Distant-Atom Mutation for Better Earth-Abundant Light Absorbers: A

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Distant-atom Mutation for Better Earth-abundant Light Absorbers: A Case Study of Cu2BaSnSe4 Zewen Xiao, Weiwei Meng, Jian Li, and Yanfa Yan ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00577 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on November 28, 2016

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Distant-Atom Mutation for Better Earth-Abundant Light Absorbers: A Case Study of Cu2BaSnSe4 Zewen Xiao,†,* Weiwei Meng,† Jian V. Li,‡ and Yanfa Yan†,* †

Department of Physics and Astronomy, and Wright Center for Photovoltaic Innovation and

Commercialization, The University of Toledo, Toledo, Ohio 43606, USA ‡

Department of Physics, and Materials Sciences Engineering and Commercialization, Texas

State University, San Marcos, Texas, 78666, USA AUTHOR INFORMATION Corresponding Author *Z.X.: [email protected] *Y.Y.: [email protected]

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ABSTRACT: Thin-film Cu(In,Ga)Se2 and CdTe solar cells have demonstrated high power conversion efficiencies, but they cannot provide a sustainable clean energy pathway due to the scarcity of Te and In. Here, we propose a distant-atom concept to mutate In by a group-II element (Ba) and a group-IV element (Sn) that are at rather different locations on the periodic table. Due to the very different electronic properties between the cations, the resultant earthabundant orthorhombic Cu2BaSnSe4 absorber does not have the detrimental cation–cation disorder issue seen in earth-abundant kesterite Cu2ZnSnSe4 absorber. We anticipate that Cu2BaSnSe4 solar cells should not have large open-circuit voltage deficits as seen in CuZnSnSe4 solar cells. Density-functional theory calculation on the electronic and defect properties of Cu2BaSnSe4 confirms these expectations.

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Solar photovoltaic and solar-driven water splitting for hydrogen production technologies are attractive choices for producing clean energy.1–4 A key component of these technologies is the light absorber that converts sunlight into electrons and holes. To provide sustainable clean energies, thin-film absorbers made of earth-abundant elements are necessary. Thin-film Cu(In,Ga)Se2 (CIGSe) and CdTe solar cells have demonstrated high power conversion efficiencies,5–10 but they cannot provide a sustainable energy pathway for producing clean energy at the terawatt level due to the scarcity of Te and In.11–13 Therefore, the need of replacing Te and In by earth-abundant elements has attracted enormous research attention. While no good solution has been found for replacing Te, In can be mutated by two earth-abundant elements, Zn and Sn, to form kesterite Cu2ZnSnSe4 (CZTSe). However, while CIGSe solar cells have achieved a record efficiency of 22.6%,6 the record efficiency of kesterite solar cell has been kept at 12.6%14 after years of extensive study,15–23 significantly lower than the theoretical efficiency limit for kesterite solar cell based solely on bandgap consideration. The efficiencies of kesterite solar cells are mainly limited by the large open-circuit voltage (VOC) deficit, defined as Eg/q – VOC, where Eg is the band gap of the absorber and q is the electron charge.24,25 For CZTSe and Cu2ZnSnS4 (CZTS) cells, the VOC deficit can be as large as 650 mV,14,22 comparing to a typical value of less than 500 mV for CIGSe cells.5,6,26 The low VOC deficits of CIGSe thin-film solar cells are primarily attributed to the relatively low non-radiative recombination rate in CIGSe absorbers.27 Theoretical studies have shown that the antibonding coupling between fully occupied Cu 3d and Se 4p orbitals raises the valence band maximum (VBM) of CIGSe, which consequently makes Cu vacancies (VCu) very shallow acceptors.28 Furthermore, the antibonding nature energetically favors the formation of Cu vacancies, making them the dominant defects in CIGSe.28 CZTSe and CZTS also exhibits antibonding coupling


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between fully occupied Cu 3d and Se 4p/S 3p orbitals and the Cu vacancies are also shallow acceptors.29,30 However, because Cu and Zn are the nearest neighbors on the periodic table and share similar structural environments in CZTSe and CZTS, it is easy to form Zn-on-Cu (ZnCu) and Cu-on-Zn (CuZn) antisite defects, which are not shallow defects.31,32 Recent hybrid density functional studies reveal that Sn-on-Zn (SnZn) and Sn-on-Cu (SnCu) antisites also can be recombination centers with defect states.33,34 Such cation−cation disorder may introduce absorption band-tail states and high density of non-radiative recombination centers,35–37 leading to the large VOC deficits for CZTSe and CZTS solar cells. We have previously used an ionic size (i.e., Shannon Radius38) mismatch argument to mitigate the above cation−cation disorder issue and studied the electronic and defect properties of trigonal Cu2−II−Sn−VI4 (II=Ba, Sr and VI =S, Se) quaternary compounds.39 We predicted that trigonal Cu2BaSnS4 (CBTS) and Cu2BaSn(S,Se)4 (CBTSSe) can be promising candidates for solar cell and photoelectrochemical water splitting. However, this argument cannot explain why CIGSe solar cells have small VOC deficits but CuGaSe2 (CGSe) solar cells have large VOC deficits. The Shannon radius of Ga3+(0.47 Å), In3+ (0.62 Å), Zn2+ (0.6 Å), and Sn4+ (0.55 Å) ions and their differences with that of Cu1+ (0.6 Å) would suggest that Ga−Cu antisite defects should have higher formation energies than In−Cu antisite defects, and therefore, CGSe solar cells should have smaller VOC deficits than CIGSe solar cells, which contradicts with experimental observations. Here, we propose a new concept–distant-atom mutation–to mitigate the above shortfalls. If two cations are located at rather distant locations on the periodic table, the formation of cation–cation antisite defects will be difficult, due to their very different electronic properties. The distant-atom concept now explains why a Ga-on-Cu antisite defect, which is a deep donor, has a lower formation energy than an In-on-Cu antisite defect, which is a shallow


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donor.40 It is also consistent with experimental report that CGSe absorbers typically exhibit much shorter minority carrier lifetimes than CIGSe absorbers.27 Using orthorhombic Cu2BaSnSe4 (CBTSe) as a case study, we investigate the effects of distant-atom mutation using density-functional theory (DFT) calculation. Cu, Ba, and Sn are positioned at rather different locations on the periodic table and have very different electronic properties. Ba is significantly more ionic than Cu and Sn. As a result, Ba has a rather different structural environment than Cu and Sn in orthorhombic CBTSe, i.e., Ba atoms are eight-fold coordinated whereas Cu and Sn are tetrahedrally coordinated (4-fold). Ba atoms do not make any contributions to the conduction band minimum (CBM) and valence band maximum (VBM). Therefore, Ba vacancies produce very shallow states. This is similar to the effects of the organic molecular cations in the organic−inorganic lead halide perovskites.41–44 Due to antibonding coupling between fully occupied Cu 3d and Se 4p orbitals, VCu’s are shallow acceptors and the dominant defects under Cu-poor synthesis conditions. The cation-on-cation defects such as Baon-Cu (BaCu) and Cu-on-Ba (CuBa) have high formation energies and are difficult to form at all growth conditions. Furthermore, we find that there is a large chemical potential range for stabilizing orthorhombic CBTSe, indicating that single-phase absorbers can be easily formed, which is very desirable for thin-film solar cell fabrications. The chemical potential range is much larger than that for its trigonal CBTS counterpart.39 Our results suggest that orthorhombic CBTSe is a promising candidate for solar energy harvesting. Its bandgap of ~1.75 eV is not ideal for efficient single-junction solar cell applications, but is ideal for top cell in tandem solar cells and solar-driven water splitting for hydrogen production. The distant-atom mutation concept also provides an insightful guidance for designing new light absorbers with desirable photovoltaic properties.


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We first describe the effects of distant-atom mutation on the crystal structure of CBTSe. Due to the large difference of electronic properties between Ba and Cu/Sn, Ba and Cu/Sn prefer rather different local structures and coordination. Therefore, CBTSe crystallizes in space group Ama2 with a = 11.1215(13) Å, b = 11.2373(13) Å, c = 6.7531(8) Å, and Z = 4.45 Figure 1a shows the crystal structure of CBTSe, which comprises [BaSe8] square antiprisms connected via opposite faces to linear chains running along [001]. The smaller cationic elements, Cu and Sn, are 4-fold coordinated. The [CuSe4] tetrahedra are pairwise interconnected, and isolated [SnSe4] tetrahedra connect them via corner-sharing to form a three-dimensional network and onedimensional channels along [001]. The Ba cations are located in the channels, leading to cagelike spaces surrounded by two Ba atoms, two Cu atoms, two Sn atoms, and eight Se atoms, as shown in Figure 1b.

Figure 1. (a) Crystal structure of Cu2BaSnSe4. (b) Local symmetry of the cage-like spaces involved in the tetrahedra channels along [001].


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The distant-atom mutation also has important effects on the electronic properties of CBTSe. Because Ba is very ionic, Ba atoms do not contribute to the lower conduction band (CB), which resembles the situation of the methylammonium (MA) ions in the organic-inorganic lead halide perovskites.41–43,46 Figures 2a and 2b show the band structure and densities of states (DOSs) for CBTSe, respectively, calculated with the Heyd−Scuseria−Ernzerhof (HSE)47,48 hybrid functional. The band structure exhibits a direct bandgap of 1.38 eV at the Γ-point, which is slightly smaller than the reported experimental value (1.72 eV).49 The calculated effective masses for holes are 1.87 m0, 0.62 m0, and 1.20 m0 along Γ−Z, Γ−Y, and Γ−S, respectively, and the effective masses for electrons are 0.23 m0, 0.37 m0, and 0.20 m0 along Γ−S along Γ−Z, Γ−Y, and Γ−S, respectively. The VBM is composed of antibonding states of Se 4p and Cu 3d orbitals. The CB consists of antibonding states of Sn 5s and Se 4p orbitals and has an unusual narrow band width of 0.84 eV. Interestingly, the “upper” CB, which is composed of Ba 5d, 6s, Cu 4s, Sn 5p and Se 4p orbitals, separates from the CB with a forbidden gap of 0.69 eV. On the other hand, the bonding states of Sn 5s and Se 4p orbitals are deeply located at around 7.6 eV below the VBM and form a narrow band with 0.27 eV in width. These features of the band structure of CBTSe are similar to that of the molecular iodosalt semiconductor Cs+2[SnI6]2−, where strong covalency exists in the [SnI6]2− units.50,51 To estimate numerically the bonding nature and the electron redistribution in CBTSe, we carried out a Bader analysis. The Bader effective charges of CBTSe can be expressed as Cu+0.352 Ba+1.51Sn+1.13Se−0.844. The Bader effective charge of Ba (+1.51) in CBTSe is close to that of +1.42 in a typical Ba2+-based ionic compound BaTiO3,52 indicating that the bonding nature between Ba2+ cations and the [Cu2SnSe4]2− network is mainly ionic, consistent with the fact that the highly-ionic Ba2+ cations have little contributions to the lower CB. The Bader effective


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charge of Cu (+0.35) is much smaller than that of typical Cu+-based oxide Cu2O (+0.53),53 indicating strong covalency of the Cu−Se bonds. As seen in Figure 2b, the Cu 5s orbitals covalently bond with Se 4p orbitals; therefore, Cu 5s orbitals are partially occupied, leading to the low Bader effective charge. The Bader effective charge of Sn (+1.13) is much smaller than those for typical Sn4+-based compounds such as SnO2 and SnF4 (+2.40 and +2.71, respectively),50 but close to Sn2+-based compounds such as SnO and SnF2 (1.25 and 1.56, respectively),50 indicating that the nominal effective charge of Sn in CBTSe is closer to +2, rather +4, like the Cs2SnI6 case.50 This is consistent with the fact that Sn 5s orbitals are mainly occupied and Sn 5p orbitals are also partially occupied (see Figure 2b). Also because of the strong covalency in the [Cu2SnSe4]2− network, the Bader effective charges of the Se anions (−0.83, −0.89, and −0.80 for Se1, Se2, and Se3 in CBTSe,45 respectively) are much smaller than the nominal value of −2.

Figure 2. (a) Band structure and (b) total and projected DOS of CBTSe.


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We find that there is a rather large chemical potential window that can thermodynamically stabilize orthorhombic CBTSe. Figure 3 shows the calculated chemical potential window of CBTSe (see computational methods in the Supporting Information), which is a polyhedron surrounded by the thermodynamic equilibrium planes of elemental Cu, Sn, Se, binary compounds BaSe, BaSe2, BaSe3, Cu2Se, CuSe, SnSe, and SnSe2, and ternary compounds BaCu2Se2, BaSn2Se3, Ba7Sn3Se13, and Cu2SnSe3. It is noted that the chemical potential window of CBTSe is much larger than that of kesterite Cu2ZnSnSe4 reported in the literature,30,34 indicating CBTSe should exhibit better thermodynamic stability than CZTSe. This explains the easiness of synthesis of pure CBTSe phase. The large-∆µSe side (i.e., near the TUVWX plane) indicates Se-rich and metal cation-poor chemical conditions, where the competing secondary phases are Se, SnSe2 and some non-stoichiometric Se-rich selenides such as BaSe2, BaSe4, CuSe, and BaSn2Se5 (based on the nominal +2, +1, and +4 oxidation states for Ba, Cu, and Sn cations). The small-∆µSe side (i.e., near the A, B, E, and F points) indicates Se-poor and metal cation-rich chemical conditions, where metallic Cu and Sn are the competing phases. The large size of chemical window also allows the control of the formation of defects for optimizing photovoltaic properties, as will be discussed later.


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Figure 3. Chemical potential (∆µCu, ∆µSn, ∆µSe) window diagram. The polyhedron shows the region where CBTSe is stabilized against possible competing phases. The distant-atom mutation affects the defect properties of CBTSe. Because of the high ionicity, Ba-related defects such as Ba vacancy (VBa), Ba interstitial (Bai), Ba-Cu antisites (BaCu, CuBa) produce shallow states. This is similar to MA-related defects in MAPbI3.41–43 Figures 4a and 4b show the calculated charge-state transition levels for intrinsic acceptors and donors, respectively. It is seen that VBa is a shallow accepter with the (0/1−) and (1−/2−) transitions at 0.04 eV below the VBM and 0.04 eV above the VBM, respectively. Ba interstitial (Bai) is also a shallow donor with the (0/2+) transition even above the VBM. Cu vacancy (VCu) is a shallow acceptor with the (0/1−) transition just at the VBM. Similar to other Cu+-based semiconductor absorbers such as CIGSe and CZTS,28–30 the shallow nature of VCu is attributed to the antibonding coupling between the high-lying Cu 3d orbitals and Se 4p orbitals, which derive the VBM. Cui is a shallow donor with the (0/1+) transition at 0.14 eV below the CBM. Similarly,


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CuBa shows a shallow (0/1−) transition at 0.02 eV above the VBM and acts as an acceptor, while BaCu is a shallow donor with the (0/1+) transition at 0.17 eV above the CBM. In contrast to VBa and VCu, VSn exhibits (0/n−) transitions in the bandgap. Similar to VSn, CuSn and BaSn have deep (0/n−) transitions in the bandgap. VSe is expected to be a donor; however, its (0/1+) and (1+/2+) transitions are far from the CBM, but very close to the VBM. As a result, VSe is neutral. The behavior of cation-on-Se antisites can be qualitatively understood by viewing it as a pair of cation interstitial and VSe. Because of the neutral VSe at most Fermi levels (EF), the CuSe and BaSe antisites can ionize to +1 and +2 charge states, similar to simple Cui and Bai, respectively. Both the (0/1+) transition of CuSe and the (0/2+) transition of BaSe are as shallow acceptors above the VBM. In contrast to CuSe and BaSe, SnSe is a deep donor with (0/n+) transitions below the mid-bandgap. Sni has very shallow (0/1+) and (1+/2+) transitions, but very deep (2+/3+) and (3+/4+) transitions. As a result, for most EF, Sni is ionized in the charge state of 2+, which further supports that it is difficult for Se to oxidize Sn to the nominal +4 oxidation state. For SnCu, the (0/1+) transition is above the CBM, and the +1 charge state is stabilized almost in the whole bandgap. SnBa is stabilized in the neutral state almost in the whole bandgap. The +1 charge state of SnCu and neutral state of SnBa, to some degree, further support the above-discussed “+2” nominal effective charge of Sn. Se interstitial (Sei ) has (0/1−) and (1−/2−) transitions at 0.64 eV and 1.04 eV above the VBM, respectively, the origin of its deep nature will be discussed later.


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Figure 4. Calculated charge-state transition levels of (a) intrinsic acceptors and (b) intrinsic donors in CBTSe. The distant-atom mutation also affects the formation of the dominant defects under specific growth conditions. First, due to the large difference on electronic properties of Ba and Cu, the formation energies of Ba−Cu antisite defects are very high under all synthesis conditions, in contrast to the Cu−Zn antisite defects in CZTSe. Figure 5 shows the calculated formation enthalpies for intrinsic defects under three representative chemical potential points A, U and L shown in Figure 3. The formation energies of CuBa and BaCu are much higher than other defects such as VCu, Sei, Cui, SnBa, etc. This will ensure a low degree of cation–cation disorder seen in CZTSe, a main cause for the large Voc deficits of CZTSe solar cells. Second, under Cu-poor and Se-moderate conditions (Figure 5a), VCu’s are still the dominant defects. The equilibrium Fermi level is determined by the compensation between VCu and SnCu, and is located at 0.38 eV above the VBM, leading to CBTSe with a weak p-type conductivity. Because the calculated average election effective mass (around 0.25 m0) is much smaller than the average hole effective mass


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(around 1.01 m0), a weak p-type CBTSe is desirable for solar cell applications. CBTSe films grown under other conditions are not suitable for solar cell applications. Under Cu-rich and Sepoor conditions (Figure 5b), SnBa has the lowest formation energy, which, however, does not significantly affect the electrical and photovoltaic properties because it keeps at the neutral charge state in the bandgap. The acceptors VCu and VBa and the donors SnCu, Sni, and Cui have relatively low formation energies and are the dominant defects. The acceptors and donors compensate each other, leading to a Fermi level pinned slightly below the mid-bandgap (e.g., 0.66 eV above the VBM) and resulting in a near-insulating conductivity, not desirable for solar cell applications. Under Cu-poor and Se-rich conditions (Figure 5c), Sei has the lowest formation energy (3.7 Å). The increase of effective charge of the interstitial Se atom should increase the anion−anion repulsion, leading to a high energy. Further, the “−0.97” charge state of the interstitial Se atom in the Sei2− structure is higher than its surrounding and bulk Se atoms, which is beyond the oxidation power of the Se atoms in CBTSe. Therefore, the charged states for Sei are not so energetically favored, leading to the very deep (0/1−) and (1−/2−) transitions at 0.64 eV and 1.04 eV above the VBM, respectively.


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Figure 6. Local symmetry around the cages of (a) host, (b) Sei0, (c) Sei−, and (d) Sei2−. The Bader effective atomic charges are shown. (e) Crystal structure of a Se-rich compound Ba2SnSe5, where Ba atoms are 9-coordinated with Se atoms. In summary, to solve the scarcity of In in CIGSe solar cells, we have proposed a distantatom mutation concept to mutate In by a group-II element (Ba) and a group-IV element (Sn) that are at rather different locations on the periodic table to form earth-abundant orthorhombic CBTSe photovoltaic absorber. Due to the very different electronic properties between Ba and Cu, the orthorhombic CBTSe absorber exhibits better photovoltaic properties than its CZTSe counterpart, as confirmed by DFT calculations. We found that due to the high ionicity, dominant Ba related defects, such as VBa, Bai, CuBa, and BaCu create only shallow defect levels. We further found that CBTSe synthesized under Cu-poor and Se-rich conditions should exhibit a moderate p-type conductivity suitable for being absorbers and low degree of cation–cation disorder. Our results suggest that CBTSe solar cells may have smaller VOC deficits than CZTSe solar cells. The distant-atom mutation concept provides insights for designing new light absorber materials with desirable photovoltaic properties. ASSOCIATED CONTENT


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Supporting Information. Computational methods. AUTHOR INFORMATION Present Address Materials Research Center for Element Strategy, Tokyo Institute of Technology, Yokohama 2268503, Japan Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The work was also partially supported by the National Science Foundation under contract no. CHE−1230246 and DMR−1534686, and the Ohio Research Scholar Program. This paper presents results from an NSF project (award number CBET−1433401) competitively−selected under the solicitation “NSF 14−15: NSF/DOE Partnership on Advanced Frontiers in Renewable Hydrogen Fuel Product via Solar Water Splitting Technologies”, which was co−sponsored by the National Science Foundation, Division of Chemical, Bioengineering, Environmental, and Transport Systems (CBET), and the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office. This research used the resources of the Ohio Supercomputer Center and the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DEAC02-05CH11231.

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