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

Abstract

The methylammonium lead halides have become champion photoactive semiconductors for solar cell applications; however, issues still remain with respect to chemical instability and potential toxicity. Recently, the Cs2AgBiX6 (X = Cl, Br) double perovskite family has been synthesized and investigated as stable nontoxic replacements. We probe the chemical bonding, physical properties, and cation anti-site disorder of Cs2AgBiX6 and related compounds from first-principles. We demonstrate that the combination of Ag(I) and Bi(III) leads to the wide indirect band gaps with large carrier effective masses owing to a mismatch in angular momentum of the frontier atomic orbitals. The spectroscopically limited photovoltaic conversion efficiency is less than 10% for X = Cl or Br. This limitation can be overcome by replacing Ag with In or Tl; however, the resulting compounds are predicted to be unstable thermodynamically. The search for nontoxic bismuth perovskites must expand beyond the Cs2AgBiX6 motif.

Capturing the Sun: A Review of the Challenges and Perspectives of Perovskite Solar Cells

“…Recently, silver/bismuth-based elpasolites (more commonly known as double perovskites) with the general formula A 2 AgBiX 6 , (A = MA, FA or Cs; X = Cl, Br, I, Figure 16) have been suggested as a promising lead-free and air-stable alternative. [277,278] Single crystals of these double perovskites have shown good optoelectronic properties and long charge carrier lifetimes. However, the www.advancedsciencenews.com preparation of high-quality films has shown to be challenging, and therefore photovoltaic devices have not been published so far.…”

Research Update: Bismuth-based perovskite-inspired photovoltaic materials

“…The directionality of the Ag 4d orbitals results in the Ag 4d and Bi 6s states having high charge density at different k-points in the conduction band minimum, giving a flatter conduction band and wider bandgap than CsPbBr 3 . 12 This results in higher effective masses. 12,23 The mixing of Br p orbitals with Ag d z 2 orbitals at the X point in reciprocal space results in the valence band maximum occurring at the X point rather than at the L point (where the conduction band minimum is), giving an indirect bandgap.…”

“…12 This results in higher effective masses. 12,23 The mixing of Br p orbitals with Ag d z 2 orbitals at the X point in reciprocal space results in the valence band maximum occurring at the X point rather than at the L point (where the conduction band minimum is), giving an indirect bandgap. Calculations have shown that the lower absorption coefficients resulting from the indirect bandgap limit the maximum achievable efficiencies.…”

“…Calculations have shown that the lower absorption coefficients resulting from the indirect bandgap limit the maximum achievable efficiencies. 12 Defect calculations have also shown that not all bismuth-based compounds with s orbitals at the valence band maximum form shallow defects. Transition levels are Fermi energies at which there is a change in the charge of a defect and are energies within the bandgap at which a defect is expected to be able to trap electrons or holes.…”

“…[8][9][10][11] Another strong emphasis has been on finding compounds with the same crystal structure, resulting in numerous investigations into double perovskites, e.g., Cs 2 AgBiBr 6 , which is also well-documented in recent reviews and articles. 8,[12][13][14] An approach that is contemporary to these is to find compounds that could replicate the tolerance of lead-halide perovskites to intrinsic defects. Computations of methylammonium lead iodide (CH 3 NH 3 PbI 3 ) have shown that it primarily forms shallow defects, which would lead to low rates of Shockley-Read-Hall recombination.…”