Marina Filip

Abstract:

Formamidinium lead triiodide (FAPbI₃) is the leading candidate for single-junction metal-halide perovskite photovoltaics, despite the metastability of this phase. To enhance its ambient-phase stability and produce world-record photovoltaic efficiencies, methylenediammonium dichloride (MDACl₂) has been used as an additive in FAPbI₃. MDA²⁺ has been reported as incorporated into the perovskite lattice alongside Cl^-. However, the precise function and role of MDA²⁺ remain uncertain. Here, we grow FAPbI₃ single crystals from a solution containing MDACl₂ (FAPbI₃-M). We demonstrate that FAPbI₃-M crystals are stable against transformation to the photoinactive δ-phase for more than one year under ambient conditions. Critically, we reveal that MDA²⁺ is not the direct cause of the enhanced material stability. Instead, MDA²⁺ degrades rapidly to produce ammonium and methaniminium, which subsequently oligomerizes to yield hexamethylenetetramine (HMTA). FAPbI₃ crystals grown from a solution containing HMTA (FAPbI₃-H) replicate the enhanced α-phase stability of FAPbI₃-M. However, we further determine that HMTA is unstable in the perovskite precursor solution, where reaction with FA⁺ is possible, leading instead to the formation of tetrahydrotriazinium (THTZ-H⁺). By a combination of liquid- and solid-state NMR techniques, we show that THTZ-H⁺ is selectively incorporated into the bulk of both FAPbI₃-M and FAPbI₃-H at ∼0.5 mol % and infer that this addition is responsible for the improved α-phase stability.

Abstract:

Although sulfide perovskites usually require high-temperature syntheses, we demonstrate that organosulfides can be used in the milder syntheses of halide perovskites. The zwitterionic organosulfide, cysteamine (CYS; ⁺NH₃(CH₂)₂S^-), serves as both the X^- site and A⁺ site in the ABX₃ halide perovskites, yielding the first examples of 3D organosulfide-halide perovskites: (CYS)PbX₂ (X^- = Cl^- or Br^-). Notably, the band structures of (CYS)PbX₂ capture the direct bandgaps and dispersive bands of APbX₃ perovskites. The sulfur orbitals compose the top of the valence band in (CYS)PbX₂, affording unusually small direct bandgaps of 2.31 and 2.16 eV for X^- = Cl^- and Br^-, respectively, falling in the ideal range for the top absorber in a perovskite-based tandem solar cell. Measurements of the carrier dynamics in (CYS)PbCl₂ suggest carrier trapping due to defects or lattice distortions. The highly desirable bandgaps, band dispersion, and improved stability of the organosulfide perovskites demonstrated here motivate the continued expansion and exploration of this new family of materials, particularly with respect to extracting photocurrent. Our strategy of combining the A⁺ and X^- sites with zwitterions may offer more members in this family of mixed-anion 3D hybrid perovskites.

Abstract:

Interlayer organic cations in quasi-two-dimensional halide perovskites are a versatile tuning vehicle for the optoelectronic properties of these complex systems, but chemical intuition for this design route is yet to be established. Here, we use density functional theory, the GW approximation, and the Bethe-Salpeter equation approach to understand the contribution of the organic cation to the quasiparticle band gap and exciton binding energy of layered perovskites. We show that organic cations in quasi-two-dimensional perovskites contribute significantly to the dielectric screening in these systems, countering quantum confinement effects on the quasiparticle band gap and the exciton binding energy. Using a simple electrostatics model inspired by parallel-plate capacitors, we decouple the organic cation and inorganic layer contributions to the effective dielectric constants and show that dielectric properties of layered perovskites are broadly tunable via the interlayer cation, providing a direct means of tuning photophysical properties for a variety of applications.