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Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. TETHERED ALKYL SUBSTITUTED POLYTHIOPHENES, COPOLYMERS AND COMPOSITES IN MEMORY APPLICATIONS
  5. POLY(3,4-ETHYLENEDIOXYTHIOPHENE):POLY(STYRENESULFONATE) (PEDOT:PSS) IN MEMORY APPLICATIONS
  6. MECHANISM IN POLYTHIOHENE-BASED MEMORY DEVICE
  7. SUMMARY AND OUTLOOK
  8. ACKNOWLEDGMENT
  9. REFERENCES

Polymeric materials used in memory devices have attracted significant scientific interest due to their several advantages, such as low cost, solution processability, and possible development of three-dimensional stacking devices. Polythiophenes, including tethered alkyl substituted polythiophenes and block copolymers, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and composites, are one of the most attractive polymeric systems for memory applications because of their commercial availability, high conductivity, and mechanical strength. In this article, recent studies of functional polythiophene for memory applications are reviewed, mostly focusing on the role of the materials in the memory functionality, optimizing the chemical structure of the polythiophene and the component of each layer in memory device. A critical summary of the proposed mechanisms, including filament formation, electric field-induced charge transfer and reduction-oxidation (redox) driven, is given to explain the resistive switching phenomena in the polythiophene system. In addition, the challenges facing the research and development in the field of polythiophene electronic memories are summarized. POLYM. ENG. SCI., 54:2470–2488, 2014. © 2013 Society of Plastics Engineers


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. TETHERED ALKYL SUBSTITUTED POLYTHIOPHENES, COPOLYMERS AND COMPOSITES IN MEMORY APPLICATIONS
  5. POLY(3,4-ETHYLENEDIOXYTHIOPHENE):POLY(STYRENESULFONATE) (PEDOT:PSS) IN MEMORY APPLICATIONS
  6. MECHANISM IN POLYTHIOHENE-BASED MEMORY DEVICE
  7. SUMMARY AND OUTLOOK
  8. ACKNOWLEDGMENT
  9. REFERENCES

Over the past few decades, thin films of polymer semiconductors are being intensively investigated for electronics applications because of their remarkable advantages including their adaptability to low-temperature processing on flexible substrates, low cost, amenability to high-speed fabrication, and tunable electronic properties that are not feasible to produce using standard inorganic electronics. Polymer semiconductor devices such as light-emitting diodes [1], photovoltaic cells [2, 3], thin-film transistors [4, 5], chemical and photo sensors [6-8], organic/polymer nonvolatile memory device [9] are potential candidates for future flexible electronic-device applications. Among these applications, polymer nonvolatile memory device appears highly attractive owing to its potential usage in data storage media. In particular, polymer memory devices have attracted a lot of attention due to their simple structure, three-dimensional stacking capability, good scalability, high mechanical flexibility [10, 11]. Unlike current memory devices storing data by means of encoding “0” and “1” as the amount of charge stored in electric circuits, polymer nonvolatile devices store information in an entirely different manner, utilizing the conductivity response of the active layer to the applied voltage, in which the low and high resistive states are assigned to “ON” (or “LRS”) and “OFF” (or “HRS”), respectively [12]. Among the nonvolatile memory types, the write-once read-many times (WORM) memory and the hybrid rewritable (write-read-erase-read) memory are usually observed in polymeric memory performance. Depending on the electrical polarity required for resistance switching, the switching behaviors can be classified into two types including polarity dependent “bipolar switching” and polarity independent “unipolar switching” [13, 14]. In the former bipolar switching, a resistance change from OFF to ON occurs at certain voltage polarity, and an inverse process from ON to OFF at reversed voltage polarity. In contrast, in the latter unipolar switching, the switching procedures do not depend on the polarity of the voltage and current signals. Moreover, it is worth notice that unipolar memory is more favorable than bipolar memory because it can simplify the circuit in integration of resistive random-access memory device [15].

As far as we know, a wide variety of polymeric materials have been extensively investigated for their promising memory potential, including poly(N-vinylcarbazole) and carbazole polymeric derivatives [10, 16], polyimides [17, 18], polythiophene and their derivatives [19, 20], nonconjugated and conjugated copolymer containing chelated europium complexes [10, 16], polyfluorene derivative [21], polyaniline composites [22], and conjugated-polymer-functionalized graphene oxide [23]. Considering the preparation and fabrication process requirements among the entire studied polymer systems, polythiophene is thought to be very promising materials for future information storage due to high conductivity, outstanding stability, chemical resistance, and mechanical strength in both the neutral and the doped states. In general, electrochemical synthesis and chemical synthesis were carried out to obtain the polymer from thiophene and its oligomers [24]. However, because of its rigid-rod backbone, the application of polythiophenes has been limited due to their intractability and insolubility, especially in the doped state. A widely accepted strategy to overcome such problems consists of the incorporation of substituents into the 3-position of the thiophene ring, which produced not only processable conducting polymers but also allowed the complete chemical and physical characterization of the prepared materials. Specifically, it was found that the introduction of long alkyl, alkoxy and carboxylic acid into π-conjugated backbones can greatly improve the solubility in common organic solvents and thus ensure processibility of conjugated polymers [25-30]. By this method, the characteristic energy levels (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) are tuned. Meanwhile, the intermolecular packing behavior highly depends on the film-forming process. Both phenomena might significantly change the electrical performance of the polymer. Therefore, the design of a soluble polythiophene, while maintaining a high thermal stability, mechanical strength and exhibiting good memory performance, is highly required.

Early in 2001, Taylor and Mills [31] reported diodes formed by electrodeposition of a carbon-bridged dithiophene polymer poly[4-dicyano methylene-4H-cyclopenta[2,1-b:3,4-b']dithiophene (PCDM) (Fig. 1a, inserted) onto indium tin oxide (ITO) glass slides and furnished with evaporated aluminum (Al) counter electrodes exhibited the displacement currents (curves do not pass through the origin in their current-voltage (I–V) sweeping in Fig. 1a), which was a reversible bistability characteristics. In 2003, Pal et al. [32] firstly reported soluble highly regioregular polythiophene (P6OMe) consisting of 6-(methoxy)hexyl as side group, showed hysteresis behavior in capacitance–voltage (C–V) characteristics, which allowed the polymer to be used as nonvolatile memory devices. Later, the memory property of P6OMe was evaluated by I–V characteristics of a typical structure of ITO/P6OMe/Al sandwiched devices [33] (Fig. 1b). Unoriented films of this polymer showed hysteresis behavior, while oriented versions exhibited switching characteristics depending on sweep direction of voltage scans. Photoluminescence measurements during conductance switching in oriented P6OMe film revealed that the bistability of the device was related to the metal/film interface. The space charge stored at the polymer layer near the metal/polymer interface controlled charge injection and resulted in hysteresis-type I–V characteristics [32-36]. However, the relaxation of the space charges to its initial value has been proved to be only 2 h [32], which cannot satisfy the requirements of practical application.

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Figure 1. (a) The low-bias section of the I–V characteristics of an ITO/PCDM/Al diode showing two conductance states. Reprinted from Ref. 31, with permission from American Institute of Physics. (b) I–V characteristics of devices based on oriented P6OMe film. Inset shows the ratio between the ON and OFF state current as a function of applied voltage for the same sweeps. Reprinted from Ref. 32, with permission from Elsevier.

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Also in 2003, Moller et al. [9, 37] firstly demonstrated WORM memory characteristics in polymeric devices based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). A thin PEDOT:PSS film was deposited on a p-i-n silicon structure by spin coating method, as shown in Fig. 2. The as-fabricated device showed a high conductivity property and can be programmed to low conductivity state after applying a bias. However, high-voltage pulses and large current densities were needed to program the memory arrays. Since then, polythiophene and its block copolymers and composites attracted considerable attention due to versatile electrical switching characteristics, such as WORM memory switching, bipolar resistive switching and unipolar resistive switching (write-read-erase-read memory) characteristics.

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Figure 2. (a) Generalized architecture of the WORM memory, chemical structural formula of the two-component PEDOT:PSS conductive polymer. (b) Data for an integrated 40-nm thick PEDOT fuse/thin-film Si p-i-n diode WORM switching characteristics. Reprinted from Ref. 9, with permission from Nature Publishing Group.

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Although large number of research have indicated that various kinds of nonvolatile memory properties (WORM and write-read-erase-read) and operation modes (unipolar and bipolar) have been observed in polythiohene-based memory devices, it still cannot provide a satisfactory explanation for the electrical conduction mechanism, because it is much more complex than in ordered inorganic materials, as most polymers are amorphous in nature [10]. Understanding the relationship between the chemical structure of polythiophenes and memory properties is a subject of utmost importance in the development of polymeric memory materials. In this article, recent progress in the development of a resistive type polythiophene memory is reviewed, mainly focusing on the effects of chemical structures and device architecture on the memory properties. The Tethered Alkyl Substituted Polythiophenes, Copolymers and Composites in Memory Applications section and Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) (PEDOT:PSS) in Memory Applications section will be classified depending on the chemical structures, which are tethered alkyl substituted polythiophenes, their block copolymers and composites, and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and its composites, respectively. In Mechanism in Polythiohene-Based Memory Device section, the mechanisms underlying the operation of the various devices and materials will be summarized, because it is impossible to assess their ultimate potential if without a fundamental understanding of the mechanisms involved in the switching, retention, endurance and sensing processes. In Summary and Outlook section, a summary and the current prospects for polythiophenes in resistive memory devices will be addressed.

TETHERED ALKYL SUBSTITUTED POLYTHIOPHENES, COPOLYMERS AND COMPOSITES IN MEMORY APPLICATIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. TETHERED ALKYL SUBSTITUTED POLYTHIOPHENES, COPOLYMERS AND COMPOSITES IN MEMORY APPLICATIONS
  5. POLY(3,4-ETHYLENEDIOXYTHIOPHENE):POLY(STYRENESULFONATE) (PEDOT:PSS) IN MEMORY APPLICATIONS
  6. MECHANISM IN POLYTHIOHENE-BASED MEMORY DEVICE
  7. SUMMARY AND OUTLOOK
  8. ACKNOWLEDGMENT
  9. REFERENCES

As one of the most widely used tethered alkyl substituted polythiophenes, poly(3-hexylthiophene) (P3HT), as well as its block copolymers and composites, were successfully synthesized and investigated their applications in memory device due to easy processibility and excellent memory performance. Regioregular and regiorandom P3HT imbedded in copper (Cu) and aluminum (Al) electrodes were thoroughly studied by Joo group [38, 39]. Typical switching behavior of the regiorandom P3HT device is represented in Fig. 3a. A sudden increase in the current was observed at −2 V to form ON state during the negative bias sweeping. The ON state could be retained for several months and a bias of 4 V could switch the device back to the initial low conductive state (OFF state). This indicated that the device exhibited a nonvolatile rewritable memory. The response time of OFF-to-ON switching is presented in Fig. 3b. When converted the current curve into the resistance one, the initial resistance was rapidly decreased to 350 ohm with response time of 2.5 × 10−8 s and then it slowly approached to 30 ohm with 3.0 × 10−7 s. These switching times could be fast enough to be applied for memory application but they suffered from a large variation in the switching delay time (∼100 ms). The write-read-erase-read cycles were performed with the writing pulse of −2.5 V, the reading pulse of 0.5 V, and the erasing pulse of 8 V. The excellent switching endurance was over 3.0 × 104 cycles without any switching failure. It was proposed that the electrically controllable bistability originated from the metal filament formation (ON state) and breakdown (OFF state) between two electrodes, which will be discussed in detail in Filament Formation in Polythiophene Memory Materials section. To solve the large variation in the switching delay time, an embossed structure into Cu/P3HT/Al device was proposed to add polystyrene (PS) beads into the device leading to rather larger structural nonuniformity [40]. This modification can significantly decrease in average delay time and narrow the distribution that occurred in OFF-to-ON switching. However, the embossed structure did not significantly decrease the average magnitude of the delay time. Therefore, more and more studies focused on the organic materials, the modification of interface between metal and organic layer, and other device structures.

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Figure 3. (a) I–V curve of Cu/P3HT/Al device. Reprinted from Ref. 39, with permission from American Chemical Society. (b) Switching speed comparison between regioregular and regiorandom P3HT devices. Reprinted from Ref. 38, with permission from American Chemical Society.

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Electrical bistability was also demonstrated in a polymer memory device with an active layer consisting of P3HT and gold nanoparticles (AuNPs) capped with 1-dodecanethiol sandwiched between aluminum bottom and top electrodes [41]. This Al/P3HT:AuNPs/Al device exhibited a remarkable write-reed-erase-read memory behavior. I–V curves showed that an increase in conductivity by more than three orders of magnitude occurred at a threshold voltage of around 3 to 4 V. The device can be returned to the low conductivity state by applying a reverse voltage of −10 V, which is relatively high when compared with the later research. Switching cycles between the two conductivity states can be up to 100 with the ON current higher by more than three orders of magnitude than the OFF current. However, fluctuation of the OFF current appeared after 1500 cycles. The electronic transition was attributed to an electric-field-induced charge transfer between the two components in the system, details of which will be discussed in Electric Field-Induced Charge Transfer Effects in Polythiophene Memory Materials section.

After the above report, polymer memory devices based on P3HT and nanoparticles, carbon nanotubes (CNTs) or nanorods were rapidly developed. Pradhan et al. [42] added butyl groups functionalized multi-walled carbon nanotubes (CNTs) into P3HT matrix to form homogeneous CNT:P3HT films. To study the role of CNTs in a polymer matrix on the electrical bistability of devices, the I–V characteristics of three devices: ITO/P3HT/Al, ITO/P3HT:CNT(3.3%)/Al and ITO/P3HT:CNT(33%)/Al, have been compared. It was found that the device with pristine P3HT did not show any step change in conductivity in both sweep directions. Both of the devices with CNTs exhibited reproducible bistability and transition from OFF state to ON state, thus they achieved random-access memory effects under the write-read-erase-read voltage sequence. The ratio between the high and low conductance states increased with an increase in CNT concentration in the polymer matrix. The transitions between the two states had been explained in terms of charge transfer from CNTs to conjugated polymer chains. However, when they studied the effect of conductance switching of TiO2 nanorods in a P3HT matrix on photovoltaic parameters, they found that the switching process of ITO/P3HT:TiO2/Al device was a redox-driven one and the open-circuit voltage acted as a probe parameter to evidence read-only and random-access memory applications [43].

The formation of donor-acceptor organic semiconductor heterostructure thin films as an active layer in organic devices in optoelectronic application such as organic light-emitting diodes and solar cells have been intensively investigated. The control of charge transport properties through incident light and gate bias can lead to the development of new concepts in photo-controlled memory devices. Chen et al. [44, 45] fabricated an organic thin-film transistor (OTFT) configuration memory device based on P3HT:CdSe [44] (Fig. 4a and b) and P3HT:CdSe@ZnSe (comprising CdSe cores and ZnSe shells) [45] quantum dots (QDs). Both devices can be optical programming and electrical erasing. Dynamic responses of the optical programming and electrical erasing of typical P3HT:CdSe device and P3HT:CdSe@ZnSe device are shown in Fig. 4c and d, respectively. For P3HT:CdSe device, upon illumination with white light (2.75 mW/cm2) for 10 s, the drain current of the P3HT/CdSe device increased from 1.5 to 415 nA. After turning off the white light, the drain current dropped slowly and eventually settled at a metastable state of 260 nA. This metastable state could be erased efficiently using a single pulse of a gate voltage for a short duration (−15 V, 100 ms). The ON/OFF ratio was only of ∼100 and the memory effect of this device was maintained for only 1 h without a gating voltage. When compared with P3HT:CdSe@ZnSe device, the latter exhibited a higher ON/OFF ratio of 2700 and maintaining this value for 8000 s without noticeable decay. It was proposed that both CdSe and CdSe@ZnSe QDs can be served as trap centers upon illumination so that the spatially separated holes and electrons can move differently—the holes drifting toward the channel and then reaching the drain electrode, the electrons mostly confined in the QDs and at the P3HT/SiO2 interface. After the light was turned off, the devices existed in a nonequilibrium state; some of the photogenerated holes presumably recombined with some residual electrons that were not confined in QDs, causing a reduction in the drain current, eventually reaching a metastable state. The reason for the longer retention time in P3HT:CdSe@ZnSe device was that the ZnSe shell layer between the CdSe core and the P3HT polymer could result in an additional tunneling barrier that prevented the electrons from tunneling back to P3HT, leading to a smaller decrease in the drain current and a larger retention time relative to those of the CdSe QDs device.

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Figure 4. (a) Schematic representation of the bottom-gate OTFT configuration with an active polymer layer and interdigitated source and drain (S for source, D for drain, and G for gate). (b) Schematic representations of the P3HT-only and P3HT:CdSe blend films. (c) Dynamic responses of the optical programming and electrical erasing of a typical P3HT:CdSe device. Reprinted from Ref. 44, with permission from American Institute of Physics. (d) Dynamic responses of the optical programming and electrical erasing of a typical P3HT:CdSe@ZnSe device. Reprinted from Ref. 45, with permission from Elsevier. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Embedding nanocrystals in P3HT matrix was also investigated in two-terminal memory devices. Lin et al. [46] demonstrated memory devices based on n-type FeS2 nanocrystals (NCs) embedded in a P3HT matrix with type II band alignment. ITO/P3HT:FeS2/Al device showed nonvolatile switching behavior at a writing voltage of 3.5 V, an erasing voltage at of −8 V, ON/OFF ratio of 103, and retention time up to 105. The operation mechanism was proposed that small amount of FeS2 NCs proceed as charge capture and storage centres inside the P3HT polymer and induced electrical bistable behaviour. Lee and Park [47] developed a cross-bar nonvolatile hybrid memory cell with P3HT embedded with Ni nanocrystals surrounded by NiO. This memory cell also demonstrated nonvolatile program-and-erase memory characteristics with a program of 3.2 V and an erase of 8.0 V, but ON/OFF ratio less than 100. More specifically, it showed a negative differential resistive (NDR) region after being programmed, where the current conduction mechanism was related to Fowler-Nordheim tunneling conduction since the current of the memory cell was proportional to V2exp(−b/V), where b is the constant corresponding to tunneling barrier height. This indicated that the tunneling barrier for the carrier charge on Ni nanocrystals was almost the same as that for carrier discharge on Ni nanocrystals.

In addition to embedding nanoparticles in P3HT film, a promising bulk-heterojunction (BHJ) composite system consisting of P3HT as electron donors and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as electron acceptors were investigated for electrical bistable memory cells [48, 49]. Huang et al. demonstrated excellent WORM effect in two-terminal devices with reduced graphene oxide (rGO) as bottom electrode and aluminum as top electrode, namely rGO/P3HT:PCBM/Al, as shown in Fig. 5a. The memory device demonstrated a high ON/OFF ratio up to 106 and low switching threshold voltage of 0.5 to 1.2 V (Fig. 5b), which were dependent on the sheet resistance of rGO electrode. The polarization of PCBM domains and the localized internal electrical field formed among the adjacent domains were suggested to explain the electrical transition of the memory device, which is shown in Fig. 5c. The PCBM domains in the P3HT matrix formed by thermal annealing can be polarized under an applied electrical field. The polarization of PCBM domains led to the increase of the localized internal electrical field among the adjacent domains and induced the conductive switching. After switching from OFF to ON, the polarized states and the localized internal electrical fields could be maintained, even when the power was turned off due to the nonvolatile properties of devices.

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Figure 5. (a) Schematic diagram of rGO/P3HT:PCBM/Al and the chemical structure of P3HT and PCBM. (b) I–V characteristics of the rGO/P3HT:PCBM/Al device. Inset: the ON/OFF ratio as a function of applied voltage in the negative sweep. (c) A schematic illustration of the polarization of PCBM domains in P3HT matrix under an applied electrical field. Reprinted from Ref. 48, with permission from American Chemical Society. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Mohamad et al. [50] investigated the optical response and memory effects in OTFT n+-Si/SiO2/P3HT:PCBM/Au device. The P3HT:PCBM film was deposited by spin coating from 1:1 weight ratio of P3HT and PCBM solution. The optical-electro memory effect based on the OTFTs can be programmed optically and erased electrically, which is similar to that of OTFT Au/P3HT:CdSe/Au device. The memory effects were attributed to the photo current induction and electron trapping-detrapping capability of PCBM nanoparticles. However, Gao et al. [51] reported dynamic formation/rupture of metallic filament processes in ITO/P3HT:PCBM/Cu structure, which exhibited a typical bipolar resistive switching effect with good retention property longer than 106 s and fast switching speed of 300 ns. Under illumination, an open circuit voltage of −0.15 V existed in high resistance state, yet it vanished in low resistance state owing to the emergence of Cu filament. By combining the symmetry of I–V curves with corresponding energy band diagrams in different resistance states, it was demonstrated that the Cu filament grew from Cu/organics interface, ended at organics/ITO interface, and ruptured near organics/ITO interface.

In the field of polythiophene memory, device based on organic field effect transistor (OFET) is especially attractive owing to its nondestructive read-out, complementary metal oxide semiconductor (CMOS) architectural compatibility, and single transistor application [52]. Several of polymer electrets (Fig. 6) are utilized in this type of memory devices because of their charge-storing ability. Early in 2004, Dutta and Narayan [53] fabricated an OFET memory device utilizing P3HT as the active layer and poly(vinyl alcohol) (PVA) as a charge-trapped gate dielectric. In this device, it was observed that information can be introduced optically and erased electrically. Under photoexcitation, the drain current in the device was significantly increased, which would be generated by the photoinduced-charge transport of more mobile positive charges and immobile negative charges localized in deep-trap sites. However, upon termination of the light, the drain current decayed and settled to a metastable state, which was the store/read state as a function of time. The decay of the drain current was owing to the recombination of the proximal charge carriers which characterized the early stage of the relaxation, and then limited by poor recombination cross-section of the trapped electrons within the depletion region in the presence of a field. When applied a negative gate bias at Vg = −60 V, the electrons were trapped by the electric field to move out of the trap states, leading to an increase in the rate of recombination between the detrapped electrons and holes, resulting in an erase of the store/read state. Therefore, the memory operation could be interpreted in terms of writing, storing, repeated reading and erasing. Kaneto et al. [54] demonstrated OFET devices based on P3HT, a metal floating gate of Al or Ca, and a polyimide (PI) insulator film, which memory effects were also induced by light illumination in the cell having the floating gate with a decay time in the range of 1500 to 2500 s. Mohamad et al. [55] researched OTFT memory devices based on poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)−2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} [P(NDI2OD-T2)]n with additional P3HT films on a poly(methyl methacrylate) (PMMA) organic dielectric layer. In this case, P3HT films played a role as hole acceptor-like storage layers resulted in reversible Vth shift upon the application of external gate bias (Vbias). Therefore, devices manifested a memory window of 10.7 V (ΔVth = 10.7 V) upon the application of an external gate bias without suffering any major degradation of charge transport properties. However, all of the above memory devices based OFET-structured with polymer electrets needed a relative high erasing voltage up to tens of volts.

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Figure 6. Chemical structure of some polymers in P3HT memory devices.

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Hwang et al. [56] reported a flexible ferroelectric-gate field-effect transistors (Fe-FET) consisting of P3HT active layer, polyvinylidene fluoride (PVDF) and poly[(vinylidenefluoride-co-trifluoroethylene] (PVDF-TrFE) electrolyte layer, an Au source-drain (S/D) electrode and Al gate (G) electrode (Au/P3HT/PVDF-TrFE/Al, Fig. 7a), which demonstrated nonvolatile memory with multilevel operation. Multilevel switching behavior of device was precisely controlling the degree of remnant polarization of the PVDF-TrFE gate insulator which was dependent on the applied gate voltage. The hysteresis curve was saturated when a VG above ± 80 V was applied. After the device was turned on, there was an increase of the IDS with a negative bias in VG due to accumulation of excess holes in the P3HT layer. When the VG returned to zero, the IDS still remained saturated due to the nonvolatile H-F dipoles, with the fluorine atoms pointing to the P3HT layer. The subsequent application of a positive VG on the device gradually switched the H-F dipoles, leading to a decrease in the IDS. The nonvolatility of the polarization caused the current to remain constant after the removal of the positive voltage, as shown in Fig. 7b. Four levels of discrete nonvolatile source-drain currents were readily achieved with the application of an appropriate programming/reading gate voltage, and the memory exhibited excellent data retention of more than 105 s (Fig. 7c) and a notable multiple write/erase endurance of 102 cycles. Since being fabricated on a polymer substrate, these Fe-FET devices achieved a dominant position in mechanical flexibility, which the characteristic 4-level reliable switching was realized with more than 1000 bending cycles at a bending radius of 5.8 mm.

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Figure 7. (a) Schematic of the Fe-FET memory device. SEM image (top) and photograph (bottom) showing cross-sectional and plan views of the Fe-FET, respectively. (b) The multilevel IDSVG transfer curves of an Fe-FET after various program VG values. The schematic in the inset shows nonvolatile IDS levels controlled by the ferroelectric polarization at different program gate voltages. (c) The time-dependent retention characteristics. Reprinted from Ref. 56, with permission from John Wiley and Sons. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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When investigated the memory effects of P3HT and PMMA hybrid layer, Park et al. found that memory performance was dependent on the device structure [57, 58]. Memory device fabricated using P3HT molecules embedded in a PMMA layer, namely ITO/PMMA:P3HT/Al [57], was observed a WORM memory characteristics from a low current to a high current at a threshold voltage of 5 V and an ON/OFF ratio of 104. Current–time (I–t) measurements of devices indicated a perfect retention performance under ambient conditions. The mechanism was proposed that when the applied voltage was above 5 V, many electrons injected from the Al electrode were captured in the P3HT molecules, resulting in an increase in the device conductivity. Even though a large positive voltage or a large negative voltage was applied to the device, the electrons captured in P3HT molecules cannot be emitted because of the deeper LUMO of P3HT than that of PMMA. Therefore, the P3HT molecules in the PMMA layer acted as a charge storage medium. However, due to the very different hydrophilicity characteristics of P3HT and PMMA, the P3HT/PMMA hybrid layer could be spontaneously phase separated during the baking process of the sample. Therefore, an ITO/P3HT/PMMA/Al device could be obtained [58]. It presented write-read-erase-read consecutive bistabilities with a writing voltage of −2.4 V and an erasing voltage of 3.4 V, and a maximum ON/OFF ratio up to 104. The retention cycle testing indicated that both ON and OFF states could be maintained 105 cycles without significant degradations. However, the mechanism of the memory characteristic was proposed to trapping and detrapping processes of electrons into and out of the P3HT/PMMA heterointerfaces. The electrons at a writing voltage of −2.4 V penetrated into the PMMA layer, resulting in the ON state. This ON state can be maintained in spite of electrons escaping at the interfacial region between the P3HT and the PMMA layers. When the erasing voltage of 3.4 V was applied, almost all of the electrons trapped in the interfacial region between the P3HT and the PMMA layers escaped, indicative of the OFF state.

Memory effects of polythiophene-plasticizer composites also have attracted lots of attention. In 2005, Smits et al. [59] reported the device with two stacked polymer layers sandwiched between an ITO bottom electrode and an Al top electrode, as shown in Fig. 8a. The first polymer layer was poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) containing lithium triflate (LiCF3SO3) and a plasticizer ethylene carbonate (EC). The second layer was P3HT blended with the same plasticizer. An applied reverse bias (≤−4 V) increased the conductivity and a forward bias (≥+4 V) reduced the conductivity. However, this electrically nondestructive rewritable memory only showed an retention about 10 min at room temperature. The electrical bistability was rationalized in term of migration of dopants in and out the P3HT depletion layer at the aluminum Schottky contact. In another case, memory devices consisting of a layer of a sexithiophene-poly(ethylene oxide) (6T-PEO) block copolymer with an PEDOT:PSS bottom electrode and an Al top electrode was investigated by this group [60]. In this study, an aqueous dispersion of PEDOT:PSS containing NaCl (20 wt% relative to the weight of dry material in the dispersion) and plasticizer EC was spin-coated on the transparent ITO electrode (Fig. 8b). Introduction of inorganic salt (NaCl) in the PEDOT layer resulted in resistive switching behavior under forward bias while retaining the diode character. The electrochemical doping of 6T occurred at both electrodes under a forward bias. Near the Al electrode, 6T was reduced by the migration of sodium ions toward the metal electrode (n-type doping), while the oxidation of 6T occurred near the interface with the PEDOT:PSS electrode and was associated with migration of the chloride ions in the vicinity of the interface (p-type doping) (Fig. 8c). This mechanism was supported by the temperature dependence of the I–V characteristics. At low temperatures, the hysteresis was significantly reduced because ions mobility was inhibited. Conversely, the mobility of ions was higher at high temperature and relaxation to the thermodynamically favored state was faster. Hence, the hysteresis was suppressed as well. However, this battery-like mechanism with ion-diffusion was generally slow-switching rate and the retention time of the information was still very short (∼10 s) [60]. In fact, the current density of the device was decay without sufficient electrical-charging. To improve on this, a block copolymer with a very steep dependence of the ion mobility on electric field is needed.

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Figure 8. (a) J–V characteristics of ITO/PEDOT:PSS:LiCF3SO3:EC/P3HT:EC/Al memory cell. (b) Chemical structure of the 6T-PEO block copolymer and I–V characteristics of ITO/PEDOT:PSS/6T-PEO/Al devices. Reprinted from Ref. 59, with permission from John Wiley and Sons. (c) Schematic layout of the ITO/PEDOT:PSS(NaCl,EC)/6T-PEO/Al device (left) and a band level diagram for the diode under forward bias stress (middle) and reverse bias stress (right). Reprinted from Ref. 60, with permission from American Chemical Society. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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All-conjugated diblock copolythiophenes and their blends with PCBM have also been investigated the potential application in electrical bistable memory devices. Fang et al. [61] synthesized donor-acceptor diblock copolymers of regioregular poly(3-hexylthiophene)-block-poly(2-phenyl-5-(4-vinylphenyl)−1,3,4-oxadiazole) (P3HT-b-POXD) (Fig. 9a). This rod-coil copolymer exhibited a nonvolatile memory depending on the P3HT/POXD block ratio and the resulting morphology. Due to the quasi-semiconducting nature on a high P3HT composition, the P3HT44-b-POXD5 device exhibited a single state with a moderate conductance diode behavior. The current just increased linearly with the applied voltage bias. The P3HT44-b-POXD18 device could exhibit distinctly tristable conductance states with NDR effect due to the charge trapped POXD block. It can be repeatedly written, read and erased with a switch-ON voltage of 4 V, a read-ON voltage of 0.5 V, a switch-OFF voltage of 7 V, but an ON/OFF current ratio just around 102 (Fig. 9b). As the ratio of POXD segment increase, the I–V characteristics of P3HT33-b-POXD25 showed no obvious increased drain current when applying the gate voltage bias, which was attributed to the decrease on the mobility with higher POXD ratio. The voltage-controlled multilevel states in ITO/P3HT44-b-POXD18/Al device was proposed to depend on the charge hopping conduction between the P3HT blocks and the deep charge traps in the POXD blocks. Lai et al. [62] also explored the memory effects of a series of diblock copolythiophenes, poly(3-hexylthiophene)-block-poly(3-phenoxymethylthiophene) (P3HT-b-P3PT) (Fig. 9c), and their blends with PCBM. P3HT52-b-P3PT39 and P3HT102-b-P3PT37 exhibited dynamic random access memory (DRAM) behavior in the sandwich configuration of ITO/P3HT-b-P3PT/Al, whereas P3HT only showed semiconductor characteristics, suggesting the significant trapping effect of the amorphous P3PT segments on the electrical switching behavior. It was proposed that the charge trapping may have occurred within the amorphous P3PT domains dispersed in the block copolythiophene by preventing charge transport. However, when blending a small amount (5–10 wt%) of PCBM into P3HT-b-P3PT of different block ratios (P3HT52-b-P3PT39, P3HT102-b-P3PT37, (Fig. 9d), and P3HT89-b-P3PT23), the memory devices showed a WORM behavior with the switching voltages of −2.6 to −3.3 V and high ON/OFF ratios of 105 to 107. The mechanism associated with the memory characteristics was due to the charge transfer from the P3HT-b-P3PT donor to the PCBM acceptor, which stabilized the charge-separated state and retained the high conductance state for a long time during the ON stage. Details of the mechanism will be discussed in Electric Field-Induced Charge Transfer Effects in Polythiophene Memory Materials section.

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Figure 9. (a) Chemical structure of P3HT-b-POXD copolymers. (b) I–V characteristics of ITO/P3HT44-b-POXD18/Al device. Reprinted from Ref. 61, with permission from John Wiley and Sons. (c) Chemical structure of P3HT-b-P3PT copolymers; (d) I–V characteristics of ITO/P3HT102-b-P3PT37: 5 wt% PCBM/Al device. Reprinted from Ref. 62, with permission from The Royal Society of Chemistry. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Battery-inspired and rewritable memory architecture based on conducting polymer poly(3,3″′-didodecylquaterthiophene) (PQT) have also been explored the conductance switching characteristics in the OFET-structured memory devices [19]. The device structure consisted of a PQT layer and an ethyl viologen diperchlorate (EV(ClO4)2) doped poly(ethylene oxide) (PEO) electrolyte layer between the source/drain and the gate electrode. The chemical structure of PQT, PEO, and EV(ClO4)2 are shown in Fig. 10a, while device structure is shown in upper right of Fig. 10b. This OFET-structured memory device exhibited an inherent advantage in separating the “write/erase” circuit (Source and Gate, denoted as SG circuit) from the “read” operation (Source and Drain, denoted as SD circuit). I–V curve of SG circuit of Au/PQT/PEO + EV(ClO4)2/C/Au device (red line) showed apparent hysteresis when compared with that of Au/PQT/PEO/C/Au device (black line). The hysteresis was due to oxidation of PQT accompanied by ethyl viologen reduction to accompany the cell reaction (Eq. (1)) under the bias sweep.

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Figure 10. (a) Chemical structures of the neutral form of PQT, PEO, and EV(ClO4)2. (b) I–V curve for a three-terminal Au/PQT/PEO+EV(ClO4)2/C/Au device scanned between VSG = −2 and +2 V (red line). (c) Ten write-read-erase-read cycles of Au/PQT/PEO+EV(ClO4)2/C/Au device. Reprinted from Ref. 19, with permission from American Chemical Society. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Write/erase sequences of 10 W/E cycles are shown in Fig. 10c, using +4 V write pulse and −4V erase pulse of 2 s duration. After each write or erase pulse, ISD values were recorded with VSD = 0.5 V at 2 s intervals. Note that the switching voltage and time yielded an ON/OFF current ratio of ∼>104, and there was no degradation in this ratio even after 200 complete cycles. However, the switching speed of this device was relatively slow with the switching time of millisecond range. Raman spectroscopy had revealed reversible polythiophene oxidation to its conducting polaron form throughout the polymer layer. Details of the polaron formation and propagation will be discussed in Redox-driven mechanism and Raman spectroscopic monitoring of oxidation state in polythiophene memory devices section. There was slow decay in the ON and OFF conductance apparent in Fig. 10c, which was most likely a recombination reaction between the PQT+ polaron and the reduced EV+ species at their common interface. In a conventional battery, the two redox phases are always separated by an ionic conductor to prevent such recombination, and such a separator was lacking in the current design.

The development of alkyl polythiophene materials for application to solution processing, flexible, and nonvolatile data storage devices is an emerging and interesting area. To summarize, the various alkyl polythiophene and device structures proposed for resistive nonvolatile memory can be conveniently separated into five categories according to the switching type, which are collected in Table 1. The notation of the tables gives layers, from bottom to top, separated by a slash, and components of a blend separated by a colon. Also, without exception, these memory properties show extremely high endurance during long term operation, which again makes tethered alkyl substituted polythiophenes a very suitable type for memory applications.

Table 1. Switching behavior observed in alkyl polythiophenes, composites, and block copolymers.
Switching typeStructureReferences
  1. The slash separates layers in the stack, starting with the bottom electrode.

Hysteresis, without threshold or NDRITO/PCDM/Al[31]
ITO/unoriented P6OMe/Al[33]
ITO/P3HT:FeS2 NCs/Al[46]
Au/P3HT/PVDF-TrFE/Al[56]
ITO/PEDOT:PSS:LiCF3SO3:EC/P3HT:EC/Al[59]
ITO/PEDOT:PSS:NaCl/6T-PEO/Al[60]
Au/PQT/PEO:EV(ClO4)2/C/Au[19]
Reverse polarity switching, no NDRITO/oriented P6OMe/Al[33]
PS beads embossed Cu/P3HT/Al[40]
Al/P3HT:Au NPs/Al[41]
Cu/P3HT/Al[38, 39]
ITO/P3HT:CNT/Al[42]
ITO/P3HT:PCBM/Cu[51]
p+-Si/PMMA/P3HT/P(NDI2OD-T2)/Au[55]
ITO/P3HT/PMMA/Al[58]
Optical programming and electrical erasingAu/P3HT:CdSe/Au[45]
Au/P3HT:CdSe@ZnSe QDs/Au[44]
n+-Si/SiO2/P3HT:PCBM/Au[50]
Al/PVA/P3HT/Au[53]
Al/PI/Al or Ca floating gate/P3HT/Au[54]
Switching by either polarity, NDRAl/P3HT:Ni/Al[47]
ITO/P3HT44-b-POXD18/Al[61]
WORM depending on conditionsITO/P3HT:TiO2/Al[43]
rGO/P3HT:PCBM/Al[48, 49]
Al/PMMA:P3HT/ITO[57]
ITO/P3HT102-b-P3PT37: 5 wt% PCBM/Al[62]

POLY(3,4-ETHYLENEDIOXYTHIOPHENE):POLY(STYRENESULFONATE) (PEDOT:PSS) IN MEMORY APPLICATIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. TETHERED ALKYL SUBSTITUTED POLYTHIOPHENES, COPOLYMERS AND COMPOSITES IN MEMORY APPLICATIONS
  5. POLY(3,4-ETHYLENEDIOXYTHIOPHENE):POLY(STYRENESULFONATE) (PEDOT:PSS) IN MEMORY APPLICATIONS
  6. MECHANISM IN POLYTHIOHENE-BASED MEMORY DEVICE
  7. SUMMARY AND OUTLOOK
  8. ACKNOWLEDGMENT
  9. REFERENCES

Poly(3,4-ethylene-dioxythiophene):polystyrenesulfonate, commonly referred to PEDOT:PSS, is composed of PEDOT oxidatively p-doped by PSS (Fig. 2a), with a PEDOT to PSS weight ratio of 1:2.5. In the p-doped state, the PEDOT film exhibits highly stable conductivity of up to 1 S/cm [63]. It can be switched at 2 μs to a permanent OFF state with an ON/OFF current ratio up to 103 [9, 37], which demonstrated a nonvolatile WORM memory device. After these reports, a lot of investigations have followed on this material. Bipolar and unipolar and nonpolar switching characteristics of PEDOT:PSS film have been intensively investigated and provided some flexibility for memory device operations. Moreover, other crucial elements for real application, such as the ON/OFF current ratios, threshold and read voltages, retention and reproducibility characteristics also have been investigated in PEDOT:PSS-based memory devices.

It was reported that the electronic conductivity of PEDOT:PSS could be modified under a high switching current because of the phase segregation near the cathode/polymer interface [64]. Different switching properties can be observed in devices with different electrode materials, which implied there existed different memory characteristics and switching mechanisms in various devices. Bipolar write-read-erase-read characteristics of PEDOT:PSS film were demonstrated in two-terminal sandwiched p+-Si/PEDOT:PSS/Al and n+-Si/PEDOT:PSS/Al [65], ITO/PEDOT:PSS/Al [66], and ITO/PEDOT:PSS/ITO [67] devices. Typical I–V curve of ITO/PEDOT:PSS/Al device is shown in Fig. 11a. The bias voltage was swept from 2 to −3 V and then from −3 to 2 V. It was observed that the turn-ON and turn-OFF voltages were at 0.7 and −1.55 V, respectively. The ON/OFF current ratio could be up to 103, write-read-erase-read cycle test was operated over 104 times and the retention time was up to 16 h. These characteristics were explained by the formation and destruction of current paths by the reduction and oxidation of PEDOT chains in a PEDOT:PSS thin film. However, unipolar resistive switching was reproducibly demonstrated in Al/PEDOT:PSS (60 nm)/Al devices [68] with the turn-ON and OFF voltages of about 2 to 3 and 0.5 V, respectively. The ON/OFF current ratio of the device was up to 104 to 109. The write-erase cycle test was operated 27 times and the retention time was 6 h. This unipolar switching was not only ascribed to the formation of current paths due to the redox behavior of PEDOT chains, but also ascribed to the existence of a native thin Al2O3 layer sandwiched between the PEDOT:PSS thin film and Al bottom electrode (BE), which acted as an energy barrier that interrupted the injection of carriers from the polymer thin film into the BE. This Al2O3 layer was verified by transmission electron microscope (TEM) of a cross section of an Al/PEDOT:PSS/Al device. Interestingly, nonpolar nonvolatile resistive switching was found in Au/PEDOT:PSS/Au devices [69]. This device was initially in the ON state with a resistance of hundreds of ohms, and can be erased to the OFF state at about 1 V in either polarity (Fig. 11b). In order to turn the device back to the ON state, a current-limited set voltage in either polarity was added to the device. The resistive ratio between the ON and OFF state could be on the order of 103 and retention time could be over 104 s. The authors proposed that the reduction and oxidation of PEDOT: PSS film might be the switching mechanism, but Au atoms penetrating into the film during metal deposition and other defects might also influence the formation of conductive paths.

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Figure 11. (a) Bipolar I–V characteristics of ITO/PEDOT:PSS/Al device. Reprinted from Ref. 66, with permission from AIP Publishing LLC. (b) Nonpolar I–V characteristics of Au/PEDOT:PSS/Au device. Reprinted from Ref. 69, with permission from Elsevier. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Typically, the effect of various electrode materials for nonvolatile memory devices with PEDOT:PSS thin film as active layer was thoroughly probed by Ha and Kim [70]. The bottom electrodes (BEs) were ITO and Al, while the top electrodes (TEs) were Al, Ti, Cr, ITO, Au, Ni, Pd, and Pt. The ITO/PEDOT:PSS/TE devices showed only a bipolar switching behavior, while the Al/PEDOT:PSS/TE devices did not show any switching behavior unless a compliance current was used in the write-operation process. Under the compliance current, devices presented unipolar switching behaviors regardless of the TE material. Furthermore, it was revealed that switching characteristics of the ITO/PEDOT:PSS/Al and Al/PEDOT:PSS/Al devices varied as functions of the turn-ON CC. The resistance of the current path in the PEDOT:PSS thin film decreased linearly as the turn-ON CC was increased. However, the bipolar switching device always had lower turn-OFF current than the turn-ON CC (turn-OFF/ON maximum current ratio: 0.8–1.0), and the unipolar switching device had higher turn-OFF current than the turn-ON CC (turn-OFF/ON maximum current ratio: 1.3–1.6), which indicated that the switching from ON to OFF state in the bipolar devices was eventually caused by the amount of carrier injection into the PEDOT:PSS thin film and reduction of the current paths and the switching in the unipolar devices by a current that was much larger than the turn-ON CC flowing through the device and rupturing the current paths. Therefore, both the BE material and the current compliance played crucial roles in the switching behavior and characteristics.

Bipolar and unipolar switching mechanism can basically be explained as the formation of current paths due to the redox behavior of the PEDOT:PSS film [66, 68, 69]. When a voltage is applied to the electrode, the PEDOT chains will be oxidized to PEDOT+ chains by the injected carriers. Then, current paths will be formed by PEDOT+ chains, and the devices will switch from OFF to ON state. However, in the bipolar switching devices, the PEDOT+ chains are reduced to PEDOT0 chains by injection carriers when a voltage is scanned in the opposite direction. Then, the current paths are destroyed, and the devices switch from ON to OFF state. In the unipolar switching devices, the current flowing through the current path in the devices is increased beyond the CC when a voltage is applied with a maximum CC. Then, the current paths are ruptured by a very large turn-OFF current [69], and the devices switch from ON to OFF state.

However, in some PEDOT:PSS-based devices, the memory mechanism was not ascribed to redox behavior of the PEDOT:PSS film. Typically in two-terminal configurations of Al/PEDOT:PSS (∼70 nm)/Al [71] and Al/PEDOT:PSS/Cu device [72], the bipolar switching was proposed to arise from metal filamentary switching. Take Al/PEDOT:PSS/Cu device [72] for an example, I–V curves indicated write-read-erase-read resistive memory characteristics (Fig. 12a and b) with switching-ON threshold voltage less than 1.5 V, the switching-OFF threshold voltage less than 1.0 V, the ON/OFF current ratio as large as 104. However, cycling endurance of the switching characteristics in Fig. 12b showed that there was partly degradation of the ON states when sweep over 150 switching cycles. The devices presented good thermal stability that the resistive switching can be observed even at temperature up to 160°C (Fig. 12c). Retention test showed that the two resistance states were stable for over 106 s without serious deterioration (Fig. 12d), which was much longer than in other PEDOT:PSS-based devices [9, 66, 68]. It was proposed that the Cu TE was the source for Cu filament growing through the PEDOT:PSS layer during the forming process. Details of the Cu filament formation will be discussed in Filament Formation in Polythiophene Memory Materials section. It was also found that when the devices were heated to 180°C, they could be switched to the ON state but could not be maintained. The cross-sectional SEM image of Cu TE at 180°C showed that it cracked in some extent and its grain size became smaller than that at 25°C, which indicated Cu was oxidized. And surface XPS spectra clearly found that Cu2+ appeared intensively after the sample was heated at 180°C. Therefore, The failure of the Al/PEDOT:PSS/Cu memory at higher temperatures was due to the oxidation of the Cu TE.

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Figure 12. (a) Typical I–V curve for Al/PEDOT:PSS/Cu devices. (b) Cycling endurance of the switching characteristics. (c) I–V curves of the Al/PEDOT:PSS/Cu memory devices when heated from 25 to 160°C. (d) Retention time of the device. Reprinted from Ref. 72, with permission from American Chemical Society. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Design and preparation of electrodes were emphasized to modulate the resistive switching properties, aiming to adjust the energy alignment at the interface of metal-organic semiconductor to control the injection of holes or electrons. However, modifying the active layer also can be applicable to modulate the resistive switching properties of memory devices. Yang et al. [73] mixed PEDOT:PSS with poly(vinylpyrrolidone) (PVP) with diluted content to form continuous network of the conducting polymer. It was found that additive of PVP in PEDOT:PSS could enlarge the ON/OFF ratio from 103 to 105 and keep a long retention time over 105 s. There were typically two main functions of PVP. One was inducing more charge traps to decrease the OFF state current. The other was changing the coil conformation to linear conformation of PEDOT+, and thus increased the ON-state current. Besides, PVP molecules were obstacles that occupied the position of PEDOT and blocked the charge transportation. Ávila-Nino et al. [74, 75] constructed memory devices consisting a small quantity of functionalized carbon nanoshells (f-CNSs) or multiwalled carbon nanotubes (f-MWCNTs) embedded in PEDOT:PSS active matrix. Al/PEDOT:PSS:f-CNSs/Al showed irreversible WORM memory characteristics, which was explained by the electrons trapped in the f-CNS, and by the redox process occurring in the PEDOT backbones [74]. However, ITO/PEDOT:PSS:f-MWCNT/Al exhibited rewritable volatile memory characteristics with the threshold voltage for OFF to ON switching changing by adjusting the f-MWCNTs concentration, which suggested that the nanotubes might produce an inhomogeneous electric field playing a role in the electroforming (dielectric breakdown) of the aluminum oxide layer at the Al2O3/(PEDOT:PSS) interface [75]. Park et al. [76] carried out an investigation about glycerol-modified PEDOT:PSS (G-PEDOT:PSS) film-based polymer memory devices. ITO/G-EDOT:PSS/Al showed an nonvolatile bipolar switching characteristics with a write voltage pulse of around −0.5 V and an erase pulse of around +2 V. The ON/OFF current ratio was of 103 to 104 and the write-read-erase-read cycle was operated over 105 times. Most importantly, the ON-retention time was largely dependent on the glycerol to PEDOT:PSS ratio and annealing temperature. When PEDOT:PSS was modified by 5% glycerol and annealed at 180°C, a better ordered morphology could be obtained on the G-PEDOT:PSS films, and the corresponding device showed relative long retention time up to 20 days with 70 to 80% of ON state current level in air. This study suggested that well-ordered organic layer representing more stable morphology can provide improved electrical bistability, and the surface morphology control of polymer material can be an important factor to improve the device performance in polymeric memory devices.

The other investigation concerning the improved memory effects of PEDOT:PSS thin film have been performed to insert a buffer layer between the active PEDOT:PSS layer and the bottom electrode. Son et al. [77] designed ITO/PMMA/PEDOT:PSS/Al device on polyethylene terephthalate flexible substrates, which showed write-read-erase-read memory capability with an enlarged ON/OFF current ratio of 103 and endurance number of the ON/OFF switching above 105. It was proved that the surface of the PEDOT:PSS on the PMMA buffer layer was significantly improved due to the uniform molecular structure of the PMMA, resulting in the more uniform interfacial contact between the PEDOT:PSS layer and Al top electrodes. Therefore, the device performance was improved due to the decrease of the leakage current.

WORM memory characteristics also have been observed in the PEDOT:PSS memory devices. Brito et al. [78] fabricated vertical PEDOT/PSS fuses in patterned microholes and sandwiched between bottom and top gold contacts. Fuses with diameters from 10 to 100 μm can be programmed with ultralow power. The switching mechanism in this PEDOT:PSS-based WORM devices was suggested that electrode delamination by gas formation upon electrolysis of water, and that the switching was voltage-driven. ITO/PEDOT:PSS/ZnO/Al devices fabricated by Wang et al. [79, 80] also have demonstrated WORM memory characteristics with a very low power consumption. In this case, wide-bandgap colloidal semiconductor ZnO nanoparticles acted as a hole blocking layer. Therefore, injection of electrons into PEDOT:PSS films was dominated. The conductive p-doped conjugated polymer was permanently dedoped by injected electrons, producing an insulating state. Using Raman spectroscopy and in situ absorbance measurements, it was directly observed the change of doping level of PEDOT during the device switching, which was the proposed switching mechanism. Electrode delamination was not the primary switching mechanism, since the switching voltage was dependent on device thickness and sweep rate.

In summary, all the switching behaviors observed in PEDOT:PSS complexes were collected in Table 2. Also, it was separated into four categories according to the switching type. The notation of the tables gives layers, from bottom to top, separated by a slash, and components of a blend separated by a colon.

Table 2. Switching behavior observed in PEDOT:PSS and composites.
Switching typeStructureReferences
Hysteresis, without threshold or NDRp-i-n Si/PEDOT:PSS/Au[9, 37]
p+ or n+-Si/PEDOT:PSS/Al[65]
Reverse polarity switching, no NDRITO/PEDOT:PSS/Al[66]
Al/PEDOT:PSS (60 nm)/Al[68]
ITO/PEDOT:PSS/ITO[70]
ITO/PEDOT:PSS/TE (TE: Al, Ti, Cr, ITO, Au, Ni, Pd, and Pt)[70]
Al/PEDOT:PSS (∼70 nm)/Al[71]
Al/PEDOT:PSS/Cu[72]
Al/PEDOT:PSS:PVP/Al[73]
ITO/G-EDOT:PSS/Al. PSS/ITO[76]
Switching by either polarityAu/PEDOT:PSS/Au[69]
Al/PEDOT:PSS/TE (TE: Al, Ti, Cr, ITO, Au, Ni, Pd, and Pt)[70]
ITO/PEDOT:PSS:f-MWCNT/Al[75]
WORMAl/PEDOT:PSS:f-CNSs/Al[74]
ITO/PMMA/PEDOT:PSS/Al[77]
Au/PEDOT:PSS/Au[78]
ITO/PEDOT:PSS/ZnO/Al[79, 80]

MECHANISM IN POLYTHIOHENE-BASED MEMORY DEVICE

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. TETHERED ALKYL SUBSTITUTED POLYTHIOPHENES, COPOLYMERS AND COMPOSITES IN MEMORY APPLICATIONS
  5. POLY(3,4-ETHYLENEDIOXYTHIOPHENE):POLY(STYRENESULFONATE) (PEDOT:PSS) IN MEMORY APPLICATIONS
  6. MECHANISM IN POLYTHIOHENE-BASED MEMORY DEVICE
  7. SUMMARY AND OUTLOOK
  8. ACKNOWLEDGMENT
  9. REFERENCES

Despite the number of studies about polythiophene memory effects increased rapidly, the mechanisms responsible for the different switching effects were not well understood. In the past decade, researchers have paid great efforts to explore the mechanisms underlying the switching phenomena of the functional polythiophenes. In this section, three dominant theories, namely filament formation, field-induced charge transfer effects and redox-driven will be addressed to explain the memory characteristics.

Filament Formation in Polythiophene Memory Materials

A widely accepted mechanism in resistive random access memories is filamentary conduction mechanism, which is confined to that arising from physical damages in devices. In general, when the ON state current is highly localized to a small fraction of the device area, one has a phenomenon generally termed “filamentary” conduction [81]. The simplest form of filament is, of course, a metallic bridge connecting the two electrodes. If filaments are formed in a device, (i) the ON state current will exhibit metallic I–V characteristics and will increase as the temperature is decreased, and (ii) the injected current will be insensitive to the device area or show a random dependence, because the dimension of the filaments is much smaller when compared with the device area. Most of the devices with artifact memory phenomena from filamentary conduction are difficult to control and reproduce. However, some controllable filamentary conduction situations have been demonstrated in polymers for nonvolatile memory application. The Cu/P3HT/Al device has electrically controllable bistability, which was proposed to arise from the metal filament formation (ON state) and breakdown (OFF state) between two electrodes [39], as shown in Fig. 13. The filament formation can be divided into three processes. First of all, the copper electrode was ionized to copper ions by the high positive voltage, and then copper ions injected into polymer layer. Assisted by coordination to the heteroatom (Sulfur) of the P3HT, copper ions were distributed uniformly throughout the polymer layer. Then, copper ions were metallized to form the filament by the injected electrons under negative voltage bias. The formation of metal filament was confirmed by time-of-flight secondary ions mass spectroscopy (TOF-SIMS) (Fig. 13e), which measured the depth profile of copper ions before and after the positive voltage application over the threshold value. To avoid the broadening of Cu profile caused by the ion beam mixing effect during the SIMS measurement, inverse devices structure of Al/P3HT/Cu on wafer substrate was fabricated. Three samples were prepared with different voltage bias conditions; the first device was virgin and the second and the third ones were biased for 100 s with lower and higher voltages than the threshold value (5 and 13 V, respectively). From Fig. 13e, the density of copper ions was much higher near the aluminum electrode for the device applied with the high voltage. The SIMS analysis strongly indicated that ionize copper electrode and to drift the ions within a P3HT layer indeed occured by applying the positive voltage over the threshold.

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Figure 13. (a) Device structure. (b) Ionization and drift processes of copper caused by positive voltage. (c) Metal filament formation by the reduction of copper ions. (d) The breakdown of copper filament by joule heating. (e) Copper profiles in wafer/Cu/P3HT/Al device applied by the lower and higher voltages (5 and 13 V, respectively) than the threshold using TOF-SIMS analysis. Reprinted from Ref. 39, with permission from American Chemical Society.

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Similarly, in Al/PEDOT:PSS/Cu device [72], the top Cu electrode acted as the source of the redox ions that was injected through the PEDOT:PSS layer during the filament forming process. The Cu filament was verified by the cross-sectional TEM image of a memory cell in ON state as well as by corresponding energy-dispersive X-ray (EDX) spectrum of Cu (Fig. 14a). The Cu filament connected robustly with Cu top electrode and extended to Al bottom electrode, but did not connect to Al BE. From the log-log I–V plot and its linear fitting curve for the device after electroforming in Fig. 14b, the logI-logV curve in the OFF state had a linear region with a slope of 1.0 at low voltage. At voltages higher than 0.3 V, the logI–logV curve was mostly linear with a slope of 1.2. This indicated a strong space-charge-limited-conduction (SCLC) effect changed to a weak SCLC effect after the forming process, and the current was bypassed by the metal filament and then weak charge accumulation occured. A slope of 1.0 was observed in the ON state, which clearly showed Ohmic conduction behavior. Relationship between resistance and cell size from Fig. 14c indicated that a smaller dependence of RLRS on the cell size than that of RHRS, which implied that the filament conducting path in the ON state was localized. Moreover, the Arrhenius plot of the current in the ON state showed a negative dependence on temperature (Fig. 14d), which is a feature of a metallic conduction mechanism. On the basis of these observations, the switching behavior in Al/PEDOT:PSS/Cu memory devices can be explained by a redox-controlled Cu bridge creation and rupture process.

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Figure 14. (a) Cross-sectional TEM images of memory cells in ON state; the insert is EDX spectrum for Cu element profiles along the dotted lines marked. (b) Double logarithmic plots and their linear fitting of the I–V curves in the positive voltage sweep region for the devices after electroforming. (c) Cell size dependence of the resistances in the HRS and LRS. (d) Temperature dependence of the currents in the HRS and LRS. Reprinted from Ref. 72, with permission from American Chemical Society. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Electric Field-Induced Charge Transfer Effects in Polythiophene Memory Materials

Electric field-induced charge transfer effect is an electronic transition(s) to an excited state in which there is a partial transfer of electronic charge from the donor to the acceptor moiety. Electric field-induced charge transfer effects between the donor and acceptor has been exploited in polythiophene-based bistable nonvolatile memory system. In 2006, Prakash et al. [41] performed comparative experiments between Al/P3HT/Al device and Al/P3HT:AuNPs/Al device. They found the conduction mechanism for the Al/P3HT/Al device was charge injection-limited current at low bias and changed to space charge-limited current at high voltage. Whereas, it was observed a linear relation between log(I/V) and V1/2 for Al/P3HT:AuNPs/Al device in the ON state at room temperature, which was fitting Poole-Frenkel emission due to the presence of the gold nanoparticles in the polymer film. The electronic transition in Al/P3HT:AuNPs/Al was attributed to an electric field-induced charge transfer between P3HT and AuNPs. When the external electric field was high enough, electron on the HOMO of P3HT could gain enough energy and tunnel through 1-dodecanethiol into the core of gold nanoparticles (Fig. 15). Consequently, the gold nanoparticles were negatively charged while P3HT were positively charged. The effect of charge transfer on the electronic structure of P3HT was similar to that of chemical oxidation of a conducting polymer. After the charge transfer, the negative charge on a gold nanoparticle could be stabilized due to the insulator 1-dodecanethiol shell, and the positive charge (hole) delocalized in the whole P3HT chains so that the entire system was stabilized. The returning of the device from ON to OFF state was due to the turning back of the electron from gold nanoparticle to P3HT.

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Figure 15. (a) Energy diagram of the core of gold nanoparticle, 1-dodecanethiol (DT), and P3HT. The two dots on the HOMO of P3HT represented two electrons, E indicated the direction of the electric field, and the arrow from the electrons on the HOMO of P3HT indicated the electron transfer from P3HT to the core of gold nanoparticle. Reprinted from Ref. 41, with permission from AIP Publishing LLC.

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Nonvolatile switching characteristic of all-conjugated diblock copolythiophenes blending with donor also have been attributed to the electric field-induced charge transfer effects, such as P3HT-b-P3PT blends with PCBM [62]. As previously mentioned, the memory device characteristics of the polymer composites with 5 and 10 wt% PCBM in the P3HT52-b-P3PT39, P3HT102-b-P3PT37, or P3HT89-b-P3PT23 matrices were all the WORM type. A high electric field, however, may facilitate electron transfer from the HOMO (−4.82 eV) of P3HT-b-P3PT to the LUMO (−3.7 eV) of PCBM. Consequently, the LUMO of PCBM and the HOMO of P3HT-b-P3PT were partially filled, and PCBM and polymer were negatively and positively charged, respectively. Therefore, carriers were generated and the device exhibits a sharp increase in the conductivity after charge transfer [82]. When compared the photoluminescence (PL) spectra of the studied P3HT102-b-P3PT37 and P3HT102-b-P3PT37/PCBM in the thin films excited at the corresponding absorption λmax of the studied polymers, it could be observed that the fluorescence of P3HT102-b-P3PT37/PCBM was significantly quenched, indicating the charge transfer from the P3HT102-b-P3PT37 to PCBM. When the voltage scanned over the threshold voltage, the electrondonating P3HT102-b-P3PT37 probably formed a charge transfer with the electron acceptor PCBM and the generated carriers consequently induce a sharp increase in the current density. The stabilized charge separated state may not easily be recombined even under the reverse field and the high conductance state can be retained for a long time in the observed WORM device. For samples with different concentration of PCBM, both TEM and AFM images indicated the observable PCBM clusters, where P3HT102-b-P3PT37:10 wt% PCBM film showed denser PCBM clusters than that of P3HT102-b-P3PT37:5 wt% PCBM film. The PCBM cluster in P3HT102-b-P3PT37 matrix could be polarized, and functioned as those in rGO/P3HT:PCBM/Al device [57], thus demonstrating the nonvolatile properties. However, the variation in the device performance was not observed in the devices containing PCBM between 5 and 10 wt%.

Redox-Driven Mechanism and Raman Spectroscopic Monitoring of Oxidation State in Polythiophene Memory Devices

Redox reaction is an element of a memory device gains or loses electrons to cause a significant change in conductivity [83]. Redox active functionality can occur rapidly in the submicrosecond range, which enable data storage at fast rates. The redox of polythiophene can be accomplished by the introduction of charges either by electron-removal (oxidation or p-doping) or electron-injection (reduction or n-doping) into the polymer chain [84]. The removal of one electron from the polythiophene chain produces a mobile charge in the form of a radical cation, also called a polaron. The positive charge tends to induce local atomic displacements ("clothing with phonons''), leading to the polaronic behavior. Further oxidation can either convert the polaron into a spinless bipolaron or introduce another polaron (double polaron). In either case, introduction of each positive charge also means introduction of a negatively charged counterion. This counterion is one important constituent in redox-based polythiophene memories, because it can support the polymer oxidation and reduction processes and simultaneously stabilize the charged form of the polymer. Without such components, the polymer oxidation would result in a space charge that would limit oxidation and provide a strong driving force for reduction of the polaron and therefore short retention.

For thin films, polaron formation in polythiophenes can be monitored by spectroscopic, such as transient absorption spectroscopy, near infrared and Raman spectra. In particularly, Raman spectroscopy can provide detailed information on the structures of neutral and doped polymers. Two key features in Raman characterization of doped polymers are as follows. (1) Structural information on charged domains generated by doping can be obtained by using exciting laser lines in resonance with doping-induced electronic absorptions appearing in the region from visible to infrared. (2) The Raman spectra thus obtained can be analyzed by referring to those of the radical ions and divalent ions, which are viewed, respectively, as polarons and bipolarons confined to limited sequences of repeating units. Raman spectroscopy had revealed reversible PQT oxidation to its polaron form to accomplish bias-induced switching between two metastable states having different conductivity [19]. The dynamics of the PQT/EV system were investigated by Raman spectrum in both open face geometry and three-terminal structure (Fig. 16a and b, respectively). Before bias was applied, the Raman spectrum from the as-prepared open face device showed a major peak at 1460 cm−1 corresponding to neutral, undoped PQT. When applied a positive bias, VSD = +2 V, a major peak at 1405 cm−1 corresponding to the PQT polaron appeared. For the opposite bias (VSD = −2 V), the S was negatively biased and polarons were reduced back to the neutral form, with the major peak at 1460 cm−1 for reduced PQT reappearing. Compared to the initial spectrum, the spectrum for VSD= −2 V showed two additional bands at 1028 and 1528 cm−1 (marked with *), which were attributable to the one electron-reduced species EV+. Further support for the redox mechanism was provided by in situ Raman spectroscopy of the three-terminal device under different VSG conditions (Fig. 16b). With VSG = +2 V, PQT was oxidized and a polaron peak appeared at 1405 cm−1. For negative bias with VSG = −2 V, the neutral PQT peak reappeared, indicating reduction of polaron back to the neutral state. Under the same bias condition (VSG = −2 V), EV+ peaks were also observed over the S electrode (marked with *). For spectra acquired over the D electrode, the neutral PQT peak (1460 cm−1) showed initially, due to the undoped PQT used to fabricate the device. However, the polaron peak was observed over the D terminal with VSG = +2 V, even though the D electrode was floating while VSG is applied. The polaron at both S and D reverted to its neutral state when VSG = −2 V. The EV was partially reduced in the Raman beam because it was close to the negatively biased S and reduced PQT. It can be concluded that the two-terminal geometry exhibited opposite oxidation states at S and D under either bias polarity, while in the three-terminal device the oxidation states of both the S and D electrodes tracked the applied bias between the S and G electrodes.

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Figure 16. (a) Raman spectra obtained at the source for the indicated VSD values in the two-terminal open face geometry containing stacked layers of PQT and EV(ClO4)2 in PEO. The upper right shows the overlay of Raman spectra on the same scale for VSD = +2 V (black curve) and VSD = −2 V (red curve). (b) Effect of SG bias on Raman spectra at S and D electrodes in the Au/PQT/EV(ClO4)2 + PEO/C/Au device. (c) Proposed polaron propagation mechanism in the Au/PQT/EV(ClO4)2 + PEO/C/Au device. Reprinted from Ref. 19, with permission from American Chemical Society. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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The relationship between the polaron and the memory characteristics were proposed by the polaron propagation mechanism, shown schematically in Fig. 16c, where A was perchlorate ion, EV was ethyl viologen, P was polythiophene and arrows indicated electron transfers occuring in the polythiophene film. Under a positive SG bias, the polaron first formed at the S electrode. This resulting polaron may undergo redox exchange with adjacent reduced PQT, resulting in propagation of an oxidized “front” of polarons moving away from the S electrode. Since PQT+ is a conductor, this front may propagate quickly and the S electrode effectively “expands” as the conducting front propagates through the SD gap and onto the D electrode, leading a conductance change. This moving front may be presumably accompanied by transport of ClO4 ions into the PQT layer from PEO to form electrochemical doping, thus polarons in PQT could be electrostatically balanced by ClO4 counterions. The process of electrochemical doping is much slower in comparison with the response time of the polymer electrolyte dielectric operating as an electric double layer capacitor. This is due to the much slower mobility of ions in the semiconducting polymer as compared with that in the ion conducting PEO [85]. Therefore, the relatively slow write and read speeds of the PQT-based device is likely due to the time required for heterogeneous electron transfer to the redox agents and the transit time of mobile anions to compensate space charge. A permeable semiconductor and a solid electrolyte with ionic liquid [86] or poly(ionic liquid)s as polyelectrolytes [87] are proposed to improve the W/E speed rates for this battery-inspired memory device.

SUMMARY AND OUTLOOK

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. TETHERED ALKYL SUBSTITUTED POLYTHIOPHENES, COPOLYMERS AND COMPOSITES IN MEMORY APPLICATIONS
  5. POLY(3,4-ETHYLENEDIOXYTHIOPHENE):POLY(STYRENESULFONATE) (PEDOT:PSS) IN MEMORY APPLICATIONS
  6. MECHANISM IN POLYTHIOHENE-BASED MEMORY DEVICE
  7. SUMMARY AND OUTLOOK
  8. ACKNOWLEDGMENT
  9. REFERENCES

This article reviewed the various types of nonvolatile memory devices based on tethered alkyl substituted polythiophenes and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) composites. We highlighted the role of the materials in the memory functionality, with the intention to provide helpful indications for further improvement of the material properties. To date, I–V characteristics show a conductance change of at least three orders of magnitude with the switching voltage varied for different voltage scans. These transitions between the ON and OFF states can be triggered on a range of millisecond to nanosecond time scale depending on the transition mechanism. Although these devices have demonstrated promising performance, they are still in the early stages of research. The effect of active layer components, nanoscale dopants, electrode materials, structural configurations and electrolyte layers are the main issues involved in polythiophene memory device.

Some theories, such as filament formation, field induced charge transfer effects, and redox reaction have been exploited to explain electrical bistability and memory phenomenon in polythiophene memory devices. Although they have been working well to a great extent, there still remain a number of issues to be solved. One of the big problems is that the considerations of the resulting memory properties and their mechanism include many speculations. For example, what determines the switching voltage and why the switching voltage varies slightly for different voltage scans? What determines the switching speed and how to eliminate the delay response in polymeric memory device? Where is the charge stored for very long term retention? The active mechanisms are still the subject of heated debate and controversy, complicated by the fact that the experimental data are not always reproducible, even within the same lab. Thus it is not possible to analyze the trade-offs between, for example, switching voltages, ON and OFF state current and retention times. Endurance also was very different in a system that involves mass transport compared to one in which only charges move.

The next great challenge in polythiophene memory is to clarify conclusively the relevant switching and transport mechanisms in any particular device structure. The memory devices now need to be made more elegantly and to perform more elaborate information processing. This will require painstaking work to identify and characterize any localized conduction paths that may be responsible. The second challenge is to elucidate the device-failure mechanisms and the ultimate in device performance. The third challenge is to scale up their function in terms of application, such as program/erase speed, retention time, endurance and three-dimensional stacking capability in integration circuit. The integration with three-dimensional stacking circuitry for read-out and programming is a subject of further research. Successful research on fabricating various kinds of devices like three-dimensional, small and dense, and flexible devices [56] will accelerate the practical use of polythiophenes as memory materials. All of these issues need to be addressed in the future to aid the design of high performance devices. All in all, polythiophene-based memories have received extensive attention and promise attractive application prospects. It is hoped that further research will successfully resolve the problems that polymeric memories are currently encountering, allowing their wide use in organic electronics.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. TETHERED ALKYL SUBSTITUTED POLYTHIOPHENES, COPOLYMERS AND COMPOSITES IN MEMORY APPLICATIONS
  5. POLY(3,4-ETHYLENEDIOXYTHIOPHENE):POLY(STYRENESULFONATE) (PEDOT:PSS) IN MEMORY APPLICATIONS
  6. MECHANISM IN POLYTHIOHENE-BASED MEMORY DEVICE
  7. SUMMARY AND OUTLOOK
  8. ACKNOWLEDGMENT
  9. REFERENCES