(C6H5C2H4NH3)2GeI4: A Layered Two-Dimensional Perovskite with

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(CHCHNH)GeI: A Layered Two-Dimensional Perovskite with Potential for Photovoltaic Applications Pengfei Cheng, Tao Wu, Jiangwei Zhang, Yajuan Li, Junxue Liu, Lei Jiang, Xin Mao, Rui-Feng Lu, Wei-Qiao Deng, and Keli Han J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01985 • Publication Date (Web): 31 Aug 2017 Downloaded from http://pubs.acs.org on September 1, 2017

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The Journal of Physical Chemistry Letters

(C6H5C2H4NH3)2GeI4: A Layered Two-Dimensional Perovskite with Potential for Photovoltaic Applications Pengfei Cheng,†,‡ Tao Wu,†,§ Jiangwei Zhang,¶ Yajuan Li,†,‡ Junxue Liu,† Lei Jiang,† Xin Mao,†,‡ Rui-Feng Lu,§ Wei-Qiao Deng,*,† and Keli Han*,† †

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics,

Chinese Academy of Science, Dalian 116023, P. R. China. ‡

University of the Chinese Academy of Sciences, Beijing 100039, P. R. China.

§

Department of Applied Physics, Nanjing University of Science and Technology, Nanjing

210094, P R China. ¶

Gold Catalysis Research Center, State Key Laboratory of Catalysis, Dalian Institute of

Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT: Recently, two-dimensional organic-inorganic perovskites have attracted increasing attention due to their unique photophysical properties and high stability. Here we report a leadfree,

two-dimensional

perovskite,

(PEA)2GeI4

(PEA

=

C6H5(CH2)2NH3+).

Structural

characterization demonstrated that this 2D perovskite structure is formed with inorganic germanium iodide planes separated by organic PEAI layers. (PEA)2GeI4 has a direct band gap of 2.12 eV, in agreement with 2.17 eV obtained by density functional theory (DFT) calculations, implying that it is suitable for a tandem solar cell. (PEA)2GeI4 luminesces at room-temperature with a moderate lifetime, exhibiting good potential for photovoltaic applications. In addition, 2D (PEA)2GeI4 is more stable than 3D CH3NH3GeI3 in air, owing to the presence of hydrophobic organic long-chain. This work provides a direction for the development of 2D Ge-based perovskites with potential for photovoltaic applications.

TOC GRAPHICS


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Recently, two-dimensional (2D) organic-inorganic halide perovskites, which can be derived from three-dimensional (3D) perovskites by slicing along specific crystallographic planes,1,2 have attracted a lot of attention due to their unique properties and promising applications.3-5 For 2D layered perovskites, inorganic and organic layers stack alternately to construct the frameworks. The inorganic moiety provides the potential for high carrier mobility,6 while the organic moiety offers the potential for intense photoluminescence due to the quantum confinement effect.7-9 2D layered perovskites based on Pb2+, Sn2+, and Bi3+ have been extensively studied for decades,10-13 and they have been applied to field-effect transistor (FET),14,15 light-emitting diode (LED),16 photodetector17 and solar cells.1,2,18 However, as a similar element to Pb and Sn in IV A group, Ge has been rarely used to develop two-dimensional perovskites. In 1996, a layered Ge-based perovskite, (C4H9NH3)2GeI4, was reported by Mitzi to investigate its crystal structure and optical properties, but the photoelectric behavior of this compound remains unclear.19 Recently, Kanatzidis et al. synthesized a series of Ge-based perovskites with 3D frameworks and 1D infinite chains, but the 2D layered structure is not formed.20 The lack of sufficient structural and photoelectric information of 2D Ge-based perovskites prompts us to investigate this family of perovskites, to explore their potential applications in photovoltaic field. In this article, we reported the crystal structure, photoelectric properties and stability of a leadfree, two-dimensional perovskite, (PEA)2GeI4, which can be obtained as precipitates through cooling down a HI solution in the presence of H3PO2. Density functional theory (DFT) calculations show that (PEA)2GeI4 has a direct band gap of 2.17 eV, in good agreement with the experimental results, implying that (PEA)2GeI4 is suitable for tandem solar cell applications. Furthermore, we find that compared to the 3D analogue, 2D layered (PEA)2GeI4 is more stable


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and exhibit room-temperature photoluminescence with a lifetime exceeding 1 ns. We hope our research will provide guidelines for the development of lead-free two-dimensional perovskites. (PEA)2GeI4 crystallize in P1 space group of triclinic crystal system, with the lattice parameters a=11.9977(7) Å, b=12.0245(6) Å and c=17.4686(11) Å. The layered crystal structure along the crystallographic c-axes is shown in Figure 1a. The organic PEA+ cations were embedded in

Figure 1. Crystal structure of (PEA)2GeI4. (a) Structural illustration of the layered 2D perovskite (PEA)2GeI4, viewed along the b axis. Violet, yellow, grey, blue and white spheres represent Ge, I, C, N and H atoms, respectively. (b) The distorted [GeI6]4- octahedral, viewed along the c axis. (c)


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PXRD patterns. (d) SEM images of (PEA)2GeI4 showing the layered structure. (e) TEM images of a thin (PEA)2GeI4 sheet. the structure as a chain between the germanium iodide planes composed of corner-sharing [GeI6]4- octahedral. In (PEA)2GeI4, Ge is in a divalent state with a pair of nonbonding electrons. Consequently, the lone pair of electrons reduces the coordination symmetry around Ge2+ cations and has obvious stereochemical activity. For instance, the six Ge-I bonds in [GeI6]4- octahedral split into three short ones and three long ones with length ranging from 2.78 Å to 3.37 Å, and the bond angles of I-Ge-I deviate from 90° obviously. As shown in Figure 1b, the adjacent [GeI6]4octahedra alternately tilt along the crystallographic a-axes with Ge–I–Ge angles of 158.34°, which is more distorted than the 3D MAGeI3 structure with the angle of 167.65°.20 More complete information are presented in Table S1 and the structure comparison between the 2D (PEA)2GeI4 and the 3D MAGeI3 are shown in Table 1. Table 1. Structural details for 2D (PEA)2GeI4 compared with the 3D MAGeI3 (ref. 19) (PEA)2GeI4 P1 (triclinic) a = 11.9977(7) b = 12.0245(6) c = 17.4686(11) Cell angles α = 80.085(5)° β = 73.521(5)° γ = 89.962(4)° Cell volume (Å3) 2377.4(2) Z 2

Compound Space group Cell length (Å)

MAGeI3 R3m (trigonal) a = 8.5534(13) b = 8.5534(13) c = 11.162(2) α = 90.00° β = 90.00° γ = 120.00° 707.2(2) 3

Powder XRD of (PEA)2GeI4 was presented in Figure 1c. Obviously, this material is well crystallized and highly-oriented, with seven sharp and equally spaced peaks. Miller indices obtained from single crystal date were marked on experimental diffraction peaks, all of which belonged to the (00l) reflections. The observation of unitary (00l) reflections is similar to the


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features of lead-based 2D perovskite,2,21 implying that 2D layered structure of (PEA)2GeI4 is formed. The distance between the (001) plane is approximately 1.65 nm, calculated from the Bragg law. The layered structure can be observed by a scanning electron microscope, and Figure 1d shows the edge of a thick (PEA)2GeI4 crystal sheet, which is composed of stacks of sheets. Chemical analysis was carried out on the crystal by energy dispersive spectroscopy (EDS). The obtained average Ge:I atomic ratio is 3.9:1, in good agree with the stoichiometry. In addition, TEM images of a exfoliated (PEA)2GeI4 thin sheet from the crystal are presented in Figure 1e, confirming that the structure is composed of stacked layers connected by weak van der waals forces. To evaluate whether (PEA)2GeI4 is suitable for photovoltaic applications, UV−vis diffuse reflectance spectra was used to determine the optical band gap. The absorption spectra converted from reflectance data through Kubelka−Munk equation is presented in Figure 2a. As can be seen,

Figure 2. (a) Absorption spectra and corresponding tauc plot of (PEA)2GeI4. (b) The electronic band structure and (c) DOS of (PEA)2GeI4. The fermi level is set to be zero. (PEA)2GeI4 has excellent absorbing performance with an onset at 620 nm. Corresponding Tauc plot in the inset of Figure 2a, obtained by assuming a direct band gap, as indicated by subsequent DFT calculations, reveals an Eg of 2.12 eV. Though this material is not suitable for a single-


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junction solar cell, it can be coupled with silicon as the absorber for a tandem solar cell, as has been demonstrated for CH3NH3PbBr3 (Eg = 2.26 eV).22 To further investigate the electronic structure of (PEA)2GeI4, DFT with hybrid functional HSE06 is used to calculate the band structure and local density of states (LDOS). As shown in Figure 2b, the valence band maximum (VBM) and conduction band minimum (CBM) are both located at Γ point, revealing a direct band gap of 2.17 eV, in good agreement with the experimental results. It is worth noting that the energies at Z point are very close to Γ point in valence band and conduction band. The bands between Z and Γ point are almost on a horizontal line, indicating that electrons are localized along the Γ- Z direction. This is consistent with the fact that the layered structure of (PEA)2GeI4 is along Z direction. Apart from that, LDOS with the contributions of different atoms are shown in Figure 2c. Similar with the situation in CH3NH3PbI3, Ge2+ cation in (PEA)2GeI4 mainly contributes to the bottom of conduction band, and its contribution to the top of valence band is little. On the contrary, the contribution of Ianion is mainly concentrated on the top of valence band. We find that (PEA)2GeI4 shows room-temperature photoluminescence and the spectrum is presented in Figure 3a. When excited by a pump wavelength of 400 nm, the sample show a wide

Figure 3. (a) Room-temperature PL spectrum of 2D (PEA)2GeI4 and 3D MAGeI3. (b) PL decay profile of (PEA)2GeI4 single crystals at room temperature.


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PL peak centered at 630 nm. As a comparison, the PL of MAGeI3 is too weak to detect under the same condition. For 2D layered organic-inorganic perovskites, alternate stacks of high dielectric constant organic layer and low dielectric constant inorganic layer produce a dielectric confinement effect, generating stable excitons with enhanced exciton binding energy.23 As a result, these stable excitons in layered perovskites lead to an intense PL, which exits even at room temperature. The observed PL for 2D (PEA)2GeI4, indicating lower losses in nonradiative recombination path, which is desirable for photovoltaic applications, and may therefore give rise to promising open-circuit voltages for solar cells.24 Besides, the high PL for (PEA)2GeI4 makes it attractive to optoelectronic applications, including LED and laser.25 The photoluminescence quantum efficiency (PLQE) of (PEA)2GeI4 is 0.6%, which is slightly higher than that of the MA3Bi2I9 perovskite (~0.4%)26 and should be improved for photovoltaic application in the future. We use a 405 nm wavelength laser to excite the single crystal and the corresponding PL delay curve was recorded using a time-correlated single photon counting (TCSPC) setup (Figure 3b). The raw data can be fitted by a bi-exponential delay curve, with a short-lifetime of 408 ps and a long-lifetime of 1.28 ns. Given the instrument respond function (400 ps), analysis of the shortlifetime process is limited. The long-lifetime process exceeding 1 ns indicates that (PEA)2GeI4 is worth further exploring as a new solar absorber.26,27 Compared with the conventional MAPbI3 perovskite, the carrier lifetime is too short for (PEA)2GeI4 currently. The improvement of lifetime in (PEA)2GeI4 may be obtained by eliminating defects and impurities (such as Ge4+ cations generated during the synthesis and measurement process, Figure S1), as has been shown in other materials.27,28


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In order to compare the stability of MAGeI3 and (PEA)2GeI4 to moisture, we exposed powders of both perovskites to a relative humidity level of 60% at 25 °C. After stored in the humidity environment for two days, MAGeI3 showed pronounced MAI and GeI4 peaks (Figure 4a), as a

Figure 4. PXRD of MAGeI3 (a) and (PEA)2GeI4 (b), which are exposed to 60% relative humidity at 25 °C. Asterisks denote signals from GeI4. result of decomposition and oxidation. The oxidation of Ge2+ during the aging can be observed by the XRS measurement, as shown in Figure S2. We speculate that the moisture degradation of MAGeI3 may be related to the transformation of MAGeI3 to MAI and GeI2. Then MAI can decompose into HI and CH3NH2, and GeI2 can be oxidized to GeI4. However, the diffraction pattern of (PEA)2GeI4 remained almost unchanged as our expected (Figure 4b). For (PEA)2GeI4, the long hydrophobic chain of PEA cations can prevent the direct contact of water with perovskite and thus may delay the moisture degradation and the subsequent oxidation of perovskite. When sored in the inert atmosphere, (PEA)2GeI4 exhibit rather high stability without signs of decomposition (Figure S3). The thermogravimetric analyses (TGA) curve measured under a nitrogen atmosphere is shown in Figure S4a. (PEA)2GeI4 is thermally stable (up to 250 °C) and exhibits a single step degradation. However, slight weight loss (< 1%) occurs at low temperature (about 150 °C), most likely due to the volatilization of PEA and HI. Two distinct peaks were observed at 174 °C and


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195 °C respectively, as shown in the differential scanning calorimetric analysis (DSC) curve in Figure S4b. The endothermic peak at 195 °C is close to the decomposition point of the material, thus it corresponds to a melting transition. Another endothermic peak at 174 °C is likely to be caused by a structure transition, which has been observed in other layered organic-inorganic perovskites.29-31 Besides, the endothermic and exothermic peaks in the heating and cooling process indicate that the phase transition is reversible. In spite of the phase transition, (PEA)2GeI4 is relatively stable in the range of device operating temperatures .32 In summary, we have investigated the structure and photoelectric properties of a new lead-free, 2D layered perovskite, (PEA)2GeI4, to evaluate its prospects in photovoltaic applications. The new compound, crystallizing in P1 space group, has a direct band gap of 2.12 eV. We found that (PEA)2GeI4 luminesces at room temperature, with an long-lifetime exceeding 1 ns , exhibiting good potential for photovoltaic applications. In addition, introducing long-chain PEA+ cations to form layered structure can improve the stability of Ge-based halide perovskite. Our findings suggest that a series of new 2D germanium-halide perovskites should be developed with a view to exploring their photoelectrical properties and potential applications. ■ EXPERIMENTAL METHODS Synthesis. HI (57 wt. % in water), H3PO2 (50 wt. % in water) and GeO2 (>=99.99%) were purchased from Sigma-Aldrich. CH3NH3I (MAI, 99%) and C6H5(C2H4)NH3I (PEAI, 99%) were purchased from Xi’an Polymer Light Technology Corp. All the chemicals were used as received. (PEA)2GeI4 was prepared by mixing 1 mmol GeO2 with 2 mmol PEAI in 3 ml HI and 3 ml H3PO2 aqueous solution. The solution was stirred at 100 °C for 1 hour，turning to a yellow solution. The stirring was then stopped and the solution was cooled to room temperature to afford red flake-like crystals. For MAGeI3, 1 mmol GeO2 and 1 mmol MAI were dissolved in 3


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ml HI and 3 ml H3PO2 aqueous solution. The solution was stirred at 130 °C using an oil bath until half of its original volume has been evaporated, then the stirring was discontinued and the solution was cooled to room temperature to produce the dark red crystals. The general procedures mentioned above were done in N2 atmosphere. Finally, the crystals were washed with a minimum quantity of dried ethanol and dried in vacuum at 70 °C for 24 hours. Characterization. PA Nalytical Empyrean using Cu Kα radiation (λ = 1.54056 Å) was operated for X-ray analysis at room temperature, and the acquisition was done for every 0.04° increment over the Bragg angle range of 10°−70°. A UV-Vis (JASCO V-550) spectrometer equipped with an integrating sphere was used to collect absorption data. Field Emission Scanning Electron Microscope (FESEM, JEOL, JSM-7800F, 3 kV) equipped with an Oxford XMax Silicon Drift Detector were used to record surface morphology and perform chemical analysis, respectively. TEM images were recorded with a JEOL JSM-7800F transmission electron microscope. PL spectra were recorded on the Horiba Jobin Yvon FluoroMax-4P. The fluorescence lifetime measurement setup used in this study was based on the homemade timecorrelated single photon counter system. The excitation beam was picosecond pulse diode laser with 405 nm output wavelength and 50-ps pulse width. The optical detector was single photon counting module. XPS measurement was done with a Thermo Scientific Escalab 250 Xi instrument using monochromatic Al Kα radiation (hν = 1486.7 eV). Thermo gravimetric analyses (TGA) were performed with a Netzsch STA 449 F3Jupiter Thermo-Microbalance at a heating rate of 10 °C/min, using 11.56 mg samples in alumina pans. PL quantum efficiency (PLQE) measurement was performed using an absolute PL quantum yield spectrometer (Hamamatsu C11347).


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X-ray Crystallographic Study. Suitable single crystals were selected. Data collections were performed on Agilent Atlas CCD diffractometer with the use of graphite-monochromated MoKα radiation (λ = 0.71073 Å). Data reduction, cell refinement and experimental absorption correction were performed with the software package of Agilent Gemini Ultra CrysAlisPro (Ver 1.171.35.11). The structures were solved by direct methods and refined against F2 by full-matrix least-squares. All non-hydrogen atoms were refined anisotropically. All calculations were carried out by the program package of SHELXTL ver 5.1 and Olex2 ver 1.2.8. Theoretical calculation. The ground state of structure is determined by using density functional theory (DFT) implemented in the Vienna ab initio simulation package (VASP).33 All calculations were performed by Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) exchange-correlation.34 Projector augmented wave method (PAW) was used to describe the interaction between core electrons and valence electrons.35,36 In particular, the kinetic energy cutoff for plane wave basis set was set to 400 eV. The atomic positions were fully relaxed until the force convergence on atoms of 0.1 eV/Å were achieved with the experimental cell parameters constrained. The Brillouin zone was sampled with 2×2×2 Monkhorst-Pack grid for the bulk structure. Importantly, because the PBE functional underestimates the band gaps of semiconductors, all electronic structures were further calculated by using the HSE06 hybrid function to improve the accuracy of electronic structure.37 It is worth noting that the spin-orbit coupling effect is not considered in the electronic structure calculation, because metal anion Ge2+ is not the heavy metal anion compared with Pb2+. Thus, the spin-orbit coupling effect in (PEA)2GeI4 is not important an can be neglected in the calculation. ASSOCIATED CONTENT AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the Ministry of Science and Technology of China (grant number 2017YFA0204800), National Natural Science Foundation of China (grant number 21525315 and 21533010), DICP DMTO201601, the Science Challenging Program (JCKY2016212A501). Supporting Information. Crystal data and structure refinement information for (PEA)2GeI4, XPS narrow scans of (PEA)2GeI4 and MAGeI3 for Ge 2P, XRD data of (PEA)2GeI4 stored in N2 for 51days, Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) data of (PEA)2GeI4. (PDF) REFERENCES (1) Smith, I. C.; Hoke, E. T.; Solis-Ibarra, D.; McGehee, M. D.; Karunadasa, H. I. A Layered Hybrid Perovskite Solar-cell Absorber with Enhanced Moisture Stability. Angew. Chem. Int. Ed. 2014, 53, 11232-11235. (2) Cao, D. H.; Stoumpos, C. C.; Farha, O. K.; Hupp, J. T.; Kanatzidis, M. G. 2D Homologous Perovskites as Light-Absorbing Materials for Solar Cell Applications. J. Am. Chem. Soc. 2015, 137, 7843-7850. (3) Saparov, B.; Mitzi, D. B. Organic-Inorganic Perovskites: Structural Versatility for Functional Materials Design. Chemical reviews 2016, 116, 4558-96. (4) Chen, S.; Shi, G. Two-Dimensional Materials for Halide Perovskite-Based Optoelectronic Devices. Adv. Mater. 2017, 1605448.


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(C6H5C2H4NH3)2GeI4: A Layered Two-Dimensional Perovskite with

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