In recent years, flexible display has been one of great interest especially in mobile applications. However, flexible substrates cannot sustain under high temperature fabrication process, the development of low temperature process of electronics is necessary. Organic semiconducting materials have high potential due to the following advantages: 1) low-cost; 2) solution fabrication process on a large area array system; 3) low fabrication temperature (< 200 ℃); and 4) the applications are on a flexible substrate.
Recently, organic materials are extensively applied on electronic devices, such as light emitting diode(LED), thin film transistors(TFT), solar cells, and sensors. Also, organic chemical products are closely related to our life, such as foods, medicines, paper, plastics, and fibers. There are two kinds of organic semiconducting materials such as polymer and small molecular weight materials. We introduce these two kinds
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of organic materials as following.
1.2.1. Polymers
The conventional polymer is extensively used due to their plastic deformability, mechanical strength, low weight, and usually high resistivity. In 1970, the Japanese chemist Shirakawa found that it was possible to synthesize polyacetylene (CH)n in a new way. Shirakawa synthesized trans-polyacetylene by accidentally adding “a thousand-fold too much catalyst” to the reaction vessel. Shirakawa was stimulated by this discovery. The corresponding reaction at another temperature gave a copper-colored film instead, and it appeared to consist of almost pure cis-polyacetylene. Around the same time chemist Alan G. MacDiarmid andphysicist Alan J. Heeger were experimenting with a metallic-looking film of the inorganic polymer sulphur nitride, (SN)x. When MacDiarmid heard about Shirakawa'sdiscovery at a seminar in Tokyo, he invited Shirakawa to the University of Pennsylvania in Philadelphia. After Shirakawa and MacDiarmid modifiedpolyacetylene by oxidation with iodine vapor, they knew that the optical properties changed in the oxidation process and asked Heeger to have a look at the films. Aftermeasured the conductivity of the iodine-doped trans-polyacetylene, the incredibleincrease of ten million times the original conductivity was discovered. In the summerof 1977, Alan Heeger, Alan MacDiarmid and Hideki Shirakawa, and co-workers,published their discovery in the article "Syndissertation of electrically conducting organic polymers: Halogen derivatives of polyacetylene (CH)n" in The Journal of Chemical Society, Chemical Communications.[17] The discovery was considered a majorbreakthrough, and Alan Heeger et al. have beenawarded the Nobel Prize in Chemistry in the year 2000 for showing how plastic canbe made to conduct electric current.[18] Since the field has grown immensely, and also given rise to many new and exciting applications. The
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properties of a polymer are strongly related to the physical arrangement ofmonomers along the backbone of the chain. Polymers which contain only one type ofmonomer are known as homopolymers, while polymers containing a mixture ofmonomers are known as copolymers.
The conducting conjugated polymer consists of a long chain of carbon atoms with alternating single and double bonds between them, each with one hydrogen atom. The structure of polyacetylene is shown in Figure 1.3(a) as a typical example.
Polyacetylene is usually prepared in the cis- form which can be converted into the thermodynamically more stable tans- form by thermal isomerization. Other semiconducting conjugated polymers shown in Figure 1.3 are commonly used in organic light-emitting diodes (OLEDs), organic field-effect transistors (FETs), and organic solar cells. As shown in Figure 1.3(b), PEDOT:PSS is a water-soluble transparent conducting polymer, which enabled the fabrication of all plastic polymer light-emitting diodes (PLEDs). PEDOT:PSS can be used as a transparent anode.
Currently, it serves as the hole transport layer to develop PLEDs for commercial products. Structures of PPV and PPV derivatives (MEH-PPV) are shown in Figure 1.3(f) and 1.3(h). The most commonly used PPV is typically deposited by spin coating a precursor polymer, and then thermal treatment is used to convert the precursor to PPV. PPV also used as hosts for low gap emitter.[19] PFO is also the material used in the blue PLEDs as shown in Figure 1.3(g). Following the first blue PFO-based PLED was developed in 1991 [20], efforts was conducted on developing commercially viable devices based on these polymer. Polythiophenes and P3HT-based PLEDs and FETs are widely studied.[21], [22] Due to it relatively low gap, the polythiophenes are red emitters. However, the relatively poor lifetime of polythiophene-based PLEDs inhibits their commercialization. On the other hand, P3HT is a commonly used material in FETs. Under proper treatment, the mobility can
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be increased and the performance of the FETs can be optimized.
Figure 1.3 Molecular structure of widely used conjugated polymers: (a) polyacetylene;(b) poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT:PSS); (c)poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB);(d)poly(9,9-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,N-phenyl-1,4-p
henylenediamine) (PFB); (e)
poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine] (polyTPD);
(f)poly(p-phenylenevinylenes) (PPV); (g) poly[9,9-dioctylfl uorenyl-2,7-diyl] (PFO);
(h)poly-[2-methoxy,(5-2'-ethyl-hexyloxy)-p-phenylenevinylene] (MEH-PPV);
(i)poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT); (j) poly[3-hexylthiophene](P3HT); (k) poly[5,5'-bis(3-alkyl-2-thienyl)-2,2'-bithiophene) (PQT); (l)poly[(9,9-dioctylfluorene-co-bithiophene] (F8T2); (m)
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poly(9-vinylcarbazole) (PVK).
1.2.2. Small molecular weight materials
The phenomenon of organic electroluminescence was first discovered by Pope in 1963.[23] However, the development of organic light-emitting diode actually began in the late 1970s by Tang and his coworkers. Their research led eventually to the discovery of the first efficient multi-layered organic electroluminescent device.[24]
Since then, tremendous progress has been made in the field of organic electroluminescence. Among all efforts to improve the performance of organic light-emitting diode, the continuing discovery of new and improved electroluminescent materials is the most essential one. Small molecular weight materials consist of molecules with several to a few hundred atoms. Small molecular weight materials were the initial focus of physicists and engineers who seeking to understand the optoelectronics properties of organic materials. Structure of some small molecular weight materials are shown in Figure 1.4. Figure 1.4(a) shows the structure of CuPC which is widely used as an hole transport layer. However, depending on the other layer, it may inhibit hole injection [25] or enhance it [26].
TPD is another material commonly used as hole transport layer as shown in Figure 1.4(b). But, its relatively low glass transition temperature around 65℃ causes a failure of OLED as TPD recrystallized. The recrystallization may be suppressed by adding guest molecule such as rubrene. However, it may result in red electroluminescence from rubrene. Hence, NPB is developed with a structure similar to TPD but the methylphenyl groups are replaced by naphthylphenyls. The modification significantly enhances the stability of the OLED due to the increased glass transition temperature around 95℃.
Alq3 is the most widely used electron-transport and host emitting material in
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OLEDs. It is still one of the most robust electron-transport backing layers in OLED, particularly with the help of the hole blocking layer to trap the hole carriers from injecting into Alq3.[27] It is not only commonly used as a green emitter, but also as a host for lower-gap emitter guest molecules. It has been found by the time-of-flight technique that the drift mobility of electrons in Alq3 is increased by about two orders of magnitude (to 10-4 cm2/Vs) as the deposition rate decreased from 0.7 to 0.2 nm/s.
Figure 1.4 Molecular structure of widely used small molecular weight materials: (a)
copper phthalocyanine (CuPC); (b)
N,N'-Bis(3-methylphenyl)-N,N'-bis-(phenyl)-benzidine (TPD); (c) N,N-bis(1-naphtalenyl)-N-N’-bis(phenylbenzidine) (α-NPB);
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(d)N,N’-bis(2-naphtalenyl)-N-N’-bis(phenylbenzidine) ( β -NPB); (e) 4,4',4"
-Tris(N-(1-naphthyl)-N-phenyl-amino)triphenylamine (1T-NATA); (f) 4,4',4"
-Tris(N-(2-naphthyl)-N-phenyl-amino)triphenylamine (2T-NATA); (g) α-sexthiophene (α-6T);(h) Pentacene; (i) Tris(8-hydroxyquinoline) Aluminum (Alq3T); (j) (5,6,11,12)-Tetraphenylnaphthacene (Rubrene); (k) 2-(4-Biphenylyl)-5-
(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD); (l)
4-(Dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyr an (DCJTB).