Abstract:
A thin-film transistor uses a semiconducting layer comprising a semiconducting material of (A): 
                                 
where X, Ar, Ar′, R 1 , R 2 , R 3 , R 4 , R 5 , a, b, m, and n are as defined herein. The transistor has improved performance.

Description:
BACKGROUND 
     The present disclosure relates, in various embodiments, to compositions and processes suitable for use in electronic devices, such as thin film transistors (“TFT”s). The present disclosure also relates to components or layers produced using such compositions and processes, as well as electronic devices containing such materials. 
     Thin film transistors (TFTs) are fundamental components in modern-age electronics, including, for example, sensors, image scanners, and electronic display devices. TFT circuits using current mainstream silicon technology may be too costly for some applications, particularly for large-area electronic devices such as backplane switching circuits for displays (e.g., active matrix liquid crystal monitors or televisions) where high switching speeds are not essential. The high costs of silicon-based TFT circuits are primarily due to the use of capital-intensive silicon manufacturing facilities as well as complex high-temperature, high-vacuum photolithographic fabrication processes under strictly controlled environments. It is generally desired to make TFTs which have not only much lower manufacturing costs, but also appealing mechanical properties such as being physically compact, lightweight, and flexible. Organic thin film transistors (OTFTs) may be suited for those applications not needing high switching speeds or high densities. 
     TFTs are generally composed of a supporting substrate, three electrically conductive electrodes (gate, source and drain electrodes), a channel semiconducting layer, and an electrically insulating gate dielectric layer separating the gate electrode from the semiconducting layer. 
     It is desirable to improve the performance of known TFTs. Performance can be measured by at least three properties: the mobility, current on/off ratio, and threshold voltage. The mobility is measured in units of cm 2 /V·sec; higher mobility is desired. A higher current on/off ratio is desired. Threshold voltage relates to the bias voltage needed to be applied to the gate electrode in order to allow current to flow. Generally, a threshold voltage as close to zero (0) as possible is desired. 
     BRIEF DESCRIPTION 
     The present disclosure is directed, in various embodiments, to a thin film transistor having a semiconducting layer comprising a specific genus of semiconducting material. The transistors have good mobility and the semiconducting layer is stable in air. 
     Disclosed are thin-film transistors comprising a semiconducting layer, wherein the semiconducting layer comprises a semiconducting material selected from Formula (A): 
     
       
                 
         
             
             
         
      
         
         wherein X is independently selected from S, Se, O, and NR 6 ; 
         R 1 , R 2 , R 3 , R 4 , R 5 , and R 6  are independently selected from hydrogen, alkyl, substituted alkyl, aryl, substituted aryl, heteroaryl, halogen, —CN, and —NO 2 ; 
         Ar and Ar′ are independently a conjugated divalent moiety; 
         a and b are integers of from 0 to about 10; 
         m is 0 or 1; and 
         n is an integer and is at least 2. 
       
    
     In more specific embodiments, the semiconducting material is of Formula (I), (II), (III), or (IV): 
     
       
                 
         
             
             
         
      
     
     In some embodiments, (a+b)&gt;0. In others, the semiconducting material has a weight average molecular weight of from about 2,000 to about 200,000. 
     Ar and Ar′ may independently comprise a moiety selected from 
                                
and combinations thereof, wherein R′ is independently selected from hydrogen, alkyl, substituted alkyl, aryl, substituted aryl, heteroaryl, halogen, —CN, and —NO 2 .
 
     Ar and Ar′ may both be 
                                
and a and b are each from 1 to about 5.
 
     Each X may be sulfur. At least one of R 1 , R 2 , R 3 , R 4 , and R 5  may be alkyl. In other embodiments, R 1 , R 2 , R 3 , R 4 , and R 5  are hydrogen. 
     In some embodiments of Formula (I) or (II), each X is sulfur and R 1 , R 2 , and R 3  are hydrogen. In others, each X is sulfur; one of R 1 , R 2 , and R 3 , is alkyl, and the other two of R 1 , R 2 , and R 3  are hydrogen. 
     In some embodiments of Formula (III) or (IV), each X is sulfur and R 1 , R 2 , R 3 , R 4 , and R 5  are hydrogen. In others, each X is sulfur; one of R 1 , R 2 , R 3 , R 4 , and R 5  is alkyl, and the other four of R 1 , R 2 , R 3 , R 4 , and R 5  are hydrogen. 
     Also included in further embodiments are semiconducting compositions comprising the semiconducting material of Formula (A); the semiconducting layers and/or thin film transistors produced by this process. 
     These and other non-limiting characteristics of the exemplary embodiments of the present disclosure are more particularly described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following is a brief description of the drawings, which are presented for the purpose of illustrating the exemplary embodiments disclosed herein and not for the purpose of limiting the same. 
         FIG. 1  is a first exemplary embodiment of a TFT of the present disclosure. 
         FIG. 2  is a second exemplary embodiment of a TFT of the present disclosure. 
         FIG. 3  is a third exemplary embodiment of a TFT of the present disclosure. 
         FIG. 4  is a fourth exemplary embodiment of a TFT of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     A more complete understanding of the components, processes, and apparatuses disclosed herein can be obtained by reference to the accompanying figures. These figures are merely schematic representations based on convenience and the ease of demonstrating the present development and are, therefore, not intended to indicate relative size and dimensions of the devices or components thereof and/or to define or limit the scope of the exemplary embodiments. 
     Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the embodiments selected for illustration in the drawings and are not intended to define or limit the scope of the disclosure. In the drawings and the following description below, it is to be understood that like numeric designations refer to components of like function. 
       FIG. 1  illustrates a first OTFT embodiment or configuration. The OTFT  10  comprises a substrate  20  in contact with the gate electrode  30  and a dielectric layer  40 . Although here the gate electrode  30  is depicted within the substrate  20 , this is not required. However, of some importance is that the dielectric layer  40  separates the gate electrode  30  from the source electrode  50 , drain electrode  60 , and the semiconducting layer  70 . The source electrode  50  contacts the semiconducting layer  70 . The drain electrode  60  also contacts the semiconducting layer  70 . The semiconducting layer  70  runs over and between the source and drain electrodes  50  and  60 . Optional interfacial layer  80  is located between dielectric layer  40  and semiconducting layer  70 . 
       FIG. 2  illustrates a second OTFT embodiment or configuration. The OTFT  10  comprises a substrate  20  in contact with the gate electrode  30  and a dielectric layer  40 . The semiconducting layer  70  is placed over or on top of the dielectric layer  40  and separates it from the source and drain electrodes  50  and  60 . Optional interfacial layer  80  is located between dielectric layer  40  and semiconducting layer  70 . 
       FIG. 3  illustrates a third OTFT embodiment or configuration. The OTFT  10  comprises a substrate  20  which also acts as the gate electrode and is in contact with a dielectric layer  40 . The semiconducting layer  70  is placed over or on top of the dielectric layer  40  and separates it from the source and drain electrodes  50  and  60 . Optional interfacial layer  80  is located between dielectric layer  40  and semiconducting layer  70 . 
       FIG. 4  illustrates a fourth OTFT embodiment or configuration. The OTFT  10  comprises a substrate  20  in contact with the source electrode  50 , drain electrode  60 , and the semiconducting layer  70 . The semiconducting layer  70  runs over and between the source and drain electrodes  50  and  60 . The dielectric layer  40  is on top of the semiconducting layer  70 . The gate electrode  30  is on top of the dielectric layer  40  and does not contact the semiconducting layer  70 . Optional interfacial layer  80  is located between dielectric layer  40  and semiconducting layer  70 . 
     The semiconducting layer comprises a semiconducting material of Formula (A): 
                                
wherein X is independently selected from S, Se, O, and NR 6 ; R 1 , R 2 , R 3 , R 4 , R 5 , and R 6  are independently selected from hydrogen, alkyl, substituted alkyl, aryl, substituted aryl, heteroaryl, halogen, —CN, and —NO 2 ; Ar and Ar′ are independently a conjugated divalent moiety; a and b are integers of from 0 to about 10; m is 0 or 1; and n is an integer and is at least 2. In additional embodiments, n is from 2 to about 5,000.
 
     Specific embodiments of Formula (A) include those of Formula (I), (II), (III), and (IV): 
                                
wherein X is independently selected from S, Se, O, and NR 6 ; R 1 , R 2 , R 3 , R 4 , R 5 , and R 6  are independently selected from hydrogen, alkyl, substituted alkyl, aryl, substituted aryl, heteroaryl, halogen, —CN, and —NO 2 ; Ar and Ar′ are independently a conjugated divalent moiety; a and b are integers of from 0 to about 10; and n is an integer of at least 2, including from 2 to about 5,000.
 
     Generally, the alkyl group contains 1 to about 20 carbon atoms and the aryl or heteroaryl group contains from about 5 to about 20 carbon atoms. Although a and b may each be zero, in embodiments, a and b are generally each at least one. In particular embodiments, (a+b)&gt;0. In some embodiments, R 1 , R 2 , R 3 , R 4 , R 5 , and R 6  are C 1 -C 20  alkyl. 
     Exemplary heteroaryl groups include thienyl, furanyl, pyridinyl, oxazoyl, pyrroyl, triazinyl, imidazoyl, pyrimidinyl, pyrazinyl, oxadiazoyl, pyrazoyl, triazoyl, thiazoyl, thiadiazoyl, quinolinyl, quinazolinyl, naphthyridinyl, and carbazoyl, and the heteroaryl group may be substituted with alkyl, aryl, a heteroatom-containing group with zero to about 36 carbon atoms, or halogen. 
     Ar and Ar′ may independently comprise a moiety selected from 
                                
and combinations thereof, wherein R′ is independently selected from hydrogen, alkyl, substituted alkyl, aryl, substituted aryl, heteroaryl, halogen, —CN, and —NO 2 . In specific embodiments, R′ is alkyl.
 
     In specific embodiments Ar and Ar′ are both 
                                
and a and b are each from 1 to about 5.
 
     Desirably, each X in the semiconducting formula is sulfur. At least one of R 1 , R 2 , R 3 , R 4 , and R 5  may be alkyl. In other embodiments, R 1 , R 2 , R 3 , R 4 , and R 5  are hydrogen. 
     The semiconducting material may have a weight average molecular weight of from about 2,000 to about 200,000, including from about 5,000 to about 100,000. 
     In some embodiments of Formula (I) or (II), each X is sulfur and R 1 , R 2 , and R 3  are hydrogen. In other embodiments, each X is sulfur; one of R 1 , R 2 , and R 3 , is alkyl, and the other two of R 1 , R 2 , and R 3  are hydrogen. 
     In some embodiments of Formula (III) or (IV), each X is sulfur and R 1 , R 2 , R 3 , R 4 , and R 5  are hydrogen. In others, each X is sulfur; one of R 1 , R 2 , R 3 , R 4 , and R 5  is alkyl, and the other four of R 1 , R 2 , R 3 , R 4 , and R 5  are hydrogen. 
     Polymers of Formula (A) can be prepared by any appropriate method. For example, they can be produced by Suzuki coupling polymerization of dihalogen compound (1) or (2) with diboronic acid compound (3), as shown in Scheme 1. 
     
       
                 
         
             
             
         
      
     
     If desired, the semiconducting layer may further comprise another organic semiconductor material. Examples of other organic semiconductor materials include but are not limited to acenes, such as anthracene, tetracene, pentacene, and their substituted derivatives, perylenes, fullerenes, oligothiophenes, other semiconducting polymers such as triarylamine polymers, polyindolocarbazole, polycarbazole, polyacenes, polyfluorene, polythiophenes and their substituted derivatives, phthalocyanines such as copper phthalocyanines or zinc phthalocyanines and their substituted derivatives. 
     The semiconducting layer is from about 5 nm to about 1000 nm thick, especially from about 10 nm to about 100 nm thick. The semiconducting layer can be formed by any suitable method. However, the semiconducting layer is generally formed from a liquid composition, such as a dispersion or solution, and then deposited onto the substrate of the transistor. Exemplary deposition methods include liquid deposition such as spin coating, dip coating, blade coating, rod coating, screen printing, stamping, ink jet printing, and the like, and other conventional processes known in the art. 
     The substrate may be composed of materials including but not limited to silicon, glass plate, plastic film or sheet. For structurally flexible devices, plastic substrate, such as for example polyester, polycarbonate, polyimide sheets and the like may be used. The thickness of the substrate may be from about 10 micrometers to over 10 millimeters with an exemplary thickness being from about 50 micrometers to about 5 millimeters, especially for a flexible plastic substrate and from about 0.5 to about 10 millimeters for a rigid substrate such as glass or silicon. 
     The gate electrode is composed of an electrically conductive material. It can be a thin metal film, a conducting polymer film, a conducting film made from conducting ink or paste or the substrate itself, for example heavily doped silicon. Examples of gate electrode materials include but are not restricted to aluminum, gold, silver, chromium, indium tin oxide, conductive polymers such as polystyrene sulfonate-doped poly(3,4-ethylenedioxythiophene) (PSS-PEDOT), and conducting ink/paste comprised of carbon black/graphite or silver colloids. The gate electrode can be prepared by vacuum evaporation, sputtering of metals or conductive metal oxides, conventional lithography and etching, chemical vapor deposition, spin coating, casting or printing, or other deposition processes. The thickness of the gate electrode ranges from about 10 to about 500 nanometers for metal films and from about 0.5 to about 10 micrometers for conductive polymers. 
     The dielectric layer generally can be an inorganic material film, an organic polymer film, or an organic-inorganic composite film. Examples of inorganic materials suitable as the dielectric layer include silicon oxide, silicon nitride, aluminum oxide, barium titanate, barium zirconium titanate and the like. Examples of suitable organic polymers include polyesters, polycarbonates, poly(vinyl phenol), polyimides, polystyrene, polymethacrylates, polyacrylates, epoxy resin and the like. The thickness of the dielectric layer depends on the dielectric constant of the material used and can be, for example, from about 10 nanometers to about 500 nanometers. The dielectric layer may have a conductivity that is, for example, less than about 10 −12  Siemens per centimeter (S/cm). The dielectric layer is formed using conventional processes known in the art, including those processes described in forming the gate electrode. 
     If desired, an interfacial layer may be placed between the dielectric layer and the semiconducting layer. As charge transport in an organic thin film transistor occurs at the interface of these two layers, the interfacial layer may influence the TFT&#39;s properties. Exemplary interfacial layers may be formed from silanes, such as those described in U.S. patent application Ser. No. 12/101,942, filed Apr. 11, 2008. 
     Typical materials suitable for use as source and drain electrodes include those of the gate electrode materials such as gold, silver, nickel, aluminum, platinum, conducting polymers, and conducting inks. In specific embodiments, the electrode materials provide low contact resistance to the semiconductor. Typical thicknesses are about, for example, from about 40 nanometers to about 1 micrometer with a more specific thickness being about 100 to about 400 nanometers. The OTFT devices of the present disclosure contain a semiconductor channel. The semiconductor channel width may be, for example, from about 5 micrometers to about 5 millimeters with a specific channel width being about 100 micrometers to about 1 millimeter. The semiconductor channel length may be, for example, from about 1 micrometer to about 1 millimeter with a more specific channel length being from about 5 micrometers to about 100 micrometers. 
     The source electrode is grounded and a bias voltage of, for example, about 0 volt to about 80 volts is applied to the drain electrode to collect the charge carriers transported across the semiconductor channel when a voltage of, for example, about +10 volts to about −80 volts is applied to the gate electrode. The electrodes may be formed or deposited using conventional processes known in the art. 
     If desired, a barrier layer may also be deposited on top of the TFT to protect it from environmental conditions, such as light, oxygen and moisture, etc. which can degrade its electrical properties. Such barrier layers are known in the art and may simply consist of polymers. 
     The various components of the OTFT may be deposited upon the substrate in any order, as is seen in the Figures. The term “upon the substrate” should not be construed as requiring that each component directly contact the substrate. The term should be construed as describing the location of a component relative to the substrate. Generally, however, the gate electrode and the semiconducting layer should both be in contact with the dielectric layer. In addition, the source and drain electrodes should both be in contact with the semiconducting layer. The semiconducting polymer formed by the methods of the present disclosure may be deposited onto any appropriate component of an organic thin-film transistor to form a semiconducting layer of that transistor. 
     The resulting transistor may have, in embodiments, a mobility of 0.01 cm 2 /V·sec or greater, including 0.1 cm 2 /V·sec or more. The current on/off ratio may be 10 4  or greater. 
     While particular embodiments have been described, alternatives, modifications, variations, improvements, and substantial equivalents that are or may be presently unforeseen may arise to applicants or others skilled in the art. Accordingly, the appended claims as filed and as they may be amended are intended to embrace all such alternatives, modifications variations, improvements, and substantial equivalents.