Polythiophene polymers have been studied broadly since the late 1980's as suitable materials for organic solar cells (OSCs),2 thin film transistors (TFTs),1 organic light-emitting diode (OLEDs).3 However, Polythiohephene is insoluble in most organic solvents and can not be further processed into thin films, resulting in poor electron mobility 10-3 cm2/Vs.5 The solubility can be increased by attachment of alkyl or chains such as regioregular poly(3-hexylthiophene) (P3HT). The hexyl chains of the regiorandom poly (3-hexylthiophene) (P3HT) were found to improve the polymer solubility, and facilitate thin film formation without improvement of electron mobility, which is only 0.001 cm2/Vs.5 Furthermore, the regiogularly oriented alkyl chains of the regioregular (P3HT) have shown to allow the molecules to self-organized into crystalline lamella through intermolecular orbital overlap. This can lead to increasing π- conjugation, and carrier mobilities, which is typically between 0.05 to 0.1 cm2/Vs, compared with the regiorandom (P3HT).5,6 It has been investigated that alkyl-substituted oligothiophenes can achieve mobility ranging from 0.07- 1.1 cm2/Vs, when consider the appropriate alkyl chain lengths as additional substitutions.6 Moreover, many derivatives have been formed through molecular modification to enhance the polymer's properties.1
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High performance of organic semiconductors in field-effect transistor a has been extensively pursued, because such FETs can potentially be fabricated on flexible substrates,7 leading to development of inexpensive organic electronic devices. Two of the most commonly promising small molecules for OFETs are derivatives of p-type semiconductors, acenes and thiophenes.1 Significant effort has been performed on pentacene molecule in order in increase its stability in air, and thus providing better properties for organic electronic applications. Recently, Anthony and co-workers successfully designed soluble, and stable pentacence derivatives such as triethylsilylethynyl anthradithiophene (TESADT) ( 56,58), and difluoro-triethylsilylethynyl anthradithiophene (diF-TESADT) 59. Both of these acenes have improved air stability due to functionalization with TIPS group at the central ring, preventing the formation of oxidation by product. All from chemreview linear10regiopdf, Single crystal of these functionalized pentacene molecules were found to have extremely high hole mobilities ranging from 35 to 58 cm2/Vs at cm2/Vs at room temperature, and 225 K, respectively, making them essential components of organic field-effect transistors (OFET). At the same time, substantial progress has been made on many oligo- and polythiophenes in order to meet the criteria for obtaining improved intrinsic FET properties or high FET performance under ambient conditions. Talk about regioular p3heyl thiophene. 1 The regioregular (P3HT) is generally stable to air in the dark, but tends to degrade easily under ambient conditions, leading to inefficient FET performance. Ref Linear 1Nonetheless, regioregular polythiophenes are generally stable to oxygen in the dark, they are only susceptible to oxidative doping and photobleaching in air when exposed to ambient light. Modified regioregular polythiophenes with different side chains such as 5, 6, and 7 had also been studied in OTFTs without improvement in FET performance. e,g linear 11 These results clearly demonstrate the formidable challenge in fundamental design of high-mobility polymer semiconductors for OTFTs, which is additionally compounded by the needs to consider materials solution processability and sensitivity to ambient conditions. Many groups have attempted to enhance the stability as well as the electrical properties of regioregular thiophenes through molecular ordering and structural modifications with different side chains. (ref linear 1,3,4) such as 5, 6, and 7. However no improvements of FET performance were observed, which highlight the formidable challenges in preparing semiconducting polymers with high mobilities ,ease solution processabilites, and excellent stabilites under ambient conditions. 27 from linear 1
Research interest has then focused on synthesizing other type of oligothiphenes that imitate acence structure, which could lead to environmentally stable semiconducting thiophene molecules for better performance in OFET devices or exhibit both interesting electrical and optical properties and have excellent thermal and chemical stability. Or successful synthetic routes used to improve pentacene properties, give rise to researchers to apply similar strategy on thiophenes in order to improve its stability in air. For instance, Wex et al reported the design of two new embedded thiophenes, pure syn and anti isomers of thieno [f,f 9]bisbenzothiophenes, consisting of alternating thiophene and benzene rings as shown in figure .. linear 5. These new isomers are prominsing n-type of organic semiconductors that show increase stability in air up to 340 0 C with high field effect mobilites of o.12 cm2/Vs, that are slightly lower than greatest known organic transistors based on silicon dioxide gate insulator. The good stability and the high mobilities of these compounds are due in part to efficient solid-state packing, which promotes the charge transport and decreases the amount of oxygen dispersing into the bulk materials. In addition, the sensitivity of regioregular polythiophenes to air and light has been overcome by simple structural development, schematically described by structure b (Fig. 1), in which the spacing between alkyl side chains on the conjugating moiety M are shorten. This approach leads to improve the polymer processability, molecular organization, and electrical properties in the solid state. The reduction of pi conjugation length of the polythiophene by a conjugating moiety, enhance its stability M by lowering its highest occupied molecular orbital (HOMO) level.
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An interesting member of this class is poly (2,5-bis (2-thienyl)-3,6-dialkylthieno [3,2-b]thiophene), in figure ( ), which have shown to be solution- processed at low temperatures under ambient conditions and offer a high-mobility of up to 0.25 cm2/Vs, promoting semiconductor for high-performance TFTs.
It was found that the use of oligothiophenes instead of polythiophenes provides a higher mobility due to the formation of more ordered structural assemblies. From pp for example, two of the most widely studied p-type oligothiophenes, are hexathiophene (6T), and the more soluble dihexylhexathiophene (DH6T), have been shown to reveal mobilities similar to P3HT 15 with enhanced self- organization into more ordered films. NEED Ee.g p-type
The majority of ploy/oligthiophenes derivatives explored to date have been exhibiting hole-transporting (p-type) semiconductors. In general, n-type organic semiconductors are hard to obtain and easily degraded by air, resulting in poor performance when compared to the p-type. On other hand, there is a great interest in developing electron-transporting (n-type) oligothiophenes in recent years, due to their tunable chemical and physical properties. Recently, it has been demonstrated that introduction of electron-withdrawing groups into already prepared p-type oligothiophenes, and modifying the surface properties usually lower the LUMO level and therefore permit electron injection. This strategy allows n-type oligothiophene derivatives to reach performance equal to their p-type counterparts. A few examples of common n-type thiophene semiconductors are depicted in figure ( )
Mishra, A.; Ma, C.; Bauerle, P. Functional oligothiophenes: molecular design for multidimensional nanoarchitectures and their Applications. Chem. Rev. 2009, 109, 1141-1276
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(227) Facchetti, A.; Mushrush, M.; Katz, H. E.; Marks, T. J. AdV. Mater.
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(228) Yoon, M.-H.; DiBenedetto, S. A.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 1348-1349.
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From oligothiophenes pdf// Marks et al. synthesized and studied the properties of the first fluoroalkyl-functionalized thiopene-based materials as the efficient n-type semiconductors. Attachment of terminal perfluorinated hexyl chains to the oligothiophene core significantly improved the thermal stability, and electron affinity when compred to the parent R,R′-dihexyl-substituted oligothiophenes. Thermal and XRD studies indicated that fluoro substituted derivatives or diperflurorohexylquarter-thiophene (DFH-4T))), show strong intermolecular π-π stacking, improved stability and solubility (pp). The presence of fluorohexyl groups in 2 give sufficient electron-withdrawing ability and lowered the HOMO/LUMO energy levels, allowing electron injection to exceed hole injection. The sufficient electron-withdrawing ability of fluorohexyl groups in 2.126, lowered the HOMO/LUMO energy levels, allowing electron injection to exceed hole injection.
Oligothiophenes pdf and linear 4 : The addition of carbonyl groups between the quaterthiophene and perfluohexyl chains, gives (DFHCO-4T) as shown in 2.127 (Scheme 2.21), which further lowered the LUMO level and showed enhanced stability and unique charge transport properties .228,229 Polarized optical microscopy (POM) and differential scanning calorimetry (DSC) analyses revealed that incorporation of carbonyl groups increased the intermolecular packing and imposed long-range ordering. Fabricated from 2.127 were found to be air sensitive and showed n-type character with improved charge carrier mobilities of 0.32 cm2 V-1 s-1 compared to 6 - 10-2 cm2/ Vs for analogous quaterthiophene 2.126 (n ) 0) without carbonyl groups.230
Look at linear 4 Similarly, DFHCO-4TCO ( ) provides efficient electron injection to the LUMO level, and has shown to exhibit very high electron mobility as well as good air and thermal stability .
From linear 3 DFH-6T derivatives ( ) have shown to exhibit enhanced thermal stability, improved solubility, when compared to DFH-6T, due to the presence of flouroalkyl substitutions all over the entire molecule. This allows for better π overlap and facilitates thin film formation, making these materials useful n- type semicondutors.
Sayuri Handa,† Eigo Miyazaki,† Kazuo Takimiya,*,†,‡ and Yoshihito Kunugi§J. AM. CHEM. SOC. 9 VOL. 129, NO. 38, 2007 11685. Solution-Processible n-Channel Organic Field-Effect Transistors Based on Dicyanomethylene-Substituted Terthienoquinoid Derivative ( cyano-thiophene) (oligothiophenes)
Dicyanomethylene-substituted quinoidal oligothiophenes (1) are one of the common types of n-channel organic semiconductors, high-performance n-channel semiconductors. However, the solubility of these oligomers has found to be rapidly decreased by the presence of crowded solubilizing units side chains, which reduces the intermolecular interaction in solid state. Analogous of these compounds as showing in figure ( ) was synthesized to maintain such high solubility as well as better intermolecular interactions in the solid state. The thin films of this oilgomer has exhibited an efficient n- semiconductive behavior in OFETs, with high electron mobilities of up to 0.16 cm2 / Vs.
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Organic solar cells (OSC) have attracted a great deal of interest in recent years due to their ability to be fabricated at low temperatures, making them promising canadiates for OPVs. In recent years, bulk heterojunctions (BHJ) solar cells utilizing solution-processable small molecules as the active layer have been widely developed.
An example of small molecules for highly efficient organic photovoltaics (OPVs) is:a naphtho [2,3-b: 6,7-b-]dithiophene (NDT)-based donor molecule substituted by strongly absorbing and electron-deficient units (TDPP) as shown in ( ). Loser et al. synthesized NDT(TDPP)2, D-A-D oligomer, using Stille cross-coupling of NDT(SnMe3)2 with TDPP-Br in 80% yield. Expanding π-conjugation system of NDT with TDDP has found to be useful in increasing the intermolecular orbital overlap through π- stacking. This molecule has reached a successful 4.06% power conversion efficiency (PCE), excellent for use in OPVs applications. For the development of low-band-gap materials, Steinberger and coworkers created A-D-A-D-A type oligomers ( ) that contain bithiophene units as donor, benzothiadiazole (BTDA) as core, and trifluoroacetyl (TFA) as terminal acceptor units. (BTDA) is one of the most popular acceptor units utilized in low band gap materials for OSCs applications. It has been shown that when the heterocyclic BTDA is incorporated in π-conjugated systems, it provides thermal stability and strong intermolecular overlap through π- stacking. The oligomers were found to have a broad UV absorption from 466 nm-1 and 518 nm-1 with high molar absorptivity, and PCE of 1.56%. With such interesting optical and electrochemical behaviors, theses oligomers are promising for use in solar cells.
Recently, A-D-A type oligothiophenes 40 and 41 as shown in figure, were successfully prepared by including BTDA, or thiadiazolopyridine (TDAPy) as terminal acceptors for applications in OSCs. Thses two oligomers were synthesized to study the effect of replacing the benzene of BTDA with a pyridine in thiadiazolopyridine (TDAPy) on electronic and device properties. Replcement of BTDA in ( ) with TDAPy in ( ), results in lowing both HOMO and LUMO energy levels, due to better acceptor strength of the pyridine moiety.
In compared to oligomer 40, 41 oligomer has shown a PCE of 3.15%, with more well-ordered thin film due to intermolecular H-bonding interactions caused by the nitrogen atom of the pyridine ring. In 2010, Mei and coworkers prepared a D-A-D type oligothiophene for use in BHJ solar cells. (120 from sara). The oligomer was constructed by incorporating isoindigo as a molecular acceptor unit and two bithiophene units as molecular donor. The resulting polymer has exhibited a broad absorption in the visible spectrum, and a power conversion efficiency up to 1.76%, which make it suitable for applications in solar cells.
For semiconducting behaviors with high conductivity, the HOMO-LUMO gap must be reduced; this can be accomplished with increasing π -conjugated area, or incorporating heteroatoms with lone electron pairs. This reduced band gap allows electrons to more simply hop between conduction and valence bands, resulting in the semiconductive characteristics.