polymers Review DevelopmentReview of Conjugated Polymers for Memory DeviceDevelopment Applications of Conjugated Polymers for Memory Device Applications Hung-Ju Yen 1,*, Changsheng Shan 1, Leeyih Wang 2, Ping Xu 3, Ming Zhou 4 1, andHung-Ju Hsing-Lin Yen 1, *,Wang Changsheng * Shan 1, Leeyih Wang 2, Ping Xu 3, Ming Zhou 4 1, and1 Physical Hsing-Lin Chemistry Wang and* Applied Spectroscopy (C-PCS), Chemistry Division, Los Alamos National 1 Laboratory,Physical Chemistry Los Alamos, and NM Applied 87545, Spectroscopy USA; [email protected] (C-PCS), Chemistry Division, 2 CenterLos Alamos for Condensed National Matter Laboratory, Science, Los National Alamos, Taiwan NM 87545, University, USA; [email protected] 1 Roosevelt Road, 4th Sec., Taipei 10617, 2 Taiwan;Center [email protected] for Condensed Matter Science, National Taiwan University, 1 Roosevelt Road, 4th Sec., 3 SchoolTaipei of 10617, Chemistry Taiwan; and [email protected] Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China; 3 [email protected] of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China; 4 [email protected] of Chemistry, Northeast Normal University, Changchun 130024, China; 4 [email protected] of Chemistry, Northeast Normal University, Changchun 130024, China; [email protected] * Correspondence:Correspondence: [email protected] [email protected] (H.-J.Y.); (H.-J.Y.); [email protected] [email protected] (H.-L.W.); (H.-L.W.); Tel.:Tel.: +1-505-310-2898 +1-505-310-2898 (H.-J.Y.); (H.-J.Y.); +86-0755-8801-8901 +86-0755-8801-8901 (H.-L.W.) (H.-L.W.)

Academic Editor: Do-Hoon Hwang Received: 20 November 2016; Accepted Accepted:: 8 January 2017; Published: date 12 January 2017

Abstract: ThisThis review review summarizes summarizes the the most most widely widely used used mechanisms mechanisms in inmemory memory devices devices based based on conjugatedon conjugated polymers, polymers, such such as charge as charge transfer, transfer, space space charge charge traps, traps, and and filament filament conduction. conduction. In addition,In addition, recent recent studies studies of conj ofugated conjugated polymers polymers for memory for memory device applications device applications are also reviewed, are also discussed,reviewed, discussed,and differentiated and differentiated based on based the mech on theanisms mechanisms and structural and structural design. design. Moreover, Moreover, the electricalthe electrical conditions conditions of conjugated of conjugated polymers polymers can can be be further further fine-tuned fine-tuned by by careful careful design design and synthesis based on the switching mechanisms. Th Thee review also emphasizes and demonstrates the structure-memory properties relationship of donor-acceptordonor-acceptor conjugated polymers for advanced memory device applications.

Keywords: conjugatedconjugated polymer; polymer; memory memory device; device; dynamic dynamic random random access access memory memory (DRAM); (DRAM); static randomstatic random access access memory memory (SRAM); (SRAM); write-once write-once read-many-times read-many-times (WORM); (WORM); flash flash

1. Introduction

1.1. Conjugated Polymers (CPs) Conjugated polymers (CPs) are organic macromoleculesmacromolecules characterized by a backbone chain consisting ofof alternating alternating single- single- and double-bonds.and double-bonds. Their overlapping Their overlappingp-orbitals createp-orbitals a delocalization create a delocalizationsystem of π-electrons system thus of resultingπ-electrons in interesting thus resulting and useful in interesting optoelectronic and properties. useful optoelectronic The simplest properties.CP, polyacetylene, The simplest constitutes CP, polyacetylene, the core of all constitu conjugatedtes the polymers core of although all conjugated it is itself polymers too unstable although for itany is practicalitself too applications unstable for (Scheme any practical1). Owing applications to its structural (Scheme and electronic 1). Owing simplicity, to its structural polyacetylene and electronicis well suited simplicity, to ab initiopolyacetylene and semi-empirical is well suited calculations, to ab initio whichand semi-empirical has played a calculations, critical role which in the hastheoretical played aspectsa critical of role CPs in [1 the]. theoretical aspects of CPs [1].

Scheme 1. 1. StructuresStructures of of conjugated conjugated polymers polymers (a (a) )trans- trans- and and (b (b) )cis-polyacetylene; cis-polyacetylene; and and (c ()c polythiophene.) polythiophene.

Polymers 2017, 9, 25; doi:10.3390/polym9010025 www.mdpi.com/journal/polymers Polymers 2017, 9, 25; doi:10.3390/polym9010025 www.mdpi.com/journal/polymers Polymers 2017, 9, 25 2 of 17 Polymers 2017, 9, 25 2 of 16 Polymers 2017, 9, 25 2 of 17 Attention on π-CPs has increased over the years [2–6]. The extensive delocalization of π electrons in CPs is well recognized to be responsible for their remarkable characteristics, including Attention on π-CPs has increased over the years [2 [2–6].–6]. The extensive delocalization of π electronic conductivity, interesting optical characteristics, and exceptional mechanical properties electrons in CPs is well recognized to be responsibleresponsible forfor theirtheir remarkableremarkable characteristics,characteristics, including [7,8] such as high tensile strength and resistance to harsh environments. CPs comprising aromatic or electronic conductivity,conductivity, interesting interesting optical optical characteristics, characteristics, and and exceptional exceptional mechanical mechanical properties properties [7,8] hetero-aromatic ring structures have also been considered particularly as outstanding materials. [7,8]such such as high as high tensile tensile strength strength and and resistance resistance to to harsh harsh environments. environments. CPs CPscomprising comprising aromaticaromatic or Moreover, applications of CPs in advanced aerospace technology have provided a powerful hetero-aromatic ring structures have also been co considerednsidered particularly as outstanding materials. materials. motivation for the development of functionalized polyhetero-aromatics [9]. Later investigations Moreover, applicationsapplications of of CPs CPs in advanced in advanced aerospace aerospace technology technology have provided have provided a powerful a motivationpowerful were directed to rigid rod polybenzobisazoles (Scheme 2). motivationfor the development for the development of functionalized of functionalized polyhetero-aromatics polyhetero-aromatics [9]. Later investigations [9]. Later were investigations directed to wererigid roddirected polybenzobisazoles to rigid rod polybenzobisazoles (Scheme2). (Scheme 2).

Scheme 2. Structures of typical rigid rod polybenzobisazoles. Scheme 2. Structures of typical rigid rod polybenzobisazoles. From an electrochemical perspective, the most important aspect of CPs is their capability as electronic conductors. Therefore, π-electron polymers have been the focus of extensive research [10], From an electrochemical perspective, the most im importantportant aspect of CPs is their capability as ranging from applications of conventional CPs (as shown in Scheme 3) in energy storage devices, to electronic conductors. Therefore, Therefore, π-electron polymers have been been the the focus focus of of extensive extensive research research [10], [10], novel CPs with special electronic properties such as low band-gap and intrinsic conductivity. ranging from from applications applications of of conventional conventional CPs CPs (as (as shown shown in in Scheme Scheme 3)3 in) in energy energy storage storage devices, devices, to Indeed, many successful commercial applications of CPs have been available for more than fifteen novelto novel CPs CPs with with special electronicelectronic properties properties such such as lowas low band-gap band-gap and intrinsic and intrinsic conductivity. conductivity. Indeed, years, including capacitors, batteries, magnetic storage, electrostatic loudspeakers, and Indeed,many successful many successful commercial commercial applications applications of CPs of have CPs been have available been available for more for thanmore fifteen than fifteen years, anti-static bags. years,including including capacitors, capacitors, batteries, magneticbatteries, storage, magnetic electrostatic storage, loudspeakers, electrostati andc loudspeakers, anti-static bags. and anti-static bags.

Scheme 3. Structures of conventional conjugated polymers. Scheme 3. Structures of conventional conjugated polymers. Scheme 3. Structures of conventional conjugated polymers. Recently, CPs have been considered as electroactiveelectroactive memory materials and reported as revealingRecently, electrically CPs have volatile volatile been and and considered non-volatile non-volatile as memory memoryelectroactive characteristics. characteristics. memory Thematerials The conjugated conjugated and reportedbackbone backbone asis revealingisthe the most most important electrically important component volatile component and since sincenon-volatile it contributes it contributes memory to most to characteristics. most of the of theelectrical electrical The pr conjugatedoperties. properties. In backbonegeneral, In general, the is theincorporation most incorporation important of different of component different electron electron since acceptors it acceptors contributes into into toCPs CPs most significantly significantly of the electrical affects affects pr theoperties. the memory memory In general, properties, properties, the whichincorporation can either either of differentcreate create a a trappingelectron trapping acceptorssite site or or provide provide into CPs the the significantlycharge charge transf transfer affectser (CT) (CT) the conducting conductingmemory channel.properties, channel. In whichInaddition, addition, can the either the stability stability create of ofacharge chargetrapping trapping trapping site or or orprovide the the CT CT complex the complex charge process, process, transf furtherer further (CT) determines determinesconducting the the channel. volatility volatility In addition,of the memory the stability device. of charge trapping or the CT complex process, further determines the volatility of the memory device. 1.2. Resistor-Type ElectronicElectronic MemoriesMemories 1.2. Resistor-TypeThe basic basic goal goal Electronic of of a amemory memory Memories device device is to is provid to providee a means a means for storing for storing and accessing and accessing binary binarydigital digital data sequences of “1” and “0”, being one of the core functions (primary storage) of modern data Thesequences basic goal of of“1” a memoryand “0”, device being is oneto provid of thee acore means functions for storing (primary and accessing storage) binary of modern digital computers. An electronic memory is a form of semiconductor storage, fast in response, compact in size, datacomputers. sequences An electronicof “1” and memory “0”, being is a form one of of se thmiconductore core functions storage, (primary fast in response,storage) compactof modern in and can be read and written when coupled with a central processing unit (CPU). In the conventional computers.size, and can An beelectronic read and memory written is awhen form coupledof semiconductor with a central storage, processing fast in response, unit (CPU). compact In the in silicon-based electronic memory, data are stored based on the amount of charge stored in the memory size,conventional and can silicon-basedbe read and electronicwritten when memory, coupled data with are astored central based processing on the unitamount (CPU). of chargeIn the cells. On the contrary, organic/polymer electronic memory stores data in an entirely different way, conventionalstored in the memorysilicon-based cells. electronicOn the contrary, memory, orga danic/polymerta are stored electronic based onmemory the amount stores dataof charge in an stored in the memory cells. On the contrary, organic/polymer electronic memory stores data in an

Polymers 2017, 9, 25 3 of 17 Polymers 2017, 9, 25 3 of 16 entirely different way, for instance, based on the different electrical conductivity states (ON and OFFfor instance, states) in based response on the to differentthe applied electrical electric conductivity field. In particular, states (ON polymeric and OFF memory states) devices in response with anto theelectrically applied bi-stable electric field.behavior In particular, have received polymeric considerable memory atte devicesntion recently with an due electrically to their attractive bi-stable characteristicsbehavior have receivedsuch as considerable rich structure attention flexib recentlyility, duelow to theircost, attractivesolution characteristicsprocessability, such and as three-dimensionalrich structure flexibility, stacking low capability. cost, solution Therefore, processability, the organic/polymer and three-dimensional electronic memory stacking is capability. likely to beTherefore, alternative the or organic/polymer supplementary electronicto the conv memoryentional is semiconductor likely to be alternative electronic or memory. supplementary to the conventionalElectronic semiconductor memories can electronic be generally memory. divided into two primary categories according to the storageElectronic type: volatile memories and can non-volatile be generally memories divided into(Figure two primary1). Volatile categories memory according eventually to the loses storage the storedtype: volatileinformation and non-volatileunless it is provided memories with (Figure a constant1). Volatile power memory supply eventually or refreshed loses periodically the stored withinformation a pulse; unless non-volatile it is provided memory with ais constant capable power of holding supply ordata refreshed permanently periodically and withbeing a pulse;read repeatedly.non-volatile memoryAmong isthese capable types of holdingof electronic data permanently memories, andwrite-once being read read-many-times repeatedly. Among (WORM) these memorytypes of electronic[11], hybrid memories, non-volatile write-once and rewritable read-many-times (flash) memory (WORM) [12], memory static random [11], hybrid access non-volatile memory (SRAM)and rewritable and dynamic (flash) memory random [12 access], static memory random access(DRA memoryM) are the (SRAM) most andwidely dynamic reported random polymer access memoriesmemory (DRAM) [13,14]. are the most widely reported polymer memories [13,14].

Electronic Memory

Non-volatile Memory Volatile Memory

ROM Hybrid RAM

WORM EPROM Flash EEPROM DRAM SRAM

Figure 1. ClassificationClassification of electronic memories.memories. ROM: ROM: read-only memory; RAM: random-access memory; EPROM: EPROM: erasable programmable read-onlyread-only memory; EEPROM: electrically erasable programmable read-only memory.

Memory devices incorporating switchable resistive materials are generally classified classified as resistor-type memory, or resistiveresistive randomrandom access memory (RRAM). Unlike transistor and capacitor memories, aa resistor-type resistor-type memory memory does does not requirenot require a specific a specific cell structure cell structur (e.g., field-effecte (e.g., field-effect transistor; transistor;FET) or to FET) be integrated or to be withintegrated the complementary with the complementary metal-oxide-semiconductor metal-oxide-semiconductor (CMOS) technology. (CMOS) technology.The electrical The bi-stability electrical ofbi-stability resistor-type of resistor-type memories memories usually results usually from results the changesfrom the inchanges intrinsic in intrinsicproperties properties of electroactive of electroactive materials inmaterials response in to re thesponse applied to voltagethe applied or electric voltage field, or suchelectric as charge field, suchtransfer, as charge phase transfer, change, conformationphase change, change, conformation and redox change, reaction and [redox15]. reaction [15]. The important parameters to to the memory perf performanceormance include include switching switching (write (write and and erase) time, ON/OFF ON/OFF current current ratio, ratio, read read cycles, cycles, and and retent retentionion time. time. The The switching switching time time influences influences the rate the torate write to write and andaccess access the thestored stored information; information; the the ON/OFF ON/OFF current current ratio ratio defines defines the the control control of of the misreading raterate duringduring devicedevice operation; operation; while while the the number number of of read read cycles cycles and and long long retention retention time time are arerelated related to the to stabilitythe stability and reliabilityand reliability of the of memory the memory devices. devices. For practical For practical applications, applications, other factors, other factors,such as powersuch as consumption power consumption and cost, and structural cost, structural simplicity simplicity and packing and density, packing as density, well as mechanical as well as mechanicalstiffness and stiffness flexibility, and are flexibility, of equal importance are of equa whenl importance designing when and fabricatingdesigning newand memoryfabricating devices. new memoryConsiderable devices. efforts Considerable have been efforts devoted have to developbeen devoted novel to CPs develop for information novel CPs andfor information communication and communicationtechnologies [16 –techno21]. logies [16–21].

1.2.1. Operation Mechanism Many research works have been dedicated to understanding the electric switching phenomena of memory memory devices. devices. Although Although this this field field is still is still controversial, controversial, researchers researchers have have proposed proposed several several well wellestablished established switching switching mechanisms mechanisms based based on theoretical on theoretical simulations, simulations, experimental experimental results, results, and advancedand advanced analytical analytical techniques techniques [15,22–27]. [15,22 –In27 this]. In review, this review, we summarize we summarize the most the mostwidely widely used

Polymers 2017, 9, 25 4 of 16 used mechanisms in CP resistive memory devices, such as charge transfer, space charge traps, and filamentary conduction.

Charge Transfer (CT) CT can be clarified as a process of partial transfer of electronic charge from the donor (D) to the acceptor (A) moiety in the electron D-A system by applying a suitable voltage, which can result in a sharp increase in conductivity [28]. In order to obtain a better understanding of switching mechanisms, several study methods, such as density functional theory (DFT) calculations, ultraviolet-visible (UV/Vis) absorption spectra, in-situ fluorescence spectra, and transmission electron microscope (TEM) techniques, can be used to investigate and explain the CT phenomenon [29–32]. CT is anticipated to occur most frequently in D-A polymers [33–35]. The memory behaviors based on the D-A polymers can be tuned adequately through modification of the polymer structures. By tuning the electron-donating or -accepting capability of D-A polymers, different memory behaviors can be achieved [36]. The strong dipole moment in a polymer is also beneficial to maintain the conductive CT state, usually leading to non-volatile memory behavior. Otherwise, the conductive CT state is not stable after removing the electric field and a volatile memory characteristic will be observed if the dipole moment is not strong enough.

Space Charge Traps When the interface between electrode and polymer is ohmic and the polymer is trap-free, the carriers near the electrode will accumulate and build up a space charge channel. Mutual repulsion between individual charges restricts the total charge injected into the polymer, and the resulting current is defined as space charge-limited current (SCLC). Space charges in materials may occur from several sources, such as (1) electrode injection of electrons and/or holes; (2) ionized dopants in interfacial depletion regions; and (3) accumulation of mobile ions at electrode interfaces. Traps may be present in the bulk materials or at interfaces, and result in lower carrier mobility. When present at interfaces, they may also affect charge injection into the materials. The electrical switching behaviors of some polymeric materials have been reported to be associated with space charges and traps [30].

Filament Conduction Particularly, when the ON state current is highly localized to a small area within the memory device, the phenomenon can be termed “filament conduction”. It has been suggested that filament conduction is confined to device physical damage in RRAMs. Two types of filament conduction have been widely reported in polymer resistive memory devices, and the formed filaments could be observed under an optical microscope or scanning electron microscope [37,38]. One type is carbon-rich filaments formed by local degradation of polymer films [38,39]. The other is associated with metallic filaments that result from migration of electrodes through the polymer films [40,41]. For filamentary conduction, the polymers with both the coordinating atom and π-conjugation can bind to metal ions, regardless of the binding sites as side chain or main chain, are essential for the production of metal filaments [15,42]. Therefore, the filamentary conduction mechanism has been often suggested to explain switching phenomena observed in a variety of polymer memory devices. However, the severe current leakage caused by the filament effect is the main factor in restricting the exploration of memory mechanism. Therefore, some literature examples discussing how to reduce the filament effect have been reported [43,44]. Unfortunately, although the device concept is simple, the physics is anything but. There are controversies surrounding the conductive filament and the role of the top and the bottom electrodes. The mobility, energy, and stability of the oxygen vacancies remain topics of intense study. As a result of these open issues, projection of device reliability becomes difficult. Furthermore, the switching mechanisms in different references of similar structures are always various, and they need to be unique from both an understanding point of view as well as for application. RRAMs have issues on reproducibility of their electrical characteristics; there are large resistance variations not just Polymers 2017, 9, 25 5 of 17 16 Polymers 2017, 9, 25 5 of 17 of programming of the same device. Therefore, selecting the switching material and deposition of programming of the same device. Therefore, selecting the switching material and deposition methodbetween also devices, plays but an alsoimportant between role. cycles of programming of the same device. Therefore, selecting the methodswitching also material plays an and important deposition role. method also plays an important role. 2. CPs for Volatile Memory Devices 2. CPs CPs for for Volatile Volatile Memory Devices For volatile memory effects, the device cannot be kept at the ON state and will relax to the OFF For volatile memory effects, effects, the device cannot cannot be be kept kept at at the the ON ON state state and and will will relax relax to the OFF state after the power is turned off. Nevertheless, the ON state can be maintained by refreshing the statestate after the power is turned off. Nevertheless, Nevertheless, the the ON ON state state can can be be maintained by by refreshing the voltage pulse. Volatile memory effects can be divided into DRAM and SRAM, depending on the voltage pulse. Volatile Volatile memory memory effects effects can can be be divided into into DRAM and SRAM, depending on on the retention time of the ON state after removal of the applied voltage. For DRAM behavior (Figure 2) retentionretention timetime ofof thethe ON ON state state after after removal removal of of the th appliede applied voltage. voltage. For For DRAM DRAM behavior behavior (Figure (Figure2)[ 30 2)], [30], the ON state can only be retained for a short time (less than 1 min) after the removal of the [30],the ON the stateON state can only can beonly retained be retained for a shortfor a timeshort (less time than (less 1 min)than after1 min) the after removal the removal of the applied of the applied voltage. For SRAM (Figure 3) [45], the device can stay in the ON state for a longer period of appliedvoltage. voltage. For SRAM For (FigureSRAM (Figure3)[ 45], 3) the [45], device the device can stay can in stay the in ON the state ON forstate a longerfor a longer period period of time of time after turning off the power than in DRAM devices. Despite the longer retention time of the ON timeafter after turning turning off the off power the power than than in DRAM in DRAM devices. devi Despiteces. Despite the longer the longer retention retention time time of the of ON the stateON state in SRAM memory devices, it is still volatile, and the ON state will relax to the OFF state without statein SRAM in SRAM memory memory devices, devices, it is stillit is still volatile, volatile, and and the ONthe ON state state will will relax relax to the to the OFF OFF state state without without an an erasing process. anerasing erasing process. process.

Figure 2. Current-voltage (I–V) characteristics of the indium tin oxide (ITO)/polymer/Al memory Figure 2. Current-voltage (I–V) characteristics of thethe indiumindium tintin oxideoxide(ITO)/polymer/Al (ITO)/polymer/Al memory device as a representative of DRAM characteristic. (The third sweep was conducted about 1 min after device as a a representative representative of DRAM characteristic characteristic.. (The (The third third sweep was conducted conducted about 1 1 min min after after turning off the power). turningturning off off the power).

Figure 3.3. Current-voltageCurrent-voltage (I–V) (I–V) characteristics characteristics of the ITO/polymer/AlITO/polymer/Al memory memory device device as as a Figure 3. Current-voltage (I–V) characteristics of the ITO/polymer/Al memory device as a representative ofof SRAMSRAM characteristic.characteristic. (The (The third third sweep sweep was was conducted conducted about about 50 50 min min after after turning turning off representative of SRAM characteristic. (The third sweep was conducted about 50 min after turning offthe the power). power). off the power).

2.1. Dynamic Random Access Memory (DRAM) Properties 2.1. Dynamic Random Access Memory (DRAM) Properties P1 containing oxadiazole and bipyridine as acceptor units was synthesized by Suzuki coupling P1 containing oxadiazole and bipyridine as acceptor units was synthesized by Suzuki coupling and exhibited DRAM memory behavior with an ON/OFF ratio of more than 106 (Scheme 4) [14]. and exhibited DRAM memory behavior with an ON/OFF ratio of more than 106 (Scheme 4) [14].

Polymers 2017, 9, 25 6 of 16

2.1. Dynamic Random Access Memory (DRAM) Properties

PolymersP1 2017containing, 9, 25 oxadiazole and bipyridine as acceptor units was synthesized by Suzuki coupling6 of 17 and exhibited DRAM memory behavior with an ON/OFF ratio of more than 106 (Scheme4)[ 14]. The memorymemory effect effect was was volatile volatile due due to spaceto space charge char andge traps,and traps, resulting resulting in the short in the retention short retention ability of itsability ON of state. its ON The state. ON state The currentON state could current be electrically could be electrically sustained by sustained a refreshing by a voltage refreshing pulse voltage every 10pulse s. every 10 s. Devices with thethe sandwichsandwich structurestructure ofof ITO/ITO/P2/Al exhibited exhibited volatile volatile DRAM property with bi-stable electrical electrical switching switching characteristics, characteristics, which which is due is due to the to existence the existence of trapping of trapping sites in sites the inpoly(3-phenoxymethylthiophene) the poly(3-phenoxymethylthiophene) domains, domains, whereas whereas poly(3-hexylthiophene) poly(3-hexylthiophene) devices only devices showed only showedsemiconductor semiconductor characteristics characteristics [30]. This [30]. result This result suggested suggested the theimportance importance of of the the amorphous poly(3-phenoxymethylthiophene) segments segments on on the electrical switching behavior. Both the ON and OFF states of P2 are stablestable upup toto 101088 read cyclescycles underunder aa constantconstant voltagevoltage stressstress ofof −−1.0 V with a high ON/OFF current current ratio ratio of of about about 10 1066. . Ree etet al.al. alsoalso investigatedinvestigated the the memory memory characteristics characteristics of of arylamine-linked arylamine-linked poly(2,7-carbazole)s poly(2,7-carbazole)sP3, P4P3, andP4, P5and[46 P5]. These[46]. These polymers polymers are amorphous are amorphous but slightly but sl orientedightly oriented in the film in plane.the film All plane. polymers All withpolymers the sandwich with the structuresandwich ofstructure ITO/polymers/Al of ITO/polymers/Al were found were to exhibitfound to similar exhibit DRAM similar behaviors DRAM withoutbehaviors polarity. without The polarity. devices The are devices programmable are programmable at low voltage at withlow voltage a high ON/OFFwith a high current ON/OFF ratio upcurrent to 10 ratio9 as theup thicknessto 109 as the ranged thickness between ranged 8 and between 60 nm. 8 The and memory 60 nm. The behaviors memory are behaviors governed are by SCLCgoverned and by local SCLC filament and local formation, filament which formation, might originatewhich might from originate the electron-donating from the electron-donating carbazole and triphenylaminecarbazole and triphenylamine units in the polymer units in backbones. the polymer backbones.

Scheme 4. Chemical structures of some polymers with DRAM memory properties.

2.2. Static Random Access Memory (SRAM) Properties Chen et al. reported a D-A CP,CP, poly(arylenevinylene),poly(arylenevinylene), consisting of carbazole (Car) with pendent phenanthro[9,10-d]imidazole]imidazole ( (P6-CarP6-Car)) (Scheme (Scheme 55))[ [45].45]. The The flexible flexible P6-Car device with the sandwich configurationconfiguration ofof poly(ethylene-2,6-naphthalate)poly(ethylene-2,6-naphthalate) (PEN)/Al/(PEN)/Al/P6-Car/Al revealed revealed volatile SRAM characteristic, characteristic, which which can can be be operated operated at low at low voltages voltages with with high highON/OFF ON/OFF current current ratios (more ratios (morethan 10 than4) and 104 )excellent and excellent durability. durability. The Thehigh high steric steric hindrance hindrance between between carbazole carbazole donor donor and phenanthro[9,10-d]imidazole]imidazole side side chain chain leads leads to to a weak electric charge separated state and easy recombination after turning off thethe electricalelectrical power,power, resultingresulting inin volatilevolatile memorymemory characteristics.characteristics.

Polymers 2017, 9, 25 7 of 16 Polymers 2017, 9, 25 7 of 17

Polymers 2017, 9, 25 7 of 17 Polymers 2017, 9, 25 7 of 17

SchemeScheme 5. 5. ChemicalChemical structure structure of of polyme polymerr with with SRAM SRAM memory property. Scheme 5. Chemical structure of polymer with SRAM memory property. Scheme 5. Chemical structure of polymer with SRAM memory property. 3. CPs CPs for Non-Volatile Memory Devices 3. CPs for Non-Volatile Memory Devices 3. CPs for Non-Volatile Memory Devices Non-volatileNon-volatile memory memory memory devices devices devices can cancan stay stay stay in in inthe the the ONON ON state state steadily steadily steadily without without without an anapplied applied an voltage applied voltage bias. voltage bias. Non-volatile memory devices can stay in the ON state steadily without an applied voltage bias. Non-volatilebias.Non-volatile memory memory memory behaviors behaviors behaviors can can canbe be divided bedivided divided into into twotwo two classeclasse classes,s, s,namely namely namely WORM WORM WORM memory memory memory and and Non-volatile memory behaviors can be divided into two classes, namely WORM memory and rewritablerewritable (flash) (flash) (flash) memory, memory, depending depending on on whether whether a suitable voltage voltage can can switch switch the theON ON state state to the to the rewritable (flash) memory, depending on whether a suitable voltage can switch the ON state to the OFF stateOFF stateor not. or not.If the If theON ON state state can can be be switched switched baback to thethe OFF OFF state state by by applying applying a suitable a suitable voltage, voltage, OFF stateOFF state or not. or not. If the If the ON ON state state can can be be switched switched backback to to the the OFF OFF state state by byapplying applying a suitable a suitable voltage, voltage, which is an erasing process, the memory effect is called rewritable memory (Figure 4) [47]. However, whichwhich is an is erasing an erasing process, process, the the memory memory effect effect effect is isis called called rewritablerewritable rewritable memory memory memory (Figure (Figure (Figure 4) [47]. 44))[ [47]. However,47]. However, However, WORM is capable of maintaining the ON state (holding data) permanently, even applying a reverse WORMWORM is capable is capable of maintaining of maintaining the the ON ON state state (hol(holding(holding data)data) permanently, permanently, even even applying applying a reverse a reverse voltage (Figure 5) [48]. voltagevoltage (Figure (Figure5 5))[ [48].48 5)]. [48].

Figure 4. Current-voltage (I–V) characteristics of the ITO/polymer/Al memory device as a FigureFigure 4. Current-voltage 4. Current-voltage (I–V) characteristics(I–V) characteristics of the ITO/polymer/Alof the ITO/polymer/Al memory memory device asdevice a representative as a representative of Flash characteristic. of Flashrepresentative characteristic. of Flash characteristic. Figure 4. Current-voltage (I–V) characteristics of the ITO/polymer/Al memory device as a representative of Flash characteristic.

Figure 5. Current-voltage (I–V) characteristics of the ITO/polymer/Al memory device as a Figure 5. Current-voltage (I–V) characteristics of the ITO/polymer/Al memory device as a Figurerepresentative 5. Current-voltage of WORM characteristic. (I–V) characteristics (The third ofsweep the was ITO/polymer/Al conducted more than memory one hour device after as a representative of WORM characteristic. (The third sweep was conducted more than one hour after representativeturning off ofthe WORM power). characteristic. (The third sweep was conducted more than one hour after turning off the power). turning off the power).

Figure 5. Current-voltage (I–V) characteristics of the ITO/polymer/Al memory device as a representative of WORM characteristic. (The third sweep was conducted more than one hour after turning off the power).

Polymers 2017, 9, 25 8 of 17

Polymers3.1. WORM2017, 9Properties, 25 8 of 16 The non-volatile memory effect was investigated in other fluorene-acceptor push-pull 3.1.polymeric WORM systems Properties [14,49–55]. P7 consisting of 9,9-didodecylfluorene, pendent triphenylamine donors,The and non-volatile pyridine acceptors memory exhibited effect was WORM investigated memory behavior in other (Scheme fluorene-acceptor 6) [49]. Hole push-pull injection polymericfrom ITO systemsinto the [14highest,49–55 occupied]. P7 consisting molecular of 9,9-didodecylfluorene, orbital (HOMO) of pendentP7 is an triphenylamine energetically favored donors, andprocess pyridine due to acceptors the low energy exhibited barrier WORM between memory the behaviorwork function (Scheme of ITO6) [ 49 and]. Hole the HOMO injection level from of ITO P7. intoThe injected the highest hole occupied migrates molecular through the orbital continuous (HOMO) positive of P7 is electrostatic an energetically potential favored channel process along due the to thepolymer low energy chain barrierand becomes between trapped the work by functionthe electron of ITO acceptor and the group HOMO (nitrogen level of atomP7. The in the injected pyridine hole migratesring) but throughcannot theget continuousactivated by positive a reverse electrostatic voltage potentialbias, resulting channel in along the WORM the polymer type chain memory and becomesbehavior. trapped by the electron acceptor group (nitrogen atom in the pyridine ring) but cannot get activatedNonvolatile by a reverse WORM voltage memory bias, resulting behavior in the WORMwith tristable type memory property behavior. was obtained by poly(2,6-diphenyl-4-((9-ethyl)-Nonvolatile WORM memory9H behavior-carbazole)-pyridinyl- with tristablealt property-2,7-(9,9-didodecyl)- was obtained by9H poly(2,6-diphenyl-4--fluorenyl) (P8) ((9-ethyl)-[51]. The device9H-carbazole)-pyridinyl- switched from thealt initial-2,7-(9,9-didodecyl)- low-conductivity9H -fluorenyl)(OFF) state (P8to )[the51 first]. The high-conductivity device switched from(ON-1) the state initial at low-conductivity a threshold voltage (OFF) of state 1.8 toV, theand first subsequently high-conductivity to the (ON-1)second statehigh-conductivity at a threshold voltage(ON-2) ofstate 1.8 V,at anda higher subsequently voltage to of the 2.4 second V, which high-conductivity is also elucidated (ON-2) by state the at aenhanced higher voltage UV-Vis of 2.4absorption. V, which Under is also elucidatedan applied byfield, the the enhanced process UV-Vis of conformational absorption. Underorder, anaris applieding from field, charge theprocess carrier ofdelocalization-induced conformational order, D-A arising interaction from charge to form carrier a partial delocalization-induced or complete face-to-face D-A interaction conformation to form of the fluorene and carbazole units, can be generated throughout the polymer layer. An effective a partial or complete face-to-face conformation of the fluorene and carbazole units, can be generated charge transport channel originated from the electron hopping between the ordered structures throughout the polymer layer. An effective charge transport channel originated from the electron effectively switched the device from the OFF to ON-1 state. The coordinating ability of the nitrogen hopping between the ordered structures effectively switched the device from the OFF to ON-1 state. atoms in the carbazole pendant moieties and the indium atoms of ITO also promoted the charge The coordinating ability of the nitrogen atoms in the carbazole pendant moieties and the indium atoms transfer at the polymer/ITO interface. of ITO also promoted the charge transfer at the polymer/ITO interface.

Scheme 6. Chemical structures of some polymers with WORM memory properties. Scheme 6. Chemical structures of some polymers with WORM memory properties.

3.2. Flash Properties Chen et al. reported a flexible flexible bipolar resistive memory device with reliable performance in response to electric and mechanicalmechanical stimuli by usingusing aa conjugatedconjugated poly(fluorene-thiophene)poly(fluorene-thiophene) donor tethered phenanthro[9,10-phenanthro[9,10-dd]imidazole]imidazole acceptor acceptor (P9 (P9) as) theas the active active layer layer (Scheme (Scheme7)[ 54 ].7) The[54].P9 Thedevice P9 exhibiteddevice exhibited low threshold low threshold voltages, voltages, large ON/OFF large ON memory/OFF memory windows, windows, and good and retention good retention time.In time. the backboneIn the backbone of P9, fluorene of P9, fluorene and thiophene and thiophene donors act donors as hole act transporters/trapping as hole transporters/trapping centers, while centers, the phenanthro[9,10-while the phenanthro[9,10-d]-imidazoled]-imidazole acceptor servesacceptor as anserves electron as an transporter/trapping electron transporter/trapping center. Both center. the donorBoth the and donor acceptor and acceptor act as trapping act as trapping sites that sites depend that depend on the chargeon the charge association association and polarity and polarity of the electricof the electric field. As field. the applied As the voltage applied approaches voltage a thepproaches threshold the voltage, threshold the majority voltage, of the trapped majority charges of aretrapped filled charges to create are a filled trap-free to create environment. a trap-free The environment. captured chargesThe captured and the charges charged and states the charged can be maintainedstates can be due maintained to a large due energy to a barrierlarge energy for the barr backier transfer for the ofback charges, transfer and of the charges, deep trappingand the deep sites maytrapping not besites easily may recovered not be easily even afterrecovered turning even off theafter power. turning However, off the under power. a reverseHowever, voltage under bias, a thereverse trapped voltage charges bias, canthe betrapped extracted charges and can then be return extracted the device and then back return to the the OFF device state, back leading to the to bipolarOFF state, flash-type leading memory to bipolar behavior. flash-type Chen and memory Li designed behavior. a fluorene-acceptor Chen and copolymer Li designedP10 fora resistorfluorene-acceptor memory where copolymer 9,9-bis[4-(4-phenoxyl)phthalonitrile] P10 for resistor memory where pendant 9,9-bis[4-(4-phenoxyl)phthalonitrile] groups at the C-9 position of thependant fluorene groups unit at are the electron C-9 position acceptors of the [55 fluorene]. A strong unit dipoleare electron moment acceptors in P10 (10.71[55]. A Debye) strong isdipole also beneficialmoment in for P10 maintaining (10.71 Debye) the is conductive also beneficial state. for maintaining the conductive state.

Polymers 2017, 9, 25 9 of 16

Polymers 2017, 9, 25 9 of 17

Polymers 2017, 9, 25 9 of 17

Scheme 7. Chemical structures of some polymers with Flash memory properties. Scheme 7. Chemical structures of some polymers with Flash memory properties. 3.3. Negative Differential Resistance (NDR) Properties 3.3. Negative DifferentialNon-volatile Resistance resistive (NDR)memory Propertiesproperties based on polyfluorenes have been statically characterized [56,57]. The working mechanism was attributed to metallic filaments with a write Non-volatilepulse at 4 resistive V and an erase memory pulse ranging properties from 8–10 based V. Lee et on al. used polyfluorenes two different polyfluorenes, have been statically characterizedP11 [56 and,57 P12]. The, andScheme workingtwo 7. Al Chemical and mechanismAu structures electrodes of someto was fabricate polymers attributed the with memory Flash to memory metallic devices, properties. respectively filaments (Scheme with a write pulse at 4 V and an8) erase [58]. Bi-stability pulse ranging was observed from 8–10in all V.devices Lee etwith al. deposited used two Al differentas electrode. polyfluorenes, On the contrary, P11 and P12, bi-stable3.3. Negative switching Differential was Robservedesistance (NDR)only when Properties Au was deposited on the oxidized polyfluorene P12. and two Al andPresumably, Au electrodes both the internal to fabricate trap site the and memory organic/metal devices, interface respectively were responsible (Scheme for the 8electric)[ 58 ]. Bi-stability Non-volatile resistive memory properties based on polyfluorenes have been statically bi-stability of these memory devices. Gomes’ group also reported resistive memory devices based was observedcharacterized in all devices [56,57]. with The depositedworking mechanism Al as electrode.was attributed On to themetallic contrary, filaments bi-stable with a write switching was on poly(spirofluorene) P13 using small signal impedance measurements [59,60]. The device observed onlypulse when at 4 AuV and was an depositederase pulse ranging on the from oxidized 8–10 V. polyfluorene Lee et al. used twoP12 different. Presumably, polyfluorenes, both the internal remained highly resistive but the low frequency capacitance increased by several orders of P11 and P12, and two Al and Au electrodes to fabricate the memory devices, respectively (Scheme trap site andmagnitude. organic/metal Higher external interface applied were voltages responsible led to an increased for the electrical electric stress bi-stability across the ofoxide, these memory 8) [58]. Bi-stability was observed in all devices with deposited Al as electrode. On the contrary, which reduced the resistance, hence the switching. devices. Gomes’bi-stable group switching also was reported observed only resistive when Au memory was deposited devices on the based oxidized on polyfluorene poly(spirofluorene) P12. P13 Chen et al. have reported the synthesis of novel D-A rod-coil diblock copolymer P14 and its using small signalPresumably, impedance both the internal measurements trap site and [ 59organi,60].c/metal The interface device remainedwere responsible highly for the resistive electric but the low memory device [61]. The highest occupied molecular orbital (HOMO) energy level at −6.08 eV for bi-stability of these memory devices. Gomes’ group also reported resistive memory devices based frequency capacitancepolyoxadiazole increased was employed by several as a charge orders trap of for magnitude. electrical switching Higher memory external devices. applied The voltages led on poly(spirofluorene) P13 using small signal impedance measurements [59,60]. The device ITO/P14/Al memory device exhibited nonvolatile memory property with a NDR effect due to the to an increasedremained electrical highly stress resistive across but the the low oxide, frequency which capacitance reduced increased the resistance, by several hence orders the of switching. polyoxadiazole charge trapped block (Figure 6). Chen etmagnitude. al. have Higher reported external the applied synthesis voltages of led novel to an increased D-A rod-coil electrical diblock stress across copolymer the oxide, P14 and its memory devicewhich [ 61reduced]. The the highestresistance, hence occupied the switching. molecular orbital (HOMO) energy level at −6.08 eV Chen et al. have reported the synthesis of novel D-A rod-coil diblock copolymer P14 and its for polyoxadiazolememory device was [61]. employed The highest asoccupied a charge molecular trap orbital for (HOMO) electrical energy switching level at −6.08 memoryeV for devices. The ITO/P14polyoxadiazole/Al memory was device employed exhibited as a charge nonvolatile trap for electrical memory switching property memory with devices. a NDRThe effect due to the polyoxadiazoleITO/P14/Al memory charge device trapped exhibited block nonvolatile (Figure 6memo). ry property with a NDR effect due to the polyoxadiazole charge trapped block (Figure 6).

Scheme 8. Chemical structures of some polymers with negative differential resistance (NDR) memory properties.

Scheme 8. Chemical structures of some polymers with negative differential resistance (NDR) Scheme 8. memoryChemical properties. structures of some polymers with negative differential resistance (NDR) memory properties. Polymers 2017, 9, 25 10 of 17

Figure 6. Current-voltage (I–V) characteristics of the ITO/polymer/Al memory device as a Figure 6. Current-voltagerepresentative of NDR (I–V)characteristic. characteristics of the ITO/polymer/Al memory device as a representative of NDR characteristic. 4. Effects of the Molecular Design on Volatility

4.1. Donor Effect The tunable electrical switching characteristics of the vinylene-based CPs [47], P6-Car, P6-TH, and P6-TPA, consisting of various donors, such as carbazole, thiophene, and triphenylamine, respectively, with the pendant acceptor of phenanthro[9,10-d]imidazole were demonstrated (Scheme 9). The donor structures not only affected the polymer conformation, but also the D-A interaction and LUMO energy levels for stabilizing the charge separation. The PEN/Al/P6-Car/Al flexible device revealed SRAM behavior while the P6-TPA device exhibited WORM property, both memory devices can be operated at low voltages with high ON/OFF current ratios and excellent durability under repeated bending tests. However, the P6-TH device only exhibited a diode-like electrical behavior. Ree et al. reported the programmable memory characteristics of fully π-conjugated polymers P15 and P16 [62,63]. The memory properties of P15 were investigated as a function of temperature and film thickness. P15 with a thickness of 15–30 nm showed excellent unipolar DRAM behavior with a high ON/OFF ratio up to 108, which was mainly governed by filament formation supported by the metallic properties of the P15 film. The ON state current was dominated by Ohmic conduction, and the OFF state current appeared to undergo a transition from Ohmic to space charge limited conduction with a shallow-trap distribution. On the other hand, P16 with a thickness of 30 nm exhibited a very stable WORM memory property with an ON/OFF ratio of 106. Both the ester units and the conjugated double bonds of the P16 polymer backbone acted as efficient charge trapping sites. Li et al. also reported two donor-acceptor type poly(azomethine)s, incorporating an oxadiazole group either acting as an electron acceptor in P17 with the triphenylamine donor, or serving as a donor in P18 with the 3,3′-dinitro-diphenylsulfone acceptor [64]. The variation in the role of the oxadiazole group in the D-A polymers resulted in different memory properties of the prepared poly(azomethine)s. The P17-based memory device with Pt/P17/Pt sandwiched structure showed rewritable memory behavior but poor endurance of less than 20 cycles, while the P18-based device exhibited WORM memory behavior. The different memory properties are attributed to the different band gaps of the poly(azomethine)s, indicating the different degree of intra- and intermolecular charge transfer interaction between the electron donor and acceptor. The stronger electron push-pull interaction in P17 facilitates the charge transfer effect, resulting in a lower switching threshold voltage than that of the P18-based device. For the memory devices with Al/polymer/Al sandwiched structure, both P17 and P18 demonstrate a much improved resistive switching effect, and the endurance of the P18-based device is better than that of the P17-based device. The difference in the electronic transport and memory properties of the four devices may originate from the different charge injection/extraction and electron transfer processes of the sandwich systems.

Polymers 2017, 9, 25 10 of 16

4. Effects of the Molecular Design on Volatility

4.1. Donor Effect The tunable electrical switching characteristics of the vinylene-based CPs [47], P6-Car, P6-TH, and P6-TPA, consisting of various donors, such as carbazole, thiophene, and triphenylamine, respectively, with the pendant acceptor of phenanthro[9,10-d]imidazole were demonstrated (Scheme9). The donor structures not only affected the polymer conformation, but also the D-A interaction and LUMO energy levels for stabilizing the charge separation. The PEN/Al/P6-Car/Al flexible device revealed SRAM behavior while the P6-TPA device exhibited WORM property, both memory devices can be operated at low voltages with high ON/OFF current ratios and excellent durability under repeated bending tests. However, the P6-TH device only exhibited a diode-like electrical behavior. Ree et al. reported the programmable memory characteristics of fully π-conjugated polymers P15 and P16 [62,63]. The memory properties of P15 were investigated as a function of temperature and film thickness. P15 with a thickness of 15–30 nm showed excellent unipolar DRAM behavior with a high ON/OFF ratio up to 108, which was mainly governed by filament formation supported by the metallic properties of the P15 film. The ON state current was dominated by Ohmic conduction, and the OFF state current appeared to undergo a transition from Ohmic to space charge limited conduction with a shallow-trap distribution. On the other hand, P16 with a thickness of 30 nm exhibited a very stable WORM memory property with an ON/OFF ratio of 106. Both the ester units and the conjugated

Polymersdouble 2017 bonds, 9, 25 of the P16 polymer backbone acted as efficient charge trapping sites. 11 of 17

Scheme 9. ChemicalChemical structures structures of some polymers with donor effect.

4.2. Acceptor Effect Li et al. also reported two donor-acceptor type poly(azomethine)s, incorporating an oxadiazole groupPolyfluorene-based either acting as an electroncopolymer acceptor P19 in P17containingwith the triphenylamine electron-don donor,or triphenylamine or serving as a donorand electron-acceptorin P18 with the 3,3 9,9-bis[3,4-bis(3,0-dinitro-diphenylsulfone4-dicyanophenoxy)phenyl] acceptor [64]. The side variation chains in at the the role C-9 of position the oxadiazole of the fluorenegroup in theunit D-A was polymers applied resultedfor a nonvolatile in different WORM memory memory properties device of the (Scheme prepared 10) poly(azomethine)s. [52], meanwhile P20The P17consisting-based memoryof poly[9,9-bis(4 device with-diphenylaminophenyl)-2,7-fluor Pt/P17/Pt sandwiched structureene] showeddonors rewritablewith end-capped memory Dispersebehavior Red but poor1 exhibited endurance bi-stable of less conductive than 20 cycles,states and while rewritable the P18-based memory device behavior exhibited [50]. Under WORM a lowmemory positive behavior. voltage The sweep, different favorable memory hole propertiesinjection and are migration attributed resulted to the different in the formation band gaps of ofa high current state. As the hole injection process was underway, the positive charges on the triphenylamine moieties in P19 were rapidly consumed by the cyano groups as a result of the irreversible switching operation. On the contrary, the active area- or temperature-independent current density in P20 indicated the absence of sample degradation or breakdown and excluded the metallic filamentary conduction effect.

Scheme 10. Chemical structures of some polymers with acceptor effect.

Polymers 2017, 9, 25 11 of 17

Polymers 2017, 9, 25 11 of 16 the poly(azomethine)s, indicating the different degree of intra- and intermolecular charge transfer interaction between the electron donor and acceptor. The stronger electron push-pull interaction in P17 facilitates the charge transfer effect, resulting in a lower switching threshold voltage than that of the P18-based device. For the memory devices with Al/polymer/Al sandwiched structure, both P17 and P18 demonstrate a much improved resistive switching effect, and the endurance of the P18-based device is better than that of the P17-based device. The difference in the electronic transport and memory properties of the four devices may originate from the different charge injection/extraction and electron transfer processes of the sandwich systems. Scheme 9. Chemical structures of some polymers with donor effect. 4.2. Acceptor Effect 4.2. Acceptor Effect Polyfluorene-basedPolyfluorene-based copolymer copolymerP19 containing P19 containing electron-donor electron-don triphenylamineor triphenylamine and electron-acceptor and 9,9-bis[3,4-bis(3,4-dicyanophenoxy)phenyl]electron-acceptor 9,9-bis[3,4-bis(3,4-dicyanophenoxy)phenyl] side chains at the side C-9 chai positionns at the of C-9 the position fluorene of unit the was appliedfluorene for a unit nonvolatile was applied WORM for a nonvolatile memory device WORM (Scheme memory 10 device) [52 ],(Scheme meanwhile 10) [52],P20 meanwhileconsisting of poly[9,9-bis(4-diphenylaminophenyl)-2,7-fluorene]P20 consisting of poly[9,9-bis(4-diphenylaminophenyl)-2,7-fluor donors with end-cappedene] donors Disperse with end-capped Red 1 exhibited bi-stableDisperse conductive Red 1 exhibited states and bi-stable rewritable conductive memory states behavior and rewritable [50]. Under memory a low behavior positive [50]. voltage Under sweep,a favorablelow positive hole injection voltage andsweep, migration favorable resulted hole injection in the and formation migration of aresulted high current in the formation state. As of the a hole injectionhigh process current was state. underway, As the thehole positive injection charges process on was the triphenylamineunderway, the positive moieties charges in P19 were on the rapidly triphenylamine moieties in P19 were rapidly consumed by the cyano groups as a result of the consumed by the cyano groups as a result of the irreversible switching operation. On the contrary, irreversible switching operation. On the contrary, the active area- or temperature-independent the activecurrent area- density or temperature-independent in P20 indicated the absence currentof sample density degradation in P20 orindicated breakdown the and absence excluded of the sample degradationmetallic orfilamentary breakdown conduction and excluded effect. the metallic filamentary conduction effect.

SchemeScheme 10. 10.Chemical Chemical structures structures ofof some polymers polymers with with acceptor acceptor effect. effect.

4.3. Thickness Effect

Ree et al. reported donor-acceptor polymers, P21, P22, and P23, which were composed of fluorene, triphenylamine, dimethylphenylamine, alkyne, tetracyanoethylene (TCNE), and 7,7,8,8- tetracyanoquinodimethane (TCNQ) adducts (Scheme 11)[65]. The TCNE and TCNQ units were found to enhance the π-conjugation lengths and intramolecular charge transfer of P22 and P23, respectively, despite their electron-acceptor characteristics. The TCNE and TCNQ units enabled the authors to fine-tune the memory properties and widen the thickness window of the polymer layer. In the memory device with Al/polymer/Al sandwiched structure, P21 exhibited stable unipolar permanent memory behavior with high reliability. On the other hand, P22 and P23 devices showed stable unipolar permanent memory behavior over only a narrow film thickness window of 10–20 nm while DRAM behavior can be obtained with a wider thickness window of 10–30 nm at higher operation voltages. The memory behavior of P23 was observed to be driven by both hole and electron injection in which the electron donor and acceptor groups both acted as charge trapping sites, indicating that the memory devices can be operated at relatively low voltages. Polymers 2017, 9, 25 12 of 17

4.3. Thickness Effect Ree et al. reported donor-acceptor polymers, P21, P22, and P23, which were composed of fluorene, triphenylamine, dimethylphenylamine, alkyne, tetracyanoethylene (TCNE), and 7,7,8,8-tetracyanoquinodimethane (TCNQ) adducts (Scheme 11) [65]. The TCNE and TCNQ units were found to enhance the π-conjugation lengths and intramolecular charge transfer of P22 and P23, respectively, despite their electron-acceptor characteristics. The TCNE and TCNQ units enabled the authors to fine-tune the memory properties and widen the thickness window of the polymer layer. In the memory device with Al/polymer/Al sandwiched structure, P21 exhibited stable unipolar permanent memory behavior with high reliability. On the other hand, P22 and P23 devices showed stable unipolar permanent memory behavior over only a narrow film thickness window of 10–20 nm while DRAM behavior can be obtained with a wider thickness window of 10– 30 nm at higher operation voltages. The memory behavior of P23 was observed to be driven by both Polymershole2017 and, 9, 25electron injection in which the electron donor and acceptor groups both acted as charge12 of 16 trapping sites, indicating that the memory devices can be operated at relatively low voltages.

SchemeScheme 11. 11.Chemical Chemical structures structures ofof some polymers polymers with with thickness thickness effect. effect.

5. CPs5. ContainingCPs containing Metal Metal Complexes Complexes Conjugated polyfluorenes with cationic Ir(III) complexes, P24–26, were selected as active Conjugated polyfluorenes with cationic Ir(III) complexes, P24–26, were selected as active memory memory materials for the functionalities of flash memory devices [33,66]. The memory device based materialson P24 for containing the functionalities Ir(III) complex of flash exhibited memory low readin devicesg, writing, [33,66]. and The erasing memory voltages device with based a high on P24 containingON/OFF Ir(III) current complex ratio of exhibited more than low 105 reading, (Scheme writing,12). Both and ON erasingand OFF voltages states were with stable a high under ON/OFF a currentconstant ratio voltage of more stress than of 10 −1.05 (Scheme V up to 10128 ).read Both cycles. ON The and flash OFF memory states werebehavior stable was under attributed a constant to voltagethe stresspolarized of −charge1.0 V transfer up to 10between8 read the cycles. fluorene The donor flash and memory the cationic behavior Ir(III) wascomplex attributed acceptors to the polarizedPolymersunder charge an2017 applied, 9, transfer25 field. between Furthermore, the fluorene through donorthe modi andfication the cationic of the ligand Ir(III) st complexructures acceptorsof the13 Ir(III) of 17 under an appliedcomplexes, field. the Furthermore, resulting polymers through theP25 modificationand P26 also of showed the ligand excellent structures memory of the behavior Ir(III) complexes, with the resultingresultingdifferent polymersthresholdin a lower voltageP25 energyand and barrierP26 currentalso between showedat the theconductive excellent work function state memory [33]. of the behavior ITO anode with and different the HOMO threshold CPs containing Pt(II) complexes have also been fabricated in resistor memory devices with voltagelevel and of P28 current as well at theas an conductive easier charge state transfer [33]. compared to that of P27. ITO/polymer/Al sandwiched structure [67,68]. Conjugated polyfluorene and polycarbazole with Pt(II) complexes in the side chain (P27 and P28, respectively) exhibited excellent flash memory behaviors with a high ON/OFF current ratio and excellent stability with repetitive read cycles (107) [67]. According to the redox properties and theoretical calculation results, the memory mechanism can be attributed to the formation and dissociation of a charge transfer state induced by negative and positive voltages, respectively. When applying an electric field over the threshold voltage, charge transfer from the polymer main chain to the side chain Pt(II) complex units occurs and switches the device to the ON state. According to the stable charge transfer complex, the ON state was still maintained even after the driving power was turned off. However, the device can be returned to the original OFF state as a reverse bias voltage is applied thus dissociating the charge transfer state. In addition, the main chain structures had significant influence on the threshold voltages. The threshold voltages of the P28-based device were lower than that of the P27-based device due to the lower oxidation potential of polycarbazole (0.43 V) than polyfluorene (0.96 V),

SchemeScheme 12. 12.Chemical Chemical structures structures ofof some polymers polymers containing containing metal metal complexes. complexes.

CPs6. Flexible containing CP-Based Pt(II) Memory complexes Devices have also been fabricated in resistor memory devices with ITO/polymer/AlFlexible polymer sandwiched memory structure devices were [67,68 also]. Conjugateddemonstrated polyfluorene by using CPs andas active polycarbazole layers as in with Pt(II)other complexes organic electronics in the side [69]. chain Ueda, (P27 Liu,and and P28Chen, respectively)fabricated a typical exhibited memory excellent device based flash on memory a 7 behaviorsflexible with polyethylene a high ON/OFF terephthalate current ratio(PET) and substrate excellent [47,54], stability which with repetitiveshowed a read highly cycles stable (10 )[67]. Accordingnonvolatile to the memory redox propertiesbehavior even and after theoretical bending calculationup to 1000 bending results, cycles the memory at a radius mechanism curvature of can be attributed5 mm. toAlso, the the formation flexible memory and dissociation device reported of a charge by our transfer group [70] state was induced tested under by negative severe bending and positive voltages,at various respectively. curvature When radiiapplying of 11, 9, 7, an and electric 5 mm, field respectively, over the threshold showing no voltage, crack or charge deform transfer upon from the polymerbending. mainThe reliable chain and to the reproducible side chain switching Pt(II) complex memory units behavior occurs of CP and film switches in the device the devicecan also to the ON state.be obtained According under tomechanical the stable bending charge stress. transfer Similar complex, to other the organic ON state electronics, was still such maintained as organic even transistors, organic photovoltaics, etc., the development of device fabrication for practical application is well underway, and the performance of polymer memory devices can be further improved by optimizing the associated processing parameters. Therefore, there is still ample opportunity for improving the electroactive materials and polymer memory devices.

7. Conclusions and Perspectives Conjugated polymers for memory devices is an emerging area of intense research interest as it encompasses low cost, high mechanical strength, facile processability, and high-density data storage. This review summarized the most widely studied mechanisms in CP resistive memory devices, such as charge transfer, space charge traps, and filament conduction. Further refinements in structural design and preparation methods, enhancement in device fabrication, measurement, characterization, and integration techniques, are essential to advance polymeric memory technology.

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Roncali, J. Conjugated poly(thiophenes): Synthesis, functionalization, and applications. Chem. Rev. 1992, 92, 711.

Polymers 2017, 9, 25 13 of 16 after the driving power was turned off. However, the device can be returned to the original OFF state as a reverse bias voltage is applied thus dissociating the charge transfer state. In addition, the main chain structures had significant influence on the threshold voltages. The threshold voltages of the P28-based device were lower than that of the P27-based device due to the lower oxidation potential of polycarbazole (0.43 V) than polyfluorene (0.96 V), resulting in a lower energy barrier between the work function of the ITO anode and the HOMO level of P28 as well as an easier charge transfer compared to that of P27.

6. Flexible CP-Based Memory Devices Flexible polymer memory devices were also demonstrated by using CPs as active layers as in other organic electronics [69]. Ueda, Liu, and Chen fabricated a typical memory device based on a flexible polyethylene terephthalate (PET) substrate [47,54], which showed a highly stable nonvolatile memory behavior even after bending up to 1000 bending cycles at a radius curvature of 5 mm. Also, the flexible memory device reported by our group [70] was tested under severe bending at various curvature radii of 11, 9, 7, and 5 mm, respectively, showing no crack or deform upon bending. The reliable and reproducible switching memory behavior of CP film in the device can also be obtained under mechanical bending stress. Similar to other organic electronics, such as organic transistors, organic photovoltaics, etc., the development of device fabrication for practical application is well underway, and the performance of polymer memory devices can be further improved by optimizing the associated processing parameters. Therefore, there is still ample opportunity for improving the electroactive materials and polymer memory devices.

7. Conclusions and Perspectives Conjugated polymers for memory devices is an emerging area of intense research interest as it encompasses low cost, high mechanical strength, facile processability, and high-density data storage. This review summarized the most widely studied mechanisms in CP resistive memory devices, such as charge transfer, space charge traps, and filament conduction. Further refinements in structural design and preparation methods, enhancement in device fabrication, measurement, characterization, and integration techniques, are essential to advance polymeric memory technology.

Conflicts of Interest: The authors declare no conflict of interest.

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