recent advances,” IBM J. Res. & Dev. Vol. 45, No. 1, Jan. 2001. [8] http://www.drproctor.com/os/ [9] www.st-andrews.ac.uk/~osc/org-semi.html [10] C. Reese, M. Roberts, M. M. Ling and Z. Bao, “Organic thin film transistors,” materialstoday, pp.20-27, Sept. 2004. [11] Gao Z Q, Lee C S, Bello I, Lee S T, Chen R M, Luh T Y, Shi J and Tang C W, Appl. Phys.Lett. pp.74, 1999 [12] Kulkarni A P, Gifford A P, Tonzola C J and Jenekhea S A, Appl. Phys. Lett. pp.86, 2005 [13] Gebeyehu D, et al., Synth. Met. Pp.148-205, 2005 [14] Hosokawa C, Higashi H, Nakamura H and Kusumoto T, Appl. Phys. Lett. pp. 67, 1995 [15] Liao C H, et al., Appl. Phys. Lett. pp. 86, 2005 [16] Wu Y Z, et al. Appl. Phys.Lett. pp.83, 2003 [17] Morin J F, Leclerc M, Adès D and Siove A, Macromol. Rapid Commun. pp. 761, 2005 [18] http://www.oea-osc.com/persp_overview.htm [19] http://www.phys.unsw.edu.au/~arh/background/Organic%20Electronics/ (b) Organics.html Figure. 4 Organic TFT (a) basic schematics [18] (b) I-V curves [19] [20] A.Tsumura, H. Koezuka, and T. Ando, Appl.Phys.Lett. pp. 49, 1986 [21] http://pubs.acs.org/hotartcl/ci/00/apr/0650wallace.html [22] Y. Inoue, et al., “Organic Thin-file transistors with high electron IV. FUTURE WORK mobility based on perfluoropentacene,” Japan Society of Applied Organic semiconductors based photoconductors are Physics, Vol. 44, No. 6A pp. 3663-3668, 2005 >ECE 423 4