Layered and Pb-Free Organic–Inorganic Perovskite Materials for

Sep 30, 2016 - The evident photoresponse of the FTO/TiO2/MA2MnCl4/carbon ... Huawei ZhouXuejing LiuGuohang HeLin FanShaozhen ShiJiazhen ...
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Layered and Pb-free Organic–inorganic Perovskite Materials for Ultraviolet Photoresponse: (010)-Oriented (CHNH)MnCl Thin Film 3

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Zhonghao Nie, Jie Yin, Huawei Zhou, Ning Chai, Baoli Chen, Yingtian Zhang, Konggang Qu, Guodong Shen, Huiyan Ma, Yuchao Li, Jinsheng Zhao, and Xianxi Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08962 • Publication Date (Web): 30 Sep 2016 Downloaded from http://pubs.acs.org on September 30, 2016

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Layered and Pb-free Organic–inorganic Perovskite Materials for Ultraviolet Photoresponse: (010)-Oriented (CH3NH3)2MnCl4 Thin Film Zhonghao Nie,† Jie Yin,† Huawei Zhou,†,* Ning Chai,† Baoli Chen,† Yingtian Zhang,† Konggang Qu,† Guodong Shen, † Huiyan Ma,† Yuchao Li,† Jinsheng Zhao,† Xianxi Zhang†,* †

Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, School of Chemistry and Chemical Engineering, Liaocheng University; College of Materials Science and Engineering, Liaocheng University; Liaocheng 252059, China.

ABSTRACT: Organic–inorganic lead perovskite materials show impressive performance in photovoltaics, photodetectors, light-emitting diodes, lasers, sensors, medical imaging devices, and other applications. Although organic–inorganic lead perovskites have shown good performance in numerous fields, they contain toxic Pb, which is expected to cause environmental pollution in future large-scale applications. Thus, the photoelectric properties of Pb-free organic–inorganic perovskite materials should be developed and studied. In this paper, we report on the photoresponse of Pb-free organic– inorganic hybrid manganese perovskite (CH3NH3)2MnCl4. To the best of our knowledge, this study demonstrates the first time that organic–inorganic hybrid manganese perovskites are used for this type of application; we found that the solution-processed MA2MnCl4 thin film tends to be oriented along the b-axis direction on the TiO2 surface. The evident photoresponse of the FTO/TiO2/MA2MnCl4/carbon electrode devices was observed under 10–30 Hz flashlight frequencies and a 330 nm light beam. This simple, green, and low-cost photoresponsive device is beneficial for the future industrial production of optical recorders or optical memory devices.

Keywords: organic–inorganic perovskite, Pb-free, organic–inorganic hybrids manganese perovskite, ultraviolet photoresponse, (CH3NH3)2MnCl4

Introduction Organic–inorganic perovskite materials have attracted significant attention as new light sensitizers for thirdgeneration thin-film solar cells because of their low exciton 1–4 5, 6 binding energy (50–37 meV) , large absorption coefficient , 7, 8 and high charge carrier mobility . Recently, organic– inorganic perovskite-based solar cells have demonstrated high power-conversion efficiencies (PCEs) of more than 20%. Therefore, organic–inorganic perovskites have emerged as a

promising material for photovoltaic applications. Organic– inorganic perovskite materials not only perform well in the field of solar cells but also show impressive performance in 9–11 12–15 16, 17 , senphotodetectors , light-emitting diodes , lasers 18, 19 20 sors , medical imaging devices , and other applications. Thus, the optoelectronic properties of organic–inorganic perovskite materials have attracted considerable attention in different research fields worldwide. Three-dimensional organic–inorganic perovskite can be represented by the general + formula ABX3, where A is an organic cation, such as CH3NH3 6 + 21–24 2+ 6 2+ 25– , and HC(NH2)2 , B is a metal ion, such as Pb , Sn 2+ 29, 30 − 31 − 32 28 , or Ge , and X is a halogen atom, such as Cl , Br , − 33 − 34, 35 I , or SCN . The sizes of organic or inorganic ions are not arbitrary in the organic–inorganic perovskite. We can decide the organic–inorganic perovskite structure simply 1/2 through the tolerance factor (t), t = (rA+rX)/[(2 (rB+rX))], where rA, rB, and rX are the ionic radii of A, B, and X, respec21 tively . The value of t should be between 0.8 and 1 to form a stable three-dimensional structure, such as the structure in classical CH3NH3PbI3 (MAPbI3). Although 3D organic– inorganic lead perovskites have shown good performance in numerous fields, many problems exist. On one hand, organic–inorganic lead perovskites contain toxic Pb, which is expected to cause environmental pollution in future large-scale applications. On the other hand, the 3D structure of perovskites is easily destroyed upon exposure to external moisture, which eventually leads to degradation of the device. Thus, the photoelectric properties of Pb-free organic–inorganic perovskite materials should be developed and studied. Lead-free organic–inorganic tin perovskite analogues (MASnI3, FASnI3) can function as light harvesting materials, and solar-cell PCEs in the range of 5%−6% have been 25–28 acheived. Computational screening based on density functional theory calculations reveal germanium as an alternative element to replace Pb in organic–inorganic lead perovskites that are suitable for light absorption. Experimentally, three AGeI3 (A = Cs, MA, or FA) halide perovskite materials are stable up to 150 °C. Among these

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Figure 1 (a) Schematics of the solution-processed TiO2 electronic transport layer (ETL) and MA2MnCl4 thin film. (b) SEM images of the MA2MnCl4 thin films on the TiO2 surface. (c) cross-sectional SEM images of FTO/ETL-TiO2/MA2MnCl4 thin films. (d) amplifying cross-section SEM image of (c). materials, CsGeI3-based solar cells display higher photocur29 rents. CH3C(NH2)2GeI3, C-(NH2)3GeI3, (CH3)3NHGeI3, and 30 (CH3)2C(H)NH3GeI3 analogues were also prepared. In these compounds, 3D perovskite frameworks are formed for AGeI3 (A = Cs, MA, FA, and CH3C(NH2)2GeI3), whereas 1D infinite chains are formed for AGeI3 (A = C-(NH2)3, (CH3)3NH, and (CH3)2C(H)NH3). A two-dimensional, one-dimensional, or zero-dimensional structure is formed because of a tolerance factor of t > 1 with increasing organic cation radii. Compared with 3D organic–inorganic perovskite materials, the 2D organic–inorganic perovskite materials exhibit high moisture 36 stability. In addition, the 2D organic–inorganic perovskite material is easily solution-processed and crystallized into a high-quality thin film, which can lower the device manufacturing costs. Two-dimensional Pb-free (C6H5C2H4NH3)SnI4 materials are successfully applied in field-effect transistors as 37 a semiconductor channel. However, Ge and Sn are easily oxidized, which leads to degradation of the device. Twodimensional Pb-free organic−inorganic manganese perovskite (pyrrolidinium) MnCl3 exhibits excellent ferroelectricity 2 with a saturation polarization of 5.5 μC/cm , as well as intense red luminescence with a high quantum yield of 56% 38 under UV excitation. The excellent ferroelectricity and luminescence under UV excitation will probably result in photoresponse properties. The structure of (CH3NH3)2MnCl4 has been studied, but its photoelectric behavior remains unclear. With the aforementioned motivation, we started to investigate the photoresponse of organic–inorganic hybrid manganese perovskite (CH3NH3)2MnCl4 for opto-electronic applications. In this paper, we report on the photoresponse of Pb-free organic–inorganic hybrids manganese perovskite (CH3NH3)2MnCl4. To the best of our knowledge, this study represents the first time that organic–inorganic hybrid manganese perovskites have been used for this type of application; we found that the solution-processed MA2MnCl4 thin film tends to be oriented along the b-axis direction on the

TiO2 surface. The evident photoresponse of the FTO/TiO2/MA2MnCl4/carbon electrode devices was observed between 10–30 Hz and under a 330 nm light beam. This simple, green, and low-cost photoresponsive device is beneficial for the future industrial production of optical recorders or optical memory devices. Experimental Section Synthesis of CH3NH3I: CH3NH3I were synthesized followed 39 our previous literature. Fabrication and Characterizations of FTO/TiO2/MA2MnCl4/carbon electrode Devices: Patterned FTO-coated glass substrates with a sheet re-1 sistance of 15 Ω sq were coated with a TiO2 compact layer by spin-coating the TiO2 organic sol at 3,000 r.p.m. for 30 s, followed by drying at 450 ℃ for 30 minutes, followed our previ40 ous literature. The MA2MnCl4 layer was prepared by spin -1 -1 coating 0.1 molL MnCl2 and 0.333 molL CH3NH3Cl precursor solution at 3,000 r.p.m. for 30 s, followed by gradually heated to 70℃, baked at this temperature for 30 min. Finally, the carbon electrodes were prepared followed our previous 40 literature, which was coated by doctor-blade coating conductive-carbon paste on MA2MnCl4 layer, using adhesive tapes as pattern and spaces, followed by drying at 70 ℃ for 40 minutes. The size of the FTO/TiO2/MA2MnCl4/carbon 2 electrode devices was 1 cm . Nanostructures of MA2MnCl4 films were characterized by scanning electron microscopy (SEM, Hitachi SEM S-4800). UV−vis absorption spectrum and diffuse reflectance spectrum was obtained using UV/vis/NIR spectrometer (Perkinelmer, lambda, 750S). The photocurrent − time performance of FTO/TiO2/MA2MnCl4/carbon electrode devices was carried out by an electrochemical workstation

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Figure 2 (a) experimental and calculated powder XRD profiles of as-prepared MA2MnCl4 single crystal and MA2MnCl4 thin films on the TiO2 surface. (b) the microscope image of millimeter-sized and pyramid-liked MA2MnCl4 single crystals. (c) schematics of MA2MnCl4 thin film preferably oriented along b axis direction on the TiO2 surface. (d) TGA of MA2MnCl4 single crystals. (e) DSC of MA2MnCl4 single crystals. system (CHI760, Chenhua, and Shanghai) and equipped with a solar simulator (IV5, PV Measurements, Inc., USA). The photocurrent−time with incident photon (wavelength) and frequency response were measured by an electrochemical workstation system (CHI760, Chenhua, and Shanghai) and equipped with a the quantum efficiency/spectral response(SR)/incident photon to current conversion efficiency (IPCE) measurement system (QEX10, PV Measurements, Inc., USA). The photocurrent of devices were measured under the bias voltage with 0.00 V. Growth and Characterizations of MA2MnCl4 Single Crystals: In a typical procedure, A 50 ml round bottom flask was charged with 3 ml methanol and 3 ml ethanol. The liquid was degassed by passing a stream of nitrogen through it for 1 min and keeping it under a nitrogen atmosphere throughout the experiment. 126 mg MnCl2, 135 mg CH3NH3Cl were dissolved ° in the above solution upon heating the flask to 80 C using an oil bath, forming a colorless and transparent solution. Then ° the above solution was transferred to 80 C oven. The single crystal will be observed by slow evaporation solvent and gra° ° dient cooling (from 80 C to 40 C during 20 h). Single crystal XRD Characterization: Data was collected on a Bruker SMART CCD diffractometer with the use of Mo-Kα radiation (λ = 0.71073 Å). The structures were solved by direct method and expanded with the use of difference Fourier techniques with the SHELXL-97 program. Single Crystal Film Characterization: Powders XRD measurements were obtained using PANalyticalX´Pert diffractometer (Cu Kα radiation at λ = 1.54 Å) sampling at 2°/ min, 40 ekV and 100 mA. UV-vis absorption spectrum spectrum was obtained using UV/Vis/NIR spectrometer (Perkinelmer, lambda, 750S).

Results and Discussion The thin-film quality is a crucial factor in determining the performance of opto-electronic devices. To probe the thin-film quality of MA2MnCl4, the layer was prepared by −1 −1 spin-coating 0.1 mol L MnCl2 and 0.333 mol L CH3NH3Cl precursor solutions on the TiO2 electronic transport layer (ETL) (as shown in Figure 1a). The scanning electron microscopy (SEM) images of the MA2MnCl4 thin films are shown in Figure 1. Evidently, several micrometer-sized MA2MnCl4 nanosheets are formed on the TiO2 film, as shown in Figure 1b. In addition, several small crystals can be observed. The presence of a large number of grain boundaries between the small crystals is predicted to cause defects and traps, which may hurt the photoelectric performance. Therefore, we intend to optimize experimental conditions to reduce the presence of grain boundaries in the other paper. To measure the uniformity and thickness of the thin film, we provide the cross-sectional structure, as shown in Figures 1c and 1d. The island-like film forms on the smooth TiO2 film. This islandlike film possesses higher surface roughness, which is conducive to the interfacial contact between the back electrode and MA2MnCl4 thin films. The amplified cross-sectional SEM image is shown in Figure 1d. The thin-film thickness is in the range of 45–210 nm. In addition, the thickness of the uniform ETL–TiO2 film is approximately 5 nm. To determine the structural phase of the MA2MnCl4 thin film, MA2MnCl4 single crystal was synthesized by slow evaporation of a mixed precursor solution of MnCl2 and CH3NH3Cl. Single-crystal diffraction data were collected on a Bruker Smart CCD diffractometer with the use of Mo-Kα radiation (λ = 0.71073 Å). The structures were solved by a

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Figure 3 (a) schematics of fabrication of carbon electrode by the doctor-blade coating conductive-carbon paste on MA2MnCl4 layer. (b) photocurrent density-time characteristics of the FTO/TiO2/MA2MnCl4/carbon electrode device under illumination of 2 AM1.5 (1 Sun; 100 mW/cm ) by using a solar simulator. (c) amplifying on/off signal in photocurrent density-time characteristics. (d) the charge quantity (Q) generated by light is estimated from the integration of the area under the measured photocurrent density decay curve in initial five seconds. between the two adjacent titanium atoms is approximately 7.568 Å. The good lattice matching induced a preferential direct method and expanded using difference-Fourier techniques with the SHELXL-97 program. The single-crystal diforientation of the MA2MnCl4 thin film along the b-axis direcfraction data, which revealed the structure of MA2MnCl4, tion. To investigate the thermal stability of MA2MnCl4, we were assigned to the orthorhombic crystal system (space conducted thermogravimetric analysis (TGA), as shown in group = cmca, unit cell parameters of a = 7.2179 Å, b = Figure 2d. The TGA spectrum shows that MA2MnCl4 is ther19.4266 Å, c = 7.2777 Å, α = 90.000, β = 90.000, and γ = mally stable up to 210 °C but quickly decomposes above this temperature. The entire decomposition process can be divid90.000). A unit cell contains two molecules with the chemical formula of MA2MnCl4. The detailed crystal data and ed into two steps. The first step is from 210 °C to 295 °C, and structural refinement for the MA2MnCl4 single crystals can the weight fraction is approximately 26%. The second step is from 295 °C to 350 °C, and the weight fraction is also approxbe observed in the supporting information. The phase purity of the MA2MnCl4 single crystals was confirmed by powder imately 26%. This result is due to the sequential decomposiXRD from a large batch of single crystals (Figures 2a). The tion of the CH3NH3Cl component in MA2MnCl4. The differexperimental patterns are consistent with calculated curves ential scanning calorimetry (DSC) curve is shown in Figure from single-crystal diffraction data. Each diffraction peak of 2e. The enthalpy of the two steps is estimated from the intethe crystal powder is shown in Figure 2a. Strong peaks at gration of the fitting peak area. The values of enthalpy for the 9.40, 18.16, 22.37, 24.77, 27.90, 31.03, and 35.23 are assigned to first and second steps for the decomposition of CH3NH3Cl in −1 the (010), (111), (131), (200), (060), (240), and (202) planes, MA2MnCl4 are −353.48 and −215.44 kJ mol , respectively. respectively. The microscope image of millimeter-sized and Therefore, the thermodynamic equations for the entire depyramid-like MA2MnCl4 single crystals is shown in Figure 2b. composition process are as follows. The diffraction patterns of the MA2MnCl4 film on the TiO2 The photoresponse of organic−inorganic manganese layer are clearly different from those of the single crystal, as perovskite is unclear. Thus, photoresponsive devices based shown in Figure 2a. The MA2MnCl4 film on the TiO2 layer on MA2MnCl4 were fabricated. Traditionally, the high cost of showed only two diffraction peaks. Compared with the single the Au electrode as the back electrode requires its deposition crystal diffraction patterns, the two diffraction peaks were by a high-vacuum evaporation technique in photoresponse 41, 42 attributed to the (010) and (060) planes. The results suggest devices, thereby limiting its future application. Low-cost that ETL–TiO2 induced MA2MnCl4 growth along the b-axis, carbon is an ideal material to substitute Au as the back conas shown in Figure 2c. The distance between the two chlorine tact in photoelectric devices because its function and conatoms in an octahedral arrangement is 7.278 Å. The distance ductivity are similar to that of Au. Therefore, we prepared

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Figure 4 photocurrent density-time characteristics of the FTO/TiO2/MA2MnCl4/carbon electrode device under different wavelengths (300, 330, 360 and 390 nm) with flashlight frequency 1.33 Hz.

carbon electrode as the back electrode on the MA2MnCl4 thin film by a doctor-blade method that followed the previ40 ous literature , as shown in Figure 3a. The simple, green, and low-cost FTO/TiO2/MA2MnCl4/carbon electrode device is beneficial for future industrial production. The photoresponse properties of the device were studied by recording the photocurrent–time characteristics under illumination of 2 AM1.5 (1 Sun; 100 mW/cm ) using a solar simulator. An evident photoresponse with a maximum photocurrent density −2 of 148 nA cm was observed, as shown in Figure 3b. The pho−2 tocurrent density gradually decreased to 20 nA cm in five seconds, as shown in Figure 3c. The gradual decrease in photocurrent density may be due to interfacial mismatch. The interfacial mismatch may come from two sources. One part is between the TiO2 and MA2MnCl4 layers. The other part is between the MA2MnCl4 layer and the carbon electrode. If an appropriate hole transport layer is added between the MA2MnCl4 layer and the carbon electrode, interfacial electron recombination can be reduced. The photocurrent densi-

ty stayed constant after an initial drop. When the light was switched off, the current dropped immediately to its lowest point, and then increased to a stable platform. The photocurrent density suddenly increased to the maximum value when the light was switched on again. Although the maximum photocurrent of the device gradually decreased in the 1000 s cycle test, the stable photocurrent density virtually remained constant. To calculate the electric charge (Q) in the device, we estimated the Q generated by light by integrating the area under the measured photocurrent decay curve in the initial five seconds, as shown in Figure 3d. Q in the initial five seconds is calculated to be 155.84 nC. The evident on/off signal ratio indicates that the material may be potentially used in optical recording, optical storage and optical detection. Q=It=∫ J0 s dt

Figure 5 photocurrent density-time characteristics of the FTO/TiO2/MA2MnCl4/carbon electrode device under different flashlight frequencies (10, 20, 30 and 40 Hz) with 330 nm light-beam.

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The UV-Vis absorption spectra of MA2MnCl4 can be observed in Figure S2. The spectra show strong absorption at 332, 357, and 418 nm. This type of absorption may result in ultraviolet photoresponse. To probe the monochromatic light response, we determined the photoresponse under different wavelengths of light with a flashlight frequency of 1.33 Hz, as shown in Figure 4. The photoresponse is not observed under 300 nm light irradiation. In the case of 330 nm light irradiation, a clear photocurrent is observed. High Ion/Ioff ratios of the photocurrent intensity when the light is on (Ion) to the photocurrent intensity when the light is off (Ioff) are observed, as required in optical recording, optical storage or optical detection. When the light wavelength is tuned to 360 nm, the Ion/Ioff ratios clearly decreased. The photoresponse is weak when the light wavelength is tuned to 390 nm. Therefore, the 330 nm monochromatic light can be used as an effective light beam for optical recorders or optical memory devices. As a reference, the photoresponse of an FTO/TiO2/ carbon electrode device was measured. The photocurrent density-time characteristics of the FTO/TiO2/carbon electrode device under a 330 nm light-beam at 1.33 Hz are shown in Figure S3. The results indicate that photoresponse was not observed from the device with an FTO/TiO2/carbon electrode. Thus, the photoresponse of the device with the FTO/TiO2/MA2MnCl4/carbon electrode is attributed to the MA2MnCl4 layer. For optical recorders or memory devices, the light beam frequency determines the efficiency of the device. We investigated the photoresponse of the device at 330 nm with different flashlight frequencies, as shown in Figure 5. The Ion/Ioff ratio gradually reduces with increasing flashlight frequency. The Ion/Ioff signal ratios for frequencies of 10, 20, 30, and 40 Hz are 120, 75, 50, and 35, respectively. Considering the influence of wavelength and frequency, we can obtain high and effective Ion/Ioff ratios at 10–30 Hz and under a 330 nm light beam. It is possible to change the halide in this system to tune the optical band gap for a broader waveband response. The UV-vis absorption spectra of sam−1 −1 ples prepared by 0.1 mol L MnCl2 and 0.333 mol L MAX (X=Cl, Br, I) are shown in Figure S4. The color of the samples gradually changed from white to black (inset in Figure S4). The results indicate that changing the halide leads to modification of the absorption characteristics, which can result in bandgap tuning. We will investigate the effects of different halogens on the structure and properties of organic−inorganic manganese perovskite in another paper. Conclusion In conclusion, we have shown that layered and Pb-free organic–inorganic perovskite (CH3NH3)2MnCl4 materials exhibit effective ultraviolet photoresponse. Our preliminary studies demonstrate that the MA2MnCl4 thin film is preferentially oriented along the b-axis direction on the TiO2 surface. A high and effective Ion/Ioff ratio can be obtained at 10– 30 Hz and under a 330 nm light beam. The simple, green, and low-cost photoresponsive device based on the FTO/TiO2/MA2MnCl4/carbon electrode is beneficial for the future industrial production of optical recorders or optical memory devices.

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ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Figures: Schematics of thermal ellipsoid gived by Xshell in Shelxtl 97; The UV-Vis absorption spectra of MA2MnCl4; Photocurrent density-time characteristics of the FTO/TiO2/carbon electrode device under 330 nm light-beam with 1.33 Hz. UV-vis absorption spectra of sample prepared by mixing 0.1 mol L−1 MnCl2 and 0.333 mol L−1 MAX (X=Cl, Br, I); Tables: Crystal data and structure refinement for MA2MnCl4; Selected Bond lengths / Å and angles / ° for MAMnCl4

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] *E-mail: [email protected] ACKNOWLEDGMENT This work was financially supported by Shandong Province Natural Science Foundation (Grant No. BS2015NJ013; ZR2014BQ010), Research Fund for the Doctoral Program of Liaocheng University (Grant No. 31805), National Natural Science Foundation of China (Grant No. 21171084; 21402079; 21503104; 21601078; 51303076; 51407087), National Basic Research Program of China (Grant No. 2011CBA00701), Liaocheng science and technology development project fund (Grant No. 2014GJH08).

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