The surface termination -a key factor to influence electronic and optical

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C: Energy Conversion and Storage; Energy and Charge Transport

The Surface Termination -A Key Factor to Influence Electronic and Optical Properties of CsSnI 3

Zhuo-Liang Yu, Qi-Rui Ma, Yu-Qing Zhao, Biao Liu, and Meng-Qiu Cai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11976 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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The Journal of Physical Chemistry

The surface termination -a key factor to influence electronic and optical properties of CsSnI3 Zhuo-Liang Yu1§, Qi-Rui Ma1§, Yu-Qing Zhao1, Biao Liu1, Meng-Qiu Cai1,2* 1

School of Physics and electronics Science, Hunan University, Changsha, Hunan

410082, People’s Republic of China 2

Synergetic Innovation Center for Quantum Effects and Applications (SICQEA),

Hunan Normal University, Changsha 410081, China ABSTRACT Despite great efforts have been devoted to the unusual optoelectronic properties for the bulk inorganic halide perovskites. How to overcome the surface effects and bring about selective growth in the specified surface-termination is still a challenge. In this paper, we investigate the electronic structures, effective masses, carriers mobility and optical properties of γ-CsSnI3 with different terminations by employing density functional theory (DFT) calculations. The calculated results show that the range of values of holes mobility is from 370.50 to 584.39 cm2/Vs. Our results are close to the experimental data 400 cm2/Vs. Moreover, we further predicted that the perfect CsI-termination may exhibit better photovoltaic characteristics than SnI2-termination. Based on the stability of different surfaces and surface vacancies, an appropriate condition was obtained to suppress the I vacancies and promote the growth of perfect CsI-termination surface. This work also indicates that the electronic and optical properties of inorganic halide perovskites are tuned by selecting proper surface, which is an important technique in design of other optoelectronic devices. ∗ §

Corresponding author. E-mail address: [email protected](M.Q. Cai) The author's contribution is equal. 1

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1. Introduction Organic−inorganic halide perovskites have attracted considerable attention in fascinating applications of optoelectronic devices on account of their excellent photoelectric performance, for example tunable direct band gap, long electron−hole diffusion lengths, high carrier mobility, and large absorption coefficients.1-10 However, there are toxicity with lead and great instability of being pregnable to the heat, moisture, oxygen, electrical field, and light for the the organic−inorganic halide perovskite materials.11-13 For the sake of the efficient applications of optoelectronic devices, pure inorganic perovskites are highly critical and desirable.14-16 Recently, people are beginning to investigate the pure inorganic halide perovskites CsSnX3 (X = Cl, Br, and I) because of their unusual characteristics.17 In particular, more attention has been paid to the black-phase CsSnI3 in the bulk configuration, which have great potentiality for the excellent photoelectric applications due to the high stability, lead-free property, near-infrared emission at room temperature, strong absorption at shorter wavelengths, and large hole mobility.17-19 In the experimental aspect, Chung and co-workers investigated the detailed

crystal

structures,

high

hole

mobility,

and

strong

near-infrared

photoluminescence of CsSnI3.18 They found that there are four temperature-dependent crystal structures: B-α, B-β, B-γ, and Y forms for CsSnI3. The black orthorhombic configuration of CsSnI3 with p-type characteristics shows room-temperature stability, a direct optical band gap 1.3 eV, and a high hole mobility of ∼400 cm2/Vs. Moreover, the power conversion efficiency of the Schottky solar cell consisting of a layer structure of indium-tin-oxide/CsSnI3/Au/Ti on glass substrate has been also studied.19 2


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In theory, Huang and Lambrecht explored the electronic band structure, phonons, and exciton binding energies of inorganic halide perovskites CsSnX3 (X = Cl, Br, and I) with bulk configurations.17 Jung et al. studied the the effect of the Rb:Cs ratio on the octahedral distortion, band gap, and thermodynamic properties by first-principles calculations.20 Ross Hatton et al. findings raise the prospect that lead-free perovskite photovoltaic may prove viable for applications.21 Bo Wu et al predict a PCE of ≈23% foroptimized CsSnI3 single crystal solar cells, highlighting their great potential.22 As we know, most of the previous researches are about the bulk of halide perovskites. Generally speaking, perovskite materials are made into solar cells in the form of thin films. In fact, dimensionality and surface have usually great influence on electronic and optical properties. For example, some two dimension (2D) halide perovskites exhibit efficient photoluminescence, and are used as light emitting device.23,24 Xiao et al put forward the new concept of electronic dimensionality and revealed the difference of optical and electronic properties between 2D and 3D halide perovskite.25 Recently, there are some experimental reports about influence of surface in GaN and ZnO.26,27 Generally, surface termination is critical factor affecting the electronic properties. To date, in spite of some researches about organic−inorganic perovskites surfaces such as cubic CH3NH3PbBr3 (001) surface and tetragonal CH3NH3PbI3 surfaces.28,29 There are very few investigations for the pure inorganic halide perovskites surfaces. It is thus necessary and important to investigate the surface properties of CsSnI3. In this paper, the electronic structures, carries mobility and optical absorption coefficients of different terminations surface in the (001) direction are investigated by 3



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employing density functional theory (DFT) calculations. The calculated results show that the range of values of holes mobility is from 370.50 to 584.39 cm2/Vs. The results are close to the experimental data 400 cm2/Vs.18 We analyzed reasons of low electron mobility of CsSnI3 and offered a proposal to compensate the defect with perfect CsI-termination in (001) surface. In addition, we discussed the stability of different surfaces and surface vacancies, and the negative influence of I vacancies on photovoltaic properties. Finally, an appropriate condition was obtained to suppress the I vacancies and promote the growth of perfect CsI-termination surface. This work also indicates that the electronic and optical properties of inorganic perovskites are tuned by selecting proper surface, which is an important technique in design of other optoelectronic devices.

2. Computational details In this work, we chose γ-CsSnI3 as the prototype structure, which exhibits a orthorhombic structure with the Pnma space group symmetry.17 The optimized lattice parameters (a0=8.92Å, b0=8.70Å, and c0=12.51Å) agree well with experimental parameters (a0=8.67Å, b0=8.64Å, and c0=12.38Å).17,20 The (001) surface of γ-CsSnI3 was chosen.20 Because the nonpolar (001) surface consist of alternate stacking of the neutral [CsI]0 and [PbI2]0 planes.29 And by comparing with the the grand potential Ω data of Xin et al, our data calculated in the followed text are in an order of magnitude with the data of the Xin et al.28 Thus, γ-CsSnI3 (001) surface may be stable. We constructed two different slabs (CsI-termination and SnI2-termination) consisting of 1× 1× 5 and 1× 1× 6 unit cells respectively and select 30 Å vacuum as 4


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shown in the figure 1. When optimizing the structure, we fixed the C axis (direction of vacuum layer). Then ISIF=3 is set (The atoms are fully relaxed in all directions and the lattice can relax in the direction of A and B ). The perovskite is used as a solar cell, its thickness is much larger than that of 5 atomic layers and the value of band gap will converge to the band gap value of bulk (Detailed data are shown in table S1). And the width and length of the surface are far greater than the thickness. In order to make our models much closer to the actual case, we chose 5 or 6 bilayers and kept the periodicity of surface. Relatively thick atomic layers and vacuum region in [001] direction were selected to avoid strong interaction via both the bulk and vacuum.30 For the calculation of surface vacancies, total-energy defect calculations about CsI-termination were performed with the Г-only k-mesh adopted 2 × 2 × 5 super cell.25,31 And by comparing the energy of different defect systems, the positions of I and Cs vacancies were determined.

5



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Cs Sn I

(c) VCs

(d) VI

Figure 1. The structure of γ-CsSnI3 (001) slabs with two different surfaces: the CsI-termination (a) and the SnI2-terminations (b). (green: Caesium; purple: iodine; dark gray: tin). The positions of Cs and I vacancy on CsI-terminated surface are shown in the (c) and (d) respectively.

The calculations were carried out using the Vienna ab-initio Simulation Package (VASP).32,33 The projected augmented wave (PAW) method was employed to describe the core-valence interaction. The wave functions were expanded in a plane-wave basis set with a 350 eV cutoff. Structure relaxation was stopped when the force on each atom is

The surface termination -a key factor to influence electronic and optical

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