Electrical Memory Materials and Devices
CHAPTER 1: Organic Electronic Memory Devices
Published:16 Oct 2015
Special Collection: 2015 ebook collection , 2011-2015 materials and nanoscience subject collectionSeries: Polymer Chemistry Series
With the rapid development of the electronics industry in recent years, information technology devices, such as personal computers, mobile phones, digital cameras and media players, have become an essential part of our daily life. From both the technological and economic points of view, the development of novel information storage materials and devices has become an emergent issue facing the electronics industry. Due to the advantages of good scalability, flexibility, low cost, ease of processing, 3D-stacking capability and high capacity for data storage, organic-based electrical memory devices have been promising alternatives or supplementary devices to conventional inorganic semiconductor-based memory technology. The basic concepts and historical development of electronic memory devices are first presented. The following section introduces the structures and switching mechanisms of organic electronic memory devices classified as transistors, capacitors and resistors. Subsequently, the progress in the field of organic-based memory materials and devices is systematically summarized and discussed. Finally, the challenges posed to the development of novel organic electronic memory devices are summarized.
As the performance of digital gadgets for information technology advances, the complexity of data storage devices increases correspondingly. Conventional memory devices are implemented on semiconductor-based integrated circuits, such as transistors and capacitors. In order to achieve greater density of data storage and faster access to information, more components are deliberately packed onto a single chip. The feature size of transistors has decreased from 130 nm in the year 2000 to 32 nm at present.1,2 Silicon-based semiconductor devices become less stable below 22 nm, and the reliability to store and read individual bits of information will be substantially reduced by severe “cross-talk” issues. Moreover, power consumption and unwanted heat generation are also of increasing concern, and the fidelity of addressing the memory units diminishes correspondingly. Therefore, the current state-of-the-art memory technologies are no longer capable of fulfilling the requirements for information storage of the near future.3
Regarding the aspiration for new data storage technologies, ferroelectric random access memory (FeRAM),4 magnetoresistive random access memory (MRAM),5 phase change memory (PCM),6 and organic/polymer memory have appeared on the scene of the information technology industry.7–9 Instead of information storage and retrieval by encoding “0” and “1” as the amount of stored charge in the current silicon-based memory devices, the new technologies are based on electrical bistability of materials arising from changes in certain intrinsic properties, such as magnetism, polarity, phase, conformation and conductivity, in response to the applied electric field. The advantages of organic and polymer electronic memory include good processability, molecular design through chemical synthesis, simplicity of device structure, miniaturized dimensions, good scalability, low-cost potential, low-power operation, multiple state properties, 3D stacking capability and large capacity for data storage.10–16
Extensive studies toward new organic/polymeric materials and device structures have been carried out to demonstrate their unique memory performances.17–22 This chapter provides an introduction to the basic concepts and history of electronic memory, followed by a brief description of the structures and switching mechanisms of electrical memory devices classified as transistors, capacitors and resistors. Then, the progress of organic-based memory materials and devices is systematically summarized and discussed. Lastly, the challenges posed to the development of novel organic electrical memory devices are summarized.
1.2 Basic Concepts of Electronic Memory
The basic goal of a memory device is to provide a means for storing and accessing binary digital data sequences of “1’s” and “0’s”, as one of the core functions (primary storage) of modern computers. An electronic memory device is a form of semiconductor storage which is fast in response and compact in size, and can be read and written when coupled with a central processing unit (CPU, a processor). In conventional silicon-based electronic memory, data are stored based on the amount of charge stored in the memory cells. Organic/polymer electronic memory stores data in an entirely different way, for instance, based on different electrical conductivity states (ON and OFF states) in response to an applied electric field. Organic/polymer electronic memory is likely to be an alternative or at least a supplementary technology to conventional semiconductor electronic memory.
According to the storage type of the device, electronic memory can be divided into two primary categories: volatile and non-volatile memory. Volatile memory eventually loses the stored information unless it is provided with a constant power supply or refreshed periodically with a pulse. The most widely used form of primary storage today is volatile memory. As shown in Figure 1.1, electronic memory can be further divided into sub-categories, as read only memory (ROM), hybrid memory, and random access memory (RAM). ROM is factory programmable only; data is physically encoded in the circuit and cannot be programmed after fabrication. Hybrid memory allows data to be read and re-written at any time. RAM requires the stored information to be periodically read and re-written, or refreshed, otherwise the data will be lost. Among these types of electronic memory, write-once read-many-times (WORM) memory,7 hybrid non-volatile and rewritable (flash) memory,8 static random access memory (SRAM) and dynamic random access memory (DRAM) are the most widely reported polymer memory devices.23,24
A WORM memory device can be used to store archival standards, databases and other massive data where information has to be reliably preserved for a long period of time. Conventional CD-Rs, DVD±Rs or programmable-read-only-memory (PROM) devices are examples of WORM memory. Flash memory is another type of non-volatile electronic memory. Different from WORM memory, its stored state can be electrically reprogrammed and it has the ability to write, read, erase and retain the stored state. Thus it is mutable or rewritable in nature. Due to its non-volatility, no power is needed to maintain the information stored in flash memory. DRAM is a type of volatile random access memory that stores each bit of data in a separate capacitor within an integrated circuit. Since real-world capacitors have charge-leaking tendencies, the stored data eventually fade unless the device is refreshed periodically. Because of this periodical refresh requirement, it is a volatile and dynamic memory. The volatility, ultrafast data access time and structural simplicity hold great promise for high density and fast responding performance, making DRAM memory the main memory for most computers. SRAM is another type of volatile memory. The term “static” differentiates it from “dynamic” RAM (DRAM) which must be periodically refreshed. SRAM exhibits data remanence, but it is still volatile and the stored data are eventually lost when the memory remains in the power-off state. SRAM is faster and more reliable than the more common DRAM. Due to its high cost, SRAM is often used only as a memory cache.
Parameters of importance to the performance of a memory cell include switching (write and erase) time, ON/OFF current ratio (or memory window), read cycles, and retention ability. The switching time influences the rate of writing and accessing the stored information, the ON/OFF current ratio defines the control of the misreading rate during device operation, with a higher value being essential for the device to function with minimal misreading error, while the number of read cycles and retention ability are related to the stability and reliability of the memory devices. For practical applications, other factors, such as power consumption and cost, structural simplicity and packing density, as well as mechanical stiffness and flexibility, are of equal importance when designing and fabricating new memory devices.
1.3 History of Organic/Polymer Electronic Memory Devices
Different forms of storage, based on various natural phenomena, have been reported since the 1940s. A computer system usually contains several kinds of storage, each with an individual purpose. In the 1960s, there was a great interest in the electrical properties of amorphous semiconductors and disordered structures, arising from their unusual electrical properties which also make them promising materials for device applications.25,26
In 1968, Gregor observed bistable negative resistance in polymer materials and noted that a Pb/polydivinylbenzene/Pb bistable electrical switching device is capable of acting as an information storage device.27 In 1969, Szymansk et al. reported bistable electrical conductivity phenomena in thin tetracene films sandwiched between metal electrodes.28 In 1970, Sliva et al. reported that devices based on Saran® wrap, phthalocyanines and polystyrene all exhibited bistable switching behavior.29 Subsequently, Segui et al. demonstrated reproducible bistable switching in polymer thin films prepared by glow-discharge polymerization.30 Inspired by these pioneering studies, a wide variety of organic and polymer materials have been explored for threshold and memory switching effects.31–33 Many of the observed electrical memory effects were due to the formation of filamentary conduction paths, and the performance was not satisfactory for practical applications. Memory switching effects in polymethylmethacrylate, polystyrene, polyethylmethacrylate and polybutylmethacrylate films were ascribed to field-controlled polymer chain ordering and disordering.34 Memory switching effects in poly(N-vinylcarbazole) (PVK) thin films were attributed to trapping–detrapping processes associated with impurities in PVK.35
Studies of the transition behavior of some ferroelectric polymers began in the 1980s.36–38 Thin films of ferroelectric materials can be repeatedly switched between two stable ferroelectric polarization states, and are capable of exhibiting non-volatile memory effects. Polymer films obtained by solution processing techniques were so thick that some devices required operating voltages of at least 30 V. Bune et al. reported a major breakthrough in the fabrication of ferroelectric films by the Langmuir–Blodgett (LB) technique in 1995.39 The resulting ferroelectric films are as thin as 1 nm and can be switched using a voltage as low as 1 V.40 Rapid progress in polymer ferroelectric random access memory (FeRAM) as a promising memory technology has since been achieved.41–43
An organic transistor memory device using a sexithiophene oligomer as the conductor and an inorganic ferroelectric material as the gate insulator was demonstrated in 2001 by Velu et al.44 Subsequently, ferroelectric organic and polymer materials have also been utilized as gate insulators in field-effect transistors (OFETs).45–48 High performance all-organic or polymer transistor memory devices have been demonstrated by Naber et al.49–51 Transistor memory devices can be faster and more readily integrated with traditional electronics. However, they are not able to meet the high density and low-cost requirements since an additional terminal is required between the gate and the semiconducting channel. A WORM type memory device based on polymer fuses was demonstrated by Forrest and coworkers in 2003.7 The memory element consists of a thin film p–i–n silicon diode and a conductive polymer fuse, composed of poly(ethylene dioxythiophene) (PEDOT) oxidatively p-doped by poly(styrene sulfonic acid) (PSS).
Bistable electrical switching and memory effects involving charge transfer (CT) complexes were first reported in an electronic device based on a copper (electron donor) and 7,7,8,8-tetracyanoquinodimethane (TCNQ, electron acceptor) complex.52 Subsequently, a wide variety of organometallic and all-organic CT complexes have also been explored for non-volatile electronic memory applications.52 Polymer memory devices based on CT effects from doping of a polymer matrix by electron donors, such as 8-hydroxyquinoline (8HQ), tetrathiafulvalene (TTF), polyaniline (PANI), poly-3-hexylthiophene (P3HT), or electron acceptors such as gold nanoparticles, copper metallic filaments and phenyl C61-butyric acid methyl ester (PCBM), have been reported.8,9,53–55 Carbon nanotubes (CNTs) possess intense π-conjugation and strong electron-withdrawing ability. The CT complexes of CNTs and P3HT, a conjugated copolymer or poly(N-vinylcarbazole) (PVK) have been reported to exhibit a bistable electrical memory effect.56–58 By utilizing copolymers containing both donor (D) and acceptor (A) moieties in the basic unit, phase separation and ion aggregation could be effectively avoided in a single-component polymer film, resulting in uniform film morphology and improved device performance. D–A polymer-based electrically bistable memory devices have received considerable attention. The molecular design-cum-synthesis approach has allowed several polymer electronic memory devices, including flash memory, WORM memory and DRAM to be realized.59–68
In order to achieve ultrahigh density memory devices, organic materials with multilevel stable states are highly desirable. In 2004, Pal et al. reported multilevel conductivity and conductance switching in supramolecular structures of Rose Bengal.69 Subsequently, they observed one low- and three high-conducting states in ultra-thin film devices, and all four accessible states have associated memory effects for data-storage applications.70 Multilevel conductance switching in poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) films was first reported by Lauters et al. in 2005.71 They observed that the ITO/MEH-PPV/Al device had the ability to store a continuum of conductance states. These states were non-volatile and could be switched reproducibly by applying appropriate programing biases above a certain threshold voltage. Devices demonstrating multistability where more than two conducting states can be programmed into a single switching element will dramatically increase the amount of data stored per area or volume. Further progress in the development of multilevel organic/polymer memory has been made in recent years.72–76
In 1971, Chua proposed a new circuit element, a memristor, which is the fourth passive circuit element beyond the fundamental resistor, capacitor and inductor.77 A memristor is capable of processing information in the same way as biological systems, mimicking the function of a mammalian synapse, with the ability to learn and memorize new information. According to the redefinition of Chua in 2011, all two-terminal non-volatile resistive switching memory devices are memristors, regardless of the device material or the physical operating mechanism.78 A polymer memristor was first reported in cobalt(iii)-containing conjugated (CP) and non-conjugated (NCP) polymers with an azo-aromatic backbone by Higuchi et al. in 2011.79 Single crystals of a cyclodextrin-based metal–organic framework (MOF) infused with an ionic electrolyte and flanked by silver electrodes can act as memristors.80 The metal/single-crystal MOF/metal heterostructure can be switched between high and low conductivity states due to the self-limiting oxidative reactions of the metal anode.
The International Technology Roadmap for Semiconductors (ITRS) has identified polymer memory as an emerging memory technology since the year 2005. Figure 1.2 indicates the number of related publications each year worldwide since the year 2000. Research work on polymer memory before 2008 was summarized in a comprehensive review by Ling et al.22 Liu and Chen highlighted recent developments in the field of D–A polymers for resistive switching memory device applications.20 Chen et al. reviewed the application of electrically, thermally and chemically modified graphene and polymer-functionalized graphene derivatives for switching and information storage applications.19 Most recently, Huang and coworkers summarized recent progress concerning the use of polymers or polymer composites as active materials for resistive memory devices.17 The impetus for the research effort in this area arises from the fact that organic/polymer electronic memory devices have been a promising alternative or supplementary device to conventional memory technologies facing the problem of miniaturization from microscale to nanoscale.
1.4 Classification of Electrical Memory Devices
According to the device structure, electronic memory devices can be divided into three primary categories: transistors, capacitors and resistors. With their respective ability to amplify electronic signals, to store charges, and to produce proportional electric currents, electronic memory devices can be constructed from transistors, capacitors and resistors.
1.4.1 Transistor-Type Electronic Memory
Inorganic transistors are widely used in conventional semiconductor memory. For example, a group of six or more field-effect transistors can be integrated to assemble a SRAM cell, while a floating gate transistor can be integrated to assemble a flash memory cell. Organic (including polymer) transistors are also of great potential for memory applications.81–84 Organic field-effect transistor (OFET) type memory devices have attracted considerable research interest due to their easily integrated structure and the non-destructive reading of a single transistor.85–87 In addition, the mechanical flexibility of organic materials makes them compatible with plastic substrates for lightweight and flexible device design.
220.127.116.11 Device Structure
The organic transistor inherits its design features from inorganic MOSFET precursors.88,89 It is composed of three main components: source, drain and gate electrodes, a dielectric insulator layer and an active semiconductor layer, as illustrated in Figure 1.3a.22 The electrodes can be n- or p-Si, ITO, PEDOT:PSS, TaN, Au, Pt, Al, Cu, Cr or other metals. Among these, Au electrodes are often used for OFETs because the work function of gold is close to the ionization potential of many polymer materials, which leads to an ohmic contact in the device. An OFET memory device consists of at least one polymeric material either in its dielectric insulator layer or active semiconductor layer or both. The device is usually supported by a glass, wafer, or plastic substrate. Within the basic MOSFET design, there are two types of device configuration: top contact and bottom contact (Figure 1.3a and b). In the former, the source and drain electrodes are fabricated on a preformed semiconductor layer, whereas the latter is constructed by depositing the organic materials over the contacts.90
18.104.22.168 Operation Mechanism
A voltage applied to one pair of the transistor’s terminals (either source–drain or source–gate) can change the current flowing through another pair of terminals. The voltage applied between the source and drain is referred to as the source–drain voltage, VD. The current flowing through the semiconductor film from source to drain is referred to as the source–drain current, ID. For a lightly doped or undoped semiconductor, the concentration of free charge carriers in the channel is very low. When VG = 0 V, ID is very low and the transistor is initially in the OFF state. With an increase in VG, a layer of mobile charges from the source can accumulate at the interface between the semiconductor and the insulator. Due to the increased charge carrier concentration in the semiconductor, ID increases significantly and turns the transistor to the ON state.91 The gate threshold voltage (Vth) is defined as the voltage necessary to induce mobile charges, that is, the gate voltage at which the square root of the saturation ID begins to increase substantially. Alternatively, field-effect transistor memory can also be operated at constant VG and sweeping VD.
To be a non-volatile transistor for memory applications, the charge must be stored or polarized in domains within the bulk of the dielectric layer, or at interfaces between the gate contact and the semiconductor channel. An additional voltage, via charge storage or polarization, is thus introduced between the gate and the semiconductor channel to alter the charge distribution in the transistor. On the basis of charge storage and polarization methods in the dielectric layer or interfaces, OFET memory devices can be divided into three categories: floating gate OFET memory, charge trapping OFET memory, and ferroelectric OFET memory (Figure 1.3c and d).22
1.4.2 Capacitor-Type Electronic Memory
Capacitors can store charges on two parallel plate electrodes under an applied electric field. Based on the amount of charge stored in the cell, the bit level (either “0” or “1”) can be encoded accordingly. When the medium between the electrodes is merely a dielectric, the stored charge will be lost eventually.92 Thus, DRAM using a dielectric capacitor is volatile memory, and the information stored in DRAM eventually fades unless the capacitor charge is refreshed periodically. On the other hand, if the medium is ferroelectric in nature, permanent electric polarization can be maintained and longer retention time can be achieved. A ferroelectric material can maintain permanent electric polarization that can be repeatedly switched between two stable states by an external electric field. Thus, memory based on ferroelectric capacitors (FeRAM) is non-volatile memory.4 FeRAM needs no periodic refreshing and it still retains its data in the case of power failure. Organic and polymeric ferroelectric materials can also be used in DRAM and FeRAM applications.93–97
22.214.171.124 Device Structure
Several FeRAM structures, including 1T1C (T – transistor, C – capacitor),98 2T2C,99 1T2C100 and others, have been developed. The simplest DRAM and FeRAM cells have similar structures, both utilizing 1T1C as the building components. Figure 1.4a shows an image of a 1T1C FeRAM device, while a schematic circuit diagram of the cell is shown in Figure 1.4b.98 The plate line (PL) of a FeRAM device has a variable voltage level to enable the switching of the polarization of the ferroelectric capacitor. The upper electrode of the capacitor is made of Pt, Ir or Ru, and the lower electrode is Pt/Ti.101 The local interconnect between the access transistor and the storage node of the capacitor is TiN. These materials are refractive and can form conductive oxides, such as IrOx or RuOx.
126.96.36.199 Operation Mechanism
Ferroelectric materials exhibit polarization–electric field hysteresis loops. As shown in Figure 1.5, when voltages are applied from 0 V to +Vcc, the polarization state changes from point A to B to C progressively.22 Similarly, the polarization state changes progressively from point D to E to F, with the applied voltages increasing from 0 V to −Vcc. When voltages are applied from +Vcc to 0 V and from −Vcc to 0 V, the polarization state changes, moving from point C to D and from point F to A, respectively. The amount of polarization charge can be maintained well, without reversing their direction. Under such a situation, zero-voltage remnant polarization states with opposite direction (Pr, points A and D) can be obtained by a large saturation voltage (±Vcc), and can be reversed or switched by the coercive voltage (±Vc), the minimum value of the voltage necessary to reverse, or switch, the polarization state. Thus, “0” and “1” can be defined as the two stable states, “upward polarization” and “downward polarization”, to apply ferroelectric characteristics to electronic memory.22
“1” or “0” data can be written to a FeRAM cell by applying a voltage +Vcc or −Vcc to both electrodes of the ferroelectric capacitor. For instance, to write “0”, the word line (WL) is turned on (meaning that the access transistor is on), the bit line (BL) is held at 0 V and the plate line (PL) is cycled from 0 V to Vcc to 0 V (Figures 1.4b and 1.5).22 This polarizes the ferroelectric capacitor in the “0” state. After writing, data is retained even if the selected WL becomes unselected (meaning that the transistor is off). When reading “0” or “1” data from a cell, prior to selecting the WL, the BL must be precharged to 0 V to retain the high-impedance condition. Next, the WL is selected and Vcc is applied to the PL (Figures 1.4b and 1.5). For the cell in the “0” state, there will be a minimal voltage change on the BL since the capacitor does not switch the polarization direction. For the cell in the “1” state, there will be a larger voltage variation on the BL since the capacitor switches the polarization direction and the difference in compensating charge flows onto the BL. When reading the “1” state, the reversal in polarity causes the data to be destroyed, creating a “0” state. Therefore, it is necessary for the capacitor to be re-polarized to the “1” state before closing the WL and moving on to the next operation. To rewrite the “1” state, the BL voltage level is set at Vcc and the PL voltage level is set at 0 V. Finally, the WL is turned off and the “1” state is stored again (Figures 1.4b and 1.5).22 When reading the “0” state, since no reversal of polarity occurs, the datum is not destroyed and the re-writing process is not required.
1.4.3 Resistor-Type Electronic Memory
Devices incorporating switchable resistive materials are generically classified as resistor-type memory, or resistive random access memory (RRAM). Unlike transistor and capacitor memory devices, resistor-type memory does not require a specific cell structure (e.g. FET) or to be integrated with the CMOS (complementary metal-oxide-semiconductor) technology. Resistor-type memory devices store data in an entirely different form, for instance, based on different electrical conductivity states (ON and OFF states). Electrical bistability usually arises from changes in the intrinsic properties of materials, such as charge transfer, phase change, conformation change and reduction–oxidation (redox) reaction, in response to an applied voltage or electric field.22
188.8.131.52 Device Structure
Resistor-type electronic memory usually has a simple structure with an organic/polymer thin film sandwiched between two electrodes on a supporting substrate (glass, silicon wafer, plastic or metal foil). The configuration of the top and bottom electrodes can be either symmetric or asymmetric, with aluminum, gold, copper, p- or n-doped silicon, and ITO being the most widely used electrode materials. Test structures usually consist of a spin-coated polymer thin film on the bottom electrode, for instance, ITO, with the top electrodes deposited through a shadow mask via thermal evaporation in a vacuum chamber. The area covered by the top electrode forms the active device area. The basic configuration of a test memory device is shown in Figure 1.6a. The individual memory cells can be integrated into a cross-bar (two dimensional or 2D) memory array (Figure 1.6b), and further stacked into three-dimensional (3D) data storage devices (Figure 1.6c). Each cell in the 2D memory array or 3D stacked device can be identified by its unique Cartesian coordinates. Due to the two terminal simple structure and the nanoscale active organic/polymer thin film, high data storage density can be realized in organic/polymer memory.
184.108.40.206 Operation Mechanism
Resistor-type memory is based on the change of the electrical conductivity of materials in response to an applied voltage (electric field). Various mechanisms have been proposed to explain electrical conductance switching in organic/polymer memory devices. Among them, the most widely reported mechanisms include filament conduction, space charges and traps, charge transfer effects, and conformational changes.
220.127.116.11.1 Filament Conduction
Filament conduction is used to describe the high conductivity (ON) state where the current is highly localized in a small fraction of the device area. It is believed that filament conduction is normally associated with physical damage to the device, and thus results in artifact memory effects which are difficult to control and reproduce. Two types of filament conduction, carbon-rich filament and metallic filament conduction, are widely reported in polymer memory.102 The former is caused by local degradation of polymer films and will increase the mobility of charge carriers only.30 The latter results from local fusing, migrating or sputtering of electrode metal through the films, leading to an increase in both the charge carrier mobility and concentration.103
18.104.22.168.2 Space Charges and Traps
The intrinsic electrical conductivity of organic and polymer materials is far lower than that of metals. When the electrode–film contact is ohmic, charge carriers will be easily injected from the electrode into the organic thin film and accumulated near the interface to form a space charge buildup. The electrostatic repulsion between individual charges can screen the applied electric field and further limit charge injection into the film.104 Consequently, hysteresis in the current–voltage (I–V) characteristics of the film is observed. Space charges in materials may arise from several sources, such as electrode injection of charge carriers, ionized dopants in interfacial depletion regions and accumulation of mobile ions at electrode/organic film interfaces. Capacitance–voltage (C–V) characteristics can also show hysteresis arising from space charges.105 The hysteric behavior, either in I–V or C–V characteristics, can be utilized to create data storage devices. A device can be programmed by applying a voltage pulse to write a state, and read by measuring the device current under a small probe voltage.
When traps are present either in the bulk of the material or at the interface regions, the charge carrier mobility will be significantly reduced. Adsorbed oxygen molecules in organic films,35 intra-molecular donor–acceptor structures106 and semiconductor or metal nanoparticles107 can act as charge trapping centers. As greater numbers of charge carriers are injected with increasing voltage, the traps in the organic thin film are gradually filled.35 When all of the traps are eventually filled, the newly injected charge carriers will no longer be affected by the fully filled traps. An abrupt increase in the current is observed, and the transition from the OFF to the ON state is related to the level of occupancy of the charge traps. The current is limited by re-excitation (de-trapping) of the trapped carriers in the trap-filled state.108 Both space charges and traps play an important role in the electronic processes and switching behavior of organic electronic devices.109
22.214.171.124.3 Charge Transfer Effects
A charge transfer (CT) complex is defined as an electron donor–acceptor (D–A) complex, characterized by an electronic transition to an excited state in which a partial transfer of charge occurs from the donor moiety to the acceptor moiety. The conductivity of a CT complex is dependent on the ionic binding between the D–A components.110 As illustrated in Figure 1.7,22,111 if the donor is characterized by small size and low ionization potential, a strongly ionic salt forms and a complete transfer of charge (or with the CT degree value, δ > 0.7) occurs from the donor to the acceptor, making the ionic salt insulating. When the donor is very large and has a high ionization potential, a neutral molecular solid (δ < 0.4) forms, which is also insulating. If the donor has intermediate size and ionization potential, it tends to form a weakly ionic salt with the acceptor, which possesses incomplete CT (0.4 < δ < 0.7) and thus is potentially conductive. The formation of a conductive CT complex can be employed to design molecular electronic devices. Many organic CT systems, including organometallic complexes, carbon allotrope (fullerene, carbon nanotubes and graphene)-based polymer complexes, gold nanoparticle–polymer complexes, and single polymers with intra-molecular D–A structures have been explored for memory applications.112–115
126.96.36.199.4 Conformational Changes
The spatial conformation of a material can significantly affect the distribution of electron density within the macromolecule and/or the π-conjugation of the system, thus effectively controlling the material’s electrical conductivity. Electrical bistability arising from electric field-induced conformational changes has been reported in organic and polymeric materials.116–118 For instance, carbazole containing polymers are capable of exhibiting field-induced conformational change via rotations of randomly oriented carbazole groups to form a more regioregular arrangement for facilitated carrier delocalization and transport.117 A transition in the molecular conformation of anthracenyl moieties can tune the energy levels of anthracene -containing polymers, and improve the bulk charge recombination.118 Substituted biphenyl or bipyridine molecules have been inserted as the active material between a pair of metallic electrodes, to control the longitudinal conduction in response to an applied electric field.119,120 The mechanism of operation is based on the action of an electric field perpendicular to the ring–ring bond on the torsional angle and, as a consequence, the inter-ring conjugation. With the inclusion of suitable substituents on the aromatic rings, the transverse electric field can increase the dihedral angle, and thus change the conformation from one in which electrons can flow freely from side to side, to one in which this flow is hindered. As a result, the conductance of the molecule is varied, providing a means for potential data storage.
To further understand the device transition from the OFF state to the ON state, the current density–voltage data in both states can be fitted to theoretical models. Various mechanisms have been proposed to explain the generation, trapping and transport of charge carriers in organic and polymer memory devices.121,122
1.5 Types of Organic-Based Electrical Memory Devices
1.5.1 Organic Molecules
Organic electronic memory devices based on organic molecules were first reported in several acene derivatives including naphthalene, anthracene, tetracene, pentacene, perylene, p-quaterphenyl and p-quinquephenyl.123–126 Ma et al. reported an organic electrically bistable device with the structure of a single layer of N,N′-di(naphthalene-l-yl)-N,N′-diphenyl-benzidine (NPB) embedded between ITO and Ag electrodes.127 Subsequently, a negative differential resistance (NDR) in organic devices consisting of 9,10-bis-(9,9-diphenyl-9H-fluoren-2-yl)-anthracene (DPFA) sandwiched between two electrodes was observed.128 The device exhibited reproducible NDR and can be electrically switched between the ON state and the OFF state.
Organometallic and all-organic CT complexes have been explored for use in organic memory (Scheme 1.1). Electrical memory phenomena of CT complexes were first reported in a copper and 7,7,8,8-tetracyanoquinodimethane (TCNQ) complex (Cu-TCNQ).52 Stable and reproducible current-controlled bistable electrical switching was observed in a device with the structure Cu/Cu-TCNQ/Al. Inspired by this discovery, many other organometallic CT complexes with different metals and organic acceptors have been prepared and explored for memory effects over the past few decades.129–132 A series of all-organic CT complexes were prepared by alternative, mixed or dual deposition in a vacuum chamber. All films of these organic CT complexes exhibited electrically bistable states at room temperature and a short transition time from high to low resistance.133–137 These CT complexes consisted of methanofullerene 6,6-phenyl C61-butyric acid methyl ester (PCBM) as the organic electron acceptor, and tetrathiafulvalene (TTF) as the organic electron donor. Electrical bistability was demonstrated in devices with a sandwich structure. Devices in the OFF state could be switched to the ON state by applying a 5 V pulse of width shorter than 100 ns, while the ON state could be converted to the OFF state by applying a −9 V pulse of width shorter than 100 ns. Pal et al. demonstrated a WORM memory effect in a monolayer of a copper(ii) phthalocyanine (CuPc) and fullerenol CT complex.136 Ma et al. observed a reproducible NDR and memory effect in a 1,4-dibenzyl C60 (DBC) and zinc phthalocyanine (ZnPc) CT complex.137 The introduction of DBC enhanced the ON/OFF current ratio and significantly improved the memory stability. The ON/OFF current ratio was up to 2 orders of magnitude. The number of write–read–erase–reread cycles was greater than 106, and the retention time reached 10000 s without current degradation.
Organic dyes, such as phthalocyanines (Pc), porphyrins (Por) and xanthene derivatives have also been explored for electrical memory effects. Ray et al. fabricated bistable memory devices by using 70 nm thick spun films of PbPc molecules sandwiched between an ITO substrate and Al top electrodes.138 The bistable electrical switching effects were attributed to the existence of a depletion layer at the ITO/PbPc interface and an exponential distribution of the energetics of traps in the non-active region of the PbPc films. Similar phenomena can also be observed for some other phthalocyanine derivatives, such as NiPc,105 CuPc139 and ZnPc.140 Pal et al. observed high electrical conductance switching (ON/OFF ratio = 105) in single-layer sandwich structures based on the organic molecule Rose Bengal (RB) at room temperature.141 A molecular switching device utilizing LB monolayer films containing ZnPor as a redox-active component has also been reported.142 Devices with the structure metal/ZnPor LB monolayer/metal exhibited outstanding switching diode and tunneling diode behavior at room temperature.
Organic memory devices with a triple-layer structure sandwiched between two outer metal electrodes have been reported.143 The active layer of the organic bistable device consists of a 2-amino-4,5-imidazoledicarbonitrile (AIDCN)/Al/AIDCN trilayer structure interposed between an anode and a cathode. This device has two distinctive states of conductivity that can be achieved by applying voltage pulses with different polarities. This memory device is non-volatile and rewritable, with an ON/OFF ratio of more than 106. The device demonstrates good rewritability characteristics during cycle testing. More than one million write–read–erase cycles were performed on the device without failure. When this organic bistable device is integrated with an organic light-emitting diode, the device can be read out optically.144
Small organic molecules containing both an electron donor and an electron acceptor are an important type of material for organic electronic memory devices. As shown in Scheme 1.2, a number of conjugated D–A molecules have been investigated as electrically active materials in binary devices. The triphenylamine-containing D–A molecule (BDOYM) was designed to improve the intrinsic storage performance.145 By using scanning tunneling microscopy (STM), stable, reliable, reversible data storage was demonstrated.
Subsequently, the effect of different D–A structures on the electronic switching properties of triphenylamine-based molecules was investigated (Figure 1.8).146 Based on different arrangements of the donor and acceptor units, D–A molecules of two structural types, viz., D–π–A–π–D and A–π–D–π–A were designed. Devices based on the D–π–A–π–D molecule exhibited excellent write–read–erase characteristics with a high ON/OFF ratio of up to 106, while the device based on the A–π–D–π–A molecule exhibited irreversible switching behavior and a relatively low ON/OFF ratio. The arrangement of the donor and acceptor within the molecular backbone plays a key role in the electrical memory behavior of devices.
Lu and coworkers reported a simple DRAM device based on SNACA where the electron acceptor naphthalimide (NA) and electron donor carbazole (CA) were linked by a hydrazone bond (Figure 1.9a).147 The electrical switching behavior of a Pt/SNACA/Al device is demonstrated in Figure 1.9c. From theoretical calculations, it was found that during the HOMO to LUMO transition, the electron density underwent a minor shift from the electron donor to acceptor in the SNACA backbone, resulting in poor stability of the ON state of the memory device after removal of the electrical field, and thus leading to volatile DRAM characteristics. To alter the distribution of electron density throughout the molecular backbone and thereby tune the corresponding memory type of the device, Lu et al. introduced a π-spacer pyridyl acetylene into the conjugated molecular skeleton to replace the original hydrazone bond between the carbazole unit and the naphthalimide moiety (Figure 1.9b and d).148
Small D–A molecules with multilevel stable states can lead to an increased device capacity of 3n or larger. As illustrated in Scheme 1.3, a series of novel small organic molecules have been designed and synthesized for ternary data-storage devices.74–76,149–152 Figure 1.10a shows the I–V characteristics of an ITO/DPKAZO/Al device.76 The device demonstrates typical non-volatile ternary WORM memory behavior. As shown in Figure 1.10b, the I–V characteristics of the ITO/FKAZO/Al device also exhibit non-volatile ternary WORM characteristics. The two switching threshold voltages of FKAZO are markedly lower than those of DPKAZO (−1.05 and −1.81 V compared to −1.50 and −2.61 V). The low switching threshold voltages are due to the fact that FKAZO has a planar conformation (Figure 1.10c), which can generate a highly ordered arrangement in the film and decrease the hole injection energy barrier. The charge carrier transport process in the D–A molecule FKAZO (Figure 1.10d) was proposed to explain the switching mechanism of the ternary memory device.
Song et al. reported a meta-conjugated donor–bridge–acceptor (DBA) organic molecule for electronic multilevel storage.153 This DBA molecule is exploited as the storage medium (Figure 1.11a) in which TPA acts as an electron donor and TCF acts as an electron acceptor. The D–A pair is separated by a meta-conjugated bridge of a 2,7,12-trisubstituted truxene unit. As shown in Figure 1.11b and c, the device with the structure of ITO/DBA/Al shows good electrically bistable switching behavior between “0” (OFF) and “2” (HC-ON) states in the dark, with a large ON/OFF ratio of more than 106. In cooperation with UV light, a new “1” (LC-ON) state is addressable. This “1” state is “written” from the “0” state, by a combination of UV light and a positive voltage of +1.0 V, or from the “2” (HC-ON) state, by a combination of UV light and a negative voltage of −3.4 V.
Certain organic materials and biomacromolecules possess great potential for application in biocompatible, low cost and disposable electronic devices.154–156 Chen et al. used the sericin protein as the functional material for the fabrication of flexible multilevel memory devices.154 Kundu et al. reported the fabrication of transparent bio-memristor devices using natural regenerated silk fibroin protein.155 As shown in Figure 1.12, by sweeping the dc bias in 4 steps, the ITO/silk/Al device exhibits typical pinch hysteresis-like I–V characteristics on a linear scale. The observed rectifying nature in the I–V characteristics suggests that the silk fibroin can make a rectifier contact with either electrode. Thus the bipolar memristive switching with rectifying characteristics of the silk fibroin memristor was demonstrated.
1.5.2 Polymeric Materials
The molecular structure of polymeric materials can be tailored using electron donors and acceptors of different strengths, spacer moieties with different steric effects, and electroactive pendant groups to induce different switching behaviors for electronic memory applications.
188.8.131.52 Functional Polyimides
Functional polyimides (PIs) are one of the most attractive polymeric materials for organic electrical memory applications due to their good solution processability, high thermal stability and mechanical strength.18 In functional PIs, phthalimide acts as the electron acceptor, and electron donors (triphenylamine or carbazole moieties) are introduced to form a D–A structure. An electric field-induced CT state can be formed, which is the main mechanism responsible for the memory behavior.157 Electronic memory devices based on soluble PIs were first reported in 2006.59 As shown in Figure 1.13a, the functional TP6F-PI contains triphenylamine as electron donor and phthalimide as the acceptor. The hexafluoroisopropylidene (6F) group plays an important role in increasing the solubility of the PIs due to its bulkiness and low surface energy. TP6F-PI exhibited excellent thermal stability, with a 10% weight-loss temperature of 524 °C and a glass transition temperature of 316 °C. The Al/TP6F-PI/ITO device exhibits dynamic random access memory (DRAM) behavior with an ON/OFF current ratio of up to 105 (Figure 1.13b). Field-induced charge transfer from triphenylamine to phthalimide is considered to dominate the switching behavior. Since 2006, a large number of memory effects, including volatile and non-volatile switching behavior, have been observed in a number of functional PIs.158–167
Flexible dual functional devices with multi-colored electrochromic and volatile memory characteristics have been fabricated from a solution-processable TPA-containing PI (Figure 1.14a).166 Introduction of highly electron-donating starburst TPA moieties into the polymer main chain not only stabilizes its radical cations but also leads to good solubility and film-formation properties of the PI. As shown by the I–V characteristics of Figure 1.14b, the device can switch from the OFF state to the ON state during the negative sweep. The fifth sweep was conducted after turning off the power for about 3 min. It was found that the ON state had relaxed to the steady OFF state without an erasing process. The device could be reprogrammed to the ON state again at a threshold voltage of −3.6 V. Thus the ITO/9Ph-6FDA/Al flexible memory device exhibits SRAM memory behavior. Reliable and reproducible switching memory behavior of this flexible memory device (Figure 1.14c) can be maintained under mechanical bending stress. Furthermore, the polymer device showed electrochromism with a high contrast ratio, high coloration efficiency, low switching time, and outstanding stability for long-term electrochromic operation in both the visible and near-infrared regions. Ree et al. reported the fabrication and operation of an electrically programmable non-volatile memory device based on a thermally and dimensionally stable PI containing carbazole moieties (6F-HAB-CBZ PI).167 The fabricated Al/6F-HAB-CBZ/Al devices exhibited excellent unipolar switching behavior, and could be repeatedly written, read, and erased.
184.108.40.206 Non-Conjugated Polymers with Pendants
Non-conjugated polymers with pendent electroactive donors, acceptors and chromophores are another kind of polymer material favorable for electronic memory.168–172 Chen et al. developed D–A random copolymers P(VTPAxBOXDy) and P(CNVTPAxBOXDy) containing pendent electron-donating TPA and electron-accepting 1,3,4-oxadiazole units for memory device applications (Figure 1.15).66 The tunable switching behavior (SRAM or DRAM) was explored by using different ratios of the pendent TPA donor and the BOXD acceptor. Similarly, the electrical switching behavior based on pendent polymers containing electron-donating carbozole units (VPK) and electron-withdrawing oxadiazole-containing units (OXD or BOXD) can be tuned using the donor/acceptor ratio or acceptor trapping ability.168 The electrical I–V characteristics change between the diode, the volatile memory, and the insulator, depending on the relative donor/acceptor ratios. The unstable ON state in the P(VPK8OXD2) or P(VPK8BOXD2) device was due to shallow hole traps with spontaneous back transfer of charge carriers when the electric field was removed to produce volatile SRAM behavior.
Novel random and block copolymers with controlled morphologies and electrical properties have been developed to prevent aggregation and thus improve the reproducibility and stability of memory devices.169,171 Lee et al. synthesized D–A type block copolymers of poly(9-(4-vinylphenyl)carbazole)-b-poly(2-vinylpyridine) (denoted PVPCz-b-P2VP).169 By adjusting the block ratios of PVPCz and P2VP, various morphologies, ranging from spherical to lamellar, as well as different electrical behaviors including metallic, bistable switching and insulating, were observed.
For the Disperse Red 1-functionalized PVK copolymer (PVDR), in which carbazole entities serve as electron donors, and DR1 moieties serve as electron acceptors,68 both inter- and intrachain electron transfer can occur easily. PVDR nanoaggregates self-assemble via π–π stacking interactions of the carbazole groups in the polymer system after adding a solution of PVDR in N,N′-dimethylformamide (DMF) to dichloromethane (DCM). The ITO/PVDR/Al device fabricated from the pre-assembled PVDR film exhibited a typical WORM memory effect.
The use of azobenzene moieties as spacers between electron donor and acceptor groups in the pendant chains of vinyl-based polymers has been investigated.173–177 The reversibility of the ON state was found to be dependent on the terminal moiety of the azobenzene chromophores.173 Polymers with electron-accepting terminal moieties (–Br or –NO2) in the pendent azobenzene exhibited WORM memory behavior, while those with electron-donating terminal moieties (–OCH3) showed rewritable memory behavior. The switching threshold voltage of the devices varied almost linearly with the functional azobenzene moiety content in the copolymer, which was attributed to the reduced energy barrier between the HOMO of the copolymer film and the ITO work function.174 The ON/OFF ratios of the WORM devices (104–106) were also higher than those of the rewritable devices (103–104). Two poly(N-vinylcarbazole) derivatives with pendant donor–trap–acceptor (D–T–A) structures, PVK-AZO-2CN and PVK-AZO-NO2, have been prepared for memory devices.175,176 The Al/polymer/ITO devices all exhibited WORM memory effects with a low switching threshold voltage and a high ON/OFF current ratio, with the Al/PVK-AZO-2CN/ITO device with stronger electron acceptors (2CN) exhibiting an even higher ON/OFF ratio. The charge transfer and trapping processes were further stabilized by the conformational relaxation of the total energy of the D–T–A system through donor–acceptor electrostatic interaction. When the terminal acceptor moieties were removed from the azobenzene chromophores, pure insulators were obtained.
220.127.116.11 Conjugated Polymers
Considerable effort has been devoted to develop novel conjugated polymers for information and communication technology.178–180 The incorporation of different electron acceptors into conjugated polymer donors significantly affects the memory properties, while the induced trapping environment or charge transfer channel determines the volatility of the memory device.20 D–A type conjugated polymers (Scheme 1.4) have been utilized to fabricate different types of memory device, such as volatile DRAM and SRAM devices, and non-volatile WORM and Flash devices.181–186 A device with the sandwich structure ITO/PFOxPy/Al could write, read, erase and refresh its electronic states, fulfilling the functionality of a DRAM device. Its memory behavior was attributed to space charge and de-trapping effects.
A device based on a fluorene conjugated polymer with electron-accepting oxidiazole (PCFO) exhibited WORM memory effects,67 with a switching threshold voltage of −2.3 V and an ON/OFF current ratio of 105. Another polyfluorene-based copolymer (PFTPACN) containing electron-rich TPA and electron-poor cyano substituents on the side chain also exhibited typical WORM memory characteristics.184 In addition, non-volatile rewritable memory behavior was observed in a device based on conjugated poly[9,9-bis(4-diphenylaminophenyl)-2,7-fluorene] donors covalently bridged with Disperse Red 1 acceptors (DR1-PDPAF-DR1).181
Conjugated polymers with good solution processability have been used to fabricate flexible memory devices. A conjugated polymer with fluorene and thiophene donors in the main chain and the phenanthro[9,10-d]-imidazole (PFT-PI) acceptor in the side chain was employed as the active layer for a flexible device.183 The fabricated PEN/Al/PFT-PI/Al flexible device exhibited non-volatile rewritable memory behavior. The J–V characteristics did not change significantly before and after continuous bending stress. Subsequently, the tunable electrical switching characteristics of the vinylene-based conjugate polymers, PVC-PI, PVT-PI, and PVTPA-PI, consisting of different donors of carbazole (C), thiophene (T) and triphenylamine (TPA) and the acceptor phenanthro[9,10-d] imidazole (PI) were demonstrated.185 The donor structure affects the polymer conformation, the D–A electrical interaction, and the LUMO energy levels for stabilizing the charge separation. The PEN/Al/PVC-PI/Al flexible device exhibited SRAM behavior while the PVTPA-PI device exhibited WORM behavior. However, the PVT-PI device only exhibited diode-like electrical behavior. The PVC-PI and PVTPA-PI flexible memory devices could operate at low voltages (less than <2.5 V) with high ON/OFF current ratios (>104) and exhibited excellent durability in repeated bending tests.
Benzodithiophene (BDT)-based conjugated polymers that exhibit good performance in both organic field-effect transistors and solar cells have also been used in polymer memory.186 A series of BDT-based D–D and D–A conjugated polymers have been prepared (Figure 1.16). Reliable and reproducible WORM memory properties were demonstrated in ITO/CPs/Al flexible memory devices. Conjugated polymers with different backbones and side chains exhibited different stabilities of their charge-separated states. Conjugated polyelectrolytes,187 poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)188,189 and doped conjugated polymers73,190 have also been used as switching media for flexible devices. McCreery et al. fabricated redox-based polymer memristors using polythiophene as the electroactive component and ethylviologen perchlorate as the redox counter-electrode material.190 The devices exhibited switching immediately after fabrication and did not require the “electroforming” step required in many types of memory device. The conductivity of the polythiophene layer could be reversibly switched between high and low conductance states with a “write–erase” (W–E) bias, with only minor degradation of response after 200 W–E cycles.
A poly(Schiff base) is a conjugated polymer with imine groups (C=N) in its backbone. Li et al. fabricated an organic resistive switching memory device based on protonic-acid-doped polyazomethine (PA-TsOH).73 The resistance of this memory device can be controlled gradually and exactly via manipulation of the doping level of the PA:TsOH system. The uniformity in the device performance is superior to that reported for RRAM devices, suggesting that controllable protonic doping is an effective way to optimize the uniformity of the resistive switching behavior. By applying different RESET voltages to the Pt/PA-TsOH/Pt devices, the memory device can be programmed into four resistive states consecutively for more than 700 switching cycles, increasing the data storage capacity exponentially.
18.104.22.168 Polymers Chemically Modified with Fullerenes or Graphene
Carbon nanomaterials such as fullerenes, graphene and their derivatives have exhibited good performance in optoelectronic devices. PVK with covalently attached fullerene (PVK–C60),58 in which the carbazole group serves as the electron donor and hole-transporting moiety, and C60 serves as the electron acceptor, has been synthesized. The fabricated ITO/PVK–C60/Al device exhibited non-volatile rewritable memory behavior. Ree et al. also attached a fullerene acceptor covalently to a carbazole containing polymer in a controlled manner.191 With C60 acting as a charge trap, poly(2-(N-carbazolyl)ethyl methacrylate) end-capped with fullerene (PCzMA-C60) exhibited both bipolar and unipolar rewritable memory characteristics. By introducing 10 wt% PCBM into poly-4-methoxytriphenylamine,61 Liou et al. demonstrated a WORM memory device with lower switching-ON voltage (0.9 V) and much higher ON/OFF ratio (109) than those of the blended composites.
Chemical functionalization also plays a key role in tailoring the structure, processability, and physicochemical and electronic properties of GO nanosheets. By using the “grafting to” or “grafting from” method, a variety of electroactive polymers have been covalently incorporated onto the surface as well as the edge of GO nanosheets to fabricate graphene-based polymer memory devices.113–115,192–195 Triphenylamine (TPA)-based conjugated polyazomethine, covalently grafted to GO (TPAPAM–GO),113 has been used directly for fabricating memory devices (Figure 1.17). With efficient hole injection, high carrier mobilities and the low ionization potential of the TPAPAM polymer chains, the devices exhibited bistable electrical switching and non-volatile rewritable memory effects, with a small switch-on voltage of about −1 V and an ON/OFF current ratio of more than 103. The CT state of TPAPAM–GO is effectively stabilized by electron delocalization in the graphene nanosheets, which leads to the non-volatile nature of the memory device.
A PANI-functionalized GO derivative (GO–PANI) was prepared via in-situ oxidative graft polymerization of aniline on GO nanosheets.195 After growing PANI from the surface of GO, spindle-shaped PANI nanofibers with large length-to-diameter ratios appear to surround the GO nanosheets and act as a tunneling barrier for electrons moving from one GO sheet to another. The Al/GO–PANI/ITO device exhibited typical bistable electrical switching and non-volatile rewritable memory effects, which can be attributed to electric field-induced charge transfer between the PANI chains and the GO sheets. Covalent functionalization of GO with electroactive polymers is thus an effective and versatile approach to tuning the electronic properties of GO. In addition, the conductance of PANI can be further manipulated by ionic doping with mineral as well as organic acids, demonstrating great potential for the development of novel ionic electronics.
22.214.171.124 Polymers Containing Metal Complexes
When introduced into polymer backbones or pendants, the electrical affinity of transition-metal complexes can improve the stability of conductive states, and their reversible redox properties can make a polymer memory device more suitable for practical applications.196 Among various transition-metal complexes, ferrocene (Fe2+) is of particular interest because of its well-studied reversible redox behavior, and its stable oxidized form ferrocenium (Fe3+), which renders the possibility of non-volatility for memory applications. Choi et al. synthesized a new conjugated polymer (PFT2-Fc) with redox-active functionality, ferrocene, in the polymer main chain (Figure 1.18).197 The fabricated Al/LiF/PFT2-Fc/ITO device exhibited non-volatile rewritable memory behavior. Its switching mechanism involves ferrocene acting as a voltage-dependent in-situ dopant during the redox process to result in the enhancement of conductivity of the polymer film.
Ling et al. coupled 1.3 mol% of a rare-earth metal complex (Eu-complex) with a carbazole donor to synthesize the D–A type functional copolymers, PKEu (Scheme 1.5).112 The as-fabricated transparent and uniform film avoided the phenomena of ion aggregation and phase separation commonly found in mixed or doped systems. The Al/PKEu/ITO device exhibited non-volatile rewritable memory behavior with an ON/OFF ratio as high as 104. Transient current density versus time measurements indicated that the device transition occurred in less than 20 µs. They further introduced 1,3,4-oxadiazole moieties (electron acceptors) into the polymer chains, to obtain a new triblock copolymer, PCzOxEu.198 The device based on PCzOxEu exhibited much better rewritable memory performance, with an ON/OFF current ratio up to 105, a switching response time of ∼1.5 µs, more than 106 read cycles and a retention time of more than 8 h.
Non-volatile flash memory devices using polyfluorenes containing Ir(iii) complexes in the main chain (iamP1–iamP3) as the active material have been reported.60 The fluorene moieties act as electron donors and the Ir(iii) complex units as the electron acceptors. Charge transfer and traps in the polymers are probably responsible for the conductance-switching behavior and the memory effect. Polycarbazole and polyfluorene containing cationic Ir(iii) complexes in their side chains also exhibited electrical bistability, and non-volatile flash memory devices based on them have been realized.199 Polymer iamP6 comprises flexible spacers (O=C–O–C–C units) bridging the electroactive pendent chromophore carbazoles to permit conformational change under an electric field and a donor–acceptor system containing carbazole as donor and an Ir(iii) complex as acceptor to permit charge transfer. The ITO/iamP6/Al device exhibited excellent ternary memory performance, including low reading, writing and erasing voltages, and good stability for the three states.
Higuchi et al. studied the memory behavior of a Co(iii)-containing conjugated (CP) and non-conjugated polymer (NCP) with an azo-aromatic backbone (Figure 1.19).79 The I–V characteristics of the CP device traced in the voltage range ±5 V showed the current to jump from the initial low level to a high level at −5 V, as well as another jump from the high level to the initial level at +5 V. This bistable behavior with a “pinch-off” in the I–V characteristics is a typical feature of a memristive device. The CP device could be operated at different voltage levels with different current responses, therefore providing multilevel storage capability to replace the existing binary computing devices.
1.5.3 Organic–Inorganic Hybrid Materials
Generally, organic–inorganic hybrid materials are composed of organic layers containing fullerenes, carbon nanotubes, graphene, metal nanoparticles, semiconductor nanoparticles or inorganic quantum dots (QDs).
126.96.36.199 Organic–Carbon Allotrope Hybrid Materials
Fullerene and its derivatives possess high electron-withdrawing ability, and are able to capture up to six electrons. For organic electronic memory applications, they have been widely used as electron acceptors to form CT complexes with polymer-containing electron donors, such as thiophene, fluorene, carbazole and aniline derivatives.200–204 A hybrid material-based electrical memory device with the configuration rGO/P3HT:PCBM/Al exhibited electrically bistable behavior and the WORM memory effect, with an ON/OFF ratio of more than 104.200 The memory effect was attributed to the polarization of PCBM domains and the formation of a localized internal electrical field among the adjacent domains.
Chen et al. reported the fabrication of a memory device with the structure of poly[4-(9,9-dihexylfloren-2-yl)styrene]-block-poly(2-vinylpyridine) (P(St-Fl)-b-P2V):PCBM composites sandwiched between the top Al electrode and the bottom ITO electrode.201 The switching mechanism could be explained by a charge injection dominated thermionic emission model (OFF state) and a charge transfer dominated Poole–Frenkel (PF) emission model (ON state).
In the absence of electron donor moieties in hybrid materials, C60 molecules can be used as the charge storage medium.205–208 By simply changing the concentration of C60 in the insulating polystyrene (PS) matrix, the device with the structure Al/PS:C60/Al exhibits three distinctly different electrical behaviors, namely a true insulator (C60%(wt) = 1.0–5.0%), a rewritable memory device (C60%(wt) = 5.0–7.5%) with an ON/OFF current ratio larger than 104 and a WORM type memory device (C60%(wt) = 7.5–20%). Memory effects have also been observed in a memory device with C60 molecules dispersed in a poly(vinyl phenol) (PVP) matrix. This simple device exhibits distinct electrical hysteresis with low and high conductivity states. Charge transfer and retention in the C60 molecules are proposed to account for the electrical switching.
Polyimide (PI) has been used as a dielectric and mechanical-support material in the electronics industry due to its thermal stability, good chemical resistance, and excellent mechanical properties. Kim et al. fabricated organic non-volatile memory devices on PET flexible substrates using PI:PCBM hybrid materials as active layers.209 These flexible memory devices showed successful rewritable switching properties even under bent conditions. Three-dimensional (3D) stacking of memory devices provides a way to achieve a great increase in memory cell density. A 3D-stacked 8 × 8 cross-bar array of polymer resistive memory devices with the PI:PCBM hybrid material as the memory element has been fabricated using a spin-coating process.210 The memory cells in each layer exhibited excellent non-volatile memory performance, and they can be independently written, read and erased with a high ON/OFF ratio, good endurance and good retention capability.
Organic/polymer:PCBM hybrid materials have been integrated into one diode–one resistor (1D–1R) arrays to demonstrate practical implementation of organic memory.211,212 For instance, Kim et al. reported the first 64-bit organic flexible memory cell array in a 1D–1R architecture with photo-patternable hybrid memory (PS:PCBM) and diode (P3HT) systems.212 Figure 1.20 shows the integrated structure of the PEN/Al/PS:PCBM/Al/P3HT/Au/Al 1D–1R cell. A P3HT/Bis-FB-N3 solution was chosen as the diode material, and a PS/PCBM/Bis-FB-N3 mixture was used for the unipolar memory component. The 1D–1R cell array can avoid cross-talk problems and encode letters based on the standard ASCII character code.
Controllable electrical conductance switching and non-volatile memory effects have been observed in Al/PVK–CNT/ITO sandwich structures.57 Unique device behavior, including (i) insulator behavior, (ii) bistable electrical conductance switching effects (rewritable memory and WORM memory effects), and (iii) conductor behavior, can be realized in an ITO/PVK–CNT/Al sandwich structure by increasing the CNT content in the PVK–CNT composite film. The turn-on voltage of the bistable devices decreased with the increase in CNT content of the composite film.
Park et al. developed resistive-type non-volatile polymer memory devices for high temperature operation using hybrid nanocomposites of the conjugated block copolymer poly(styrene-block-paraphenylene) (PS-b-PPP) and single-walled carbon nanotubes (SWNTs) by simple solution blending.213 With certain SWNT concentration regimes, the nanocomposites exhibited bipolar non-volatile rewritable memory characteristics. High temperature operation of the device was realized at up to 100 °C without significant degradation of the memory performance.
Kim et al. fabricated mechanically flexible, multilevel switching resistive memory by solution casting of PS–chemically-doped multiwalled CNT composites.214 The controlled work function and high dispersibility of the substitutionally doped CNTs significantly improved the resistive memory performance of the PS:CNT composite devices. As shown in Figure 1.21, devices consisting of quadruple layers of Al/PS:BCNT/PS:NCNT/Al were prepared on polyimide substrates (BCNT = boron-doped CNT, NCNT = nitrogen-doped CNT). The flexible device with a quadruple layer exhibited stable triple state memory characteristics (i.e., low resistance state (LRS), high resistance state (HRS) and interstate (IRS)), unlike triple layer devices consisting of Al/PS:BCNT or PS:NCNT/Al. The ON/OFF ratios for LRS/IRS and IRS/HRS were 50 and 15, respectively. The switching mechanism of the CNT:PS composite memory devices follows a charge storage mechanism by trapping and de-trapping.
Graphene nanosheets have been of particular interest for application in organic–inorganic hybrid material-based memory devices.215–221 Figure 1.22 shows a schematic structure of the hydrogen bonding between poly(styrene-block-4-vinylpyridine) (PS-b-P4VP) and GO sheets.217 The ITO/PS-b-P4VP:7 wt% GO composite/Al device exhibits a WORM memory effect with an ON/OFF ratio of 105 at −1.0 V. The switching mechanism was attributed to the charge trapping environment operating across the PS-b-P4VP/GO interface and due to the GO intrinsic defects. Controlling the physical interaction of the copolymers and functional GO nanosheets can generate a well-dispersed charge storage composite device for future flexible information technology.
An interconnected, one-dimensional/two-dimensional lamellar film was produced through encapsulation of self-assembled tetracene-derived organic wires by GO nanosheets.215 By combining the hole transporting ability of the p-type tetracene semiconductor and the charge trapping properties of GO, the memory devices exhibited reproducible and non-volatile electrical bistability. Zhang et al. demonstrated resistive memory effects in a device with the structure ITO/PVK:graphene/Al.218 Devices with 2 wt% graphene in the composite exhibited the WORM memory effect, while devices with 4 wt% graphene showed rewritable memory effects. The conductance switching effects of the composites can be attributed to electron trapping in the graphene nanosheets of the electron-donating/hole-transporting PVK matrix.
Figure 1.23 shows a schematic diagram of an Al/polymer/graphene/polymer/ITO memory device, which was fabricated by laminating two glass substrates coated with patterned electrodes and spacers.216 Devices based on the Al/PS/graphene/PS/ITO sandwich structure exhibit WORM memory behavior, while devices with the structure Al/PVK/graphene/PVK/ITO show volatile memory behavior. The distinct electrical behavior of the two devices is probably related to the different depths of charge traps formed between the graphene sheets and polymer matrix, and provides a new route to tailor the memory effects of hybrid bistable devices.
Li et al. systematically investigated the rewritable memory effects of devices based on polyimide (PI) and graphene oxide (GO) with the structure Ag/PI/GO:PI/PI/ITO.220 The stacked layer structure for the GO–PI film indicates that the GO nanosheets are well-packed during the spin coating process. The I–V curves for the as-fabricated device exhibit multilevel resistive-switching properties under various reset voltages. Multilevel conduction states arose from the varying filling degrees of traps in the active layer at different reset bias.
188.8.131.52 Organic–Inorganic Nanocomposites
Hybrid electronic memory devices have been reported in some organic composites containing metal nanoparticles (NPs), quantum dots (QDs) and metal oxide NPs.222–227 Yang et al. fabricated a memory device with the structure of a Au NP and 8-hydroxyquinoline-containing PS film sandwiched between two metal electrodes.8 Electronic transitions were attributed to electric field-induced charge transfer between the Au NPs and 8-hydroxyquinoline. Au NPs can also be introduced into electroactive polymers such as P3HT,54 PANI224 and PVK226 to realize memory behavior. A rewritable memory effect was demonstrated in a hybrid memory device with an active layer consisting of P3HT and Au NPs capped with 1-dodecanethiol sandwiched between two metal electrodes.54 The device was fabricated through a simple solution processing technique and exhibited remarkable electrically bistable behavior.
In such electroactive polymer–metal nanoparticles hybrid systems, the electroactive polymers usually act as electron donors while the metal nanoparticles act as electron acceptors; the electrical bistability is related to electric field-induced charge transfer between the two components. Thus, the compatibility of the two components is crucial for optimal device performance. Yang et al. reported a non-volatile rewritable memory device based on polyaniline (PANI) nanofibers decorated with Au NPs.55 The active polymer layer was created by growing nanometer sized gold particles within the PANI nanofibers using a redox reaction with chloroauric acid. The solution syntheses of PANI nanofibers with different sizes of autoreduced Au NPs were also investigated and the relationship between Au NP size and bistable memory response was evaluated (Figure 1.24).224 The performance of devices made from four different solutions, with the Au NPs in four distinct size ranges is shown in Figure 1.24g.
Metal NP–polymer hybrids have also been used in organic polymer floating-gate transistor memory.82,85 Organic memory devices consisting of P3HT electrospun nanofiber transistors functionalized with surface-modified Au NPs have been reported.82 Figure 1.25 illustrates a prototypical electrospun P3HT:Au hybrid nanofiber-based transistor memory device on a PEN substrate with a bottom-gate top-contact configuration using thermally evaporated Au as back gate. The device remains reliable and stable even under bending conditions (radius: 5–30 mm) or after 1000 repetitive bending cycles. The high performance P3HT:Au hybrid nanofiber-based transistor memory devices allow their integration in future flexible logic circuits.
Other kinds of inorganic nanomaterials, such as ZnO, TiO2 and MoS2, have also been introduced into the organic polymer layer for hybrid memory devices. For instance, Kim and co-workers investigated the electrical properties and carrier transport mechanisms of memory devices fabricated by utilizing ZnO NPs embedded in PVK or PMMA layers as active materials.223,225 Kim et al. fabricated an 8 × 8 array structured device using TiO2 NPs embedded in a PVK film.228 By varying the concentration of TiO2 NPs, the ON/OFF ratio of the devices could be modulated. More recently, Liou et al. fabricated memory devices using hydroxy-containing fluorene-based polyimide hybrids with tunable TiO2 contents.229 The hybrid memory devices with different TiO2 concentrations from 0 wt% to 50 wt% exhibited tunable memory properties from DRAM to SRAM to WORM with an ON/OFF current ratio as high as 108. The unique energy level and quantum confinement effects also render MoS2 as a charge trapping component for memory applications. Zhang et al. developed a facile method for exfoliation and dispersion of MoS2 with the aid of polyvinylpyrrolidone (PVP).230 The flexible device fabricated by a solution processing technique with the configuration rGO/PVP:MoS2/Al exhibited electrical bistability and a non-volatile rewritable memory effect.
Yolk–shell type nanospheres, consisting of a movable Au nanocore in the hollow cavity of a hairy electroactive polymer shell (Au@air@PTEMA-g-P3HT hybrid nanorattles, PTEMA = poly(2-(thiophen-3-yl)ethyl methacrylate), P3HT = poly(3-hexylthiophene)) can be readily dispersed in toluene and uniformly integrated into polystyrene (PS) thin films (Figure 1.26).231 By controlling the nanorattle content in the composite film, the corresponding devices are capable of exhibiting unique device behaviors, including insulating behavior (with 5 wt% nanorattles), WORM (with 10 wt% nanorattles) and rewritable memory effects (with 25 wt% nanorattles), and conductive behavior (with 50 wt% nanorattles). In the structure of the Au@air@PTEMA-g-P3HT hybrid nanorattles, the PTEMA-g-P3HT shell can act as an electron donor, with the Au nanocore as an electron acceptor. The switching mechanism of the hybrid devices can probably be attributed to electric field-induced charge transfer from the P3HT shell to the Au nanocore.
Kim et al. investigated the electrical bistability and operating mechanism of an organic–inorganic hybrid device consisting of CdSe–ZnS core–shell nanoparticles embedded in a PVK layer.232 The devices exhibited non-volatile electrically bistable behavior. Similar electrical behavior was also observed in memory devices involving CuInS2(CIS)–ZnS core–shell type QDs blended with a PVK layer.233 Pal et al. investigated the bistability of a memory device based on core–shell CdS:PB (phloxine B) hybrid nanoparticles.234 The device based on core–shell nanoparticles exhibited a higher ON/OFF ratio and a lower switching threshold voltage than those based on the individual components.
1.6 Conclusions and Outlook
Research on novel memory materials, device structures and mechanisms is currently at a rapid growth stage, since it is recognized that organic-based electrical memory devices may be an alternative or supplemental technology to the conventional memory technologies facing the problem of miniaturization from micro- to nanoscale. By using organic/polymeric materials as storage media, flexible and miniaturized memory devices with simple structure can be fabricated with particular ease through solution processing. More importantly, the electronic properties of organic/polymeric materials, and thus the device performance, can be tailored through molecular design cum chemical synthesis. This chapter provided an introduction to the basic concepts, history, device structures and switching effects associated with organic/polymer memory, and systematically summarized recent advances in organic/polymer memory materials and device performance.
Although significant advances have been made in the field of organic/polymer electronic memory over the past decade, it is still at the early stage of development in comparison to commercially viable inorganic devices. The immediate challenges facing organic electronic memory devices are the fabrication of organic thin film devices with reproducible switching and transport properties, and their integration into addressable memory arrays. The relationships among the materials structure, device parameters and electrical transition phenomena should be further explored to understand the solid-state physics and electronics. Refinements in materials design and preparation methods, and improvements in device fabrication, characterization and integration techniques will be needed to advance organic memory technology. In spite of the great challenges faced by memory devices, organic/polymer electronic memory proves to be one of the most promising electronic technologies of the 21st century.