Marina Filip

Abstract:

Layered hybrid organic–inorganic lead halide perovskites have intriguing optoelectronic properties, but some of the most interesting perovskite systems, such as defective, disordered, or mixed perovskites, require multiple unit cells to describe and are not accessible within state-of-the-art ab initio theoretical approaches for computing excited states. The principal bottleneck is the calculation of the dielectric matrix, which scales formally as O(N⁴). We develop here a fully ab initio approximation for the dielectric matrix, known as IPSA-2C, in which we separate the polarizability of the organic/inorganic layers into minimal building blocks, thus circumventing the undesirable power-law scaling. The IPSA-2C method reproduces the quasi-particle band structures and absorption spectra for a series of Ruddlesden–Popper perovskites to high accuracy, by including critical nonlocal effects neglected in simpler models, and sheds light on the complicated interplay of screening between the organic and inorganic sublattices.

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:

The accurate prediction of the band gaps of halide perovskites within density functional theory is known to be challenging. The recently developed Wannier-localized optimally tuned screened range-separated hybrid functional was shown to be highly accurate for fundamental band gaps of standard semiconductors and insulators. This was achieved by selecting the parameters of the functional to satisfy an ansatz that generalizes the ionization potential theorem to the removal of charge from a state that corresponds to a Wannier function. Here, we present applications of the method to the band gaps of typical halide perovskites. We find a satisfyingly small formal mean absolute error of ∼0.1 eV with respect to experimental band gaps and very good agreement with previous many-body perturbation theory calculations.

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.