Abstract:
The present invention provides flexible polymer diodes in the form of a printable polymer sandwich configuration similar to that found in electroactive polymer transducers. The inventive flexible polymer diodes comprise a dielectric layer sandwiched between a pair of electrodes. With appropriate optional additives introduced in the electrode formulation and the proper electrical properties in the electrode, a device may be constructed which allows current to pass through for only one polarity of applied voltage.

Description:
RELATED APPLICATIONS 
     This application is the U.S. National Stage application filed under 35 U.S.C. §371(c) of International Application No. PCT/US2013/066504, filed on Oct. 24, 2013, which claims the benefit, under 35 USC §119(e), of U.S. Provisional Application No.: 61/717,780 filed Oct. 24, 2012 entitled “POLYMER DIODE”, the entirety of both applications are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed in general to polymer-based electronic devices and in particular to polymer diodes for use in flexible electronic devices. 
     BACKGROUND OF THE INVENTION 
     There has been a desire in recent years for flexible electronic devices which in turn has driven a need for flexible electronic components that can be applied to flexible (polymer) substrates at low temperatures. Although polymer and organic light emitting diodes are generally available, reliable, easy to process polymer diodes are not readily available for flexible electronics applications. Some work has been done with wet electrolytic systems. Both solutions require good sealing for long lifetimes. There has been a movement to use high speed. printing and other deposition methods rather than subtractive lithographic methods. 
     Approaches have generally centered on solution processable semiconductors such as those used in polymer light emitting diodes (PLEDs) sandwiched between electrodes that can be deposited, and optionally sintered, at low temperatures. Many of the semiconducting materials are difficult to process and can have lifetime issues. Some may chemically de-dope and become inactive. In addition, such materials can be sensitive to atmospheric moisture and need to be sealed. Multilayer structures may be difficult to fabricate. Many of these electrode systems require sintering temperatures that can cause damage to the polymer substrate and need to be tailored to have the correct work function for diode operation. Some methods have introduced pressure-annealing or lamination steps to improve the performance of the devices. 
     For example, Yoshida et al., in  Jpn. Appl. Phys.  50 (2011) 04DK16 describe a pressure-annealing method for fabricating printed low-work-function metal patterns and printed metal alloy patterns. The pressure-annealed metal electrodes of Yoshida et al., are used as bottom electrodes of printed polymer diodes. 
     Reports on the development of solution-state polymer diodes with nanogap electrodes that support intra-chain-dominant conduction are provided at http://nanotechweb.org/cws/article/lab/50114. 
     In all these cases, the diode mechanism has moving charges—electrons and holes—which flow through an electrically (semi-)conductive layer with similar mobilities (relative to the mobilities of any of the molecular species in the layer between the electrodes). Ionic diodes are known in the art but typically these require fluidic electrolytes to enable ionic mobility. 
     Lee et al., in U.S. Published Patent Application Nos. 2007/0221926 and 20120025174 describe the production of solution-processed titanium oxide layer containing polymer diodes. 
     There continues to be a need in the art for polymer diodes that are suitable for use in flexible electronics applications. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides such flexible polymer diodes in the form of a printable polymer sandwich configuration similar to that found in electroactive polymer transducers. The inventive flexible polymer diodes comprise a dielectric layer sandwiched between a pair of electrodes. With appropriate optional additives introduced in the electrode formulation and the proper electrical properties in the electrode, a device may be constructed which allows current to pass through for only one polarity of applied voltage. 
     These and other advantages and benefits of the present invention will be apparent from the Detailed Description of the invention herein below. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The present invention will now be described for purposes of illustration and not limitation in conjunction with the figures, wherein: 
         FIG. 1  is a plot showing current vs. time vs. cycle with alternating polarity; 
         FIG. 2  is a plot showing displacement vs. time vs. cycle with alternating polarity; 
         FIGS. 3A, 3B and 3C  show a possible mechanism for the present invention; 
         FIG. 4  provides a plot of current vs. time vs. cycle for a standard electrode material with alternating polarity for 10 cycles; 
         FIG. 5  shows a plot of displacement vs, time vs. cycle for 10 cycles; 
         FIG. 6  is a plot of current vs. time at voltage; 
         FIG. 7  converts the data from  FIG. 6  to a plot of resistance vs. time; 
         FIG. 8  shows the response to positive polarity pulses with a plot of current vs. time vs. cycles; 
         FIG. 9  shows the response to positive polarity pulses with a plot of displacement vs. time vs. cycle; 
         FIG. 10  illustrates response to negative polarity pulses with a plot of current vs. time vs. cycles; 
         FIG. 11  illustrates response to negative polarity pulses with a plot of displacement vs. time vs. cycle; 
         FIG. 12  shows response to cyclic negative polarity with a plot of current vs. time vs. cycles; 
         FIG. 13  shows response to cyclic negative polarity with a plot of displacement vs. time vs. cycle; 
         FIG. 14  illustrates pulse response to cyclic negative polarity with a plot of current vs. time vs. cycles; 
         FIG. 15  illustrates pulse response to cyclic negative polarity with a plot of displacement vs. time vs. cycle; 
         FIG. 16  shows response to cyclic positive polarity with a plot of current vs. time vs. cycles; 
         FIG. 17  shows response to cyclic positive polarity with a plot of displacement vs. time vs. cycle; 
         FIG. 18  illustrates pulse response to cyclic positive polarity with a plot of current vs. time vs. cycles; and 
         FIG. 19  illustrates pulse response to cyclic positive polarity with a plot of displacement vs. time vs. cycle. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described for purposes of illustration and not limitation. 
     Examples of electroactive polymer devices and their applications are described, for example, in U.S. Pat. Nos. 6,343,129; 6,376,971; 6,543,110; 6,545,384; 6,583,533; 6,586,859; 6,628,040; 6,664,718; 6,707,236; 6,768,246; 6,781,284; 6,806,621; 6,809,462; 6,812,624; 6,876,135; 6,882,086; 6,891,317; 6,911,764; 6,940,221; 7,034,432; 7,049,732; 7,052,594; 7,062,055; 7,064,472; 7,166,953; 7,199,501; 7,199,501; 7,211,937; 7,224,106; 7,233,097; 7,259,503; 7,320,457; 7,362,032; 7,368,862; 7,378,783; 7,394,282; 7,436,099; 7,492,076; 7,521,840; 7,521,847; 7,567,681; 7,595,580; 7,608,989; 7,626,319; 7,750,532; 7,761,981; 7,911,761; 7,915,789; 7,952,261; 8,183,739; 8,222,799; 8,248,750; and in U.S. Patent Application Publication Nos.; 2007/0200457; 2007/0230222; 2011/0128239; and 2012/0126959, the entireties of which are incorporated herein by reference. 
     The present inventors have surprisingly discovered that a polymer diode may be constructed from a simple, printed electroactive polymer material stack as such stacks show consistent difference in measured current based on the polarity of the applied voltage. 
     This current difference may be enhanced by the inclusion of additives. Mobile, electrically active additives added to the electrode formulation can significantly improve the performance of electroactive polymer material stack. Such additives do not need to be ionic. Although not wishing to be bound to any particular theory, the present inventors speculate that a portion of these electrically active additives diffuse into the dielectric layer. These diffusants may chemically interact with the functional groups of the dielectric layer material, particularly after photo- or thermal exposure. 
     Chemical modifications of the dielectric film to increase interaction between the polymer matrix and the electrically active additives may enhance performance and long-term stability. The diffusivity of the electrically active additives and their fragments may relate to molecular size and also to their charge or induced charge. The electrically active additives and their fragments may have functional groups that can react or interact with the dielectric matrix to limit their diffusivity. This can enable the creation of permanent concentration gradients in the dielectric layer which may enhance performance. 
     Additives containing iodonium salts, sulfonium salts and phthalocyanines are preferred as electrically active additives in the present invention. As iodonium salts, the following may be mentioned, phenyl iodonium hexafiuorophosphate, diphenyl iodonium hexafluoroantimonate, diphenyl iodonium tetrafluoroborate, diphenyl iodonium tetrakis(pentafluorophenyl)borate, bis(dodecylphenyl)iodonium hexafluorophosphate, bis-(dodecylphenyl)iodonium hexafluoroantimonate, bis(dodecylphenyl)iodonium tetrafluoroborate, bis(dodecylphenyl)iodonium tetrakis(pentafluorophenyl)borate, 4-methylphenyl-4-(1-methyl-ethyl)phenyl iodonium hexafluorophosphate, 4-methylphenyl-4-(1-methylethyl)phenyl iodonium hexafluoroantimonate, 4-methylphenyl-4-(1-methylethyl)phenyl iodonium tetrafluoroborate, and 4-methylphenyl-4-(1-methylethyl)phenyl iodonium tetrakis(pentafluorophenyl)borate. 
     As a sulfonium salt, examples include, but are not limited to, bis[4-(diphenylsulfonio)phenyl]sulfide bishexafluorophosphate, bis[4-(diphenylsulfonio)phenyl]sulfide bishexafluoroantimonate, bis[4-(diphenylsulfonio)phenyl]sulfidebistetrafluoroborate, bis[4-(diphenylsulfonio)phenyl]sulfide tetrakis(pentafluorophenyl)borate, diphenyl-4-(phenylthio)phenylsulfonium hexafluorophosphate, diphenyl-4-(phenylthio)phenylsulfonium hexafluoroantimonate, diphenyl-4-(phenylthio)phenylsulfonium tetrafluoroborate, diphenyl-4-(phenylthio)phenylsulfonium tetrakis(pentafluorophenyl)borate, triphenylsulfonium hexafluorophosphate, triphenylsulfonium hexafluoroantimonate, triphenylsulfonium tetrafluoroborate, triphenylsulfonium tetrakis(pentafluorophenyl)borate, bis[4-(di-(4-(2-hydroxyethoxy))phenylsulfonio)phenyl]sulfide hishexafluorophosphate, bis[4-(di-(4-(2-hydroxyethoxy))phenylsulfonio)phenyl]sulfide bishexafluoroantimonate, bis[4-(di-(4-(2-hydroxyethoxy))phenylsulfonio)phenyl]sulfidebistetrafluoroborate, and bis[4-(di-(4-(2-hydroxyethoxy))phenylsulfonio)phenyl]sulfide tetrakis(pentafluoro-phenyl)horate, tris({4-[(4-acetylphenyl)sulfanyl]phenyl})sulfanium hexafluorophosphate (commercially available from BASF as IRGACURE PAG270), tris({4-[(4-acetylphenyl)sulfanyl]phenyl})sulfanium tetrakis(pentafluorophenyl)borate (commercially available from BASF as IRGACURE PAG290). 
     In some embodiments, mixtures of electrically active additives may be used to balance performance, time response, and long-term stability as needed for a particular application. Also, many of these compounds are photo- and thermally labile, and in some embodiments, the polymer film may be photo- or thermally treated to release fragments that are more effective as electrically active additives or that may react with functional groups in the dielectric matrix material. In some embodiments, the photo- or thermal treatment may be used to create permanent compositional gradients within the dielectric layer to reduce diffusional effects. 
     A particularly preferred additive in the present invention is sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (Formula I); 
     
       
                 
         
             
             
         
      
     
     As can be appreciated by reference to  FIG. 1 , a plot showing current vs. time vs. cycle with alternating polarity for 10 cycles illustrates the current measured when a positive voltage is applied across the diode (odd cycles) or when a negative voltage is applied across the diode (even cycles). During the odd cycles, it is evident that current changes with time while the voltage is applied indicating that electrical charges are being transferred during the cycle. During the even cycles with negative polarity, the current is constant and the diode behaves as a resistor. 
       FIG. 2 , a plot depicting displacement vs. time vs. cycle with alternating polarity for 10 cycles, shows that displacement is observed only during the odd (positive polarity) cycles. Little or no displacement is observed during the even (negative polarity) cycles. 
       FIGS. 3A, 3B and 3C  illustrate a possible mechanism of the present invention. There are orders of magnitude difference in the diffusivities of the anions and cations. The polarity effect is not seen with anion/cation pairs that are more comparable in size and diffusivities 
     As shown in  FIG. 3A , with no voltage flowing, anions  30  and cations  32 , are associated with each other. One electrode is grounded  36 ; the other is active  34  and has high resistance. Charging is current limited—it takes a finite amount of time to transfer charge (electrons) onto the high resistance electrodes. 
     As shown in  FIG. 3B , when imposing a negative voltage, electrons are transferred to the active electrode  34  (rather than to the grounded electrode  36 ). The cations  32  can diffuse almost instantaneously and are in sufficient quantity to balance the incoming electrons. The rest of the material in the capacitor stack does not experience an electric field. 
     When imposing a positive voltage as depicted in  FIG. 3C , electrons are removed from the active electrode  34  faster than the large anions  30  can diffuse, enabling the imposition of an electric field across the pair of electrodes ( 34 ,  36 ). 
     The opposite polarity effect should occur when the anions  30  diffuse more easily than the cations  32 . 
       FIG. 4  provides a plot of current vs. time vs. cycle for a standard electrode material with alternating polarity and  FIG. 5  shows a plot of displacement vs. time vs. cycle for 10 cycles. As can be appreciated by reference to  FIGS. 4 and 5 , the lines for the first through 10 th  cycles essentially overlay each other. The same response to either positive or negative polarity was observed. 
       FIGS. 6 and 7  show plots of current vs. time and resistance vs. time for samples conditioned at different voltages for 80 seconds. As can be appreciated by reference to  FIGS. 6 and 7 , there seems to be charge transfer. 
       FIGS. 8 and 9  show the response to positive polarity pulses with  FIG. 8  showing a plot of current vs. time vs. cycles with the first pulse  80  being uppermost, the second  82  below that, etc.  FIG. 9  provides a plot of displacement vs. time vs. cycle with the first pulse  90  being the lowermost line, the second pulse  92  being directly above that, etc. As can be appreciated by reference to  FIGS. 8 and 9 , the material converts from a resistor to a capacitor. 
       FIGS. 10 and 11  illustrate response to negative polarity pulses.  FIG. 10  is a plot of current vs. time vs. cycles and  FIG. 11  is a plot of displacement vs. time vs, cycle. As can be appreciated by reference to  FIGS. 10 and 11 , the material remains a resistor, there is no displacement. 
       FIGS. 12 and 13  show response to cyclic negative polarity.  FIG. 12  is a plot of current vs. time vs. cycles and  FIG. 13  is a plot of displacement vs. time vs. cycle. The conditions were 75 Hz for 15 sec; negative polarity; data taken at 1 sec., 7 sec. and 14 sec. As can be appreciated from  FIGS. 12 and 13  the material appears to condition quickly. 
       FIGS. 14 and 15  illustrate pulse response to cyclic negative polarity, 10 pulses with negative polarity. The material immediately reverts back to a resistor.  FIG. 14  is a plot of current vs. time vs. cycles and  FIG. 15  is a plot of displacement vs. time vs. cycle. 
       FIGS. 16 and 17  show response to cyclic positive polarity. Conditions were: 75 Hz for 15 sec; positive polarity; data taken at 1 sec., 7 sec. and 14 sec.  FIG. 16  is a plot of current vs. time vs. cycles and  FIG. 17  is a plot of displacement vs. time vs. cycle. As can be appreciated by reference to  FIGS. 16 and 17 , the material appeared to condition quickly. 
       FIGS. 18 and 19  illustrate pulse response to cyclic positive polarity. 10 pulses with positive polarity.  FIG. 18  is a plot of current vs. time vs. cycles and  FIG. 19  is a plot of displacement vs. time vs. cycle. As can be appreciated by reference to  FIGS. 16 and 17 , the material remained conditioned. 
     An exemplary application of the present invention is as a component in an electrical circuit. A positive voltage applied across the polymer diode results in a displacement of a portion of the diode which mechanically closes a switch or relay elsewhere in the circuit. If the voltage has a negative polarity, no displacement of the polymer diode occurs and there is no change in the state of the circuit. 
     Various aspects of the subject matter described herein are set o in the thllowing numbered clauses in any combination thereof: 
     1. A flexible polymer diode comprising: a dielectric elastomer material; a first electrode material on a first side of the dielectric elastomer material; and a second electrode material on a second side of the dielectric elastomer material. 
     2. The flexible polymer diode according to claim  1  further including an electrically active additive. 
     3. The flexible polymer diode according to claim  2 , wherein the electrically active additive comprises one or more compounds selected from the group consisting of ionic salts, iodonium salts and sulthnium salts. 
     4. The flexible polymer diode according to claim  2 , wherein the electrically active additive comprises one or more compounds selected from the group consisting of (4-tert-Butylphenyl) diphenyl sulfonium triflate, Tris(pentafluorophenyl)boron, 4-isopropyl-4′-methyldiphenyliodonium tetrakis(pentafluorophenyl)borate, Sodium tetraphenylborate, sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, bis(4-tert-butylphenyl)iodonium triflate, tris({4-[(4-acetylphenyl)sulfanyl]phenyl})sulfanium tetrakis(penta-fluorophenyl)borate, sodium chloride and a phthalocyanine. 
     The foregoing examples of the present invention are offered for the purpose of illustration and not limitation. It will be apparent to those skilled in the art that the embodiments described herein may be modified or revised in various ways without departing from the spirit and scope of the invention. The scope of the invention is to be measured by the appended claims.