Nonvolatile bistable memory device based on polyfluorene with Ag NPs doping materials

Journal Pre-proof Nonvolatile bistable memory device based on polyfluorene with Ag NPs doping materials

Jiahe Huang, Hongyan Zhang, Xiaofeng Zhao, Ju Bai, Yanjun Hou, Shuhong Wang, Cheng Wang, Dongge Ma PII:

S1566-1199(19)30576-2

DOI:

https://doi.org/10.1016/j.orgel.2019.105549

Reference:

ORGELE 105549

To appear in:

Organic Electronics

Received Date:

12 October 2019

Accepted Date:

01 November 2019

Please cite this article as: Jiahe Huang, Hongyan Zhang, Xiaofeng Zhao, Ju Bai, Yanjun Hou, Shuhong Wang, Cheng Wang, Dongge Ma, Nonvolatile bistable memory device based on polyfluorene with Ag NPs doping materials, Organic Electronics (2019), https://doi.org/10.1016/j. orgel.2019.105549

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Journal Pre-proof

Journal Pre-proof Nonvolatile bistable memory device based on polyfluorene with Ag NPs doping materials Jiahe Huanga, Hongyan Zhanga, Xiaofeng Zhaob, Ju Baia, Yanjun Houa, Shuhong Wanga*, Cheng Wanga, c, d*, Dongge Mad a

School of Chemical Engineering and Materials, Heilongjiang University, Harbin

150080, P. R. China b

School of electronic engineering, Heilongjiang University, Harbin 150080, P. R.

China c

Key Laboratory of Functional Inorganic Material Chemistry,(Heilongjiang

University), Ministry of Education, Harbin 150080, P. R. China d

School of Materials Science and Engineering, South China University of

Technology, Guangzhou 510640, P. R. China Corresponding author: Shuhong Wang, e-mail: [email protected], Cheng Wang, e-mail: [email protected] Abstract Organic electric memory devices have broad application prospects. In the present paper, a new kind of polyfluorene-based material containing methoxytriphenylamine groups was synthesized, which exhibited good Flash-type storage characteristics. The material was further doped with Ag NPs to improve storage performance. With appropriate amount of Ag NPs, the device presented an ON/OFF current ratio up to 1.1×104 with excellent stability. In addition, the switching characteristics of the device were discussed through data fitting and molecular orbital calculation.

Journal Pre-proof Keywords: PF-MeOTPA; Ag NPs; Flash memory device; Nonvolatile behavior 1. Introduction With the development of information technology, storage technology has been extensively applied in human life. Conventional memory devices are made by silicon-based semiconductor, and the research on these devices will approach the theoretical and physical limits. Therefore, in order to meet the requirement of information technology, it is particularly important to develop a new generation of storage technology. Among them, organic resistance random access memory (ORRAM) has been attracting increasing attention because of its exclusive advantages. Information storage is implemented by applying a certain external voltage on ORRAM between the low conductive state (OFF state) and the high conductive state (ON state), which correspond to the "0" (OFF state) and "1" (ON state) of digital storage respectively [1-3]. The active layers of ORRAM are mainly organic small molecules and polymers. Organic small molecules have the advantages of low cost, flexibility and adjustability of structural design[4,5]. But due to the inherent weak inter-molecular interaction of organic small molecules, the devices prepared by them have low thermal stability, high operating voltage, and poor long-term environmental stability hinders their potential applications. Therefore, polymer materials with advantages such as easy processing, low cost, good stability, and low power consumption have attracted the interest of researchers and become a hot research topic. In recent years, many polymers have been synthesized for the preparation of active thin films, for example,

Journal Pre-proof poly(N-vinylcarbazole)

(PVK)[6],

poly(methyl

methacrylate)

(PMMA)[7],

polyvinylpyrrolidone (PVP)[8]. However, the storage properties of the fabricated devices are insufficient to produce a satisfactory storage performance. For the sake of an improvement, researchers doped inorganic nanoparticles into the polymer to prepare organic polymer/inorganic nanocomposites, such as poly(vinyl alcohol) (PVA)： CNTs[9], β-phase poly(9,9-dioctylfluorene) (PFO)： Au NPs[10], polyethylene oxide(PEO):Ag NPs[11], poly (styrene-block-4-vinyl-pyridine (PS-b-P4VP)： Ag NPs[12]. Devices made of these materials perform much better than devices made of a single polymer. Takeshi Kondo et al doped Ag NPs with poly (N-vinylcarbazole) (PVK), and found that the ON/OFF current ratio increases to 104 after doping 10 wt % Ag NPs[13]. Although many literatures have reported the improved electrical switching and memory effects of inorganic nanoparticles/polymers, the effects of doping amount on the conductivity of polymer/inorganic nanocomposites is yet an open problem. Polyfluorene and its derivatives, as an excellent hole transport materials, have prominent carrier transport capability along its skeleton, which makes it one of the most promising candidate materials for ORRAM[14]. 4-methoxy-triphenylamine group can be embedded as an electron donor, due to its facilitation in the stability and hole transport capacity of the polymer[15]. Therefore, copolymers of fluorene and triphenylamine have attracted wide concern. In

this

study,

we

designed

and

synthesized

monomer

4-bromo-N-(4-bromophenyl)-N-(4-methoxyphenyl)benzenamine (Br-MeOTPA) and copolymer PF-MeOTPA through Suzuki reaction, and the compounds were

Journal Pre-proof characterized in terms of a variety of relevant properties. Finally, the storage performances were evaluated for the fabricated sandwich devices. 2. Experimental Details 2.1 Materials For the current investigation, all chemicals were purchased from Aladdin without any further purification. The Ag NPs were prepared by soluble starch as a dispersing agent and β-D-glucose as a reducing agent[16], and kept at low temperature under vacuum. The Ag NPs were first investigated by X-Ray Diffraction (XRD) and transmission electron microscopy (TEM). The XRD pattern of silver nanoparticles as shown in Fig. 1(a), and the peaks of diffraction at 2θ values of 38°, 44°, 64° and 77° are consistent with (110), (200), (220) and (311) planes, respectively. According to the JCPDS.04-0783 diffraction data card, the material assumes a cubic crystal structure. The TEM image of the Ag NPs dispersed in the PF-MeOTPA, as shown in Fig. 1(b), exhibits a diameter of 20 to 40 nm.

Fig.1 (a) The XRD diffraction pattern of Ag NPs;(b) TEM image of the Ag NPs dispersed in the PF-MeOTPA toluene solution 2.2 Synthesis of Br-MeOTPA

Journal Pre-proof Scheme 1(a) shows the synthetic route and the structure of Br-MeOTPA. 4-methoxyaniline (1.6728 g, 13.5834 mmol), 1-bromo-4-iodobenzene (8.0825 g, 28.5691

mmol),

Pd2(dba)3

(0.1244

g,

0.1358

mmol),

1,1'-Bis(diphenylphosphino)ferrocene (DPPF) (0.3014 g, 0.5436 mmol), sodium tert-butoxide (t-BuONa) (3.9158 g, 40.7471 mmol) and 25 mL toluene were added to the 500 mL Schlenk bottle in turn. The reaction was in a nitrogen atmosphere and the temperature was gradually raised to 110 °C and then maintained constant for 6 hours. The mixture was extracted with dichloromethane and concentrated, and the crude product was purified by column chromatography (dichloromethane: n-hexane = 1:3).

Scheme 1 Synthesis routes and the chemical structure of (a) Br-MeOTPA and (b) PF-MeOTPA Fig. 2 shows the 1H-NMR of Br-MeOTPA. 1H-NMR (400 MHz, CDCl3) δH(ppm): δ7.276(4H, He, Hf, Hi, Hj), 7.001(2H, Hb, Hm), 6.829(6H, Hc, Hd, Hg, Hh, Hk, Hl), 3.796(3H, Ha).

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Fig. 2 The 1H-NMR spectrum of Br-MeOTPA Fig. 3 shows the

13C-NMR

of Br-MeOTPA. 13C-NMR (100MHz, CDCl3), δ C

(ppm):55.433 (C1), 114.494 (C3, C19), 114.989 (C9, C15), 124.204 (C4, C18), 127.344 (C7, C11, C13, C17), 132.107 (C8, C10, C14, C16), 139.643 (C5), 146.743(C6, C12), 156.681(C2).

Fig. 3 The 13C-NMR spectrum of Br-MeOTPA

Journal Pre-proof 2.3 Synthesis of PF-MeOTPA Scheme 1 (b) shows the synthetic route and the chemistry structure of PF-MeOTPA.

First

of

all,

9,9´-Dioctyfluorene-2,7-dibornic

acid

bis(1,3-propanediol)ester (0.5000 g, 0.8954 mmol), Br-MeOTPA (0.4432 g, 0.8954 mmol), Pd(PPh3)4 (0.0310g, 2.686×10-2 mmol) and toluene (15 mL) were added into Schlenk successively. After complete dissolution, Na2CO3 (3 mol·L-1, 15 mL) was further added. Subsequently, with a stirring in a nitrogen atmosphere, the mixture was gradually heated up to a temperature of 105 ℃. And then, after 48 hours of complete reaction, the solution was cooled to room temperature. Afterwards, the organic extract, which was obtained through several times of washing with deionized water, was dropped into methanol to precipitate a product. Soxhlet extraction was performed for the product with acetone as solvent and lasted 48 hours. Dehumidification of 48 hours was carried out for the final product in a vacuum oven, and the ultimate synthesis was inspected with by 1H-NMR, 13C-NMR and FT-IR spectroscopy. The FT-IR of PF-MeOTPA is shown in Fig. 4, the C-H stretching vibration of the polymer at 2925.95 cm-1 is observed. The vibration of the C=C bond of the polymer is at 1506.73 cm-1 and 1464.73 cm-1. Besides, 1241.84 cm-1 is the Ar-O stretching vibration on the polymer. Fig. 5 shows the 1H-NMR of PF-MeOTPA. 1H-NMR (400 MHz, CDCl3) δH(ppm): 7.730 (2H, Hh), 7.756 (8H, He, Hg, Hf), 7.187 (6H, Hb, Hd), 6.970 (2H, Hc), 3.835 (3H, Ha), 2.025 (4H, Hi), 1.125 (24H, Hj, Hk, Hl, Hm, Hn, Ho), 0.750 (6H, Hp). The 13C-NMR of PF-MeOTPA is shown in Fig. 6. 13C-NMR (100MHz, CDCl3),

Journal Pre-proof δ C (ppm):155.408 (C2), 150.599 (C15), 146.144 (C6), 139.422 (C10), 138.671 (C13), 138.340 (C5), 134.088 (C14), 126.690 (C8), 126.410 (C11), 124.435 (C12), 122.040 (C11), 124.435 (C12), 122.040 (C4), 119.840 (C9), 118.870 (C7), 113.869 (C3), 54.470 (C1), 39.501 (C16), 30.749 (C17), 29.851 (C22), 29.029 (C19), 28.119 (C20), 28.179 (C21), 22.800 (C18), 21.556 (C23), 13.039 (C24).

Fig. 4 The FT-IR spectra of PF-MeOTPA

Fig. 5 1H-NMR spectrum of the PF-MeOTPA

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Fig. 6 13C-NMR spectrum of the PF-MeOTPA In addition, we analyzed the molecular weight of the polymer using GPC. The number average molecular weight (Mn ), weight average molecular weight (Mw ), molecular weight distribution index (D), and the average polymerization degree (n) of the PF-MeOTPA were 46023, 70276, 1.527 and 69.6, respectively. 2.4 Preparation of electrical storage devices The indium tin oxide (ITO) (2cm×2cm in size, Rs = 7-10 Ω) was pretreated. Toluene, acetone, ethanol and deionized water were used for ultrasonic cleaning of ITO for 40 minutes respectively, and then vacuum drying for 60 minutes. PF-MeOTPA was dissolved in toluene to prepare a solution of 3 mg·mL-1 concentration, and a following stir lasted for 24 hours. Ag NPs was dispersed in trichloromethane and produced a solution of 0.75 mg·mL-1. With dilution, dissolvent with different concentrations was produced, and further stirred for 24 hours. The PF-MeOTPA solution was mixed with the diluted Ag NPs solution of equivalent

Journal Pre-proof amount, as shown in Table 1. Then the mixture was subjected to an ultrasonic stirring for 3 hours. Table 1 Different ratios of PF-MeOTPA: Ag NPs doping samples Sample

Ag NPs content in

Volume of Ag

Volume of

Ag NPs

chloroform

NPs solution

PF-TPA toluene

content in the

solution(mg·mL-1)

(mL)

solution (3

composite film

mg·mL-1)(mL)

(wt%)

A

0

1

1

0

B

0.13

1

1

4.15

C

0.16

1

1

5.06

D

0.19

1

1

5.95

E

0.23

1

1

7.12

F

0.26

1

1

7.97

G

0.30

1

1

9.09

H

0.33

1

1

9.91

I

0.41

1

1

12.05

Fig. 7 (a) Cross-section SEM image of the device; (b) The schematic of the memory device.

Journal Pre-proof The prepared solutions were spin-coated on ITO glass substrates (20s with 800 rpm and 40s with 3000 rpm). The prepared samples were dried in vacuum at 60 ℃ for 3 hours. A three-layer structure was observed by scanning electron microscopy (SEM), as illustrated by Fig. 7 (a). Finally, the Al electrode of 250 μm diameter was plated on the thin film by vacuum evaporation. The device structure is presented by Fig. 7 (b). Keithley 4200-SCS semiconductor was employed to measure the IV characteristic curves of these devicesfor a further analysis and discussion. 3. Results and discussion 3.1 Characterization of the ITO/PF-MeOTPA/Al memory device To demonstrate the memory performance of the device and the effect of doping levels on memory performance, a single undoped ITO/PF-MeOTPA/Al (Sample A) was also prepared. As shown in Fig. 8(a), the entire I-V characteristic curve consists of four voltage sweep steps, 0 to -6 V, -6 to 0 V, 0 to 6 V, and 6 to 0 V. In the first step, the voltage is gradually increased and the current increases slowly. With a voltage up to -1.79 V, the current suddenly rises (from 5.369 × 10 -5 A to 4.495 × 10 -2 A). This indicates that the memory device is converted from a low conduction state (OFF) to a high conduction state (ON). Thus it produces a "write" process in digital storage, and the voltage that causes a change of the device from an OFF state to an ON state is termed as threshold voltage. In such case, the device exhibits an ON state, with an ON/OFF current ratio of 837, and the device maintains an OFF state in the second step, which involves a voltage change from -6 to 0 V, corresponding to the "read" process of digital storage. During step 3, the voltage reaches is increased from

Journal Pre-proof 0 to 6 V. At voltage of 2.65 V, the device suddenly switches back from a highly conductive state to a low conduction state, equivalent to an "erasing" process. The fourth scan step is a scan voltage of 6 to 0 V, in which the device returns to a low conduction state, which corresponds to "rewrite". And it marks the end of a cycle. for the next cycle, the device repeats the entire process described above, and thus it presents the characteristics of the Flash type electrical memory[17].

Fig. 8 (a) The I-V characteristic curve; (b) Retention behavior and (c) durability behavior of Sample A. The stability is critical for the device, so two different tests were performed to confirm its retention and endurance performance. The results are presented by Fig. 8(b) and (c). Fig. 8(b) shown the retention time of Sample A. It can be seen that the current of the device remains stable for more than 3 hours at a constant reading voltage of 2V, indicating that the device has pronounced data retention capability and

Journal Pre-proof can store information for a long time. Fig. 8(c) shows the number of cycles of sample A at a pulse voltage of 2 V (2 ms pulse period, 1 ms pulse width). It can be seen from the test data that the current of the device remains stable after 3×104 cycles, indicating that the device has favorable write-read endurance.

Fig.9 (a) I-V curves of ITO/PF-MeOTPA/Al with Ag NPs doping concentration; (b) Retention time and (c) Cycle number of device based on Sample F. In order to study the effect of Ag NPs content on device performance,

samples

with different doping concentrations were compared in terms of the current-voltage characteristic curves, as exhibited by Fig. 9(a). The entire scan period is the same with that for sample A. It can be observed from the figure that the threshold voltage of sample for B-H are -1.40 V, 1.35 V, -1.30 V, 1.20 V, - 1.10 V, -1.15 V, and -1.20 V, respectively, with their corresponding ON/OFF current ratio of 5.3×103, 6.4×103, 7.2×103,

8.9×103,

1.1×104,

8.1×103

and

4.8×103.

Compared

with

Journal Pre-proof ITO/PF-MeOTPA/Al without Ag NPs, the I-V characteristic curve with a Ag NPs content of 7.97 % (sample F) presented the best performance. Moreover, we tested the retention time and endurance of devices made of the sample F, as exhibited by Fig. 9(b) and (c). Fig. 9(b) presents the retention time performance of the device. With a constant voltage of 2V, the device remains stable for over 3 h, indicating excellent information retention. Fig. 9(c) shows the number of cycles of the device. At a pulse voltage of 2 V(2 ms pulse period, 1 ms pulse width), the device remains stable after more than 3×104 cycles, implying excellent durability of write-read performance. 3.2 Mechanism analysis of switching behavior For the purpose of clarifying the governing mechanism of the electrical storage device, the "set" portions of the devices ITO/PF-MeOTPA/Al (Sample A) and ITO/PF-MeOTPA:Ag NPs/Al (Sample F) were subjected to double logarithmic processing, as shown in Fig.10. The conduction mechanism can be understood using space charge limited current (SCLC)[18].The switching mechanism of the device with sample A is shown in Fig. 10(a). Below the transition voltage (Vtr), the device assumes a high-resistance state (HRS), and current transfer follows Ohm's law (slop = 1.13). When the voltage exceeds Vtr, conduction is associated with the trap filling process. With gradual filling of the trap, conduction becomes a space charge limit, according to Child's law (slop = 2.05). With a voltage above the threshold voltage(Vth), the trap is filled and the device switches to a low resistance state (LRS). The fitted curve are consistent with the aforementioned discussion of the electron transfer process. Fig. 10(b) shows the fitted curve of the ITO/PF-MeOTPA:Ag NPs/Al device

Journal Pre-proof with Sample F. It indicates that a linear relationship is still present between current and voltage for the ON state. The slope at the low voltage in the OFF state is 0.98, in accordance with Ohm's law. The slope is 2.10 at higher voltages, in accordance with Child's law. Consequently, the conduction mechanism could be explained with the SCLC.

Fig. 10 (a) Linear fitting and corresponding slopes for ITO/PF-MeOTPA/Al and (b) ITO/PF-MeOTPA: Ag NPs/Al.

Fig. 11 Resistance temperature dependence for (a) HRS and (b) LRS of device with Sample A. Fig. 11 exhibits the resistance temperature dependence test of the device with Sample A for both the high (HRS) and the low resistance state (LRS). According to Fig. 11 (a), the resistance of the device in the HRS reduces non-linearly with

Journal Pre-proof temperature, and such characteristic of the device is similar to a semiconductor. Fig. 11 (b) shows the temperature dependence of the device in the LRS. The resistance of the device increases linearly with rise in temperature. The device meets Ohm's law in such case[19]. The consistence with with the linear fit of Fig. 10, further verifies that conduction mechanism of the device is consistent with the SCLC mechanism.

Fig. 12 Resistance temperature dependence for (a) HRS and (b) LRS of device with Sample F In order to verify the accuracy of the fitting results after doping Ag NPs, we also tested the resistance temperature dependence of the device doped with Ag NPs. Fig. 12 shows the resistance temperature dependence of sample F in the ON and OFF states. It can be seen that when the device is in a high resistance state (OFF State), the device resistance decreases as the temperature rises, and at the same time exhibits a non-linear relationship. In the low resistance state (ON State), the resistance of the device increases with the temperature increasing, and exhibits a linear relationship. This is consistent with the fitting result of Fig. 10 (b). 3.3 Stored process of the devices To investigate the electronic structure and transfer process of the polymer, the

Journal Pre-proof values of HOMO and LUMO levels of PF-MeOTPA were simulated by DFT (B3LYP/6-31G). The size of EHOMO represents the molecular capability to generate electrons. With a stronger HOMO level, molecules tend to supply more electrons to receptors with low energy molecular orbitals. The size of ELUMO denotes the molecular capability to receive electrons. with a weaker LUMO level, molecules are more likely to accept electrons. The interaction between molecules could be determined by a combination of these two variables, expressed by the band gap, ΔE=ELUMO−EHOMO. A larger band gap indicates a better transport ability for the carrier. The theoretical computation of the PFO level is shown in Table 2. The EHOMO, ELUMO and ΔE of the polymer PF-MeOTPA were -5.04 eV, -2.22 eV and 2.82 eV, respectively. The electron cloud of the HOMO part is primarily distributed on methoxytriphenylamine (MeOTPA), which is favorable for obtaining stronger hole mobility, and the electron cluster of LUMO is primarily positioned on the conjugated chain of polymer. Table 2 Molecular simulation result for PF-MeOTPA Frontier molecular Energy level (eV) orbital HOMO

-5.04

LUMO

-2.22

Journal Pre-proof In order to verify the results of the quantum theoretical calculation of PF-MeOTPA, the polymer was measured with UV-vis and cyclic voltammetry (CV). The measured data are provided by Fig. 13. It can be seen from Fig. 13(a) that the absorption peak of PF-MeOTPA in the solution is 434 nm, and thus Egopt can be calculated by the eqn (1) to be 2.86 eV. Egopt = 1240/λonset

(1)

The cyclic voltammetry measurements in Fig. 13 (b) show that initial potential of the oxidation (Eonset) is 0.97 eV. And corresponding EHOMO and ELUMO can be calculated according to eqn (2) and (3) as -5.39 eV and -2.53 eV, respectively. The results of these experimental calculations are close to the theoretical calculation results, and the error may be because the theoretical calculation results are derived from a single molecular structure, while the experimental calculation results are obtained from complete long-chain polymers, and the involved solvent in the test and electrolytes are likely to contribute to the imprecision of the experimental results. The experimental calculation results are used for discussion of the electrical memory storage process below. EHOMO= - (Eonset -Eox (ferrocene)+ 4.80) eV (Eox (ferrocene) =0.38 V versus Ag/AgCl) ELUMO = EHOMO + Egopt

(2) (3)

Journal Pre-proof Fig. 13 (a) Ultraviolet visible absorption of PF-MeOTPA; (b) cyclic voltammetry curve of PF-MeOTPA film. In the first scanning step (0 to -6 V) of the device, the conductive glass (ITO) is the anode (-4.8 eV) with the aluminum electrode as the cathode (-4.27 eV). The storage process of ITO/PF-MeOTPA/Al is presented by Fig. 14 (a). It produces an energy barrier of 1.74 eV between the aluminum electrode and LUMO level of PF-MeOTPA, which is significantly higher compared with the anode and HOMO. The lowest energy barrier between the energy levels (0.59 eV) indicates that hole injection to HOMO level from the anodic side is more likely to occur throughout the process, with conduction being dominated by hole injection. The barrier between LUMO level and the cathodic side hinders the transmission of electrons, which leads to a weak increase in current at the beginning of the rescan. if the applied voltage is elevated to a certain value (Vtv), the external energy enables the electron to surpass the barrier between the cathodic side and the LUMO level of the PF-MeOTPA, so that a large amount of charge injection into the film exhibits a sudden increase in current. The off state is converted to the ON state for the device. In contrast, when voltage is reversed, the device resumes the off state. The charge injected during this process is separated from the PF-MeOTPA film, while the filled trap eliminates the creation of new traps for the charge carriers to jump, allowing the device to switch back to a OFF state[20]. In addition, we also simulated the electrostatic surface potential (ESP) of polymer molecules, and the results are shown in Fig. 14 (b). It can be seen that there is

Journal Pre-proof a continuous whole ESP (blue) on the surface of the entire molecule, through which the charge carriers can be transported smoothly, and the negative ESP region generated (red) by the methoxy group may behave as a "charge trap" to block the transport of charge. When the voltage is low, it is difficult for the charge carriers to obtain sufficient energy to resist injection barrier between the donating and accepting part, so it produces the OFF state. with elevated voltage, the charge carriers possess enough energy and the accumulated charge are injected from the donating side to the accepting side to fill the trap, giving rise to a switch from the OFF state to the ON state. During the transition from HOMO to LUMO, electrons could be transported from donor to acceptor moiety and tend to be more concentrated. The trapped carriers can form a charge separation state by an intra-molecular transfer of charge. Furthermore, only one electronegative group is present in its backbone of PF-MeOTPA, and the DFT calculations also revealed the absence of twisted configuration in the backbone of these molecules. Hence, the charge transfer process cannot maintain the charge separation state[21]. Therefore, when a reversed voltage is applied, the trapped carriers can be restored to the initial state, thereby exhibiting a Flash-type switching behavior.

Fig. 14 (a) The storage process of (a) ITO/PF-MeOTPA/Al; (b) The ESP molecular

Journal Pre-proof simulation result of PF-MeOTPA. After doping Ag NPs into the active layer, Ag NPs as a capture center for electrons is favorable for charge injection and capture. In addition, Ag NPs can also form a metal conductive filament to increase the ON/OFF current ratio of the device, as shown in Fig.15.

Fig. 15 Schematic diagram of the switching mechanism of ITO/PF-MeOTPA:Ag NPs/Al In the initial state, Ag NPs are freely distributed in the polymer film as shown in Fig. 15 (a). When a voltage is applied, an oxidation reaction on the surface of Ag NPs generates Ag+ ions. These Ag+ ions aggregate around the Al electrode due to coulomb on the surface of Ag NPs, as shown in Fig. 15 (b). With sufficiently high voltage, a large amount of Ag+ ions are generated and connected to adjacent Ag NPs to form conductive filaments. When the conductive filaments connect the two electrodes, a multitude of charge are conducted through the filaments, and the device is switched from the ON state to the OFF state. The ON state is as shown in Fig. 15 (c). the application of a reverse voltage produces an electrochemical dissolution somewhere in the filament, and leads to a breakdown of the filament and thus a switch from the ON state to the OFF state, as demonstrated by Fig. 15(d)[22,23].

Journal Pre-proof Different doping concentration of Ag NPs will affect device performance. In order to quantify the influence of doping concentrations of Ag NPs on storage performance, we calculated the average and standard deviation of the ON/OFF current ratio and the threshold voltage with 30 samples of each group for a variety of doping concentration, as shown in Fig.16. The threshold voltage and electrical storage performance of the device present a clear dependence on doping concentration. With rise in doping level of Ag NPs increases, the threshold voltage of the device gradually decreases, until a minimum value of -1.10 V at a doping concentration of 7.97 wt%. The ON/OFF current ratio gradually increases with respect to the doping level of Ag NPs, and shows a reversed tendency after the maximum peak at doping concentration of 7.97 wt%. This phenomenon is possibly due to dispersion of Ag NPs in the active layer, conductive filaments formed by external voltage, and transport of electrons along the conductive filaments. As the doping level of Ag NPs increases, more filaments are formed after voltage excitation, which increases the ON/OFF current ratio of the device and decreases the threshold voltage[24]. Fig. 17(a) shows the ON/OFF current ratio window for devices with sample A and sample F. It demonstrates apparently that with doping of Ag NPs, the memory performance of the device is greatly improved. The switch ratio is increased by an order of approximately 102. However, the figure clearly reveals that the ON/OFF current ratio tends to decrease after the maximum value. It is possibly attributed to that the excessive amount of Ag NPs due to redundant doping in the active layer produce a high conductivity throughout the entire active layer and reduce the ON/OFF current ratio

Journal Pre-proof of the device. Consequently, the device exhibits a less favorable memory performance with a further increase in doping level, which is the current-voltage characteristic of the device with a doping concentration of 12.05 wt% (Sample I), as shown in Fig. 17(b). The curve shows that electrical storage performance is lost[25]. The device shows conductor behavior, mainly because the shorter distance between the isolated Ag NPs and the carriers forms a current pathway.

Fig.16 (a) The current ratio for ON/OFF state and (b) threshold voltage of the devices with different content of Ag NPs.

Fig. 17 (a) The current ratio for ON/OFF state of ITO/PF-MeOTPA/Al and ITO/PF-MeOTPA: Ag NPs/Al; (b) I-V curve of device with Ag NPs doping concentration of 12.05wt% 4. Conclusions

Journal Pre-proof In conclusion, the present study prepared the polymer PF-MeOTPA through the Suzuki reaction successfully and applied it to the storage devices by spin coating. In addition, a doping with an appropriate amount of Ag NPs could improve the memory performance of the device, and the influence of doping concentrations on device performance were investigated. The device memory performance is optimal at a doping concentration of 7.97 wt%, the ON/OFF current ratio is improved (837 to 11000), and the threshold voltage is reduced (-1.79 V to -1.10 V). Furthermore, the current of the device possesses stability after a duration over 3 hours at 2 V, or repeated cycles over 30,000 at 2 V pulse voltage. Favorable stability, small threshold voltage and large ON/OFF current ratio contribute to the potential application of PF-MeOTPA: Ag NPs materials in the field of organic electrical storage devices. Acknowledgements The authors are grateful to the support of the National Natural Science Foundation of China (grant numbers 51527804 and 51973051), Heilongjiang University JMRH project (HDJMRH201913), and Natural Science Foundation of Heilongjiang Province of China (grant number B2017010) References [1] S. H. Jo and W. Lu, CMOS compatible nanoscale nonvolatile resistance switching memory, Nano. Lett. 8 (2008) 392-397. [2] J. Y. Ouyang, C. W. Chu, C. R. Szmanda, et al, Programmable polymer thin film and nonvolatile memory device, Nat. Mater. 3 (2004) 918-922. [3] Y. C. Lai, D. Y. Wang, I. S. Huang, et al, Low operation voltage macromolecular

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

There are no known competing financial interests or personal relationships that influence the work reported in this paper.

Journal Pre-proof Highlights 

Synthesis of a novel triphenylamine-fluorene copolymer and preparation of its memory device.



A series of memory devices were prepared by doping different concentrations of Ag NPs into the copolymer.



After doping Ag NPs, the switching ratio of the device is increased to 104, and the threshold voltage is reduced by 0.69V, thus greatly reducing the misreading rate and reducing the power consumption in the application.