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perovskite heterojunction toward perovskite solar cell applications
Journal of Molecular Graphics and Modelling 89 (2019) 96e101
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Understanding structures and properties of phosphorene/perovskite heterojunction toward perovskite solar cell applications Lei Zhang a, b, *, Shuai Lin a, b, Bo Wu a, b, Qingfang Li a, b, Jingfa Li a, b a Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean, Nanjing University of Information Science & Technology, Nanjing, 210044, China b School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing, 210044, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 September 2018 Received in revised form 18 February 2019 Accepted 8 March 2019 Available online 12 March 2019
Two-dimensional black phosphorus (phosphorene) has drawn much attention in recent years due to its excellent electronic and optical properties. In this manuscript, we employ ab initio calculations to investigate the structural origin of the phosphorene/perovskite heterostructure. The calculations suggest that the chemical stability and the mechanical stability depend on the surface terminations, and the mechanical stability of the phosphorene/perovskite heterojunction should be further improved. The weak interactions between the P atoms in the phosphorene and the under-coordinated Pb atoms at the perovskite surfaces, as well as the weak interfacial charge transfer characters, are proposed to be mainly responsible for the moderate heterostructure stability. Suggestions to improve the stability of the heterojunction are provided. This study helps the fundamental understanding of the interaction between the phosphorene and the halide perovskite materials, and could provide a foundation for the better understanding of the low-dimensional materials in perovskite-based optoelectronic devices. © 2019 Elsevier Inc. All rights reserved.
Keywords: Perovskite solar cell Phosphorene Ab initio First principles
1. Introduction Perovskite solar cells (PSCs) have demonstrated to be a disruptive technology in the solar cell industry, and have shown potentials to substitute the silicon solar cells due to their excellent power conversion efficiencies, lower cost and facile synthesis [1e13]. The PSCs are functionalized by the halide perovskite materials that act as the light absorber to convert the light into the electricity. The halide perovskite materials are sandwiched between the electron transporting layer (ETL) and the hole transporting layer (HTL) that are responsible to selectively conduct electrons and holes, respectively [14,15]. The nanoscopic structures and properties of surfaces and interfaces of halide perovskite materials, such as the perovskite/HTL interface, are important to achieve high device performances [16e20]. Recent developments of PSCs are facilitated by the inclusion of the two-dimensional (2D) materials [21e24]. The phosphorene is an exemplar 2D semiconductor based on the black phosphorus that have been applied to many optoelectronic devices
* Corresponding author. Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean, Nanjing University of Information Science & Technology, Nanjing, 210044, China. E-mail address: [email protected] (L. Zhang). https://doi.org/10.1016/j.jmgm.2019.03.012 1093-3263/© 2019 Elsevier Inc. All rights reserved.
owing to its interesting electronic and optical properties [25e31]. Recently, the phosphorene has been incorporated into perovskite solar cells and works as an alternative HTL [32,33]. Nevertheless, the optimization of 2D materials such as the phosphorene in PSCs are dependent on the detailed nanoscopic structural understanding of the heterojunction formed by the halide perovskite surface and the 2D materials, which requires further investigation at the moment. The detailed nanoscopic structures of the phosphorene/perovskite interfacial structures remain elusive, and the better understanding of the phosphorene/ perovskite heterostructure should be obtained to suggest future materials engineering routes [32,34]. In this manuscript, we employ ab-initio calculations to understand the structures and properties of the perovskite/phosphorene heterojunction. The prototypical CH3NH3PbI3 is used for the perovskite modeling. We obtained the heterostructure with an excellent mismatch between the two semiconductor materials. It is found that moderate interactions exist between the halide perovskite material and the phosphorene material, and such interactions are strongly dependent on the perovskite surface terminations. The projected density of states (PDOS), orbitals distributions, electron density difference are UV-vis absorption spectra acquired from the ab initio calculations are employed to probe the electronic and
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optical properties of the phosphorene/perovskite interface. The interactions between the water molecules and the heterostructures are probed by the classical molecular dynamics (MD) simulation to understand the water stability of the phosphorene/perovskite systems. 2. Computational details The halide perovskite CH3NH3PbI3 exposing the (001) surface is focused in this study since the halide perovskite surface along this direction is stable, and has been selected as the model surface in a number of PSC studies [35e38]. Two perovskite surface terminations are investigated, including a bare PbI-terminated surface (1) and a bare cation-terminated surface (2). Other surfaces such as those based on the iodine dimer and the defected surfaces also exist, but those surfaces are considered to be defects and do not serve as an ideal modeling platform; therefore they are not included in this manuscript [19,37,39,40]. The phosphorene monolayer (3) is placed on top of the bare perovskite surfaces 1 and 2 for the geometry optimization, resulting in 4 (combining 3 and 1) and 5 (combining 3 and 2), respectively. A unit cell with the surface area of 26.4 Å 8.8 Å and the caxis of 43.6 Å is prepared. The vacuum layer is set to be 20 Å in an effort to minimize the undesirable interlayer interactions. The PBE functional and 340 eV cutoff energy are employed at the Gamma point due to the large unit cell and large number of atoms. The van der Waals effects are included to account for the intermolecular interactions [43]. The optimized perovskite-based heterojunctions are more difficult to obtain than the molecular additives-based perovskite counterparts [41,42], since the stringent lattice mismatch rules are required. An excellent lattice match is successfully achieved between the phosphorene and the perovskite layer (within 0.49% and 0.47% along different axes). A 0.03 Å 1 separation is used for the properties determination including the PDOS, orbital distribution and UV-vis absorption spectra in CASTEP [44]. The potential energy surface (PES) plots are obtained by moving the phosphorene layer either vertically or horizontally in the unit cell. The classical molecular dynamics calculation is performed using the universal force field with the constant temperature ensemble (NVT) at 298 K in Materials Studio. Eight water molecules are randomly placed on top of the perovskite/phosphorene heterojunction at the starting point. Four snapshots are presented to reveal the structural revolution and the interactions between the water molecules and the halide perovskite surfaces. 3. Results and discussion The optimized heterostructures of 1e5 obtained from the abinitio calculations are depicted in Fig. 1. The calculations indicate that the phosphorene monolayer resides on halide perovskite
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surfaces with the nearest interatomic distance at 3.15 Å between the phosphorus atom in the phosphorene layer and the lead atom in the perovskite surface (3.22 Å between the phosphorus atom in the phosphorene and the hydrogen atom in the perovskite surface). The average P-P bond lengths in 4 and 5 are 2.25 Å, which are similar with the phosphorene bond lengths in the free form [29,45e47]. The distance values between the phosphorene and the perovskite surfaces are similar in 4 and 5, while the adsorption energies defined by the difference between the energy of the overall system and the energies of individual systems are different: 4 has an adsorption energy of 6.3 eV, while 5 has an adsorption energy of 2.8 eV. This might be ascribed to the larger interactions between the electron-rich phosphorus atom in the phosphorene and the electrophilic under-coordinated lead atom in the perovskite surface, which is available in 4 but not present in 5. By moving the phosphorene at the perovskite surface along different directions, the potential energy surfaces (PESs) are constructed for 4 and 5 (Fig. 2). It is observed that the obtained PES is highly dependent on the perovskite surface terminations: larger energy changes are present in 4 than those in 5 when the phosphorene is moving along the perovskite surface. This is ascribed to the larger interaction between the phosphorene and the PbIterminated perovskite surface in 4 than that in 5. More energy input is required to move the phosphorene vertically toward the perovskite surface; for example, a 24 eV energy value is needed to move the phosphorene toward the PbI-terminated perovskite surface by 1 Å. On the other hand, smaller energy values are required to move the phosphorene layer away from the perovskite surfaces; for example, a 3 eV (2 eV) energy is necessary to move the phosphorene away from the PbI-terminated (the cation-terminated) perovskite surface by 1 Å. Moving the phosphorene horizontally with respect to the perovskite surface involves relatively small energy inputs; for example the energy change is 0.5 eVe1.5 eV (or 0.08 eV ~ 0.6 eV) when the phosphorene is moved horizontally by 1 Å from the equilibrium position in 4 (or 5). The energy inputs required to move the phosphorene vertically or horizontally with respect to the perovskite surfaces are smaller than many other heterosystems based on the perovskite surfaces [38,48]. The energy inputs are especially smaller in the cationterminated perovskite case. This could indicate that the phosphorene/perovskite heterojunction might suffer from poor stability and mechanical performance. Since the cation-terminated perovskite surface is prone to the passivation from Lewis acids and halogen-containing additives [15,42,49,50], it is expected that doping phosphorene with halogen and electron-deficient element could improve the stability of the phosphorene/perovskite heterojunctions. In addition, the PbI2 the considered to self-passivate the perovskite surfaces and beneficial for the overall device performance, while the cation-rich region is considered prone to the cation removal and charge trap formation [49]. Consistently, The PES calculation in this manuscript suggests that the PbI-terminated
Fig. 1. Optimized structures of 1e5.
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Fig. 2. PES of 4 (top) and 5 (bottom) by moving the phosphorene along various directions with respect to the perovskite surface. For each system, the phosphorene is moved along three directions (Vertical, Horizontal_LongAxis and Horizontal_ShortAxis). Vertical: the phosphorene is moved along the vertical axis (c-axis) in the unit cell. The negative sign in the reaction coordinate means the phosphorene is moving toward the perovskite surface while the positive sign in the reaction coordinate means the phosphorene is moving away from the perovskite surface. Horizontal_LongAxis: the phosphorene is moved along the horizontal longer axis parallel to the perovskite surface. Horizontal_ShortAxis: Move along the horizontal shorter axis parallel to the perovskite surface.
surface might be better than the cation-termination surface in the perovskite/HTL heterojunctions when HTL is the phosphorene. The electronic properties of functional semiconductor materials could be revealed by the PDOS spectra near the band gap (see Fig. 3). The PDOS of 4 and 5 demonstrate that the conduction band is mainly contributed by the phosphorene moiety, while the valence band is predominantly contributed by the perovskite p orbitals. The top of the valence band at 0 eV of 4 is mainly from the phosphorene, which is not observed in 5. In addition, the conduction band is mixed with the perovskite s orbitals. As a result, charge transfer characters could exist from the perovskite to the phosphorene; nevertheless, the impact could be minimal since both of the top of valence band and the bottom of the conduction band are contributed by the P atoms in the phosphorene of 4. The band gaps of 4 and 5, formed by the distance between the conduction band and the valence band, are 0.92 eV and 0.87 eV, respectively, which are smaller than those of the bare perovskite surfaces [41]. From the spatial distributions of frontier orbital, the valence band maximum (VBM) and the conduction band maximum (CBM) for 4 are localized predominantly in the phosphorene (Fig. 4). On the other hand, the VBM is localized in the bottom of the perovskite layer while the CBM is localized in the phosphorene of 5. Therefore
the phosphorene/perovskite heterojunction is suggested to be stabilized by the weak charge transfer characters. The electron density difference is an indicator of the degree of the interfacial charge transfer of the materials. The electron density difference plots of 4 and 5 (Fig. 5) reveal that the charge transfer is highly dependent on the perovskite surface termination, since the plots are much more easily visualized in 4 than 5. In addition, the electron density difference plots indicate the small but nonnegligible interfacial charge transfer characters that stabilize the heterostructures. Future engineering methods could be devoted to enhancing the charge transfer characters in the heterojunction to improve the structural stability. The optical properties of the phosphorene/perovskite heterostructure are also dependent on the perovskite surface terminations. The calculated UV-vis absorption spectra (see Fig. 6) demonstrate better absorption intensity in the visible region of 4 than that of 5. Therefore, it is expected that the optical properties of the phosphorene/perovskite heterostructures could be manipulated by the surface engineering techniques that modify the perovskite termination characters. Since the stability is of paramount importance in PSCs, the interactions between water molecules and the phosphorene/
Fig. 3. PDOS plots of 4 (left) and 5 (right). Phosphorene: the PDOS from the phosphorene layer. Perovskite: the PDOS from the perovskite layer. s: the PDOS from the s orbital. p: the PDOS from the p orbital. d: the PDOS from the d orbital. Total: the total density of states. The top of the occupied states is set to 0 eV.
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Fig. 6. UV-vis absorption spectra of 4 and 5.
Fig. 4. Spatial distributions of VBM and CBM for 4 (top) and 5 (bottom). The isovalue is 0.01.
Fig. 7. MD simulations of eight water molecules placed on the perovskite systems 1, 2, 4 and 5. Four snapshots are obtained at 0 ps, 20 ps, 50 ps and 100 ps. The red arrows indicate the overall water molecule movement as time passes by. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 5. Electron density difference plots of 4 and 5. The isovalue is 0.008. The blue and yellow regions refer to an increase and a decrease of electronic density, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
perovskite systems are investigated via the MD simulation (Fig. 7). Eight water molecules are randomly placed on top of either the bare perovskite surfaces of the heterostructures at the starting point. Based on the snapshots at 0 ps, 20 ps, 50 ps and 100 ps, it is observed that in the bare perovskite surfaces especially the PbIterminated system 1, the water molecules clearly approach to the perovskite surfaces as the time passes by. This is probably due to the interaction between the electron-rich oxygen atoms in the
water molecules and the electron-deficient under-coordinated lead atoms at the perovskite surfaces. The movement of the water molecules toward the perovskite surfaces indicates severe water instability of the perovskite solar cells. Many studies have shown that the surface passivation techniques can successfully inhibit the water-induced degradation in the perovskite surfaces [51e53], but the MD simulation in this study indicates that the phosphorene layer does not enhance the perovskite stability. In particular, when the phosphorene reside on the perovskite surfaces in the cases of 4 and 5 (Fig. 7), the movement of the water molecules toward to the heterojunctions is not significantly retarded, which might ultimately detriment the heterojunction stability. This agrees with the experimental result that the phosphorene/perovskite materials is prone to the water degradation [32]. 4. Conclusions The heterostructure consisting of the phosphorene layer and the CH3NH3PbI3 surface are investigated via the ab initio calculations.
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The heterostructure prepared in this study exhibit excellent mismatch values that are below 0.5%. The stabilities as well as the electronic and optical properties of the heterostructures strongly depend on the perovskite surface terminations. The PES features are found to be dependent on the perovskite surface terminations. Consistent with the experimental results, the classical MD simulations confirm that the water stability properties of the phosphorene/perovskite heterojunctions are not ideal and should be further engineered with surface passivation techniques. The calculations indicate that the application of the phosphorene into the perovskite HTL material is prohibited by the weak interactions between the phosphorene and the halide perovskites. Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 51702165, 11704195 and 11547030) and the Jiangsu Provincial Natural Science Foundation (Grant No. BK20160942 and BK20160941). The authors acknowledge computational support from NSCCSZ Shenzhen, China. References [1] Q. Lin, A. Armin, R.C.R. Nagiri, P.L. Burn, P. Meredith, Electro-optics of perovskite solar cells, Nat. Photon. 9 (2014) 106e112, https://doi.org/10.1038/ nphoton.2014.284. [2] I. Chung, B. Lee, J. He, R.P.H. Chang, M.G. Kanatzidis, All-solid-state dyesensitized solar cells with high efficiency, Nature 485 (2012) 486e489, https://doi.org/10.1038/nature11067. [3] J. Lin, M. Lai, L. Dou, C.S. Kley, H. Chen, F. Peng, J. Sun, D. Lu, S.A. Hawks, C. Xie, F. Cui, A.P. Alivisatos, D.T. Limmer, P. Yang, Thermochromic halide perovskite solar cells, Nat. Mater. (2018), https://doi.org/10.1038/s41563-017-0006-0. [4] W.-J. Fan, J.-W. Cai, G.-J. Yang, J.-W. Chi, D. Zhou, D.-Z. Tan, R.-Q. Zhang, Aggregation of metal-free organic sensitizers on TiO2(101) surface for use in dyesensitized solar cells: a computational investigation, Comput. Theor. Chem. 1093 (2016) 1e8, https://doi.org/10.1016/j.comptc.2016.08.006. [5] P. Salvatori, A. Amat, M. Pastore, G. Vitillaro, K. Sudhakar, L. Giribabu, Y. Soujanya, F. De Angelis, Corrole dyes for dye-sensitized solar cells: the crucial role of the dye/semiconductor energy level alignment, Comput. Theor. Chem. 1030 (2014) 59e66, https://doi.org/10.1016/j.comptc.2013.12.012. [6] M. Liu, M.B. Johnston, H.J. Snaith, Efficient planar heterojunction perovskite solar cells by vapour deposition, Nature 501 (2013) 395e398, https://doi.org/ 10.1038/nature12509. [7] F. Hao, C.C. Stoumpos, D.H. Cao, R.P.H. Chang, M.G. Kanatzidis, Lead-free solidstate organic-inorganic halide perovskite solar cells, Nat. Photon. 8 (2014) 489e494, https://doi.org/10.1038/nphoton.2014.82. [8] N.J. Jeon, J.H. Noh, W.S. Yang, Y.C. Kim, S. Ryu, J. Seo, S. Il Seok, Compositional engineering of perovskite materials for high-performance solar cells, Nature 517 (2015) 476e480, https://doi.org/10.1038/nature14133. [9] M.A. Green, A. Ho-Baillie, H.J. Snaith, The emergence of perovskite solar cells, Nat. Photon. 8 (2014) 506e514, https://doi.org/10.1038/nphoton.2014.134. [10] N.J. Jeon, J.H. Noh, Y.C. Kim, W.S. Yang, S. Ryu, S. Il Seok, Solvent engineering for high-performance inorganiceorganic hybrid perovskite solar cells, Nat. Mater. 13 (2014) 897e903, https://doi.org/10.1038/nmat4014. [11] J. You, L. Meng, T.-B. Song, T.-F. Guo, Y.Michael Yang, W.-H. Chang, Z. Hong, H. Chen, H. Zhou, Q. Chen, Y. Liu, N. De Marco, Y. Yang, Improved air stability of perovskite solar cells via solution-processed metal oxide transport layers, Nat. Nanotechnol. 11 (2015) 1e8, https://doi.org/10.1038/nnano.2015.230. [12] M.A. Becker, R. Vaxenburg, G. Nedelcu, P.C. Sercel, A. Shabaev, M.J. Mehl, € ferle, J.G. Michopoulos, S.G. Lambrakos, N. Bernstein, J.L. Lyons, T. Sto , A.L. Efros, Bright triplet exR.F. Mahrt, M.V. Kovalenko, D.J. Norris, G. Raino citons in caesium lead halide perovskites, Nature 553 (2018) 189e193, https://doi.org/10.1038/nature25147. [13] A. Marchioro, J. Teuscher, D. Friedrich, M. Kunst, R. van de Krol, T. Moehl, M. Gr€ atzel, J.-E. Moser, Unravelling the mechanism of photoinduced charge transfer processes in lead iodide perovskite solar cells, Nat. Photon. 8 (2014) 250e255, https://doi.org/10.1038/nphoton.2013.374. [14] W. Wang, J. Su, L. Zhang, Y. Lei, D. Wang, D. Lu, Y. Bai, Growth of mixed-halide perovskite single crystals, CrystEngComm 20 (2018) 1635e1643, https:// doi.org/10.1039/C7CE01691C.
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Topical Perspectives
Understanding structures and properties of phosphorene/perovskite heterojunction toward perovskite solar cell applications Lei Zhang a, b, *, Shuai Lin a, b, Bo Wu a, b, Qingfang Li a, b, Jingfa Li a, b a Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean, Nanjing University of Information Science & Technology, Nanjing, 210044, China b School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing, 210044, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 September 2018 Received in revised form 18 February 2019 Accepted 8 March 2019 Available online 12 March 2019
Two-dimensional black phosphorus (phosphorene) has drawn much attention in recent years due to its excellent electronic and optical properties. In this manuscript, we employ ab initio calculations to investigate the structural origin of the phosphorene/perovskite heterostructure. The calculations suggest that the chemical stability and the mechanical stability depend on the surface terminations, and the mechanical stability of the phosphorene/perovskite heterojunction should be further improved. The weak interactions between the P atoms in the phosphorene and the under-coordinated Pb atoms at the perovskite surfaces, as well as the weak interfacial charge transfer characters, are proposed to be mainly responsible for the moderate heterostructure stability. Suggestions to improve the stability of the heterojunction are provided. This study helps the fundamental understanding of the interaction between the phosphorene and the halide perovskite materials, and could provide a foundation for the better understanding of the low-dimensional materials in perovskite-based optoelectronic devices. © 2019 Elsevier Inc. All rights reserved.
Keywords: Perovskite solar cell Phosphorene Ab initio First principles
1. Introduction Perovskite solar cells (PSCs) have demonstrated to be a disruptive technology in the solar cell industry, and have shown potentials to substitute the silicon solar cells due to their excellent power conversion efficiencies, lower cost and facile synthesis [1e13]. The PSCs are functionalized by the halide perovskite materials that act as the light absorber to convert the light into the electricity. The halide perovskite materials are sandwiched between the electron transporting layer (ETL) and the hole transporting layer (HTL) that are responsible to selectively conduct electrons and holes, respectively [14,15]. The nanoscopic structures and properties of surfaces and interfaces of halide perovskite materials, such as the perovskite/HTL interface, are important to achieve high device performances [16e20]. Recent developments of PSCs are facilitated by the inclusion of the two-dimensional (2D) materials [21e24]. The phosphorene is an exemplar 2D semiconductor based on the black phosphorus that have been applied to many optoelectronic devices
* Corresponding author. Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean, Nanjing University of Information Science & Technology, Nanjing, 210044, China. E-mail address: [email protected] (L. Zhang). https://doi.org/10.1016/j.jmgm.2019.03.012 1093-3263/© 2019 Elsevier Inc. All rights reserved.
owing to its interesting electronic and optical properties [25e31]. Recently, the phosphorene has been incorporated into perovskite solar cells and works as an alternative HTL [32,33]. Nevertheless, the optimization of 2D materials such as the phosphorene in PSCs are dependent on the detailed nanoscopic structural understanding of the heterojunction formed by the halide perovskite surface and the 2D materials, which requires further investigation at the moment. The detailed nanoscopic structures of the phosphorene/perovskite interfacial structures remain elusive, and the better understanding of the phosphorene/ perovskite heterostructure should be obtained to suggest future materials engineering routes [32,34]. In this manuscript, we employ ab-initio calculations to understand the structures and properties of the perovskite/phosphorene heterojunction. The prototypical CH3NH3PbI3 is used for the perovskite modeling. We obtained the heterostructure with an excellent mismatch between the two semiconductor materials. It is found that moderate interactions exist between the halide perovskite material and the phosphorene material, and such interactions are strongly dependent on the perovskite surface terminations. The projected density of states (PDOS), orbitals distributions, electron density difference are UV-vis absorption spectra acquired from the ab initio calculations are employed to probe the electronic and
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optical properties of the phosphorene/perovskite interface. The interactions between the water molecules and the heterostructures are probed by the classical molecular dynamics (MD) simulation to understand the water stability of the phosphorene/perovskite systems. 2. Computational details The halide perovskite CH3NH3PbI3 exposing the (001) surface is focused in this study since the halide perovskite surface along this direction is stable, and has been selected as the model surface in a number of PSC studies [35e38]. Two perovskite surface terminations are investigated, including a bare PbI-terminated surface (1) and a bare cation-terminated surface (2). Other surfaces such as those based on the iodine dimer and the defected surfaces also exist, but those surfaces are considered to be defects and do not serve as an ideal modeling platform; therefore they are not included in this manuscript [19,37,39,40]. The phosphorene monolayer (3) is placed on top of the bare perovskite surfaces 1 and 2 for the geometry optimization, resulting in 4 (combining 3 and 1) and 5 (combining 3 and 2), respectively. A unit cell with the surface area of 26.4 Å 8.8 Å and the caxis of 43.6 Å is prepared. The vacuum layer is set to be 20 Å in an effort to minimize the undesirable interlayer interactions. The PBE functional and 340 eV cutoff energy are employed at the Gamma point due to the large unit cell and large number of atoms. The van der Waals effects are included to account for the intermolecular interactions [43]. The optimized perovskite-based heterojunctions are more difficult to obtain than the molecular additives-based perovskite counterparts [41,42], since the stringent lattice mismatch rules are required. An excellent lattice match is successfully achieved between the phosphorene and the perovskite layer (within 0.49% and 0.47% along different axes). A 0.03 Å 1 separation is used for the properties determination including the PDOS, orbital distribution and UV-vis absorption spectra in CASTEP [44]. The potential energy surface (PES) plots are obtained by moving the phosphorene layer either vertically or horizontally in the unit cell. The classical molecular dynamics calculation is performed using the universal force field with the constant temperature ensemble (NVT) at 298 K in Materials Studio. Eight water molecules are randomly placed on top of the perovskite/phosphorene heterojunction at the starting point. Four snapshots are presented to reveal the structural revolution and the interactions between the water molecules and the halide perovskite surfaces. 3. Results and discussion The optimized heterostructures of 1e5 obtained from the abinitio calculations are depicted in Fig. 1. The calculations indicate that the phosphorene monolayer resides on halide perovskite
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surfaces with the nearest interatomic distance at 3.15 Å between the phosphorus atom in the phosphorene layer and the lead atom in the perovskite surface (3.22 Å between the phosphorus atom in the phosphorene and the hydrogen atom in the perovskite surface). The average P-P bond lengths in 4 and 5 are 2.25 Å, which are similar with the phosphorene bond lengths in the free form [29,45e47]. The distance values between the phosphorene and the perovskite surfaces are similar in 4 and 5, while the adsorption energies defined by the difference between the energy of the overall system and the energies of individual systems are different: 4 has an adsorption energy of 6.3 eV, while 5 has an adsorption energy of 2.8 eV. This might be ascribed to the larger interactions between the electron-rich phosphorus atom in the phosphorene and the electrophilic under-coordinated lead atom in the perovskite surface, which is available in 4 but not present in 5. By moving the phosphorene at the perovskite surface along different directions, the potential energy surfaces (PESs) are constructed for 4 and 5 (Fig. 2). It is observed that the obtained PES is highly dependent on the perovskite surface terminations: larger energy changes are present in 4 than those in 5 when the phosphorene is moving along the perovskite surface. This is ascribed to the larger interaction between the phosphorene and the PbIterminated perovskite surface in 4 than that in 5. More energy input is required to move the phosphorene vertically toward the perovskite surface; for example, a 24 eV energy value is needed to move the phosphorene toward the PbI-terminated perovskite surface by 1 Å. On the other hand, smaller energy values are required to move the phosphorene layer away from the perovskite surfaces; for example, a 3 eV (2 eV) energy is necessary to move the phosphorene away from the PbI-terminated (the cation-terminated) perovskite surface by 1 Å. Moving the phosphorene horizontally with respect to the perovskite surface involves relatively small energy inputs; for example the energy change is 0.5 eVe1.5 eV (or 0.08 eV ~ 0.6 eV) when the phosphorene is moved horizontally by 1 Å from the equilibrium position in 4 (or 5). The energy inputs required to move the phosphorene vertically or horizontally with respect to the perovskite surfaces are smaller than many other heterosystems based on the perovskite surfaces [38,48]. The energy inputs are especially smaller in the cationterminated perovskite case. This could indicate that the phosphorene/perovskite heterojunction might suffer from poor stability and mechanical performance. Since the cation-terminated perovskite surface is prone to the passivation from Lewis acids and halogen-containing additives [15,42,49,50], it is expected that doping phosphorene with halogen and electron-deficient element could improve the stability of the phosphorene/perovskite heterojunctions. In addition, the PbI2 the considered to self-passivate the perovskite surfaces and beneficial for the overall device performance, while the cation-rich region is considered prone to the cation removal and charge trap formation [49]. Consistently, The PES calculation in this manuscript suggests that the PbI-terminated
Fig. 1. Optimized structures of 1e5.
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Fig. 2. PES of 4 (top) and 5 (bottom) by moving the phosphorene along various directions with respect to the perovskite surface. For each system, the phosphorene is moved along three directions (Vertical, Horizontal_LongAxis and Horizontal_ShortAxis). Vertical: the phosphorene is moved along the vertical axis (c-axis) in the unit cell. The negative sign in the reaction coordinate means the phosphorene is moving toward the perovskite surface while the positive sign in the reaction coordinate means the phosphorene is moving away from the perovskite surface. Horizontal_LongAxis: the phosphorene is moved along the horizontal longer axis parallel to the perovskite surface. Horizontal_ShortAxis: Move along the horizontal shorter axis parallel to the perovskite surface.
surface might be better than the cation-termination surface in the perovskite/HTL heterojunctions when HTL is the phosphorene. The electronic properties of functional semiconductor materials could be revealed by the PDOS spectra near the band gap (see Fig. 3). The PDOS of 4 and 5 demonstrate that the conduction band is mainly contributed by the phosphorene moiety, while the valence band is predominantly contributed by the perovskite p orbitals. The top of the valence band at 0 eV of 4 is mainly from the phosphorene, which is not observed in 5. In addition, the conduction band is mixed with the perovskite s orbitals. As a result, charge transfer characters could exist from the perovskite to the phosphorene; nevertheless, the impact could be minimal since both of the top of valence band and the bottom of the conduction band are contributed by the P atoms in the phosphorene of 4. The band gaps of 4 and 5, formed by the distance between the conduction band and the valence band, are 0.92 eV and 0.87 eV, respectively, which are smaller than those of the bare perovskite surfaces [41]. From the spatial distributions of frontier orbital, the valence band maximum (VBM) and the conduction band maximum (CBM) for 4 are localized predominantly in the phosphorene (Fig. 4). On the other hand, the VBM is localized in the bottom of the perovskite layer while the CBM is localized in the phosphorene of 5. Therefore
the phosphorene/perovskite heterojunction is suggested to be stabilized by the weak charge transfer characters. The electron density difference is an indicator of the degree of the interfacial charge transfer of the materials. The electron density difference plots of 4 and 5 (Fig. 5) reveal that the charge transfer is highly dependent on the perovskite surface termination, since the plots are much more easily visualized in 4 than 5. In addition, the electron density difference plots indicate the small but nonnegligible interfacial charge transfer characters that stabilize the heterostructures. Future engineering methods could be devoted to enhancing the charge transfer characters in the heterojunction to improve the structural stability. The optical properties of the phosphorene/perovskite heterostructure are also dependent on the perovskite surface terminations. The calculated UV-vis absorption spectra (see Fig. 6) demonstrate better absorption intensity in the visible region of 4 than that of 5. Therefore, it is expected that the optical properties of the phosphorene/perovskite heterostructures could be manipulated by the surface engineering techniques that modify the perovskite termination characters. Since the stability is of paramount importance in PSCs, the interactions between water molecules and the phosphorene/
Fig. 3. PDOS plots of 4 (left) and 5 (right). Phosphorene: the PDOS from the phosphorene layer. Perovskite: the PDOS from the perovskite layer. s: the PDOS from the s orbital. p: the PDOS from the p orbital. d: the PDOS from the d orbital. Total: the total density of states. The top of the occupied states is set to 0 eV.
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Fig. 6. UV-vis absorption spectra of 4 and 5.
Fig. 4. Spatial distributions of VBM and CBM for 4 (top) and 5 (bottom). The isovalue is 0.01.
Fig. 7. MD simulations of eight water molecules placed on the perovskite systems 1, 2, 4 and 5. Four snapshots are obtained at 0 ps, 20 ps, 50 ps and 100 ps. The red arrows indicate the overall water molecule movement as time passes by. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 5. Electron density difference plots of 4 and 5. The isovalue is 0.008. The blue and yellow regions refer to an increase and a decrease of electronic density, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
perovskite systems are investigated via the MD simulation (Fig. 7). Eight water molecules are randomly placed on top of either the bare perovskite surfaces of the heterostructures at the starting point. Based on the snapshots at 0 ps, 20 ps, 50 ps and 100 ps, it is observed that in the bare perovskite surfaces especially the PbIterminated system 1, the water molecules clearly approach to the perovskite surfaces as the time passes by. This is probably due to the interaction between the electron-rich oxygen atoms in the
water molecules and the electron-deficient under-coordinated lead atoms at the perovskite surfaces. The movement of the water molecules toward the perovskite surfaces indicates severe water instability of the perovskite solar cells. Many studies have shown that the surface passivation techniques can successfully inhibit the water-induced degradation in the perovskite surfaces [51e53], but the MD simulation in this study indicates that the phosphorene layer does not enhance the perovskite stability. In particular, when the phosphorene reside on the perovskite surfaces in the cases of 4 and 5 (Fig. 7), the movement of the water molecules toward to the heterojunctions is not significantly retarded, which might ultimately detriment the heterojunction stability. This agrees with the experimental result that the phosphorene/perovskite materials is prone to the water degradation [32]. 4. Conclusions The heterostructure consisting of the phosphorene layer and the CH3NH3PbI3 surface are investigated via the ab initio calculations.
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The heterostructure prepared in this study exhibit excellent mismatch values that are below 0.5%. The stabilities as well as the electronic and optical properties of the heterostructures strongly depend on the perovskite surface terminations. The PES features are found to be dependent on the perovskite surface terminations. Consistent with the experimental results, the classical MD simulations confirm that the water stability properties of the phosphorene/perovskite heterojunctions are not ideal and should be further engineered with surface passivation techniques. The calculations indicate that the application of the phosphorene into the perovskite HTL material is prohibited by the weak interactions between the phosphorene and the halide perovskites. Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 51702165, 11704195 and 11547030) and the Jiangsu Provincial Natural Science Foundation (Grant No. BK20160942 and BK20160941). The authors acknowledge computational support from NSCCSZ Shenzhen, China. References [1] Q. Lin, A. Armin, R.C.R. Nagiri, P.L. Burn, P. Meredith, Electro-optics of perovskite solar cells, Nat. Photon. 9 (2014) 106e112, https://doi.org/10.1038/ nphoton.2014.284. [2] I. Chung, B. Lee, J. He, R.P.H. Chang, M.G. Kanatzidis, All-solid-state dyesensitized solar cells with high efficiency, Nature 485 (2012) 486e489, https://doi.org/10.1038/nature11067. [3] J. Lin, M. Lai, L. Dou, C.S. Kley, H. Chen, F. Peng, J. Sun, D. Lu, S.A. Hawks, C. Xie, F. Cui, A.P. Alivisatos, D.T. Limmer, P. Yang, Thermochromic halide perovskite solar cells, Nat. Mater. (2018), https://doi.org/10.1038/s41563-017-0006-0. [4] W.-J. Fan, J.-W. Cai, G.-J. Yang, J.-W. Chi, D. Zhou, D.-Z. Tan, R.-Q. Zhang, Aggregation of metal-free organic sensitizers on TiO2(101) surface for use in dyesensitized solar cells: a computational investigation, Comput. Theor. Chem. 1093 (2016) 1e8, https://doi.org/10.1016/j.comptc.2016.08.006. [5] P. Salvatori, A. Amat, M. Pastore, G. Vitillaro, K. Sudhakar, L. Giribabu, Y. Soujanya, F. De Angelis, Corrole dyes for dye-sensitized solar cells: the crucial role of the dye/semiconductor energy level alignment, Comput. Theor. Chem. 1030 (2014) 59e66, https://doi.org/10.1016/j.comptc.2013.12.012. [6] M. Liu, M.B. Johnston, H.J. Snaith, Efficient planar heterojunction perovskite solar cells by vapour deposition, Nature 501 (2013) 395e398, https://doi.org/ 10.1038/nature12509. [7] F. Hao, C.C. Stoumpos, D.H. Cao, R.P.H. Chang, M.G. Kanatzidis, Lead-free solidstate organic-inorganic halide perovskite solar cells, Nat. Photon. 8 (2014) 489e494, https://doi.org/10.1038/nphoton.2014.82. [8] N.J. Jeon, J.H. Noh, W.S. Yang, Y.C. Kim, S. Ryu, J. Seo, S. Il Seok, Compositional engineering of perovskite materials for high-performance solar cells, Nature 517 (2015) 476e480, https://doi.org/10.1038/nature14133. [9] M.A. Green, A. Ho-Baillie, H.J. Snaith, The emergence of perovskite solar cells, Nat. Photon. 8 (2014) 506e514, https://doi.org/10.1038/nphoton.2014.134. [10] N.J. Jeon, J.H. Noh, Y.C. Kim, W.S. Yang, S. Ryu, S. Il Seok, Solvent engineering for high-performance inorganiceorganic hybrid perovskite solar cells, Nat. Mater. 13 (2014) 897e903, https://doi.org/10.1038/nmat4014. [11] J. You, L. Meng, T.-B. Song, T.-F. Guo, Y.Michael Yang, W.-H. Chang, Z. Hong, H. Chen, H. Zhou, Q. Chen, Y. Liu, N. De Marco, Y. Yang, Improved air stability of perovskite solar cells via solution-processed metal oxide transport layers, Nat. Nanotechnol. 11 (2015) 1e8, https://doi.org/10.1038/nnano.2015.230. [12] M.A. Becker, R. Vaxenburg, G. Nedelcu, P.C. Sercel, A. Shabaev, M.J. Mehl, € ferle, J.G. Michopoulos, S.G. Lambrakos, N. Bernstein, J.L. Lyons, T. Sto , A.L. Efros, Bright triplet exR.F. Mahrt, M.V. Kovalenko, D.J. Norris, G. Raino citons in caesium lead halide perovskites, Nature 553 (2018) 189e193, https://doi.org/10.1038/nature25147. [13] A. Marchioro, J. Teuscher, D. Friedrich, M. Kunst, R. van de Krol, T. Moehl, M. Gr€ atzel, J.-E. Moser, Unravelling the mechanism of photoinduced charge transfer processes in lead iodide perovskite solar cells, Nat. Photon. 8 (2014) 250e255, https://doi.org/10.1038/nphoton.2013.374. [14] W. Wang, J. Su, L. Zhang, Y. Lei, D. Wang, D. Lu, Y. Bai, Growth of mixed-halide perovskite single crystals, CrystEngComm 20 (2018) 1635e1643, https:// doi.org/10.1039/C7CE01691C.
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