Hybrid perovskites are currently the fastest growing photovoltaic technology, having reached a solar cell efficiency of over 20%. One possible strategy to further improve the efficiency of perovskite solar cells is to tune the degree of octahedral tilting of the halide frame, since this in turn affects the optical band gap and carrier effective masses. It is commonly accepted that the ion sizes are the main control parameter influencing the degree of tilting in perovskites. Here we re-examine the origin of octahedral tilts in halide perovskites from systematic first-principles calculations. We find that while steric effects dominate the tilt magnitude in inorganic halides, hydrogen bonding between an organic A-cation and the halide frame plays a significant role in hybrids. For example, in the case of MAPbI 3 , our calculations suggest that, without the contribution from hydrogen bonding, the octahedra would not tilt at all. These results demonstrate that tuning the degree of hydrogen bonding can be used as an additional control parameter to optimize the photovoltaic properties of perovskites.

The discovery of lead-free hybrid double perovskites provides a viable approach in the search for stable and environmentally benign photovoltaic materials as alternatives to lead-containing systems such as MAPbX 3 (X = Cl, Br, or I). Following our recent reports of (MA) 2 KBiCl 6 and (MA) 2 TlBiBr 6 , we have now synthesized a hybrid double perovskite, (MA) 2 AgBiBr 6 , that has a low band gap of 2.02 eV and is relatively stable and nontoxic. Its electronic structure and mechanical and optical properties are investigated with a combination of experimental studies and density functional theory calculations.

IN A SEARCH FOR LEAD-FREE MATERIALS THAT COULD BE USED AS ALTERNATIVES TO THE HYBRID PEROVSKITES, (MA)PbX 3 , in photovoltaic applications, we have discovered a hybrid double perovskite, (MA) 2 KBiCl 6 , which shows striking similarities to the lead analogues. Spectroscopic measurements and nanoindentation studies are combined with density functional calculations to reveal the properties of this interesting system.The light-harvesting, semiconducting hybrid inorganic-organic perovskites (HIOPs) have recently attracted a great deal of attention in the photovoltaic community, with their solar cell efficiencies rising from ~4% to over 20% in just six years. 1, 2 The most extensively studied materials are the lead-containing systems, APbX 3 , where A is alkyl ammonium cation (e.g. CH 3 NH 3 + (methylammonium, MA) or NH 2 CHNH 2 + (formamidinium, FA)) and X is Cl -, Bror I -. However, the toxicity of lead to the environment could become a major drawback in their commercialization and the quest for lead-free alternatives is therefore attracting a lot of attention. Other group IV metals such as Ge and Sn are being explored, but the chemical instability of Sn 2+ and Ge 2+ presents challenges for their practical utilization. 3,4 Alternatively, the replacement of Pb 2+ by isoelectronic ions also seems attractive because the strong light absorption and long carrier life-times exhibited by MAPbX 3 are believed to be related to the 6s 2 6p 0 electronic configuration of Pb 2+ . 5 While Tl + is also toxic, Bi 3+ is an interesting option because coordination complexes of bismuth are used in over-the-counter medicines such as Pepto-Bismol. 6 However, this strategy poses challenges because Bi 3+ has a different valence state from Pb 2+ and cannot therefore be simply substituted into phases such as (MA)PbX 3 . In the present work, we show that the incorporation of Bi 3+ into a HIOP can be achieved by synthesizing a hybrid double perovskite of general formula (MA) 2 M I M III X 6 .There has been significant recent progress in the incorporation of Bi 3+ into hybrid perovskiterelated halides. For example, (MA) 3 Bi 2 I 9 can be readily obtained by using a synthetic route analogous to that used for MAPbI 3 , 7 and an ammonium bismuth iodide phase, (NH 4 ) 3 Bi 2 I 9 , was recently reported to show a bandgap of 2.04eV. 8 A number of alkali metal systems of

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 .

air-stability of some compositions has motivated researchers to find lead-free alternatives. [7,8] A wide range of materials classes have recently been explored computationally and experimentally. [8,9] In evaluating the potential of these new materials for photovoltaics, much of the focus has been on the bandgap, stability, absorption coefficient, and phase. [8,[10][11][12][13] The minority-carrier lifetime has received less attention, yet has historically limited the development of new photovoltaic materials. [14,15] However, the reported lifetimes of lead-free alternatives to the perovskites have typically ranged from <0.1 to ≈10 ns. [15][16][17] Silver-bismuth double perovskites (e.g., Cs 2 AgBiBr 6 and Cs 2 AgBiCl 6 ) have recently been found to be an exception. Time-resolved photoluminescence (TRPL) measurements of these materials show an initial drop in photo luminescence (PL) over a nanosecond timescale by 0.5-2 orders of magnitude, followed by a slow tail in PL decay. These PL decay traces are fit with a bi-or triexponential model and the longest time constant is attributed to the fundamental lifetime, which has been found to be ≥100 ns. [18][19][20] Given that these materials have also been found to be more stable than methylammonium lead iodide, [18,20] they have attracted significant interest, with many recent investigations of new families of double perovskite compounds with novel properties.There is current interest in finding nontoxic alternatives to lead-halide perovskites for optoelectronic applications. Silver-bismuth double perovskites have recently gained attention, but evaluating their carrier lifetime and recombination mechanisms from photoluminescence measurements is challenging due to their indirect bandgap. In this work, transient absorption spectroscopy is used to directly track the photocarrier population in Cs 2 AgBiBr 6 by measuring the ground state bleach dynamics. A small initial drop is resolved in the ground state bleach on a picosecond timescale, after which the remaining photocarriers decay monoexponentially with a lifetime of 1.4 µs. The majority of the early-time decay is attributed to hot-carrier thermalization from the direct transition to the indirect bandgap, and the 1.4 µs lifetime represents the recombination of most photocarriers. From this lifetime, a steady-state excess carrier density of 2.2 × 10 16 cm −3 under 1 sun is calculated, which is an order of magnitude larger than that for methylammonium lead iodide, suggesting that charge transport and extraction can be efficient in Cs 2 AgBiBr 6 solar cells. Double PerovskiteLead-halide perovskites display remarkable optoelectronic properties, with long diffusion lengths >1 µm, [1,2] strong optical absorption on the order of 10 5 cm −1 , [3] and high photoluminescence quantum efficiencies >80%. [4] These properties have led to rapid increases in the efficiency of perovskite solar cells (up to a certified power conversion efficiency of 22.7%) [5] and light emitting diodes (>11% external quantum efficiency) [6] over a sho...