Structural Dynamics of Two-Dimensional Ruddlesden-Popper Perovskites: A Computational Study.

Recently two-dimensional (2D) hybrid organic-inorganic perovskites have attracted a lot of interest as more stable analogues of their three-dimensional counterparts for optoelectronic applications. However, a thorough understanding of the effect that this reduced dimensionality has on dynamical and structural behavior of individual parts of the perovskite is currently lacking. We have used molecular dynamics simulations to investigate the structure and dynamics of 2D Ruddlesden-Popper perovskite with the general formula BA2MA n-1Pb n I3n+1, where BA is butylammonium, MA is methylammonium, and n is the number of lead-iodide layers. We discuss the dynamic behavior of both the inorganic and the organic part and compare between the different 2D structures. We show that the rigidness of the inorganic layer markedly increases with the number of lead-iodide layers and that low-temperature structural phase changes accompanied by tilting of the octahedra occurs in some but not all structures. Furthermore, the dynamic behavior of the MA ion is significantly affected by the number of inorganic layers, involving changes both in the reorientation times and in the occurrence of specific preferred orientations.

Directing charge transfer in perylene based light-harvesting antenna molecules.

Directing energy and charge transfer processes in light-harvesting antenna systems is quintessential for optimizing the efficiency of molecular devices for artificial photosynthesis. In this work, we report a novel synthetic method to construct two regioisomeric antenna molecules (1-D2A2 and 7-D2A2), in which the 4-(n-butylamino)naphthalene monoimide energy and electron donor is attached to the perylene monoimide diester (PMIDE) acceptor at the 1- and 7-bay positions, respectively. The non-symmetric structure of PMIDE renders a polarized distribution of the frontier molecular orbitals along the long axis of this acceptor moiety, which differentiates the electron coupling between the donor, attached at either the 1- or the 7-position, and the acceptor. We demonstrate that directional control of the photo-driven charge transfer process has been obtained by engineering the molecular structure of the light-harvesting antenna molecules.

Trapping and Detrapping in Colloidal Perovskite Nanoplatelets: Elucidation and Prevention of Nonradiative Processes through Chemical Treatment.

Metal-halide perovskite nanocrystals show promise as the future active material in photovoltaics, lighting, and other optoelectronic applications. The appeal of these materials is largely due to the robustness of the optoelectronic properties to structural defects. The photoluminescence quantum yield (PLQY) of most types of perovskite nanocrystals is nevertheless below unity, evidencing the existence of nonradiative charge-carrier decay channels. In this work, we experimentally elucidate the nonradiative pathways in CsPbBr3 nanoplatelets, before and after chemical treatment with PbBr2 that improves the PLQY. A combination of picosecond streak camera and nanosecond time-correlated single-photon counting measurements is used to probe the excited-state dynamics over 6 orders of magnitude in time. We find that up to 40% of the nanoplatelets from a synthesis batch are entirely nonfluorescent and cannot be turned fluorescent through chemical treatment. The other nanoplatelets show fluorescence, but charge-carrier trapping leads to losses that are prevented by chemical treatment. Interestingly, even without chemical treatment, some losses due to trapping are mitigated because trapped carriers spontaneously detrap on nanosecond-to-microsecond timescales. Our analysis shows that multiple nonradiative pathways are active in perovskite nanoplatelets, which are affected differently by chemical treatment with PbBr2. More generally, our work highlights that in-depth studies using a combination of techniques are necessary to understand nonradiative pathways in fluorescent nanocrystals. Such understanding is essential to optimize synthesis and treatment procedures.

Overcoming the exciton binding energy in two-dimensional perovskite nanoplatelets by attachment of conjugated organic chromophores.

In this work we demonstrate a novel approach to achieve efficient charge separation in dimensionally and dielectrically confined two-dimensional perovskite materials. Two-dimensional perovskites generally exhibit large exciton binding energies that limit their application in optoelectronic devices that require charge separation such as solar cells, photo-detectors and in photo-catalysis. Here, we show that by incorporating a strongly electron accepting moiety, perylene diimide organic chromophores, on the surface of the two-dimensional perovskite nanoplatelets it is possible to achieve efficient formation of mobile free charge carriers. These free charge carriers are generated with ten times higher yield and lifetimes of tens of microseconds, which is two orders of magnitude longer than without the peryline diimide acceptor. This opens a novel synergistic approach, where the inorganic perovskite layers are combined with functional organic chromophores in the same material to tune the properties for specific applications.

Tuning the Structural Rigidity of Two-Dimensional Ruddlesden-Popper Perovskites through the Organic Cation.

Two-dimensional (2D) hybrid organic-inorganic perovskites are an interesting class of semi-conducting materials. One of their main advantages is the large freedom in the nature of the organic spacer molecules that separates the individual inorganic layers. The nature of the organic layer can significantly affect the structure and dynamics of the 2D material; however, there is currently no clear understanding of the effect of the organic component on the structural parameters. In this work, we have used molecular dynamics simulations to investigate the structure and dynamics of a 2D Ruddlesden-Popper perovskite with a single inorganic layer (n = 1) and varying organic cations. We discuss the dynamic behavior of both the inorganic and the organic part of the materials as well as the interplay between the two and compare the different materials. We show that both aromaticity and the length of the flexible linker between the aromatic unit and the amide have a clear effect on the dynamics of both the organic and the inorganic part of the structures, highlighting the importance of the organic cation in the design of 2D perovskites.

The Relation between Rotational Dynamics of the Organic Cation and Phase Transitions in Hybrid Halide Perovskites.

The rotational dynamics of an organic cation in hybrid halide perovskites is intricately linked to the phase transitions that are known to occur in these materials; however, the exact relation is not clear. We have performed detailed model studies on methylammonium lead iodide and formamidinium lead iodide to unravel the relation between rotational dynamics and phase behavior. We show that the occurrence of the phase transitions is due to a subtle interplay between dipole-dipole interactions between the organic cations, specific (hydrogen bonding) interactions between the organic cation and the lead iodide lattice, and deformation of the lead iodide lattice in reaction to the reduced rotational motion of the organic cations. This combination of factors results in phase transitions at specific temperatures, leading to the formation of large organized domains of dipoles. The latter can have significant effects on the electronic structure of these materials.