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AFM2 2017

Phase Separation in Organic Semiconducting/Ferroelectric Blend Films with Air-Stable Resistive Properties

Junhui Wenga, Qiusong Chena, Weilin Liua, Xunlin Qiub,*, Guodong Zhua,*

aDepartment of Materials Science, Fudan University, Shanghai, ChinabInstitute for Print and Media Technology, Technische Universität Chemnitz, Chemnitz, Germany

Abstract

Organic semiconducting/ferroelectric blend films integrate both resistive and rectifying properties and thus well solve the cross-talk problem usually suffered in resistive memories. Till now several kinds of semiconducting/ferroelectric blend systems have been developed with good retention and ON/OFF properties. However, seldom work concerned their air stability of electrical properties. Here we reported one kind of blend resistive films with air-stable Poly[(9,9-dioctylfluorenyl-2,7-diyl) -alt-co-(bithiophene)] (F8T2) as semiconducting material and the copolymer of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)) as ferroelectric material. We introduce AFM based nanoscale electrical measurements into the observation of phase separation in F8T2/P(VDF-TrFE) blend films. Conductive AFM measurements present discrete high-conductivity circular domains embedded in continuous low-conductivity matrix. Surface Potential Microscopy results further indicate that these discrete circular domains show much lower surface potential than those from their surrounding matrix, implying the difference in chemical structures. It is reasonable to attribute those continuous surrounding matrix to ferroelectric P(VDF-TrFE) phase and those discrete circular domains to semiconducting F8T2 phase who penetrates the whole films resulting in large electrical conductivity. F8T2/P(VDF-TrFE) resistive blend films present good restive performance with high ON/OFF ratio up to 3.5×10 3. Such blend films show excellent electrical stability in air even after aging of 200 days.

© 2018 Elsevier Ltd. All rights reserved.Selection and/or Peer-review under responsibility of 2017 International Workshop on Atomic Force Microscopy for Advanced Functional Materials.

Keywords: semiconducting/ferroelectric blend; resistive memory; AFM; phase separation

1. IntroductionIn recent years organic materials and devices have obtained dramatic development due to their potentials in low-

cost and flexible electronics, among which nonvolatile memories are one of essential modules for long-time storage * * Corresponding authors. Tel.: +86-21-65642872 (Prof. Zhu).E-mail address: xunlin.qiu@mb.tu-chemnitz.de (Prof. Qiu); gdzhu@fudan.edu.cn (Prof. Zhu)

2214-7853 © 2018 Elsevier Ltd. All rights reserved.Selection and/or Peer-review under responsibility of 2017 International Workshop on Atomic Force Microscopy for Advanced Functional Materials.

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of information. Currently the work on nonvolatile random memories mainly focuses on four types of devices: ferroelectric memories based on ferroelectric polarization, phase-change memories based on electric-induced phase transition, magnetic memories based on magnetoresistance effect, and resistive memories [1]. Resistive memory works based on the voltage-induced change of electrical resistance across dielectric films and possesses those advantages of such as excellent scalability, fast reading and writing speeds and ease of production, and has been regarded as a promising candidate for next-generation memory devices [2]. Resistive memory is usually structured by sandwiching resistive films between crossed word and bit lines. Electrical selection of different word and bit lines results in the operation of specific memory cells. Though such cross-bar array structure greatly simplifies the fabrication of resistive memory, it also results in cross-talk problem which may results in the misleading identification of high-resistance state during reading operation. One solution is to integrate transistors or diodes into resistive structures to eliminate cross-talk problem which surely complicates the production process of those devices [3, 4].

Organic semiconducting/ferroelectric blend films integrate both resistive and rectifying properties into blend films and well solve the cross-talk problem [5]. Semiconducting/ferroelectric blend films are deposited from their co-dissolved solution and during the evaporation of organic solvent, phase separation occurs with discrete semiconducting phase embedded in continuous ferroelectric matrix. Semiconducting phase electrically connects with both top and bottom electrodes forming conductive channels, whose conductivity is tuned by the polarization states of the surrounding ferroelectric phase. In such blend system, electrode materials should be carefully selected in order to build up a potential barrier with proper height between electrode/semiconductor interfaces [6]. Thus, for the virgin blend films with no poling process in ferroelectric phase, this potential barrier restricts the charge injection through the interface and charge transport is injection limited (IL). However, once the ferroelectric phase is polarized, one polarization state can effectively decreases the barrier and switches the charge transport to space charge limited (SCL) current, while the opposite polarization state shows no positive influence on this barrier and the carrier transport still remains injection limited. Thus resistive and rectifying characteristics are simultaneously realized in the same blend films. Currently in such resistive blend films ferroelectric material is usually the copolymer of vinylidene fluoride and trifluoroethylene, P(VDF-TrFE), and the reported semiconducting materials include P3HT (poly(3-hexylthiophene)) [5,7], PFO (poly(9,9-dioctylfluorenyl-2,7-diyl) end-capped with dimethylphenyl groups) [8,9], PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) [10], F8BT (Poly[(9,9-din-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)]) [11,12] and P3EPT (poly[3-(ethyl-5-pentanoate)thiophene-2,5-diyl]) [13,14]. Usually electrical property of most of organic semiconductors was not air stable. Though in such blend systems the content of semiconducting phase was lower than, for example, 10 wt. %, the conductivity of the whole blend films was controlled mainly by this semiconducting phase. Thus the electrical stability in semiconducting phase might greatly influence the resistive property of the whole blend films. However seldom work concerned the electrical stability of such blend devices for long-time exposure in air. One well accepted understanding of air instability in organic semiconductors is that these conjugated polymer semiconductors are susceptible to the ambient oxygen-induced doping resulting in larger OFF-state current, lower ON/OFF ratio and worse device performance [15]. Generally increasing the ionization potential of organic semiconducting polymers, that is lowering the highest occupied molecular orbital (HOMO) level, can improve environmental stability by inhibiting the oxygen doping [15,16]. Therefore organic semiconducting polymers with lower HOMO level is expected to fabricate semiconducting/ferroelectric blend films with high ON/OFF ratio and air-stable electrical properties. Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(bithiophene)](F8T2) is such kind of p-type semiconducting polymer with HOMO level as low as -5.5 eV and shows good air stability of electrical property [17-19]. Here we report our work on fabrication and characterization of air stable F8T2/P(VDF-TrFE) resistive blend films with high ON/OFF ratio, up to 3.5×103.

2. ExperimentalF8T2 and P(VDF-TrFE) with 70/30 VDF/TrFE molar ratio were bought from American Dye Source and Kunshan

Hisense Electronic Co. Ltd., respectively, and used as received. Structural formulas of both materials are shown in Fig. 1a. P(VDF-TrFE) powder was dissolved in tetrahydrofuran (THF) solvent to obtain a solution of 3.0 wt. %. Then F8T2 was added into P(VDF-TrFE)/THF solution to get the blend solutions with weight ratio between F8T2 and P(VDF-TrFE) of 1:20. Finally the blend solution was centrifuged to obtain a homogeneous solution.

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Ag bottom electrodes were vacuum deposited on cleaned glass substrates via hard mask resulting in stripe-like electrodes with thickness of about 50 nm and width of 0.5 mm. Blend solution was spin coated onto such Ag-coated substrates. Thickness of these as-coated blend films was about 310 nm. Films were further annealed at 135 °C to promote the crystallization of P(VDF-TrFE) phase. Finally stripe-like top Ag electrodes were vacuum-deposited to form the sandwiched resistive devices with effective electrode area of 0.25 mm2 for each memory cell.

Morphologies of blend films were characterized by both atomic force microscope (AFM, Dimension Edge, Bruker) and scanning electron microscope (SEM, XL30FEG, Philips). AFM worked in tapping mode for morphology measurements. Conductive AFM (C-AFM) measurements were performed based on tunneling AFM mode (TUNA, Dimension Edge, Bruker). Surface potential microscope (SPoM, Dimension Edge, Bruker) measurements were operated in lift mode with lift height of 60 nm. During both C-AFM and SPoM measurements, conductive probe was electrically grounded while external voltage was applied to the bottom electrode. Current-voltage (I-V) and retention measurements were performed by a sourcemeter (B2900A, Agilent). In retention measurements, reading voltage is applied only during reading operation, otherwise reading voltage was removed to minimize the influence of reading voltage on stored information. All electrical measurements were conducted in air and at room temperature.

3. Results and discussionStructural formulas of ferroelectric P(VDF-TrFE) and semiconducting F8T2 are shown in Fig. 1a. After

evaporation of organic solvent, F8T2/P(VDF-TrFE) blend films are expected to form phase separation structure with discrete and conductive F8T2 channels surrounded by continuous P(VDF-TrFE) phase, as schematically illuminated in Fig. 1b. Since resistive mechanism in semiconducting/ferroelectric blend films has been comprehensively investigated and well accepted via both experimental observations [5-14] and theoretical simulations [20], here we hope to first give a brief demonstration on this resistive mechanism in F8T2/P(VDF-TrFE) blend films. HOMO level of F8T2 is about -5.5 eV [17]. To reduce the OFF-state current during resistive operation, Ag electrodes with work function of 4.26 eV are intentionally selected resulting in an injection barrier of about 1.24 V between F8T2/Ag interfaces. Due to this large injection barrier, charge transport is injection limited for as-prepared blend films with no poling process in ferroelctric phase, as is demonstrated in the band diagram in Fig. 1c. However, when ferroelectric phase is polarized, one polarization state induces the accumulation of holes at ferroelectric/semiconducting interface resulting in the band bending and therefore the decrease of the injection barrier (here the polarization state is denoted as positive polarization state), as is shown in the band diagram of Fig. 1d. Charge transport is switched from injection limited to space charge limited modes and large ON-state current is expected [5]. However, the other opposite polarization state shows no positive influence on reducing the injection barrier and thus the low OFF-state current remains. Thus such polarization state is denoted as negative polarization state to differentiate from the positive one.

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Fig. 1. (a) Structural formulas of P(VDF-TrFE) and F8T2, (b) expected phase separation structure in the blend films and (c) and (d) the band diagrams at semiconductor/electrode interface for blend film (c) without poling and (d) with positive poling of ferroelectric phase. The band diagrams in (c) and (d) are positioned at the interface of F8T2 and bottom Ag electrode, where local sandwiched structure of P(VDF-TrFE)/F8T2/Ag is expected to be formed [5] and the potential barrier at F8T2/Ag interface can be modulated by the polarization state of P(VDF-TrFE).

First phase separation in F8T2/P(VDF-TrFE) blend systems was verified since it is the base for resistive mechanism. SEM image shown in Fig. 2a shows the typical morphology of such semiconducting/ferroelectric blend films: discrete disk-like semiconducting domains embedded in continuous ferroelectric phase with needle-like crystallites [8,11,12]. AFM morphology in Fig. 2b also shows such phase separation structure though to some extent the large surface roughness blurs the observation of phase separation. Phase separation was further proved by AFM-based measurements of nanoscale electrical properties. Surface potential microscope (SPoM) observation (Fig. 2c) clearly present such phase separation in the corresponding potential image where discrete black holes are randomly embedded in continuous matrix. Since both phases have different chemical structures, it is reasonable to attribute the black holes to F8T2 phase and the continuous matrix to P(VDF-TrFE) phase. C-AFM measurement was also performed based on TUNA mode of AFM. Since semiconducting phase is expected to connect with both top and bottom electrodes, thus in C-AFM measurements semiconducting disks should show much larger current than its surrounding insulating P(VDF-TrFE) phase. The corresponding C-AFM image in Fig. 2d shows undoubted difference of conductivity between both semiconducting and ferroelectric phases. Blurred topography in Fig. 2d should be due to the scratching of AFM tip on film surface in contact mode.

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Fig. 2. (a) SEM and (b) AFM morphologies and (c) SPoM and (d) C-AFM characterizations of 1:20 F8T2/P(VDF-TrFE) blend films.F8T2/P(VDF-TrFE) blend films show good electrical stability in air. Typical results are shown in Fig. 3. The I-V

curve of as-prepared F8T2/P(VDF-TrFE) blend film shows obvious butterfly shape indicating the resistive switching property. According to the resistive mechanism of semiconducting/ferroelectric blend system, during the application of small positive voltage between 0 and +12.5 V, small OFF-state current occurs with the order of magnitude of 10-9 A due to previously polarized ferroelectric phase which set the current transport injection limited in semiconducting channels. With the further increase of positive voltage to +55 V, polarization reversal occurs in ferroelectric phase at about 15.3 V indicating a coercive field of about 49.4 MV/m, and thus electrical transport in F8T2 channels is switched to space charge limited one resulting in distinct increase of current to the order of magnitude of 10-6 A (ON-state). When the voltage sweeps back from 55 to 0 V, positive polarization state remains

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in ferroelectric phase and thus large ON-state current is still observed, as also proves the nonvolatile nature of such semiconducting/ferroelectric blend memory. For the application of negative voltage, the same analysis is applicable.

Much interesting is that the resistive switching property of the film shows no obvious degradation even after an aging time of as long as 200 days (see the open circles in Fig. 3a). Here aging just means that the film is stored in air and in room temperature for long enough time. The butterfly loop after 200-day aging still follows the shape of the I-V curve of the same film measured just after preparation, indicating excellent air stability of the electrical property in such F8T2/P(VDF-TrFE) blend films.

Retention performance of the blend film before and after aging process is presented in Fig. 3b. Before retention measurements, the film was pre-polarized by +30 V (-30 V) voltage for 10 s in order to switch the film to ON (OFF) state and then +5 V voltage was applied to read the ON-state (OFF-state) current for retention measurements. Note that here, to avoid any potential influence of this reading voltage on ON or OFF states and also to imitate the actual operation of memory devices [21], the reading voltage is only applied during reading operation; otherwise reading voltage was removed. For the as-prepared film, both ON- and OFF-state current keeps nearly constant with ON/OFF ratio of 3.5×103 during the whole retention measurement of 2000 s. Even after 200-day aging, the same film still retains good retention property though its ON-state current fluctuates to some extent between 8.9×10 -6 and 3.4×10-6

A and the OFF-state current is slightly larger than that of the original film. Good retention performance further verifies both nonvolatile memory function and the air stability of electrical performance of such blend films.

Fig. 3. (a) Current-voltage curves and (b) retention properties from one 1:20 F8T2/P(VDF-TrFE) blend film before and after aging process of 200 days. Arrows in Figure (a) indicated the direction of voltage sweeping.

4. ConclusionIn summary, we reported air stable semiconducting/ferroelectric blend resistive films with F8T2 as

semiconducting phase and P(VDF-TrFE) as ferroelectric phase. Phase separation in such blend films was characterized and proved via various microscopic techniques. Electrical measurements indicated that such F8T2/P(VDF-TrFE) blend films presented ON/OFF ratio of about 3.5×103 and good retention performance even after aging of 200 days.

AcknowledgementThis work was financially supported by NSFC [Grant No. 61774043], NSAF [Grant No. U1430106] and STCSM

[Grant No. 17142201900].

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