High-Performance Transparent Conducting Metal Network Electrodes

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High-Performance Transparent Conducting Metal Network Electrodes for Perovksite Photodetectors Jie Yang, Chunxiong Bao, Kai Zhu, Tao Yu, and Qingyu Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15205 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 27, 2017

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ACS Applied Materials & Interfaces

High-Performance Transparent Conducting Metal Network Electrodes for Perovksite Photodetectors Jie Yang†‡, Chunxiong Bao§‡, Kai Zhu†, Tao Yu§ and Qingyu Xu†§* †

School of Physics, Southeast University, Nanjing 211189, China

§

National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

‡

These authors contributed equally to the work.

* Address correspondence to [email protected] ABSTRACT: Transparent conducting electrodes with high transparency and conductivity are necessary components for optoelectronic devices. In this work, a facile wet-chemical lift-off process is firstly introduced to fabricate Au network flexible transparent electrodes with electrospun polymer fiber network as mask. Low resistance (5.18 Ω sq-1) of the transparent electrode was obtained when the transparency was around 90%, which was comparable to the state-of-the-art transparent electrodes. Low root-mean-square roughness of 23 nm was obtained when the Au nanowire thickness was 30 nm. The perovskite CH3NH3PbI3 photodetector based on the 30-nm-thickness Au network electrode shows large linear dynamic range of 138 dB, high detectivity over 1012 Jones and displays better flexibility than that based on the commercial ITO electrode, which demonstrates that the Au network electrode is a promising flexible transparent conducting electrode for optoelectronic devices.

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Keywords: metal network, transparent conducting electrode, lift-off, flexibility, photodetector, perovskite. 1. INTRODUCTION Transparent conducting electrodes (TCEs) have become an indispensable component for the portable optoelectronic devices such as touch screen panels1,2, light emitting devices,3 photodetectors2,4 and photovoltaic cells5,6. Nowadays, the most widely used TCEs are based on transparent conducting oxides (TCOs) such as ITO (In2O3:Sn), FTO (SnO2:F) and AZO (ZnO:Al) for their high optical transparency (about 90% transparency), high conductivity (10–25 Ω sq-1) and large-scale fabrication capability. However, inherent brittleness of these TCOs cause their unsatisfactory performance in flexible devices.7 To fabricate highly flexible TCEs, some novel materials with high flexibility, such as carbon materials,8-10 conductive polymer,11,12 and metal network13,14 have been developed. For the carbon materials and conducting polymer, low conductivity (more than 100 Ω sq-1) is the main drawback which hinders their application in optoelectronic devices. Metal network electrodes have attracted much attention in the field of TCEs for their superior optical transparency, electrical conductivity and flexibility. There are mainly two kinds of metal network transparent electrodes: randomly dispersed metal nanowires electrodes and patterned metal network electrodes. Randomly dispersed metal nanowires electrodes, which are usually fabricated by dispersing metal nanowire solution on substrates, show lower conductivity for their lower length to diameter ratio and higher wire-to-wire junction resistances.15-18The sheet resistances of the randomly dispersed nanowire network electrodes are over 20 Ω sq-1 when the transparency is 90%. Patterned metal network electrodes are patterned from continuous metal films rather than composed with separate nanowires, which can not only avoid the junction resistance between separate nanowires, but also can realize that transparency


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and electrical conductivity improve simultaneously, exhibiting better optical and electrical performance than randomly dispersed metal nanowire electrodes. The transparency and conductivity of patterned metal network electrodes can be easily controlled by changing the nanowire width, space and thickness. However, to obtain patterned networks usually needs complex methods such as nano-imprint lithography19-21 or electron beam lithography,22 which is time and energy consuming. Electrospinning is an economic method to prepare polymer fiber network which can be used as mask to prepare continuous metal network.23,24 Cui et al. fabricated patterned metal network electrodes by depositing metal on the electrospun polymer fiber networks and removing the polymer fibers after transferring them to substrates.25 They fabricated TCEs with sheet resistance 2 Ω sq-1when the transparency was 90%. However, the transfer process during the fabrication will inevitably damage the metal network, and the adhesion of metal network to the substrates is weak. Besides, the high roughness of the metal network electrodes also hinders their applications in thin-film optoelectronic devices. In our previous work, we fabricated patterned metal network electrodes by ion beam etching metal films with electrospun polymer networks as shadow masks.2 This method is compatible and transfer-free, and can fabricate TCEs with sheet resistance of 2.2 Ω sq-1 when the transparency was 91.1 %. However, the ion beam etching method needs vacuum environment, which is energy consuming. Here, we firstly use a more facile and economical wet-chemical lift-off method to prepare transfer-free patterned metal network electrodes. Large length to width, sharp edge, low roughness metal wires can be obtained for the metal network. The as-fabricated metal networks exhibit excellent performance in the terms of high conductivity, transparency and flexibility. We use the patterned metal network electrodes as the TCEs for the perovskite CH3NH3PbI3


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photodetectors. The prepared CH3NH3PbI3 photodetectors based on our TCEs showed comparable optoelectronic performance and better flexibility than those based on the commercial flexible ITO electrodes, which demonstrates the potential application prospect for our prepared TCEs. 2. RESULTS AND DISCUSSION

Figure 1. Schematic for the preparation process of the patterned metal network electrode. a) The transparent substrate. b) Electrospinning polymer network on the substrate. c) Vapor-depositing Al film on the polymer network coated substrate. d) Dissolving polymer network in ethanol. e) Depositing Au film on the processed substrate. f) Putting the Au coated substrate in the HCl solution and removing Al film, leaving a Au network electrode.

The schematic diagram for the preparation procedures using wet-chemical lift-off method are present in Figure 1. The details are described in the experimental section. Briefly, a polymer network was transferred onto a transparent substrate (Figure 1b). The polymer network was formed by combining two electrospun polymer fiber arrays in prependicular directions together. Then a metal shadow mask was fabricated for preparing the ultimate metal network. When to choose the category of the metal to be used for shadow mask, the following two factors should


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be taken into consideration: i) the metal should be able to dissolve in caustic solution for the following wet-chemical lift-off process. ii) the thickness of the metal film should be smaller than the diameter of the polymer fiber and larger than the thickness of the final metal network electrode. In our work, we choose Al as the metal shadow mask material for its corrosion in acid solution. As seen in Figure 1c, a continuous Al film of about 80-300 nm was vapor-deposited on the polymer network coated substrate. The thickness of the deposited aluminum layers is varied depending on the thickness of final metal network needed to prepare. For final metal network electrode of 30 nm thickness, the thickness of the deposited aluminum layer is 80 nm. The polymer network was removed after washed with ethanol, and the Al film with troughs was left on the substrate as the shadow mask for the following process (as seen in Figure 1d). In principle, any metal network electrode which can be resistant to caustic solution can be fabricated using our method. In order to match the work function of the contact electrodes for organolead halide perovskite devices, Au was chosen as the metal network material. As seen in Figure 1e, a Au film, which is resistant to corrosion in HCl solution, was vapor deposited on the Al shadow mask. To improve the adhesion of the Au film on the substrates, a thin Ti underlayer of about 5 nm was firstly deposited. Finally, a lift-off process was carried out by immersing the substrate in the 10% HCl solution to remove Al film as well as the covering Au, leaving a Au network on the substrate (Figure 1f). The scanning electron microscope (SEM) images of the processed substrate in every step of the preparation process are shown in Figure 2. The polymer fiber covered with Al film is shown in Figure 2a, which shows that the diameter of the fiber is about 500 nm. After removing the polymer network, an Al film with nanowire-shaped grooves was formed (Figure 2b). The SEM image is blurring due to the low conductivity of the discontinuous Al film. When depositing Au


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film on the discontinuous Al film, the film is connected by the Au film in the groove, which improves the conductivity of the substrate, as seen in Figure 2c. The as-fabricated Au network after dissolving the Al mask in HCl solution is shown in Figure 2d. From the figure, we can see the intersection of the Au network has no extra junction resistance and the edge of the nanowires is very sharp without any obvious damage. All these indicate that the as-prepared metal network electrodes are expected with good conductivity and transparency.


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Figure 2. a) SEM image of Al film on the polymer coated substrate. b) SEM image of the Al coated substrate after removing the polymer fiber. c) SEM image of the substrate after coating with Au film on Al film. d) SEM image of the as-prepared Au network.

Figure 3a displays the large-scale SEM image of the Au network electrode. As seen in the figure, the Au network is very intact and shows large length to width ratio, which demonstrates the feasibility and advantage of the wet-chemical lift-off method. As seen in Figure S1 and S2, the average line width of our Au network is 463.9 nm with the standard deviation of 117.9 nm, and the average area of the Au aperture for the Au network with electrospinning time 3 min is 441.5 µm2 with standard deviation of 424.0 µm2. The deviation is mainly for not uniform size of electrospun polymer fibers as synthesized before. By changing the density of polymer network and the thickness of Au nanowires, Au network electrodes with different conductivity and transparency can be obtained. Figure 3b plots the transparency versus sheet resistance for the Au network electrodes with different thicknesses of 30 nm, 90 nm and 180 nm. As seen in the figure, the performance of the Au network electrode improves as the thickness of Au network increase, with the sheet resistance 30.8 Ω sq-1 (T=91.2%) for 30 nm, 14.9 Ω sq-1 (T=88.4%) for 90 nm and 5.2 Ω sq-1 (T=90.1%) for 180 nm. It’s worth noting that the thickness increase of the Au network doesn’t decrease their optical transmission obviously, which means the conductivity and transparency can be improved simultaneously by increasing the thickness of metal network and eliminating the network density. The reason is that the transmission of metal network electrode is mainly determined by the apertures. When the metal thickness is increased, the apertures of network will not be changed, which is very different from that of the continuous thin film transparent electrode. The transmittance spectrum of Au network electrode on PEN substrate can also reflect different light transmission mechanism from ITO/PET electrode. The transmission spectrum of 30-nm-thickness Au network electrode (17.9 Ω sq-1) was presented in Figure 3c with


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its transparency of about 80% (T ≈80%). A plateau was shown in the visible range for the Au network/PEN electrode, while obvious absorption in the UV and near infrared range can be seen for the ITO/PET electrode. The plateau for the Au network electrode can also be attributed to the direct light transmission. In order to present a sensory awareness of our Au network electrode, the photograph of the as-prepared Au network/PEN electrode as an electric conductive medium for LED lighting was given in Figure 3d. As seen in the figure, the picture under the Au nanowire network electrode can be clearly seen through our electrode. The flexibility of the Au nanowire network/PET electrode was tested by measuring the resistance change when we bent it with different radii. For comparison, the resistance change of the ITO/PEN electrode was also tested. As seen in Figure S3, when bending them to radii from 100 to 1 mm, the resistance of Au nanowire network/PET electrode showed negligible change, while the resistance of the ITO/PEN electrode increased obviously under the bending radius from 25 mm to 1 mm. The bending test indicated that, the Au nanowire network/PET electrode possesses better flexibility than the ITO/PEN electrode. The adhesion of the metal network to the substrate is crucial when it is utilized as electrode in optoelectronic devices. To study the adhesion of the Au network with Ti underlayer, the resistance of Au/Ti network electrode vs the tape cycles was displayed in Figure S4. As seen in the figure, the resistance increase of our Au network electrodes is small after we continually attach and peel the tape on the electrodes. When the Au network electrode was attached and peeled for 100 times, the resistance increase was around 50%, while the resistance of Au network electrode without Ti layer increase rapidly without conductivity for 1 time.


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Figure 3. a) The large-scale SEM image of the Au network. b) The optical transmission at 550 nm of the Au nanowire networks electrodes versus sheet resistance with different Au thickness of 30, 90 and 180 nm. c) The transmittance spectra of the as-prepared Au network electrode and the commercial flexible ITO electrode. d) The photograph of the flexible Au network electrode used as conduction media for LED lighting.

The aim of our work is to fabricate high-performance metal network transparent electrodes which can be used in some thin-film optoelectronic devices such as organic light emitting diodes and solar cells and perovskite photodetectors and solar cells. Generally, the thickness of the active layer for this thin-film optoelectronic devices is always very small, which seriously hinders the application of many network based TCEs with high conductivity and transparency but with large roughness. Therefore, small roughness is a critical factor for the TCEs of thin-film optoelectronic devices. Figure 4 shows the atomic force microscope (AFM) surface morphology


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of the 30-nm-thick Au network electrode. As seen in the figure, the surface of the Au network is very smooth with root-mean-square roughness (Rms) of about 23.00 nm. The value is much smaller than that of many other reported metal networks.25,26

Figure 4. AFM image of the Au nanowire network electrode. The root-mean-square (Rms) roughness of Au network electrode exacted from AFM image is about 23.00 nm.

Organolead halide perovskite photodetectors based on the as-prepared Au network electrodes were fabricated with the structure schematically illustrated in Figure 5a. In order to fabricate high quality perovskite film, 30 nm-thick Au network electrode with roughness of about 23 nm was chosen as the TCE of the perovskite photodetector. Before fabricating the perovskite absorber layer, the Au network electrode was coated with a PEDOT:PSS with a thickness of about 30 nm as hole transport layer. The PEDOT:PSS layer can not only make the surface of the Au network electrode smoother, but also improve the hole collection by the Au network electrode. A perovskite CH3NH3PbI3 layer was then spin-coated on the substrate as the absorber layer using the previous reported method.4 The PCBM with a thickness of about 100 nm and Al with a thickness of 100 nm were sequentially deposited as the electron transporting layer and contact electrode on the perovskite film, respectively. Figure 5b presents the photograph of the as-prepared perovskite photodetector bent in a certain angle, showing its good flexibility. The


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thickness for the devices is about 1 μm. As seen the cross profile in Figure S2a and surface morphology of the perovskite film in Figure S2b, the perovskite film is very smooth prepared on the Au network electrode, demonstrating the feasibility of our TCEs in thin-film optoelectronic devices. Figure 5c shows the photocurrent density through the CH3NH3PbI3 photodetector versus the applied bias voltages from -0.9 V to 0.9 V when it was under illumination of 500 nm monochrome light and in dark. The dark current is very small with the value in the order of 8.16×10−5 mA cm−2 when the bias voltage is 0 V. The small dark current can be mainly attributed to the existence of electron and hole blocking layers and the high quality of the perovskite film in the perovskite photodetector. When the device was illuminated by a monochromatic light of 500 nm, the photocurrent of the device increased significantly. An open circuit voltage (VOC) could be found from the J−V curves of device when the device was under illumination, demonstrating the device is a self-powered device and can work as a photovoltaic device. The reason for this is that the built-in electric fields generated in the heterojunction of the photodetector forces the photogenerated carriers in the perovskite layer to transfer to different electrodes. The External quantum efficiencies (EQE) at 0 V bias as a function of wavelength for our Au network electrode based perovskite photodetector is displayed in Figure 5d. The peak EQE of the device is around 70% in the wavelength range from 400 nm to 800 nm, which is in accordance with the light absorption range of the Au network electrode and the CH3NH3PbI3 film.


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Figure 5. a) Schematic structure of the perovskite photodetector. b) Photograph of the perovskite photodetector. c) Photocurrent density of the perovskite photodetector versus different voltage from -0.9 V to 0.9 V. d) External quantum efficiency (EQE) of the CH3NH3PbI3 photodetector based on the Au network/PEN electrode at 0 V bias. e) Photocurrent density and detectivity of the Au network/PEN electrode based CH3NH3PbI3 photodetector versus different incident light intensities at 0 V bias. f) Photocurrent of Au network/PEN electrode and ITO/PET electrode based CH3NH3PbI3 photodetectors under different bending radius at 0 V bias.


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Linear dynamic range (LDR) is one of the important figure-of-merit parameters for a photodetector, which reflects the detectable light intensity range. The calculation can be expressed as follows: LDR= 20lg௉

௉ೠ೛ ೏೚ೢ೙

where Pup is the largest light intensity and Pdown is the smallest light intensity, between which the photocurrent changes linearly with the light intensity.4 As seen in Figure 5e, our Au network electrode based CH3NH3PbI3 photodetector shows a linear response at light intensities ranging from 5×10−9 to 0.04 W cm−2, with the LDR of about 138 dB. This value is comparable to many other perovskite photodetectors27,28 and Si photodetector (~120 dB), and higher than InGaAs photodetectors (~60 dB). Based on the measured dark current and photocurrent at different light intensity, the specific detectivity can be expressed as follows: ‫= ∗ܦ‬

௃೛೓ /௅೗೔೒೓೟ (ଶ௤௃೏ )భ/మ

,

where Jph is the photocurrent, Llight is the incident light intensity, q is the elementary charge and Jd is the dark current density. Specific detectivity at 0 V bias versus different incident light intensities was plotted in Figure 5e. The specific detectivity keeps almost constant in the order of 1012 Jones from 5×10−9 to 0.04 W cm−2. This value is comparable to those of some other reported perovskite photodetectors based on ITO electrodes,

28,29

demonstrating that our Au

network electrode with high conductivity, transparency and low roughness plays no negative influence on the performance of the Au network electrode based perovskite photodetector. And the detectivities of the prepared perovskite photodetectors are also comparable to polymer photodetectors30, PbS based photodetectors31 and higher than InGaAs32,33 based photodetectors, indicating the outstanding optoelectronic performance for perovskite photodetector.


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To study the flexibility of the Au network electrode based devices, we fabricated the CH3NH3PbI3 photodetector based on two kinds of transparent flexible electrodes: ITO/PET electrode and Au network/PEN electrode, and bent them in different bending radius. From Figure 5f, we can see that the photocurrent through the Au network electrode based perovskite photodetector began to decrease when the device was bent with the radius of about 5 mm, while the photocurrent through the ITO/PET based perovskite photodetector network began to decrease at 10 mm. For the same structures of the two photodetectors, the phenomenon demonstrates that our Au network electrode possesses better flexibility than commercial ITO electrodes. The good flexibility of the Au network/PEN electrodes is mainly attributed to the extensibility of the Au network. The photocurrent decay after 5 mm for the Au network/PEN based CH3NH3PbI3 photodetector is mainly because of the brittleness of the CH3NH3PbI3 film.4 The device stability with the bending radius of 10 mm was investigated and displayed in the supporting information. As seen in Figure S6, no obvious current decay was detected when the incident light was on and off for multicycles in 120 s. 3. CONCLUSION In conclusion, transparent Au network electrodes were in-situ fabricated on transparent substrates using wet-chemical lift-off method without transfer process. By increasing the thickness of Au film to 180 nm, low resistance of 5.18 Ω sq-1 can be obtained when the transparency is around 90 %. When the thickness of Au nanowire is 30 nm, low roughness of 23 nm is obtained, and the resistance of Au network electrode is around 17.58 Ω sq-1 with the transparency of around 80 %, which is comparable to the commercial ITO electrodes. Perovskite CH3NH3PbI3 photodetector based on the Au network electrode shows high LDR of about 138 dB, high detectivity over 1012 Jones and better flexibility than the perovskite photodetector based on


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the commercial ITO electrode, which indicates the Au network electrodes good optoelectronic performance and flexibility. 4. EXPERIMENTAL SECTION Materials: All the chemicals were used as received without further purification, including poly (vinyl pyrrolidone) (PVP, weight-average molecular weight of 1300000, Aladdin), Hydrochloric acid (37% ， Aladdin), PbI2 (99.9983%, Aladdin), N,N-dimethylformamide (DMF, >99%, Aladdin), dimethyl sulfoxide (DMSO, >99%, Aladdin), [6,6]-phenyl C61 butyric acid methyl ester (PCBM, >99%, Aladdin), PEDOT:PSS aqueous solution (Clevios PH1000, Aladdin). CH3NH3I was synthesized in our laboratory according to the previous report. Electrospining polymer networks on transparent substrates: 14 wt% of PVP solution in ethanol was prepared for electrospining. The polymer fiber collection electrode was a rectangle steel frame. The PVP solution was spun through a needle of a plastic syringe at a rate 0.2 mL h-1 pushed by a syringe pump. The voltage between the needle and the collection electrode was set as 15 kV. The distance between the needle and the collection electrode was about 25 cm. Polymer fiber array was collected on the collection electrode in one direction. Different densities of the polymer fiber arrays on the rectangle steel frames were fabricated by changing the electrospining time from 1 to 5 min. The spinning time for 30-nm-thickness Au network electrode (with resistance of about 17.9 ohm) which was used for the TCE of the perovskite photodetector is 3 min. The polymer fiber arrays were transferred to the transparent substrates for twice in vertical directions and formed polymer network. Deposition of Al and Au films: The Al continuous films were deposited on the transparent substrates by an electron beam evaporation system with the cavity pressure of 1×10-6 torr and the deposition rate of about 1 Å s-1. The thickness of the deposited aluminum layers is varied from


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80 to 300 nm depending on the thickness of Au network needed to prepare. For Au network electrode of 30 nm thick, the thickness of the deposited aluminum layer was 80 nm. Larger thickness of the Au network electrode will need larger thickness of aluminum layer. The Au films were deposited on the Al mask with the same method. The thickness of the Au film was varied with 30 nm, 90 nm and 180 nm. Lift-off Al from the Au coated substrates: The corrosion solution for lift-off is 10% (v/v) HCl aqueous solution. The Au coated Al substrate was put in the HCl solution with the Au side down. Different immersion time is needed for different Au thickness deposited on the Al substrate until the substrate was almost transparent. The substrate was then ultrasonically washed in the HCl solution for a few seconds until the peeled off Au film was totally separated from the substrate. Finally, the substrate was washed with ethanol for several times and dried with N2 flow. Fabrication of perovskite CH3NH3PbI3 photodetector: The Au network/PEN electrodes were ultrasonically cleaned in ethanol for 15 min, and then cleaned with UVozone for 30 min. The eletrodes were put into a glovebox filled with N2 with O2 content less than 0.1ppm, H2O content less than 0.1 ppm. A 30-nm-thickness PEDOT:PSS layer was first spin-coated on the Au network/PEN electrode at 3000 rpm for 30 s, then heated on a heating plate at 120 °C for 20 min. After cooling, the precursor N,N-dimethylformamide (DMF) solution (1M) (PbI2:CH3NH3I: dimethyl sulfoxide(DMSO) = 1:1:1 mol %) was spin-coated on the PEDOT:PSS layer at 4000 rpm for 30 s. During spin-coating process, about 0.5 mL of diethyl ether was dripped on the rotating electrode, forming a transparent film on the electrode. By heating it at 65 °C for 5 min and 100 °C for 15 min, the transparent film transfered to a dark perovskite CH3NH3PbI3 film. 2% PCBM chlorobenzene solution was then spin-coated on the CH3NH3PbI3 film at 1000 rpm for 30 s. The processed electrode was then evaporated with an Al film of about 100 nm thick as the


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back contact electrode using a shadow mask, controlling the active area of the perovskite photodetector about 0.3 × 0.3 cm2. Characterization: The surface morphologies of Au network electrodes were characterized by a scanning electron microscope (SEM) system (FEI Inspection F50) and an AFM system (MFP3D-SA, Asylum Research, USA). The transmittance spectra were measured by a Shimadzu UV 3600 UV-IR spectrophotometer. The thickness of the network electrodes were tested by a profilemeter (DEKTAK 150, Veeco Instruments Inc, USA). The sheet resistances were measured with 4 probes using a Keithley 2400 source meter. The photocurrent through the devices were measured using a Keithley 2400 source-meter illuminated by 630 nm LED and lock-in amplifier. The light intensity was varied by changing the power current through the LED light and measured with a Si photodetector. EQE data were collected by recording the photocurrent of the photodetector illuminated by Xe lamp through a monochromator and by recording the incident light power measured with a power meter (Newport 2936-C). ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website, which includes Au network line width distribution and aperture distribution, the comparison of resistance increase for the Au network/PEN and ITO/PET transparent electrodes under different bending radii, the adhesion of the Au/Ti network on the PEN substrate, the cross profile SEM image of the perovskite photodetector based on Au network electrode, top view SEM images of the perovskite film and the stability of the Au network electrode based perovskite photodetector (PDF) ACKNOWLEDGEMENTS


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This work is supported by the National Natural Science Foundation of China (51771053, 51471085), the Natural Science Foundation of Jiangsu Province of China (BK20151400, BK20170657), the Postdoctoral Science Foundation of China (2016M601690), the Fundamental Research Funds for the Central Universities and the open research fund of Key Laboratory of MEMS of Ministry of Education, Southeast University. This work is also supported by the Innovation Foundation of Qixia Qu, Nanjing (201517) and the Green Low-carbon Science Program of Nanjing (2015sc320005).

REFERENCES (1) Lee, J.; Lee, P.; Lee, H. B.; Hong, S.; Lee, I.; Yeo, J.; Lee, S. S.; Kim, T.; Lee, D.; Ko, S. H. RoomTemperature Nanosoldering of a Very Long Metal Nanowire Network by Conducting-Polymer-Assisted Joining for a Flexible Touch-Panel Application. Adv. Funct. Mater. 2013, 23, 4171-4176. (2) Bao, C.; Yang, J.; Gao, H.; Li, F.; Yao, Y.; Yang, B.; Fu, G.; Zhou, X.; Yu, T.; Qin, Y.; Liu, J.; Zou, Z. In Situ Fabrication of Highly Conductive Metal Nanowire Networks with High Transmittance from DeepUltraviolet to Near-Infrared. ACS Nano 2015, 9, 2502-2509. (3) Gaynor, W.; Hofmann, S.; Christoforo, M. G.; Sachse, C.; Mehra, S.; Salleo, A.; McGehee, M. D.; Gather, M. C.; Lüssem, B.; Müller-Meskamp, L.; Peumans, P.; Leo, K. Color in the Corners: ITO-Free White OLEDs with Angular Color Stability. Adv. Mater. 2013, 25, 4006-4013. (4) Bao, C.; Zhu, W.; Yang, J.; Li, F.; Gu, S.; Wang, Y.; Yu, T.; Zhu, J.; Zhou, Y.; Zou, Z. Highly-Flexible Self-Powered Organolead Trihalide Perovskite Photodetectors with Gold Nanowire Networks as Transparent Electrodes. ACS Appl. Mater. Interfaces 2016, 8, 23868-23875. (5) Gaynor, W.; Burkhard, G. F.; McGehee, M. D.; Peumans, P. Smooth Nanowire/Polymer Composite Transparent Electrodes. Adv. Mater. 2011, 23, 2905-2910. (6) Li, Y.; Meng, L.; Yang, Y.; Xu, G.; Hong, Z.; Chen, Q.; You, J.; Li, G.; Yang, Y.; Li, Y. High-efficiency robust perovskite solar cells on ultrathin flexible substrates. Nature Communications 2016, 7:10214.


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(7) Chen, Z.; Cotterell, B.; Wang, W.; Guenther, E.; Chua, S.-J. A mechanical assessment of flexible optoelectronic devices. Thin Solid Films 2001, 394, 201-205. (8) Wu, Z.; Chen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard A.F.; Rinzler A.G. Transparent, Conductive Carbon Nanotube Films. Science, 2004, 305, 1273-1276. (9) Du, J.; Pei, S.; Ma, L.; Cheng, H.-M. 25th Anniversary Article: Carbon Nanotube- and Graphene-Based Transparent Conductive Films for Optoelectronic Devices. Adv. Mater. 2014, 26, 1958-1991. (10) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J.-H.; Kim, P.; Choi, J.-Y.; Hong, B. H. Large-Scale Pattern Growth of Graphene Films for Stretchable Transparent Electrodes. Nature 2009, 457, 706-710. (11) Xia, Y.; Sun, K.; Ouyang, J. Solution-Processed Metallic Conducting Polymer Films as Transparent Electrode of Optoelectronic Devices. Adv. Mater. 2012, 24, 2436-40. (12) Yang, Y.; Heeger, A. J. Polyaniline as a Transparent Electrode for Polymer Light-Emitting Diodes: Lower Operating Voltage and Higher Efficiency. Appl. Phys. Lett. 1994, 64, 1245-1247. (13) Garnett, E. C.; Cai, W.; Cha, J. J.; Mahmood, F.; Connor, S. T.; Christoforo, M. G.; Cui Y.; McGehee, M. D.; Brongersma, M. L. Self-Limited Plasmonic Welding of Silver Nanowire Junctions. Nat. Mater. 2012, 11, 241-249. (14) Lu, H.; Zhang, D.; Ren, X.; Liu, J.; Choy, W. C. H. Selective Growth and Integration of Silver Nanoparticles on Silver Nanowires at Room Conditions for Transparent Nano-Network Electrode. ACS Nano 2014, 8, 10980-10987. (15) Layani, M.; Gruchko, M.; Milo, O.; Balberg, I.; Azulay, D. and Magdassi, S. Transparent Conductive Coatings by Printing Coffee Ring Arrays Obtained at Room Temperature. ACS Nano 2009, 3, 3537-3542. (16) Lee, J.-Y.; Connor, S. T.; Cui, Y.; Peumans, P. Solution-Processed Metal Nanowire Mesh Transparent Electrodes. Nano Lett. 2008, 8, 689-692. (17) Leem, D.-S.; Edwards, A.; Faist, M.; Nelson, J.; Bradley, D. D. C.; de Mello, J. C. Efficient Organic Solar Cells with Solution-Processed Silver Nanowire Electrodes. Adv. Mater. 2011, 23, 4371-4375.


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(18) Maurer, J. H. M.; González-García, L.; Backes, I. K.; Reiser, B.; Schlossberg, S. M.; Kraus, T. Direct Nanoimprinting of a Colloidal Self-Organizing Nanowire Ink for Flexible, Transparent Electrodes. Advance Materials Technologies 2017, 2, 1700034 (1-8). (19) Kang, M.-G.; Kim, M.-S.; Kim, J.; Guo, L. J. Organic Solar Cells Using Nanoimprinted Transparent Metal Electrodes. Adv. Mater. 2008, 20, 4408-4413. (20) Kang, M.-G.; Guo, L. J. Nanoimprinted Semitransparent Metal Electrodes and Their Application in Organic Light-Emitting Diodes. Adv. Mater. 2007, 19, 1391-1396. (21) Kang, M.-G.; Park, H. J.; Ahn, S. H.; Guo, L. J. Transparent Cu Nanowire Mesh Electrode on Flexible Substrates Fabricated by Transfer Printing and its Application in Organic Solar Cells. Sol. Energ. Mat. Sol. C. 2010, 94, 1179-1184. (22) van de Groep, J.; Spinelli, P.; Polman, A. Transparent Conducting Silver Nanowire Networks. Nano Lett. 2012, 12, 3138-3144. (23) Hsu, P.; Wang, S.; Wu, H.; Narasimhan, V. K.; Kong, D.; Lee, H. R.; Cui, Y. Performance enhancement of metal nanowire transparent conducting electrodes by mesoscale metal wires. Nature Communications, 2013, 4, 2522 (1-7). (24) He, T.; Xie, A.; Reneker, D. H.; Zhu, Y. A Tough and High-Performance Transparent Electrode from a Scalable and Transfer-Free Method. ACS Nano 2014, 8, 4782-4789. (25) Wu, H.; Kong, D.; Ruan, Z.; Hsu, P.; Wang, S.; Yu, Z.; Carney, T. J.; Hu, L.; Fan, S.; Cui, Y. A Transparent Electrode Based on a Metal Nanotrough Network. Nat. Nanotechnol. 2013, 8, 421-425. (26) Hu, L.; Kim, H. S.; Lee, J.-Y.; Peumans, P.; Cui, Y. Scalable Coating and Properties of Transparent, Flexible, Silver Nanowire Electrodes. ACS Nano 2010, 4, 2955-2963. (27) Dou, L.; Yang, Y. M.; You, J.; Hong, Z.; Chang, W.; Li, G.; Yang, Y. Solution-processed hybrid perovskite photodetectors with high detectivity. Nat. Commun. 2014, 5, 5404. (28) Lin, Q.; Armin, A.; Lyons, D. M.; Burn, P. L.; Meredith, P. Low Noise, IR-Blind Organohalide Perovskite Photodiodes for Visible Light Detection and Imaging. Adv. Mater. 2015, 27, 2060-2064. (29) Fang, Y.; Huang, J. Resolving Weak Light of Sub-picowatt per Square Centimeter by Hybrid Perovskite Photodetectors Enabled by Noise Reduction. Adv. Mater. 2015, 7, 2804-2810.


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(30) Gong, X.; Tong, M.; Xia, Y.; Cai, W.; Moon, J. S.; Cao, Y.; Yu, G.; Shieh, C.-L.; Nilsson, B.; Heeger, A. J. High-Detectivity Polymer Photodetectors with Spectral Response from 300 nm to 1450 nm. Science 2009, 325, 1665-1667. (31) Jiang, J.; Tsao, S.; O’Sullivan, T.; Zhang, W.; Lim. H. High detectivity InGaAs/InGaP quantum-dot infrared photodetectors grown by low pressure metalorganic chemical vapor deposition. Appl. Phys. Lett. 2004, 84, 2166-2168. (32) Konstantatos, G.; Clifford, J.; Levina, L.; Sargent. E. H. Sensitive solution-processed visiblewavelength photodetectors. Nature Photonics 2007, 1, 531-534. (33) Chakrabarti, S.; Adhikary, S.; Halder, N.; Aytac, Y. and Perera. A. G. U. High-performance, long-wave (∼ 10.2 µ m) InGaAs/GaAs quantum dot infrared photodetector with quaternary In0. 21Al0. 21Ga0. 58As capping. Appl. Phys. Lett. 2011, 99, 181102 (1-3).


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Table of Contents Graphic 100 90

Transmittance (%)

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30 nm 50 nm 80 nm

80 70 60 50 40 1

10

100 -1

Sheet Resistance (Ω sq )


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High-Performance Transparent Conducting Metal Network Electrodes

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