Stable High-Performance Flexible Photodetector ... - ACS Publications

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Stable High-Performance Flexible Photodetector Based on Upconversion Nanoparticles/Perovskite Microarrays Composite Jianbo Li,† Yingli Shen,† Yucheng Liu,† Feng Shi,*,† Xiaodong Ren,† Tianqi Niu,† Kui Zhao,*,† and Shengzhong Frank Liu*,†,‡ †

Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of Education; Shaanxi Key Laboratory for Advanced Energy Devices; Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China ‡ Dalian National Laboratory for Clean Energy; iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China S Supporting Information *

ABSTRACT: Methylammonium lead halide perovskite has emerged as a new class of low-temperature-processed highperformance semiconductors for optoelectronics, but with photoresponse limited to the UV−visible region and low environmental stability. Herein, we report a flexible planar photodetector based on MAPbI3 microarrays integrated with NaYF4: Yb/Er upconversion nanoparticles (UCns) that offers promise for future high performance and long-term environmental stability. The promise derives from the confluence of several factors, including significantly enhanced photons absorption in the visible spectrum, efficient energy transition in the near-infrared (NIR) region, and inhibition of water attack by the hydrophobic UCns capping layer. The UCns layer aided in remarkably enhanced photodetection capability in the visible spectrum with detectivity (D*) reaching 5.9 × 1012 Jones, among the highest reported values, due to the increased photocarrier lifetime and decreased reflectivity. Excellent NIR photoresponse with spectral responsivity (R) and D* as high as 0.27 A W−1 and 0.76 × 1012 Jones were obtained at 980 nm, respectively, superior to the reported values of state-of-the-art organic-perovskite NIR photodetectors. Moreover, the hydrophobic UCns capping layer serving as a moisture inhibitor allowed significantly enhanced long-term environmental stability, e.g., 70% vs 27% performance retained after 1000 h exposure in 30−40% RH humidity air without encapsulation for the bilayer and the neat MAPbI3 devices, respectively. These results suggest that the composite based on perovskite and UCns is promising for constructing high-performance broadband optoelectronic devices with long-term stability. KEYWORDS: photodetector, perovskite, upconversion nanoparticles, broadband photoresponse, long-term stability

1. INTRODUCTION Methylammonium lead halide perovskites (MAPbX3) have attracted a great deal of attention due to their remarkable physical properties, such as broadband light absorption,1,2 long carrier diffusion length,3−5 and low trap state density.4,6 These outstanding characteristics have brought about a wide range of applications, including solar cells,7−10 light emitting devices,11,12 lasers,13−15 and photodetectors.16−20 Specifically, photodetectors are fundamental devices in optoelectronics enabling the conversion of light to electric signals and being widely applied in sensors, cameras, etc.21 As a result, both a © 2017 American Chemical Society

solar-cell-type multilayer structure with the best photodetection capability and a planar-structured photodetector that is easy to fabricate have been presented based on MAPbX3 single crystals and microcrystalline films which offer flexible capability and potential of large-scale fabrication (X = I, Cl, Br).22−28 Furthermore, composites based on MAPbX3 perovskites and two-dimensional (2D) materials such as WS2,29 MoS2,30,31 Received: March 6, 2017 Accepted: May 18, 2017 Published: May 18, 2017 19176

DOI: 10.1021/acsami.7b03229 ACS Appl. Mater. Interfaces 2017, 9, 19176−19183

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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic device structure of the MAPbI3/UCns bilayer photodetector fabricated on flexible PET substrate with top gold electrodes. (b) Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of the neat UCns layer, MAPbI3 arrays, and the bilayer. (c) X-ray diffraction (XRD) patterns of the neat MAPbI3 film and the MAPbI3/UCns bilayer. (d) Absorption spectra of the MAPbI3 film and the neat NaYF4:Yb/Er UCns as well as photoluminescence (PL) spectrum of the neat NaYF4:Yb/Er UCns. (e) Time-resolved photoluminescence (TRPL) of the neat MAPbI3 film and the MAPbI3/UCns bilayer.

WSe2,20,32 and graphene33 have been developed to suppress dark current and demonstrate excellent photoresponsivity and detectivity. However, MAPbX3 (X = I, Cl, Br) perovskite materials can only absorb the photons in the NIR region caused by their wide band gaps,34 preventing their application in the near-infrared (NIR) region. One interesting approach to solve this NIR energy loss problem is to broaden the light absorption spectrum by introducing materials absorbing NIR light. Very recently, Shi et al. realized NIR photoresponse for MAPbI3 material by introducing a narrow-band-gap-conjugated polymer that has strong absorption in the NIR spectrum. The planar composite photodetector exhibited a photoresponsivity (R) of 5.5 mA W−1 and detectivity (D*) of 3.2 × 109 Jones at 937 nm, respectively.35 An organic bulk-heterojunction (BHJ) layer can also be integrated with the MAPbI3 film to offer a vast array of other interesting possibilities, such as higher open circuit voltage (Voc) and NIR photoresponse.36,37 Especially, a solarcell-type multilayer photodetector based on a low-band-gap poly{2,5-bis(2-hexyldecyl)-2,5-dihydropyrrolo[3,4-c]pyrrole1,4-dione-3,6-di(5-thiophen-2-yl)yl-alt-N-(2-ethylhexyl)-dithieno-[3,2-b:2,3-d]pyrrole-2,6-diyl}:phenyl-C61-butyric acid methyl ester (PDPPTDTPT:PCBM) blends and the MAPbI3 layer extended the photoresponse to ca. 1000 nm with the corresponding D* reaching 1 × 1011 cm Hz1/2 W−1 at 900 nm and a superior response time of 5 ns.36 In addition to organic molecules, upconversion nanoparticles (UCns) can be solutionprocessed and preferably harvest NIR photons to generate visible light.38 The field of photon upconversion has inspired the design of new composites based on UCns for many potential applications. For example, the composites of UCns and MAPbX3 materials have recently been successfully applied in solar cells.39−41 The exploitation of NaYF4:Yb/Er UCns as mesoporous electrode in MAPbI3 perovskite solar cells

imparted the realization of an NIR perovskite solar cell under the 980 nm laser irradiation resulting in a PCE of 0.35% by harvesting only NIR photons in the solar spectrum.39 Therefore, in the current work, we focus beyond the wellstudied low-band-gap organics and MAPbI3 composite systems and seek to consider the coupling between new structural concepts and optoelectronic applications based on UCns and perovskite materials. In this work, we fabricated and characterized a flexible photodetector made of a NaYF4:Yb/Er UCns monolayer and solution-sheared MAPbI3 microarrays. Owing to the energy transfer in the NIR region, superior NIR photodetection capability was obtained. Importantly, the introduction of the UCns layer was found to decrease reflectivity and enhance PL decay for MAPbI3 crystals, which yields 3 times higher D* in the visible spectrum which reaches 5.9 × 1012 Jones. Finally, the bending duration and long-term environmental stability as well as water resistance tests were explored. The hydrophobic property of the UCns capping monolayer contributes an excellent long-term environmental stability of the device.

2. RESULTS AND DISCUSSION Figure 1a presents a schematic device structure of the MAPbI3/ UCns photodetector fabricated on a flexible poly(ethylene terephthalate) (PET) substrate with top gold electrodes. The fabrication of the bilayer structure involves solution-shearing MAPbI3 microarrays, followed by spin-casting a NaYF4:Yb/Er 20−30 nm capping layer. The large-area solution-sheared MAPbI3 films exhibit excellent crystalline quality and wellcontrolled macroscopic alignment (Figure 1b and Figure S1) compared to the polycrystalline film prepared using spincasting.42 The NaYF4:Yb/Er UCns are spin-coated on MAPbI3 films from the hexane, a nonsolvent for MAPbI3 material. These UCns exhibit high upconversion performance43 and a 19177


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Figure 2. (a) The highest photoresponse parameters of the bilayer photodetector as a function of wavelength. Inset showing the photo of the flexible integrated photodetector. (b) Schematic model showing the detailed energy transitions from the NaYF4:Yb/Er UCns to the MAPbI3 perovskite. (c) Responsivity R and (d) detectivity D* of the bilayer photodetector as a function of NIR irradiance intensity at 980 nm, respectively. (e) Single photocurrent response cycle with light switched ON and OFF, showing the response speed of the photodetector at 980 nm.

While the MAPbI3/NaYF4:Yb/Er UCns bilayer described above has a fairly identical crystalline structure with the neat MAPbI3 microarray film, one important parametercharge lifetimehas to be considered for the successful design of a high-performance perovskite-derived device. Time-resolved photoluminescence (TRPL) of the films on glass shows a two-exponential decay with a fast photocarrier lifetime (τ1 = 3.6 ns) and a slow one (τ2 = 104.6 ns) in the neat MAPbI3 film (Figure 1e), assigned to surface and bulk photocarrier lifetime,50 respectively. Interestingly, both surface and bulk lifetimes are increased to 16.2 and 188.3 ns in the MAPbI3/ UCns bilayer, suggesting lower trap-state density in the MAPbI3/UCns bilayer which is definitely beneficial to enhanced photoresponse in photodetector. The lower trap density in the MAPbI3/UCns bilayer is likely due to trap passivation, which might result from two possibilities: alleviated low-quality MAPbI3 crystals on the film surface during spincasting UCns/hexane solution, and further crystallization during thermal annealing under protection of the UCns capping layer. However, more research is necessary to fully unravel the precise mechanism for the low trap density in the bilayer. We have designed and fabricated flexible planar-type photodetectors on PET using both neat MAPbI3 microarrays and the MAPbI3/UCns bilayer (Figure 2a, inset). Each device consists of 118 groups, and each group is made up of 5 pieces of Au pads (Figure 1a). The dimensions of the active channel between two adjacent Au pads are 1 mm in length, 40 μm in width, and ∼300 nm in thickness. The effective illuminated device area is about 2.7 × 10−7 m2. The current−time (I−t) curves of the MAPbI3/UCns device measured under illumination (560 nm LED light, 22.9 mW cm−2) at 2 V bias shows a stable photocurrent as high as 7.6 μA (Figure S4). The responsivity (R), external quantum efficiency (EQE), and detectivity (D*) were further determined. R is defined as the photocurrent generated per unit of incident power, EQE is defined as the number of carriers produced in

proper particle size of 20−30 nm (Figure 1b) which is beneficial for achieving good surface coverage on the MAPbI3 film. The plan-view scanning electron microscopy (SEM) images indicate that the MAPbI3 macroscopic alignment is well retained after solution-casting UCns, and small gaps between arrays are effectively filled with the UCns (Figure S2). The cross-sectional SEM images further demonstrate that a monolayer of NaYF4:Yb/Er UCns is formed on the MAPbI3 film (Figure 1b). X-ray diffraction (XRD) patterns of the neat MAPbI3 film and the MAPbI3/UCns bilayer have similar features, showing three peaks at 14.12, 28.25, and 40.62° representing the tetragonal (110), (220), and (400) planes, respectively (Figure 1c).44 An additional small peak appearing at 17.02° in the MAPbI3/UCns bilayer is ascribed to the crystalline NaYF4:Yb/ Er UCns. These results confirm that the introduction of NaYF4: Yb/Er does not affect the crystalline structure of MAPbI3. Figure 1d shows the absorption spectra of the MAPbI3 film and the neat NaYF4:Yb/Er UCns, as well as the PL spectrum of the neat NaYF4:Yb/Er UCns. The NaYF4:Yb/Er UCns show a strong NIR absorption region in 850−1033 nm, locating outside the spectral coverage of the MAPbI3 film. The PL spectrum of the UCns shows four dominant peaks in the region of 400−670 nm, making it promising for fabrication of a vis− NIR broadband photodetector. Interestingly, the introduction of UCns endows lower reflectivity for the neat MAPbI3 layer in the visible spectrum, e.g., decreasing from 18% to 13% at 530 nm (Figure S3). It is important to note that the decreased reflectivity has a significant impact on the photons absorption for a light absorber. This is best illustrated by the example of Au-based nanoparticles,45,46 which have been widely used in organic solar cells for improving the light absorption and, therefore, short circuit current (Jsc).47−49 In simple terms, this can be rationalized by the fact that the plasmon effect can be introduced to the perovskite-based photovoltaic and optoelectronic devices, which results in a decrease in photon loss and thus yields higher current. 19178


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ACS Applied Materials & Interfaces the external circuit for each absorbed incident photon, and D* is commonly used as a figure of merit reflecting the sensitivity of the photodetector. These three parameters are defined by the following equations51−53 R=

ΔI PA

(1)

EQE = R

D* =

efficient charge transfer at the perovskite/organics interface, therefore, lead to a low photocurrent collected in the device. Figure 2b illustrates detailed energy transitions from the NaYF4:Yb/Er UCns to MAPbI3. In the upconversion process of the NaYF4:Yb/Er system, the energy in Yb3+ excited by NIR light is transferred to Er3+ ions and then released as visible light. Four dominant Er3+ emission peaks at 408, 523, 543, and 655 nm were assigned to 2H9/2 → 4I15/2 (408 nm), 2H11/2 → 4I15/2 (523 nm), 4S3/2 → 4I15/2 (543 nm), and 4F9/2 → 4I15/2 (655 nm) transitions.38,57 The incorporation of the NaYF4:Yb/Er UCns layer provides a promising route to harvest NIR photons that is transparent in the MAPbI3 film. Note that the incident photons with energy higher than the band gap of the perovskite are effectively harvested by the perovskite active layer. The incident photons with energy lower than the perovskite band gap are absorbed in the UCns, followed by upconversion into higher-energy photons that can be absorbed in the perovskite to generate photocurrent.36 The bilayer photodetector exhibits sensitive photoresponse to NIR irradiance intensity. The photodetector performance at a fixed bias voltage of 2 V was illustrated as a function of irradiance intensity. Decreasing irradiance intensity leads to a decreased photocurrent and an increased noise current (see Figure S6). When the irradiance intensity is lower than 1.0 mW cm−2, it is too low to enable the bilayer photodetector to work well. Meanwhile, increasing the applied electric filed from 2 to 10 V results in an increased dark current (Figure S6). We observed a trend that the values of R and D* decrease linearly with increased irradiance intensity (Figure 2c,d). This phenomenon is consistent with those reported in the literatures for photodetectors based on the neat perovskites.58 Moreover, the bilayer photodetector at 980 nm responds fast as shown from the typical ON/OFF cycle (Figure 2e). The rise time and decay time were measured to be ca. 52 ms and ca. 67 ms, respectively. We further turn our attention to the influence of the introduction of UCns on the photodetection capability in the visible region. Interestingly, the bilayer photodetector shows higher photocurrent than the neat MAPbI3 at 560 nm and an irradiance intensity of 22.9 mW cm−2 (see Figure S4), suggesting the possibility that the UCns capping layer aids enhancement in photoresponse. To verify this possibility, we evaluated the D* and R as a function of irradiance intensity at 560 nm and 2 V bias for both the MAPbI3/UCns bilayer and MAPbI3 (see Figure S7). We observed a similar trend that the values of D* and R decrease linearly with increased irradiance intensity following the characteristic behavior of external quantum efficiency (EQE; see Figure S8). The values of R and D* are always higher for the MAPbI3/UCns bilayer photodetector (Figure 3a). We extended the investigations to the entire visible spectrum and found that D* of the MAPbI3/ UCns bilayer photodetector is ∼3 times higher in contrast to the neat MAPbI3 device (Figure 3b), e.g, 5.9 × 1012 Jones vs 1.9 × 1012 Jones at 730 nm. The corresponding R is also higher for the bilayer photodetector, 0.87 mA W−1 vs 0.73 mA W−1 at 730 nm. The enhanced photoresponse performance can be explained by the fact that the enhanced photocarrier lifetime and increased photons absorption in the bilayer contribute to higher efficient collection of photogenerated charges (Figure S3). In addition, we found that the addition of UCns has a negligible impact on the photoresponse speed, with the rise time and decay time measured to be 115 μs vs 159 μs and 253

hc λe

R 2qJdark

(2)

(3)

where ΔI is the photocurrent defined as the difference between the current in the dark and under illumination, A the effective illuminated area, P the laser power density, h the Planck’s constant, c the velocity of light, λ the wavelength of incident light, e the elementary charge (1.6 × 10−19 coulomb), q the absolute value of electron charge (1.6 × 10−19 coulomb), and Jdark the dark current density. The photoresponse parameters were calculated at different irradiance intensities (from 0.11 to 80 mW cm−2), and the highest values are illustrated in Figure 2a as a function of wavelength for the bilayer photodetector (see Figures S5 and S6). The R value is as high as 0.55−0.86 A W−1 in the visible region. We noted that the highest R appears at 730 nm regardless of the UCns layer rather than previously reported 650 nm which was observed for the organic-MAPbI3 photodetector based on spin-coated polycrystalline MAPbI3 film.32 However, there remains a question as to whether the crystalline quality determines peak position of the R value, because the peaks at ca. 600 nm and ca. 470 nm were also observed in other reports for the MAPbI3-based photodetectors.26,40 Interestingly, the R value significantly declines to the lowest 0.08 A W−1 at 850 nm, while rising to 0.27 A W−1 at 980 nm (see Figure S6), suggesting a promising NIR photoresponsivity. The corresponding EQE and D* values show a similar trend with the maximum at 730 nm. The EQE value is in the range of 120− 180% in the visible spectrum, which drops to 14% at 850 nm, followed by rising to 48% at 980 nm. The D* value stands in the range of 3.8−5.9 × 1012 Jones in the visible spectrum. We noted that the D* value is among the highest values for the perovskite-film-based planar photodetectors,40,54,55 which combine facile fabrication and high performance together, although photodetectors based on a single crystal or solar-celltype multilayer structure exhibited higher D*, e.g., ∼1014 Jones.56 The D* value drops to 0.46 × 1012 Jones at 850 nm, followed by increasing to 0.76 × 1012 Jones at 980 nm. The observed high values of R, EQE, and D* for the bilayer photodetector in the visible spectrum, apparently, are ascribed to well-known strong absorption of the MAPbI3 film. Due to the low intensity at absorption edge 850 nm, the MAPbI3/ UCns bilayer photodetector shows 5−10 times lower in all figures-of-merit compared to the visible spectrum. The bilayer extended the photons absorption to a wavelength of ca. 1030 nm. The efficient energy transition between UCns and MAPbI3 leads to an excellent photoresponse with R and D* as high as 0.27 A W−1 and 0.76 × 1012 Jones at 980 nm (Figure S6), respectively. As far as we know, this is superior to the reported values of state-of-the-art organic-perovskite NIR photodetectors.33,34 Indeed, most of the low-band organics themselves exhibit low absorption in the NIR region and have low charge mobility in films. Such drawbacks combining with a low19179


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yields a deteriorated transparency and a reduced photocurrent of the bilayer photodetector in the visible spectrum. Further increasing the thickness of the UCns layer to ca.100 nm results in difficulty in getting the bilayer photodetector to work; this can be explained by the fact that the densely packed thick UCns layer blocks charge transfer from MAPbI3 to the Au electrode. The thickness-dependent performance, therefore, confirms that the optimum UCns thickness for the bilayer photodetector should be ca. 20−30 nm. Flexible durability and environmental stability are another two major concerns for flexible devices in practical applications.59,60 The flexibility of the MAPbI3/UCns bilayer photodetector was studied by flexing the device in a vernier caliper at an angle of 60° as shown in Figure 4a. The devices were measured under illumination with 560 nm light (irradiance intensity of 22.9 mW cm−2) at 2 V bias. The evolution of the ON/OFF ratio is essentially unchanged before and after flexing for 2000 times, demonstrating excellent flexibility and a robust bending durability. The air-stable UCns layer may serve as a protecting layer to improve environmental stability for the MAPbI3/UCns bilayer device. To confirm the hypothesis, we recorded the long-term environmental stability for both the MAPbI3/UCns bilayer and the neat MAPbI3 photodetectors without encapsulation under ambient condition with 30−40% relative humidity (RH). The devices were measured under LED illumination at 560 nm at 2 V bias (see Figure S10). The long-term stability results show a faster degradation to 60% in the first 100 h, followed by a slowed degradation afterward for these solution-sheared MAPbI3 microarrays (Figure 4b). For the test period of 1000 h, D* declined by 73% from 1.90 × 1012 to 0.53 × 1012 Jones; and R dropped by 77% from 0.30 to 0.07 mA W−1. For the MAPbI3/UCns bilayer device, D* degraded from 3.02 × 1012 to 2.12 × 1012 Jones with a loss of 30%, and R decreased from 0.43 to 0.31 mA W−1, lost by 28%. It is apparent that the

Figure 3. (a) Detectivity and responsivity for both bilayer photodetector and neat MAPbI3 photodetector as a function of irradiance intensity at 560 nm excitation. (b) The highest photoresponse parameters of both bilayer and neat MAPbI3 photodetectors in the visible regions.

μs vs 165 μs for the neat MAPbI3 and the bilayer photodetectors, respectively (see Figure S9). The thickness effect of the NaYF4:Yb/Er UCns film on the MAPbI3/UCns bilayer photodetector was also evaluated. The UCns monolayer is ca. 20−30 nm thick. Increasing thickness

Figure 4. (a) ON/OFF ratio of the flexible MAPbI3/UCns bilayer photodetectors as a function of flexing. (b) Detectivity and responsivity as a function of time showing long-term environmental stability for the MAPbI3/UCns bilayer and the neat MAPbI3 photodetectors exposure under ambient condition with 30−40% relative humidity (RH). (c) Water soaking showing strong water resistance of the MAPbI3/UCns bilayer in contrast to the neat MAPbI3 film. (d) Contact angle tests for MAPbI3 films without and with UCns layer. 19180


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resulting mixture was stirred for 60 min. The temperature was then increased to 60 °C for evaporating methanol from the reaction mixture; in succession, the solution was heated to 300 °C in an argon atmosphere for 60 min and then cooled to room temperature naturally. The resulting solid products were precipitated by addition of ethanol and collected by centrifugation at 8000 rpm for 10 min, and then washed with ethanol for three times, and finally redispersed in hexane. Optical metrology: UV−visible absorption and reflection spectra were acquired on a Cary 5000 (Varian) instrument. Steady-state photoluminescence (PL) (excitation at 532 nm) and time-resolved photoluminescence (TRPL) (excitation at 405 nm and emission at 760 nm) were measured with an Edinburgh Instruments Ltd. (FLS980). Optical microscopy was performed on an OLYMPUS BX51instrument. Electron microscopy: Scanning electron microscopy (SEM) imaging was conducted on a field emission scanning electron microscope (FE-SEM; SU-8020, Hitachi) at an acceleration voltage of 8 kV. Transmission electron microscopy (TEM) imaging was conducted on a FEI Tecnai G2 F20 electron microscope. X-ray diffraction (XRD) measurements were carried out in a θ−2θ configuration with a scanning interval of 2θ between 10° and 60° on a Bruker D8 Discover (X-ray Source: Cu Kα; λ = 1.54 Å). Optoelectronic characterizations: The photocurrent response characteristics for the photodetectors were collected by a Keithley 2400 SourceMeter with a series of LED lasers. The high speed characteristics of the device were measured by an optical chopper and a Digital Storage Oscilloscope (Agilent DSO-X 2012A). Contact angle measurements were contacted on a DSA100 instrument.

photodetector performance retains much higher values based on the MAPbI3/UCns bilayer (70%) compared to the neat MAPbI3 film (27%). The greatly improved long-term stability is presumably ascribed to the function of the hydrophobic UCns layer inhibiting water attack. To verify this proposed mechanism, we dipped two films into water for ca. 5 s to study water resistance. We observed a quick and thorough color change from dark to yellow for the neat MAPbI3 film after water soaking. In contrast, the MAPbI3/UCns bilayer retains its dark appearance without a noticeable color change. These findings demonstrate that the presence of UCns yields superior resistance to water damage for the MAPbI3. The water contact angle was measured to be ca. 54° for the neat MAPbI3 film and ca. 102° for the MAPbI3/UCns bilayer (Figure 4d), further confirming the superior hydrophobic nature of the UCns that contributes to the long-term environmental stability for the bilayer device.

3. CONCLUSION In conclusion, we demonstrate a stable high-performance flexible planar photodetector by integrating a layer of NaYF4: Yb/Er UCns onto the top of the solution-sheared MAPbI3 microarrays as photosensitizer. The efficient upconversion of NIR photons from the UCns gives an excellent NIR photoresponse with R and D* reaching as high as 0.27 A W−1 and 0.76 × 1012 Jones, respectively. As far as we know, these NIR photoresponse properties are the highest among what is reported for the organic-perovskite-based NIR photodetectors. Meanwhile, the incorporation of the UCns capping layer gives a higher photocarrier lifetime and lower reflectivity, leading to 3 times higher detectivity D* in the visible spectrum, with the high number reaching 5.9 × 1012 Jones, which is among the highest values for the perovskite-film-based planar photodetectors. Furthermore, the highly hydrophobic nature of the UCns capping layer endows superior long-term stability and water resistance for the flexible devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b03229. Additional information and figures including TEM, SAED, and HRTEM images of the confined MAPbI3 crystals; SEM image of the solution-sheared MAPbI3 microarrays; reflection spectroscopy of the MAPbI3/ UCns bilayer and the neat MAPbI3 film; I−t curves of the MAPbI 3 /UCns bilayer and the neat MAPbI 3 photodetectors; on/off ratio of the MAPbI3/UCns bilayer photodetector as a function of wavelength; EQE of the MAPbI3/UCns bilayer photodetector; the response speed of the photodetector for the MAPbI3/ UCns bilayer and the neat MAPbI3 photodetectors; photocurrent response of the MAPbI3/UCns bilayer and the neat MAPbI3 photodetectors (PDF)

4. EXPERIMENTAL SECTION Solution preparation and device fabrication: The solution preparation was conducted under an inert atmosphere inside a nitrogen glovebox. Perovskite precursor solution, MAI (1-Material), and PbI2 (Aldrich) were dissolved (1:1 molar ratio) in N,N-dimethylformamide (DMF, Aldrich) in 1 M concentration for solution-shearing. The solution was stirred overnight at 60 °C and filtered with a 0.2 μm PTFE filter before solution-casting. Glass or poly(ethylene terephthalate) (PET) substrates were cleaned through sequential sonication in ethanol, acetone, and isopropanol for 15 min each and then dried with a N2 blow gun. Substrate cleaning was completed through exposure to oxygen plasma for 5 min. The solution-shearing and spin-coating were conducted under ambient condition. The solution-shearing was performed with a speed of 10 mm/min. The as-cast films were annealed at 100 °C for 5 min, followed by spin-coating NaYF4:Yb/Er upconversion nanoparticles (UCns) with a spin speed of 2000 rpm. The as-cast films were further annealed at 100 °C for 5 min, followed by evaporation of the 100 nm gold electrode. NaYF4:Yb/Er upconversion nanoparticles (UCns) synthesis: OAcapped β-NaYF4:20%Yb,2%Er NPs were synthesized via a solvothermal route adapted from the literature.61 Briefly, 1 mmol of RECl3· 6H2O (RE = 78 mol % Y, 20 mol % Yb, and 2 mol % Er) was dissolved in a mixture of ODE (15 mL, Alfa) and OA (6 mL, Alfa). The solution was heated to 160 °C for 30 min under argon protection to form the lanthanide oleate complexes. The temperature was then cooled to room temperature with a gentle flow of argon gas through the reaction flask. During this time, a solution of NH4F (4 mmol) and NaOH (2.5 mmol) dissolved in methanol (10 mL) was added to the flask, and the

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (F.S.). *E-mail: [email protected] (K.Z.). *E-mail: [email protected] (S.L.). ORCID

Shengzhong Frank Liu: 0000-0002-6338-852X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China (No. 61604092, No. 21641001), Fundamental Research Funds for the Central Universities (GK201603055, GK201703028, and GK201701007), the 19181


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Innovative Research Team (IRT_14R33), the National Key Research Program of China (2016YFA0202403), and the Chinese National 1000-Talent-Plan program (Grant No. 111001034).

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