Sterically Suppressed Phase Segregation in 3D Hollow Mixed-Halide Wide Band Gap Perovskites

Abstract

Band gap tuning in mixed-halide perovskites enables efficient multijunction solar cells and LEDs. However, these wide band gap perovskites, which contain a mixture of iodide and bromide ions, are known to phase segregate under illumination, introducing voltage losses that limit stability. Previous studies have employed inorganic perovskites, halide alloys, and grain/interface passivation to minimize halide segregation, yet photostability can be further advanced. By focusing on the role of halide vacancies in anion migration, one expects to be able to erect local barriers to ion migration. To achieve this, we employ a 3D “hollow” perovskite structure, wherein a molecule that is otherwise too large for the perovskite lattice is incorporated. The amount of hollowing agent, ethane-1,2-diammonium dihydroiodide (EDA), varies the density of the hollow sites. Photoluminescence measurements reveal that 1% EDA in the perovskite bulk can stabilize a 40% bromine mixed-halide perovskite at 1 sun illumination intensity. These, along with capacitance–frequency measurements, suggest that hollow sites limit the mobility of the halide vacancies.

Physics-driven discovery and bandgap engineering of hybrid perovskites

“…The use of GA ions has been reported to passivate defects in the perovskite systems, which can reduce non-radiative recombinations. 41,43,68 The steric suppression of phase segregation can stabilize the bandgap under illumination, 69 which is an indicator that the phase separation observed in the XRD results does not impact the bandgap model in a mixed GA and MA system. GA can serve as a modifying agent within the perovskite structure by inuencing phase stability and ion migration by various mechanisms.…”

Stable and Efficient Mixed‐halide Perovskite LEDs

“…Sargent et al erected the local barriers to ion migration by focusing on the role of halide vacancies in anion migration. [71] They employed a hollowing agent, ethane-1,2-diammonium dihydroiodide (EDA) to construct a 3D "hollow" perovskite structure, [72] and verified that hollow sites limit the mobility of the halide vacancies. Photoluminescence measurements reveal that 1 % EDA in the perovskite bulk can stabilize a 40 % bromine mixed-halide perovskite at 1 sun illumination intensity.…”

Enhanced Photostability of “Hollow” Mixed Halide Wide-Bandgap Perovskite Films

“…Compositional engineering, mainly through the substitution of a portion of iodide with bromide, is a conventional approach for bandgap widening in these materials. , Synthesis of wide-bandgap perovskites (>1.65 eV) for use in multi-junction solar cells typically requires 20% or more Br substitution. This configuration is known to result in halide segregation under light irradiation. − …”

“…The so-called hollow perovskites are a special class reported to have better air stability than conventional 3D perovskites that also allow bandgap widening without increasing the amount of Br. − The hollow perovskite structure forms by the introduction of a diamine (e.g., ethylene­diamine; en) as a cation, which has a size that violates the geometric tolerance factor rule, resulting in the expulsion of M and X atoms, causing metal and halide vacancies in the existing octahedral framework but retaining the overall 3D perovskite structure. In hollow perovskites, the ion migration behavior is altered as the activation energy of the migration is increased in MAPbI 3 , but decreased in FAPbI 3 and FAPbBr 3 , as shown using experimental methods and density functional theory (DFT) calculations (MA = methyl­ammonium, FA = form­amid­inium). , This demonstrates that the ion migration behavior changes differently during hollow perovskite formation depending on the A-site cation and octahedron cage dynamics.…”