We theoretically characterize the unusual white-light emission properties of two-dimensional (2D) hybrid organic-inorganic perovskites with an APbX4 structure (where A is a bidentate organic cation and X=Cl, Br). In addition to band-structure calculations including corrections due to spin-orbit couplings and electron-hole interactions, a computationally intensive molecular-cluster approach is exploited to describe the excitonic and polaronic properties of these 2D perovskites at the atomistic level. Upon adding or removing an electron from the neutral systems, we find that strongly localized small polarons form in the 2D clusters. The polaron charge density is distributed over just ~1.5 lattice sites, which is consistent with the calculated large polaron binding energies, on the order of ~0.4-1.2 eV. TOC GRAPHICSOver the past few years, three-dimensional (3D) hybrid organic-inorganic perovskites with general formula APbX3 (where A is a monovalent cation and X=Cl, Br, I) have demonstrated outstanding optoelectronic properties that have led to thin-film solar cell and light-emitting diode applications.1-3 Hybrid perovskites with low-dimensionality also display remarkable electronic and optical characteristics.4 The reduction in dimensionality, such as in the case of layered twodimensional (2D) perovskites APbX4, leads to strong quantum/dielectric confinement effects responsible for significant changes in the optical properties.5-9 A particularly interesting feature is the broadband, white-light photoluminescence (PL) at room temperature shown by a series of 2D perovskites such as (API)PbBr4 (API=N-(3-aminopropyl)imidazole),10 (EDBE)PbX4 (EDBE=2,2-(ethylenedioxy)bis(ethylammonium), X=Cl, Br),11 (C6H11NH3)2PbBr4,12 and (N-MEDA)[PbBr4−xClx] (N-MEDA=N1-methylethane-1,2-diammonium)13. While earlier works attributed the origin of such a broadband photoluminescence either to deep-trap surface states14-15 or to energy transfer from the inorganic to the organic layers,10 the formation of self-trapped excitons is now generally accepted as the main mechanism underlying the large Stokes shift and emission broadening.11-13, 16-17 Similarly, it has been proposed that self-trapping phenomena could affect the homogeneous PL broadening of 3D MAPbI3,18 where the existence of small polarons triggered by the collective reorientation of MA+ cations was theoretically predicted.19Recently, a first-principles study of exciton self-trapping in 2D hybrid perovskites has shown that local perturbations of specific crystal lattice sites induce the localization of self-trapped species in the perovskite inorganic framework, and result in the formation of small polarons.17 However, the standard computational methods used in that work, relying on the local perturbation of lattice parameters within a supercell, do not allow the assessment of the actual response calculations. This approach has been previously used to reproduce successfully the optical properties of hybrid perovskite systems with spin-orbit interactions.43 ASSOCIATED CONTENT Supporti...

This study presents a combined experimental and theoretical study of the electronic structure of two two-dimensional (2D) metal halide perovskite films. Ultraviolet and inverse

We have carried out nonadiabatic molecular dynamics simulations combined with time-dependent density functional theory calculations to compare the properties of the two-dimensional (2D) (BA) 2 (MA)Pb 2 I 7 and three-dimensional (3D) MAPbI 3 (where MA = methylammonium and BA = butylammonium) materials. We evaluate the different impacts that the 2D-confined spacer layer of butylammonium cations and the 3D-confined methylammonium cations have on the charge carrier dynamics in the two systems. Our results indicate that, while both the MA + and BA + cations play important roles in determining the carrier dynamics, the BA + cations exhibit stronger nonadiabatic couplings with the 2D perovskite framework. The consequence is a faster hotcarrier decay rate in 2D (BA) 2 (MA)Pb 2 I 7 than in 3D MAPbI 3 . Thus, tuning of the functional groups of the organic spacer cations in order to reduce the vibronic couplings between the cations and the Pb−I framework can offer the opportunity to slow down the hot-carrier relaxations and increase the carrier lifetimes in 2D lead-halide perovskites.

As a supplement of the droop control, the concept of secondary controlled microgrid (MG) has been extensively studied for voltage and frequency restoration. However, the low band-width communication (LBC) channels are needed to exchange information between the primary and secondary controllers, and the performance of the secondary controller degrades due to the uncertain communication delay and data drop-out in the LBC lines. Recently, a washout filter-based power sharing method was presented without communication lines and additional control loops. In this paper, the equivalence between secondary control and washout filter-based power sharing strategy for islanded microgrid is demonstrated, and the generalized washout filter control scheme has been obtained. Additionally, the physical meaning of control parameters of secondary controllers is also presented. Besides, a complete small-signal model of the generalized washout filter-based control method for islanded MG system is built, which can be utilized to design the control parameters and analyze the stability of MG system. Finally, extensive simulation and experimental results are provided to confirm the validity and effectiveness of the derived equivalent control scheme for islanded MG.

While the search for two-dimensional (2D) organic semimetallic Dirac materials displaying like graphene a Dirac cone at the Fermi level remains active, attention is also being paid to the quantum phase transition from semimetal to antiferromagnet. Such a transition in graphene-like materials was predicted based on theoretical investigations of 2D honeycomb lattice; it occurs (within a Hubbard model) when the on-site electron-electron Coulomb repulsion (U) is much larger than the nearestneighbor inter-site electronic coupling (t). Here, we consider monomers carrying long-lived radicals and use them as building blocks to design 2D hexagonal π-conjugated covalent organic frameworks (COFs). We evaluate both the nonmagnetic semimetallic phase and in magnetically ordered phases.