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First-principle Study of Lead Iodide Perovskite Tetragonal and Orthorhombic Phases for Photovoltaics Wei Geng, Le Zhang, Yan-Ning Zhang, Woon-Ming Lau, and Li-Min Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp504951h • Publication Date (Web): 08 Aug 2014 Downloaded from http://pubs.acs.org on August 12, 2014
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The Journal of Physical Chemistry
First-Principle Study of Lead Iodide Perovskite Tetragonal and Orthorhombic Phases for Photovoltaics Wei Geng1, Le Zhang1, Yan-Ning Zhang1,2, Woon-Ming Lau1,2, Li-Min Liu1* 1
Beijing Computational Science Research Center, 3 Heqing Street, Haidian District, Beijing 100084, China 2 Chengdu Green Energy and Green Manufacturing Technology R&D Center, Chengdu, Sichuan, 610207, China Email:
[email protected] Tel: +86-10-8268 7086
Abstract: Methylammonium lead iodide perovskite, CH3NH3PbI3, has attracted particular attentions because of its fast increase in efficiency as solid-state solar cells. We performed firstprinciples calculations with non-local van der Walls (vdW) correlation to investigate the crystal structures, electronic and optical properties of CH3NH3PbI3. The calculated results show that the distribution of methylammonium ions, which further changes the vdW interaction and hydrogen bonds of organic and inorganic matrices, plays a vital role in both the geometry stability and electronic structure. The vdW correlation is critical to provide appropriate descriptions on the interaction between the organic and inorganic part. The phase transform from orthorhombic to tetragonal phase causes the decrease of band gap and the red shift of optical absorption coefficient.
Key Words: DFT, CH3NH3PbI3, van der Walls interaction, electronic property
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1. INTRODUCTION Dye-sensitized solar cells (DSCs) based on nanocrystalline TiO2 are a promising photovoltaic device for renewable energy source because of their easy fabrication, low cost, and flexibility1-2. The light-harvesting molecular dyes in an organic solvent is widely used as the TiO2 sensitizer3. However, the liquid electrolytes lead to several technological problems such as dye degradation, solvent evaporation and leakage of the solvent, which greatly limit the stability of DSC device for photovoltaic applications. Currently, many efforts have been made to search new solid hole transport materials, so as to replace liquid electrolytes and get solid-state dye-sensitized solar cells (ssDSCs) with good stability and high efficiency4-8. Methylammonium lead iodide perovskites (CH3NH3PbI3), which was proposed by Miyasaka et al., is considered as a promising material for ssDSCs with an expected photoconversion efficiency of 20%9-11. CH3NH3PbI3 was firstly used as sensitizing materials in liquid DSC in 2009, with a very low power conversion efficiency (PCE) of 3.8%12-13. In 2012, Chung et al. introduced a related inorganic CsSnI3 perovskite in ssDSCs as hole conduction, in the present of ruthenium dyes N719, and the ssDSCs showed a 8.5% efficiency14. In the same year, Kim et al. used spiroMeOTAD based perovskite ssDSCs to reach a high photovoltaic efficiency of 9.7%15. Recently, researches of perovskite-based solar cells steeply increased and an impressive efficiency value of up to 15% were reported in 201316-17. The extremely fast progress in perovskite photovoltaic materials have attracted increasing attentions, both experimentally and theoretically. Many theoretical efforts have been performed on the structural, electronic, and optical properties of CH3NH3PbI3 perovskite based on density functional theory (DFT) calculations18-25, so as to understand the fundamental mechanisms behind experimental observations. However, some questions are still unclear. For example, what are the roles of the organic molecule and inorganic matrix in photovoltaics, and what affects the electron pathway and transmission speed? To provide further insight into these questions, here we perform DFT calculations with the inclusion of vdW correction for both the tetragonal and orthorhombic phases of CH3NH3PbI3 perovskites that have been well observed in experiments26. Furthermore, the relation between structure, electronic and optical properties were fully investigated during phase change. 2
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2. COMPUTATIONAL DETAILS The DFT calculations were performed by applying the Vienna Ab Initio Simulation Package (VASP) code27-28. The electron-ion interaction was described by the projector augmented wave (PAW) method29-31. Electronic orbitals 5d6s6p, 5s5p, 2s2p, 2s2p and 1s were considered in valence for Pb, I, C, N and H atoms, respectively. The basis set cutoff was 400 eV, and the k-space integration was done with a 4 × 4 × 4 k-mesh in the Monkhorst-Park32 scheme. Further increasing the energy cutoff and k-points showed little difference in the results. All the structures considered in this study were relaxed with a conjugate-gradient algorithm until the energy on the atoms were less than 1.0 × 10−4 eV. Periodic boundary conditions were applied in all three dimensions. The cage made by four PbI6 octahedron was rather large and there was no obvious chemical bond formation between the organic molecule and inorganic matrix. Therefore, non-local density functional, vdW-DF33, was employed to take account in the weak interaction between them, as implemented in VASP by J. Klimeš et al34-35. In this method, the exchange-correlation energy takes the form of = + +
(1)
the semilocal GGA correlation such as PBE correlation is replaced by the nonlocal form of “vdW correlation” ( + correlation energy). Here, the Perdew-Burke-Ernzerhof (PBE)36 exchange functional was employed, along with non-local vdW density functional, except we noted below.
3. RESULTS AND DISCUSSION 3.1. Geometric structures of CH3NH3PbI3 CH3NH3PbI3 has two typical crystal structures: the tetragonal (tet) phase at room temperature and the orthorhombic (ort) phase at low temperature (
