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CH3NH2BiI3 Perovskites - A New Route to Efficient Lead Free Solar Cells Zhuo Wang, Binglong Lei, Xiaohong Xia, Zhongbing Huang, Kevin Peter Homewood, and Yun Gao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11849 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

CH3NH2BiI3 Perovskites - A New Route to Efficient Lead Free Solar Cells Zhuo Wang2,3, Binglong Lei1, Xiaohong Xia1*, Zhongbing Huang1, Kevin Peter Homewood1, Yun Gao1* 1

Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional

Materials, Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, School of Materials Science & Engineering, Hubei University, Wuhan 430062, China 2

State Center for International Cooperation on Designer Low-carbon & Environmental

Materials, Zhengzhou University, 100 Kexue Avenue, Zhengzhou 450001, China 3

Zhengzhou Materials Genome Institute, Zhongyuanzhigu, Xingyang 450100, China

Corresponding Authors *[email protected], *[email protected]

ACS Paragon Plus Environment

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ABSTRACT:

CH3NH3PbI3 based perovskite solar cells have achieved great success in the last several years. However their further large scale application is hindered by the toxicity of the Pb. Here, the existence of a stable perovskite structure with close to optimum optical properties for solar cells with Pb replaced by Bi was shown by theoretical modelling. It was found that the cubic perovskite BiI3 framework is maintained only when the polar organic group of CH3NH3+ is dissociated into neutral CH3NH2 molecule and hydrogen. The band gap of the neutral molecular filled CH3NH2BiI3 perovskite structure gives a value of 1.61eV which matches closely the solar spectrum with a maximum possible efficiency of 28.5 % close to the Shockley-Queisser limit of 33% for a single stage cell, offering a new and promising lead free route for perovskite solar cells.


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1. INTRODUCTION

CH3NH3PbI3 perovskite material has attracted much attention since the first report of its application in solar cells. The photovoltaic efficiency of CH3NH3PbI3 perovskite solar cell has kept increasing ever since and achieved a certified 21.2% by April of 2017.1 The excellent performance of the CH3NH3PbI3 perovskite solar cell is mainly due to two advantages of the perovskite material. One is the suitable band gap of around 1.5 eV, close to the ShockleyQuiesser limit, 1.4eV, which enables the absorption and efficient use of a large part of the solar spectra from visible to the near infrared region (320-1000 nm).2 The other advantage is the large diffusion length of charge carriers which is greater than 175µm3 and its light effective masses.4-5 Although the functional properties of CH3NH3PbI3 are outstanding the toxicity of Pb impedes its large scale application. Thus, it is urgent to explore environment-friendly replacements for Pb while retaining the excellent performance. Many attempts have been made to achieve this by substituting Pb2+ with Sn2+, Sr2+, Cd2+ and Ca2+, 6 as well as Ge2+.7 Among which, Sn2+, an element from the same group of Pb has the advantage of easy fabrication and low cost. However, Sn2+ is chemical active and easily oxidizes,8 greatly restricting its photovoltaic applications. As an alternative, Bi with three valence electrons has been considered for several reasons as a suitable candidate to substitute Pb. Firstly, the atomic radius of Bi is close to that of Pb making it possible to sustain the perovskite configuration after substitution of Pb by Bi. Secondly, the BiI3 crystal has an octahedral microstructure which could generate a similar perovskite structure. Thirdly, as Bi is also a heavy element like Pb, it should maintain the original light effective mass of carriers after substitution. In previous reports on Bi substitution, most of the absorbers are (CH3NH3)3Bi2I9,9-10 in which


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Bi2I93- network presents as a cluster structure.10 Recently, our group has successfully prepared (CH3NH3)3Bi2I9 film-based solar cells. However, the efficiency is low, only 1.64%. This may be partly caused by the discontinuous cluster structure of (CH3NH3)3Bi2I9,11 disrupting continuous carrier transport. Cubic phase lead-free halide double perovskite has also been reported, in which Pb2+ is substituted by Bi3+ and with a low valence metal ion (Ag+, Cu+, or K+). However, the double perovskite has a separated conduction band that is isolated far away from the conduction bands.12-14 The limited number of conduction band may have a problem of containing abundant excited electrons, or those isolated bands would be considered as impurity bands lying in the band gap area. Besides, all of the double perovskite present an indirect band gap larger than 2.0 eV, which lowers the maximum efficiency from 33% to 21% even for an otherwise perfect cell. In this sense, Bi substitution of Pb to produce an efficient Bi based perovskite stills requires significant improvement. In this work, density functional theory (DFT) modelling was adopted to help searching for the optimized cubic perovskite material structure with complete replacement of Pb by Bi. The perovskite structure of CH3NH3PbI3 is constructed with the inorganic network (PbI3−) frame and the organic molecule filling (CH3NH3+). The electron states of the heavy elements (Pb and I) both contribute components to the conduction and valence bands density of states15-18 that determines the optical and electrical properties. In order to obtain accurate effective masses of the carriers, the spin orbit coupling (SOC) must be taken into consideration. However, SOC was usually neglected in early DFT calculations for the band gap of perovskite.16,19 When SOC was taken into account, the band gap estimated by generalized gradient approximation (GGA) is about 0.53eV for tetragonal CH3NH3PbI320, much smaller than the experimental results. Subsequently, many-body GW-SOC calculations21-22 and hybrid functional calculations


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including HSE0623,24 and PBE025,26 plus SOC15 have been adopted to rectify the underestimation of the band gap energy. PBE0+SOC with an optimal fraction 0.188 for the exchange energy coefficient was used to predict the band gap of orthorhombic CH3NH3PbI3 perovskite as 1.63eV15 close to the experimental values. Since the hybrid functional method has the advantage of memory saving and is more versatile than the GW method, PBE0+SOC has been used in this work. We chose CsxBiyIz to search the best cubic framework first due to the fact that the CH3NH3+ group could not supply spherical-like support to the BiyIz frame as the rotation state of CH3NH3+ could not be expressed in the DFT at 0K. Direct simulation with the structure of CH3NH3BiyIz will bring structural and energetic derivation in the global searching of USPEX. Meanwhile, CH3NH3+ is the metastable state, the most stable state of this organic group is CH3NH2+H+, so we choose Cs+ to stabilize the framework first and then put the CH3NH3+ in. It was found that the inserting of CH3NH3+ indeed has brought structural changes, moreover, the two neighbor CH3NH3+ groups incline to approach to each other and produce H2 and neutral CH3NH2 molecules. 2. METHODS Theoretical calculations are performed using the Vienna Abinitio Simulation Package (VASP),27-28 with the ionic potentials including the effect of core electrons being described by the projector augmented wave (PAW) method.29-30 The orbitals of 5d106s26p2, 5s25p5, 2s22p3, 2s22p2, 1s1, and 5d106s26p3 are explicitly treated as valence electrons for Pb, I, N, C, H, and Bi. In geometric relaxation part, the Perdew−Burke−Ernzerhof (PBE) GGA exchange−correlation functional30-31 combined with pairwise dispersion interactions of the Tkatchenko−Scheffler (TS) pairwise dispersion scheme32 have been used to relax the configurations. The DFT-TS method takes the long range van der Waals (vdW) contributions of atoms into account according to their


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local chemical environment. Dispersion coefficients and damping function are charge-density dependent and is beneficial for correcting the lattice parameters. We employ two steps to relax the initial configuration. In the first step, the atomic positions of the inorganic frame BiI3 are fixed and the configuration of the molecule in the interstice was sufficiently relaxed. In the second step, both cell parameters and atomic positions are optimized. The iterative relaxation of atomic positions will be stopped when the forces on the atoms are less than 0.01 eV Å−1 and a Gaussian smearing with kBT = 0.1365 eV is applied. A 4×4×4 Monkhorst–Pack grid is chosen for sampling the Brillouin zone. The valence electron wave functions are expanded on a plane wave basis with a kinetic energy cutoff of 600 eV. PBE0+SOC (parameter HFSCREEN=0.02) and HSE06+SOC (parameter HFSCREEN=0.2) are used to predict the value of band gaps. We employ convergence criteria of 10-6 eV for the electronic self-consistent cycle and the maximum cutoff equals to 600 eV. The Brillouin zone has been sampled with 3 × 3 × 3 Γ centered grid for the hybrid functional. It has shown that the 3 × 3 × 3 grid will allow the total energy to converge within 0.005eV/atom and the band gap converges within 0.01eV from testing k-points grid up to 5 × 5 × 5. The climb image nudged elastic band (CI-NEB) method with Limited-memory BroydenFletcher-Goldfarb-Shanno (LBFGS) optimizer33-35 has been used to search for the CH3NH3+ dissociation path and the saddle points. The initial (reactant) and final (product) configurations are obtained after full-relaxation. The number of inserted images used in the CI-NEB calculations depends on the reaction coordinates between reactant and product. Ab initio molecular dynamics (AIMD) simulation36 is used to investigate the CH3NH3+ dissociative trajectory in the CH3NH3BiI3 framework. The AIMD simulations are performed


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using the PBE GGA functional. To keep the computational cost at a reasonable level, smaller plane wave energy cut-offs of 400 eV have been chosen. A minimal Γ-centered 1×1×1 k-point grid is used for the 3×3×3 supercell. The time step of molecular dynamics is chosen to be 0.25 fs for a 10 ps AIMD evolution with a NVT ensemble and a Nosé–Hoover thermostat. The universal structure predictor (USPEX)37-38 based on a genetic algorithm has been employed to predict global stable or metastable structures in the phase diagram of CsxBiIy with variable-composition at a temperature of 0K and pressure of 0GPa. USPEX, in which VASP is linked to relax the initial configurations built by Matlab, performs the calculation of the enthalpy of formation. For each composition, the population of 200 possible structures is created randomly according to the possible symmetries in the first generation. As the full-relaxation finished, the most stable and metastable structures are inherited into the next generation. In the following, the new generation is created through heredity, lattice mutation and permutation operators, and the population in each new generation is set to be 60. USPEX continues screening until the most stable structure stays unchanged for the next 20 generations. The energy (enthalpy) of formation for each CsaBibIc compound is defined with respect to the chemical potentials of constituent elements as = ( − − − )/( + + ), where is the total energy for the compound CsaBibIc and chemical potentials , and are total energy per atom for Cs, B and I2 (We used the ground state stable structure as the reference for each element as listed in the Materials Project webpage https://www.materialsproject.org)


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The phonon frequency spectrum is typically used for examining the dynamical stability of a theoretically predicted structure. The super cell method in the PHONOPY package39-40 has been employed to perform the relevant frozen-phonon calculations based on the harmonic approximation. For cubic BiI3, XeBiI3 and CH3NH2BiI3, a 2 × 2 × 2 super cell has been set up for the phonon calculations. The stability criterion is that the amplitude of the imaginary frequency is less than 0.3 THz,41-42 which is caused by a known numerical error in phonon calculations. Experiments were carried out to synthesize the Bi based perovskite solar cell materials. In the glove box, BiI3 (99.99%, Aladdin reagent) and CH3NH3I are mixed at a mole ratio of 1:1 and then placed into a quartz tube, which is pumped to 1 x10-3 Pa and sealed by an oxygen-hydrogen flame gun. The sealed tube was placed into a vacuum furnace and annealed at 120 °C for 2 hours at a heating rate of 5°C /h, and then cooled to room temperature at a cooling rate of 10°C /h. The gas product was characterized by gas chromatography. Standard H2 was used as reference. Trace amounts of hydrogen were found in the gas product of the annealed CH3NH3I and BiI3 mixture. Cubic-like structure has been detected by XRD with excess CH3NH3I remains. Perfect and pure perovskite structure is expected to obtain through adjusting the experimental parameters. 3. RESULTS AND DISCUSSION

The simulation starts from a global search for possible stable structures of cubic like BiI3 with interstitial Cs ions supporting the BiI3 framework. USPEX,38, 43 a genetic algorithm, is employed for the global search of Bi-I structures at specified composition ratios (defined as I/Bi+I). As the total energy of CH3NH3+ is higher than the more stable CH3NH2+H+ at 0K, the splitting of CH3NH3+ into CH3NH2+H+ will lead to incorrect structural predictions and errors in the estimation of energy in the search process. Thus, here we put the Cs+ ion in the place of


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CH3NH3+ group as an alternative spherical-like support and charge carrier similar to that of CH3NH3+ group do44 in the interstice of the inorganic BiI3 framework. The phase diagram of the extended convex hull diagram for both stable and metastable states of CsxBiIy has been presented in Fig. 1. In detail, five different compositions are studied, (a) pure BiI3 frame work, (b) CsBiI3, (c) CsBiI4, (d) Cs3Bi2I9, and (e) Cs3BiI6. The formation enthalpies of (a), (c), (d) and (e) locates at the edge of the extended convex hull.CsBiI3 was found to have an even smaller enthalpy of formation than BiI3 under the edge of the convex hull, which suggests that it would be more energy stable than BiI3 and could be a possible existing phase.

Figure 1. Extended Convex hull phase diagram of Bi-I framework on the I-rich side with different compositions of BiI3 (a), CsBiI3 (b), CsBiI4 (c), Cs3Bi2I9 (d) and Cs3BiI6 (e). Black dot stands for the lowest enthalpy of formation of different structures, and blue dot indicates the metastable phase of different structures. The formation enthalpies of (a), (c), (d) and (e) locates at the edge of the extended convex hull. CsBiI3 was found to have a smaller enthalpy of formation than that of BiI3 under the edge of the convex hull, which suggests that it would be more energy stable than BiI3, and could be a possible existing phase.


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The most stable configurations in the extended convex hull phase diagram corresponding to different compositions as (a) BiI3 (b) CsBiI3 (c) CsBiI4 (d) Cs3Bi2I9 and (e) Cs3BiI6 are displayed in Fig. 2. The structure in the most stable phase for pure BiI3 is a two dimensional (D) layered structure with P-31M symmetry as shown in Fig. 2 (a). An obvious phase transition is seen to take place as a Cs+ is inserted, and the most stable state of CsBiI3 is found to present as a cubic structure with PM-3M symmetry as shown in Fig. 2 (b). It is a typical perovskite structure remaining in the cubic phase. This structure is conductive in all three dimensions enabling charge carrier transport freely along each direction. This configuration also provides multiple routes for carriers. CsBiI4 (Bi:I=1:4) as shown in Fig. 2 (c) presents a quasi-one-dimension structure that fits with the relevant reported chainlike BiI4-.45 As the iodine concentration continues to increase (Bi : I = 2 : 9), Cs3Bi2I9 will emerge with symmetry of P-6, in which a cluster structure is formed via two nearest neighbor Bi sharing three I atoms (Fig. 2 (d)). This microstructure is the same as the local configuration reported in reference.10 It is worth emphasizing that the cluster structure of Bi2I9- is discontinuous and quite different from the three dimensional connected perovskite structure (Fig. 2 (b)). The large blank area in the cluster structures would highly impede the charge transport. With further increase of the concentration of I (Bi : I = 1 : 6), the size of the cluster will be further reduced to the isolated octahedral configuration of BiI63- (Fig. 2 (e)). From this structure diagram, we obtain a general idea that the configuration of the product could be adjusted by controlling the source concentration of Bi and I. Cubic CsBiI3 could be produced only with strict control of the Bi and I ratio at 1:3. Further addition of iodine will cause undesired phase transitions.


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Figure 2. The most stable configurations in the extended convex hull phase diagram, top and side views of BiI3 (a), CsBiI3 (b), CsBiI4 (c), Cs3Bi2I9 (d) and Cs3BiI6 (e). They display two dimensional layered structure with P-31M symmetry (a), cubic structure with PM-3M symmetry (b), quasi-one-dimension structure (c), a cluster structure with symmetry of P-6 (d) and isolated octahedral configuration (e) respectively.

Following the energy of formation trends in Fig. 1, the formation enthalpy per atom of CsBiI3 is even lower than that of BiI3. Although the difference in the enthalpy of formation between CsBiI3 and Cs3Bi2I9 is small, their different iodine concentration causes big difference in structure. CsBiI3 is in a typical perovskite structure while the Cs3Bi2I9 has an isolated octahedral configuration. The structures of metastable state with the variation of Iodine concentration are displayed in FigS.1, which has similar phase transition as that indicated in Fig.2. The symmetry information for the most stable and metastable states in the extended convex hull of the phase diagram are list in TableS 1 and TableS2. From the universal search test results above, the ratio between Bi and I is the main factor determining the structural characteristic of the metal-iodine framework. As a result, the traditional method of mixing BiI3 and CH3NH3I to prepare perovskite solar cells may need to be improved. Excessive iodine (I:Bi=4:1>3:1) would induce undesirable phases and may be the


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reason why the most experimentally reported structure seen with Bi substitution in (CH3NH3)3Bi2I9 is the cluster structure.9-10 To solve this problem, two possible solutions could be proposed. One is heating the sample at an appropriate temperature for evaporating the redundant iodine. The second is to use just Bi metal plus CH3NH3I as reaction sources to enable the control of the Bi and I rate at 1:3, followed by annealing at a higher temperature to trigger the reaction. After confirmation of the existence of the perovskite structure CsBiI3, CH3NH3+ was put into the BiI3 frame work to take the place of Cs+. It could be observed that charge densities are mainly accumulated at the N site in the CH3NH3+ group (as shown in FigS. 2). The charge difference between N-H and C-H will causes stronger polarization of the Hσ+ on the -NH3 side than that on the –CH3 side, which induces a strong electric field between I- on the framework and the Hσ+ on the –NH3 side. This electric field will attract the CH3NH3+ closer to the Iodine site on the BiI3 framework, resulting in the away of CH3NH3+ from the cube center. Deviation of CH3NH3+ from the cube center will unbalance the framework, large lattice parameter change will happen and the bond angles in the frame of BiI3 also changes (as list in TableS 3). This implies that it is difficult to maintain the original standard cubic phase using the CH3NH3BiI3 composition. A 2×1×1 super cell is employed to investigate the transition from the unstable CH3NH3BiI3 composition to a more stable structure. Two CH3NH3+ groups in Fig. 3 (a) are arranged starting from the lowest energy construction in the [110] direction, in which the two CH3NH3+ groups locating in the center of the cubic BiI3 framework. Due to the charge difference and the resulted strong electric field between I- and Hσ+ on the –NH3 side as described above, the CH3NH3+ deviated from the center of BiI3 cube, thus significantly reduce the distance between the


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neighboring CH3NH3+ groups. The short distance between two CH3NH3+ groups will activate the following reaction: 2CH3NH3++2BiI3- →2CH3NH2BiI3+H2. Change in the charge state of Bi occurs from Bi2+ to Bi3+ after dehydrogenation as the tetravalent Bi3+ is the more stable state, forming a neutral CH3NH2 filling in the BiI3 framework. In Fig. 3 (b) to (e), transition states of the reaction are described and Fig. 3 (f) draws the whole reaction path searched by CINEB. As the two CH3NH3+ groups approach each other (Fig. 3 (b)), one hydrogen splits off the -NH3 (Fig.3 (c)), the H then diffuses through the crystal face and bridges with the hydrogen atom on the neighboring -NH3 (Fig. 3 (d)). The formation of H-H bond will weaken the interaction between the N and H in the bridged -NH3, a hydrogen molecule will then escape and leave two neutral CH3NH2 molecules behind as shown in (Fig. 3 (e)). Looking at the energy evolution along the path between the states, marked on Fig. 3 (f), we find that the whole reaction is exothermic with a large energy drop. The total energy of the final product (e) is 3.26 eV lower than that of the reactant (b), which indicates that this reaction is very likely to take place and will be irreversible. The activation barrier for dehydrogenation (from state (b) to state (c)) is only 0.76 eV, a low temperature anneal could make it occur. A similar dehydrogenation reaction has been reported to exist in the -NH3 based organic group,46 in which on-site hydrogen generation in BH3NH3 takes place at 383K. The reaction processes for dehydrogenation and hydrogen generation have been observed in AIMD simulations when heating the CH3NH3BiI3 system at 600K (See FigS.3 and FigS.4 in the supplementary information). Experimentally, anneal at 120 º C is enough to overcome the activation barrier to make CH3NH3+ dehydrogenate. We have detected H2 generation by gas chromatography when annealing the mixture of BiI3 and CH3NH3I with the ratio of 1:1. A cubic-like structure has been obtained although excess CH3NH3I remains, since excessive iodine source was added. The XRD results are shown in FigS.5.


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Figure 3. Transition process of the dehydrogenation reaction with the primary composition as CH3NH3BiI3. Two neighbor CH3NH3+ groups stay at the center of each cubic BiI3 framework (a), two neighbor CH3NH3+ groups approach to each other, deviating from the center of BiI3 cube (b), one hydrogen splits off the -NH3 (c), H diffuses through the crystal face and bridges with the hydrogen atom on the neighboring -NH3 (d), The formation of H-H bond will weaken the interaction between the N and H in the bridged -NH3, a hydrogen molecule will then escape and leave two neutral CH3NH2 molecules behind (e). Fig. 3 (f) shows the whole reaction path searched by CINEB, from which an obvious exothermic process was observed. Energy difference between the state (b), (c), (d), (e) and state (a), the starting point, is indicated along the path.

As the CH3NH3+ groups turns into neutral CH3NH2 molecule, charge distributes symmetrically on the N and C sites as presented in Fig. 4 (a). Consequently the degree of polarization of H on the –CH3 and –NH2 would be nearly equivalent, resulting in the CH3NH2 staying in the center of the cube. Analyzing the electron density of states (DOS) of CH3NH2BiI3 in Fig. 4 (b), the edge of valence band is mainly constructed by the molecular orbital of CH3NH2, different from the case of CH3NH3BiI3 in which the DOS of CH3NH3+ is far away from the band gap area. The total density of electronic states of the CH3NH2 molecule is approximately equivalent to that of I (p)


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near VBM. The CBM is very curved, showing high electron mobility of this material. VBM contributed from I(p) is also curved, indicating high hole mobility. From the band structure we could observe an intermediate band located at 1.61 eV below the conduction band edge, contributed mainly from the neutral CH3NH2, and one of 2.04 eV originating mainly from I (p). The intermediate mini band in our materials gives the opportunity for increasing the efficiency by up conversion of lower energy photons in the solar spectrum. Moreover, the small effective masses of electrons at CBM and hole effective mass at I(p) could guarantee the easy movement of charge carriers, and the intermediate band brought by the neutral CH3NH2 will offer a springboard for electrons excited from I(p) to CBM (Detail effective mass data and the function of the springboard is list and discussed in TableS 4). It suggests that the perovskite CH3NH2BiI3 would absorb in the visible light region, showing very promising application as a solar absorber.


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Figure 4. Charge density distribution of CH3NH2BiI3 (a), after the dehydrogenation process, the neutral CH3NH2 turned back to the center of the BiI3 framework, and the cubic structure was maintained. Predicted density of states and the calculated band structure of CH3NH2BiI3 (b), the total density of states are shown in black and the contributions of the Bi (p), I, N, C, H, an Bi(s) are shown in red, blue, yellow, dark yellow, dash blue and dash red respectively. The total density of electronic states of the CH3NH2 molecule is approximately equivalent to that of I (p) near VBM. An intermediate band located at 1.61 eV below the conduction band edge, contributed mainly from the neutral CH3NH2, and one of 2.04 eV originating mainly from I (p).

In contrast, when polar organic CH3NH3+ is filled into the BiI3 framework along different directions as [110], [100] and [111], the associated band structures present n-type semiconductor or semimetal characteristics where the Fermi level shifts high into the conduction band area as presented in FigS.6. As the bands below the Fermi level are fully occupied there will be no band gap for light absorption in the CH3NH3+ filled system. Finally, analysis of phonon band structures is made to illuminate the dynamical stability of the BiI3 framework with neutral molecule fillings in the center, performed within the harmonic approximation. Low-frequency regions of the calculated phonon band structures are predicted for BiI3 (Fig. 5(a)) and CH3NH2BiI3 (Fig. 5(b)), in which 0.3 THz is used as the known numerical error in the phonon calculations.47 As displayed in Fig. 5 (a), there are obvious imaginary frequencies below -0.3 THz for a pure cubic BiI3 framework, meaning the cubic BiI3 structure is not dynamically stable. As the neutral CH3NH2 molecule was filled into the center, the imaginary frequencies decreased close to 0THz as illustrated in Fig. 5 (b). This indicates that the neutral molecule CH3NH2 could provide strong enough support to the BiI3, and could keep the whole structure dynamically stable.


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Figure 5. Low-frequency regions of the calculated phonon band structures of the thermodynamically stable structures predicted for BiI3 (a) and CH3NH2BiI3 (b). There are obvious imaginary frequencies below -0.3 THz for a pure cubic BiI3 framework, the imaginary frequencies decreased close to 0THz as the neutral CH3NH2 molecule was filled into the center.

4. CONCLUSIONS In conclusion, we focus on the construction of lead-free perovskite materials, aiming to substitute the toxic Pb element in perovskite solar cells with Bi. The existence of stable perovskite structure composed of Bi and I was searched with Cs+ filling and the perovskite structure with the lowest formation enthalpy has been globally searched by USPEX under a defined iodine concentration as CsBiI3. When replacing the Cs+ by CH3NH3+, a dehydrogenation reaction is likely to occur as: 2CH3NH3++2BiI3-→2CH3NH2BiI3+H2. The active barrier along this reaction path is only 0.76eV, and the large energy difference between reactant and product makes the reaction irreversible. The reaction is verified by AIMD simulation as well as primary experimental results. The neutral CH3NH2 molecular can provide enough support to the inorganic BiI3 framework making the whole system dynamically stable. The band gap of 1.61eV for the CH3NH2BiI3 is close to ideal for a solar cell. Bismuth contained cubic perovskite


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materials with neutral molecule filling in the interstice now offers a promising new approach to produce novel and efficient lead-free perovskite solar cells.

ACKNOWLEDGEMENT This work was financial supported by the National Nature Science Foundation of China (Nos: 11374091, 11574076, 11274100, 11674087, 51602094).

Supporting Information. Brief statements on the symmetry information for the most stable and metastable states of CsxBiIy, the charge difference density and band structure of CH3NH3BiI3, the dehydrogenation reaction on the –NH3 side in CH3NH3 group and H2 generation as well as the effective mass of holes and electrons are supplied as Supporting Information.

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