Letter www.acsami.org
Photodetectors Based on Two-Dimensional Layer-Structured Hybrid Lead Iodide Perovskite Semiconductors Jiachen Zhou,† Yingli Chu,† and Jia Huang*,† †
School of Materials Science and Engineering, Tongji University, Shanghai, 201804, P. R. China
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S Supporting Information *
ABSTRACT: Hybrid lead iodide perovskite semiconductors have attracted intense research interests recently because of their easy fabrication processes and high power conversion efficiencies in photovoltaic applications. Layer-structured materials have interesting properties such as quantum confinement effect and tunable band gap due to the unique two-dimensional crystalline structures. ⟨100⟩-oriented layerstructured perovskite materials are inherited from threedimensional ABX3 perovskite materials with a generalized formula of (RNH3)2(CH3NH3)n−1MnX3n+1, and adopt the Ruddlesden−Popper type crystalline structure. Here we report the synthesis and investigation of three layer-structured perovskite materials with different layer numbers: (C4H9NH3)2PbI4 (n = 1, one-layered perovskite), (C4H9NH3)2(CH3NH3)Pb2I7 (n = 2, two-layered perovskite) and (C4H9NH3)2(CH3NH3)2Pb3I10 (n = 3, three-layered perovskite). Their photoelectronic properties were investigated in related to their molecular structures. Photodetectors based on these two-dimensional (2D) layer-structured perovskite materials showed tunable photoresponse with short response time in milliseconds. The photodetectors based on three-layered perovskite showed better performances than those of the other two devices, in terms of output current, responsivity, Ilight/Idark ratio, and response time, because of its smaller optical band gap and more condensed microstructure comparing the other two materials. These results revealed the relationship between the molecular structures, film microstructures and the photoresponse properties of 2D layer-structured hybrid perovskites, and demonstrated their potentials as flexible, functional, and tunable semiconductors in optoelectronic applications, by taking advantage of their tunable quantum well molecular structure. KEYWORDS: photodetector, layer-structured perovskites, high sensitivity, sensor, lead iodide
R
devices.16−18 Several works have been done to investigate the electronic properties tuning by changing the chemical compositions of perovskite CH3NH3PbI3.19,20 Dinesh Kabra et al. have studied the CH3NH3Pb (Br1−xClx)3 hybrid perovskite systems and investigated the structural, optical and electronic properties of them using both experimental and theoretical tools. They found out that replacing Br− with Cl− would reduce the lattice parameters, increase the optical band gap and decrease the dielectric constant.21,22 Because modifying the crystalline structures of these materials can tune their optical band gaps, it is thus appealing to investigate optoelectronic properties of perovskite materials with a 2D layered molecular structure. Ruddlesden−Popper 2D layer-structured perovskite materials are inherited from three-dimensional (3D) ABX3 perovskites, which have a generalized formula of (RNH3)2(CH3NH3)n−1MnX3n+1, where R is an organic cation, M is a metal ion (Cu2+, Mn2+, Sn2+, Pb2+, etc.), X is a halide (Cl, Br, I) and n is the number of inorganic layers between the
ecently, there have been growing interests in hybrid lead halide perovskite materials (e.g., CH3NH3PbI3) because of their excellent properties such as long charge carrier lifetime, low recombination rate of charge carriers, low trap density, as well as high external quantum efficiency (EQE) in the wide range of spectrum. These materials show promising potential for photovoltaic and photoelectronic devices, such as solar cells, light-emitting diodes (LEDs), field-effect transistors (FETs), and photodetectors.1−8 Among these applications, photodetectors, which harvest light and convert it into electrical signals, have been widely applied in imaging, optical communication, chemical, and biomedical sensing devices.9−11 The commercialized photodetectors are mainly made by semiconductors such as PbTe, GaN, HgCdTe, InGaAs, etc.12−15 The perovskite materials could also have been fabricated into photodetectors because of their extraordinary electrical properties mentioned above. Two-dimensional (2D) layer-structured materials such as graphene, transition metal dichalcogenides (TMDs), and black phosphorus have attracted the attention of researchers to investigate their unique properties, such as spatial quantum effect and tunable optical band gap presented by their 2D molecular structures, which are essential for a wide range of optoelectronic and semiconductor © 2016 American Chemical Society
Received: August 1, 2016 Accepted: September 16, 2016 Published: September 16, 2016 25660
DOI: 10.1021/acsami.6b09489 ACS Appl. Mater. Interfaces 2016, 8, 25660−25666
Letter
ACS Applied Materials & Interfaces organic chains.23,24 To be more specific, when n is equal to 1, the crystalline structure turns out to be an ideal quantum well in which one inorganic atomic layer is separated by one layer of organic chains. When n tends toward infinity, the structure approaches to the 3D perovskite structure. Only if the organic chain is longer than propyl amine could it isolate inorganic atomic layers from electric coupling.25 Therefore, long organic molecules, butylamine, for example, could be selected to “slice” the lattice planes along ⟨100⟩ crystallographic directions of lead iodide to obtain the 2D layer-structured perovskite materials, where the inorganic PbnI3n+1 layer is sandwiched between organic butylamine molecular layers. The thickness of the inorganic layers could be controlled simply by tuning the stoichiometric ratio of the precursors.23 Yang et al. have synthesized thin 2D organic−inorganic hybrid perovskite (C4H9NH3)2PbBr4 and its derivatives by using a solution method, and photoluminescence (PL) shifts could be observed by tuning the thicknesses and the ratios of halides.24 Liang and co-workers improved this one-step solution method by combining solution method and vapor-phase conversion method, and produced many different 2D perovskite materials, which exhibit excellent optoelectronic properties.18 Mercouri G. Kanatzidis et al. have investigated the structural evolution and electronic properties of Ruddlesden−Popper hybrid lead iodide perovskites, and found the intermediate phases (n = 2− 4) of (CH3(CH2)3NH3)2(CH3NH3)n−1PbnI3n+1 are quite different from the end-members (n = 1 or ∞) in terms of optoelectronic properties, which define a new class of super lattice semiconductors with potential applications in lightemitting diodes.26 Here in this work we have synthesized three different 2D layer-structured perovskite materials with different layer numbers (n = 1, 2, and 3, as shown in Figure 1) and various colors (yellow, red and brown, respectively as shown in Figure 2a−c), namely (C4H9NH3)2PbI4 (n = 1, one-layered perovskite), (C4H9NH3)2(CH3NH3)Pb2I7 (n = 2, two-layered perovskite), and (C4H9NH3)2(CH3NH3)2Pb3I10 (n = 3, threelayered perovskite). Their optoelectronic properties were investigated in related to their molecular structures. Those materials were used to fabricate photodetectors with a lateral two-electrode structure, and their photoresponse properties have been carefully studied and compared, which demonstrated their capability for detecting different ranges of wavelengths and exhibited fast and reproducible response to light illumination. The experimental details are given in Materials and Methods. Briefly, the layer-structured perovskite materials were synthesized by reacting lead iodide, C4H9NH3I and CH3NH3I at 1:2:0, 2:2:1, and 3:2:2 stoichiometric ratio. The perovskite films were fabricated by spin-coating at 500 rpm for 5 s and then 2000 rpm for 17 s, followed by thermal annealing at 100 °C for 30 min. The resulted one-layered perovskite, two-layered perovskite, and three-layered perovskite films have thicknesses of 260, 690, and 560 nm, respectively. Gold electrodes were thermally evaporated onto these films through shadow masks with channel length of 15 μm and channel width of 1 mm. The schematic crystal structures of one-layered perovskite, twolayered perovskite, and three-layered perovskite are illustrated in Figure 1a−c, respectively. CH3NH3+ lies in the corner of a unit cell. Pb2+ stands at the body-centered site while I− occupies the face-centered site, and the [PbI6]4− octahedrons are connected by I− to form a layered structure. By controlling different stoichiometric ratios among PbI2, C4H9NH3I and
Figure 1. Schematic diagrams of different layer-structured perovskite materials: (a) one-layered perovskite; (b) two-layered perovskite; (c) three-layered perovskite (atom colors: Pb = yellow; I = violet; C = gray; N = light yellow; H = light blue). (d) XRD spectra of the layerstructured perovskite materials (1, 2, and 3 represent one-layered perovskite, two-layered perovskite, and three-layered perovskite, respectively).
CH3NH3I, different layer-structured network could be achieved with inorganic perovskite layers of [PbI6]4− at various thicknesses being sandwiched between C4H9NH3+ layers, and these unit layers are held together by weak intermolecular forces and hydrophobic forces during a self-assembling process.26 In addition, the inorganic layer defines the ab plane while the organic bilayers intercalate along the c-axis. These alternating organic−inorganic structures as shown in Figure 1 resemble multiple quantum wells in which the inorganic layers are potential “wells”, whereas the organic layers are potential “barriers”. And these intrinsic crystalline structure will endow them with unique photoelectrical properties.25 To further confirm the crystal structures of these layerstructured perovskite materials, X-ray diffraction (XRD) measurement was conducted. Figure 1d shows the XRD spectra of the 2D layer-structured perovskite materials. Figure S1 shows the magnified diffraction pattern for three-layered perovskite with the angles ranging from 3 to 13°. Diffraction angles, (hkl) values, and interplanar spacing dhkl are listed in Table S1. Compared with the 3D perovskite, these 2D layerstructured perovskite materials clearly demonstrate preferential orientations along the c-axis, because (00l) diffraction peaks seem to dominate the diffraction patterns, and the higher order diffraction peaks reveal the good crystallinity of these materials.27 The diffraction peak of (008), for example, shifts toward lower diffraction angle with the increase of n, which could be explained by the Bragg law. Several diffraction peaks 25661
DOI: 10.1021/acsami.6b09489 ACS Appl. Mater. Interfaces 2016, 8, 25660−25666
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Figure 2. (a−c) Photo images showing the different colors of layer-structured perovskite materials for one-layered perovskite, two-layered perovskite, and three-layered perovskite, respectively. (d−f) SEM images of layer-structured perovskite materials for one-layered perovskite, twolayered perovskite, and three-layered perovskite, respectively, scale bar: 5 μm. (g−i) AFM images of layer-structured perovskite materials for onelayered perovskite, two-layered perovskite, and three-layered perovskite, respectively, scale bar: 10 μm. (j) UV−vis absorption spectra for layerstructured perovskite materials.
have been successfully indexed for one-layered perovskite, and the orthorhombic lattice parameters were a = 8.874 Å, b = 8.692 Å, c = 27.331 Å. For two-layered perovskite and threelayered perovskite, the long cells dimensions along the c-axis were 37.068 and 48.347 Å. The expansion along the c-axis demonstrated that the CH3NH3+ cations layers have been successfully “inserted” into the inorganic layers and different thicknesses could be obtained by controlling the concentration of the precursors. The monotonic trend in the lattice parameter along the c-axis, as depicted in Figure S2, confirms the progressive expansion of the unit cell.19 Besides, the absence of the characteristic peak at 12.8°of PbI2 indicates the high purity of these layer-structured perovskite materials. Scanning electron microscopy (SEM) was carried out to investigate the morphological evolution of these layerstructured perovskite materials. As depicted in Figure 2d−f, the morphology of one-layered perovskite is different from that of the other two materials, which presents an irregular and partially discontinuous film and may result in insufficient light harvest. From the two-layered perovskite and three-layered perovskite films, we observed flat and smooth surfaces. We also utilized the atomic force spectroscopy (AFM) to study the
micromorphologies of these materials. Figure 2g shows the morphology of a one-layered perovskite film, which is consistent with the SEM image of the same material. Figure 2h shows that two-layered perovskite has an interconnected network structure, while AFM image of three-layered perovskite in Figure 2i shows a clear crystalline texture network. These micromorphologies, together with different optical band gaps of these materials, can have a strong impact on their photoelectronic properties, which will be discussed below. Figure 2j presents the ultraviolet−visible (UV−vis) absorption spectrum of one-layered perovskite, two-layered perovskite, and three-layered perovskite. Three characteristic absorption peaks (515 nm, 570 and 605 nm) could be clearly observed, and their corresponding optical band gaps are 2.33, 2.11, and 2.00 eV, respectively, by using the Kubelka−Munk equation (see Figure S3).26 Different optical band gaps are derived from different molecular structures, which are directly related to photoresponse on light with different wavelengths. Table 1 shows the energy levels of these 2D layer-structured perovskite materials characterized by ultraviolet photoelectron spectroscopy (UPS). The spectra of each layer-structured material are presented in Figure S4. We noticed that compared 25662
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two-electrode photodetectors based on them. Figure 3a shows the schematic device structure of the perovskite photodetector. The perovskite solutions were spin-coated on the pretreated substrates, followed by thermal annealing. Shadow masks were used to pattern gold electrodes on perovskite films. The experimental details are given in Materials and Methods. Under light illumination, the electron−hole pairs are generated in layer-structured perovskite materials and rapidly separated and collected by the gold electrodes under the applied bias. I−V curves of the 2D layer-structured perovskite photodetectors measured in dark and under white light irradiation
Table 1. Energy Levels for Layer-Structured Materials layer-structured materials
LUMO (eV)
HOMO (eV)
Fermi levels (eV)
one-layered perovskite two-layered perovskite three-layered perovskite
−5.63 −5.61 −4.57
−7.96 −7.72 −6.57
−5.02 −6.35 −6.49
to one-layered perovskite and two-layered perovskite materials, the energy level of three-layered perovskite is closer to that of the CH3NH3PbI3 perovskite material.28,29 To investigate the photoresponse of these 2D layerstructured perovskite materials, we have fabricated lateral
Figure 3. (a) Schematic diagram of the perovskite-based photodetector. (b−d) I−V curves for one-layered perovskite, two-layered perovskite, and three-layered perovskite, respectively, in the dark and under light illumination ranging from 0.2 to 4.7 mW/cm2. (e) Ilight/Idark ratio vs voltage for layer-structured perovskite photodetectors. (f) Linear relationship between the photocurrent and the incident light intensities under the bias of 30 V for layer-structured perovskite photodetectors. (g) Wavelength-dependent photocurrent of the devices with a bias of 30 V. 25663
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Figure 4. Time-dependent photocurrent response of layer-structured perovskite photodetectors of: (a, b) one-layered perovskite; (c, d) two-layered perovskite; (e, f) three-layered perovskite with an incident light intensity of 3.0 mW/cm2 at the bias of 30 V.
from 0.2 to 4.7 mW/cm2 are shown in Figure 3b−d. In the dark, the current was as low as approximately 1 × 10−12 to 1 × 10−11 A at the voltage bias of 30 V. The output current increased significantly under light illumination. Under 3.0 mW/ cm2 illumination, for example, the photocurrents increased to 1 × 10−9 A for the one-layered perovskite and two-layered perovskite, and 1 × 10−8 A for three-layered perovskite. The slightly nonlinear I−V characteristics of these photodetectors, especially for three-layered perovskite, are mainly due to the mismatch between the work function of Au electrode (4.9−5.2 eV) and the energy levels of these layer-structured materials, as shown in Table 1. Another factor might be the defects in the semiconductor layers, which induce charge trapping effect. The Ilight/Idark ratio of the one-layered perovskite and two-layered perovskite photodetectors were on the order of 1 × 102 under voltage bias of 10 V, whereas the three-layered perovskite photodetector exhibited a Ilight/Idark ratio on the order of 1 × 103, as shown in Figure 3e. Obviously, the three-layered perovskite photodetector showed much higher Ilight/Idark ratio than that of the other two materials. Responsivity, indicating the efficiency of a photodetector responds to an optical signal, is another important parameter for photodetectors. It is defined as R = (Ilight − Idark)/(PinA),
where Ilight is the photocurrent, Idark is the dark current, Pin is the incident light intensity, and A is the active area. Under 3.0 mW/cm2 white light illumination and bias voltage of 30 V, the responsivities of one-layered perovskite, twolayered perovskite, and three-layered perovskite photodetectors are 3.00, 7.31, and 12.78 mA/W, respectively. These responsivities are rather low compared to 3D perovksitebased photodetectors due to several factors.6 First, these 2D layer-structured perovskite materials have larger optical band gaps than that of the 3D perovskite CH3NH3PbI3 (∼1.56 eV). Second, they absorb only a small fraction of white light in the UV−vis absorption spectrum (see Figure 2j), which is different from the 3D counterpart.29 In addition, the insulating butylammonium layers can act as barriers for charge transportation, and limit the charge carrier mobility. Figure 3f shows the relationship between the photocurrent and incident light intensities, which manifests that the photocurrent is linearly dependent on light intensities for these layer-structured perovskite photodetectors. This linear properties show their potential in determining the intensity of incident light simply by measuring the photocurrent in practical applications. Figure 3g shows the wavelength-dependent output current of the layer-structured perovskite photodetectors, under fixed 25664
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ACS Applied Materials & Interfaces illumination intensity at 1.0 mW/cm2. For all three devices, the photocurrents remained almost constant with illuminating wavelength larger than their corresponding “threshold” wavelength, and then increased substantially when the wavelength was below the “threshold” wavelength. The wavelengthdependent photoresponses of these devices are induced by the energy band structure of the perovskite materials. The electron−hole pairs could only be excited by the incident light whose energy is higher than optical band gap (Eg), thus resulting in a wavelength-dependent photocurrent.30 Generally speaking, though the 3D perovskite-based devices throw the 2D ones into the shade in terms of light absorptivity, responsivity and photocurrents as discussed above, this “selective absorption” characteristics of lights for layer-structured materials will endow them with potential applications such as detecting or “filtrating” lights at specific wavelengths. Fast and reproducible responses to light illumination are crucial to high-performance photodetectors for commercial applications. Figure 4a, c, and e presents the photoresponse of 2D layer-structured perovskite photodetectors upon the onand-off switching of light illumination at 3.0 mW/cm2 intensity. The results indicate that all these layer-structured perovskite photodetectors have highly reproducible and stable photoresponses and are therefore of great interest from a practical application point of view. We analyzed the response times for these layer-structured perovskite photodetectors as shown in Figure 4b, d, and f, and the rise and decay times for the onelayered perovskite photodetector are 28.4 and 27.5 ms, for the two-layered perovskite photodetector are 8.4 and 7.5 ms, and for the three-layered perovskite photodetectors are 10.0 and 7.5 ms, respectively. From the discussions above, we conclude that the threelayered perovskite photodetectors has a better performances than one-layered perovskite and two-layered perovskite photodetectors, in terms of output current, responsivity, Ilight/Idark ratio and response time. We believe it is partially because the optical band gap of the three-layered perovskite is narrower and it absorbs larger portion of light than the one-layered perovskite and two-layered perovskite do. In addition, the microstructure of three-layered perovskite might be more favorable to achieve higher output current than that of the other materials. It could be expected that with the increase of the inorganic PbnI3n+1 layer thickness within the layered-perovskite material structure, the photoelectronic properties will be further improved. In conclusion, 2D layer-structured perovskite materials, (C 4 H 9 NH 3 ) 2 PbI 4 , (C 4 H 9 NH 3 ) 2 (CH 3 NH 3 )Pb 2 I 7 and (C4H9NH3)2(CH3NH3)2Pb3I10 were synthesized and characterized. Lateral two-electrode photodetectors based on these materials with different microstructures have been fabricated and tested. The optical band gaps for one-layered perovskite, two-layered perovskite, and three-layered perovskite are 2.33, 2.11, and 2.00 eV, respectively, which give rise to the potential for detecting and responding to light at different wavelengths. These photodetectors present highly reproducible on-and-off switching photoresponse properties, and the rise and decay times are 28.4 and 27.5 ms for one-layered perovskite, 8.4 and 7.5 ms for two-layered perovskite, and 10.0 and 7.5 ms for three-layered perovskite, respectively. The three-layered perovskite photodetectors has better performance than the onelayered perovskite and the two-layered perovskite photodetectors, in terms of output current, responsivity, Ilight/Idark ratio and response time, which can be attributed to smaller optical band gap and better condensed and more compact
microstructure of the three-layered perovskite than others. From the perspective of crystalline structures, because inorganic layers act as “wells” that help the charge transport, whereas the insulating butyl-ammonium layers are “barriers” that hinder the charge transport, it could be expected that with the increase of inorganic layers (n) numbers, these layer-structured perovskitebased devices exhibit better optoelectronic performance because inorganic layers prevail organic ones. And when n tends toward infinity, the performances of these devices are close unlimitedly to 3D perovskite-based devices. These results revealed the relationship between the molecular structures, film microstructures and the photoresponse properties of 2D layerstructured hybrid perovskites, and suggested that these materials can be new candidates for flexible, functional, and tunable semiconductors in optoelectronic applications, by taking the advantage of the tunable quantum well molecular structure of these 2D layer-structured hybrid perovskites.
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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.6b09489. Materials and Methods; detailed XRD information on diffraction peak angles, (hkl) values, and interplanar spacing of layered-structured materials; relationship between the lattice parameter along the c-axis and the thickness of inorganic layer (n); detailed XRD pattern for three-layered perovskite with the angles ranging from 3 to 13°; optical absorption spectrum of layer-structured materials converted by using the Kubelka−Munk equation; UPS spectra of layer-structured materials(PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding
National Natural Science Foundation of China (Grants 21302142 and 51373123), Science & Technology Foundation of Shanghai (14JC1492600). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Professor Jianguo Wu and the characterization and testing center of School of Materials Science and Engineering at Tongji University. This work was supported by the National Nature Science Foundation of China (Grants 21302142 and 51373123), Science & Technology Foundation of Shanghai (14JC1492600), and the 1000 youth talent plan.
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REFERENCES
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DOI: 10.1021/acsami.6b09489 ACS Appl. Mater. Interfaces 2016, 8, 25660−25666
