Solution‐processable organic semiconductors for thin‐film transistors: Opportunities for chemical engineers

Imagine households with wallpapers whose patterns and colors we can change at the click of a switch. Imagine newspapers and books whose contents automatically update daily or on request. Or imagine a disposable strip that monitors a child's temperature and dispenses the minimum amount of medication for fever relief. Central to these applications will be electronic devices that are based on organic molecules and polymers. Compared to silicon or germanium, organic molecules and polymers — with a wide range of functionalities as building blocks — are chemically versatile, and can be synthesized in large quantities at moderate reaction conditions. They are mechanically flexible, and can be easily melt- or solution processed over large areas. The combination of these unique attributes has fueled research and development in organic electronics, with hopes that, one day, we can realize the low-cost, large-area, mechanically-flexible applications mentioned earlier. This review focuses on the materials development for the organic thin-film transistor — a small, yet critical, component that makes the realization of these applications possible. The organic thin-film transistor has been proposed as the fundamental unit for backplane driving circuits in flexible displays and low-cost memory applications. In an active matrix-type display, for example, arrays of organic thin-film transistors — each individually controllable — would turn on and off the corresponding pixels. An organic thin-film transistor is shown in Figure 1. This device consists of sequentially-deposited layers of conductors (gate, source and drain electrodes), an insulator (gate dielectric), and an organic semiconductor. This device architecture — known as the bottom-contact geometry — resembles that of a silicon transistor, with the exception that the semiconductor is exposed and deposited last. Since the organic semiconductor is frequently chemically- and mechanically fragile, depositing it last eliminates its exposure to the various solvents and chemical environments that are needed to define the other device components. A bottom-contact thin-film transistor. The thin-film transistor effectively functions as a switch; biasing the gate electrodes switches the transistor between its “on” and “off” states. Specifically, when a bias is applied on the gate electrode, charges accumulate at the organic semiconductor-dielectric interface capacitively, as illustrated in Figure 1. It is this charge accumulation layer that allows charge carriers to flow from the source to the drain when a bias is applied between the two electrodes. The organic semiconductor layer therefore plays the role of an insulator in the absence of any gate bias, while allowing current to flow between the source and drain electrodes only in the presence of an electric field induced by an applied gate bias. Analogous to a valve in fluid flow, the gate voltage thus controls the flow of current between the source and drain electrodes in these devices. Depending on the nature of the organic semiconductor — p-channel characteristics if it has a tendency to transport holes, and n-channel characteristics if it has a tendency to transport electrons — the gate electrode must be biased negatively or positively to induce hole- or electron transport, respectively. In order for thin-film transistors to be successfully incorporated as driving circuits in display applications, they need to exhibit high current output, good switching speeds, and high contrast between the “on” and “off” states. These requirements translate to several important parameters that are used to characterize organic thin-film transistors, including the mobility (also known as the charge-carrier mobility or the field-effect mobility; this quantity measures the average charge-carrier drift velocity per unit electric field, and has units of cm2/V-s); the threshold voltage (which measures the voltage required to turn “on” the transistor); and the on/off current ratio (a measure of contrast between the device “on” and “off” states). Because the threshold voltage is controlled by subtleties of the organic semiconductor-dielectric interface that are neither well understood nor under control, the community has — to a large extent — used mobility and on/off current ratio as the primary and secondary benchmarks for thin-film transistor performance. For display applications, organic thin-film transistors in driving circuits need to exhibit a minimum mobility of 0.1 cm2/V-s, and an on/off current ratio of at least 103. Given the promise of organic thin-film transistors, significant research effort has been dedicated to the design and synthesis of environmentally-stable organic semiconductors that exhibit high mobility. Figure 2 contains the chemical structures of representative organic semiconductors. For example, pentacene (Figure 2a), an organic semiconductor with p-channel characteristics, has been reported to exhibit mobilities of order 1 cm2/V-s.1 This value is comparable to the mobilities exhibited by amorphous silicon thin-film transistors and exceeds the requirements for driving circuits in display applications. Thin-film transistors made with numerous other organic semiconductors with p-characteristics, including anthracene, oligo-thiophenes, copper phthalocyanine (Figure 2b), etc., have been reported to exhibit mobilities of order 10−3 − 10−1 cm2/V-s. While organic semiconductors with p-channel characteristics have been explored extensively, organic semiconductors that exhibit n-channel characteristics are also needed to realize p-n junctions and complementary logic circuits. Organic semiconductors more typically exhibit p-, rather than n-channel characteristics, because of their low oxidizing and high-reducing potentials. As such, a common practice to realize organic semiconductors with n-characteristics is to modify existing materials with strong electron-withdrawing groups, like -CN, -F, and -Cl, to lower their lowest unoccupied molecular orbital (LUMO) energy levels for aiding electron injection and electron transport. For example, substituting fluorines for hydrogens along the periphery of copper phthalocyanine (Figure 2c), can alter the organic semiconductor from having p-channel to having n-channel characteristics; devices made with fluorinated copper phthalocyanine exhibit a typical electron mobility of 10−2 cm2/V-s.2 By attaching perfluoroalkyl chains to oligothiophenes, the electronic character can also be altered so they transport electrons instead of holes.3 More recently, families of fluorinated organic semiconductors based on naphthalene diimide (Figure 2d) and perylene diimide that are environmentally stable and exhibit electron mobilities of order 10−2 − 10−1 cm2/V-s4 have also been reported. Charge-carrier mobilities of thin-film transistors based on organic semiconductors. Organic semiconductors with p-characteristics are colored blue, while those that exhibit n-characteristics are colored red: (a) pentacene, (b) copper phthalocyanine, CuPC, (c) fluorinated copper phthalocyanine, FcuPC, (d) fluorinated naphthalene diimide, F-NDI; (e) regioregular poly(3-hexyl thiophene), P3HT; (f) poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene), PBTTT, and (g) triethylsilylethynyl anthradithiophene, TES ADT. There is thus a wide variety of organic semiconductors that satisfy the performance criteria of thin-film transistors. The majority of these organic semiconductors, however, need to be deposited on a device platform by thermal evaporation in a high-vacuum chamber. To truly realize the low-cost aspects of organic electronics, organic semiconductors must be deposited from solution with more economical and versatile techniques, such as spin coating, drop casting, inkjet printing and screen printing. This need has in turn driven the development of solution-processable organic semiconductors. To date, there are three classes of solution-processable organic semiconductors: polymer semiconductors, soluble organic semiconductor precursors, and organic semiconductors with solubilizing side chains. This review highlights recent advances and developments of soluble electrically-active polymers and organic molecules for thin-film transistor applications. Early solution-processable organic semiconductors were largely based on substituted oligomers or polymers. By incorporating electrically-active repeat units along a continuous backbone and introducing solubilizing alkyl side chains, polymer semiconductors are soluble in common organic solvents, and have excellent film formability when spun or cast from dilute solutions. Among the first polymers to exhibit field-effect characteristics was poly(3-alkyl thiophene), or P3HT (<10−5 − 10−4 cm2/V-s)5. This early P3HT was amorphous because its synthesis did not allow the controlled placement of the alkyl side chains. While the amorphous nature of P3HT films has been suggested to limit the mobility of these materials, experiments conducted on P3HT of varying alkyl side chain lengths suggested a strong correlation between side chain length and mobility.6 Specifically, thin-film transistors with spun films of poly(3-butylthiophene) exhibit a mobility of 1 × 10−4 cm2/V-s while analogously processed thin-film transistors with poly(3-decylthiophene) exhibit a mobility of 6 × 10−7 cm2/V-s. The decrease in mobility likely stems from an increase in the average spacing between the electrically-active moieties on neighboring polymer chains, which effectively increases the intermolecular charge transport spacing. This comparison provided initial evidence that the mobility is limited by intermolecular transport and implicates the importance of molecular organization within the organic semiconductor thin film on device characteristics. The mobility of P3HT thin-film transistors increased dramatically with the discovery of routes to control the placement of the alkyl side chains.7 For example, thin-film transistors fabricated with regioregular P3HT (e in Figure 2) with > 98.5% head-to-tail linkages routinely exhibit mobilities in the range of 0.05 – 0.1 cm2/V-s.8 When spun from chloroform solutions, these materials tend to adopt a microcrystalline morphology consisting of two-dimensional (2-D) conjugated layers with strong π-π interchain interactions separated by layers of insulating alkyl chains. On a silicon dioxide dielectric surface, these lamellae are oriented edge-on so the π-π stacking direction is in-plane. This orientation facilitates fast in-plane charge transport between the source and drain electrodes at the polymer semiconductor-dielectric interface. More recently, polythiophene derivatives, such as poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene), or PBTTT (Figure 2f), have been designed to contain chemical moieties that can pack with closer π-π intermolecular interactions, so they can assemble into large crystalline domains on crystallization from a liquid crystal phase. Thin-film transistors containing PBTTT routinely exhibit a hole mobility between 0.2 – 0.6 cm2/V-s when the devices are tested in nitrogen.9 As the organic electronics community is quickly discovering, the molecular characteristics of the electrically-active components alone do not govern the electrical performance of devices comprised of these materials. The way in which the organic semiconductor is deposited can dramatically alter the way in which polymer chains pack, which can in turn alter its electrical characteristics. For example, drop casting (slow solvent evaporation) instead of spin coating (almost instantaneous solvent evaporation) P3HT on the same thin-film transistor platforms can induce the molecules to adopt an edge-on rather than a face-on orientation, thereby increasing the mobility in these devices by more than an order of magnitude.10 This instance, along with many others reported in the literature, implies the ability to exert significant morphological control — which influences device characteristics — through proper channels of processing. The ease with which polythiophenes can be synthesized has prompted scientists to copolymerize other conjugated monomers (e.g., fluorene) with thiophene in the hope of accessing a wider range of structure and properties. Copolymers containing thiophene and fluorene units typically exhibit liquid-crystalline characteristics, and with proper substrate alignment, thin-film transistors comprising these copolymers can exhibit mobilities of order 10−4 − 10−3 cm2/V-s.11 In addition to providing main chain rigidity, the incorporation of fluorene units is reported to enhance the stability of the polymer against oxidative doping. Thiophene units have also been incorporated in donor-acceptor-type conjugated copolymers recently. The copolymerization of donor and acceptor-type moieties along a single backbone allows one to tune the electronic and optoelectronic properties of the resulting copolymer through manipulating the extent of intramolecular charge transfer. Indeed, high-electron mobility and ambipolar-type thin-film transistors, based on thiophene-containing conjugated copolymers have been reported.12 Blending two or more electrically-active materials to achieve the desired properties is a straightforward alternative to copolymerization. By incorporating an air-stable polymer semiconductor, poly(benzobisimidazobenzophenanthroline), BBL, that exhibits n-characteristics with copper phthalocyanine (Figure 2b; p-characteristics), Babel and coworkers13 were able to fabricate ambipolar thin-film transistors that transport both holes and electrons. The hole mobilities were reported to be 10−6 − 10−4 cm2/V-s and electron mobilities10−6 − 10−5 cm2/V-s for the range of compositions explored. As with P3HT homopolymers and copolymers, processing history was shown to dramatically influence the blend morphology and, accordingly, the blend electrical characteristics. In another example, Goffri and coworkers14 examined thin-film transistors consisting of polymer blends of regioregular P3HT and polyethylene. Polyethylene, a commodity plastic that is available at low-cost and has excellent film forming and mechanical properties, is added to control film morphology. Specifically, when the blend is cooled from the melt, polyethylene crystallizes first. This crystallization induces phase separation of the blend; P3HT is selectively expelled to the air-polymer and the polymer-dielectric interfaces. When cast and annealed on a device platform, only minute quantities of P3HT (as low as 3% of total polymer concentration) are needed. The bulk of the polymer film, consisting of polyethylene, effectively encapsulates the P3HT at the organic semiconductor-dielectric interface, resulting in mechanically- and environmentally-robust, high-performance thin-film transistors. While polymer semiconductors have excellent film formability, they are generally amorphous or semicrystalline and exhibit a significant amount of kinetically-trapped disorder in the solid state. Organic small molecules, on the other hand, tend to exhibit high-levels of crystallinity. Since molecular order in the organic semiconductor thin film is correlated with device mobility, there exists a strong drive to develop solution-processable, small-molecule organic semiconductors that adopt a highly ordered structure on solidification. One such common route entails the development of solution-processable organic semiconductor precursors. These precursors can be readily dissolved in common organic solvents and spun down on dielectric surfaces to form a uniform thin film before they are or to the conjugated that can For example, a pentacene was synthesized by as the This is highly soluble in organic solvents, as well as and On at – the to pentacene, thin-film transistors with mobilities in the range of 0.1 – of solution-processable organic semiconductor precursors that of which to the electrically-active form thin-film transistors with a mobility of Thin-film transistors containing substituted whose are by at – were reported to exhibit a mobility of cm2/V-s. The thermal of to its electrically-active form was by structure and a in the that is with the side the molecules to the substrate a morphology that is frequently correlated with in thin-film transistors. While soluble precursors of organic semiconductors are from a processing the of these soluble precursors is and or high are required to the to its electrically-active small-molecule organic semiconductors by of flexible side chains or that do not their electrical properties a alternative to achieve solution enhance film formability and chemical stability against and and for example, added alkyl side chains to to and thin-film transistors with exhibit mobilities of – cm2/V-s. More recently, substituted organic semiconductor two at the of the electrically-active only did the of molecules (e.g., anthracene, pentacene, dramatically increase the addition of on the the of the In the solid the molecules tend to pack in a (Figure to The addition of side to the π-π between molecules, so the substituted molecules typically exhibit or as shown in and respectively. In these the intermolecular spacing is and there exists between the of neighboring mobilities of cm2/V-s have been reported for thin-film transistors with pentacene for organic the the electrically-active of (a) (b) (c) that has a of is triethylsilylethynyl anthradithiophene, or TES in Figure pentacene, TES a in the solid state. Thin-film transistors with TES have been reported to exhibit charge-carrier mobilities of 1 Thin-film transistors fabricated with TES on the other hand, exhibit mobilities that are at least two to three of The of a TES film, and its corresponding electrical characteristics when incorporated into a thin-film transistor are shown in and respectively. other organic semiconductors, the electrical characteristics of TES are highly and on the of the processing a and straightforward to enhance the electrical characteristics of TES thin Specifically, thin films of TES that were amorphous to allows on the dielectric TES crystallizes 2 of exposure to Figure contains an that crystalline of TES exposure to the mobility and on/off current of TES thin-film transistors by more than two of mobilities > 0.1 cm2/V-s can be routinely from spin cast TES solvent with (Figure of TES and its thin-film transistor electrical characteristics when the same transistor is exposed to TES crystallizes and the electrical characteristics of the thin-film transistor dramatically the is against the with increasing gate voltage direction of The gate voltage was increased in of from to gate were used to turn on the transistor because TES has p-channel characteristics. the thin-film transistor exhibits and significant on The increase dramatically with this increase to a mobility and a on/off current also at the charge transport interface, as by the of in The addition of triethylsilylethynyl side to the of also has on its materials properties. While TES pack in a its intermolecular is limited to the of the of the of the triethylsilylethynyl The interactions that TES in its crystalline are thus and can be Indeed, experiments on TES a temperature and a small TES not easily on from the state. It is this unique of TES that has its in thin-film transistor More in the presence of can TES to from the substrate are therefore able to this to the active of TES thin-film transistors and the within the to devices with high mobility, and low off with for electrical with TES arrays of high-performance thin-film transistors can be routinely and The of solvent not only organic semiconductor it an added of with which the morphology and structure of the electrically-active can be and reported the addition of a not a solvent a that the crystallization of processing. crystallization of the organic semiconductor in a in thin-film transistors with mobilities as high as cm2/V-s, and an current ratio The field of organic electronics, in organic thin-film transistors, has in the last The of more has the design and synthesis of environmentally-stable and solution-processable organic semiconductors that exhibit excellent electrical characteristics. numerous — in of fundamental and — to be before applications can be truly fundamental such as charge transport is by molecular structure and will have to be and stability against the are that this the of organic semiconductors available to the electrical characteristics of thin-film transistors made with the same organic semiconductor can range from on with those of amorphous silicon to on the of processing. the processing dramatically influence the of the organic semiconductor, which can in turn its electrical characteristics. recently, the morphological of organic semiconductor thin films have been are with the to processing influences structure and morphology and, accordingly, electrical properties of these materials. and including electron and structure and will in the of structure over several length For example, we that the electrical of can be by more than an order of when the molecular of the polymer is The increase in electrical is by including increases in the and length of at hand, chemical are to be the can interface between the organic and the device and electrical Specifically, we are to develop the design and which will high-performance electronic devices a While it is generally that the charge transport within the first several of the organic semiconductor layer at the organic semiconductor-dielectric interface, the majority of the has been in the for example, an crystal structure in thin In however, pentacene a crystal the majority of the morphological of pentacene is on the Because the bulk structure not be representative of the structure at the organic semiconductor-dielectric interface, it is that we the initial development at that interface to charge transport To the interface, and have a that allows one to the organic semiconductor from the dielectric interface. The of structure allows one to the molecular orientation of organic semiconductors on and on charge transport is by Given that charge transport is by the interface, there have been the dielectric is in to the electrical characteristics of thin-film transistors. the dielectric with for example, can dramatically thin-film transistor device characteristics, which have been to from interactions between the polymer semiconductor and the More recently, there have been of device characteristics through the of the dielectric with having chemical and the of a gate dielectric to achieve charge in organic thin-film transistors. the numerous the of the organic semiconductor-dielectric interface is not at understood and an active of The of chemical and with on and transport in a to these Organic semiconductors are mechanically- and chemically As such, that were for silicon transistors are not for organic thin-film transistors. the and that are used to define the source and drain electrodes with the organic semiconductor, resulting in devices with electrical characteristics. There therefore exists a strong drive to develop that are with organic semiconductors for organic thin-film transistor For example, is a for components that is with organic semiconductors. allows electrical to be between organic semiconductors and electrodes that are on a More recently, the development of and dielectric have also circuits. the of the materials and the organic to with the nature of chemical including exposure to and we are to an in this Given the of charge transport within a organic the field will from molecular and an chemical have made a significant over length will not only the fundamental charge transport which are in also chain and molecular orientation at to electrical properties of the for example, and the interchain — a quantity that measures the ease with which a charge is between two molecules and is therefore correlated with mobility — can be to molecular The ability to of electrically-active organic molecules and polymers from first will the design of these materials. 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