Layered Hexagonal Oxycarbides, Mn+1AO2Xn (M = Sc, Y, La, Cr, and

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Layered Hexagonal Oxycarbides, M AOX (M=Sc, Y, La, Cr and Mo, A=Ca, X=C): Unexpected Photovoltaic Ceramics Zhenyu Wang, Xin Chen, and Chunming Niu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00905 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 11, 2018

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The Journal of Physical Chemistry

Layered Hexagonal Oxycarbides, Mn+1AO2Xn (M=Sc, Y, La, Cr and Mo, A=Ca, X=C): Unexpected Photovoltaic Ceramics Zhenyu Wang,† Xin Chen,*,† and Chunming Niu*,† †

Center of Nanomaterials for Renewable Energy (CNRE), State Key Lab of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi’an Jiaotong University, 99 Yanxiang Road, Xi’an 710054, China

ABSTRACT: A family of layered hexagonal oxycarbides and oxynitrides with the general formula, Mn+1AO2Xn (MAOX) is discovered using first-principles DFT calculations, where n=1–3, M is an early transition metal, A is an alkaline earth metal in Group IIA or a late transition metal in Groups IB and IIB, X is C and/or N. Thermodynamically, the MAOX phases are very stable. Tuning the composition, MAOX can be metals, semimetals or semiconductors. To date, five 2121 oxycarbide MAOX phases, M2CaO2C (M=Sc, Y, La, Cr and Mo) are found to be semiconductors with bandgaps from 0.39 to 1.14 eV. To our strong surprise, they have superior photovoltaic (PV) properties and their theoretical solar cell efficiencies are on par with GaAs. Particularly, the efficiency of Cr2CaO2C reaches 27.7% that is above 90% of the Schottky-Queisser (SQ) limit. Furthermore, amazingly, the five MAOX semiconductors possess outstanding strength and machinability, e.g., their Young’s moduli are comparable to ceramics and MAX phases, and Poisson’s ratios higher than MAX and even comparable to metals. MAOX semiconductors are promising multifunctional ceramics. The unique combination of the photovoltaic and mechanical properties will certainly enable the MAOX semiconductors to find broad applications in the photovoltaic industry.

1. INTRODUCTION The endeavor to seek new PV materials and develop highperformance solar cells never stops. The rapid development of computational methods and tools deepens our understanding of existing materials and provides an insightful guide for experiments.1-7 And most importantly, this makes it possible to discover novel materials with computer simulations, such as solid state electrolytes,8-10 electrocatalytic materials,11 twodimensional (2D) semiconductors,12 perovskite solar cells etc.13 In the recent years, advanced functional ceramics attract broad interests due to its outstanding mechanical properties, such as high strength and hardness, excellent wear resistance and good frictional behavior.14, 15 In addition, thermoelectricity, piezoelectricity, ferroelectricity, and ferromagnetism have been found in the functional ceramics which exhibit fascinating applications in electronic devices.16, 17Given excellent dimensional stability as well as the ability to withstand corrosive environments, the applications of functional ceramics in the semiconductor industry is desirable. Ceramic semiconductors are mostly metal oxides, but there are also other versatile ceramic materials including carbides and nitrides, such as silicon carbide (SiC) and pyrolytic boron nitride (PBN).18, 19 Recently, as a new family of 2D carbide and

nitride materials, MXene has been of increasingly significant interest. It has been demonstrated that MXenes are promising materials for applications in the areas, such as energy storage, catalysis, and composite reinforcement.20-24 The surface of etching produced MXenes is functionalized by the anionic functional group, such as O, –F, –OH. MXene phases are mostly metallic.12, 25-28 Extensive efforts have been devoted to engineering semiconducting MXenes by doping,29 surface modification,12, 30 nanoribbons,31-33 and strain effect.34 Interestingly, functionalization has turned some MXenes (M=Sc, Y, La, Ti, Zr, Hf, and Mo) into semiconductors.12, 25, 31, 35, 36 Their bandgaps are listed in Table S1. Unlike MXene, our DFT calculations show that the new family of oxycarbides, Mn+1AO2Xn (MAOX), can be intrinsic semiconductors. MAOX semiconductors are extremely stable at high temperatures. They also have outstanding mechanical and photovoltaic properties. Due to their carbide nature, we anticipate that MAOX semiconductors can have high resistance to corrosive environments. The combination of the above-mentioned properties will make MAOX stand out among the emerging semiconductors and enable it to find broad applications in the photovoltaic industry, such as solar roof tiles.

2. CALCULATION DETAILS

ACS Paragon Plus Environment

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All periodic density functional theory (DFT) calculations were performed by the Vienna Ab Initio Simulation Package (VASP),37-40 with the projector-augmented wave (PAW) method.41, 42 A plane wave cut-off energy of 440 eV was applied in the geometry optimization, electronic and optical properties calculation. Brillouin zone was sampled in Γ-centred k-point meshes of 5×5×2 and Perdew-Becke-Ernzerhof (PBE) exchange-correlation (xc) functional were used for the geometry optimization.43 Denser k-point meshes, Γ-centred 6×6×3, a hybrid HSE06 exchange-correlation functional including 25% screened Hartree–Fock exchange was implemented for the electronic structures analysis.44, 45 The path plotting band structure along the high-symmetry k-points in Brillouin zone was displayed in Figure S1. For the optical calculation, the kpoint meshes of Γ-centred 8×8×3 were set and the tetrahedron method was added with HSE06 functional.46 The valence band alignment was calculated using the core-level alignment approach developed by Wei and Zunger.47 The (110) surface was chosen for the calculation since it is nonpolar and did not produce any dangling bonds. A cut-off energy of 500 eV, kpoint meshes of Γ-centred 12×12×5 and PBE functional were utilized to calculate the elastic constants and moduli. The initial configurations of representative MAOX materials with 4×4×3 supercell were selected for AIMD simulations. Each of AIMD simulation was performed for 8 ps with the time step of 1.0 fs in the NVT ensemble, and the temperatures were controlled at 1000 K with the Nosé-Hoover method.48 Calculations for geometry optimization and electronic properties were converged to within an energy and force tolerance of 10−5 eV⸱atom-1 and 0.02 eV⸱Å-1, respectively. Tight convergence criteria of 10−8 eV⸱atom-1 and 10−5 eV⸱Å-1 were set for mechanical property calculation.

3. RESULTS AND DISCUSSION The MAOX phases have a hexagonal, laminar structure with the P3m1 symmetry. The structure of M2AO2X (2121 MAOX) is shown in Figure 1a. MAOX has a similar hexagonal lattice with MAX.49-51 Structurally, the MAOX phase can be viewed as a result of replacement of single atomic A layer in MAX by a triatomic layer of oxide AO2. In addition to 2121 phase, MAOX could have two more phases of 3122, and 4123, all of them

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Figure 2. The distribution of elements of M, A, O, X in MAOX in the periodic table. The blue represents the M elements; the orange A; the red O; the gray X. can be represented by the general formula of Mn+1AO2Xn (Figure 1). According to the distribution of M, A, X in Figure 2, for the 2121 phase alone, there are 216 different varieties theoretically. For this report, our detailed computational evaluation is limited to twelve Ca coupled 2121 MAOX phases with a general formula of M2CaO2C, where M=Sc, Ti, V, Cr, Mn, Y, Zr, Nb, Mo, La, Hf, Ta. Table 1. Theoretical structural data and thermal stability of the twelve MAOX phases Compound

a (Å)

c (Å)

α (°)

γ (°)

Stability

Sc2CaO2C Ti2CaO2C V2CaO2C Cr2CaO2C Mn2CaO2C Y2CaO2C Zr2CaO2C Nb2CaO2C Mo2CaO2C La2CaO2C Hf2CaO2C Ta2CaO2C

3.283 3.147 3.099 3.112 3.122 3.498 3.319 3.266 3.307 3.658 3.286 3.282

8.083 7.803 7.580 7.291 7.170 8.474 8.175 7.891 7.489 8.832 8.090 7.856

90.0 90.0 90.0 90.0 90.0 90.0 90.0 90.0 90.0 90.0 90.0 90.0

120.0 120.0 120.0 120.0 120.0 120.0 120.0 120.0 120.0 120.0 120.0 120.0

Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes

The stability of twelve Ca coupled 2121 MAOX phases is examined theoretically based on the formation energy (Section S1). Their lattice constants and thermal stability are presented in Table 1, the formation energy relative to several competing phases are shown in Table S2, and the corresponding relaxed structures are presented in Figure S2. Due to the close-packed structure nature of MAOX, the lattice parameters, a and c (Table 1) are strongly related to the atomic radius of M atoms. All of them have negative formation energies, indicating that they are thermally stable with respect to the corresponding elementary substances. To further evaluate the stability of these phases, ab initio molecular dynamics (AIMD) simulations at the temperatures of 1000 K were carried out for the five semiconducting MAOX phases, Sc2CaO2C, Cr2CaO2C, Y2CaO2C, Mo2CaO2C, and La2CaO2C. The calculations were performed in the NVT ensemble with 4×4×3 supercells correspondingly. The structures of the five MAOX phases at temperatures of 1000 K after 8 ps are shown in Figure S3-S7. For the five MAOX phases, their laminar hcp structures sustain in the final configurations after 8 ps, indicating they are thermally stable at ambient temperature.13, 52 For the caveat, although the stability tests of MAOX phases are common practices widely used and accepted in the current research, particularly for computational materials, it is worthwhile stating that these tests have limitations. Our results provide a computational guidance in the search for new PV materials. Certainly, the

Figure 1. The crystal structure of Mn+1AO2Xn phases (n=1–3) Plus Environment ACS Paragon with hexagonal symmetry shows AO2-layers interleaves between Mn+1Xn MXene layers: (a) M2AO2X, (b) M3AO2X2 and (c) M4AO2X3. The unit cell of M2AO2X is marked in black dash

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The Journal of Physical Chemistry ultimate tests of the new MAOX materials are the experimental validations. The density functional theory (DFT) calculations show that electronic properties of the twelve MAOX phases span from metals, semimetals, to semiconductors (both direct and indirect ones) as depicted in Figure 3. The five semiconductor MAOX phases, Sc2CaO2C, Cr2CaO2C, Y2CaO2C, Mo2CaO2C, and La2CaO2C are discovered to be fantastic photovoltaic semiconductors with theoretical efficiencies on par with the prominent semiconductors, such as crystalline Si, single-junction GaAs, Cu(In, Ga)Se2 (CIGS), CdTe, etc.

Figure 3. Elements M in the periodic table and the corresponding semiconductor MAOX are marked in orange, semimetals green, and metals blue. Table 2. The fundamental indirect and direct bandgaps (Eind and Edir) given in eV, hole and electron effective masses (mh and me) in the directions that are parallel to the MAOX layer given in m0, spectroscopic limited maximum efficiencies (SLMEs) given in % at 0.2 µm, Young’s moduli (E) given in GPa and Poisson’s ratios (ν) of M2CaO2C, where M = Sc, Y, Cr, Mo and La, based on the HSE06 calculations. m0 stands for the electron mass Compound Sc2CaO2C

Eind

Edir

mh

me

SLME

E

ν

1.14

1.55

2.01

0.47

17.5

250.0

0.22

Y2CaO2C

0.94

1.07

3.30

0.42

22.1

213.6

0.22

Cr2CaO2C

0.85

0.90

1.55

2.54

27.7

231.2

0.31

Mo2CaO2C

0.78

1.00

0.66

0.35

22.4

232.4

0.30

La2CaO2C

-

0.39

2.71

0.42

13.6

147.7

0.29

The hybrid functional HSE06 was applied to calculate the band structures and projected density of states (DOS). The details of the band structures and projected DOS are depicted in Figure 4 and S8. Table 2 shows the bandgaps of the five semiconductors, Sc2CaO2C, Cr2CaO2C, Y2CaO2C, Mo2CaO2C, and La2CaO2C, ranging from 0.39 to 1.14 eV. La2CaO2C has a direct bandgap, the remain four all have indirect bandgaps. In comparison, the bandgaps of MXene semiconductors are all reported to be indirect.12, 25, 31, 35, 36 The M transition metals in

the five MAOX semiconductors belong to Groups IIIB and VIB. The projected DOS (PDOS) reveals the atomic origin of the band structure of the MAOX semiconductors in Figure 4. The transition metal Ms’ d orbitals have significant contributions in the highest valence bands and lowest conduction bands while p orbitals of C contribute mostly to valence bands. The relative contributions of M’s d and C’s p orbitals to the highest valence bands vary. For example, in Cr2CaO2C and Mo2CaO2C, the contributions from d orbitals of Cr and Mo (VIB group) to the highest three valence bands are much larger than those from p orbitals of C. However, in Sc2CaO2C, Y2CaO2C and La2CaO2C, the contributions from d orbitals of Sc, Y and La (IIIB group) to the highest three valence bands are comparable to those from p orbitals of C. The calculations suggest that the electronic properties of the MAOX semiconductors can be tuned with the selection of M from the IIIB and VIB groups. Figure 4 (more to refer to Figure S8 and S9 in the Supporting Information) shows that the d orbitals of the transition metal M in both MAOX and MAX dominate the density of states around Fermi level.53-56 The existence of the oxygen atoms in the MAOX semiconductors leads to the localization of electron of the transition metal M due to the bonding between O and M, and therefore results in energy level splitting and opens up the bandgap. Given the laminar structure, MAOX has two distinctive directions: the parallel direction corresponding to the a1-a2 plane and the c direction perpendicular to the a1-a2 plane as shown in Figure 1. The effective masses for the five MAOX semiconductors are computed in the two directions according to the band structures plotted in Figure 4. The computational details of effective masses can be found in Section S2. The laminar structure leads to the strong anisotropic electronic transport properties. For the parallel direction, the electron effective masses range between 0.35 and 2.54 m0 and hole effective masses between 0.66 and 3.30 m0 (Table 2). In general, the electron effective mass is lighter than that of the hole, which is similar to MXenes.33, 57 As an exception, Cr2CaO2C has much heavier electron effective mass than other four MAOX semiconductors. The electron effective masses of Sc2CaO2C, Y2CaO2C, and La2CaO2C are around 0.40 m0, which are much heavier than zinc-blende III-V direct semiconductors, such as InP, InAs, InSb, GaAs, and GaSb, with electron effective masses below 0.08 m0,58 but are marginally larger than oxide semiconductors, such as In2O3, CdO, and SnO2 (around 0.3 m0),59-61 and smaller than 0.95 m0 of CuO.62 Mo2CaO2C has an indirect bandgap and its electron effective mass is 0.35 m0 comparable to those of wurtzite-type ZnO and ZnS.63 Its hole effective mass is 0.66 m0 which is the smallest among the five semiconductors. Mo2CaO2C has the most attractive electronic transport property among the five MAOX semiconductors. For the perpendicular direction, the electron effective masses range between 0.47 and 32.46 m0 and hole effective masses between 2.28 and 17.24 m0 (Table S3). Anisotropically, both electrons, and holes in the perpendicular direction have much heavier effective mass than those in the parallel direction due to the layer structure.

Figure 4. The HSE06 calculated band structures and the projected DOS of the five MAOX semiconductors: (a) Sc2CaO2C, (b) Y2CaO2C, (c) Cr2CaO2C, (d) Mo2CaO2C, and (e)ACS La2CaO 2C. Paragon Plus Environment For the semiconductors, the valence band maximum and conduction band minimum are marked in red and green circles. The dash black line indicates the Fermi level.


Figure 5. (a) Optical absorption spectra, with the visible light spectrum as a reference, (b) SLME, (c) open-circuit voltage (Voc), (d) short-circuit current density (Jsc) of the five MAOX semiconductors versus different absorber film thicknesses, (e) Voc and (f) Jsc of 0.2 μm thin film PV absorbers versus optical bandgaps of the five MAOX semiconductors, and GaAs as benchmark. The calculated optical bandgaps of the six semiconductors are, Sc2CaO2C=1.55 eV, Y2CaO2C=1.07 eV, Cr2CaO2C=0.90 eV, Mo2CaO2C=1.00 eV, La2CaO2C=0.39 eV and GaAs=1.18 eV. To compute and benchmark the solar cell efficiency, the Shockley-Queisser (SQ) and spectroscopic limited maximum efficiency (SLME) models are adopted (The details of implementations can be found in Section S3).64 The calculated absorption spectrums and SLMEs at different thicknesses are presented in Figure 5 a and b with the benchmark of GaAs. In Figure 5 c and d, open-circuit voltage (Voc) and short-circuit current density (Jsc) versus the thickness of thin films are shown, respectively. When the thickness becomes infinite, SLME will eventually converge to the SQ limit,64, 65 and the photovoltaic properties (Voc, Jsc and SLMEs) of the semiconductors with stronger absorption in the visible light region converge more quickly according to the SLME model. The absorption spectrums are calculated based on the optical linear response in which only the intraband transitions are considered since the interband ones are forbidden. The relationships Voc and Jsc of 0.2 μm thin film with the optical bandgaps are shown in Figure 5 e and f respectively for the five MAOX semiconductors and benchmark GaAs. For the semiconductors with direct bandgaps, normally, the larger optical bandgap, the higher Voc, and lower Jsc. This is true for the direct semiconductors, La2CaO2C and GaAs, but no such correlation is observed for four indirect semiconductors, Sc2CaO2C, Cr2CaO2C, Y2CaO2C and Mo2CaO2C in Figure 5 e and f, which can be explained largely by a different mechanism of electronhole-pair recombination between direct and indirect semiconductors. The Jsc is limited by the radiative recombination in

Figure 6. SLMEs of the five MAOX semiconductors and GaAs for the 0.2 μm thin film PV absorbers in comparison with 100%, 75% and 50% of the SQ limit plotted in orange, green and blue lines respectively. The calculated optical bandgaps of the six semiconductors, Sc2CaO2C=1.55 eV, Y2CaO2C=1.07 eV, Cr2CaO2C=0.90 eV, Mo2CaO2C=1.00 eV, La2CaO2C=0.39 eV and GaAs=1.18 eV. solar cells based on low defect direct bandgap semiconductors while Auger recombination is a major limiting factor for solar cells based on low defect indirect bandgap semiconductors.64, 66, 67. In comparison with the SQ limit, SLMEs of the five MAOX semiconductors and benchmark GaAs are presented in Figure 6. SLMEs are computed based on a thin film of 0.2 µm thickness and listed in Table 2. The SLMEs of the five MAOX semiconductors are all above 50% of the SQ limit as shown in Figure 6. The best MAOX semiconductor, Cr2CaO2C is an indirect one with the incredible 27.7% SLME efficiency which is above 90% of the theoretical SQ limit. We use GaAs as the benchmark in Figure 6. For caveat, the GaAs experimental convention efficiency (28.8%) is better than the one predicted by the SLME model based on the absorption calculated from HSE06 functional with tetrahedron correction.68 Thus, it needs further theoretical study and experimental work to know if Cr2CaO2C and Y2CaO2C, Mo2CaO2C can be better than GaAs in the realistic solar cell system. On the other hand, the efficiency of the narrow bandgap La2CaO2C predicted by SLME is

Figure 7. Band alignments of M2CaO2C (M = Group IIIB (Sc, Y, La) and Group VIB (Cr and Mo)), relative to the vacuum level. Electron affinities (EAs) and ionisation potentials (IP) are shown as blue and orange numbers at the top and bottom of the figure, respectively.


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The Journal of Physical Chemistry 13.6% and exceeds the SQ limit because the convention efficiency of the narrow bandgap semiconductors predicted by SLME could be overestimated,67 such as for CuInSe2. Overall, theoretical optoelectronic results suggest Mo2CaO2C is the most desirable MAOX semiconductor as a photovoltaic material. Electron affinity (EA) and ionization potential (IP) of photovoltaic materials play an important role in the band alignment and the performance of solar cells, in particularly, for electron and hole collection at interfaces.69 Figure 7 presents the band alignments of the five MAOX semiconductors where the M elements fall in Group IIIB and VIB. It shows an interesting trend for the fundamental bandgap, which decreases in the order of Sc2CaO2C, Y2CaO2C, and La2CaO2C as M metal descent from the top down in Group IIIB. This is also true for Cr2CaO2C and Mo2CaO2C as M metal descent from Cr to Mo in Group VIB. To understand the contribution of M transition elements in the band alignment, the electron affinities and first ionization potentials of M elements of Groups IIIB-VIIB for all the twelve MAOX phases are shown in Figure 8.70-78 In Group IIIB, electron affinity increases and first ionization energy decrease from Sc, Y to La while in Group VIB, both electron affinity and first ionization energy increase from Cr to Mo. The changes of

Figure 8. The electron affinities (upper number) and ionization potentials (lower number) in eV for M elements from 4th to 6th period and group IIIB to VIIB, where the first ionization energies are taken as the ionization potentials. Those marked in orange, green and blue represent the MAOX composed by them are semiconductors, semimetals and metals respectively. Elements in gray that don’t belong to M are included to show the trend of EAs and IPs. The electron affinities and first ionization energies have been taken from the literature.70-78 EA and IP are non-monotonic for the elements in group IVB, VB and VIIB and the corresponding MAOX phase is either metal or semimetal. Those groups might be avoided in the search for the MAOX semiconductors in the future. We have also studied mechanical properties of five MAOX semiconductors, and the computational details can be found in Section S4. As shown in Table 2, their Young’s moduli are comparable to those of MAX phases50, 79-82 except for La2CaO2C which has the relatively lower Young’s modulus of 147.7 GPa. Thus, the MAOX semiconductors could be elastically as stiff as the MAX phase. The shear and bulk modulus are listed in Table S4.

The Poisson’s ratio of metals, particularly steel is around 0.3, while ceramics and glasses are around 0.2.83 The 211 MAX phases have Poisson’s ratio between 0.18 to 0.21,79, 80 which makes the MAX strain-elastic property close to ceramics. To our surprise, Table 2 shows that three MAOX semiconductors have larger Poisson’s ratio than MAX indicating that they are not only elastically stiff but quite damage tolerant and easier to be machined than MAX potentially. With a combination of semiconductor, ceramic and machinable properties, MAOX semiconductors could potentially lead to many optoelectronic applications, including a new class of solar cells.

4. CONCLUSION A new family of layered hexagonal oxycarbides and oxynitrides with the general formula, Mn+1AO2Xn (MAOX) is proposed based on the first-principles calculations, where M is an early transition metal, A is an alkaline earth metal in Group IIA or a late transition metal in Groups IB and IIB, X is C and/or N and n=1–3. Detailed calculation of twelve 2121 MAOX phases, M2CaO2C (M=Sc, Ti, V, Cr, Mn, Y, Zr, Nb, Mo, La, Hf, Ta) demonstrate, different from MAX phases which are all metallic, the MAOX phases can be metals, semimetals, and semiconductors. To date, we have identified five semiconductors. Unexpectedly, they have fantastic theoretical photovoltaic properties on a par with GaAs. Electronic and photovoltaic calculations predict that Mo2CaO2C would be a balanced PV material which possesses low electron/hole effective masses, and strong optical absorption. Cr2CaO2C with an impressive 27.7% of SLME is another potential PV material candidate. The mechanical studies indicate MAOX semiconductors could be elastically stiff and as damage tolerant as ceramic MAX phases, at the same time as tough as metals. With these unique combined properties, MAOX semiconductors could open a new path for many optoelectronic applications, including a new class of solar cells. The general formula of Mn+1AO2Xn (MAOX) leads to three subgroups of 4123, 3122 and 2121 with n equal to 3, 2 and 1 respectively. There are 216 possible phases for the 2121 subgroup alone. More phases exist in the 4123 and 3122 subgroups. The current calculation only scratches the surface of the possibility. Future calculations could lead to the discovery of more new materials with interesting metallic, semimetallic or semiconducting properties, and the in-depth understanding of the atomistic origin of this new class of layered materials. A more significant step will be the experimental realization of the structure of MAOX phases and the measurement of optoelectronic properties predicted by the DFT calculations.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected];



*E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Xin Chen and Chunming Niu acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51201175 and 21773182). The support provided by China Scholarship Council (CSC) during a visit of Zhenyu Wang to University College London (UCL) is acknowledged.

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Layered Hexagonal Oxycarbides, Mn+1AO2Xn (M = Sc, Y, La, Cr, and

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