First-Principles Screening of Lead-Free Methylammonium Metal

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First-Principles Screening of Lead-Free Methylammonium Metal Iodine Perovskites for Photovoltaic Application Lei Jiang, Tao Wu, Lei Sun, Yajuan Li, Ai-Long Li, Rui-Feng Lu, Kun Zou, and Weiqiao Deng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04685 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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The Journal of Physical Chemistry

First-Principles

Screening

of

Lead-Free

Methylammonium Metal Iodine Perovskites for Photovoltaic Application Lei Jiang, †, ǁ Tao Wu†, ‡, ǁ, Lei Sun, † Ya-Juan Li, †, § Ai-Long Li, §, ⊥ Rui-Feng Lu, ‡ Kun Zou, ¶ and Wei-Qiao Deng*, † †

State Key Laboratory of Molecular Reaction Dynamics, Dalian National Laboratory for Clean

Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China. E-mail: [email protected] ‡

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

210094, P. R. China §

University of Chinese Academy of Sciences, Beijing 100049, P.R. China

⊥

State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian

Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China ¶

Hubei Key Laboratory of Natural Products Research and Development, College of Biology and

Pharmacy, China Three Gorges University, Yichang 443002, P. R. China

ABSTRACT. Although organo-halide lead perovskite solar cells have achieved outstanding photovoltaic performance in recent years, environmental concern about lead pollutant limits its large-scale commercialization. To address lead-free organo-halide perovskite materials, here we

ACS Paragon Plus Environment

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report first-principles screening of different perovskite materials MAXI3 with consideration of all possible (27 in total) divalent metal ions. Ten kinds of perovskite structures are obtained with a non-zero bandgap, and the others are metallic. Among them, five kinds of perovskite materials (MAXI3: X = Pb, Sn, Ge, Cd, Be) have appropriate bandgaps as the light-absorbing material in solar cells, in which MACdI3 and MABeI3 are reported for the first time. Various corresponding experiments have been carried out to confirm our theoretical predictions.

INTRODUCTION With the resource limitation of fossil fuel, the growing energy demand as well as the greenhouse gas CO2 emission, sustainable and clean energy is immensely important to preserve the economic and environmentally sustainable development. Photovoltaic (PV) technology is considered to play a major role to meet future clean energy demand. However, efficient harvesting and conversion of solar energy to electrical energy are urgent technical problems in PV cells. The breakthrough of recently reported organic-inorganic halide perovskite solar cell CH3NH3PbI3 (henceforth MAPbI3) is considered to be the revolutionary progress of PV technology. This main outstanding features of MAPbI3 include high absorption coefficient (~ 103–106 cm-1), solution processability, cheap and abundant starting materials, and proper direct bandgap (~1.50 eV).1-14 Besides, MAPbI3 owns really large diffusion length, which is beyond 175 µm and high carrier mobility with electron mobility about 24 cm2V-1s-1 and hole mobility about 105 cm2V-1s-1 in a single crystal.15 With all these excellent characters, high power conversion efficiency (PCE) was obtained when MAPbI3 was applied in PV solar cells. The efficiency of the device based on MAPbI3 has been increased from 3.8% to 22.1% in recent few years and the applications of perovskites have been expanded to field-effect light-emitting


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transistors (FETs),16 lasers,17 and light-emitting diodes (LEDs).18 Despite MAPbI3 has extremely outstanding performance in solar cells and various applications, current challenges associate with MAPbI3 are the instability of the lattice at the atmospheric environment and the toxicity of Pb which limits its large-scale application if there is no breakthrough in preserve technology. It is particularly gratifying that Kanatzidis first reported the solid-state perovskite solar cells based on methylammonium tin iodide (CH3NH3SnI3) perovskite semiconductor, which is an important progress for lead-free perovskite solar cells preparation.19 Though MASnI3 has a proper bandgap (about 1.3 eV) and the efficiency of the MASnI3 solar cell has exceeded to 6%, the problem is the poor atmospheric stability and a poisonous reactant of SnI2, which depress the reproducibility of the devices and limits their practical application.20,21 It is worth noting that Kanatzidis’ group has reported a series of hybrid germanium iodide perovskite semiconductors which have active lone pairs, structural distortions, direct and indirect energy gaps, and strong nonlinear optical properties. But the hybrid germanium iodide perovskites also meet the stability problem.22 Apart from that, developments of Bi3+ and Sb3+ related double perovskites both in theory and experiment have also been achieved in recent years.23-26 However, all of those perovskite face the problem of stability or low power conversion efficiency. Thus, it is still important to find the proper candidate from lots of perovskite structures. The properties determining the usefulness of a semiconductor as the harvester in photovoltaic cells include: (i) a proper bandgap which has significant absorption of light. (ii) high carrier mobilities and good charge separation which allow electrons and holes reach the contact surface. Semiconductors which match both requirements always show decent energy conversion efficiency when applied to the photovoltaic cells. However, huge amounts of semiconductor materials are difficult to be detailed studied in the experiment. Thanks for the


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rapid development of computing power, it makes high-throughput screening possible and greatly shortens the synthesis time to find target materials. Till now, there are several research on the inorganic or organic-inorganic hybrid perovskite structures by a high-throughput method with the

standard

DFT

calculation.

However,

the

predictions

of

bandgap

are

always

underestimated.27,28 Herein, we use an efficient computational screening of perovskites solar cells (CH3NH3XI3) with all the possible divalent metal ions in the element periodic table (except the radioactive elements) based on the bandgaps, and the calculations of carrier mobility can be further performed by the deformation potential method29,30 based on Boltzmann equation which is not in our consideration. In our bandgap calculations, the explicitly derivative discontinuity has been considered to provide more accurate estimates of bandgaps with respect to the standard DFT calculations.31 Besides, we also process the spin-orbit coupling (SOC) calculations to correct the bandgaps. Our prediction shows that most of the perovskite structures are metallic and only ten of them are semiconductors. And five kinds of perovskite materials (MAXI3: X = Pb, Sn, Ge, Cd, Be) have appropriate bandgaps as the light-absorbing material. Apart from the MAPbI3 material, the MA(Sn, Ge, Cd, Be)I3 are also potential candidates for perovskite solar cells. Moreover, four perovskite materials: MAPbI3, MASnI3, MAZnI3, and MAGeI3 have been synthesized experimentally to compare with theoretical result. All the optical bandgaps are consistent well with theoretical values which show that our simulation for the screening of leadfree methylammonium metal iodide perovskite is efficient and quality.

EXPERIMENTAL AND CALCULATIONAL SECTIONS Materials. GeI2 (99.99%, trace metals basis), PbI2 (99%), ZnI2 (99%), SnI2 (99%) solids and CH3NH2 40% aqueous solutions, HI (57%) were purchased from Sigma Aldrich or Alfa.


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Tetrathiafulvalene derivative (TTF-1, 99%) was purchased from TCI. CH3NH3I was synthesized according to a reported procedure.5 Synthesis. Equimolar amounts of MAI and MI2 were loaded in a pyrex tube (3×15cm) under an inert atmosphere. Typically, the scale is 2 mmol based on the Ge, Zn, Sn or Pb halide. For example, synthesis of CH3NH3GeI3. GeI2 solids (2mmol, 654mg, 99.99%) and CH3NH3I (2mmol, 318mg) were placed in an agate mortar and ground carefully with a pestle until a visually homogeneous mixture, and then added in a pyrex tube, then the tube was evacuated, sealed and heated to 200℃ for 2h to ensure the mixture of solids was heated homogeneously. The tube was then cooled down to room temperature and opened. The CH3NH3GeI3 solids were left forming a homogeneous modena solids. The Sn, Ge, Zn containing solids are air sensitive. Characterization. XRD patterns were recorded by X-ray powder diffraction (XRD, Rigaku D/max-2500/PC) with a monochromatized source of Cu Kα radiation (λ = 0.15406 nm). The morphology and structure of the prepared samples were observed by scanning electron microscopy (SEM, QUANTA 200 FEG) and transmission electron microscopy (TEM, TECNAI G2 SPIRIT). Optical diffuse-reflectance measurements were performed using a Shimadzu JASCO V-550 UV-vis spectrometer. Theoretical calculation sections. Structure optimization is the first and the most important step in quantum chemistry calculation. In this study, all structure optimizations are calculated by generalized gradient approximation (GGA) with density functional theory (DFT) implemented in the Vienna ab initio simulation package (VASP).32,33 Projector augmented-wave method (PAW) is applied to describe the interaction between the electron and ion.34 It is proven successful in predicting total-energy-related properties of many-electron systems, such as crystal structures and molecular geometries. The electron exchange-correlation is chosen to be Perdew-Burke-


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Ernzerhof (PBE) functional.35 In particular, the kinetic energy cutoff for plane wave basis set was set to 400 eV. The Monkhorst-Pack k point sampling used in our calculation is 7×7×7. As known to all, standard DFT calculations with Kohn-Sham bandgap are always seriously underrate actual bandgaps.36 The general approach to solve this problem is the use of hybrid functional or of many-body perturbation method based on Green’s functional. However, both methods consume a large amount of computation time which is not appropriate for screening calculations. Herein, we use the GLLB-SC model potential method implemented in GPAW code37-40 to predict the magnitudes of bandgaps of all the optimized perovskites structures. In this method, excess derivative discontinuity (∆xc) is considered based on Kohn-Sham bandgap to obtain the quasi- particle bandgap which includes two parts: Kohn-Sham bandgap and

derivative discontinuity ∆xc. Besides, for accurate description of bandgaps, spin-orbit coupling (SOC) calculations are also considered in our calculation as ∆SOC which can be obtained from the VASP calculation. The SOC calculation method is suited for heavy elements, for light elements SOC values approximate to zero. Thus, the calculated bandgap can be expressed as follow: KS Egap = Egap + ∆ xc − ∆ SOC

(1)

Ivano E Castelli et al. have previously used this method to predict the bandgap of hybrid organic-inorganic perovskites which was consistently well with the experiment.41 Apart from that, the GLLB-SC model potential method can also greatly shorten the computing time with respect to the many-body perturbation theory such as GW method which has been simply tested in the Supporting Information.

RESULTS AND DISCUSSION


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The organic-inorganic hybrid perovskite solar cell has a definite crystal structure with ABX3 formula. A denotes for the organic like methylammonium (MA; [CH3NH3]+), formamidinium (FA; [HC(NH2)2]+), and larger guanidinium (GA; [C(NH2)3]+), B denotes for the divalent metal ions, X denotes for the halogen element (Cl, Br, I). As for the organic cation FA and GA, the perovskite phase structure will be more complicated. In order to simplify the calculation procedure, we adopt the MA as the organic cation and I as the halogen element. As shown in Figure 1, three common symmetries of perovskites (Cubic, Tetragonal and orthorhombic phases with space groups: , / and ) were chosen as starting structure for this study. Most divalent metal elements except radioactive and rare earth elements are considered from the periodic table. We build the crystal structures of three phases MAXI3 (X = Pb, Sn ...) and the lattice constants of the initial structures are guessed according to the metallic iodides for converging to a proper structure.

Figure 1. The diagram of different phases: cubic (left), tetragonal (middle) and orthorhombic (right). The cyan ball represents the different divalent metals.


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Figure 2. The bandgaps of ten perovskite materials with non-zero bandgaps. The different divalent metal ions are shown in the horizontal ordinate and X represent other 17 kinds of metal in our study including Ag, Co, Cr, Cu, Fe, Ir, Mn, Mo, Ni, Os, Pd, Pt, Rh, Ru, Ti, V, W. From the periodic table, we consider all the possible divalent metal ions (27 in total) include group II elements: Be, Mg, Ca, Sr, Ba; group IV elements: Ge, Sn, Pb and transition metals: Ag, Cd, Co, Cr, Cu, Fe, Ir, Mn, Mo, Ni, Os, Pd, Pt, Rh, Ru, Ti, V, W, Zn. According to our calculation, most of the perovskite materials based on the transition metals are metallic except for Cd and Zn. All the perovskite materials based on the main group metals are nonmetallic with the wide bandgap ranging from 0.91 to 6.87 eV. The bandgaps of these ten nonmetallic perovskite structures are shown in Figure 2. It is obvious that the bandgaps of tetragonal phases are a little bit larger than the corresponding cubic phases except MAGeI3 and MAZnI3. And the bandgaps of orthorhombic phases seem do not change much compared with other two phases except MABeI3. The bandgap of orthorhombic MABeI3 is as high as 4.57 eV, which is caused by its large structure distortion and its positive formation energy also indicates the orthorhombic MABeI3 is unstable with respect to other perovskite structures.


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Table 1. The formation energy ∆ (eV/atom), Kohn-sham bandgap (eV), derivative

discontinuity ∆ (eV), bandgap (eV) and gap type of all ten perovskite structures with cubic, tetragonal and orthorhombic phase, respectively. MAXI3

MABaI3

MASrI3

MACaI3

MAMgI3

MAZnI3

MAGeI3

MAPbI3

MACdI3

phase

∆

cubic

-0.28

4.71

2.29

0.18

6.82

direct

tetragonal

-0.07

4.89

2.20

0.22

6.87

direct

orthorhombic

-0.09

4.83

2.25

0.28

6.80

direct

cubic

-0.20

4.59

2.13

0.18

6.54

direct

∆

∆

gap type

tetragonal

-0.09

4.88

2.16

0.21

6.83

direct

orthorhombic

-0.09

4.79

2.18

0.28

6.69

direct

cubic

-0.21

4.44

1.97

0.17

6.25

direct

tetragonal

-0.08

4.60

1.90

0.22

6.28

direct

orthorhombic

-0.09

4.63

2.10

0.22

6.51

direct

cubic

-0.19

2.43

1.07

0.19

3.31

indirect

tetragonal

-0.02

2.91

1.26

0.18

3.99

direct

orthorhombic

-0.06

2.54

1.16

0.18

3.52

indirect

cubic

-0.23

2.54

1.22

0.22

3.54

direct

tetragonal

-0.09

1.93

0.94

0.15

2.72

direct

orthorhombic

-0.08

1.71

0.78

0.16

2.33

direct

cubic

-0.08

1.40

0.60

0.13

1.87

direct

tetragonal

-0.03

1.37

0.57

0.17

1.77

direct

orthorhombic

-0.08

0.94

0.43

0.25

1.12

direct

cubic

-0.20

1.77

0.81

1.01

1.57

direct

tetragonal

-0.07

1.93

0.80

1.03

1.70

direct

orthorhombic

-0.09

1.95

0.84

0.98

1.77

direct

cubic

-0.20

0.85

0.42

0.16

1.11

indirect


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MASnI3

MABeI3

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tetragonal

-0.08

1.22

0.60

0.13

1.69

direct

orthorhombic

-0.08

0.74

0.34

0.09

1.01

direct

cubic

-0.21

0.95

0.40

0.29

1.05

direct

tetragonal

-0.09

1.22

0.52

0.29

1.45

direct

orthorhombic

-0.10

0.86

0.39

0.27

0.98

direct

cubic

-0.11

0.59

0.33

0.01

0.91

indirect

tetragonal

0.08

0.83

0.44

0

1.27

indirect

orthorhombic

0.01

3.24

1.47

0.14

4.57

indirect

From the Table 1. only five perovskite structural bandgaps (MAXI3, X = Pb, Sn, Ge, Cd, Be) are smaller than 2.0 eV, which are the proper bandgaps for applied in the solar cell. And the bandgaps of the rest five perovskite structures are larger than 3.0 eV which should not be good candidates for photovoltaics. The bandgaps of the MAPbI3 are 1.57 eV and 1.70 eV for cubic and tetragonal phases, respectively, which are consistently well with experimental results.1 Besides, the bandgaps of the MASnI3 are 1.05 eV and 1.45 eV for cubic and tetragonal phases, respectively, which are also consistent with the experimental result and other theoretical work.19,42 And the bandgaps of the MAGeI3 are 1.87 eV and 1.77 eV for cubic and tetragonal phases. The values are larger than the previous result of PBE calculation (1.33 eV),43 and consistent well with the result of HSE-SOC calculation (2.04 eV)44 and the experimental result (1.90 eV).22 These results indicate that our calculation is accurate enough and this method is working to calculate the bandgap of the perovskite materials. To discuss the stability of these ten perovskite structures, we performed the formation energy calculation as follow: E = EMAXI3 − EMAI − EXI2

(2)


10

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where is the total energy of MAI in gas the phase, which increases the stability of molecular and decreases the stability of perovskite when compared with MAI in bulk. is the total energies of different XI2 compounds which comes from Open Quantum Materials Database. We choose the most stable XI2 compounds as the reference states with space group: BaI2 (Pnma), SrI2 (Pnma), CaI2 (P-3m1), MgI2 (P-3m1), ZnI2 (C2/m), GeI2 (C2/m), PbI2 (P63mc), CdI2 (P1), SnI2 (C2/m), BeI2 (Ibam). The calculated formation energies (∆Ef) of all ten perovskites with different phase are listed in Table 1. All of ∆Ef are negative except for the one of MABeI3 with tetragonal and orthorhombic phases, which indicates the tetragonal and orthorhombic phases of MABeI3 are slightly unstable comparing with other perovskites.

Figure 3. Experimental (red) and calculated (black) X-ray diffraction pattern for different perovskite structures: (a) MAPbI3, (b) MASnI3, (c) MAGeI3 and (d) MAZnI3 perovskite.


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Table 2. Comparisons of our calculated bandgaps with experiment results. MAXI3

Cubic-Cal.

Tetragonal-Cal.

Cubic-Exp.-by our group

Cubic-Exp.-by references

Pb

1.57 eV

1.70 eV

1.48 eV

1.50 eV1

Sn

1.05 eV

1.45 eV

1.37 eV

1.30 eV19, 42

Ge

1.87 eV

1.77 eV

1.88 eV

1.90 eV22

Be

0.92 eV

1.27 eV

-

Cd

1.27 eV

1.69 eV

-

As for the MAPbI3, MASnI3 and MAGeI3 have been reported as the materials of the solar cell, thus MACdI3 and MABeI3 are the potential candidate with proper bandgap (Cubic 1.27 eV, 0.92 eV, Tetragonal 1.69 eV, 1.27 eV) for lead-free light-absorbing materials. For comparison, we synthesize four perovskite materials from the proposed structures: MAPbI3, MASnI3, MAZnI3 and MAGeI3. The synthesis of perovskite materials can be accomplished using solid state reactions, the morphology of prepared perovskite samples was shown in Figure S1, S2. The Xray diffraction (XRD) of these perovskites powders are presented in Figure 3. The experimental (red) and calculated (black) XRD pattern for MAPbI3, MASnI3, MAGeI3 and MAZnI3, respectively, and the experimental crystalline phase were in good agreement with the calculated results. The MAZnI3 elemental analysis was implemented using Energy Dispersive X-ray Spectroscopy (EDS), and the corresponding results were shown in Figure S3. The atomic ratio of Zn and I was approximately 1:3, which is in good agreement with the stoichiometry. UV-vis absorption spectra of the MAGeI3, MAZnI3, MAPbI3 and MASnI3 powder are shown in Figure S4. The optical bandgaps are estimated to be around 1.88 eV, 3.8 eV, 1.48 eV, 1.37 eV for the MAGeI3, MAZnI3, MAPbI3, MASnI3 perovskites, respectively (Figure S5), which were good


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consistent with the calculated results (1.87 eV, 3.76 eV, 1.57eV, 1.05 eV) completely (listed in Table 2). These experimental results indicate that our calculation method is accurate enough for the screening of candidate lead-free perovskite materials. The MACdI3 perovskite materials can not be synthesized through the existing solid state reactions methods. Then we try to use the solution method to synthesize MACdI3, but still not obtain the target product. The MABeI3 perovskite materials have not been synthesized, because the raw materials (BeI2) could not be purchased. So we synthesized the MAZnI3 perovskite materials (not reported before) to further validate the accuracy and feasibility of our theoretical predictions. CONCLUSIONS In summary, we use the first-principle method to screen the bandgaps of perovskite materials for solar cell based on all 27 possible divalent metal ions with cubic, tetragonal and orthorhombic phases, respectively. And only five kinds of perovskite materials (MAXI3: X = Pb, Sn, Ge, Cd, Be) have appropriate bandgaps for perovskite solar cells. Therefore, four kinds elements (Sn, Ge, Cd, Be) are candidates with proper bandgaps for lead-free perovskite solar cells. Series of experiments have been carried out to validate the predicted results.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Elapsed time calculation, SEM images of synthesized perovskites samples, Corresponding EDS image of MAZnI3, UV-Vis absorption spectrum and bandgaps of these perovskites samples (file type, i.e., PDF) AUTHOR INFORMATION


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Corresponding Author * E-mail: [email protected] Author Contributions ǁ

These authors contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Key R@D Program of China (Grant 2017YFA0204800), the National Natural Science Foundation of China (Grants 21525315, 91333116 and 21403211). REFERENCES (1)

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