Ultrahighly Photosensitive and Highly Stretchable Rippled Structure

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Ultra-High Photosensitive and Highly Stretchable Rippled Structure Photodetectors Based on Perovskite Nanocrystals and Graphene Ying-Huan Chen, Monika Kataria, Hung-I Lin, Christy Roshini Paul Inbaraj, Yu-Ming Liao, Han Wen Hu, Ting-Jia Chang, Cheng-Hsin Lu, Wei-Heng Shih, Wei-Hua Wang, and Yang-Fang Chen ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.9b00308 • Publication Date (Web): 30 Jul 2019 Downloaded from pubs.acs.org on July 30, 2019

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ACS Applied Electronic Materials

Ultra-High Photosensitive and Highly Stretchable Rippled Structure Photodetectors Based on Perovskite Nanocrystals and Graphene Ying-Huan Chen Δ#, Monika Kataria Δ⁋§‡ #, Hung-I Lin Δ, Christy Roshini Paul Inbaraj Δ‼†, YuMing Liao Δ, Han-Wen Hu Δ, Ting-Jia Chang Δ, Cheng-Hsin Lu₸, Wei-Heng Shih₸, Wei-Hua Wang§‡, and Yang-Fang Chen Δ¥* Δ

Department of Physics, National Taiwan University, Taipei 106, Taiwan

⁋

Department of Physics, National Central University, Chung-Li 320, Taiwan

§

Molecular Science and Technology Program, Taiwan International Graduate Program, Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 115, Taiwan

‡

Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 115, Taiwan

‼

Nanoscience and Nanotechnology Program, Taiwan International Graduate Program, Institute of Physics, Academia Sinica, Taipei 106, Taiwan

†

Department of Engineering and System Science, National Tsing Hua University, Hsinchu 300, Taiwan.

₸ Department

of Materials Science and Engineering, Drexel University, Philadelphia, PA

19104, USA ¥ Advanced

Research Centre for Green Materials Science and Technology, National Taiwan

University, Taipei 10617, Taiwan

*Corresponding author: Yang-Fang Chen [email protected] #

The authors contributed equally.

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ABSTRACT: Stretchable optoelectronic devices are the need of the hour when it comes to making present day technologies user friendly. These devices when placed conformably on the human skin or any other artificial intelligence products must function in all their capacities to get a durable and highly sensitive device performance. Stretchable photodetectors are the core fundamental constituents that fall under the umbrella of flexible optoelectronic devices. Although significant amount of research studies have been reported based on stretchable photodetectors, a good performance still remains a challenge. Nanometer-sized methylammonium lead bromide (MAPbBr3) perovskite nanocrystals can significantly emit and absorb light in the visible spectrum. With high quantum yield and stable optical and electronic properties, when these are subjected to mechanically tunable two-dimensional (2D) composites we obtain a better performance from the stretchable optoelectronic devices. Here, a hybrid photodetector composed of perovskite nanocrystals and graphene of rippled geometry is demonstrated, where a thin film of photon-absorbing perovskite nanocrystals is placed on rippled graphene that contributes to the photoresponsivity of the order of ~6 × 105 A W-1. The large value of photoresponsivity makes this hybrid stretchable device take the highest position in the devices with similar functionality. It is found that the photoresponsivity of the perovskite nanocrystals and graphene hybrid rippled structure photodetector is strain tunable with a stretchability up to 100 %. In addition, the hybrid rippled structure photodetector has wearablity and durability. All these superior functionalities lead the way in the direction of fabrication of stretchable optoelectronic devices suitable for applications in the fields of bio-medicine, defense systems, fire-detection, and flexible display panels.

KEYWORDS: stretchable photodetector, perovskites, wearable systems, optoelectronics, rippled structure, two-dimensional hybrid systems


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Introduction With rapidly progressing technologies and highly developing science, the need for stretchable and wearable optoelectronic devices for human-machine interaction is proliferating.1-6 The photodetectors, light emitting diodes, memory devices, electrically pumped lasing systems and many more have made the family of stretchable and wearable optoelectronic devices quite successful in recent times.5,7-9 Many of the important studies have been carried out on how to make highly photosensitive, stable, and wearable photodetectors. Highly stretchable photodetectors have a promising potential in medical devices and wearable electronics including watches, gaming gadgets, home appliances, defense machinery, etc. Previously, stretchable photodetectors have been successfully used in cameras, contact lenses, and pulse oximetry.10-12 Two-dimensional (2D) hybrid stretchable systems have significantly attracted attention of researchers. 2D hybrid photodetectors produce an immense amount of interest owing to their high photoresponsivity and low power consumptions.13 Graphene is a widely used 2D material that suits best for 2D hybrid stretchable systems. This single-layered carbon has an exclusive band structure, which is well defined by the Dirac equation.14 Near the Dirac point, charge carriers act like relativistic particles with rest mass equal to zero, and thus allowing graphene to have exceptionally high carrier mobility. Moreover, close to the Dirac point graphene has small of density of states; therefore its conductivity is highly sensitive to perturbations caused by materials placed in its close proximity.15 Besides, because of graphene’s low absorption coefficient, a composite structure with a light-absorbing material is an excellent strategy to make a photodetector with high photoresponsivity. Unlike flat graphene, rippled graphene has some unique properties that can make further improvements in graphene-based photodetectors. Kang et al.16 reported a photodetector derived from wrinkled graphene with stretchable properties and improved photoresponse. The uniform rippled structure of graphene can induce 3 ACS Paragon Plus Environment


an increased areal density in graphene that boosts the absorption of photons. The rippled structure is suitable for trapping photons, which can increase photoresponsivity.4 Due to these excellent features, graphene with rippled structure can be broadly used in many electronics and biomedical applications. To fabricate a 2D hybrid system, a layer of light absorbing material is placed in close conjugation with graphene. Hybrid photodetectors where rare earth doped upconversion nanoparticles, metal-organic framework nanomaterial, perovskite nanowires, graphene quantum dots, etc. together with graphene to obtain a better photoresponse.4-5,17-19 Among photoactive materials that are presently existing, perovskites lead the way.20-26 This is due to the fact that perovskites can be synthesized using plentiful low cost starting compounds as well as they possess remarkable physical characteristics. Indeed, semiconducting lead halide perovskites nanocrystals have become prominent in many different devices recently because of their high quantum efficiency and low-cost fabrication process. Methyl-ammonium lead bromide (MAPbBr3) perovskite nanocrystals with bandgap around 2.3 eV are proven to own a high optical transition probability and narrow emission bandwidth owing to the addition of the long alkyl chain ammonium cation octylammonium bromide.27 Along with the high light absorbing capacity, these perovskite nanocrystals also act as transporter of holes and electrons. With small perovskite nanocrystals (~6 nm), thin film deposition using spin coating technique is reasonably attainable.27 These perovskite nanocrystals are stable for long time in both solid and solution states. Various kinds of highly stretchable photodetectors have been reported by Ding et al.18, Kataria et al.4, Bera et al.17 and Kang et al.16 Despite the significant progress concerning the stretchable 2D hybrid photodetectors, improving the photoresponsivities and durability of the hybrid stretchable systems is still challenging. It is promising to consider a 2D hybrid system


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where methyl-ammonium lead bromide (MAPbBr3) perovskite nanocrystals serves as the light absorbing thin layer, graphene serves as a carrier transport channel and also provides for a durable rippled structure, and poly(dimethylsiloxane) (PDMS) serves as flexible supporting substrate.4 In this work, a photosensor with high stretchable properties based on rippled graphene structure with ultrahigh photoresponse is reported. We have used rippled structure graphene, which has been deliberately deformed to have a uniformly rippling three dimensional (3D) surface that induced an increase in the areal density. This increased areal density could harvest higher optical absorption per unit area which in turn provides for an increase in the photoresponsivity of the photodetector. Ripple height, pitch, and density of the ripples are regulated by applied strain. The ripples structure is entirely reversible under repeated stretching and release which leads to strain dependent photon absorption enhancement. Our hybrid device exhibits the photoresponsivity ~6 × 105 A W-1, which is the largest photoresponse ever reported in stretchable photodetectors based on rippled or crumpled graphene structures with device active area having channel length of 100 µm. Moreover, our hybrid photodetector is highly stretchable and can work well under different strain conditions ranging from 0 % to 100 %. Therefore, we achieve that a single device possesses multifunctionalities with ultra-high photoresponsivity and high stretchability simultaneously. In addition, the large area and lowcost fabrication of the hybrid rippled structure photodetector consisting of perovskite nanocrystals-graphene photodetector in rippled structure makes it convenient and accessible for the stretchable optoelectronics devices.

Results and Discussion The schematic of the perovskite nanocrystals and graphene hybrid rippled geometry 5 ACS Paragon Plus Environment


photosensor is shown in Figure 1. The detailed device fabrication is described in Figure S1. To begin with, a thin film of PDMS was initially stretched up to twice its length on a glass substrate as shown in Figure S2 (Supporting Information SII) thereby defining 100 % lateral strain. A planar graphene with polymethyl-methacrylate (PMMA) was then transferred onto the PDMS film under 100 % lateral strain condition. The detailed description of the graphene transferring method on strained PDMS film is discussed in the Experimental Section. Then, a 100 µm channel length shadow mask was used to define silver electrodes using thermal evaporation. The perovskite nanocrystals were then spin-coated over the planar graphene with PMMA on PDMS under lateral strain (Figure S3). The layer thickness of perovskite nanocrystals is observed to be ~150 nm as shown in Figure S4. The stretched PDMS film was then released thereby subjecting it to 0 % lateral strain condition. The perovskite nanocrystals decorated planar graphene with PMMA under 0 % lateral strain condition tends to form a rippled structure desired for our perovskite nanocrystals and graphene hybrid rippled structure photodetector. Figure 2a shows the cross-sectional view of the hybrid rippled structure photodetector. The lateral SEM image of the rippled graphene/PMMA structure is shown in Figure 2b. The crosssectional SEM image shown in Figure 2c indicates the average observed height of the ripples reaches 50 µm. The uniformly spaced rippled graphene with PMMA structure is desirable for enhanced durability and stability. The Raman spectrum of PDMS and planar graphene/ PDMS is shown in Figure 2d. The characteristic peaks at ~1430 cm-1 and ~1270 cm-1 corresponds to the eigenmodes of CH3 and Si-O-Si vibrations in PDMS, respectively.5,28 The vibrational signals of atoms from graphene on PDMS exhibits additionally distinctive G, 2D peaks, and a minuscule D peak around 1327 cm−1, showing that the graphene on PDMS possesses an excellent quality.14 The ratio of intensity of 2D versus G band was calculated to confirm 1-2 graphene layers on the PDMS. The ratio of intensity of 2D to G modes was achieved about 2.9 conforming that number of graphene layer is one to two.4,29 The integrated intensity ratio with 6 ACS Paragon Plus Environment

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high value corresponds to minor suspension of graphene on PDMS substrate. A detailed synthesis method of perovskite nanocrystals has been discussed below. The image of transmission electron microscopy (TEM) image of one perovskite nanocrystal is shown in Figure S5a. The size of the perovskite nanocrystal is ~ 7 nm. Figure S5b illustrates the fast Fourier transform (FFT) pattern indicating the crystalline surface of the perovskite nanocrystal.27 Figure 2e shows the absorption characteristics of the perovskite nanocrystals dispersed in organic solvent with a peak at ~460 nm. Inset of Figure 2e shows the UV-visible absorption spectra of the perovskite nanocrystals thin film with graphene. Apparently, the perovskite nanocrystals exhibit a dominant absorption in the UV and visible range. Figure 2f shows the emission behavior of perovskite nanocrystals under 374 nm pulsed laser excitation. A prominent emission peak appears at ~530 nm. The observed absorption and emission are consistent with previously reported perovskite nanocrystals by Schmidt et al. 27

The fabricated perovskite nanocrystals and graphene hybrid rippled structure photodetector was irradiated by a laser with wavelength of 325 nm to measure the performance. The laser spot was adjusted in a way that the exposed active area is kept at constant 100 µm channel length. The schematic illustration of the shadow mask (channel length 100 µm) used for depositing silver electrodes is shown in Figure 3a. Figure 3a(ii) shows the photographic image of the setup used for performing the lateral strain measurements with inset showing the actual photographic image of the perovskite nanocrystals and graphene hybrid rippled structure photodetector under 0 % lateral strain. The photoresponse or photocurrent (| ∆I |) was measured to characterize the device performance and is defined as19 | ∆I | = | I laser illumination – I dark |

(1)

The effective photoresponse with different lateral strain conditions was measured under 325 nm laser illumination at VDS (voltage drain-source) equal to 0.1 V. The device exhibited negative 7 ACS Paragon Plus Environment


photoresponse under 325 nm laser illumination (Figure 3b). When the lateral strain is increased, the photoresponse decreases. This can be explained as follows. Under 100 % lateral strain condition there exists no ripples. When such flat surface is exposed to 325 nm laser illumination, only faction of photons interact with perovskite nanocrystals–graphene hybrid system. There are high loss rate of photons due to direct transmission and reemission in all random directions without reflection. This results in the smaller photocurrent generation under 325 nm laser illumination. As we decrease the lateral strain from 100 % to 0 % in the steps of 20 %, we observe a significant improvement in photoresponse. As we decrease the lateral strain, our device tends to undertake rippled structure formations. With decreasing lateral strains, the height of the ripples increases. This makes the active area entirely covered by rippled structure with trivial hills and valleys formations. The hills and valleys formations contributed by the rippled structure device can trap photons upon laser illumination. This trapping of photons becomes more pronounced with decreasing strain. The higher the ripples, the more trapping of photons occurs. This escalates the possibility of absorption by perovskite nanocrystals integrated onto the surface of the rippled graphene.4,17 The rippled structure also enables multiple reflections of light along with the trapping of light in its vicinity, enhancing the photocurrent and photoresponse of the device under decreasing lateral strain conditions. The efficient contribution of the rippled structure in trapping laser photons is demonstrated by the transmission and absorption spectra (Figure S6). It is well observed that under 100% lateral strain, there is about 100 percent transmission. In contrast, the transmission drops to lower values as the lateral strain is reduced to 0 %. Figure 3c shows the photocurrent of the device under different lateral strain conditions and external bias voltage (VDS) with 325 nm laser irradiance. The device displays ohmic behavior. With varying VDS, photoresponse is inversely proportional to strain. To quantify the device performance, the device photoresponsivity (R) and photocurrent gain (G) were examined. The photoresponsivity of the device can be obtained


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by the expression,13,30 Rₚₕ =

|∆𝐼(𝐴)|

(2)

𝑃(𝑊)

Here, |∆𝐼(𝐴)| is the photocurrent in Amperes and P (W) is the laser illumination power at device active area in Watts. The unit for photoresponsivity is AW-1. The photocurrent gain (G) is defined as the ratio of electrons harvested by the device and photons collected by the device in a unit time. It is described by the equation,17,31

G=

|∆𝐼(𝐴)| 𝑃(𝑊)

ℎ𝜈

(3)

× 𝜂𝑞

where q represents the electron charge in Coulombs, ℎ represents Planck’s constant in Joule∙second, 𝜈 is the frequency of laser illumination in Hertz, 𝜂 represents the quantum efficiency of photogenerated charge carriers. The value of 𝜂 is taken to be 1 for simplicity in all cases. In Figure 3d it is shown that the photoresponsivity and photocurrent gain decrease with increase in lateral strain at 4.8 Wm-2 power density of laser irradiance. This behavior is due to the trapping of photons in the rippled structure as explained above. To check the stability of device performance, we have performed the experiment as shown in Figure 3e, which exhibits the photoresponsivity of the device under 0 % lateral strain stretched upto 100 % lateral strain 25, 50, 75, 100, and 125 times under laser exposure with 325 nm wavelength. The device performance was consistent under repeated back and forth stretching under 100 % lateral strain. The device consists of a multiple layered structure which includes perovskites nanocrystals, graphene, PMMA, PDMS. Even under repeated stretching, the device works robustly due to the mechanical strength provided by the graphene/PMMA/PDMS heterostructure. A dark conductivity test was performed on the device to check the wearing and tearing of the graphene under different lateral strain conditions. Figure 3f shows that with increasing lateral strain the conductivity ratio increases under dark condition at VDS = 0.1 V. This implies that as the lateral 9 ACS Paragon Plus Environment


strain is increased the flattening of graphene occurs at each step which provides a pathway for a faster flow of charge carriers on the surface of the device active area. Under lower lateral strain values the charge carriers tend to scatter more in the rippled graphene in the device active area.19 This adds to the increase in the resistivity in the device active area. This test proves that the rippled structure efficiently offers for the light confinement and the wearing and tearing of graphene have a negligible role to play. As described in Figure S1 and S2, the 100% pre-stretch was applied to PDMS in one axis. A large expansion in perpendicular axis of PDMS occurs when released after stretching. This definitely induces some less significant cracks in the graphene. But since the stretching is done to PDMS and graphene exhibits an indirect strain from the stretched PDMS on release, the lesser number of cracks are formed on the graphene in perpendicular axis. The quality of graphene was checked under different strain conditions using a dark conductivity test as discussed above. As shown in Figure 3f, the dark conductivity ratio of graphene increases with increasing strain. Therefore the conductivity of graphene is not much affected by the presence of these cracks. The significantly less cracked ripples efficiently contribute towards the confinement of light and increasing the probability of absorption arising from perovskite nanocrystals. The cracks in graphene become more and more significant when the PDMS is subjected to beyond 100 % strain which means PDMS is made to stretch more than twice its initial length. This results in the reduction of conductivity ratio of graphene beyond 100 % strain as shown in Figure 3f.19 Therefore, the device was deliberately stretched within the limits of 100 % strain conditions in order to exhibit best quality photoresponse from the hybrid rippled structure photodetector. The silver electrodes too were exposed to a variety of lateral strains while performing the strain dependent measurements for the device. The deposition of electrodes was done on a pre-strained PDMS with graphene/PMMA over it as shown in Figure S1 (Supporting Information). At 100 % strain provided to PDMS, the electrodes were subjected to no strain. The strain was induced to


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electrodes only when the PDMS was released from 100 % strain to 0 % strain condition. Therefore the electrodes could successfully survive the 100 % strain condition adding on to the quality of our highly stretchable hybrid rippled structure photodetector.

The ability of the photodetector to fast follow optically modulated signal is determined by the responsiveness of the photodetector. In this case, the responsiveness can be defined in time domain using Ꞇf (fall time or response time) and Ꞇr (rise time or recovery time) where the photodetector exhibits a rise in photocurrent when laser illumination is switched off (negative photoresponse).32 Here, the time of the dynamics of the device is defined as the time period required to fall from 90 % to 10 % of the peak value. The time to recovery is defined as the time period required to rise from 10 % to 90 % of the peak value.33 Figure 4a shows the timedependent photoresponse of the device indicating both the recovery and response times are smaller than 200 ms. The fast response and recovery time can be credited to the fast charge transfer between the very thin layer of perovskite nanocrystals and graphene. Figure 4b shows the photoresponse of the device for different optical power densities of 325 nm laser irradiance versus bias voltage (VDS). Also, Figure S7 shows the effective photoresponse of the device for different optical power densities of laser irradiance, indicating that the photocurrent of the device decreases with decreasing optical power density of laser illumination. Figure S7, also shows that with decreasing optical powers of laser illumination, response time and recovery time increases. At the interface of perovskite nanocrystals and graphene, a number of interface defects exists. The photogenerated charge carriers trapping occurs at the interface defects at very low powers of laser illumination which significantly slows down the response of the device. At higher laser power exposure, this carrier trapping becomes less significant as there is a flood of photogeneretaed charge carriers crossing the interface. This results in a faster response by the device at higher laser power exposure.32,34 Figure 4c shows the change of 11 ACS Paragon Plus Environment


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photocurrent gain and photoresponsivity versus laser power densities. We found that photoresponsivity and photocurrent gain increase with decreasing optical power densities of laser illumination. Notably, the maximum photoresponsivity and photocurrent gain under 6 × 10-4 Wm-2 power density reach ~6 × 105 AW-1 and ~2.5 × 106, respectively. This is the highest photoresponsivity value ever reported amongst stretchable devices with 100 µm channel length. Because the device performance is affected by the noise in the device, the detectivity (D*) of the hybrid rippled structure photodetector was measured to differentiate the photodetector’s performance under laser illumination from the noise in the device. The detectivity (D*) is defined by17,35 D* =

√(𝑎𝑏)

(4)

𝑁𝐸𝑃

Here, 𝑎 represents the active area, 𝑏 is the band width of the response time. NEP represents noise equivalent power, which is defined by17 𝐼

NEP = 𝑅 𝑁

(5)

𝑝ℎ

Here, 𝐼𝑁 is the noise current which corresponds to ID (dark current) and 𝑅𝑝ℎ denotes the photoresponsivity. The maximum detectivity recorded at 6 × 10-4 Wm-2 power density of laser illumination is ~ 8 × 1012 Jones. The higher value of detectivity is due to the lesser number of defect centers and higher concentration of charge carriers.

Figure 4d shows the detectivity as a function of applied laser power densities at VDS = 1 V. It is observed that the detectivity increases with decreasing optical power densities of laser illumination. The schematic illustration of light confinement in the rippled geometry is shown in Figure 5a. The underlying mechanism of observed results can be interpreted as follows. The perovskite nanocrystals film is the topmost layer of the hybrid rippled structure photodetector 12 ACS Paragon Plus Environment

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and absorbs most irradiated photons. Because the perovskite nanocrystals have band gap of about ~2.3 eV, the 325 nm laser photons can effectively generate electron/hole pairs upon illumination. Since, the perovskite nanocrystals are p-type in nature therefore the Fermi energy level is closer to valence band.36 The device exhibits a negative photoresponse (Figure 4a), thus, for a negative polarity photodetector, the Fermi energy of graphene has to be greater than the Fermi energy of perovskite nanocrystals as shown in Figure 5b in accordance with published literatures.32 Also, the charge neutrality point of graphene will change when thin film of perovskites nanocrystals is deposited on graphene and upon laser illumination.19 Figure 5b shows the band bending of the conduction band in perovskite nanocrystals because there exists a built-in field across the interface. Upon laser irradiance, electrons moved to the conduction band of the perovskites nanocrystals leaving behind holes in the valence band of the perovskites nanocrystals. The photogenerated electrons then fall from the conduction band in perovskite nanocrystals to the graphene which raised the Fermi energy level of the graphene.32 This charge transfer between perovskites nanocrystals and graphene is confirmed using photoluminescence spectra.37 The photoluminescence spectra of perovskite nanocrystals and perovskites nanocrystals/graphene flakes is shown in Figure S8 (Supporting Information). A significant photoluminescence quenching was observed when perovskite nanocrystals are in contact with graphene flakes. This is because of the charge transfer via π-π interaction occurring in-between nanocrystals and graphene.37 Under laser illumination, carriers recombine within the lifetime, resulting in the emission of light. But when the perovskite nanocrystals were placed in contact with graphene, the photogenerated electrons in nanocrystals moved towards graphene resulting in photoluminescence quenching. The photoluminescence quenching clearly indicates that the observed negative photocurrent in hybrid perovskite nanocrystals and graphene device is because of electron transfer from nanocrystals to graphene. The high photogain of 106 can be explained as follows. First, the device consisted of graphene with extremely high mobility. It 13 ACS Paragon Plus Environment


was explained that the photogenerated electrons present in perovskite nanocrystals will transfer into graphene layer because of the built-in field at perovskite nanocrystals and graphene interface, and holes remain in perovskite nanocrystals. Due to the spatially separated photogenerated electrons and holes, there was a continuous circulation of charge carriers in the active conducting channel of the device. With production of one pair of photogenerated charge carrier, a lot of electrons will circulate in the conducting channel due to high electron mobility in graphene under an external bias voltage. This leads to higher photocurrent gain value in the hybrid rippled structure photodetector. It was observed that higher power of laser illumination results in lesser photosensitivity, photocurrent gain, and detectivity. The underlying reason of this result is explained as follows. Along with the built-in field at perovskite nanocrystals and graphene interface, there will be an electric field Ec due to the accumulation of holes in the perovskite nanocrystals. At higher laser power, more electrons and holes pairs are produced, the electric field Ec is increased. As more and more electrons and holes are produced, these charge carriers are transferred to graphene in abundance, and the Fermi level of graphene moves upward, thereby reducing EB. Therefore, as laser power is increased, the net electric field (Eeffective) is reduced effectively with increase in Ec and reduction in EB. At higher laser pumping power, the built-in electric field will be screened by the high density of carriers, and more photogenerated electrons and holes will recombine in the perovskite nanocrystals, in turn, the photoresponse is reduced.32

Now we demonstrate the flexibility test of the devices under different bending strain conditions by utilizing the ripples existing in our device. Figure 6a shows the photo of the device showcasing bendable nature of the device. The photosensitivity of the device under various bending strains is shown in Figure 6b. It was observed that under 0 %, 0.2 %, 0.5 %, 0.8 %, and 2 % bending strain the photocurrent exhibits a negligible variation. Figure 6c exhibits the


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photocurrent versus bending strains. The photocurrent was slightly decreased with increasing bending strains. This was due to decrease in the height of the ripples with increasing bending strains. Inset of the Figure 6c displays the photo and schematic of the device placed on the curved body of a cylindrical object showcasing the device under bending strain. The current seems decreasing within one cycle under bending strain conditions. This is particularly due to the experimental limitations. In order to demonstrate bending strain, the device was implanted on curved surfaces with various radius. The probes were placed on the electrodes exhibiting bending strain. The contact made by the probes with the electrodes lacks a planar surface for proper and precise contacts and thus leads to decreasing currents with each cycle. On the contrary a lesser decrease in current is observed when the device is implanted on a flat plane. The photocurrent of the device under 0 % strain subjected to different cyclic stretching is shown in Figure S9, indicating a stable behavior of the device upon multiple cycles of stretching under bending strain. The time-dependent photocurrent of the device indicating the durability of the device is shown in Figure 6d. It is noteworthy that the performance stayed largely unaffected up to 120 days and more. Figure S10 shows dependence of responsivity and EQE of the device under laser exposure with different wavelength. Here, EQE is defined similar to photocurrent gain comprising the ratio between amount of electron and holes contributing the photocurrent and amount of excitation photons.33,38 The responsivity and EQE versus wavelength match well with absorption spectra of perovskites nanocrystals as shown in Figure 2e making it suitable for broadband photodetection. This made the hybrid rippled structure photodetector one of its kind highly stretchable and stable photodetector with highest ever reported photoresponsivity amongst stretchable and flexible photodetectors.

Conclusion A highly stable and stretchable perovskite nanocrystals-graphene hybrid rippled structure has



been demonstrated which is valuable for optoelectronics applications. We demonstrate the photoresponsivity of 6 × 105 AW-1 under 0 % lateral strain, which is the highest value amongst stretchable and flexible photodetectors. The rippled geometry of the photosensor is advantageous for enhancing the photoresponsivity of the device by increasing the light confinement in the rippled geometry. The device is highly durable, conducting, flexible, and stretchable including lateral and bending strain. Such kind of rippled structures can be extended to other hybrid systems with different photosensitizers and two-dimensional materials. Moreover, such technique is useful for fabricating many other stretchable optoelectronic devices. The demonstrated ultra-high performance and wearability of rippled perovskite nanocrystals-graphene hybrid photodetector via easy and low-cost fabrication is suitable for diverse internet of things applications.39

Methods Device Fabrication Technique: The fabrication process is similar to our previous report.4 The detailed synthesis procedure of single-layer graphene, electrodes deposition methods and PDMS synthesis are described in Supporting Information SIV. Perovskite Nanocrystals Synthesis: Here, the perovskite nanocrystals used have a chemical formula of CH3NH3PbBr3. These were synthesized using the procedure followed by Schmidt et al.27 The detailed synthesis procedure is discussed in Supporting Information SIV. The synthesized perovskite nanocrystals have a strong photoluminescence as shown in Figure S8.


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Electrical (IV/IT) Measurements. The electrical properties of the device with rippled geometry were measured by a 4156C semiconductor parameter analyzer from Agilent and Keithley 236 source measurement unit. All the recordings and measurements were done in ambient surroundings.



Figure 1. Schematic of perovskite nanocrystals/ graphene rippled photodetector.


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a

c

b

100 µm

100 µm

e

f 100

0.6

Perovskite nanocrystals thin film / graphene

Absorbance (a.u.)

0.06

0.5 0.4

0.05 0.04 0.03 0.02

0.3

400

500

600

700

800

Wavelength (nm)

0.2 0.1

500

600

700

Photoluminescense Spectra of Perovskite Nanocrystals

80 60 40 20 0

Perovskite Nanocrystals in cyclohexane solution 400

Intensity ( a.u. )

d Absorption (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Under 374 nm excitation

800

Wavelength (nm)

460

480

500

520

540

560

580

600

Wavelength ( nm)

Figure 2. (a) Schematic cross-sectional illustration of the hybrid stretchable device. (b) Lateral scanning electron microscopy image (SEM) of the rippled orientation. (c) Cross-sectional SEM image of the rippled orientation. (d) Raman Spectrum of graphene on PDMS {poly(dimethylsiloxane)} and PDMS. (e) Absorption spectra of perovskite nanocrystals in cyclohexane solution (Inset: Absorption spectra of perovskite nanocrystals thin film/graphene). (f) Photoluminescence spectra of perovskite nanocrystals under 374 nm laser excitation.



c

b 1 cm

0%



2.4x10-6

ii

325 nm laser illumination (Power Density = 4.8 Wm-2 ) VDS = 0.1 V

ON OFF

 40 %

1.2x10-6

 60 %  80 %

6.0x10-7

2 cm

 100 %

Photoresponsivity , Rph (AW-1)

5

10

15

20

25

30

Under 0 % Lateral Strain

Under 325 nm laser illumination Power Density = 4.8 Wm-2 VDS = 1 V

0

20

40

60

ON

2.4x10-6

OFF

1.6x10-6

Lateral Strain (%)

0.0 80

100

0.0

0% 20 % 40 % 60 % 80 % 100 %

-8.0x10-6

25th 0

50th 5

75th 10

-1.0

-0.5

100th 15

125th 20

Time (s)

0.0

0.5

1.0

VDS (V)

f

325 nm laser illumination (Power Density = 4.8 Wm-2) VDS = 0.1 V

8.0x10-7

100

Under Lateral Strain

8.0x10-6

35

Time (s)

3.2x10-6

Photocurrent Gain , G

Under 325 nm laser illumination Power Density = 4.8 Wm-2

-2.4x10-5

e

d

-5

-1.6x10-5

0.0 0

1000

1.6x10

Under Lateral Strain

 20 %

1.8x10-6

2.4x10-5



i

3.0

Conductivity Ratio

a



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.5

2.0

1.5

1.0

VDS = 0.1 V 20

40

60

80

100

120

140

Lateral Strain ( %)

Figure 3. (a) (i) Schematic of the shadow mask with channel length 100 µm used for electrode deposition. (ii) Photo of the system used for performing the lateral strain measurements (Inset: Photographic image (top view) of the device). (b) The photocurrent of the device under various lateral strain conditions and laser exposure with 325 nm wavelength. (c) Photocurrent of the device under various lateral strain conditions as a function of bias voltage (VDS) under 325 nm laser irradiance. (d) Dependence of photocurrent gain and photoresponsivity on various lateral strain conditions with 325 nm laser exposure. (e) The photoresponse of the device under 0 % lateral strain stretched upto 100 % lateral strain 25, 50, 75, 100, and 125 times under 325 nm laser irradiance. (f) Plot of dark conductivity ratio of graphene under various lateral strain conditions (VDS = 0.1 V).


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a

b

2.0x10-6

1.0x10

-6

325 nm laser illumination (Power Density = 4.8 Wm-2 ) VDS = 0.1 V

ON OFF

1.5x10-5

Under 325 nm laser illumination

-5

1.0x10

Under 0 % Strain

Recovery Time

Response Time



3.0x10-6



5.0x10-6 0.0

3.9 × 10-4 Wm-2 4.7 × 10-4 Wm-2

-6

-5.0x10

2.2 × 10-3 Wm-2 1.5 × 10-2 Wm-2

-1.0x10-5

0.0

8 × 10-2 Wm-2 0.12 Wm-2 0.31 Wm-2

-5

-1.5x10

0.6

0.9

1.2

4.2

4.5

4.8

5.1

Time (s)

c

-1.0

10

104 103 Photoresponsivity, Rph (AW -1) Photocurrent Gain, G -3

10

-2

10

-1

10

1.0


5

102

0.5

VDS = 1 V


10

0.0

VDS (V)

1013

VDS = 1 V

6

-0.5

d

107

D* ( Jones)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1012 1011 1010 109

0

10-3

10

10-2

10-1

100

101

Power Density (Wm-2)

Power Density (Wm-2 )

Figure 4. (a) Time-dependent photocurrent of the device under laser irradiance with 325 nm wavelength. (b) Photocurrent of the device under exposure of various laser power density versus external bias voltage (VDS). (c) Dependence of photocurrent gain and photoresponsivity on applied various optical power densities of laser irradiance with 325 nm wavelength. (d) Dependence of detectivity on applied various optical power densities of 325 nm laser irradiance.



a

b

Figure 5. (a) Schematic showing the multiple reflections and scattering of light confined in rippled geometry of the hybrid device. (b) Schematic energy band structure showing the transport of electrons and holes between perovskite nanocrystals and graphene under laser irradiance, where EC, EF, and EV are the energies related to conduction band, Fermi level, and valence band of the perovskite nanocrystals, respectively.


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b 4.0x10

a

-6




3.0x10-6

1.0x10-6

0.0

c 325 nm laser illumination (Power Density = 4.8 Wm-2) VDS = 1 V

10-6

0.4

0.8

1.2

1.6

0.8 % 2 %

0.5 %

10

15

20

ON OFF


2.0x10-6

1.0x10-6

2 cm 0.0

0.2 % 5

Time (s) 3.0x10-6



10-5

0% 0

d

10-4

Under Bending Strain

ON OFF 2.0x10-6

1 cm



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0 2.0

Day 1

Day 15

Bending Strain (%)

Day 30

Day 60

Day 90

Day 120

Days

Figure 6. (a) Photo indicative of the bendable nature of the device. (b) The photocurrent of the device under various bending strains and laser irradiance with 325 nm wavelength. (c) Dependence of photocurrent on various bending strains under 325 nm laser irradiance. (Inset: Schematic and photo of the device implanted on curved body of a cylindrical object thereby experiencing bending strain) (d) Time-dependent photocurrent of the device indicating the durability of the device illuminated by laser with 325 nm wavelength.



ASSOCIATED CONTENT

Supporting Information. Fabrication method of perovskite nanocrystals and graphene rippled devices, Definition of lateral strain, Optimization process for optimal amount of perovskite nanocrystals coated over rippled graphene/PMMA structure for better photoresponse, Crosssectional Scanning Electron Microscopy image of perovskite nanocrystals on silicon substrate, Transmission Electron Microscopy (TEM) image and fast Fourier transform (FFT) pattern of perovskite nanocrystals, Transmission and Absorption Spectra of graphene/PMMA under 100 % lateral strain and 0 % lateral strain, The photocurrent of the device under various optical power densities of laser irradiance, Photoluminescence of perovskite nanocrystals and perovskite nanocrystals/graphene under 374 nm laser excitation, The photocurrent of the device with 0 % strain subjected to different cyclic stretching of 2% bending strain under laser irradiance with 325 nm wavelength, Dependence of responsivity on various wavelength of laser irradiance, Dependence of EQE on various wavelength of laser irradiance, Table for comparing responsivities of different similar kind of devices, Experimental methods. The Supporting Information is available free of charge on the ACS Publications website. (PDF)

AUTHOR INFORMATION Corresponding Author *Yang-Fang Chen [email protected] Department of Physics, National Taiwan University, Taipei 106, Taiwan Advanced Research Centre for Green Materials Science and Technology, National Taiwan University, Taipei 10617, Taiwan


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Author Contributions Y-F.C. planned and supervised the overall project. #Y-H.C. and #M.K. designed and fabricated the device, and performed optical and electrical measurements. C.-H. Lu and W-H.S. have done the synthesis of perovskite nanocrystals. Y-F.C. and M.K. contributed for the idea of the project. H-I.L. and C.R.P.I. contributed for the TEM image of the perovskite nanocrystals. Y-F.C., WH.S. and W-H.W. contributed for the discussions on observed experimental results. All authors contributed thoroughly towards formalizing the manuscript. All authors have given consent to the final version of the manuscript. #These authors contributed equally.

Funding Sources This work was financially supported by the “Advanced Research Centre for Green Materials Science and Technology” from The Featured Area Research Centre Program within the framework of the Higher Education Sprout Project by the Ministry of Education (107L9006) and the Ministry of Science and Technology in Taiwan (MOST 107-3017-F-002-001).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors courteously acknowledge the Taiwan International Graduate Program, Academia Sinica for financial support.



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ABBREVIATIONS 2D,

two-dimensional;

MAPbBr3,

methyl-ammonium

lead

bromide;

PDMS,

poly(dimethylsiloxane); 3D, three-dimensional; PMMA, polymethyl-methacrylate; UV, Ultraviolet; SEM, scanning electron microscopy; TEM, transmission electron microscopy; NEP, noise equivalent power; EQE, external quantum efficiency; CVD, chemical vapor deposition.


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Ultrahighly Photosensitive and Highly Stretchable Rippled Structure

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