The adsorption postures of water on CH3NH3PbI3 surfaces: a first-principles insight

Inorganic organic perovskite solar cells (PSCs) are a kind of solar cells with rapid development in recent years, but their poor environmental stability, such as the water degradation behavior, hinders their commercialization. Here, we have thoroughly studied the adsorption postures of water on CH3NH3PbI3 surfaces. It is found that the adsorption energy of water molecules on perovskite surfaces is [-0.63, -0.59] eV, and with different initial postures, water molecules could eventually be adsorbed above Pb atom in a horizontal structure. The interaction between the perovskite surface and water molecules mainly comes from the electron exchange of Pb-O atoms. The weak interaction between H-I atoms makes the H atom approach the I atom, which could help maintain a horizontal structure of water molecules. The interactions between perovskite surfaces and water are well explained by our DFT calculations.


Introduction
Inorganic-organic perovskite solar cells (PSCs) have attracted extensive attention in the past few years. It has the advantages of high power conversion efficiencies (PCEs), large carrier mobility, long electron hole diffusion, easy preparation and low cost, and can be coated on flexible materials [1]. In the first time, Miyasaka et al. have introduced PSCs in 2009 and the PCE was 3.8% [2]. After the efforts of many researchers, the efficiency of PSCs has improved rapidly, and the latest PCE has exceeded 25% [3]. However, the PSC has a fatal defect that hinders its commercialization, that is its environmental instability.
Many researchers have a consensus that water molecules play a significant role in the decomposition of perovskites. Niu Burschka et al. have showed that in order to obtain high efficiency PSCs, the ambient humidity should be less than 1% [6]. Noh et al. have found that after PSCs were exposed to 55% humidity, the perovskite layer degraded seriously and the PCE of the cells decreased significantly [7]. Also, Frost et al. have noticed that trace water could lead to the partial decomposition of mixed perovskite [8]. These results show that water has a complex effect on the decomposition of PSCs. The first-principles method is also be used to study the effect of water on perovskite decomposition. Zhang et al. have confirmed that the water molecule could be adsorbed on the perovskite surfaces, and there are different adsorption sites on the perovskite surfaces [9]. Jiang et al. have found that the water molecule and oxygen molecule have significant impacts on the stability of XPbI3 (X= MA, FA, Cs) [10]. Diao et al. have showed that perovskite materials are prone to denaturation in high humidity environment [11]. Ouyang et al. have proposed the water molecule and iodine vacancy can reduce the energy levels of oxygen, promote the transfer of electrons from MAPbI3 to oxygen and then causes the surface photo-oxidation [12].
Many researchers use the first-principles method to study the adsorption of water molecules on the perovskite surfaces, but the effects of the initial postures of water molecules on the adsorption have not been considered. Therefore, in this work we will rotate water molecules along three directions and then study the interactions between water and perovskite surfaces. The paper also includes the following parts. Section 2 presents the computational details and the theoretical methods. In Section 3, the different postures of water molecules on the surfaces are presented and the influence of the initial postures of water molecules on the adsorption are analyzed. Section 4 provides a brief summary of the full text.

Computational Methods
We employed the Vienna Ab-initio Simulation Package (VASP) for first-principles calculations under the DFT framework [13]. Simultaneously the projector augmented wave method was used to describe the electron-ion interaction [14]. The energy cutoff for the plane wave basis was set to 500 eV in all calculations. In the simulation, an empirical pair-wise corrections proposed by Grimme in terms of DFT+D2 scheme had also been included for more precisely depicting the dispersion interactions in the systems [15]. In the structural optimization of bulk and interface systems, the pseudopotential we used was the generalized gradient approximation (GGA) in the form of the Perdew-Burke-Ernzerhof (PBE) [16].

The influence of the postures of water molecules on the adsorption
As Hao et al. work demonstrated that the water molecule was favorable to locate on the top of Pb atoms [17], we placed one water molecule above Pb atom in the super unit. To address the impact of the initial postures of water molecules on the adsorption, we firstly optimize the adsorption height on the perovskite surface, and obtain that the water molecule is 2.48 Å above the Pb atom, which is consistent with the previous calculation results. Therefore, we set the initial position of the water molecule at the height of 2.48 Å above the Pb atom, as shown in figure 1(a). Then we take the O atom as the origin and rotate the water molecule along three directions. Figure 2(b) gives the initial posture, where the water molecule is placed horizontally above the Pb atom. We label this posture as P11. Next, with the O atom as the origin, the three rotation directions (x, y and z) are marked in figure 2(c). Figure 2  Through the operation of rotating the water molecule, we can totally get twelve initial postures. By removing the repeated postures, we finally chose eight different initial postures to study the adsorption of water molecules on the surface of perovskites. The results of the adsorption position and adsorption energy are listed in Table 1. The equation of the adsorption energy ( ad ) is defined as: where slab+H 2 O is the total free energy of the system with one water molecule adsorbed on the surface, and slab and H 2 O are the total free energies of perovskite surfaces and an isolated water molecule. It can be seen from Table 1 that the initial postures of water molecules could affect their final adsorption positions. However, no matter how it rotates, the final adsorption position of water molecule is one of the postures P11-P14. When a water molecule is adsorbed on the surface of the perovskite, the adsorption energy is [-0.63, -0.59] eV. Water molecules are always adsorbed above the Pb atom in an almost horizontal posture. By comparing the four adsorption positions of P11-P14, we can obtain that their structures are very similar and when the O atom deviates slightly from the top of the Pb atom, the H atom from water could approach the I atom after the ion relaxation simulations. Taking the P32 posture as an example, we check the energy change of the system when the posture of water molecules changes from the vertical structure to the horizontal one during relaxation as shown in figure 3. It clearly demonstrates that the energy of the system decreases by 1.03 eV in the optimization process. The horizontal structure can minimize the total energy of the system, which is favorable for the stability of the water molecule.

The discussion of the electronic structures
Furthermore, the charge density differences at the perovskite surfaces are given to clarify the interactions of perovskite surfaces and water molecules. Figure 4 depicts the charge density differences (Figure 4 IOP Publishing doi:10.1088/1742-6596/2247/1/012003 5 (b)) and plane-averaged charge density differences (Figure 4 (a)) for the system of the perovskite surface absorbed by one water molecule. We can clearly observe the strong Pb-O coupling and weak H-I coupling between the perovskite surface and the water molecule. Electrons are favorable to accumulate around O/I atoms (yellow region) and deplete around Pb/H atoms (green region). Thus, it can be confirmed that the water molecule is energetically favorable to be absorbed on the perovskite surface, and the interaction mainly comes from the electron exchange of Pb-O atoms. Besides, the weak interaction between H-I atoms makes the H atom approach the I atom. Therefore, we can expect that when one water molecule is absorbed horizontally above the Pb atoms on the perovskite surface, the whole system could be relatively stable. plane-averaged charge density differences along the c direction for the system of the perovskite surface absorbed by one water molecule. The yellow area represents the electron increase, and the green area represents the electron decrease.

Conclusion
In conclusion, we used the first-principles simulation to study the adsorption position and adsorption energy of water molecules with different initial postures on the tetragonal CH3NH3PbI3 (001) surface. Our calculation results show that when a water molecule is adsorbed on the surface of the perovskite, the adsorption energy is [-0.63, -0.59] eV, and with different initial postures, water molecules could eventually be adsorbed above Pb atom in a horizontal posture. The interaction between the perovskite surface and water molecules mainly comes from the electron exchange of Pb-O atoms. The weak interaction between H-I atoms makes the H atom approach the I atom, which could help maintain a horizontal structure of water molecules. In general, the effect of different initial structures on water adsorption is relatively small. These results allow us to exclude the influence of the initial postures of water molecules during the adsorption processes. On this basis, we will further study the interaction between multiple water molecules and the perovskite surfaces.