Lead-Free, Two-Dimensional Mixed Germanium and Tin Perovskites

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Lead-Free, Two-Dimensional Mixed Germanium and Tin Perovskites Pengfei Cheng, Tao Wu, Junxue Liu, Weiqiao Deng, and Keli Han J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00871 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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The Journal of Physical Chemistry Letters

Lead-Free, Two-Dimensional Mixed Germanium and Tin Perovskites Pengfei Cheng,†,‡ Tao Wu,† Junxue Liu,† Wei-Qiao Deng,† ,§ 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.

§

Institute of Molecular Sciences and Engineering, Shandong University, Qingdao, P. R. China.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

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ABSTRACT: Hybrid two-dimensional (2D) organic–inorganic perovskites continue to draw increased attention in view of their outstanding performance in optoelectronic devices such as solar cells and light-emitting devices. Herein, for the first time, we report the synthesis and characterization of lead-free, 2D mixed Ge–Sn halide perovskites, (PEA)2Ge1-xSnxI4 (where PEA = C6H5CH2CH2NH3+), and demonstrate that the bandgaps decrease linearly with increasing Sn content. Most importantly, among them, (PEA)2Ge0.5Sn0.5I4 possesses the smallest bandgap of 1.95 eV. Density functional theory calculations confirm that Sn substitution induces a smaller bandgap and more dispersed band structure, which are desirable characteristics of lightabsorbing materials. In addition, conductivity and stability of (PEA)2Ge0.5Sn0.5I4 have also been assessed.

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In recent years, two-dimensional (2D) organic–inorganic perovskites have emerged as promising candidates for photoelectric and photovoltaic applications owing to their enhanced moisture stability and unique optical properties.1-4 In contrast to 3D perovskites, the choice of organic cations for 2D perovskites is not limited by the tolerance factor concept, thus it offers more tunability and flexibility in controlling structure and physical properties.5-8 Layered 2D perovskites have been known for decades and they have been employed in light-emitting diodes (LEDs),9,10 solar cells,1,2,4 and photodetectors.11,12 Despite the outstanding performance of leadbased 2D perovskites, environmental toxicity caused by lead continues to remain the main obstacle hampering their commercial utilization. To develop alternative 2D lead-free perovskites, most efforts are focused on tin-based 2D perovskites, and significant progress has been made in this regard.13-16 However, although germanium is another Group IVA element that has an electronic configuration similar to those of lead and tin, very few examples of 2D germanium-based perovskites have been reported.17-19 Our recent work has shown that 2D (PEA)2GeI4 is a direct bandgap perovskite with a bandgap of 2.12 eV, implying that it has the potential for photovoltaic applications.19 Considering that atomic substitution is an effective way for semiconductor band gap engineering,20,21 it is reasonable to assume that Sn-based alloying would help in further reducing the bandgap of 2D Ge-based perovskites, thereby, improving the light-absorption capacity of the material. Herein, for the first time, we reported a series of lead-free, two dimensional mixed germanium and tin perovskites. The bandgaps of these alloyed compounds, (PEA)2Ge1-xSnxI4, decrease linearly with increasing tin content, leading to a tuning range of the bandgap from 2.13 eV to 1.95 eV. Amongst the perovskites reported, (PEA)2Ge0.5Sn0.5I4 has the smallest bandgap and exhibits room-temperature photoluminescence (PL) with a lifetime over 2 ns. In addition,


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incorporation of Sn in 2D Ge-based perovskite can enhance its conductivity, thus may provide an alternative mean to further improve material performance in photovoltaic and photoelectric devices. As (PEA)2GeI4 and (PEA)2SnI4 are 2D layered perovskites, their crystal structure can be described as depicted in Figure 1a, wherein inorganic layers of the corner-sharing [MI6]4-

Figure 1. (a) General crystal structure of 2D (PEA)2GeI4 and (PEA)2SnI4. (b) XRD patterns of the compounds with different Sn content. (c) EDS elemental mapping of Ge, Sn, and I elements in the (PEA)2Ge0.5Sn0.5I4 perovskite single crystal.


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( M=Ge or Sn) octahedra are confined between interdigitating bilayers of PEA cations. When a moderate amount of Sn is incorporated into (PEA)2GeI4, the products remain its original structure, as shown with the PXRD in Figure 1b. The shift of the diffraction peaks to lower 2θ values is as expected, since Ge2+ cations are partially substituted with larger Sn2+ cations. However, instead of forming continuous solid solutions, Sn can only replace about 50% Ge before phase separation occurs in the 2D mixed Ge–Sn perovskites (Figure S1). The origin of this phenomenon may considered to be the large difference in size between Ge2+ (0.73 Å)22 and Sn2+ (1.35 Å)23 cations, thereby resulting in significant lattice perturbations when tin content exceeds a certain level in mixed Ge–Sn perovskite. Scanning electron microscope (SEM) images (Figure S2) of the (PEA)2GexSn1-xI4 perovskite crystals demonstrate that these compounds comprise stacks of sheets, confirming that they all possess layered structure, which is in agreement with PXRD data. To estimate the value of x in (PEA)2GexSn1-xI4, energy dispersive spectroscopy (EDS) was performed, the results of which are presented in Table S1. Experimental x values are very close to nominal ones. This indicates that the synthetic procedure adopted in this study was capable of exercising acceptable control over metal stoichiometry. For convenience, we considered use of stoichiometric ratios throughout the study. In addition, EDS element mapping of (PEA)2Ge0.5Sn0.5I4 is depicted in Figure 1c, wherein the homogeneous distribution of Ge, Sn and I implies that 2D mixed Ge–Sn perovskites are single-phase alloying compounds. In order to determine optical bandgaps of (PEA)2Ge1-xSnx (x = 0, 0.125, 0.25, 0.5), diffuse reflectance UV-vis spectroscopy measurements were performed, corresponding Tauc plots are presented in Figure 2a. Bandgap values were obtained by assuming that these mixed Ge–Sn perovskites are direct bandgap semiconductors, which could be verified via subsequent DFT


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calculations. The bandgap of (PEA)2Ge1-xSnxI4 decreases from 2.13 eV to 1.95 eV with a corresponding increase in x from 0 to 0.5. The change in sample color was found to be in agreement with the trend in bandgap shift, as depicted in Figure 2b. Bandgap variation as a

Figure 2. (a) Tauc plots of (PEA)2Ge1-xSnxI4 (x = 0, 0.125, 0.25, 0.5). (b) Photos of the compounds with different Sn content. (c) The variation of bandgaps as a function of x. function of x is plotted in Figure 2c, which demonstrates a linear trend (Vegard’s law).24 Notably, the bandgap of (PEA)2Ge0.5Sn0.5I4 (Eg = 1.95 eV) is smaller than that of 3D CH3NH3GeI3 (Eg = 2.0 eV),25 indicating that it is suitable for light-harvesting application in a tandem solar cell.


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Furthermore, given the better stability (discussed below) of (PEA)2Ge0.5Sn0.5I4 over CH3NH3GeI3, (PEA)2Ge0.5Sn0.5I4 was chosen for in-depth investigations in this study. Generally, 2D perovskites could be considered as natural multiple-quantum-well structures with semiconducting inorganic layers acting as potential “wells” and insulating organic layers acting as “barriers”.26,27 Excitons within inorganic layers are confined and possess high binding energy, giving rise to intense room-temperature PL.28 For the 2D mixed Ge–Sn perovskites, all samples exhibit room-temperature PL, probably owing to existence of stable excitons in these layered compounds, as described above. The PL spectra of (PEA)2Ge1-xSnxI4 perovskites show emission peaks at wavelengths of 613 (x = 0), 628 (x = 0.125), 642 (x = 0.25), and 655 (x = 0.5) nm (Figure 3a), consistent with optical bandgaps. On the other hand, increasing the Sn content


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Figure 3. The PL spectra (a) and PL decay profile (b) of (PEA)2Ge1-xSnxI4 (x = 0, 0.125, 0.25, 0.5). leads to a decrease in full width at half-maximum (fwhm). For the (PEA)2Ge1-xSnxI4 perovskites, (PEA)2GeI4 (x = 0) demonstrates broadband PL emission with a fwhm of 98.4 nm, presumably originating from large lattice deformation associated with strong electron–phonon coupling.29,30 When Ge is partly substituted with Sn, the alloyed compounds show slightly narrow peak widths (x = 0: fwhm = 98.4 nm; x = 0.125: fwhm = 84.9 nm; x = 0.25: fwhm = 84.8 nm; x = 0.5: fwhm = 84.1 nm). One possible origin of this phenomenon is the more pronounced distortion of [GeI6]4- octahedron than [SnI6]4- octahedron,17,31 leading to the narrowing of PL emission in Snincorporated perovskites.30 Time-resolved PL data was obtained using a time-correlated single-photon counting (TCSPC) setup, as shown in Figure 3b. Fitting the decay traces produced two processes: a short-lifetime process (x = 0: τ1 = 468 ps; x = 0.125: τ1 = 241 ps; x = 0.25: τ1 = 222 ps; x = 0.5: τ1 = 340 ps) and a long-lifetime process (x = 0: τ2 = 1.96 ns; x = 0.125: τ2 = 1.63 ns; x = 0.25: τ2 = 1.74 ns; x = 0.5: τ2 = 2.23 ns). The short-lifetime process may arise from charge-carrier trapping, while the long-lifetime process can be assigned to the exciton recombination.32-34 Analysis of the shortlived process was limited by the instrument response function (400 ps). The long-lifetime process over 2 ns for (PEA)2Ge0.5Sn0.5I4 indicates that this compound is worth further studying as a new light-harvesting material.35,36 Furthermore, the lifetime could be increased by better controlling the oxidation state of Sn and Ge generated during the synthesis and measurement processes (Figure S3). To further understand the evolution of electronic properties in the samples used in this study, we investigated the electronic band structure and local density of states (LDOS) for the x = 0 and


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x = 0.5 compositions using DFT with hybrid functional Heyd–Scuseria–Ernzerh of (HSE06). As shown in Figure 4, (PEA)2GeI4 and (PEA)2Ge0.5Sn0.5I4 are direct bandgap semiconductors with

Firure 4. Computed band structure and corresponding DOS of (a) (PEA)2GeI4 and (b) (PEA)2Ge0.5Sn0.5I4. both the conduction band minimum (CBM) and valence band maximum (VBM) being located at the Γ point. The computed bandgaps are 2.17 eV for (PEA)2GeI4, 1.82 eV for (PEA)2Ge0.5Sn0.5I4, respectively, in agreement with experimental bandgap values. The dispersion of the top valence band and bottom conduction band for (PEA)2Ge0.5Sn0.5I4 was found to be larger compared to that of (PEA)2GeI4, indicating higher carrier mobility in the mixed Ge–Sn perovskite. Interestingly, with the incorporation of Sn in the perovskite, two conduction bands were seen to converge at the Γ point. Figure 4 also shows the contribution of different atoms in LDOS. For (PEA)2GeI4, the Ge2+ cation predominantly contributes to the bottom conduction bands, whereas Ipredominantly contributes to the top valence bands. Incorporation of Sn introduces new Sn2+-


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derived valence band and conduction band, thereby casting an obvious influence on PDOS. Our results suggest that replacement of the metal component is an effective way to modify the electronic properties of 2D perovskites. Electronic conductivity of the mixed Ge–Sn perovskite was carried out by Hall effect measurements

with

pressed

pellets

of the

powder samples.

The conductivity of

(PEA)2Ge0.5Sn0.5I4 is about 1.1 × 10-3 S/cm, which is in the range of typical semiconductor materials and is comparable to that of other layered tin-based perovskites.37 As a comparison, the conductivity of unsubstituted (PEA)2GeI4 was too weak to be detected under the same condition. The improved conductivity in the mixed Ge–Sn perovskite indicates that despite its layered nature, charge mobility in this compound is sufficient for use in photovoltaic devices. The thermal stability of the mixed Ge–Sn perovskite was investigated by thermogravimetric analysis (TGA), and the corresponding curve is plotted in Figure S4. For (PEA)2Ge0.5Sn0.5I4, TGA results demonstrate high stability up to 250 ºC, which is stable enough in the range of device operating temperature.38 To investigate the environmental stability of the mixed Ge–Sn perovskite, we exposed the 2D (PEA)2Ge0.5Sn0.5I4 and 3D CH3NH3GeI3 to ambient air (50% relative humidity and 25ºC temperature, Figure S5). After 15 hours’ storage, CH3NH3GeI3 showed the diffraction peaks of GeI4, as a result of oxidation. In contrast, the PXRD of (PEA)2Ge0.5Sn0.5I4 remained almost unchanged. The enhanced moisture stability of 2D mixed Ge–Sn perovskite may be attributed to the existence of hydrophobic PEA cations, which prevent degradation and oxidation of the perovskite, as have been demonstrated in other 2D perovskites.2,4 In summary, we have successfully synthesized a series of 2D mixed Ge–Sn perovskites, (PEA)2Ge1-xSnxI4 (x = 0, 0.125, 0.25, 0.5), and demonstrated that the bandgaps of these


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compounds decrease linearly with the increase of x. Amongst them, (PEA)2Ge0.5Sn0.5I4 was found to possess the smallest bandgap of 1.95 eV, and exhibit better stability compared to the 3D Ge-based perovskite (CH3NH3GeI3), thereby demonstrating the potential for photovoltaic applications. We find that (PEA)2Ge0.5Sn0.5I4 emitted PL at room-temperature with a longlifetime over 2 ns, indicating that it is worth further investigating as a new light-harvester. In addition, DFT calculations confirmed the narrowing of bandgaps caused by Sn incorporation. These results highlight the effect of atomic substitution on optoelectronic properties of lead-free 2D perovskites. ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We are grateful to the National Natural Science Foundation of China (Grant No: 21533010), the National Key Research and Development Program of China (Grant 2017YFA0204800), DICP DMTO201601, DICP ZZBS201703, the Science Challenging Program (JCKY2016212A501). Supporting Information. XRD data of (PEA)2Ge1-xSnxI4 with x exceeding 0.5, summary of the EDS results, SEM images of (PEA)2Ge1-xSnxI4 perovskite crystals, XPS narrow scans of (PEA)2Ge0.5Sn0.5I4 for Ge 2p and


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Sn 3d,

Thermogravimetric analysis (TGA)

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data of (PEA)2Ge0.5Sn0.5I4, XRD data of

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