Letter pubs.acs.org/NanoLett
Ultrahigh-Responsivity Photodetectors from Perovskite Nanowire Arrays for Sequentially Tunable Spectral Measurement Wei Deng, Liming Huang, Xiuzhen Xu, Xiujuan Zhang, Xiangcheng Jin, Shuit-Tong Lee, and Jiansheng Jie* Institute of Functional Nano and Soft Materials (FUNSOM), Collaborative Innovation Center of Suzhou Nano Science and Technology (NANO−CIC), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, Suzhou Jiangsu 215123, People’s Republic of China S Supporting Information *
ABSTRACT: Compared with polycrystalline films, single-crystalline methylammonium lead halide (MAPbX3, X = halogen) perovskite nanowires (NWs) with well-defined structure possess superior optoelectronic properties for optoelectronic applications. However, most of the prepared perovskite NWs exhibit properties below expectations due to poor crystalline quality and rough surfaces. It also remains a challenge to achieve aligned growth of singlecrystalline perovskite NWs for integrated device applications. Here, we report a facile fluid-guided antisolvent vapor-assisted crystallization (FGAVC) method for large-scale fabrication of high-quality single-crystalline MAPb(I1−xBrx)3 (x = 0, 0.1, 0.2, 0.3, 0.4) NW arrays. The resultant perovskite NWs showed smooth surfaces due to slow crystallization process and moisture-isolated growth environment. Significantly, photodetectors made from the NW arrays exhibited outstanding performance in respect of ultrahigh responsivity of 12 500 A W−1, broad linear dynamic rang (LDR) of 150 dB, and robust stability. The responsivity represents the best value ever reported for perovskite-based photodetectors. Moreover, the spectral response of the MAPb(I1−xBrx)3 NW arrays could be sequentially tuned by varying the content of x = 0−0.4. On the basis of this feature, the NW arrays were monolithically integrated to form a unique system for directly measuring light wavelength. Our work would open a new avenue for the fabrication of high-performance, integrated optoelectronic devices from the perovskite NW arrays. KEYWORDS: Methylammonium lead halide perovskite, single-crystalline nanowire arrays, photodetectors, light wavelength measurement
M
crystals,16 yielding perovskite NWs with rough surfaces, lower crystallinity, and inferior optical and electronic properties. To address this issue, a two-step growth method was developed to prepare single-crystalline perovskite NWs with smooth surfaces.21,23,24 Nevertheless, this method is complicated and yields NWs with a large size distribution. Therefore, an efficient method is much needed to achieve growth of single-crystalline perovskite NWs with high quality and large-area uniformity. In addition, while prototype devices based on single perovskite NW have been explored, scale-up production of the NWs for practical applications is difficult, because growth orientation and location of the NWs are always stochastic in nature. For large-scale integrated devices, precise alignment and patterning of the perovskite NWs is a requisite. The lack of efficient techniques to grow aligned single-crystalline perovskite NW arrays has greatly hampered their use in high-performance integrated optoelectronics devices.
ethylammonium lead halide perovskite (MAPbX3, X = halogen), a new generation of solution-processable and low-cost optoelectronic materials, has recently attracted intense interest for photonic applications in solar cells,1−5 lightemitting diodes (LEDs),6−9 lasers,10 and photodetectors.11−14 The impressive advantages of the MAPbX3 materials are their high absorption coefficients, high carrier mobilities, and long electron−hole pair diffusion lengths.15 These advantages of MAPbX3 may be exploited to realize high-sensitivity photosensing devices with performance far exceeding current photodetectors.16 In addition, the bandgap of MAPbX3 can be readily tuned to cover almost the entire visible spectrum by adjusting the chemical composition,17,18 so that perovskite photodetectors can be tuned to detect light wavelength. In comparison to polycrystalline MAPbX3 perovskite films, single-crystalline perovskite nanowires (NWs) with welldefined structures possess higher photoluminescence (PL) quantum yields, larger carrier mobilities, and longer carrier diffusion lengths.16,19−21 In general, perovskite NWs are fabricated via a one-step solvent evaporation crystallization process using MAPbI3/dimethylformamide (DFM) solution in air.22 However, the moisture in air usually causes damage to © 2017 American Chemical Society
Received: January 12, 2017 Revised: February 9, 2017 Published: February 23, 2017 2482
DOI: 10.1021/acs.nanolett.7b00166 Nano Lett. 2017, 17, 2482−2489
Letter
Nano Letters
Figure 1. (a) Schematic illustration of FGAVC method for the fabrication of CH3NH3PbI3 NW array. (b) Cross-polarized optical micrograph of the CH3NH3PbI3 NW array. (c) SEM image of CH3NH3PbI3 NW array grown on the two sides of SU-8 photoresist strips. Inset in (c) is a magnified cross-sectional image of the CH3NH3PbI3 NW. (d) Magnified SEM image of the NW grown along the photoresist strip. (e) AFM image and (f) height profile taken from the edge of the SU-8 photoresist.
well, covering the entire substrate. In the FGAVC method, aligned SU-8 photoresist stripes not only offer the nucleation sites for NW growth but also act as the template to induce directional fluid motion of the hanging solution, ensuring the ordered orientations of the resultant NWs (Figure S1, Supporting Information and Supporting Video 1). The growth condition of perovskite NWs in the FGAVC method is similar to that adopted for large-size single crystals, where CH2Cl2 vapor isolates perovskite from the outside air moisture and thus ensures the high crystal quality of the resultant materials.25 Note that the FGAVC method is also applicable to flexible polyethylene naphthalate (PEN) substrate (Figure S2, Supporting Information), offering the possibility of fabricating flexible devices based on perovskite NW arrays. Polarized optical microscopy (POM) image was used to examine the large-area uniformity of the CH3NH3PbI3 NW arrays. The perfectly constant color in Figure 1b clearly shows the formation of large-area continuous and uniform NW arrays. The single-crystalline nature of the NW array is verified by the 4-fold symmetry of the optical intensity (Figure S3, Supporting Information). Scanning electron microscopy (SEM) image in Figure 1c discloses that the aligned CH3NH3PbI3 NWs are preferentially deposited on the two sides of SU-8 photoresist strips rather than the photoresist channels. This could be attributed to the wettability difference between the sides of photoresist stripes (more wettability) and photoresist channels area (less wettability). The CH3NH3PbI3 solution tends to pin at the sides of photoresist stripes, making the NWs grow along the photoresist stripes. The CH3NH3PbI3 NWs have angular end facets and well-shaped border (inset in Figure 1c). Enlarged SEM image shows that the CH3NH3PbI3 NW is crack-free with smooth surfaces (Figure 1d). The remarkable
Herein, we report a facile method to produce highly aligned single-crystalline MAPb(I1−xBrx)3 (x = 0, 0.1, 0.2, 0.3, 0.4) NW arrays with continuously tunable absorption edges from 680 to 780 nm. Owing to the high-quality crystal structure of NWs, photodetectors made from the perovskite NW array exhibited an extremely high responsivity of 12 500 A W−1, which represents the best value ever reported for perovskite-based photodetectors. By varying the components and thus the bandgap of perovskite NWs, broadband photodetectors with sequentially tunable spectral response were realized. Furthermore, monolithic integration of these photodetectors offered a unique capability to directly measure light wavelength. The large-scale production of aligned, high-quality single-crystalline perovskite NW arrays opens up opportunities for a variety of high-performance, high-integration optoelectronic devices. Results and Discussion. Single-crystalline perovskite NW arrays were fabricated by a fluid-guided antisolvent vaporassisted crystallization (FGAVC) method. Schematic illustration of the growth of CH3NH3PbI3 NW arrays via FGAVC is shown in Figure 1a. First, we used photolithography to fabricate periodically aligned SU-8 photoresist stripes on the SiO2/Si substrate, which acted as the template for the subsequently aligned growth of NW arrays. The SU-8 photoresist template was dipped into MAPbI3/DFM solution (0.05 M) for a few seconds and then was dragged out with the hanging solution and placed on a tilted glass (∼5°) in a weighing bottle together with 3 mL of CH2Cl2 solvent. Lastly, the bottle was sealed at room temperature. The saturated antisolvent (CH2Cl2) vapor within the sealed bottle would gradually diffuse into the MAPbI3/DMF solution, leading to the precipitation of CH3NH3PbI3 nanocrystals along the sides of SU-8 photoresist strips. Ordered NW arrays were simultaneously produced as 2483
DOI: 10.1021/acs.nanolett.7b00166 Nano Lett. 2017, 17, 2482−2489
Letter
Nano Letters
Figure 2. (a) XRD pattern of the CH3NH3PbI3 NW array, confirming the tetragonal phase of the CH3NH3PbI3 NWs without any impurity phases. (b) TEM image and (c−e) corresponding EDS elemental mappings of a single CH3NH3PbI3 NW. (f) Typical TEM image of the CH3NH3PbI3 NW. (g) SAED pattern and (h) corresponding HRTEM image of the CH3NH3PbI3 NW shown in (f). (i) Calculated crystal morphology of the CH3NH3PbI3 NW, showing a hexagonal (100) crystal face shape.
Figure 3. (a) Schematic representation of the experimental setup for SPCM measurement. The laser spot was focused at the left side of NW, resulting in a positive current direction from left to right. (b) SEM image of the NW-based Schottky device. Ca layer in Al/Ca electrode forms a Schottky contact with NW. (c) Schematic energy band diagram of the Ca/CH3NH3PbI3 NW Schottky junction. (d) SPCM image of the aligned CH3NH3PbI3 NW-based Schottky device. (e) Cross section of the SPCM image in (b) along the NW axis, revealing a long carrier diffusion length of 41 ± 3 μm.
Information). Atomic force microscopy (AFM) images show that the CH3NH3PbI3 NWs grow along both sides of SU-8 photoresist strips (Figure 1e,f). Note that the surface of CH3NH3PbI3 NWs is smooth with only ∼1 nm roughness, implying few defects in the crystals (Figure S6a, Supporting Information). In contrast, the CH3NH3PbI3 NWs fabricated by
size uniformity of the NWs is demonstrated by the statistics on NW width, which indicates a narrow width distribution of 400 ± 30 nm (Figure S4, Supporting Information). Energydispersive X-ray spectroscopy (EDS) analysis on the NW revealed an I/Pb ratio of ∼2.9, which is in good agreement with the CH3NH3PbI3 stoichiometry (Figure S5, Supporting 2484
DOI: 10.1021/acs.nanolett.7b00166 Nano Lett. 2017, 17, 2482−2489
Letter
Nano Letters
Figure 4. (a) Schematic illustration of CH3NH3PbI3 NW array photodetector. (b,c) SEM images of the photodetector with different magnification. (d) I−V curves of the photodetector measured under dark condition and light illumination (550 nm, 42 μW cm−2), respectively. (e) Plots of responsivity and photoconductive gain of the photodetector as a function of light wavelength. (f) Normalized photoresponse as a function of input signal frequency, showing the 3 dB cutoff frequency of ∼0.8 MHz. (g) Noise power spectrum of the photodetector measured from 1 to 1000 Hz at different bias voltage. (h) Irradiance-dependent photocurrent at a bias voltage of 5 V.
high-quality single-crystalline CH3NH3PbI3 NW arrays has been successfully achieved via the FGAVC method. To study the optoelectronic properties of the NWs, a homebuilt scanning photocurrent microscopy (SPCM) composed of a micro-Raman spectrometer and a semiconductor parameter analyzer was used to measure the photocurrent as a function of local photoinjection position. SPCM is a powerful technique for investigating charge transport, local band bending, and carrier diffusion lengths in NWs. This technique has been widely used to characterize the optoelectronic properties of Si NWs,26 carbon nanotubes (CNTs),27 CdS NWs,28 graphene,29 MoS2 nanosheet,30 and so on. A schematic representation of the experimental setup is shown in Figure 3a. Adjacent Al/Ca− Al/Ca electrodes with a gap of 100 μm were fabricated on the NW, forming a Schottky-type device (Figure 3b). A continuous wave laser (532 nm) was focused onto the NW through an objective lens with a ∼3 μm laser spot. The laser spot was then scanned along the NW surface, and the photocurrent (PC) was recorded as a function of the laser location. Upon light irradiation, the electron−hole (e−−h+) pairs are generated in the CH3NH3PbI3 NW, and then electrons and holes diffuse along the NW toward the electrodes, respectively, yielding the photocurrent at zero external bias voltage. From the energy band diagram in Figure 3c, we can see that the electrons can freely diffuse to the adjacent electrode, while the holes are blocked by a high Schottky barrier at NW/Ca interface. As a result, the SPCM image taken from one NW at one side of the photoresist strip shows an elongated PC spot near the left electrode (Figure 3d). The corresponding PC along the NW axis keeps nearly constant at a position 0−40 μm from the left electrode (Figure 3e), and then decays drastically beyond 40
previously reported methods usually have a surface roughness of ∼25 nm (Figure S6b,c, Supporting Information). Additional characterizations of the CH3NH3PbI3 NWs were carried out using X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD pattern of the NW array (Figure 2a) can be perfectly indexed to CH3NH3PbI3 with tetragonal crystal structure (space group I4/mcm, a = 8.896 Å, c = 12.707 Å). The elemental distribution shown in Figure 2b−e indicates that I and Pb elements are homogeneously distributed in the individual NW. Figure 2f shows a typical TEM image of a single CH3NH3PbI3 NW with a uniform width of about 400 nm, while the discrete diffraction spots in the selected-area electron diffraction (SAED) pattern (Figure 2g) reveals the single-crystalline nature of the NW. Meanwhile, the NW shows a clear lattice fringe of 0.220 nm in the high-resolution TEM (HRTEM) image (Figure 2h), corresponding to the (100) lattice spacing of tetragonal CH3NH3PbI3 crystal. The theoretically predicted crystal habit (Bravais−Friedel−Donnay−Harker method, BFDH) of a CH3NH3PbI3 NW indicates a hexagonal (100) crystal face shape (Figure 2i), which is consistent with the cross-sectional SEM image of the CH3NH3PbI3 NW (inset in Figure 1c). In the present growth method, the density of CH3NH3PbI3 NW arrays on the substrate can be readily tuned by adjusting the photoresist stripe intervals, such as, from 2 or 5 to 10 μm shown in Figure S7, Supporting Information. Moreover, the diameter of the NWs is found to be mainly dependent on the solution concentration and can be adjusted from 240 to 1830 nm by changing the concentration of the CH3NH3PbI3 solution from 0.01 to 0.13 M (Figure S8, Supporting Information). The totality of these results demonstrates that large-area growth of 2485
DOI: 10.1021/acs.nanolett.7b00166 Nano Lett. 2017, 17, 2482−2489
Letter
Nano Letters μm, which corresponds to a carrier diffusion length of 41 ± 3 μm in the NW. Significantly, this diffusion length is remarkably longer than that in Si NWs (10 μm),26 organic single crystals (1−10 μm)31 and even longer than that reported in large-size CH3NH3PbI3 single crystals (∼17 μm).25 As control experiments, we fabricated CH3NH3PbI3 polycrystalline film and NWs using previously reported methods,5,22 and then applied SPCM method to measure their carrier diffusion lengths (Figure S9, Supporting Information). The control CH3NH3PbI3 polycrystalline film and NWs were measured to have a diffusion length of 9 and 12 μm, respectively, which are much shorter than that (41 ± 3 μm) of the NWs fabricated by the FGAVC method. High crystal quality of the NWs with less defects and grain boundaries is considered responsible for the long carrier diffusion length, which should offer CH3NH3PbI3 NW arrays unique advantages for optoelectronic applications. Next, we fabricated photodetectors from the singlecrystalline CH3NH3PbI3 NW arrays and investigated their device performance. A schematic of the device structure is shown in Figure 4a. A layer of Au electrode (50 nm) was thermally evaporated on top of the CH3NH3PbI3 NW array by using a shadow mask, constructing an ohmic-type device (Figure 4b,c). Figure 4d depicts the current versus voltage (I− V) characteristics of the CH3NH3PbI3 NW array-based device under dark condition and 550 nm light illumination (42 μW cm−2), respectively. We can see that the current increased more than 2 orders of magnitude under 550 nm light illumination compared to the dark current. The spectral responsivity (Rλ = (Jph − Jd)/Llight, conversion rate from photons to carriers, where Jph is the photocurrent density, Jd is the dark current density, and Llight is the incident light intensity), and photoconductive gain (G = Rλ × Ehv, the ability to provide multiple electrical carriers per single incident photon, where Ehv is the energy of the incident photon (in eV)) are shown in Figure 4e. The photodetector exhibits a broad photoresponse range from 370 to 780 nm. Significantly, the maximum Rλ is approximately 12 500 A W−1, which represents the largest value ever reported for CH3NH3PbI3-based photodetectors (Table S1, Supporting Information).11−14,16,28,29,32−35 Such a large Rλ value signifies the extremely high light sensitivity of the CH3NH3PbI3 NW array. Furthermore, the maximum G is deduced to be 36 800. The high Rλ and G could be attributed to the high carrier mobility and long carrier lifetime of the single-crystalline CH3NH3PbI3 NWs. To gain statistical significance, 50 devices were fabricated and measured, and the histogram analysis of their Rλ is shown in Figure S10, Supporting Information. It is noteworthy that the Rλ values fall in a narrow distribution of 11 400−13 700 A W−1 with an average value of 12 600 A W−1, which is attributable to the remarkable uniformity of the NW arrays. We further investigated the response speed of CH3NH3PbI3 NW array photodetector to a pulsed light at different frequency. The photoresponse of the device to the pulsed light at 50 Hz to 1 MHz is very fast with excellent long-term repeatability (Figure S11, Supporting Information). Figure 4f depicts the plot of normalized response of the device versus pulse frequency, showing a fast photoresponse with 3 dB bandwidth up to 0.8 MHz. The bandwidth is much larger than the small bandwidth of only 700 Hz for bulk perovskite single crystalbased photodetectors.18 From the magnified photoresponse curve (Figure S12, Supporting Information), the rise time (tr) and decay time (td) are extracted to be 0.34 and 0.42 μs, respectively, for the perovskite NW array-based photodetector,
which are among the best results for the perovskite-based photodetectors (Table S1, Supporting Information). The high response speed originates from the fast transport of the photogenerated carriers in the single-crystalline NWs. Noise equivalent power (NEP), a key figure of merit of a photodetector, is defined as the incident light power that generates a PC equal to the noise current. The NEP is determined to be 2.83 × 10−12 W Hz−1/2 at 5 V by integrating the noise power density (NPD) spectrum in Figure 4g. Accordingly, the detectivity (D*, the ability to detect weak signals) of the photodetector is deduced to be 1.73 × 1011 Jones (cm Hz1/2 W−1) according to the following equation11 D* =
AB NEP
where A is the area of the device, B is the given bandwidth. The detectivity value is comparable with other perovskite nanostructure-based photodetectors (Table S1, Supporting Information). Figure 4h indicates that the photocurrent has a linear response versus incident light intensity and a broad linear dynamic range (LDR) of 150 dB. The LDR corresponds to a minimum detection optical power of 0.1 μW cm−2, which exceeds conventional Si-based photodetectors (120 dB),11 suggesting the great potential of the CH3NH3PbI3 NW array photodetectors for weak light detection. For practical deployment of CH3NH3PbI3 NW array photodetectors, high device stability under ambient conditions is required. In this regard, single-crystalline CH3NH3PbI3 NW array photodetectors not only exhibited highlight sensitivity but also high moisture resistance. We monitored the degradation of CH3NH3PbI3 NW array photodetector by recording dark- and photocurrents at different time and found that the Ilight/Idark ratio nearly kept constant after 13 days in air (Figure S13a,b, Supporting Information). In sharp contrast, the photocurrent of photodetectors from CH3NH3PbI3 NWs prepared by previously reported method experienced continued degradation with time, and nearly fully degraded after 6 days (Figure S13c,d, Supporting Information). The enhanced device stability of the CH3NH3PbI3 NW array photodetectors is attributed to highly crystalline structures, unique one-dimensional enclosing surfaces, fewer defects, and fewer grain boundaries of the single-crystalline NWs, which make the NWs more resistant to moisture in ambient air.4,36 Although photodetectors can well measure light intensity, precise measurement of light wavelength is difficult due to the wide spectral response of photodetectors. Traditionally, wavelength measurement is realized by using a spectrometer equipped with a diffraction grating. However, generally such a detection system is bulky and requires complicated and expensive optical systems. To perform the functions of a spectrometer, photodetectors with sequentially tunable spectral response can be integrated to form a new system that is capable of simultaneous measurement of light intensity and wavelength (Figure S14, Supporting Information). This integrated system is promising for various applications such as in portablespectrometers, sensors, and rapid monitoring at low cost and high efficiency. In this work, we first demonstrated a proof-ofconcept device for wavelength measurement by integrating the perovskite NW arrays with tunable bandgap. A series of MAPb(I1−xBrx)3 (x = 0, 0.1, 0.2, 0.3, 0.4) NW arrays were fabricated by the FGAVC method (Figure S15, Supporting Information). By varying the I/Br ratio, bandgap tuning of MAPb(I1−xBrx)3 NW arrays can be readily achieved (Figure 2486
DOI: 10.1021/acs.nanolett.7b00166 Nano Lett. 2017, 17, 2482−2489
Letter
Nano Letters
Figure 5. (a) Absorption spectra of MAPb(I1−xBrx)3 NW arrays with varied contents of x = 0−0.4. (b) Spectral response of MAPb(I1−xBrx)3 NW array photodetectors. (c) Schematic illustration of the integrated device for wavelength detection. (d) Photographic image of the integrated device (left) and diagram for the measurement circuit (right). (e) Wavelength detection of the integrated device. LEDs in the integrated device can serve as visual indicators of the incident light wavelength. Lights of 790, 770, 750, 730, 700, and 650 nm were used for excitation, respectively, to test the integrated device.
or
