First-Principles Prediction of the ZnO Morphology in the Perovskite

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First-Principles Prediction of the ZnO Morphology in the Perovskite Solar Cell Yefei Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01008 • Publication Date (Web): 24 May 2019 Downloaded from http://pubs.acs.org on May 24, 2019

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The Journal of Physical Chemistry

First-Principles Prediction of the ZnO Morphology in the Perovskite Solar Cell Ye-Fei Li* Collaborative Innovation Center of Chemistry for Energy Material, Key Laboratory of Computational Physical Science (Ministry of Education), Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, Shanghai 200433, China

Abstract The hybrid halide perovskite has attracted enormous attention due to its high photovoltaic conversion (>20%). The main concern with the practical application of this technology is the poor stability of the hybrid halide perovskite. Recent studies show that ZnO coating can substantially improve the stability of hybrid halide perovskite in the air, but the fundamental aspects, such as the epitaxial relation with perovskite and the morphology of ZnO in the coating film, remain unknown. By combining the modified phenomenological theory of Martensitic crystallography and first-principles calculations, we resolve the atomic-level structures of the interface between ZnO and MAPbI3. We show that the primary facets of ZnO, i.e., (1010) and (1120) can form coherent interfaces with MAPbI3, with low interfacial energies ranging from 0.71 to 0.89 J/m2. With the interfacial energies, we derive the equilibrium shape of ZnO nanoparticles over MAPbI3 substrate, which indicates that, in the equilibrium condition, the ZnO nanoparticles assemble loosely over the MAPbI3. The electronic structures further show that the band alignment between ZnO and MAPbI3 depends on the orientation relations of the interface. At last, we give three suggestions to improve the stability of ZnO coating layer.

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1. Introduction The hybrid halide perovskite has attracted great interest in the photovoltaic community due to the dramatic improvement in the performance of photovoltaics. To date, the perovskite solar cell (PSC) has attained a certified photovoltaic conversion efficiency over 20%.1-4 However, the main concern with the practical application of this technology is the intrinsic instability of the hybrid halide perovskite under ambient conditions, which requires to improve the long‐term stability of this material. The PSC is usually in the planar heterojunction configuration, which comprises three tandem materials, i.e., the hole transport layer (HTL), the perovskite layer, and the electron transport layer (ETL). The charge transport layer is crucial, not only for energy level matching and charge transport but also in protecting the perovskite photoactive layer from exposure to the environments. Revealing the interfacial structures between perovskite and charge transport layer may provide insight on how to improve the stability of PSC. The tetragonal methylammonium lead iodide (MAPbI3, MA = CH3NH3+) is the most popular light absorber in the PSC. Like other perovskite counterparts, this material decomposes upon exposure to air or moderate temperature increases.5 The experiments have shown that both O26, 7 and humidity8-11 can trigger the degradation of MAPbI3. Even in the inert atmosphere, the MAPbI3 would also degrade at the temperature higher than 85 °C.8,

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Using in situ

transmission electron microscopy (TEM), Fan et al. reported that the degradation of MAPbI3 occurs via a layer-by-layer pathway along the [001] axis of MAPbI3.5 The theoretical simulations of the same group further show that the degradation begins from the (001) surface of MAPbI3.5 On the other hand, wurtzite ZnO is the prevalent material for ETL,12, 13 which can be synthesized via solution-processed method at low temperature.14,

15

Recently, You et al.

reported that the solution-processed PSC with glass/ITO/NiOx/MAPbI3/ZnO/Al structure exhibits a high stability against water and oxygen degradation.15 After 60 days storage in air and room temperature, this device retains the photovoltaic performance about 90% of their original efficiency.15 The ZnO layer was found to play a vital role in preventing the degradation in the ambient condition, suggesting the importance of ZnO/MAPbI3 interface.15 Therefore, in this work, we use the ZnO/MAPbI3 composition as the model system to search the interfacial structure between perovskite and charge transport layer. In the PSC, both ZnO and MAPbI3 layers consist of their corresponding nanoparticles. In this configuration, the interface between ZnO and MAPbI3 can be formed along enormous orientation relations (OR) and thereby the interfacial structure is highly complex. Tracking the interfacial structures between ZnO and MAPbI3 is challenging in experiments, which requires a high spatial and temporal revolution. Alternatively, theoretical simulation is a valuable tool to provide the structural information in atomic level. Recently, we have developed a theoretical method, i.e., modified phenomenological theory of Martensitic crystallography (modified PTMC) to predict the OR of the low-strain interfaces with the crystal structures of two bulk phases.16 Combing with the first-principles calculations, we have applied the modified PTMC 2


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to reveal the structures of CoS2/CoOOH heterojunction.17 Here, we use this method to discover a set of low-strain interfaces between MAPbI3 and ZnO and resolve the equilibrium morphology of ZnO. Based on the calculations, we provide three suggestions to improve the stability of ZnO-coated MAPbI3. 2.

Methodology

2.1. Calculation Details Density functional theory (DFT) are utilized for computing energetics of all ZnO/MAPbI3 interfaces. The DFT calculations for geometry relaxation and interfacial energies of ZnO/MAPbI3 heterojunctions were carried out within the periodic plane wave framework as implemented in Vienna ab initio simulation package (VASP).18 The electron-ion interaction was represented by the projector augmented wave (PAW) with electrons from O, N and C 2s, 2p; H 1s; Zn 4s, 3d; Pb 6s, 6p, 5d shells explicitly included in the calculations. The kinetic energy cutoff of plane wave was set as 500 eV. The geometry optimization was based on the exchange-correlation functional GGA-PBE.19 The on-site Coulomb repulsion (PBE+U)20 was applied for Zn 3d orbital, in which the effective U-J terms (Ueff) was set as 4.7 eV for Zn, as suggested in the previous literature.21-24 The geometry convergence criterion was set as 0.04 e V/Å for the maximal component of force and 0.04 GPa for stress. The k-point mesh utilized was up to (2 × 4 × 1) in the Monkhorst−Pack scheme, which was verified to be accurate enough for these bulk systems. To obtain the band offsets at the ZnO/MAPbI3 interface, we recalculate the electronic structures of the relaxed structure by using the HSE0625 combined with the spinorbital coupling effect (denoted as HSE06+SOC). For MAPbI3, spin-orbital coupling effect is essential to generate the reliable bang dap of MAPbI3,26 while HSE06 can predict reasonable band gap for ZnO.27 In this work, we only consider the MAI-terminated MAPbI3 surface, which is more stable than the PbI2-terminated counterpart.28 The surface energy is given by

 = (EMAPbI3-MAI – EMAPbI3 – xEMAI)/2S

(1)

 denotes the surface energy, in J/m2; EMAPbI3-MAI the DFT energy of MAI-terminated MAPbI3 slab, EMAPbI3 the DFT energy of bulk MAPbI3; x the stoichiometric coefficient; EMAI the DFT energy of bulk MAI; S the surface area. The interfacial energy is calculated by

inter= (EZnO/MAPbI3 – EMAPbI3– EZnO– xEMAI)/2Sinter (2) inter is the interfacial energy, in J/m2; EZnO/MAPbI3 the DFT energy of ZnO/MAPbI3 heterojunction; EZnO the DFT energy of bulk ZnO; Sinter the area of the interface. The work of adhesion is calculated by Wa  ZnOslab +MAPbI3slabZnO/MAPbI3)/2Sinter

(3)

Wa is the work of adhesion, in J/m2; ZnOslab the DFT energy of the ZnO slab cleaved from the ZnO/MAPbI3 heterojunction; MAPbI3slab the DFT energy of the MAPbI3 slab cleaved from the 3



ZnO/MAPbI3 heterojunction. 2.2. Method to Determine the OR of Low-strain Interface The phenomenological theory of Martensitic crystallography (PTMC)29-31 is a tool to explain the habit planes in martensitic transformation. The calculations of PTMC requires lattice correspondence of two phases as input. Based on the lattice correspondence, we can build the deformation gradient. With the deformation gradient, the directions of invariant–line strain are determined, which then produce the strain invariant plane (habit plane). In our previous literature,16, 17, 32 we have demonstrated that the idea of PTMC can be generalized to predict the ORs of low-strain interfaces in heterojunction. Different from the Martensitic transformation, the lattice correspondence between two phases in a heterojunction is not related to a specified phase transition. As a result, any lattice correspondence between two phases can be utilized as the input of PTMC. The definition of a unit cell is arbitrary as long as it fulfilled the translational symmetry of lattice. The different definitions may result in different lattice correspondence and lead to different ORs. Therefore, we can screen out the ORs for low-strain interfaces by sampling the lattice correspondence between two crystals. Mathematically, the variation of the lattice correspondence can be achieved by sampling transformation matrices A and B over the primitive cell: T = AT’ M = BM’

(4)

A and B denote the transformation matrices; T’ and M’ the lattice parameters of the primitive cells; T and M the lattice parameters of the supercells. With the OR of two phases, we can construct the atomic model for heterojunction. The detailed description of the modified PTMC can be found in our previous literature.16, 32 2.3. Winterbottom Construction To predict the morphology of an isolated nanoparticle, one can use the Wulff construction.33 A crystal will arrange itself such that its surface energy is minimized by assuming a shape of low surface energy. This state is equal to minimize the surface energy of a crystal with constrained volume. In the Wulff construction, the total surface free energy (Gsurf) is calculated by summing the surface energies of exposed facets. Gsurf = Shkl.(hkl)

(5)

γ(hkl) represents the surface energy of the facet (hkl), and S is the area of the said face. The equilibrium morphology of crystal is determined by minimization of Gsurf with a volume constraint Vc.

 (Gsurf)Vc = (hkl) (Shkl)Vc = 0

(6)

Herring proves that the equilibrium morphology obeys the Gibbs-Wulff theorem:34 d(hkl): d(hkl’)= (hkl): (hkl’) 4


(7)

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d(hkl) is the "height" of the facet (hkl) drawing from the center of the crystal (referred as the Wulff point) to the face and (hkl) is the surface energy of facet (hkl). Therefore, we can derive the equilibrium morphology from a set of surface energies of a crystal. Next, let us consider the equilibrium shape of a nanoparticle in contact with a foreign substrate. In this condition, the overall surface free energy is defined as Gsurf = Shkl.(hkl)  Sinter.inter + (Ssub Sinter). sub = Shkl.(hkl) + Ssub .sub + Sinter.(intersub)

(8)

(hkl) is the surface energy of the exposed facet (hkl) on the nanoparticle; Shkl the corresponding area of the facet; Sinter the area of the solid-solid interface; Ssub the surface area of substrate before deposition; sub the surface energy of bare substrate; inter the interfacial energy. In analogy to the Wulff construction, the equilibrium morphology of a deposited nanoparticle is determined by minimization of the overall surface free energy with a volume constraint Vc:

 Gsurf)Vc= (hkl).  Shkl)Vc  sub. Ssub)Vc + (intersub) . Sinter)Vc = 0 (9) Since Ssub is a constant, the term sub. Ssub)Vc vanishes in the minimization, and we get

 Gsurf)Vc = (hkl).  Shkl)Vc + (intersub) . Sinter)Vc = 0

(10)

Thus, to resolve the shape of a deposited nanoparticle by the Wulff construction, we need to replace the surface energy of the contacted face to the term of inter minus sub. This revised Wulff construction is also known as the Winterbottom construction.35, 36 3. Results and Discussions 3.1. Equilibrium Morphology of Isolated ZnO nanoparticles Here, we use Wulff construction to predict the morphology of ZnO particles. First, we consider a free-standing ZnO nanoparticle. The experiments have shown that the (1010), (112 0), and (0001) facets are the solely exposed surfaces on ZnO nanoparticles in most synthesized methods.37-40 Thus, in the following, we only consider these three primary facets on ZnO. The surface structures are shown in Figure 1, and the corresponding surface energies are listed in Table 1. It should be noted that ZnO (0001) is a polar surface, exposes with either Zn-terminated (0001) or Zn-terminated (0001) surfaces. Previous studies have shown that both surfaces would naturally reconstruct to eliminate the net dipole of the surfaces.41-44 Here, we use the simplest Va+Va model,45 in which 1/4 surface O and 1/4 Zn are removed from O-terminated (0001) and Zn-terminated (0001), respectively. We have also examined the more complex reconstructed patterns, e.g., ADC+DY models,45 and the results show that the different structure of the polar surface would not change the conclusions. The surface energy of (0001) listed in Table 1 denotes the average of O-terminated and Zn-terminated facets since Wulff shape of ZnO only depends on the average surface energy of both facets. Table 1. Surface energies on the primary facets of ZnO and MAPbI3 ZnO

/J.m-2

MAPbI3 5


/J.m-2


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(1010)

0.91

(001)

0.05

(1120)

0.96

(100)

0.10

(0001)*

1.28

(110)

0.10

-

-

(101)

0.11

* The surface energy of (0001) denotes the average of O-terminated and Zn-terminated facets. With the surface energy, we construct the equilibrium morphology of ZnO, as shown in Figure 1b. The shape of a ZnO nanoparticle is the dodecagonal prism extended along the [0001] axis, consistent with the Wulff shape reported in the previous literature.37 The top and bottom surfaces are (0001) and (0001) surfaces. The twelve side surfaces consist of six {1010} and six {1120} facets. The area of one (1010) panel is larger than (1120), due to its slightly lower surface energy. The aspect ratio of ZnO nanoparticle (defined as the ratio of height to diameter) is ~1.4.

Figure 1. (a) The surface structures for ZnO (1010), (1120), (0001), and (0001) facets. The dashed line represents the ZnO surface. Red balls: O; Silver balls: Zn. (b) The equilibrium morphology of a ZnO nanoparticle using Wulff construction. Blue panel: (0001); Orange panels: {1010}; Green panels: {1120}. The (0001) facet is invisible in the picture. 3.2. Energetics and Structures of ZnO/MAPbI3 interfaces To establish the equilibrium morphology of deposited ZnO nanoparticles, we need to reveal the interfacial structures between ZnO and MAPbI3. To this end, we use our modified PTMC to resolve the ORs of the low-strain interfaces. The in-plane lattice parameters with reasonable strain ( 90° and (b) θeff < 90°. The blue dodecagon denotes the Wulff shape of an isolated nanoparticle; the point O the Wulff point; inter the interfacial energy; sub the surface energy of the bare substrate. From this pattern, we can find that the resulting shape is equivalent to displace the Wulff point of the isolated nanoparticle in the direction of the substrate by a distance specified by the difference of (intersub). (c-d) Schematic drawing of the morphology of film with (c) θeff > 90° and (d) θeff < 90°

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With the above analysis, we then establish the shape of deposited ZnO nanoparticle and assess the quality of the coating layer. Figure 4a and 4b displays two equilibrium morphologies of ZnO, corresponding to two adhesion modes by ZnO (1010) or (1120) facet, respectively. In the first configuration, the resulting ZnO nanoparticle exposes five {1010} facets and four {112 0} facets, as shown in Figure 4a. The eff is 118˚, larger than 90˚. Therefore, in the thermodynamical equilibrium condition, ZnO nanoparticles cannot fully cover the surface of MAPbI3 and may produce interstices at the ZnO/MAPbI3 interface. To estimate the proportion of uncovered MAPbI3 surface, we define the effective coverage Sc/Sp, where Sc is the area of contact and Sp is the projected area of the ZnO nanoparticle, as described in Figure 3a. Our results show that effective coverage is 0.76, indicating around a quarter of MAPbI3 surface is uncovered. As to the second configuration, the apparent shape is analogous to the first configuration, while the exposed faces of ZnO change to five {1120} facets and four {1010} facets. The eff is still 118˚, and the coverage is 0.79, slightly higher than that in the first configuration. Altogether, in the thermodynamical equilibrium condition, ZnO is not a good coating material over MAPbI3 substrate.

Figure 4. Equilibrium shape of a ZnO nanoparticle deposited on MAPbI3 substrate with (a) (10 10) or (b) (1120) facet. eff denotes the effective contact angle. Blue panels: {0001}; Red panels: {1010}; Green panels: {1120}; White panel: MAPbI3 substrate. From a morphological perspective, the shape of ZnO synthesized at low temperature may diverge from the equilibrium shape due to the dynamic effects,46-48 which provides the possibility to optimize the ZnO coating. While the temperature increases, the shape of ZnO nanoparticles would transform toward the equilibrium morphology, which is harmful to the stability of the coating. Therefore, in order to achieve long-term stability, the heating procedure should be avoided during the construction of the PSC device when using ZnO as ETL. Indeed, several groups have reported that ZnO coated MAPbI3 can achieve substantially improved stability in air, and all these materials are synthesized at low-temperature via the solutionprocessed strategy.12-15 3.4. Stability of ZnO-covered MAPbI3 At this point, it is intriguing to discuss how to optimize the coating strategy to improve the 10


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stability of MAPbI3 substrate. The mechanism of degradation of MAPbI3 is complex and involves heat, humidity, oxygen, and illumination. The key to improving the stability of PSC is to insulate MAPbI3 from corrosive environments. We will discuss the stability of MAPbI3 from three aspects: (i) heat; (ii) humidity and oxygen; (iii) illumination. First, the MAPbI3 itself is not stable, which may degrade at the temperature higher than 85 °C.8, 9 The in-situ TEM images have demonstrated that MAPbI3 prefers to degrade along the specified [001] axis, via a layer-by-layer exfoliation of the single-atomic-layer PbI2 from the (001)MAPbI3 surface.5 Obviously, the thermal stability of MAPbI3 lies in the adhesion between ZnO and MAPbI3, which is measured by the work of adhesion (Wa). To estimate the work of adhesion, we need to cleave a ZnO/MAPbI3 interface to ZnO and MAPbI3 surfaces. There are two ways to cleave the ZnO/MAPbI3 interface. The first way is to cleave the Zn-I, Zn-N, and O-H bonds along the ZnO surface, recovering the bare ZnO and MAI-terminated MAPbI3 surfaces. On the other hand, it is also possible to cleave the vertical Pb-I bonds along PbI2 plane, leaving the adsorbed CH3NH2 and I on the ZnO surfaces. Our result shows that the cleave of the interface along the second way is energetically more favorable than the first way. So, in the following, the work of adhesion refers to the second way. Our calculations show that the work of adhesion at the interfaces of Figure 2 are in the range of 0.11~0.19 J/m2, larger than the exfoliation of a singleatomic-layer PbI2 from the exposed (001)MAPbI3 surface, that is 0.07 J/m2. Therefore, coating ZnO can generally enhance the thermal stability of MAPbI3 against heat. Besides the adhesion, the OR of the interface may also influence the thermal stability of MAPbI3. For the OR of (1120)ZnO//(001)MAPbI3, the interface parallels to the (001)MAPbI3 surface. Once the adhesion is broken, the degradation of MAPbI3 can occur via the same reaction channel as that on the exposed (001)MAPbI3 surface. So, the contribution of the stabilization in this interface solely comes from the binding between ZnO and MAPbI3. While for other ORs, e.g., (1120)ZnO//(110)MAPbI3, (1010)ZnO//(100)MAPbI3, and (1010)ZnO// (101)MAPbI3, the interfaces intercross with the (001)MAPbI3. In this configuration, even ZnO detaches from the MAPbI3, the degradation of the as-cleaved MAPbI3 surface is expected to be more difficult than pristine (001)MAPbI3. Thus, the MAPbI3 nanoparticles which selectively exposes the less reactive faces, namely (100), (101), and (110), should benefit the thermal stability. Our calculations have shown that the difference of the surface energies among the primary facets on MAPbI3 are small, within 0.06 J/m2 (see Table 1). Experimentally, MAPbI3 single crystal with (100) as the primary facet has been attained.49 These results indicate that regulating the exposed surface of MAPbI3 by controlling the growth condition is promising. Second, to prevent the oxygen and humidity from contacting MAPbI3, we should increase the quality of the ZnO coating. To this end, we need to optimize the shape of ZnO nanoparticles to increase the effective coverage, which requires a tighter binding between ZnO and MAPbI3 substrate. One possible strategy is to replace the terminal I ions of MAPbI3 by a stronger anchor group, e.g., Cl ion.50, 51 Previous studies have shown that the Cl dopant in the MAPbI3-xClx would accumulate on the perovskite surface.50, 52 Thus, here we replace the interfacial I ions by 11



Cl, and then calculate the work of adhesion between ZnO and MAPbI3-xClx. Our result shows that the work of adhesion at the Cl-doped interface of (100)ZnO//(101)MAPbI3 is 0.52 J/m2, significantly larger than that at the pristine interface, i.e., 0.20 J/m2. Accordingly, the effective coverage increases to 0.86 (c.f. 0.76 on the pristine MAPbI3). Thus, doping Cl ions may enhance the stability of the ZnO-coated MAPbI3. Third, ultraviolet (UV) light also causes the degradation of MAPbI3,53, 54 and filtering UV light is another important procedure for the long-term stability of MAPbI3. The experimental band gap of ZnO is around 3.2 eV,55 which can selectively adsorb the UV light. In fact, ZnO has been utilized as a sun blocker in the sunscreens.56 Therefore, the ZnO coating may suppress the degradation of MAPbI3 under UV irradiation. 3.5. Electronic Structures of ZnO/MAPbI3 heterojunctions At last, the electronic structure of the ZnO/MAPbI3 interface is an important property of the PSC. Here, we investigate the electronic structures of ZnO/MAPbI3 heterojunctions with HSE06+SOC. Figure 5 illustrates the density of states (DOS) and the wavefunctions of four selected ZnO/MAPbI3 heterojunctions, namely (1120)ZnO//(001)MAPbI3, (1120)ZnO//(110)MAPbI3, (1010)ZnO//(100)MAPbI3, and (1010)ZnO// (101)MAPbI3. The total DOS (black lines in Figure 5a) show that all heterojunctions are semiconductors with a band gap of around 1.6 eV, and no trapping state exists in the band gap. With the projected DOS (PDOS), we further derive the band alignments between ZnO and MAPbI3. At the interface of (1120)ZnO//(001)MAPbI3, the conduction band minimum (CBM) of ZnO is slightly lower than that of MAPbI3, while the valence band maximum (VBM) of ZnO is significantly lower than that of MAPbI3. The wavefunctions at the interface of (1120)ZnO//(001)MAPbI3 (see Figure 5b-c) show that CBM is delocalized throughout the ZnO phase, while the VBM distributes at the boundary of MAPbI3. This band alignment benefits the charge carrier separation at the ZnO/MAPbI3 interface. In contrast, at the interfaces of (1120)ZnO//(110)MAPbI3, (1010)ZnO//(100)MAPbI3, and (101 0)ZnO//(101)MAPbI3, we found that the CBM of ZnO is higher than that of MAPbI3. For instance, at the interface of (1120)ZnO//(110)MAPbI3, the CBM of ZnO is 0.3eV higher than that of MAPbI3, indicating the injection of photo-induced electron from MAPbI3 to ZnO phase is thermodynamically unfavorable. Furthermore, the wavefunctions show that both VBM and CBM distribute in the MAPbI3 phase, suggesting the carrier may recombine at the ZnO/MAPbI3 interface. Similar band alignments were also found at the interfaces of (1010)ZnO//(100)MAPbI3 and (1010)ZnO// (101)MAPbI3, indicating the band offsets at the three interfaces are in fact improper for electron transport. This result is consistent with the experimental results that significant recombination of the carrier can occur at the ZnO/MAPbI3 interface.57, 58 Altogether, our results show that the band alignment between ZnO and MAPbI3 is sensitive to the OR of their interface.

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Figure 5. Electronic structures of four selected ZnO/MAPbI3 heterojunctions calculated by HSE06+SOC: (a) The total and projected DOS, the dashed line is the Fermi level; (b-c) 3Disosurface contour plot of the wavefunctions for VBM and CBM of the heterojunctions. The isosurface value is set as 0.0002 e/Å3. Red balls: O; Silver balls: Zn; Cyan balls: N; Brown balls: C; Purple balls: White balls: H; I; Gray balls: Pb. 4. Conclusion This work represents a comprehensive survey of ZnO structures over the MAPbI3 substrate. Using the modified PTMC and first-principles calculations, we found that ZnO and MAPbI3 can form coherent interfaces along a set of ORs, including (1120)ZnO//(110)MAPbI3, (1120)ZnO// (001)MAPbI3, (1010)ZnO//(100)MAPbI3, and (1010)ZnO//(101)MAPbI3, which corresponds to the interfacial energies of 0.74, 0.80, 0.73, and 0.71 J/m2, respectively. The interfacial energies are 13



later utilized to resolve the equilibrium shape of the deposited ZnO nanoparticle over the MAPbI3. In the thermodynamical equilibrium condition, we demonstrate that the ZnO nanoparticles cannot assemble compactly due to its large effective contact angle, i.e., 118o. Based on our calculations, we summarize three suggestions to improve the long-term stability of ZnO/MAPbI3 hetero-material: (i) Use the synthesis method operated at low temperature, since the ZnO might diverge from its unfavorable equilibrium shape due to the kinetic effect; (ii) Control the morphology of MAPbI3 to make it exposed with less reactive faces, such as (100), (101), or (110); (iii) Replace the terminal I ion on MAPbI3 surface by stronger anchor group, such as Cl. Author Information Corresponding Author Ye-Fei Li [email protected] Acknowledgment This work is supported by National Science Foundation of China (91545107, 21773032), Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institute of Higher Learning. References 1.

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Biography Ye-Fei Li is an Assistant Professor at Fudan University (Shanghai, China). Prior to joining the faculty at Fudan in 2014, he is a postdoctoral fellow in the Department of Chemistry at Princeton University. He received his Ph.D. degree from Fudan in 2012. His current research interests are mainly focused on metal oxide materials, surfaces, and interfaces; photo/electrocatalysis; and photovoltaics.

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First-Principles Prediction of the ZnO Morphology in the Perovskite

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