Exploring the properties of lead-free hybrid double perovskites using a combined computational-experimental approach

Abstract

Density functional theory screening of the hybrid double perovskites (MA) 2 B I BiX 6 (B I =K,Cu,Ag,Tl; X=Cl,Br,I) shows that systems with band gaps similar to those of the MAPbX 3 lead compounds can be expected for B I =Cu,Ag,Tl. Motivated by these findings, (MA) 2 TlBiBr 6 , isoelectronic with MAPbBr 3 , was synthesised and found to have a band gap of ~2.0eV. The remarkable performance of hybrid perovskite-based solar cells has launched a new paradigm in the area of photovoltaic research.synthesis and optical properties of a new double hybrid perovskite, (MA) 2 TlBiBr 6 , which is isoelectronic with MAPbBr 3 and has a much narrower band-gap than (MA) 2 KBiCl 6 .Exploring the properties of lead-free hybrid double perovskites using a combined computational-experimental approach Computational MethodologyThe DFT calculations were performed using the Vienna ab initio Simulation Package (VASP). 1,2 Projected augmented wave (PAW) 3 pseudopotentials were employed with the following electrons treated explicitly: H (1s 1 ), C (2s 2 2p 2 ), N (2s 2 2p 3 ), K (3s 2 3p 6 4s 1 ), Bi (5d 10 6s 2 6p 3 ), Tl (5d 10 6s 2 6p 1 ), Cu (3p 6 3d 10 4s 1 ), Ag (4p 6 4d 10 5s 1 ), Pb(5d 10 6s 2 6p 2 ), Cl (3s 2 3p 5 ), Br (4s 2 4p 5 ) and I (5s 2 5p 5 ).The non-local van der Waals density functional (vdW-DF) 4 was used with the exchange-correlation energy calculated as , where the exchange energy is obtained from the generalized gradient approximation (GGA) using the optB86b functional, the local correlation energy from the local density approximation (LDA) and the non-local correlation energy from double space integration. K-points were sampled in the first Brillouin zone using a 4×4×2 Monkhorst-Pack 5 mesh, and for electronic density of states (DOS) calculations, a finer 8×8×3 mesh was used. A 500 eV plane-wave kinetic energy cutoff was employed for all calculations. The effect of relativistic spin-orbit coupling (SOC) was included inthe DOS and electronic band structure calculations. The experimentally synthesized double perovskite (MA) 2 KBiCl 6 was used as a basis for constructing all the structures which were then relaxed until the interatomic forces were less than 0.01 eV/Å while maintaining the same rhombohedral symmetry as (MA) 2 KBiCl 6 .

From Lead Halide Perovskites to Lead‐Free Metal Halide Perovskites and Perovskite Derivatives

“…The halide double perovskites with alkaline metal B(I) cations have 0D electronic dimensionality and large bandgaps, [161][162][163] similar to the case of Cs 2 SrPbI 6 . [171] Among these compounds, Cs 2 AgBiBr 6 is the most studied one, primarily because of its relatively small bandgap (2.05-2.3 eV, [155][156][157] Figure 15a) and superior stability against heat and moisture. The last but not the least screening is for X anions.…”

“…[200][201][202][203] As shown in Figure 17a, band structures of Cs 2 TlBiBr 6 and Fr 2 InBiBr 6 , as examples, exhibit similar features to that of Cs 2 PbPbI 6 (i.e., CsPbI 3 ), particularly in the vicinity of the band edges; i) the bandgaps are located at the Γ point and of direct transition type; ii) the VBM of Cs 2 TlBiBr 6 (Fr 2 InBiBr 6 ) consists of Tl 6s (In 5s)/Bi 6s−Br 4p antibonding states, while the CBM is composed mainly of Bi 6p/Tl 6p (In 5p) states; iii) both the VBM and the CBM are dispersive, indicating 3D electronic dimensionality (see Figure 17b). [171] However, the acute toxicity of Tl makes the Tl-containing halide double perovskites not suitable for practical optoelectronic applications. [200] However, the In(I)-based halide double perovskites are extremely unstable against oxidation into In 3+ -based compounds (Figure 17d), because of the extremely highenergy-lying In 5s 2 states.…”

Can Pb-Free Halide Double Perovskites Support High-Efficiency Solar Cells?

“…These are (CH 3 NH 3 ) 2 KBiCl 6 , though like Cs 2 AgBiCl 6 , its indirect band gap was large, around 3 eV, 39 and (CH 3 NH 3 ) 2 TlBiCl 6 , possessing a direct but still large band gap of 2.16 eV. 40 Even more recent work has seen the prediction of band gaps for Cs 2 AgBiX 6 (X = Cl, Br), with many-body GW theory, 41 of 2.4 and 1.8 eV, in reasonable agreement with experiment. An assessment of the defect chemistry of Cs 2 AgBiBr 6 reported that V Bi and Ag Bi form deep acceptor levels, and thus growth under Br-poor/Bi-rich conditions could enhance performance.…”

Cu–In Halide Perovskite Solar Absorbers

“…The harmonic phonon spectrum is calculated from second-order interatomic force constants obtained by using the real-space finite-difference approach implemented in Phonopy code. 4 The 2×2×2 supercell (of the primitive cell of the double-perovskite structure) accompanying with the k-point mesh with grid spacing of 2π×0.03 Å -1 is used for these calculations. The room-temperature phonon spectrum is obtained by taking into account anharmonic phonon-phonon interaction with a self-consistent ab initio lattice dynamical (SCAILD) method.…”

“…This group of materials is designed by combining the theoretical understanding of (i) the factors that limited the performance of some of the previously considered halide perovskites (where Pb was replaced by other related elements) [1][2][3][4][5][6][7][8][9] with (ii) the special features that enabled the high performance of Cu-based ternary chalcopyrites (Cu(In,Ga)Se 2 , CIGS) as photovoltaic absorbers. [10][11][12] The significance of this work is stepping out of conventional design principles of replacing Pb 2+ with other similar ns 2 cations and considering instead transmuting two Pb 2+ to the pair of a group IB (Cu + and Ag + ) and a group III (Ga 3+ and In 3+ ) cation, exemplified by Cs 2 [AgIn]Cl 6 .…”