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Alloying and Defect Control within Chalcogenide Perovskites for Optimized Photovoltaic Application Weiwei Meng, Bayrammurad Saparov, Feng Hong, Jianbo Wang, David B. Mitzi, and Yanfa Yan Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04213 • Publication Date (Web): 11 Jan 2016 Downloaded from http://pubs.acs.org on January 16, 2016
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Chemistry of Materials
Alloying and Defect Control within Chalcogenide Perovskites for Optimized Photovoltaic Application Weiwei Meng,†,‡ Bayrammurad Saparov,§,∥ Feng Hong,†,¶ Jianbo Wang,‡ David B. Mitzi,§,∥,* and Yanfa Yan†,* †
Department of Physics and Astronomy, and Wright Center for Photovoltaic Innovation and Commercialization, The University of Toledo, Toledo, Ohio 43606, USA ‡
School of Physics and Technology, Center for Electron Microscopy, MOE Key Laboratory of Artificial Micro- and Nano-structures, and Institute for Advanced Studies, Wuhan University, Wuhan 430072, China §
Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708, USA
∥Department ¶
of Chemistry, Duke University, Durham, NC 27708, USA
Department of Physics, Shanghai University, Shanghai 200444, China
ABSTRACT: Through density-functional theory calculations, we show that the alloy perovskite system BaZr1-xTixS3 (x 0) may not be stable under thermal equilibrium growth conditions. Calculations of decomposition energies suggest that introducing compressive strain may be a plausible approach to stabilize BaZr1-xTixS3 thin films.
INTRODUCTION Organic-inorganic lead halide perovskite-based thinfilm solar cells have attracted substantial attention recently, with record power conversion efficiency (PCE) of already >20%, despite only having been in focus for a few years.1,2,3,4,5 In addition to the high efficiency, these halide perovskites consist of earth-abundant elements, as is ideal for scalable manufacturing. However, the presence of the toxic heavy-metal (Pb) and instability under typical ambient humidity levels have raised concerns for the ultimate commercialization of lead halide perovskite technologies.6,7,8 Theoretical studies have revealed that the three-dimensional (3-D) network of corner-sharing BX6 octahedra plays an important role for the superior photovoltaic properties of lead halide perovskites.9,10 The results suggest that stable alternative perovskites made of nontoxic elements deserve exploration as prospective new photovoltaic materials. Recently, chalcogenide perovskites ABX3 (X = S, Se; A, B = metals with combined valence of 6), which are more environmentally-friendly as compared to lead halide perovskites, have been proposed for photovoltaic applications.11 So far, a number of chalcogenide perovskite-related compounds have been experimentally synthesized.12,13,14,15,16,17 However, only CaZrS315, CaHfS315, BaZrS315, and BaHfS315 are found to preferably exist in the distorted perovskite phase with 3-D connected corner-
sharing BX6 octahedra. Other chalcogenide compounds preferentially form phases with either edge-sharing or isolated BX6 octahedra (the so-called “needle-like” 16,18 and “hexagonal”14,19 phases). These structures are expected to exhibit more localized conduction and valence band edges, thereby leading to heavy electron and hole masses, mainly due to the lack of connected octahedra along certain directions of the crystal structure. Therefore, from the carrier mobility point of view, CaZrS3, CaHfS3, BaZrS3, and BaHfS3 perovskites are expected to be more suitable for solar cell applications. The calculated bandgaps for these perovskites are about 1.96, 2.30, 1,76, and 2.02 eV, respectively.11 Among these sulfide perovskites, only BaZrS3 exhibits a bandgap that is close to the range suitable for making efficient single-junction solar cells. Therefore, BaZrS3 represents a material of choice for experimentally and theoretically assessing whether or not chalcogenide perovskites are promising candidates for solar cell applications. The bandgap of BaZrS3 is slightly larger than the optimal bandgap for single junction solar cells. However, the bandgap should be tunable through alloying with isovalent elements such as B = Ti and/or X = Se, an approach that has been successfully used for optimizing the analogous bandgaps of chalcopyrite Cu(In,Ga)Se2 and kesterite Cu2ZnSn(Se,S)4 absorbers.20,21,22,23 Furthermore, for optimizing solar cell performance, growth conditions should
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be identified to avoid the formation of detrimental deep level defects, and to facilitate the formation of the needed shallow level defects for appropriate doping. In this paper, we show that alloying with Ti is indeed a promising approach for optimizing the bandgap of BaZrS3, and therefore for achieving high prospective cell efficiency. DFT calculations suggest that BaZr1-xTixS3 exhibits dispersive conduction and valence band edges and ultra-high optical absorption, over an important span of the solar spectrum. A small substitution percentage (~10%) of Zr by Ti (i.e., x=0.1) is able to reduce the bandgap to 1.47 eV, close to the optimal bandgap for a single junction solar cell. Calculations of defect transition energies and formation energies reveal that BaZrS3 exhibits ambipolar self-doping properties, indicating the possibility of homo p-n junction fabrication. We further show that, for photovoltaic applications, BaZr1-xTixS3 films should be synthesized under moderate (i.e., near stoichiometric) conditions to minimize the formation of deep level defects and to optimize doping levels. The theoretical power conversion efficiencies of BaZr1-xTixS3 alloys are expected to be even higher than those of the emerging lead halide perovskites, if they can be experimentally realized. However, decomposition energy calculations indicate that synthesizing single phase BaZr1-xTixS3 perovskite thin films under thermal equilibrium conditions could be difficult, as the alloy may decompose to BaZrS3 and BaTiS3. Our experimental attempts of synthesizing BaZr1-xTixS3 perovskite alloys using a solid-state reaction approach have confirmed the tendency toward phase decomposition. Introducing compressive strain is computationally shown be a plausible approach to stabilize BaZr1-xTixS3 perovskite films. COMPUTATIONAL AND EXPERIMENTAL METHODS The DFT calculations were performed using the Vienna ab initio simulation package (VASP)24,25 code with the standard frozen-core projector augmented-wave (PAW)26,27 method. The cut-off energy for basis functions was 400 eV. The generalized gradient approximation (GGA) of Perdew-Burke-Ernzerh (PBE)28 functional was used for exchange-correlation. All atoms were relaxed until the Hellmann-Feynman forces on them were below 0.01 eV/Å. For calculations of electronic, optical and defect properties, the GGA+U 29, 30 method was employed to address the on-site Coulomb interactions in the localized d orbitals, which depend on the difference of the Coulomb (U) and exchange (J) parts. Ueff=4.5 eV (Ueff=U-J) for the Zr d orbital and Ueff=4 eV for the Ti d orbital were selected for our calculations because these U parameters produce similar bandgaps as for the BaZrS3 and BaTiS3 perovskites calculated by hybrid functional (HSE06)31,32 (Table S1). We used a dense 7x7x5 Γ−centered k-points mesh for electronic and optical calculations. For calculations of defect properties, a (222) host supercell (160 atoms) with the Γ(0,0,0) point was used.
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ground, and cold-pressed into pellets. The pellets were transferred into quartz tubes, evacuated down to ~7×10-7 Torr, and the quartz tubes were flame-sealed under dynamic vacuum. The reaction mixtures were then heated to 800 °C in 3 hours inside a box furnace, and kept at this temperature for 15 hours. This was followed by cooling of the reaction containers to room temperature by switching off the furnaces. After the first annealing step, the BaZrS3 was confirmed to be the major product through X-ray diffraction experiments performed on a PANalytical Empyrean powder X-ray diffractometer under ambient conditions and using Cu kα radiation. Minor impurities of leftover BaS and ZrS2 were also noted. Repeated annealing at 800-1000 °C after grinding, homogenizing and pelletizing steps reduces the impurity content. Finally, to determine the band gap of BaZrS3, diffuse reflectance measurements were carried out using a QE-R Quantum Efficiency/Reflectivity measurement system from Enlitech. For BaZr1-xTixS3, mixtures of BaS, ZrS2 and TiS2 (SigmaAldrich, 99.9%) with x = 0.05, 0.1, 0.2, 0.3, and 0.5 were cold-pressed, and heated in a box furnace using the conditions described above. According to the powder X-ray diffraction (PXRD) data, for all of these reactions, the products consisted of mixtures of BaZrS3 and BaTiS3. No noticeable shifts in the PXRD pattern peak positions were observed for either of these phases (which might have suggested solid solutions being formed if observed).
RESULTS AND DISCUSSION Electronic structure BaZrS3 perovskite exhibits the distorted GdFeO3-type15 phase, possessing an orthorhombic structure with the space group Pnma (No. 62) (Figure S1). The coordinates of the cations are similar to those in the ideal cubic perovskite structure: Zr is 6-fold coordinated, while Ba is 12fold coordinated. Zr and S atoms form ZrS6 octahedra with corner-sharing at the S atom vertices. For an ideal cubic ABX3 perovskite structure, the tolerance factor t (t = (RA+RX)/{√2 (RB+RX)}) should be close to 1, where RA and RB are the ionic radii of cations A and B, respectively, and RX is the ionic radius of the anion X. For cubic perovskites, the tolerance factor t lies between approximately 0.89 and 1.33 If t is larger than 1, the structure tends to adopt a configuration with face-sharing of BX6 octahedra, with BaNiO3 as an example (t = 1.13).34 If t is too small, rotation and expansion of the BX6 octahedra will occur, leading to a structure with low symmetry, such as the
For the synthetic experimental studies, solid-state reactions were employed to prepare the target compositions. For BaZrS3, stoichiometric mixtures of BaS (Materion, 99.9%) and ZrS2 (American Elements, 99%) were carefully
Figure 1. Calculated GGA+U bandgaps of (a) BaZr1-xTixS3 and (b) BaZrS3-xSex alloys.
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Chemistry of Materials
tetragonal (β phase) or orthorhombic (γ phase) structures.35 GdFeO3 (t = 0.81) is an example of the orthorhombic structure.34 For BaZrS3, the calculated t is about 0.86, manifesting the orthorhombic structure with tilted ZrS6 octahedra. It is known that DFT GGA calculations underestimate the bandgaps of crystalline materials. So far, LDA+U or GGA+U, HSE06 and GW are the most used methods for correctly predicting bandgaps of semiconducting materials. However, calculations of defect properties, which require the use of large supercells, are too computationally demanding for the HSE06 or GW methods. Therefore, in this paper, we employ GGA+U to calculate the electronic, optical, and defect properties of chalcogenide perovskites and HSE06 was used to verify the bandgaps calculated using GGA+U. The Ueff values, 4.5 eV for the Zr d orbital and 4.0 eV for the Ti d orbital, were obtained by correctly reproducing the HSE bandgaps of BaZrS3, BaTiS3, and TiO2. The GGA+U calculated lattice parameters and bandgap of BaZrS3 are shown in Table 1 and are close to the experimental values presented later in the text (i.e., 1.72 eV vs 1.85 eV). The calculated GGA+U bandgap, 1.72 eV, which agrees well with the bandgaps computed using HSE reported in Refs. 11 and 36, but is 0.1 eV smaller than the bandgap reported in Ref. 37. The GGA+U calculated band structure and BaZrS3 density of states are shown in Figure S2. It is seen that BaZrS3 exhibits a direct band gap at the Γ(0,0,0) point. The valence band is mainly derived from S 3p states, spreading between 0 and -4.5 eV, with 0 eV as the highest occupied state. The S 3s and Ba 5p states are in the same energy range, -12.5 eV and -10 eV, leading to antibonding coupling. The presence of a small component of Zr 4d in the valence band suggests that there is a weak antibonding between Zr 4d and S 3p, leading to antibonding states at the valence band maximum (VBM), which is seen from the charge density map associated with the VBM (Figure S3). Both conduction and valence band edges exhibit dispersive bands, suggesting small carrier effective masses. The dispersive bands are partially attributed to the less localized Zr 4d states as compared to 3d states.11 Furthermore, the conduction band minimum (CBM) is derived from Zr 4d-S 3p anti-bonding coupling. Our GGA+U calculated results agree well with those calculated by HSE, suggesting that GGA+U is able to provide reliable
insights into the electronic, optical, and defect properties of chalcogenide perovskites. As mentioned above, the bandgap of BaZrS3 (1.7 eV) is slightly larger than the optimal bandgap (1.1-1.45 eV) for single junction based solar cells, governed by the wellknown Shockley-Queisser limit.38 Alloying with isovalent elements has been successfully used to tune the bandgaps of solar cell absorbers, such as Cu(In,Ga)Se2 and Cu2ZnSn(S,Se)4.20,21,22,23 Because the conduction and valence bands are derived mainly from the Zr 4d and S 3p states, respectively, the bandgap of BaZrS3 can be lowered by alloying with isovalent elements that exhibit either d states lower in energy than Zr 4d or p states higher in energy than the S 3p states. Considering atomic size and electronegativity mismatch, Ti and Se are the first choices for alloying of BaZrS3. The calculated GGA+U bandgaps of BaZr1-xTixS3 and BaZrS3-xSex alloys are shown in Figure 1. It is noted that, in the calculation, the perovskite structure was considered for BaTiS3, while the experimentally synthesized BaTiS3 possesses the hexagonal structure,14,39,40 which is consistent with our calculated formation enthalpies for different structures including perovskite, hexagonal, and needle-like phases (Table S2). For each composition, various configurations with Ti occupying different Zr sites have been considered, but only the lowestenergy configuration was chosen for bandgap calculation. The calculated GGA+U bandgaps for BaTiS3 and BaZrSe3 are 0.60 eV and 1.35 eV, respectively, consistent with the values predicted by HSE calculations. It is seen that alloying with both Ti and Se can lower the bandgap of BaZrS3 effectively. The bandgap decreases almost linearly with increase of Ti concentration. Alloying with Se shows a bowing effect,41, 42 which is known for alloying with anion elements. It is seen that the bandgap reduction is more effective for Ti alloying than for Se alloying. With about 10% Zr by Ti substitution, the bandgap is already reduced to 1.47 eV. To achieve a similar bandgap, 30% S has to be substituted by Se. It should be noted that experimental results have shown that BaZr0.75Ti0.25S3 exists in the perovskite structure only at elevated temperatures (900°C) and under high pressures (60 kBar),43 with larger x compositions not being stable even under these conditions, suggesting that ambient pressure synthesis exploration for the BaZr1-xTixS3 perovskite should be limited to x
