The majority of organic semiconductors are p-type materials hole conductors, and have a HOMO level between 4.9 and 5.5 eV, resulting in ohmic contact with high work-function metals such
Literature Review of Solution-Processed P-Type Oligomers
Pentacene has consistently shown the highest charge mobility of any of the organic semiconductors when deposited by vacuum evaporation, and thus many research groups have set out to synthesize processible versions of the small molecule The first method for solubilizing pentacene was demonstrated by Herwig and Millen.® A Diels-Alder reaction on the central ring of pentacene was used to disrupt the planarity of the molecule, thus rendering it soluble (1) A similar, but more synthetically accessible route (2) was later demonstrated by Afzali et al In both cases, these precursors were spun-cast into films then heated to initiate a retro Diels-Alder reaction to convert the precursor to pentacene The performance of OFETs using this precursor route are competitive with vacuum-deposited pentacene devices, where solution processed mobilities have been reported to be as high as 0.89 cm”/V-s and an lonllog ratio above 2 x 10” when films are annealed at 200 °c.
Afzali and coworkers have also demonstrated that conversion of pentacene precursors to t The most successful case was demonstrated pentacene can be photopatterned using UV ligh by Weidkamp et al where pentacene was derivatized with an acid-sensitive N-sulfinyl-tert-butyl carbamate Co-deposition of this precursor with a photoacid generator gave efficient conversion to pentacene under UV light The ability to affect this conversion using light instead of heat is of great interest to further enhance substrate compatibility of these materials.
More recently, the Anthony group has developed elegant methods for solubilizing pentacene through the use of strategically placed substituents They found that placement of bulky silyl groups separated from the acene by a alkyne spacer at the peri positions of pentacene (3) not only gives rise to good solubility, but also disrupts edge-to-face interactions and causes the molecules to adopt either 1-D "slipped-staok" or 2-D "bricklayer" face-to-face conformations rather than the usual herringbone pattern.*” Face-to-face arrangement of the molecules significantly increases the z-orbital overlap of the pentacene rings in adjacent molecules and decreases the interplanar spacing of the aromatic rings.°” This solubilizing method has also been demonstrated with other acene derivatives such as pentacene ethers,” acenedithiophenes (4),°° and longer hexa- and heptacenes.”°
It was found that the only derivatives to give good OFET performance were those that adopted a 2-D zx-stacked arrangement Solution-deposited films of these functionalized acenes gave rise to OFET device hole mobilities up to 0.17 cm7/V-s for pentacene derivatives, and mobilities as high as 1.0 cm7/V:s for the anthradithiophene derivatives, both with high fon/lor ratios.”' These values were comparable to thermally evaporated devices made from the same materials,2 suggesting that the molecules can assemble just as well in the solution as in the vapor phase Also, notably all the measurements reported were performed in air at room temperature suggesting that the introduction of these substituents improves the stability of the acenes The higher performance of the anthradithiophene derivatives has been attributed to the increased z-interactions in the crystal where they pack ~0.2 A closer than the pentacene derivatives.°*”'
Laquindanum and coworkers had also previously investigated solution processed films of anthradithiophenes alkylated at the a-positions (6).“” They found that these materials had greater solubility and oxidative stability than pentacene but still had comparable charge mobility Alkylated derivatives were sparingly soluble but could be cast from dilute solutions of hot chlorobenzene, followed by solvent evaporation in a vacuum oven at relatively high temperatures.
Field-effect mobilities up to 0.02 cm?/V:s were obtained, but had an order of magnitude lower mobility as compared to vacuum-evaporated films of the same material.”
Finally, Tulevski and coworkers demonstrated the first monolayer solution deposition of an acene derivative.“ A tetracene derivatized at one end with ortho-hydroxy! functionalities (6) organizes into upright monolayers on the surface of aluminum oxide Assembling these molecules into nanoscale field-effect transistor channels produced gate-voltage dependant devices but only when the source-drain distances was less than 60 nm, While this idea of attaching the semiconductor to the gate dielectric could have wide reaching applications, it still remains to be seen if it is a viable approach to transistor fabrication.
By far, the most commonly investigated candidates for solution processed organic
20,45,75,76 semiconductors are oligothiophenes Like most highly conjugated molecules, unsubstituted oligothiophenes have very poor solubility as a consequence of z-r stacking. However, thiophenes offer many possibilities for functionalization in both the œ- and B- positions, and are compatible with a wide range of chemistries.7°””
One of the first solution processed oligothiophenes was reported by Dimitrakopoulos and coworkers They synthesized oligomers containing thiophenes with internal vinyl linkages (7) that were soluble in hot N-methylpyrrolidinone (NMP) Spun-cast top contact devices gave mobilities up to 0.001 cm?/V's with /„/lạạ ratios of 10°.”
The majority of the early efforts to solubilize oligothiophenes used linear alkyl chains at the œ-
79,80 24 24,81 and œ-positions Dihexyl-substituted quarter- ””, penta-“" and sexithiophenes (8) were all sufficiently soluble to fabricate transistors from solution, giving decent mobilities ranging from
0.01 to 0.05 cm?/V's However, these materials were only sparingly soluble and required the use of either hot chlorinated deposition ‘solvents, or pre-heated substrates which limits the feasibility of a large-scale manufacturing process Katz et al found that incorporating an oxygen into the alkyl chains (9) effectively doubled the solubility without decreasing the electrical properties, *" but the solubility was still low in these materials Increasing the conjugation length of these oligomers does not seem to affect the charge mobility significantly Rather, the morphology of the resulting films was found to play an important role in determining the efficiency of charge transport.
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To further increase the room temperature solubility, Afzali et al developed a method to incorporate o,a-dibutylphosphonate end groups into sexithiophene oligomers (10).°2 The incorporation of this polar moiety renders the material highly soluble and greatly simplifies the purification These oligomers can be spun-cast from a variety of solvents and exhibit charge mobilities up to 0.002 cm7/V's after annealing at 80 °C While the charge mobility is fairly low, the materials do exhibit high /on/lo¢ ratios of 10°, indicating that this synthetic method produces very pure materials.
Sandberg et al have synthesized sexithiophene oligomers end-functionalized with polyethyleneoxide (PEO) (11).°° The materials were found to have very interesting self-assembly behavior, and form alternating thiophene and PEO lamella in thin films However, PEO tends to sequester ions so it was found that washing was necessary to remove impurities and ions from the films The charge mobilities of washed films were on the order of 10 cm”/V-s with [on/log ratios up to 10°.
McCulloch and coworkers°Š followed by Huisman and coworkers” both published oligomers utilizing bisacrylate end groups on a oligothiophene core (12) to allow crosslinking of the films after processing Introduction of these end groups gave the oligomers enhanced solubility and film forming properties McCulloch et al also demonstrated that the liquid crystalline nature of these materials could be exploited to enhance order and packing." In both cases, however, the charge mobilities decreased by an order of magnitude after polymerization of the end groups, suggesting that polymerization induces a morphology change that decreases the degree of m- orbital overlap between the aromatic cores While there is much room for improvement in the charge transport of crosslinking oligomers, these approaches demonstrate an interesting way to align and pattern organic semiconductors.
A quarterthiophene oligomer with cyclohexyl end groups (13) was reported by Locklin et al. Cyclohexyl substitution gave the material high solubility in warm chlorinated solvents, and devices fabricated by drop-casting a solution of the oligomer in a closed, static atmosphere of solvent vapor gave mobilities as high as 0.06 cm2/V-s with Jon//o# ratios 10°.°”
As discussed in detail throughout this thesis, we have developed a general synthetic method for incorporating thermally removable solubilizing groups into a series of oligothiophenes containing four to seven thiophene rings.***° Branched secondary esters were introduced at the a- and œ-positions of the oligomers (14), which renders up to seven linear thiophene rings soluble in common organic solvents at room temperature.” After spin-casting films from chloroform onto
SiO, the solubilizing groups can be removed via an ester thermolysis reaction by heating to 200 °C It was found that the quarterthiophene derivative performed poorly due to incomplete surface coverage after thermolysis However, very reproducible, highly ordered films of the penta-, sexi- and heptathiophene derivatives were formed using this method No odd/even effects were observed, and charge mobility was found to slightly increase with conjugation length Measured mobilities ranged from 0.02 to 0.06 cm7/V's for the series, with /on/lo# ratios greater than 10°.°°*" It was found that surface treatments that rendered the substrate hydrophobic resulted in poor quality films and low charge mobility Further increases in mobility and on/off ratio, as well as a decrease in hysteresis in devices can be obtained by depositing ultrathin films from an inkjet printer."? Notably, high mobilities were measured even in single monolayer films, indicating that efficient charge transport can be attained in monolayer films, contrary to previous thought."*"*
Synthesis of B-Functionalized Oligothiophenes and the Thin Film Morphology, Mechanical, and Electrical Propertfi@s cuc nn nen nành he nhe 31 1 IntrOdUGẽOP cuc nh nH HH ng kế KT ĐC TK EERE EES 32 2 Results and DiscusSion cuc non HHằ by nhằ nh hen KH tà tàn 32
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2.1.1 Molecular Design The first consideration in the design of the target molecules was the necessity to have high charge mobility; therefore a thiophene backbone was chosen since oligothiophenes have been shown to exhibit hole mobilities up to 0.1 cm?/V:s.° Like most highly conjugated molecules, oligothiophenes have very poor solubility as a consequence of z-z stacking Therefore, the B-positions on the third and fifth thiophene rings were functionalized with solubilizing decyl chains In similar molecules the aliphatic chains have been shown to orient themselves vertically from the surface due to the polarity difference between the substrate and the alkyl chains, which may help direct the self-assembly of the molecules.° Most oligothiophenes are functionalized in the a- and w-position to induce perpendicular alignment of the molecule with the substrate For this system, it was decided to functionalize the B-positions in order to direct parallel alignment of the backbone with the substrate to compare the assembly with perpendicularly aligned oligomers.* Placement of alkyl chains on the third and fifth thiophene rings was chosen since crystal structures of oligothiophenes have revealed that these moiecules orient themselves in a cis-trans fashion, suggesting that B-substituents on consecutive rings lie
180° apart.° Therefore, placement of the alkyl chains on the rings adjacent to the central ring should ensure that they will be directed away from the substrate and not interfere with the self- assembly on the substrate Placement only on the third and fifth ring and not the first and seventh ring was also chosen so that the alkyl chains would not interfere in side-to-side packing of the molecules.
Since devices involving these oligomers are to be initially constructed on an metal oxide surfaces, a central functionality such as a carboxylic acid or a hydroxyl group should facilitate the horizontal alignment and self-assembly since the molecule should prefer to orient itself with this polar functionality close to the highly polar surface This functional handle also provides flexibility in the use of various techniques used to form films since both carboxylic acid and hydroxyl groups should be amenable to both dip casting as well as Langmuir-Blodgett (LB) techniques.° Finally, the central functional handle also allows for further functionalization of the molecule to fine-tune the assembly.
2.1.2 Synthesis As shown in Scheme 2.1, the synthesis of the central core of the molecules began by esterifying either 3-thiophene acetic acid or 3-thiophene methanol, using benzyl bromide or benzoyl chloride to give 1 and 2, respectively These protected cores were subsequently di-brominated using two equivalents of N-bromosuccinimide (NBS) to give 3a-b Formation of the stannate 4 was accomplished by reacting 3-decylthiophene with n-butyl lithium followed by quenching with tributyltin chloride This product contained about 10-15% of the regioisomer 3-decyl-2-tributylstannylthiophene, but was unable to be purified at this stage due to the instability of the carbon-tin bond A Stille coupling between 3a-b and the impure stannate 4 generated the alkylated terthiophenes 5a-b.
The terthiophenes 5a-b contained 10% of an inseparable regioisomer due to the impurity of the stannate 4 Therefore, reactivity differences in these isomers were used in the following bromination step to resolve them 3-alkylthiophenes selectively brominate at the 2-position using NBS, thus a sub-stoichiometric amount of NBS was added The desired regioisomers of 5a-b (pictured in Scheme 1) were the first to react, leaving the wrong regioisomers un-halogenated After column chromatography of the reaction mixture, isomerically pure terthiophenes of 6a-b were obtained.
To complete the oligomer synthesis, a final Stille coupling was performed between 6a-b and 5-tributylstannyl-2,2’-bithiophene to give 7a-b in 29% yield The low yield can be attributed to the decreasing reactivity of Stille coupling as the number of thiophene rings increases in the oligomer This reactivity decline also leads to an increase in the formation of homo-coupling products.° Also, these oligomers are acid, oxygen and light sensitive, leading to a loss of product by decomposition during purification of the reaction mixture However, the desired targets were obtained by saponification of the esters with 5% aqueous potassium hydroxide to afford the targets T7-COOH and T7-OH in quantitative yields.
2.1.3 UV-Vis The UV-vis spectrum of each target in THF was recorded The Amax for T7-COOH was 422 nm, while the Ama for T7-OH was slightly higher at 431 nm Unsubstituted heptathiophenes typically absorb around 440 nm,’ so both oligomers are blue-shifted by 10-20 nm from this value, which suggests a disruption in planarity commonly seen in B-functionalized oligothiophenes.°
Scheme 2.1 Synthesis of B-functionalized T7 molecules. benzoyl chloride
“Benzyl bromide “pyridine al ves ©ioHzi 1) n-BuLi, 78°C CagHzy
Cian @ s ` 2 OS DMF gen “D Bussncl — n-Bu BuaSnCi
1 3a-b TM 4 a: Rị = -CH,COOH ao b: Ry = -CHạOH DMF, 80 °C
2.1.4 Molecular Modeling A cartoon of T7-COOH is shown in Scheme 2.2, giving the estimated dimensions of the molecule calculated using the molecular modeling program Spartan The T7-
OH has roughly the same dimensions, so only the T7-COOH oligomer is shown In the ideal gas phase structure at 0 K, the alkyl chains should be straight and slightly tilted away from the line perpendicular to the thiophene segment However, significant conformational mobility is expected at finite temperature and as a result of interaction forces between molecules and with the substrate Assuming the thiophene rings are coplanar, with alternating orientation of the sulfur moiety the backbone forms a chain 2.7 nm long (including the van der Waals radius of the endH) The 10-carbon alkyl chains have a length of 1.2 nm and the acetic acid group is 0.35 nm long.These dimensions, together with the Van der Waals radii of the methyl end group (~0.2 nm) and the OH group of the acid (~0.1 nm), gives a total height of 2.1 nm in the fully extended conformation These molecules can bind to each other through a variety of interactions, including n-n interaction between thiophene units that favors parallel stacking, van der Waals interactions between alkyl chains and H-bonding and polar interactions from the carboxylic acid moieties.
Scheme 2.2 Molecular Modeling of T7-COOH.
Evaporated and Solution Processed Film Morphology
2.2.1 Evaporated Films Our first attempt at making films of these oligomers was done using thermal evaporation The majority of semiconducting oligomers in the literature are processed into thin films using evaporation, so this method provides a useful comparison of the morphology and properties of literature compounds with our oligomers.
An AFM image of a thermally evaporated film of T7-COOH is shown in Figure 2.1 This material forms polycrystalline films consisting of fairly small needle-like grains that do not exhibit any preferential orientation This type of morphology is similar to that seen in other evaporated films of B-substituted oligothiophenes,” and typically leads to low charge mobility due to the high barrier to charge hopping between grain boundaries.
Figure 2.1 AFM image (10 x 10 um) of a thermally evaporated film of T7-COOH.
Interestingly, the AFM images in Figure 2.2 of T7-OH reveal a strikingly different morphology from the T7-COOH These molecules tend to form long continuous ribbons Slow deposition rates were found to lead to thicker ribbons with low surface coverage, while fast deposition gave smaller ribbons but good coverage It was also discovered that the hydrophobicity of the surface dramatically changes the growth As shown in Figure 2.2c, hydrophobic surfaces such as gold promote the growth of long, thick ribbons, while small grains tend to grow on hydrophilic substrates such as SiO¿.
Figure 2.2 AFM images of thermally evaporated films of T7-OH a) 50 x 50 um image on SiO, showing the formation of small ribbons b) Higher resolution image (10 x 10 um) of the area highlighted in a) c) 50 x 50 um image demonstrating the difference in morphology depending on surface hydrophobicity Hydrophobic surfaces such as gold promote growth of thick ribbons, while smaller ribbons are formed on hydrophilic SiOz.
This morphology is quite unusual and could be either beneficial or detrimental to charge mobility Typically, when oligomers or polymers segment into individual rods the OFET mobility decreases due to the large grain boundaries created by segmentation."° However, here the ribbons span several microns and could easily span most device channels If we were able to orient the ribbons within the device channel, we might be able to get very high charge mobilities.
2.2.2 Drop Cast Films of T7-COOH To compare film morphology with the thermally evaporated films described above, drop cast films of T7-COOH were made using a 0.1% solution of the oligomer in chloroform or THF These solutions were drop cast or spin-coated on both glass and silicon at a variety of substrate temperatures As shown in Figure 2.3, fairly homogeneous, highly crystalline films could be produced by drop casting on a substrate heated at 60 °C from THF. Films produced by spin coating were of very poor quality, and were not investigated further. AFM was used to examine the drop cast films of T7-COOH as also shown in Figure 2.3 The crystallites are actually very large, in the 50 to 200 micron range and more than 500 nm thick on average Therefore, while the films are polycrystalline, the very large grain size might allow for decent charge mobility since the size of a single grain is larger than the typical channel dimensions. nm Section Analysis
Figure 2.3 Optical microscope image (50x) of a crystalline film of T7-COOH drop-cast from THF onto a glass slide heated to 60 °C On the right, an AFM image and height profile of a 10 x 10 um section of the film is shown.
Langmuir-Blodgett Monolayers of 'T7-COOH
2.3.1 LB monolayer film preparation A chloroform solution of T7-COOH was spread onto the water surface of the Langmuir trough apparatus The surface was slowly compressed by moving the barriers at a constant rate of 10 mm/min to obtain pressure-area (P-A) isotherms, as shown in Figure 2.4.
Area per molecule (A?) Figure 2.4 Compression isotherms of the amphiphilic oligothiophene T7-COOH in the Langmuir- Blodgett water trough The insets show images of films formed on mica drawn through the surface at the points marked by the arrows The insets (2.0 um x 2.5 um) show: a) disconnected small islands; b) connected monolayer films of poor quality; c) homogeneous films; d) multilayer aggregates formed at the collapse point The best quality films (c) are drawn at a surface pressure of 10 mM/mm, with a mean area/molecule of 110 A’.
Three distinct regions can be distinguished ion the isotherm curves Up to 140 A? /molecule,
P increases very slowly, indicative of a disjoined phase with molecules or rafts of molecules dispersed on the water sub-phase Below 140 A/mol, P increases noticeably, signaling the development at repulsive interaction between molecules An AFM image of a film obtained by pulling the mica through the surface at this point (P = 7.0 mN/m) is shown in the inset (a) The film consists of disconnected aggregates forming small islands Large and contiguous islands could not be obtained until the area was reduced to 120 A/molecule, where the P-A isotherm shows an inflection, as shown in inset (b) Above A = 110 A/molecule, pressure starts to increase rapidly Films obtained at this point have the highest homogeneity and quality (inset c) Higher compression gives rise to collapsed layers (inset d) We could not obtain 100% surface coverage, so all films exhibited islands and holes even under collapse conditions The films studied in the remainder of this paper were always drawn at P = 10 mN/m.
2.3.2 Mechanical properties of LB monolayers A topographic image of a freshly drawn film is shown in Figure 5a The lack of molecular scale resolution in these images makes it impossible to determine the existence of order and domain size X-ray diffraction studies of polythiophene molecules with similar alkyl side chains by Reitzel et aI."" indicate that ordered domains can form, driven by z-x interaction between thiophene units, with a size limited to a few nanometers If our molecules organize similarly, micrometer-sized islands, such as those in Figure 2.5, should be composed of multitude of domains but the resolution of the images is not sufficient to resolve the small domain structure Figure 2.5b shows the friction force image obtained simultaneously, with darker areas corresponding to low friction The lower friction on the islands relative to the surrounding mica indicates that the monolayer exposes the methyl-terminated alkyl segments, while the carboxylic group is oriented toward the interface with the mica substrate This orientation assignment is based on the well-known frictional behavior of films exposing chemically active groups such as COOH, CHạOH, NH;, etc These groups always exhibit higher friction and adhesion than the inert CH, termination.’ The carboxylic side group of T7-COOH provides bonding to the charged mica substrate due to its polar nature Bonding might also involve H- bonds to mica and to water molecules present at the interface The bonding of T7-COOH to mica is relatively strong, as manifested by its resistance to scratching by the ATM tip Only a very small amount of wear could be observed up to 40 nN load with a tip radius of about 30 nm Removal of a patch of molecules required forces of 200 nN and higher This is in contrast with films where contact with the mica is made through alkyl groups Ở In these films loads of 10 nN were sufficient to scratch the film away.
The height of the islands in the film was found to be sensitive to both applied load and humidity, as will be discussed in the following section In spite of the good resistance to scratch removal, the films were found to be unable to support loads of more than a few nanonewtons without loss of height The larger value of the monolayer thickness was found on films freshly pulled from the Langmuir trough under laboratory humidity of 40% and above The islands of these films were found to be 2.0 nm high, close to the value expected for a fully extended molecule (Scheme 2.2) when imaged under the smallest practical loads Small loads were obtained by pulling the tip away from the surface, close to the pull-off point instability, so that the negative external load compensates as much as possible the attractive adhesive force (about -60 nN in this experiment) As shown in Figure 2.6, the height decreased to a value around 1.2 nm when the applied load reached -10 nN This decrease is reversible and the film recovers its original height when re-imaged at low load The 1.2 nm height value corresponds to the acid group and the thiophene backbone oriented parallel to the surface with the alkyl chains bent-over through a gauche defect close to the thiophene unit, as shown in Scheme 2.2. showing low friction over the film relative to the surrounding mica.
This geometry does not favor strong alkyl-alkyl packing in the film, but the stronger n-m interaction between thiophene segments favors the formation of stacks of molecules, which we believe provides most of the mechanical stability This interpretation is supported by a previous study of the mechanical stability of a linear oligothiophene molecule containing 5-thiophene units, with carboxylic acid on one end and a decyl chain in the other.’ These molecules form films with the thiophene segments nearly perpendicular to the surface, which facilitate a better contact of the alkyl chains in adjacent molecules Deformation of the alkyl chain segments in these molecules occurred only under much higher applied loads.
= _- nN of oe oO Oo + ri
Figure 2.6 Height and friction versus load islands of T7-COOH The height is 2.0 + 0.1 nm at small loads and decreases to a value around 1.2 nm when the load is above —15 nN This height decrease suggests a large tilt of the alkyl chains due to gauche distortions generated near the thiophene units On the bottom graph the friction of the islands of T7-COOH increases fast initially and slower after the height decrease The friction curve of the surrounding mica is shown for comparison.
2.3.3 Phase separation induced by humidity After a 12 hour exposure of the freshly drawn films to room temperature under a relative humidity (RH) of ~23%, a morphological restructuring of the film is observed The original uniform monolayer separates into two phases, A and B that are distinguished by their topographic height, as shown in Figure 2.7 Phase A is 1.8 + 0.1 nm high and phase B is 1.2 + 0.1 nm high (imaging load is negative, to partially compensate the adhesive force) The regions covered by phase A appear as angular elongated domains 40-50 nm wide and 100 to 200 nm long surrounded by phase B Since the width is a convolution of the real width with the tip radius, typically only a few tens of nanometers, it is clear that the domains are actually quite narrow, of the order of a few nanometers The relative area of each phase depends on the humidity, with phase A dominating at high humidity and phase B dominating at low humidity In the image of Figure 2.7, acquired at 40% RH, the area occupied by phase B is about 58% of the total (Fig 2.7c), although tip size effects tend to underestimate the contributions of the low regions The lateral force image (Figure 2.7b) reveals that the friction is low and nearly uniform, indicating that in both phases the molecules expose similar chemically inert end groups. The small roughness of the friction image is related to geometric changes in the torsion of the lever as it crosses the boundaries of low and high regions Interestingly, the A domains tend to form in the interior of islands, surrounded by phase B, which also decorates the edge regions of the island This can be explained by assuming that water is involved in stabilizing the structure of phase A and that the drying and wetting processes are dominated by water transport over the mica surface In this manner the edge regions would be the first to lose or gain water when the humidity changes.
Height (A) Figure 2.7 a) Topographic and b) friction images of T7-COOH after a long exposure (12 hours) to a relative low humidity of 23% The brighter regions in (a) are 1.8 nm high while the surrounding grey regions are 1.2 nm high The uniform low friction (except at the edges of the regions, due to topographic changes) reveals that in both regions the molecules expose the same chemically inert terminal group c) Height histogram uncorrected for tip size effects.
As the humidity increases, the areas of the A-type domains grow and percolate (Figure 2.8).
At 60% humidity, phase B has disappeared completely and the areas previously occupied by it now expose bare mica, as shown in Figure 2.8b These changes are reversible and phase B reappears when the humidity is changed back to the lower values (Figure 2.8c) At high humidity the area occupied by the molecules also decreases, indicating a higher packing density of phase
Although the mechanism of the phase separation is not yet clear, the following is a plausible model We assume that phase A is stabilized by water interacting with the hydrophilic carboxylic acid group at the edges of the domains, where water could penetrate To minimize contact with water molecules, the alkyl chains could adopt an upright configuration that also maximizes their van der Waals interaction The absence of water, on the other hand, favors a tilted alkyl chain configuration, which would explain the lower height of phase B The tilt of the alkyl chains also requires space, which can be provided by expanding the separation between the ordered domains. regions are made of phase B with a height of 1.2 + 0.1 nm At 60% RH phase B has vanished and only phase A is observed.
2.3.4 Multilayers Multilayer films can be produced by multiple dipping cycles in the LB trough.Figure 2.9 shows images corresponding to a film formed by four dipping cycles Holes are often visible in these layers, with depths of about 2 nm The multilayer films are also affected by humidity At low humidity (RH = 20%), the originally smooth film breaks down into numerous small clusters protruding about 1.0 nm over the background With increasing humidity the clusters coalesce and form larger units, leaving behind holes that become larger with increasing humidity The hole near the center of the images in Figure 2.9 increases in diameter from 100 nm to nearly
4 um At 60% and 65% RH the surface becomes smoother and more homogeneous However, the holes in the film, with a depth of about 1.8 nm, are not removed or filled up during the changes of humidity. the clusters coalesce and form larger units, leaving behind holes that become larger and deeper with increasing humidity The height of holes at RH 60% is 1.8 nm.
These modifications could have a similar origin as those observed in the monolayer, where water attaching to polar carboxylic acid units could push the alkyl chains out into a stretched configuration as a result of hydrophobic interactions Another possibility for multilayer films is the intercalation of water into paired layers of molecules Pairing through R-COOH—HOOC-R interactions could drive the formation of double wall structures Such pairing has indeed been observed in molecules containing carboxylic acid terminations."
2.3.5 Heat effects Heating the sample to 50-60 °C for one hour caused the film to break down into small aggregates with a height of 1.2 nm (Figure 2.10a) After this transformation, the molecules still expose the alkyl chain ends as revealed by the same low friction force as in the original film If the temperature is increased further to 100 °C for 1 hour, the film separates into small aggregates with the same height of 1.2 nm but spread over the entire surface (Figure 2.10b) After this transformation the friction of the aggregates increases, indicating a reorganization of the molecules, which no longer exposes the alkyl chains Unlike the transformation induced by humidity, this temperature-induced change is irreversible, i.e., cooling down the sample did not restore the original film structure A similar phenomenon occurs for multilayer films, where heating to 100 °C for 1 hour completely modifies the film morphology to numerous small clusters with a height of 3.0~4.0 nm (Figure 2.10c).
Figure 2.10 Topographic maps for the morphological restructuring of monolayers and multilayers of T7-COOH after heating treatments at different temperatures: (a) monolayer after 1 h at 50 °C; (b) monolayer after 1 h at 100 °C; (c) multilayer after 1 hour at 100%.
OFET Device Construction and Performance
2.4.1 Methods and Materials OTFT devices were fabricated on low resistivity n-type silicon wafers, using thermally grown silicon dioxide as the dielectric, in bottom contact geometry Gold electrodes were patterned onto the wafers using photolithography Most devices contained a thin chrome adhesion layer under the gold, but for the LB monolayer devices the chrome layer was omitted to reduce contact resistance with the molecules In all cases the wafers were freshly oxidized with O2 plasma, then the active semiconducting layer was then applied by various deposition methods described above.
2.4.2 Thermally Evaporated Devices The thermally evaporated films were evaluated first, as films processed in this manner usually give higher semiconducting properties than their solution- processed counterparts T7-COOH films gave an average mobility of 0.001 cm?/Vs with on/off ratios of 10°, which is typical of B-functionalized oligothiophenes T7-OH films exhibited an order of magnitude decrease in mobilities than T7-COOH giving an average mobility of 0.0001 cm7/Vs with on/off ratios of 10° Current-voltage plots for both materials are given in Figure 2.11 T7-OH exhibits a much higher contact resistance (Fig 2.11b) than T7-COOH. a)-5£-08 + b) 2E%
Figure 2.11 Drain current Ip vs drain voltage Vp as a function of gate voltage for bottom-contact devices of thermally evaporated films of a) T7—COOH (300 x 5 um device) and b) T7-OH (300 x
This difference in mobility can be attributed to the film morphology discussed earlier While the grain sizes in films of the T7-COOH were small, the actual grain boundaries were much less distinct than the ribbons formed in films of T7-OH Large grain boundaries are known to inhibit charge transport and could contribute to the low mobilities in films of T7-OH.
2.4.3 Drop Cast Devices of T7-COOH Representative gate voltage-dependant IV curves are shown in Figure 2.12 The average mobility calculated through these films was three-orders of magnitude lower than through the evaporated films (~2x10* cm?/Vs) Devices turned on at positive voltages, saturated quickly, and had low on/off ratios This low performance could possibly be attributed to impurities in the material or from oxygen doping from processing the films in air The drastic decrease in mobility from the thermally evaporated films suggests that the molecules within the crystallites (shown above) are in fact disordered, and do not have good m- orbital overlap.
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Figure 2.12 A representative drain current Ip vs drain voltage Vp plot as a function of gate voltage for a bottom-contact device of a drop cast film of T7-COOH.
2.4.4 Langmuir-Blodgett Devices Since the drop-cast films above performed poorly, we wanted to investigate methods to induce order into the films As discussed above, LB is a method for ordering molecules on a water-air interface, which can then be transferred to a substrate.°
Therefore, we used this method to make an ordered monolayer T7-COOH molecules and then deposited the monolayer on silicon for device testing To prepare the silicon wafers for this technique, a 470 A thick oxide layer was thermally grown on the surface, then cleaned with UV- ozone prior to deposition in order to have a very uniform, polar surface on which to place the monolayer The wafers were dipped into the trough prior to monolayer formation then raised while the compression rod maintained a constant surface pressure This direction of deposition insures that the hydrophilic functionalities transfer from the water surface to the polar SiO; surface, thus giving a single monolayer of material on the substrate Deposition on SiO; should be similar to mica, so we can assume that the monolayers on SiO, also expose the methyl-terminated alkyl segments to the air interface, while the carboxylic group is at the interface with the silicon substrate.
The monolayers were monitored by isotherm curves and were deposited 10 mN m' below the breaking point to insure monolayers as opposed to multilayers are deposited The monolayers obtained had a contact angle of 6 = 84°, indicative of a hydrophobic surface which further verifies the orientation of the alkyl chains The films were optically clear under a microscope and did not contain any aggregates However, semiconducting behavior was seen in these devices but the total current was very low, and mobilities on the order of 10° cm”/Vs were obtained.
Langmuir-Blodgett films of T7-OH were also made by similar techniques Analysis of the films under the microscope showed pinholes and aggregation resulting in poor films As a consequence, the films had very low conductivity.
Taking all this data into consideration, it was concluded that these materials were not ideal candidates for solution-processed devices The B-functionalization of these oligomers is likely the cause of the poor organization of these molecules within a film, resulting in poor semiconducting properties.
Two target molecules consisting of a functionalized heptathiophene backbone have been synthesized and OFET devices have been constructed Using the Langmuir-Blodgett technique, monolayer and multilayer films of T7-COOH have been successfully transferred onto mica and silicon Friction imaging of the monolayers on mica with AFM revealed that the molecules in the monolayer expose the methyl-terminated alkyl segments to the air interface, while the carboxylic group is at the interface with the mica substrate The height of the islands is found to be sensitive to applied load and to humidity.
Although interesting self-assembly behaviors of these molecules from solution were observed, the desired electrical properties were not obtained when solution-processed methods of deposition were used to fabricate OTFT devices Further work is needed to redesign materials that can be both soluble and retain high semiconducting performance.
4.1 Materials All chemicals were purchased from Aldrich and used without further purification unless otherwise noted 2,2’-Bithiophene was purified by filtering through silica gel using hexane as the eluent N-bromosuccinimide was recrystallized from 1:1 acetic acid /water prior to use THF and toluene were distilled over Na/benzophenone, dichloromethane and pyridine from calcium hydride just prior to use N,N’-dimethylformamide used was anhydrous packed under No All reactions were performed under dry Nạ unless otherwise noted All extracts were dried over MgSO, and solvents were removed by rotary evaporation with aspirator pressure Flash chromatography was performed using Merck Kieselgel 60 (230-400 mesh) silica Monolayers and multilayers were drawn onto muscovite mica (KAl2(SisAlO19)-(OH)2 from Mica New York Corp 4.2 Characterization Infrared spectra were measured on neat samples unless otherwise indicated with a Mattson Genesis II FT-IR with a diffuse reflectance accessory (Pike) UV-Vis data were measured with a Varian Cary 50 spectrophotometer Emission spectra were measured with a ISA/SPEX Flourolog 3.22 equipped with a double excitation and double emission monochromators and a digital photon-counting photomultiplier 'H NMR and ‘°C NMR spectra were recorded with Bruker AMX-300, AM-400 or DRX-500 instruments using CDCl; as the solvent Matrix Assisted Laser Desorption lonization -Time Of Flight (MALDI-TOF) mass spectrometry was performed on a Perseptive Biosystems Voyager-DE instrument in positive ion mode using a-cyano-4-hydroxycinnamic acid as the matrix High Resolution Mass Spectometry(HRMS) using Fast Atom Bombardment (FAB) was done with a Micromass ZAB2-EZ double focusing mass spectrometer (BE geometry) Elemental analyses were performed at the UCBerkeley Microanalysis Laboratory.
Atomic Force Microscopy (AFM) was performed using a home-built instrument controlled with RHK Technology, Inc electronics and software The instrument was operated inside a chamber providing sound isolation and humidity control The humidity was changed by flow of dry or wet nitrogen gas Si3;N, cantilevers from NanoProbeTM, with a nominal spring constant of 0.12 N/m were used Forces were determined by multiplying the lever deflection by the nominal spring constant The X, Y and Z distance displacements were calibrated using standard samples All the reported distances deduced from measurements of AFM images have an associated absolute error of + 0.3 nm, and a relative error (within one experiment) of + 0.1 nm All of the results presented in this work were obtained in the contact mode.
4.3 Preparation of Langmuir-Blodgett films For films deposited on mica, a chloroform solution of T7-COOH (0.3% w/w) was spread onto the Milli-Q purified water subphase of a LB trough (KSV 5000) for subsequent film drawing The films were transferred onto cleaved mica by simply dipping the mica into the solution once Multilayers were formed by dipping the mica substrate several times through the trough surface.
For the films deposited on silicon, a 1.0 UM solution of T7-COOH in chloroform was prepared and filtered through a 0.20 um syringe filter 0.02 mL of the solution was added dropwise to the water trough, and the monolayer was formed at a compression rate of 30 mm min at room temperature The collapse pressure of the monolayer was found to be 35 mN m” Films were deposited on silicon with thermally grown oxide layer using the vertical dipping method at 5 mm min” while maintaining a constant surface pressure of 25 mN m'” Wafers were air dried and stored under vacuum.
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Two target molecules consisting of a functionalized heptathiophene backbone have been synthesized and OFET devices have been constructed Using the Langmuir-Blodgett technique, monolayer and multilayer films of T7-COOH have been successfully transferred onto mica and silicon Friction imaging of the monolayers on mica with AFM revealed that the molecules in the monolayer expose the methyl-terminated alkyl segments to the air interface, while the carboxylic group is at the interface with the mica substrate The height of the islands is found to be sensitive to applied load and to humidity.
Although interesting self-assembly behaviors of these molecules from solution were observed, the desired electrical properties were not obtained when solution-processed methods of deposition were used to fabricate OTFT devices Further work is needed to redesign materials that can be both soluble and retain high semiconducting performance.
4.1 Materials All chemicals were purchased from Aldrich and used without further purification unless otherwise noted 2,2’-Bithiophene was purified by filtering through silica gel using hexane as the eluent N-bromosuccinimide was recrystallized from 1:1 acetic acid /water prior to use THF and toluene were distilled over Na/benzophenone, dichloromethane and pyridine from calcium hydride just prior to use N,N’-dimethylformamide used was anhydrous packed under No All reactions were performed under dry Nạ unless otherwise noted All extracts were dried over MgSO, and solvents were removed by rotary evaporation with aspirator pressure Flash chromatography was performed using Merck Kieselgel 60 (230-400 mesh) silica Monolayers and multilayers were drawn onto muscovite mica (KAl2(SisAlO19)-(OH)2 from Mica New York Corp 4.2 Characterization Infrared spectra were measured on neat samples unless otherwise indicated with a Mattson Genesis II FT-IR with a diffuse reflectance accessory (Pike) UV-Vis data were measured with a Varian Cary 50 spectrophotometer Emission spectra were measured with a ISA/SPEX Flourolog 3.22 equipped with a double excitation and double emission monochromators and a digital photon-counting photomultiplier 'H NMR and ‘°C NMR spectra were recorded with Bruker AMX-300, AM-400 or DRX-500 instruments using CDCl; as the solvent Matrix Assisted Laser Desorption lonization -Time Of Flight (MALDI-TOF) mass spectrometry was performed on a Perseptive Biosystems Voyager-DE instrument in positive ion mode using a-cyano-4-hydroxycinnamic acid as the matrix High Resolution Mass Spectometry(HRMS) using Fast Atom Bombardment (FAB) was done with a Micromass ZAB2-EZ double focusing mass spectrometer (BE geometry) Elemental analyses were performed at the UCBerkeley Microanalysis Laboratory.
Atomic Force Microscopy (AFM) was performed using a home-built instrument controlled with RHK Technology, Inc electronics and software The instrument was operated inside a chamber providing sound isolation and humidity control The humidity was changed by flow of dry or wet nitrogen gas Si3;N, cantilevers from NanoProbeTM, with a nominal spring constant of 0.12 N/m were used Forces were determined by multiplying the lever deflection by the nominal spring constant The X, Y and Z distance displacements were calibrated using standard samples All the reported distances deduced from measurements of AFM images have an associated absolute error of + 0.3 nm, and a relative error (within one experiment) of + 0.1 nm All of the results presented in this work were obtained in the contact mode.
4.3 Preparation of Langmuir-Blodgett films For films deposited on mica, a chloroform solution of T7-COOH (0.3% w/w) was spread onto the Milli-Q purified water subphase of a LB trough (KSV 5000) for subsequent film drawing The films were transferred onto cleaved mica by simply dipping the mica into the solution once Multilayers were formed by dipping the mica substrate several times through the trough surface.
For the films deposited on silicon, a 1.0 UM solution of T7-COOH in chloroform was prepared and filtered through a 0.20 um syringe filter 0.02 mL of the solution was added dropwise to the water trough, and the monolayer was formed at a compression rate of 30 mm min at room temperature The collapse pressure of the monolayer was found to be 35 mN m” Films were deposited on silicon with thermally grown oxide layer using the vertical dipping method at 5 mm min” while maintaining a constant surface pressure of 25 mN m'” Wafers were air dried and stored under vacuum.
4.4 Solution-cast films Drop cast films were made using a 0.1% solution of the oligomers in chloroform and THF, and were filtered through a 0.45 micron syringe filter prior to use These solutions were then drop cast or spin-coated on both glass and silicon at a variety of substrate temperatures.
4.5.Thin Film Transistors Low resistivity n-type silicon wafer substrates were used with the substrate acting as a back-gate For devices with a silicon dioxide dielectric, a 1000 or 1250 A thermal oxide was grown at 900°C from steam In devices where electrodes were pre-patterned on the dielectric before deposition of the active layer, electrodes were patterned using a baseline photolithographic process Hexamethyldisilazane (HMDS) was used to coat these wafers to promote photoresist adhesion This coating was typically removed by washing and subjecting the wafer to UV-ozone cleaning prior to use.
The electrical measurements were performed in a nitrogen atmosphere using an Agilent 4156C Precision Semiconductor Parameter Analyzer As the entire backside of the substrate was used as the gate electrode, the entire thin film was accumulated during device testing To minimize gate leakage and improve isolation in this type of setup, the active layer was scratched via probe tips around groups of devices.
General Bromination Procedure The compound was dissolved in N,N’-dimethylformamide (DMF) at 0.5 M concentration The flask was protected from light and two equivalents of NBS were added The mixture was allowed to stir overnight at room temperature The mixture was diluted with ethyl acetate using four times the volume of DMF, and washed three times with equal volumes of brine followed by four water washings The organic layer was dried and the solvent evaporated.
General Stille Coupling Procedure The corresponding halide and 2.5 equivalents of the stannate were combined in anhydrous DMF at 0.5 M concentration The mixture was degassed with four freeze-pump-thaw cycles, backfilling each time with argon Four mole percent of Pd(PPhạ);Clạ was subsequently added, and the mixture was subjected to one more degassing cycle The flask was heated to 80 °C, and the mixture was stirred overnight under argon The reaction was diluted with ethyl acetate with four times the volume of DMF and washed twice with equal volumes of brine and four times with equal volumes of water The organic layer was dried and the solvent evaporated.
Benzyl thiophene-3-acetate (1) To 5.00 g (35.0 mmol) of 3-thiophene acetic acid, 9.71 g (70.0 mmol) of KạCO;, 6.00 g (35.0 mmol) of benzyl bromide and 0.56 g (2 mmol) of 18-crown-6, 100 mL of dry THF was added The heterogeneous mixture was heated at reflux for 4 hours, then cooled to room temperature The solvent was removed and the residual solid was taken up in 1:3 ethyl acetate/hexanes filtered through silica gel The solvent was removed and the white solid was dried in vacuo to give a quantitative yield of 8.12 g 'H NMR (300 MHz): 0 3.71 (s, 2H), 5.15
(s, 2H), 7.06 (dd, J = 1.2 and 5.1 Hz, 1H), 7:16 (m, 1H), 7.30 (dd, J = 2.7 and 4.8 Hz, 1H), 7.32- 7.40 (m, 5H).
Benzoic acid 3-thiophene-methyl ester (2) A mixture of 2.0 g (18 mmol) of 3-thiophene methanol, 2.1 mL (26 mmol) of pyridine and 0.64 g (5.2 mmol) of 4-dimethylaminopyridine was cooled to 0 °C in an ice bath 3.0 mL (26 mmol) of benzoyi chloride was added to this mixture slowly via syringe The mixture was allowed to stir overnight, during which a voluminous precipitate appeared The slurry was diluted with 20 mL ethyl acetate and the solid was removed by filtration The organic phase was washed with 20 mL 1 M HCl, 20 mL NaHCOs, and three times with 20 mL brine The organic phase was dried and concentrated The resulting oil was purified by flash chromatography using 1:10 ethyl acetate/hexanes as the eluent to afford 2.45 g (64%) of a clear oil IR: 3013, 3061, 3032, 2954, 2886, 1731, 1601, 1584, 1491, 1452, 1416,
1365, 1315, 1287, 637, 569 cm'" 'H NMR (300 MHz): ð 5.37 (s, 2H), 7.19 (dd, J = 1.2 and 5 Hz,
1H), 7.34 (dd, J = 2.1 and 5.1 Hz, 1H), 7.47 (dt, J = 1.2 and 6.6 Hz, 2H), 7.61 (tt, J = 1.2 and 7.5
Hz, 1H), 8.08 (dd, J = 1.2 and 6.3 Hz, 2H) ''C NMR (125 MHz): 6 166.34, 136.84, 132.99,
130.06, 129.64, 128.32, 127.56, 126.20, 124.23, 61.78 HRMS (FAB) m/z calc for (C12H1 9028S): 218.0401; found 218.0399 Anal Calcd for C;aH;oO¿S: C, 66.03; H, 4.62 Found C, 65.74; H, 4.66.
Benzyl 2,5-dibromo-thiophene-3-acetate (3a) This compound was prepared as described in the General Bromination Procedure No further purification was necessary to obtain 98% yield of an orange liquid 'H NMR (300 MHz): 6 3.61 (s, 2H), 5.16 (s, 2H), 6.93 (s,1H), 7.31-7.40 (m, 5H).
Benzoic acid 2,5-dibromothiophene-3-methyl ester (3b) This compound was prepared according to the General Bromination Procedure The product was dried in vacuo to give quantitative yields of a tan solid, mp 51 °C IR: 3089, 3067, 3034, 2953, 2924, 2852, 1724, 1601,
MHz): 6 5.24 (s, 2H), 7.06 (s, 1H), 7.47 (dt, J = 1.2 and 6.6 Hz, 2H), 7.61 (tt, J = 1.2 and 7.5 Hz,
1H), 8.08 (dd, J = 1.2 and 6.3 Hz, 2H) °C NMR (125 Mz): ð 166.14, 136.83, 133.23, 130.93,
429.72, 129.59, 128.41, 111.92, 111.50, 60.31 HRMS (FAB) m/z calc for (C12H1.O2SBrz): 373.8611;found 373.8617 Anal Calcd for C;;H;oOaSBr;: C, 38.33; H, 2.14; S, 8.53 Found C,
2-Tributylstannyl-4-decylthiophene (4) To 2.0 g (8.9 mmol) of 3-decylthiophene in 100 mL dry THF, 1.34 mL (8.9 mmol) of N,N,N,N-tetramethyl-1,2-ethylenediamine was added The flask was cooled to 0 °C and 3.56 mL (8.9 mmol) 2.5 M n-BuLi in hexanes was added dropwise via syringe The mixture was warmed to room temperature then heated at reflux for 30 minutes The flask was cooled to 0 °C, and 2.9 g (8.9 mmol) tributyltin chloride dissolved in 8 mL dry THF was added dropwise via syringe The flask was allowed to warm to room temperature overnight The solvent was evaporated and the resulting liquid was taken up in 100 mL ethyl acetate and washed twice with saturated NH,Cl and three times with water The organic layer was dried and evaporated The resulting brown liquid was distilled under reduced pressure to give 3.36 g (74%) of an orange liquid that contained a mixture of isomers that could not be separated The desired isomer was present in 67% and is notated as isomer A in the NMR data, while the regioisomer 3-decyl-2- tributylstannyl-thiophene is notated as isomer B 'H NMR (300 MHz): 60.85-0.97 (m, 18H, A and B), 1.04-1.13 (m, 9H, A and B), 1.19-1.38 (m, 32H, A and B), 1.52-1.67 (m, 14H, A and B), 2.62- 2.67 (m, 3H, A and B), 6.93 (s, 0.16H, B), 6.96 (d, J = 0.9 Hz, 1H, A), 7.00 (s, 0.11H, B), 7.09 (d,