An Origami Perovskite Photodetector with Spatial Recognition Ability

Mar 13, 2017 - Zhipeng Zhang , Fangfang Sun , Zhaohua Zhu , Jie Dai , Kai Gao , Qi Wei , Xiaotong Shi , Qian Sun , Yan Yan , Hai Li , Haidong Yu , Gui...
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An Origami Perovskite Photodetector with Spatial Recognition Ability Huajing Fang, Jiangwei Li, Jie Ding, Yue Sun, Qiang Li, Jia-Lin Sun, Liduo Wang, and Qingfeng Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b02213 • Publication Date (Web): 13 Mar 2017 Downloaded from http://pubs.acs.org on March 14, 2017

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An Origami Perovskite Photodetector with Spatial Recognition Ability Huajing Fang,a Jiangwei Li,a Jie Ding,a Yue Sun,a Qiang Li,a Jia-Lin Sun,b Liduo Wang,a and Qingfeng Yan*,a a

Department of Chemistry, Tsinghua University, Beijing, 100084, China

b

Collaborative Innovation Center of Quantum Matter, State Key Laboratory of Low-

Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing, 100084, China. KEYWORDS: perovskite, flexible photodetector, paper electronics, origami, 3D.

ABSTRACT: Flexible photodetectors are attracting substantial attention because of their promising applications in bendable display and smart clothes which cannot be fulfilled by the existing rigid counterparts. In this work, we demonstrate a newly-designed photodetector constructed on the common printing paper. Pencil trace was applied as the graphite electrode. With such a simple and convenient method, the as-prepared photodetector exhibited a satisfactory responsivity of 4.4 mA/W, on/off current ratio of 32, coupled with a high response speed of < 10 ms. It also demonstrated excellent mechanical flexibility and durability. Most inspiringly, by an ingenious origami, we created the first perovskite photodetector with a 3D configuration. The cubic photodetector array displayed an excellent spatial recognition ability

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which could not be achieved in all the previously reported 2D photodetectors. Such a fusion of materials science and the art of origami provides a robust strategy for the design of low-cost flexible electronics, especially for the applications in 3D configurations.

■ INTRODUCTION Photodetectors for light signals recognition are the vital components in modern optoelectronic systems, such as optical communications, environmental monitoring, and imaging techniques.1-5 To date, a wide variety of organic, inorganic, and organic-inorganic hybrid semiconductor materials have been explored as the light harvester in photodetectors. Among them, organolead halide perovskites, CH3NH3PbX3 (X=Cl, Br, I), have emerged as a very promising candidate due to their high charge carrier mobility, high photoconversion efficiency, and long charge diffusion length.6-10 Recently, the pursuit of flexible electronics has become a notable technological trend. In particular, flexible photodetectors are attracting substantial attention because of their promising applications in bendable display and smart clothes which cannot be fulfilled by the existing rigid counterparts.11-13 For this reason, it is highly demanded to achieve the flexibility in organolead halide perovskites based photodetectors. Fortunately, the feasibility of solution processing at a relatively low temperature enables scientists to deposit perovskite thin film on flexible substrates. Several attempts have been made on polyethylene terephthalate (PET), a typical bendable plastic substrate.14-17 However, low thermal durability and high thermal expansion coefficient of plastics are always the obstacles during manufacturing processes.18-20 Not to mention the environmental contamination caused by these non-degradable plastics waste. Hence, alternative flexible substrate with commercial perspective is the urgent need.

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Paper manufacturing as one of the four great inventions of ancient China has a history of more than 2000 years. By far, paper is still the cheapest and most ubiquitous flexible substrate in our daily life.21 Compared to the plastic substrates such as PET and polyimide (PI), paper is environmentally friendly with renewable raw materials. Besides the traditional application for writing and packing, paper has also been considered as a potential substrate for flexible electronics because of the light weight, deformability and biodegradability.22-25 More importantly, once a functional electronic device is integrated on paper, it will be easy to realize mass production by jointing the traditional printing industry. Many impressive results have been achieved in recent years, ranging from light-emission devices to thin film transistors and supercapacitors.26-30 Unfortunately, organolead halide perovskite devices constructed on such a charming substrate have not been reported yet. On the other hand, origami technique inspired from the ancient art of paper folding is now a hotspot in the scientific and engineering research communities.31,32 It is being recognized as a simple cost-effective means to develop reconfigurable engineering systems which is highly required in robotics, shipping and even space exploration.33 Some classical limitations of the flat plate devices can be circumvented after folding into a targeted 3D configuration.34 But the transformation process has rarely been realized in photodetectors. Thanks to the deformability of the paper substrate, it is feasible to introduce the origami technique if we can construct the photodetector on paper. In this report, we demonstrate the fabrication of a truly low-cost perovskite photodetector on common printing paper. Perovskite thin film was directly synthesized on the rough and porous paper surface as the light harvester. The photodetector was designed into a lateral structure to eliminate the expensive transparent electrodes which constitute to a major part of the cost in

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optoelectronic devices.35,36 Instead, commercially available graphite pencil was applied to draw the electrodes as it was the simplest way for fabricating a graphitic circuitry with arbitrary pattern on paper. This idea was inspired by the following two facts: i) pencil trace is highly conductive to act as an electrode material.37-39 ii) Carbon electrode has proved to be well compatible with perovskite thin film in solar cells.40-42 With such a simple and convenient method, the as-prepared photodetector exhibited a satisfactory responsivity of 4.4 mA/W coupled with a high response speed of < 10 ms. It also demonstrated excellent mechanical flexibility and durability. More inspiringly, the cellulose paper provides natural foldability. Hence, a cubic photodetector was created from the two-dimensional (2D) paper sheet by an ingenious origami. To the best of our knowledge, this is the first report on perovskite devices with three-dimensional (3D) structure. The six pixels located on each face of the cubic photodetector make a promising approach to 3D detection. Distinguished from the conventional photodetectors which only give the scalar information of incident light, the 3D perovskite photodetector array can also determine the direction of the incident light. This unique 3D configuration makes up the neglect of detection the direction of incident light and shows great potential for spatial information processing. We believe that the fusion of materials science and the art of origami will open a window for low-cost flexible electronics design. ■ EXPERIMENTAL SECTION Device Fabrication: All the chemicals including PbI2 (99.9985%, Alfa Aesar), CH3NH3I (>99%, Dyesol), dimethylsulfoxide (DMSO, 99.9%, Sigma Aldrich) and γ-butyrolactone (GBL, 99%, J&K Chemical) were used as received. The perovskite precursor solution was prepared by dissolving 1.7 M PbI2 and CH3NH3I in a GBL/DMSO (7:3) mixed solvent. A clear yellow solution was achieved after stirring at 90 °C for 2 h. It is worth mentioning that the precursor was

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not a liquid at room temperature, so a preheating at 70 °C was necessary before use, as shown in Figure S1, Supporting Information. The graphite electrodes were drawn on the common printing paper with a 6B type pencil. After drawn, the paper substrate was baked at 110 oC by a hot plate. Then, 0.5 µL of the liquid perovskite precursor solution was drop-casted in the channel region and annealed at 110 oC for 40 min. As solvent evaporated, CH3NH3PbI3 polycrystal was formed in the porous cellulose fibers structure, with the paper substrate changed from yellow to black color. All the above-mentioned synthetic and fabrication procedures were carried out in air. Measurements: SEM images of printing paper, pencil trace and CH3NH3PbI3 thin film were taken on the field-emission scanning electron microscopy (FESEM, JEOL JSM-6335F). The Raman spectrum of pencil trace was measured using a spectrometer (Lab-RAM HR Evolution, Horiba Jobin Yvon), whereas the I-V curves were measured by a digital sourcemeter (Keithley 2400). The sheet resistance was measured by the four point probe sheet resistance meter (Dimensions, 280SJ). A plastic tube with the outer diameter of 28 mm was used as the mold for bending state measurement. The crystal structures of the as-prepared CH3NH3PbI3 thin film were characterized through X-ray diffraction (XRD, Bruker D8-Advance) using Cu Kα radiation. The Photoluminescence (PL) spectrum was carried out by a spectrometer (Lab-RAM HR Evolution, Horiba Jobin Yvon) upon excitation at 532 nm. The output signals of the paper based photodetector were recorded by a digital sourcemeter (Keithley 2400). Laser sources with different wavelengths and optical attenuators were used for adjustable illumination. The response speed was measured by a precision source/measure unit (Agilent B2911A) and an optical chopper (C995). A parallel light source with a diameter of 15 mm was clamping on a universal mounting bracket to assess the performance of the 3D photodetector array (details can be found in Figure S2, Supporting Information).

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■ RESULTS AND DISCUSSION The first step to fabricate such a low-cost photodetector is drawing a pair of graphite electrodes on paper. As shown in Figure 1a, two squares with the side length of 2 mm were patterned by a 6B type pencil (~80 wt% graphite content). The channel between them was 2 mm in width and 1 mm in length. The scanning electron microscope (SEM) image in Figure 1b shows the surface morphology of a common printing paper. The paper was composed of disordered cellulose fibers, leading to a rough and porous surface. During drawing, this rough surface can easily peel off the graphite flakes from pencil lead to form a conductive trace. The morphology of the pencil trace was also examined by SEM (see Figure 1c). The layered feature of the graphite flakes can be observed in a magnified view of the sample. In addition, Raman spectrum of the pencil trace was recorded for the structure identification. Two prominent peaks at 1347 and 1580 cm-1 are observed, corresponding to the D and G bands of graphite respectively. The G band can be assigned to the doubly degenerated in-plane E2g vibration mode, whereas the D band arises from the edges or disordered layers.37,43 The key point of an electrode material is the conductivity. The average sheet resistance of the graphite electrode is ~265 Ω/□. We also tested the current-voltage (I-V) curves of a pencil trace on paper (1 cm in width and 4 cm in length). As shown in Figure 1e, the I-V characteristics of the pencil trace exhibit an Ohmic behavior, and the typical resistance of the given trace is on the order of a few kΩs. The resistance changes very little at a bending radius of 14 mm. More details of the resistance test can be found in Figure S3, Supporting Information. Inset in Figure 1e shows that a commercial green LED connected in series with two pencil traces was successfully lit up by a 3.7 V rechargeable battery. These results demonstrate the ability of pencil trace to carry current and act as a qualified electrode for flexible electronics.

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Figure 1. (a) The photograph of the graphite electrodes drawn on paper. (b) The SEM image of the common printing paper. (c) The SEM image of the pencil trace on paper, inset shows a magnified view. (d) Raman spectrum of the pencil trace. (e) Current-voltage (I-V) curves of the pencil trace on paper. Inset shows a photograph of a battery-powered light-emitting diode connected with two pencil traces in series.

Once the graphite electrodes have been drawn, CH3NH3PbI3 precursor solution was dropcasted at the channel, as shown in Figure 2a. Although spin-coating is the most widely used method to deposit perovskite thin film in the laboratory research, the material utilization ratio as low as 1% will certainly hinder the large scale application.44 By contrast, the material utilization ratio in our drop-casting processing is almost 100%. The CH3NH3PbI3 thin film was synthesized on the paper substrate upon a gentle annealing at 110 oC for 40 min. In general, the rough and porous surface was supposed to be detrimental for most electronic devices manufactured directly onto a paper substrate. In our case, interestingly, the porous cellulose fibers structure played an important role in mechanical support of the perovskite as a scaffold, as shown in Figure 2b. In

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fact, organolead halide perovskites can be well synthesized in a porous structure. A prominent example is the case when they were used as light harvesters in dye sensitized solar cells (DSSCs).45,46 Elemental composition was analyzed by energy dispersive spectrometry (EDS) to confirm the presence of lead and iodine with atomic percent ratio of 1:3 (Figure S4, Supporting Information). The phase structure of as-fabricated CH3NH3PbI3 thin film was characterized by the X-ray diffraction as shown in Figure 2c. All the sharp diffraction peaks can be assigned to a perovskite structure with high purity. The photoluminescence (PL) spectrum provides further evidence of the perovskite structure. The dominant peak of the PL spectrum in Figure 2d is located at 772 nm, in a good agreement with the value reported earlier.47

Figure 2. (a) Schematic illustration of fabricating the CH3NH3PbI3 thin film on the paper substrate directly. (b) The SEM image shows the morphology of the channel after drop-casting the CH3NH3PbI3. (c) XRD patterns of the as-fabricated CH3NH3PbI3 thin film. (d) Photoluminescence spectrum of the CH3NH3PbI3 thin film upon excitation at 532 nm.

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Next,we measured the response characteristics of the paper based perovskite photodetector. Figure 3a shows the I-V curves of the photodetector in the dark and illuminated with a 633 nm laser. The straight lines without any rectifying behavior implied the formation of Ohmic contact between the perovskite thin film and graphite electrodes.48 The tiny contact resistance at the interface makes pencil trace to be an ideal partner of CH3NH3PbI3 so that the applied bias can mostly drop in the active region during device operation. As expected, the CH3NH3PbI3 film became more conductive under illumination due to the generation of electron-hole (e-h) pairs through absorption of incident photons. At a fixed bias of 5 V, the photocurrents (Iph=Ilight - Idark) under different illumination intensity are summarized as the black plot in Figure 3b. In general, the photocurrent (Iph) arising from the photo-excited carriers linearly increases with the incident light intensity (P) in both log scale. The relationship between them can be expressed by a power law, Iph∝Pβ, where β is a factor determining the response of the photocurrent to intensity.49 In our system, a similar linear relation with a fitting parameter of β = 0.56 was observed. This nonunity exponent (0.5 < β < 1) can be explained by the complex process of e-h pairs generation, recombination and trapping in CH3NH3PbI3.10 Responsivity (R) is a key criteria to benchmark the performance of a photodetector. It was defined as R=Iph/PS, where S is the effective illuminated area. As shown in Figure 3b, the responsivity at 0.01 mW/mm2 illumination is calculated to be 4.4 mA/W. This value is not very high compared with the rigid perovskite photodetector reported bofore.6 The reason might be attributed to that the channel manually drawn on paper was several orders larger than those prepared by photolithography method. Nevertheless, its responsivity is still comparable to that of a flexible perovskite photodetector fabricated on PET at a same light intensity (0.01 mW/mm2).14

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Figure 3. (a) The current-voltage (I-V) curves of the photodetector in the dark and under different illumination intensities. (b) Photocurrent and responsivity as a function of illumination intensity at a bias of 5 V. (c) Time domain photoresponse of the paper based perovskite photodetector at a chopping frequency of 50 Hz. The measurements were conducted under 3.1 mW/mm2 illumination using a 633 nm laser. (d) Photo-switching characteristics of the paper based perovskite photodetector illuminated with different light sources range from UV to Nearinfrared (375 nm, 532 nm, and 785 nm). (e) The comparison of the photo-switching behaviors under the initial state, bending state (with 14 mm curvature radius) and after 1000 bending cycles.

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The paper based perovskite photodetector also possessed a high response speed. To evaluate the time domain photoresponse, the incident 633 nm laser with an intensity of 3.1 mW/mm2 was chopper modulated at 50 Hz. As shown in Figure 3c, good on-off switching with current ratio around 32 can be observed, which is similar to the value previously reported.50 Both the rise time and decay time are shorter than 10 ms, which are superior to some reported flexible photodetectors.15,51,52 The relatively higher response speed can expand the scope of application in motion detection and dynamic imaging. Moreover, our device covers a broad wavelength range derived from the narrow band gap of CH3NH3PbI3. Light sources at the UV (375 nm), the visible (532 nm) and the near-infrared (785 nm) were used to perform the photo-switching measurements. The current signal as a function of time was recorded during repetitive switching the light on and off with a period of 2 seconds. As shown in Figure 3d, the photodetector exhibits good on-off switching to all of the three incident light sources. To have a more intuitive impression on the device performance of this paper based photodetector, some results of prior reported flexible photodetectors are summarized in Table S1, Supporting Information. The paper based photodetector is basically a competent flexible device. To confirm that, it is necessary to evaluate the performance of the photodetector under a bending state. In this section, a cylindrical object with a fixed outer radius of 14 mm was used as the bending mold. And we measured the photo-switching behaviors of the photodetector under the periodic illumination of a 633 nm laser. Figure 3e shows a detailed comparison of the photo-switching behaviors at the initial state and bending state. The photocurrent at a bias of 5 V was well maintained, revealing that the photoresponse was hardly affected by external bending. Moreover, the paper based device possessed good cycling stability in the bending and releasing test. As demonstrated in Figure 3e, the photocurrent of this photodetector just showed a slight decrease from 0.57 µA to

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0.52 µA even after 1000 bending cycles. Since it demonstrates excellent mechanical flexibility and durability, the paper based perovskite photodetector offers the potential for developing shape-conforming systems such as wearable electronics and electronic skin.

Figure 4. (a) Schematic illustration of the folding steps to create a cubic photodetector from a 2D pattern. (b) A Cartesian coordinate system is built based on the cubic photodetector. (c) The signals (normalized current) of pixels on each face when the cubic photodetector was illuminated along different directions. The results calculated from Metlab software in each case when the 3D

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array was illuminated with the parallel light along (d) cube edge (0, 0, -1), (e) face diagonal (0, 1, -1) and (f) body diagonal (-1, -1, -1).

A key advantage of our paper based device is the deformability, since the cellulose paper provides natural foldability. Using the interesting origami art, complex 3D structures can be created from 2D sheets through folding along pre-defined creases. Such a shape reconfiguration will endow the devices with unprecedented functions, as proven in the design of solar panels for space deployment53 and heart stents.54 In this work, we firstly fabricated six neighboring photodetectors in one sheet, as shown in Figure 4a. After folding along the pre-defined creases, a cubic photodetector array with 1 cm edge length was created from the previous 2D pattern. Then, a Cartesian coordinate system was built as shown in Figure 4b. The six faces of the cubic photodetector array were defined as A, B, C, A’, B’ and C’ respectively. Distinguished from the conventional photodetectors only absorb light in limited directions, the 3D photodetector array responds to the incident light with arbitrary directions. More importantly, the 3D array can even determine the direction of the incident light through signal processing. As a demonstration, we list three cases when the 3D array was illuminated with the parallel light along cube edge (0, 0, 1), face diagonal (0, -1, -1) and body diagonal (-1, -1, -1), respectively. The electric signals of six pixels located on each face were recorded and shown in Figure S5, Supporting Information. The normalized current defined as (I-Idark)/(Ilight-Idark) was plotted in Figure 4c. In the case of cubeedge-oriented (0, 0, -1) light, the signal of pixel on top surface (SA) was the maximum value. The signal of pixel on bottom surface (SA’) was the minimum value. While the signals of pixels on four side face (SB, SB’, SC, SC’) were in the middle, as the black line shown (Figure 4c). To describe them in a 3D space, we introduced the space vectors nA=[0, 0, SA], nA’=[0, 0, -SA’], nB=[SB, 0, 0], nB’=[-SB’, 0, 0], nC=[0, SC, 0] and nC’=[0, -SC’, 0]. With the help of Metlab

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software (MathWorks Inc.), we drew the six space vectors in the same coordinate system and calculated their resultant vector, n=∑ni, (i= A, B, C, A’, B’ and C’). As shown in Figure 4d, the yellow arrow represents the resultant vector with the coordinates of n (-0.061, 0.023, 0.935). This resultant vector almost indicates the reverse direction of the incident light (0, 0, -1). Through a similar signal processing, the directions of the incident light in the other two cases have also been successfully indicated, see Figure 4e,f. These results confirm the excellent spatial recognition ability of such a low-cost 3D photodetector array which may expose new markets for solar tracking and optical beam steering. ■ CONCLUSION In summary, a flexible perovskite photodetector based on the common printing paper was successfully demonstrated for the first time. CH3NH3PbI3 thin film was directly synthesized on the porous paper surface as the light harvester. A pair of graphite electrodes were drawn by a 6B type pencil lead to lay out the external electrical circuit. The photodetector fabricated through such a low-cost and convenient approach exhibited a satisfactory sensitivity of 4.4 mA/W and a high response speed (< 10 ms). Meanwhile, the paper based perovskite photodetector also exhibited excellent mechanical flexibility and durability, providing a promising candidate for developing shape-conforming systems such as wearable electronics and electronic skin. More importantly, by an ingenious origami, we created the first perovskite photodetector with a 3D configuration. The cubic photodetector array displayed an excellent spatial recognition ability which could not be achieved in conventional 2D photodetectors. Overall, the fusion of materials science and the art of origami will highlight a robust strategy for the design of low-cost flexible electronics, especially for the applications in 3D configurations. ■ ASSOCIATED CONTENT

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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Performance comparison of paper based photodetector with flexible photodetectors in literatures, Photographs of the perovskite precursor at different temperature, the custom-built set to assess the performance of 3D photodetector arrays, the resistances test of pencil traces, the EDS analysis of perovskite thin film, the electric signals of six pixels. (PDF) ■ AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENT This research was supported by the National key Basic Research Program of China (973 Program) under Grant No. 2013CB632900 and the National Science Foundation of China (No. 91333109 and 21671115). The Tsinghua University Initiative Scientific Research Program (Nos. 20131089202 and 20161080165) and the Open Research Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics (No. KF201516) are also acknowledged for partial financial support. ■ REFERENCES (1) Hu, L. F.; Brewster, M. M.; Xu, X. J.; Tang, C. C.; Gradečak, S.; Fang, X. S. Heteroepitaxial Growth of GaP/ZnS Nanocable with Superior Optoelectronic Response. Nano Lett. 2013, 13, 1941-1947.

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