Poly(3,4-ethylenedioxythiophene)âPoly(styrenesulfonate) Interlayer

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PEDOT:PSS Interlayer Insertion Enables Organic Quaternary Memory Xue-Feng Cheng, Xiang Hou, Wen-Hu Qian, Jing-Hui He, QingFeng Xu, Hua Li, Na-Jun Li, Dong-Yun Chen, and Jian-Mei Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06810 • Publication Date (Web): 04 Aug 2017 Downloaded from http://pubs.acs.org on August 5, 2017

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ACS Applied Materials & Interfaces

Lu, Jian-Mei; College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Suzhou Nano Science and Technology, National United Engineering Laboratory of Functionalized Environmental Adsorption Materials, Soochow University, Soochow University, Suzhou

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PEDOT:PSS Interlayer Insertion Enables Organic Quaternary Memory Xue-Feng Cheng,‡ Xiang Hou,‡ Wen-Hu Qian, Jing-Hui He,* Qing-Feng Xu, Hua Li, Na-Jun Li, Dong-Yun Chen, Jian-Mei Lu * College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Suzhou Nano Science and Technology, National United Engineering Laboratory of Functionalized Environmental Adsorption Materials, Soochow University, Soochow University, Suzhou 215123, PR China ‡

. These authors contributed equally to this work and should be considered co-first authors.

KEYWORDS: Quaternary Memory, PEDOT:PSS, interface engineering, RRAM, organic

ABSTRACT: Herein, for the first time, quaternary resistive memory based on organic molecule is achieved using surface engineering. A layer of PEDOT:PSS was inserted in between the ITO electrode and the organic layer (SA-Bu) to form an ITO/PEDOT:PSS/SA-Bu/Al architecture. The modified resistive random-access memories (RRAM) devices achieve quaternary memory switching with the highest yield (~41%) to date. The surface morphology, crystallinity, and mosaicity of the deposited organic grains are greatly improved after the insertion of an PEDOT:PSS interlayer, which provides better contacts at the grain boundaries as well as the


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electrode/active layer interface. The PEDOT:PSS interlayer also reduces the hole injection barrier from the electrode to the active layer. Thus, the threshold voltage of each switching is greatly reduced, allowing for more quaternary switching in a certain voltage window. Our results provide a simple yet powerful strategy as an alternative to molecular design to achieve organic quaternary resistive memories.

Introduction

Resistive random-access memories (RRAMs) have attracted increasing attention in recent years due to the downscaling limit of conventional Si-based binary storage techniques.1-3 Using a simple electrode/active material/electrode sandwich-like structure, RRAM cells are able to assemble laterally in a crosspoint architecture with a scalable feature size of 4F2, where F is the minimum feature size—the linewidth and spacing of the electrodes.3 The crosspoint layers can further stack vertically in the third dimension to increase the information density. In addition, each RRAM cell is able to store, and multiple resistance levels are recognized.4-6 For example, ternary RRAMs are able to store 3n states (n is the number of cells), offering a significant expansion of the storage capability compared to the 2n states of conventional binary techniques.711

After years of research, many inorganic oxides12, polymers13-14 and small molecules10, 15 have

been found to demonstrate multilevel RRAM behaviors. Among them, small molecules have recently received increasing attention because of their tunable structures, easy purification, flexibility, and low cost.3 Compared to ternary memories, quaternary RRAMs can square the capacity of binary cells, thus further boosting the information density. In addition, quaternary storage is more compatible with the current binary technique than ternary storage. Unfortunately, quaternary RRAMs have


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rarely been reported because of the challenge in designing the active materials.16 Our group reported the first example of quaternary RRAMs using small organic molecules.17 However, strategies not heavily relying on the material are still unavailable and therefore highly desirable. In addition to active material design and selection, surface engineering is widely reported in other fields of thin-film electronics, such as solar cells18-19, organic light-emitting diodes20 and photodetectors.21 Surface engineering of organic layer is able to smoothen the Fermi levels through creation of a diffusive metal layer. Insertion of a conductive polymer layer such as poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) will lower the hole transportation barrier and increase the conductivity.22 Our recent work has demonstrated that engineering an ITO substrate by insertion of an insulating self-assembled phosphoric acid monolayer effectively increased the ternary RRAM device yield23. In this work, we demonstrated for the first time that quaternary RRAM switching can be achieved by insertion of a conductive polymer layer between the electrode and the active layer. PEDOT:PSS was inserted between the ITO electrode and the organic layer (SA-Bu) to form an ITO/PEDOT:PSS/SA-Bu/Al architecture. The modified RRAM devices are capable of quaternary memory switching with a yield of 41%, the highest reported to date. Results and Discussion Figure 1a shows the fabrication procedure of our RRAM devices. Briefly, ITO-coated glass was employed as the bottom electrode after degreasing. The squaraine molecule, 2-((4butylphenyl) amino)-4-((4-butylphenyl) iminio)-3-oxocyclobut-1-en-1-olate (SA-Bu), was thermally evaporated in vacuum and deposited on a PEDOT:PSS-covered or naked ITO surface, followed by in situ deposition of Al electrodes. SA-Bu is chosen because it is a good organic


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RRAM material.23 SA-Bu has a unique zwitterionic resonance, which offers a lower band gap, strong molecular interaction and layer-by-layer stacking crystallinity.24-27 However, without PEDOT:PSS insertion, ITO/SA-Bu/Al memory devices only show ternary memory switching with a low ternary yield. To conduct the interface engineering, a PEDOT:PSS thin layer was cast on the ITO surface before the deposition of SA-Bu through spin-coating its DMSO solution.28 The scanning electron microscopy image of the cross-section of a typical PEDOT:PSSengineered device in Figure 1b reveals a four-layer structure, noted as ITO/PEDOT:PSS (100 nm)/SA-Bu (100 nm)/Al (100 nm). After the insertion of PEDOT:PSS, the new devices show a typical quaternary switching behavior that was not observed on the original naked-ITO RRAMs. As seen in Figure 1c, the device initially exhibited a high-resistance state (“OFF” state) when a small “reading” voltage (1 V) was applied. When a negative voltage (0 to -5 V) was applied to the Al electrode (Sweep 1), three abrupt jumps of current were observed at approximate switching threshold voltages of 1.09, -2.38, and -3.52 V, named “ON1,” “ON2,” and “ON3”. Sweeps 4, 5, and 6 were performed on other fresh cells, indicating that the device can successively transit from OFF to ON1, then from ON1 to ON2, and finally from ON2 to ON3 step by step. Once the device reached the ON3 state, that state could be well retained through positive/negative sweeping voltages (Sweeps 7 and 8). These four levels are well separated by a conductance ratio of 1: 3.4×102: 5.4×104: 4.7×107, guaranteeing a low possibility of reading and writing errors. Furthermore, the stabilities of the quaternary memory devices were studied by continuous voltage stress and readout tests at room temperature (Figure 1d). During the test, no obvious degradation was observed in any of the three states for at least 10000 s despite a tiny resistance fluctuation. Therefore, the above


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ternary device is a write-once-read-many-times (WORM) device, of potential application in archive storage.29 Next, to optimize the memory performance and survey the contribution of the inserted PEDOT:PSS layer, its thickness was serially tuned through varying the spin-coating speed from 100 to 8000 rotations per minute (rpm) (Figure 2a). Eight batches of devices (50 cells per batch) with thicknesses of d=0, 33, 41, 51, 62, 108, 148 and 164 nm (determined by atomic force microscopy, Figure S1; d=0 nm refers to the device without PEDOT:PSS insertion) were fabricated. As seen in Figure 2b, without PEDOT:PSS interlayer, there is nearly no quaternary memory switching.23 By inserting a PEDOT:PSS interlayer even as thin as 33 nm, the quaternary device yield immediately increased to 26%. This yield continued to increase to 41% when d=108 nm; a slight decay occurred later when the interlayer became thicker. Potential industrialization of RRAMs calls for a high quaternary device yield and wellseparated threshold voltages that are as low as possible. Statistics of all quaternary devices confirm that the threshold voltage distributions were significantly improved by PEDOT:PSS insertion, as shown in Figure 2b (See more details in Figure S2). First, the three threshold voltages of each batch of quaternary devices are well separated, allowing each cell to be correctly written to expected states within a wide voltage window. After PEDOT:PSS insertion, all three switching voltages remarkably decrease. Particularly, Vth1 lowers from -1.83 V to -0.97 V for d=33 nm, and a constant value (-1 V) is obtained when d≥62 nm. Vth2 and Vth3 follow the same trend; they continuously decrease when d increases to 41 nm (-1.97 V) and later increase until they reach their minima (-1.85 V) at d=148 nm. Therefore, considering the quaternary yield and threshold voltage distribution variation, the PEDOT:PSS interlayer indeed contributes to the


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appearance of quaternary memory switching, and the optimal thickness for quaternary memory performance is 108 nm obtained at a spin speed of 1000 rpm. The role of the PEDOT:PSS layer can be understood firstly from its influence on the SA-Bu film, including its crystallinity, mosaicity and surface morphologies. Macroscopically, as shown in Figure 3, the evaporated SA-Bu on the PEDOT:PSS inversely has a much smoother surface (RMS=0.48 nm) than on the bare ITO (RMS=0.98 nm), although the PEDOT:PSS layer (RMS=4.55 nm) is not as smooth as the bare ITO surface (RMS=0.14 nm). In the previous work, we used self-assembled phosphoric acid monolayer to modify ITO substrate, which would not change the surface roughness too much.23 However, the SA-Bu molecule evaporated on this modified ITO has much smaller roughness. Terminated by surface modifiers of alky chains (PA or PEDOT), the surface energy of ITO substrate will decrease, and the surface adhesion to small molecules will also decrease. In this case, the molecules tend to crystalize themselves with less impact from the substrate in a conformal coating process. In addition, this molecule has a major lamellar stacking manner, grow with less conformal effect will lead to much smoother surface. Such abnormally small smoothness is likely determined by the different surface affinities toward SA-Bu molecules rather than by the morphologies. It was reported that Al deposited on organic film will form a diffusion layer, which will further influence the film quality and electronic properties.30-33 A smoother organic film surface here induce more uniform growth of Al layer, which is benifited for better contact, and in consequence lower and narrow disstribution of switching voltages. Microscopically, the PEDOT:PSS interlayer induces a crystalline phase preference during film preparation. X-ray diffraction (Figure 4a) of PEDOT:PSS on ITO indicates the amorphous nature of PEDOT:PSS, whereas the SA-Bu layer on PEDOT:PSS-modified ITO always has a


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distinct peak at ~4.2°, corresponding to approximately 21 Å (calculated from d=0.77/sinθ Å for Cu Kα beam source), which is in agreement with our previous works.23, 34 Since XRD spectra have larger uncertainties in the small angle range, we recorded their grazing-incidence smallangle scattering spectra (GISAXS, in Figures 4b-k) to resolve more details. The SA-Bu layer on naked ITO was immediately found to have two arcs at the qz direction, corresponding to highly ordered and out-of-plane lamellar structures. The one-dimensional intensity profiles along qz shows that these two arcs are centered at qz=0.27 Å-1 and 0.25 Å-1, corresponding to d=22 and 25 Å (d=2π/qz). These two d values are very close and correspond to the merged peak at ~4.2° in XRD. Since SA-Bu molecule is composed of a rigid and conjugated core and two loose terminal chains, which likely prefers to form a layer-by-layer stacking structure, whereas the cores stack via π-π stacking in layer and the alkyl chains between adjacent layers attract each other.35 Considering an SA-Bu molecule is as long as 24 Å, we proposed that the two arcs originate from two types of stacking as shown in Figure 5b and 5c, which only differ by the tilt angles respect to the substrate surface. When the PEDOT:PSS interlayer is inserted, the two arcs gradually shrink into one at approximately 25 Å; the disappearance of the other peak suggests that one phase becomes dominating as induced by the PEDOT:PSS surface (Figure 5d and 5f). The above changes of surface morphology and crystallization preference could be attributed to the special core-shell structure of PEDOT:PSS grains.36 It has been reported that a PEDOT:PSS layer on an ITO substrate consists of conductive PEDOT cores surrounded by insulating PSS chains. The contact angle of a water drop on PEDOT:PSS layer (56.4°) is larger than that on bare-ITO (26.5°), confirming the higher hydrophobicity of a PSS-dominated surface than bareITO (Figure S3). Our previous work has demonstrated that the SA-Bu molecule has a rigid conjugation plane terminated by two loose butyl chains.23, 34 While these two butyl chains prefer


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to interact with saturated alkyl chains, SA-Bu will likely contact the PEDOT:PSS shell through these butyl chains, leading to a standing-on orientation. This will depress other unfavorable crystalline phases and improve the mosaicity and surface smoothness. Different from the previous phosphonic acid modification which mainly improves the crystallinity of the organic layer, the PEDOT:PSS interlayer also functions as the hole transportation layer. Based on an energetic match between the electrode and molecular HOMOLUMO levels (Figure 5f), SA-Bu is a hole-transport material. When the HOMO of SA-Bu is 5.35 eV, replacing ITO (work function: 4.88 eV) by PEDOT:PSS (work function: 4.97 eV) will reduce the hole injection barrier from 0.47 to 0.38 eV. This reduction has been widely reported in organic optoelectronics.37-38 We plotted I-V curves in a double logarithmic scale and performed linear fitting for the four segments (states) in Figure S4. It is found that ohmic conduction starts up at an extremely low electric field for the OFF state, corresponding to a thermally activated carrier transport. While the other three states obey I∝Vn (1

Poly(3,4-ethylenedioxythiophene)âPoly(styrenesulfonate) Interlayer

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