Solution Processed DonorâAcceptor Polymer Based Electrical

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Solution Processed Donor-Acceptor Polymer Based Electrical Memory Device with High On/Off Ratio and Tunable Properties Rahul Narasimhan Arunagirinathan, Peddaboodi Gopikrishna, Dipjyoti Das, and Parameswar Krishnan Iyer ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.9b00077 • Publication Date (Web): 11 Mar 2019 Downloaded from http://pubs.acs.org on March 11, 2019

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ACS Applied Electronic Materials

Solution Processed Donor-Acceptor Polymer Based Electrical Memory Device with High On/Off Ratio and Tunable Properties Rahul Narasimhan Arunagirinathan,a Peddaboodi Gopikrishna,a Dipjyoti Dasa and Parameswar Krishnan Iyer*a,b aCentre

for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati-

781039, Assam, India bDepartment

of Chemistry, Indian Institute of Technology Guwahati, Guwahati-781039,

Assam, India. E-mail: [email protected]; Fax: +91 361 258 2349

Supporting Information Available ABSTRACT: A non-volatile “resistive random access memory” (ReRAM) device is reported with a series of four conjugated polymers (CPs) containing poly[2,7-(9,9’dioctylfluorene)-co-N-phenyl-1,8-naphthalimide (PFO-NPN) as donor and acceptor, respectively. A single layer, thin film of PFO-NPN copolymer that is sandwiched between ITO and Aluminum shows bistable property with a remarkably high Ion/Ioff ratio of 108. The charge transport of the polymer is studied by fitting I-V curves with various conduction models to realize that the trap charge limited current (TCLC) assists in switching and

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exhibiting bistable property of the memory device. Theoretical calculations were also performed on the polymer to elucidate the presence of traps on the carbonyl oxygen atoms of NPN moiety. In addition, ReRAM properties like Ion/Ioff ratio and write voltage were also tuned by changing the concentration of the acceptor moiety. Four different copolymers of acceptor concentrations (05%, 10%, 35%, and 50%) with respect to donor concentrations were characterized as a memory device. The device with high acceptor concentration (50%) showed the lowest Ion/Ioff ratio (103) and write voltage (0.8 V). It was also observed experimentally that Ion/Ioff ratio and write voltage decreases sequentially with an increase in the acceptor concentration, thereby providing flexibility in tuning the memory parameters by allowing a molecular level change in the active material. The optical studies were performed to elucidate the mechanism of the tunable memory characteristic of the polymer and the results reveal that the tunability is achieved due to the variation in the injection barrier and the strength of ICT for the different polymers.

Keywords: Conjugated polymer memory device; ReRAM; Organic electronics; Tunable devices; Donor-acceptor; Charge transfer.

1. Introduction Organic electronics has been marching along to prove worthy over archrival inorganic electronics due to its various inimitable properties like low-temperature processing,


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flexibility, solution processing, printability, low manufacturing cost, more eco-friendly and recyclability of the devices. It all began after the discovery of polyacetylene as an organic semiconductor as first reported in the year 1977.1 Since then organic semiconductors have evolved into every possible areas of electronic device applications like the emission of light as in organic light emitting diode (OLED), conversion of light energy into electrical energy as in organic photovoltaic cells (OPV) and electrical modulation through field effect as in the case of organic field effect transistor (OFET).2-5 In addition to the above applications, organic semiconductors are also used in storing and retrieving data in a binary format which are widely called as organic memories.6-9 Resistive random access memory (ReRAM) is a two terminal device like resistor but it has the ability to change the resistance with respect to the applied voltage and they are first reported in 1967.10 The bistability of the ReRAM devices are high resistive state (HRS) and low resistive state (LRS) which are analogous to binary bits logic 0 and logic 1, respectively. Initially, prior to applying voltage, the device is in the OFF state (HRS) and it remains in this OFF state for a voltage, less than the write voltage (threshold voltage), whereas, the device switches to ON state(LRS) when a voltage equal to or more than the write voltage is applied. Once the device is switched to LRS it remains in this state permanently or temporarily based on the withholding capacity of the material. Generally, ReRAM devices have a single or multiple layers of switching medium sandwiched between the two conducting electrodes as in shown Figure 1a. Various materials have been used as a switching medium which includes, perovskite



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oxides,11,12 metal oxides13,14 and certain organic materials also show memory property due to the metallic filaments15-17 formed between the two electrodes. In contrast, various donor-acceptor (D-A) based organic molecules including small molecules and polymers show bistability due to the formation of charge-transfer complexes. In general, small molecules are thermally evaporated to grow a uniform layer of the active memory layer, whereas, polymers can be solution processed owing to low fabrication cost, printability and roll-to-roll processing. Various conjugated polymers (CPs) with D-A configuration are widely experimented as a switching medium and has successfully shown bistability property.18-25 For instance a CP PFOXPY (PFO-donor,2,2’-bipyrididine(Py)-acceptor) is used as volatile (DRAM) with an Ion/Ioff ratio of 106.18 Also poly(arylene vinylene) derivatives exhibit SRAM and WORM property for various pendant substituents19 and the experimental values project a much lower Ion/Ioff ratio of 104. Likewise, poly(NVinyl carbazole)

functionalized

with cyanoacyetylated cDR1

exhibited

WORM

property20 with an Ion/Ioff ratio of 105. Carbazole-Nitrobenzene were also used as D-A in a random copolymer exhibiting two stages of switching, initially due to charge transfer (CT) between D-A and later due to filamentary conduction. The bistability is also seen in few

other

polymer

derivatives

of

polythiophenes,21 pyrrolopyrrole22 polyimides,23,24 poly(p-phenylene-vinylene), etc.25

Though many new molecules were synthesized for organic memories, a tunable electrical property of the memory device is essential to ascribe the diverse end user of


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the technology. The electrical property of a memory device can be tailored by device engineering, likewise, molecular engineering can also be used to manipulate the electrical characteristics of a memory device. Kang et al.26 synthesized a novel block copolymer (PVPCz:P2VP) with different block ratio, showing different characteristics of memory. The organic memory devices can also be tuned from volatile to nonvolatile based on varying the perylene imide composition in the copolyimide.27 The write voltage of the organic memory device can also be tuned when different acceptor derivatives of hexaazatriphenylene were separately mixed with P3HT donor.28 Thus the extraordinary tunable strength of the polymer encouraged us to fabricate a D-A based organic memory using a CP containing poly[2,7-(9,9’-dioctylfluorene)-co-N-phenyl-1,8naphthalimide (PFO-NPN) which have been synthesized by varying the N-phenyl-1,8naphthalimide (NPN) concentrations (xx = 05%, 10%, 35% and 50%) in the polyfluorene (PFO) main chain. The PFO-NPN copolymers exhibited excellent bistability and characteristics of WORM memory. Moreover, the characteristics of a memory device like Ion/Ioff ratio and write voltage are highly depended on the NPN concentration in the PFO main chain. The acceptor concentration plays a major role in altering the property of the memory device23,27,28 and by varying the acceptor concentration of the PFO-NPN, device parameters like Ion/Ioff ratio up to 108 and write voltage as less as 0.8V were achieved. In addition, to the best of our knowledge, this approach of tuning the memory properties



like Ion/Ioff ratio and write voltage is not achieved on a CP containing donor and acceptor in a single chain.

Figure.1 (a) 3D model of the fabricated device and (b) Chemical structure of PFONPNxx, where xx denotes the concentration of NPN moiety added into PFO. 2. Experimental Section PFONPNxx copolymers were synthesized using Suzuki polymerization. The synthetic procedure and the characterization of the polymer series are reported earlier.30 The chemical structure of the CP is shown in Figure 1b. The synthesized polymers were dissolved in chlorobenzene, filtered and spin coated on the anode indium tin oxide (ITO),


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with a film thickness of ~80 nm((Figure S1 of supporting information)) measured via Veeco Dektak 150 Profilometer. The residual solvent in the memory layer is removed by annealing at 120 °C for 30 min in an inert atmosphere glove box. Finally, aluminum was deposited at an average rate of 6-7 Å/s with a thickness of ~60 nm. The rate and thickness of the deposition were strictly followed in order to avoid the device getting short due to the leakage current. The active area of the device is 150mm2 and the final device is characterized under the ambient condition using a Keithley 4200 semiconductor characterization system and the best-obtained results are considered. 3. Results and Discussions 3.1 Electrical Properties of PFONPN05 Figure 2 shows the I-V characteristics of the ReRAM device for the structure ITO/PFONPN05/Al, when a positive voltage is swept from 0 to +5 V. Initially the device is in OFF state (HRS) and at around 3 V an abrupt increase in the current is seen which is the threshold voltage for the device to switch from the OFF state (HRS) to the ON state (LRS). Once the device is turned ON it remains in the LRS and showed a positive hysteresis which confirms the storage of data onto the device. When a negative dual sweep of 0 to -5 V is applied, no negative hysteresis is formed and the device remains in the LRS showing the property of a WORM memory.



Figure 2. I-V characteristics of a PFONPN05 device in a semi-log plot for a sweep voltage from 0V to + 5V (sweep 1), +5V to 0V (sweep 2), 0V to-5V (sweep 3) and -5V to 0V (sweep 4). 3.2 Experimental and fitted models The switching mechanism of the memory is likely due to the existence of traps in the copolymer. By fitting the current-voltage graph to the various transport models, the presence of traps can be validated. In the low voltage region, the transport is predominantly due to thermionic emission process. During this process, the electrons in the metal get sufficient kinetic energy to cross over the polymer layer. The thermionic emission model31 predicts the transport of current, where the natural logarithm of current is linearly proportional to the square of the voltage applied as per the equation 1. 𝑰 ∝ 𝑻𝟐𝒆𝒙𝒑

― (𝝋 ― 𝒒 𝒒𝑽/𝟒𝒅𝝅𝝐) 𝒌𝑻


(1)

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Where I, V, T, φ, q, d, ϵ and k represents Current, Voltage, Temperature, Schottky barrier height, electronic charge, thickness of the memory layer, dielectric permittivity and Boltzmann’s constant, respectively.

Figure 3. Fitting of I-V curves in various regimes based on the applied voltage (a) Ln(I) vs V1/2 for a low voltage (Voltage

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