Flexible All-Inorganic Perovskite CsPbBr3 ... - ACS Publications

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Flexible All-inorganic Perovskite CsPbBr3 Non-volatile Memory Device Dongjue Liu, Qiqi Lin, Zhigang Zang, Ming Wang, Peihua Wangyang, Xiaosheng Tang, Miao Zhou, and Wei Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15149 • Publication Date (Web): 23 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

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ACS Applied Materials & Interfaces

Flexible All-inorganic Perovskite CsPbBr3 Nonvolatile Memory Device Dongjue Liu1, Qiqi Lin1, Zhigang Zang1, Ming Wang1, Peihua Wangyang2, Xiaosheng Tang1,*, Miao Zhou1, Wei Hu1,* 1

Key Laboratory of Optoelectronic Technology and Systems of the Education Ministry of China,

College of Optoelectronic Engineering, Chongqing University, Chongqing, 400044, China 2

Sichuan Province Key Laboratory of Information Materials and Devices Application, College of

Optoelectronic Technology, Chengdu University of Information Technology, Chengdu 610225, China

ABSTRACT: All-inorganic perovskite CsPbX3 (X = Cl, Br, I) were widely used in a variety of photoelectric devices such as solar cell, light-emitting diode, laser and photodetector. However, studies to understand the flexible CsPbX3 electrical application relatively scarce, mainly due to the limitation of low-temperature fabricating process. In this study, all-inorganic perovskite CsPbBr3 films were successfully fabricated at the temperature of 75 oC through a two-step method. And, the high-crystallized films were firstly employed as a resistive switching layer in the Al/CsPbBr3/PEDOT:PSS/ITO/PET structure for flexible non-volatile memory application. The resistive switching operations and endurance performance demonstrated the as-prepared flexible ReRAM devices possess reproducible and reliable memory characteristics. Electrical reliability and mechanical stability of the non-volatile device were further testified by the robust current-voltage curves under different bending angles and consecutive flexing cycles. Moreover, the model of the formation and rupture of filaments through the CsPbBr3 layer was proposed to explain the resistive switching effect. It is believed that the study will offer a new playground to understand and design all-inorganic perovskite materials for future stable flexible electronic devices.

KEYWORDS: CsPbBr3, Perovskite, Flexible electronics, Resistive switching, Memory device

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1. INTRODUCTION Resistive switching random access memory (ReRAM) has been recognized as one of the most promising candidates for the next generation logic, adaptive and non-volatile memory devices, due to its low power consumption, rapid switching speed, and high integration densities.1-5 ReRAM generally possesses traditional metal/insulator/metal sandwich structure in which insulator acts as the active layer.3,4 Under the stimulates of voltage amplitude or bias polarity, ReRAM can be electrically switched its resistance states between low resistance state (LRS) and high resistance state (HRS) to realize memory operations. Various types of resistive switching behaviors including bipolar, unipolar, complementary, and threshold resistive switching have been observed in a variety of materials through different electrical measurements.3,6-9 With the emergence of new functional materials, perovskite materials including PrxCa1-xMnO3, BaTiO3, SrTiO3, and BiFeO3 have been broadly investigated as the active materials for resistive switching memory devices.10-13 However, the high-temperature process for preparing the aforementioned rigid ceramic perovskite oxide films limited their further applications. Recently, Gu et al. successfully manufactured hybrid organic-inorganic perovskite memory devices by utilizing the possible point defects, which provided a new cognition for a new family of perovskite materials.5 Nevertheless, owing to the inclusion of organic cations, it was commonly found that the intrinsic thermal instability of methylammonium (MA) and formamidinium (FA) lead trihalide perovskites was really a bottleneck for the development of hybrid perovskites based electronic devices.14-16 Therefore, in order to resolve this issue, the organic cations must be substituted by other ions like cesium (Cs) cations. Interestingly, there are some reports of cesium/cesium-hybridization solar cells that give us many new clues for the improved stability of all-inorganic perovskites based electronic devices. More and more publications demonstrated inorganic Cs cations based all-inorganic perovskites could be both structurally and thermally stable above 100 oC, while hybrid perovskites thermally degraded to lead iodide above 85 oC.15,17 So, it was implied that all-inorganic perovskites could be one excellent candidate for the fabrication of stable and high-efficient resistive switching memory devices by low-cost process. However, there are few reports about all-inorganic CsPbX3 perovskite materials based resistive switching memory devices. To date, flexible electronic equipment such as rollup displays,18,19 wireless sensors,20,21 and wearable devices22,23 were recognized as the next generation intelligent instruments. In order to


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achieve high performance flexible electronic devices, all the materials should be flexible. However, the poor adhesion between perovskites materials and the substrate, continuous bending would result in cracking and low attachment, which would induce destroy of the flexible devices. Recently, accompanying with the development of soft electrode-active materials and corresponding thin-film fabricating process, PEDOT:PSS were widely used in the solar cell as the charge transfer layer, which showed good adhesion between substrate and absorbance layer.24-26 In this contribution, we apply all-inorganic perovskite CsPbBr3 on resistive switching element

for

flexible

resistive

switching

memory

devices,

and

the

structure

is

Al/CsPbBr3/PEDOT:PSS/ITO/PET. The perovskite film was grown on an indium tin oxide (ITO)-coated polyethylene terephthalate (PET) substrate by a facial method under low temperature of 75 oC.27,28 And the devices exhibited reproducible and reliable non-volatile memory behaviors, which were necessary qualities in resistive switching memory fabricating technology. In addition, the statistical data of electrical characteristics under different bending angles and consecutive flexing cycles also demonstrated these flexible devices possessed good electrical reliability and mechanical stability. This study will provide an opportunity to understand and design all-inorganic perovskite materials to be used in next-generation highperformance, stable, and flexible non-volatile memory devices.

2. EXPERIMENTAL SECTION ITO/PET transparent conducting substrates were cleaned by sequential 20 min sonication in warm deionized water, acetone, and isopropanol. After drying under a nitrogen flow at atmosphere, the substrates were treated with UV-Ozone for 5 min. The CsPbBr3 films were prepared by a two-step sequential deposition technique. Firstly, 30 mg CsBr was dissolved in 2 mL methanol and heated at 60 oC for 10 min in sealed container. Subsequently, 367 mg PbBr2 in 1 mL DMF was stirred on a hot plate at 75 oC for 5 hours, and then was filtered by using a 0.22 µm pore size PTFE filter and immediately for use. The PbBr2 layer was spun-coating at 3000 rpm for 40 s on this well-cleaned preheated (75 oC) ITO/PET and then dried at 75 oC for 30 min. After drying, the PbBr2 layer were dipped for 5-15 min in a heated (50 oC) solution of 15 mg/mL CsBr and then dried under N2 stream immediately. The samples were annealed at 70 oC vacuum drying oven for 24 h, and placed inside the glovebox


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subsequently for the deposition of top electrodes. After this process, the aluminum contacts of 100 nm thickness were then evaporated through a shadow mask at a pressure of 10-5 mbar to form the Al/CsPbBr3/PEDOT:PSS/ITO structure. The cross-sectional scanning electron microscopy (SEM) image and X-ray energy dispersive spectrometry (XEDS) were recorded by ZEISS AURIGA FIB-SEM. The surface SEM image was observed by JSM-7800F. The crystal structures of the samples were characterized by X-ray diffraction (XRD) with Cu Ka radiation (XRD-6100, SHIMADZU, Japan). The current-voltage (I-V) characteristics of Al/CsPbBr3/PEDOT:PSS/ITO/PET cells were measured at room temperature in air using a Keithley 4200 semiconductor parameter analyzer.

3. RESULTS AND DISCUSSION

Figure 1. (a) The schematic drawing of the CsPbBr3-based flexible resistive switching memory. (b) Cross-sectional SEM image of the memory structure. (c) Surface SEM image of the asprepared CsPbBr3 films on flexible ITO substrate. (d) The typical XRD pattern of the CsPbBr3 samples. Peaks marked with black spots are assigned to the CsPb2Br5. We successfully fabricated flexible all-inorganic perovskite CsPbBr3 non-volatile resistive switching memory by a facial method under low temperature. Figure 1a depicts the schematic drawing of the Al/CsPbBr3/PEDOT:PSS/ITO/PET structure. The cross-sectional scanning electron microscopy (SEM) image of the memory device in Figure 1b visualizes that the CsPbBr3 film thickness is between 500 to 600 nm and the interfaces between the perovskite film


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and each electrode layer. The top-view SEM image in Figure 1c shows the CsPbBr3 micro and nano crystals arranged densely on below CsPbBr3 perovskite film. In comparison to organicinorganic hybrid perovskites, this material crystallize into big grains more easily because such kind of crystalline materials principal role play p-halogen states coordinated by heavy cations.29,30 The crystallization of the perovskite films covered on the flexible ITO substrates were analyzed by X-ray diffraction (XRD) measurements, as shown in Figure 1d. The obvious peaks located at 15.37° and 30.96°, which could be assigned to the orthorhombic CsPbBr3 film with high crystallinity. Meanwhile, the peaks marked with black spots can be assigned to the tetragonal CsPb2Br5.31 As reported, this kind of material in CsPbBr3 films can emerge in lowtemperature solution method (about 70 oC).31,32 Actually, it could be seen that the amount of CsPb2Br5 is not large, which can be confirmed by XRD result. The diffraction peak of CsPb2Br5 was weak in the final products. On the other side, we observed that the main structure of CsPb2Br5 was one kind of perovskites materials, which showed some similarity with CsPbBr3 materials.33,34 Therefore, the small amount of CsPb2Br5 show few effect to this memory device.

Figure

2. (a) A digital photograph of flexible ReRAM with the structure of

Al/CsPbBr3/PEDOT:PSS/ITO. (b) Typical I-V curves of electroforming process of these devices. (c) Typical I-V characteristics in a semi-logarithmic scale after the electroforming process. (d) Endurance performance read at -0.2 V.

Figure 2a depicts the digital photograph of the CsPbBr3 based flexible ReRAM. Generally, to obtain the resistive switching effects, an electroforming process (Figure 2b) is needed to perform


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on memory devices. In this work, the electroforming voltage was about 3.0 V and the compliance current was 1 mA. Figure 2c shows the typical I-V characteristics of the Al/CsPbBr3/PEDOT:PSS/ITO/PET memory devices in a semi-logarithmic scale after the electroforming process. It shows that with the loop of 0→-2 V→0→3 V→0 direct current (DC) voltage sweeping, the Al/CsPbBr3/PEDOT:PSS/ITO/PET memory device manifests typical bipolar resistive switching characteristics. When the negative bias was applied, a set process happened at about -0.6 V where the current dramatically increased, and the HRS was triggered to LRS. The self-compliance characteristics in set branches were also observed, which could be attributed to the existence of parasitic resistance of the PEDOT:PSS and ITO layers in the device. Of particular note is that the compliance current needlessly imposed during the aforementioned electrical measurements because of its self-compliance characteristics. It was observed that the reset process happened at around 1.7 V and the LRS switched into HRS. As shown in Figure 2d, the endurance property was obtained by applying the continuous write/erase voltage pulse of -2.0 V/+3.0 V and the read voltage pulse of -0.2 V, which indicated that the high and low resistance states were fairly stable during the transition processes.

Figure 3. All-inorganic perovskite CsPbBr3 ReRAM with different bending angles. (a) Photographs of flexible Al/CsPbBr3/PEDOT:PSS/ITO devices bending bent. (b) Schematic illustration of the flexible substrate at tensile strain (up) where R is the bending radius, D is the thickness of the devices and θ is the corresponding central angle. (c) I-V characteristics and (d) Bending stability with different bending angles of perovskite resistive switching memory with repetitive bending cycles.


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Figure 4. (a) I-V characteristics and (b) Bending stability with different bending cycles.

To further demonstrate the bending stability of all-inorganic perovskite CsPbBr3 resistive switching memory, the device was subjected to different bending angles and consecutive flexing cycles. Figure 3a displays the device was bent to a given curvature in different angles (0°→60°→120°→180°→360°) for one cycle and the size of the PET substrate is about 15×15 mm. Figure 3b shows the schematic illustration of the flexible substrate at tensile strain (up) where R is the bending radius, D is the thickness of the devices and θ is the corresponding central angle, respectively. The corresponding I-V characteristics in Figure 3c showed the slight difference of LRS and HRS current in the negative bias region, which illustrated that there was not much change in electrical properties upon different bending states. Moreover, the statistical data of the ON and OFF currents read at 0.2 V kept 0.1 mA and µA magnitude respectively (Figure 3d), which means the ION/IOFF ratio keeping about 102 robustly. In order to inspect the mechanical flexibility, the repeated bending cycles up to 100 cycles were measured (Figure 4a). It is found that the ratio of HRS current to LRS current (about 102) was maintained over 100 bending cycles in Figure 4b. Overall, all of these results indicate that CsPbBr3 shows bipolar resistive switching behavior with good electrical reliability as well as mechanical stability. Many works have been reported about the resistive switching devices of varied systems based on different materials. Some of them display good physical properties regarding with set/reset voltage, ON/OFF ratio, and endurance cycles, as shown in Table 1. Indeed, apart from the flexible nanoporous (NP) WO3−x devices, flexible CsPbBr3 devices exhibited good performance including higher ON/OFF ratio of ~102 than WO3 (

Flexible All-Inorganic Perovskite CsPbBr3 ... - ACS Publications

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