Stability of Eu2+ Dopant in CsPbBr3 Perovskites: A First Principles

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Stability of Eu2+ Dopant in CsPbBr3 Perovskites: A First Principles Study Anu Bala, and Vijay Kumar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10261 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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The Journal of Physical Chemistry

Stability of Eu2+ Dopant in CsPbBr3 Perovskites: A First Principles Study Anu Bala*,a and Vijay Kumara,b aCenter

for Informatics, School of Natural Sciences, Shiv Nadar University, NH-91, Tehsil Dadri,

Gautam Buddha Nagar -201314, Uttar Pradesh, India bDr.

Vijay Kumar Foundation, 1969, Sector 4, Gurgaon 122001, Haryana, India

ABSTRACT We study the stability of rare earth dopant, Eu, in metal halide perovskite CsPbBr3 with cubic and orthorhombic structures from first principles calculations. In these perovskites Eu is substitutionally doped on Pb sites due to their comparable ionic sizes which lead to only a small strain in the doped system. Accordingly, our results show that the cost of Eu doping is quite small compared to the values in other common hosts such as GaN. This makes these perovskites excellent candidates to develop rare earth doped semiconducting materials for bright luminescence as it has also been observed recently in the case of thin films of CH3NH3Pb1-xEuxI3, Eu doped quantum dots of CsPbBr3 and nanocrystals of CsPbCl3 in addition to their outstanding properties for applications in solar cells.

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INTRODUCTION In the past few years metal halide perovskites AMX3 with A = CH3NH3 and a few other organic molecules as well as alkalis such as Cs, M = Sn and Pb, and X = Cl, Br, and I, have attracted great attention and have emerged as excellent low cost semiconductors with tuneable direct band gap. Their high efficiency1 of up to about 22.7% for converting solar energy into electrical energy, has led to extensive research on these materials and great excitement for the development of futuristic cost effective photovoltaic devices. In contrast to the traditional solar cells based on silicon which is an indirect band gap material, these perovskites have several attractive properties such as high absorption coefficient, broad absorption spectrum, and high charge carrier mobility with long charge diffusion lengths.2,3 Also, interestingly in some cases the band gap lies in the visible range and this makes these materials potential candidates for cutting-edge optoelectronics applications such as in light-emitting diodes (LEDs) and lasers.4,5 Traditional hosts such as alumina, GaN and related systems have the problem of the stabilization of the rare earths due to significant size mismatch. However, here we show that these perovskites are also excellent hosts for the doping of rare earths. Accordingly, rare earth doped perovskites could be used to create bright luminescence of different colors for LEDs and laser applications due to intra f shell and f d transitions.6-9 It is well known that the doping of Eu, Er, and Tm in e.g. GaN leads to materials that emit bright red, green, and blue light.10-12 Yttrium aluminium garnet, YAG (Y3Al5O12), is a widely used host for Nd:YAG laser.13 However, a major challenge in these materials is the stability of rare earth ions due to the mismatch in the ionic sizes of the rare earth and the metal atom such as Al, Y, or Ga which is replaced. This creates strain around the dopant. Therefore, there is a need to find alternate materials with long life. The above mentioned organometallic and inorganic perovskites


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may offer this possibility as in these perovskite halides Pb exists in a divalent state. It is well known that Eu can exist in Eu2+ or Eu3+ state in different hosts. Detailed studies have shown doping of Eu2+ in various hosts14. This type of doping presents broad emission from the parity-allowed transition 4f65d1 → 4f7, which presents wide-emission range in the blue-green-yellow or red band. The emission band position can be tuned by selecting different host materials.15,16 The doping of Eu3+ gives luminescence that originates from its 4f6 → 4f6 transition consisting of sharp lines in the red region. The positions of its emission lines are independent of the host materials.17 Incidentally, the radius of Pb2+ ion is very close to that of Eu2+ ion. Therefore, substitution of rare earth ions can be expected on Pb sites without much strain. This type of substitution has already been observed in bulk CsEuBr3,18 CH3NH3EuI3,19 and thin films of (C4H9NH3)2EuI420 where Eu exists in 2+ state. If the doping of Eu2+ is favorable in these perovskites then the doping of Eu as Eu3+ ion as claimed in some recent experiments on nanocrystals of perovskites21-24 would also create small strain in these systems due to the smaller size of Eu3+ compared with Eu2+. Very recent experiments on various lanthanides (Ce, Sm, Eu, Tb, Dy, Er and Yb) doped nanocrystals of CsPbCl3 perovskite semiconductors have shown bright luminescence and the doping of rare earth atoms on Pb sites has been confirmed.21,24 In these nanocrystals the doping of reportedly Eu3+ leads to red color emission as also seen for Eu3+ doped GaN.10-12 Further, thin films of CH3NH3Pb122

xEuxI3

have shown enhancement in stability and efficiency due to the doping of Eu while

excellent optical properties have been observed for Eu doped cubic quantum dots of CsPbBr3.23 It is clear that the understanding of the energetics of the rare earth doping in these materials is of great interest for their further development. Here we report results of first principles calculations on Eu doping in bulk inorganic perovskites CsPbBr3 which suggest that Eu doping has only a small


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cost of about ~0.13 eV per Eu atom in contrast to Eu doping in GaN which costs about 1.84 eV per atom.17 Therefore, these materials have bright prospects for optoelectronic applications.

COMPUTATIONAL METHOD

We considered CsPbBr3 which is an intermediate case among perovskites with Cl, Br, and I to demonstrate the effects of the doping of a rare earth atom, considered to be Eu in the present study. CsPbBr3 is known to exist in orthorhombic structure below 88°C and in cubic structure above 130°C.25 We have considered both these structures to study the effects of doping because some perovskites like CsPbCl3 exist in cubic phase at around 47°C.26 The calculations have been performed by using plane wave pseudopotential method as implemented in Vienna Ab initio Simulation Package (VASP).27 We used generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE)28 for exchange-correlation functional and a cut-off of 500 eV for the plane wave expansion of the electronic wave-function. The electron-ion interactions are represented by projector augmented wave (PAW) pseudopotentials.29 We constructed a 333 (2x2x2) supercell with 135 (160) atoms for the cubic (orthorhombic) phase of CsPbBr3 (Figure 1) in order to study the doping of rare earth atoms. We tested different k-points mesh for the structural optimization and total energy calculations and the resulting values of the formation energy are given in Table S1 in supporting information. These results suggest that the choice of 2x2x2 (2x2x1) k-points mesh in the Brilluoin zone of cubic (orthorhombic) phase is suitable and accordingly all the calculations have been performed using these mesh values. The supercell shape and size as well as the ionic positions have been relaxed until the absolute value of the force on each ion becomes less than 0.005 eV/Å. For the doped systems, we replaced one of the 27 (32) Pb atoms of cubic (orthorhombic) phase by a Eu atom as shown in Figure 1. This makes doping concentration


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of 3.7% for the cubic and 3.1% for the orthorhombic phase w.r.t. Pb atoms. In experiments also ~1 at% doping values of Eu are generally used for bright luminescence. Very recent experiments22 on thin films of CH3NH3Pb1-xEuxI3 show that the highest efficiency among samples with different concentrations of Eu2+ doping is achieved with x = 0.04 i.e. about 4%. In the present study Eu behaves as a divalent cation (Eu2+) when it is substituted on a Pb site in these perovskites. This results in half-filling of the Eu 4f shell. Accordingly for the doped systems we performed spinpolarized calculations. Further, as the 4f orbitals are very localized, we have used GGA+U formalism of Dudarev et al. 30 by including on-site Coulomb interaction parameter U to describe the Eu 4f bands. In this case the role of J reduces to normalization of the U value. We can, therefore, set J = 0 and make use of an effective U which we chose to be 6 eV for these calculations. This value of U has been shown to describe the correct ground state energy of Eu doped systems.31 Spin-orbit interactions play an important role for Pb based systems and it can be important in describing the ground state stability of Eu doped compounds. Therefore, we have also included these interactions in the present study.

Figure 1: Relaxed (i) 3x3x3 (135 atoms) and (ii) 2x2x2 (160 atoms) supercells of CsPbBr3 in cubic and orthorhombic phases, respectively, for undoped and Eu doped perovskites.


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RESULTS AND DISCUSSIONS

The calculated lattice parameters of the unit cell extracted from the supercell of CsPbBr3 in cubic (5.987 Å) and orthorhombic (8.22 Å, 8.51 Å, 11.87 Å) phases are in good agreement with the experimental values (5.874 Å for the Cubic32 and 8.19 Å, 8.24 Å, and 11.735 Å for the orthorhombic33 phases). Next we calculated the formation energy (Ef) to check the stability of the undoped and rare earth doped cubic systems from Ef = [Etot(3x3x3 CsPbBr3) – 27 E(CsBr) -27 E(PbBr2)] and [Etot(Eu doped in 3x3x3 CsPbBr3) – 27 E(CsBr) – E(EuBr2) - 26 E(PbBr2)], respectively. Similar calculations of Ef have been performed for the orthorhombic phase. A negative value of Ef corresponds to a stable perovskite. These results have been given in Table 1. Further the doping energy has been calculated from [Etot(Doped system) – Etot(Undoped system) – E(EuBr2) + E(PbBr2)]. Also, we have calculated the strain energy due to doping of rare earth atom by substituting Pb atom at the rare earth site and calculating the energy without ionic relaxation. The strain energy is then the difference in energy, E(Doped systems where Eu is replaced with Pb) –E(Undoped system).

Table 1: Formation energy Ef, doping energy, strain energy, and lattice parameters for the undoped and doped systems. The values in brackets are obtained by inclusion of spin-orbit interactions which increase the stability of the systems. It is also seen that there is a very small deviation from perfect cubic or orthorhombic structure in the case of the doped systems. System

Ef /atom (eV)

Cs27Pb27Br81 (Cubic) Cs27Pb26EuBr81 (Cubic)

-0.058 (-0.062) -0.056 (-0.060)

Doping Strain energy energy (eV/ (eV/Eu atom) supercell) 0.199 (0.230)

0.005

Lattice parameters 17.96 Å α = β = γ = 90° 17.96 Å α = 89.96° , β =90.07°, γ=89.98°


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Cs32Pb32Br96 (Orthorhombic) Cs32Pb31EuBr96 (Orthorhombic)

-0.073 (-0.076) -0.072 (-0.075)

0.132 (0.132)

0.003

a = 16.44 Å, b = 17.03 Å, c = 23.74 Å, α = β = γ = 90° a = 16.46 Å, b = 17.01 Å, c = 23.73 Å, α = 89.99°, β = 90°, γ = 90°

Interestingly our results show that the doping of Eu in these perovskites costs only 0.132 eV per Eu atom in the orthorhombic phase which is the room temperature phase of the considered system. This energy is much smaller compared with the value of 1.84 eV for the doping of an Eu atom in GaN.17 Also the doped materials are almost as stable as the undoped phase. There is a slight increase in the doping cost (0.23 eV) in the cubic phase. The small value of the strain energy 0.003 (0.005) eV/supercell in the orthorhombic (cubic) phase also suggests little distortion in the structure due to doping. As mentioned earlier, this is due to the comparable sizes of Eu2+ (117 pm) and Pb2+ (119 pm) ions in comparison to other Eu doped phosphors like Al2O3 or YAG where Al3+(53 pm) is very small compared to 108.7 pm for Eu3+ for 6 coordination. Also the ionic radius of Y3+ is 104 pm and thus the difference is more significant though much less than in the case of Al. The optimized lattice parameters show (Table 1) that the lattice remains nearly unchanged in the case of the cubic structure with only a very small local contraction of ~0.01 Å around the doped ion. A similar result is obtained for the orthorhombic phase in which case there is an expansion of ~ 0.02 Å in a lattice parameter and contraction of 0.02 (0.01) Å in b (c) lattice parameter. Also octahedron angles in different planes change slightly (up to about ~0.4°) only, suggesting that the crystal structure remains nearly the same after doping with Eu2+. It has also been reported experimentally that the structure of nanocrystals of CsPbCl3 remains the same after Eu3+ doping.21 Even for layered structure (C4H9NH3)2EuI4, the lattice parameters are comparable with its Pb2+ analogue.20 We further studied the effects of the inclusion of spin-orbit (SO) interaction and found


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that it lowers the energy by 0.003 eV/atom for the undoped orthorhombic phase and 0.004 eV/atom for the undoped cubic phase. The same trend is seen for the doped cases. Figure 2 shows the total and site projected densities of states for the undoped and Eu doped orthorhombic and cubic phases of CsPbBr3. For the doped system the splitting between the spinup and spin-down states of Pb and Br is negligible. Further, the Eu 4f spin-down states lie in the energy region of about 10 eV above the top of the valence band which is beyond the range of energy shown in the figure. Accordingly we have shown only the spin-up states for the doped systems. It can be seen that the Br 4p and Pb 6s states lie near the valence band maximum similar to the case of the undoped system.34 But in Eu doped perovskites, the spin-up 4f states lie near the top of the valence band. They hybridize with the Br 4p as well as Pb 6s states and are fully occupied. The conduction band minimum is mainly composed of Pb 6p, Eu 5d (states starts at around 2.5 eV) and some Br 4p states. This would facilitates f to d transitions in these Eu2+ doped perovskites. The 4f -5d transition in Eu2+ doped phosphors has been studied in detail.14 It can be seen from Figure 2 that the energy gap between Eu 4f and 5d states is about 2.5 eV and this band gap is calculated within GGA without including SO interactions. Earlier calculations on bulk perovskites have shown that GGA (PBE) exchange-correlation functional without including SO interactions gives band gap close to the experimental value35 in these systems. Therefore, we expect blue emission (2.48-2.85 eV) from these Eu2+ doped perovskites. Therefore our results show that Eu2+ doping is energetically stable in the CsPbBr3 perovskite host with negligible strain in the system. A similar behavior is expected for other halides.


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Figure 2. Spin-up total and site as well as orbital projected densities of states for the undoped and Eu doped CsPbBr3 in cubic and orthorhombic phases.

The reported observation of Eu3+ in CsPbCl3 nanocrystals gives indication of some Cs vacancy or presence of some strongly electronegative species or moisture, deviation from stoichiometry etc. which need detailed experimental and theoretical studies. We have done some preliminary tests to explore the possibility of Eu3+ in bulk Eu doped CsPbBr3 by creating Cs vacancy and found that the magnetic moment of the supercell is ~6.8 µB. This shows that Eu does not exists in pure 3+ state, and that there is likely to be some other mechanism which may be responsible for f - f transition in bulk systems. A further detailed study of such work is beyond the scope of this paper and much further work would be needed to understand it properly. None the less, our results suggest that for Eu3+ doping which is a smaller size ion in comparison to Eu2+, the


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strain effects should be very small as the strain energy for an undersized dopant is much smaller compared with an oversized dopant.

CONCLUSIONS

In summary, the present study shows that the doping of Eu in inorganic perovskite CsPbBr3 costs only a small energy and therefore these perovskite materials give a new direction towards lowcost development of LED/laser materials in comparison to currently well studied gallium nitride (GaN) and other systems. We expect similar results for the organo-metallic perovskites which have been the subject of extensive research for solar energy applications. Our studies support the recent experiments reporting the doping of rare earths and in particular Eu2+ in perovskite materials, though a proper understanding of the reported occurrence of Eu3+ in nanocrystals needs further detailed studies and we hope that our work will stimulate further studies in this direction.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The Table S1 compares the formation energy for the cubic and orthorhombic phases by considering different k-points meshes in the Brillouin zone. AUTHOR INFORMATION *Corresponding Author Email: [email protected], [email protected] ORCID


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Anu Bala: 0000-0002-1889-2612 Vijay Kumar: 0000-0002-5283-5443 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS

A.B. acknowledges financial support from the Department of Science and Technology (DST), Government of India, through the Project Grant No. SR/WOS-A/PM-1042/2015. The calculations have been performed using the High Performance Computing facility MAGUS of Shiv Nadar University. REFERENCES (1) http://www.nrel.gov/ncpv. (2) Shi D.; Adinolfi V.; Comin R.; Yuan M.; Alarousu E.; Buin A.; Chen Y.; Hoogland S.; Rothenberger A.; Katsiev K. et al. Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystal. Science 2015, 347, 519-522. (3) Stoumpos C.C.; Malliakas C.D.; Kanatzidis M. G. Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and Near-Infrared Photoluminescent Properties. Inorganic chem. 2013, 52(15), 9019 -9038. (4) Protesescu L.; Yakunin S.; Bodnarchuk M. I.; Krieg F.; Caputo R.; Hendon C. H.; Yang R. X.; Walsh A.; Kovalenko M.V. Nanocrystals of cesium lead halide perovskites (CsPbX3, X = Cl, Br, and I): Novel optoelectronic materials showing bright emission with wide color gamut. Nano Lett. 2015, 15(6), 3692 –3696.


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Stability of Eu2+ Dopant in CsPbBr3 Perovskites: A First Principles

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