Origin of Broad Emission Induced by Rigid Aromatic Ditopic Cations in Low-Dimensional Metal Halide Perovskites

The development of broadband emitters based on metal halide perovskites (MHPs) requires the elucidation of structure–emission property correlations. Herein, we report a combined experimental and theoretical study on a series of novel low-dimensional lead chloride perovskites, including ditopic aromatic cations. Synthesized lead chloride perovskites and their bromide analogues show both narrow and broad photoluminescence emission properties as a function of their cation and halide nature. Structural analysis shows a correlation between the rigidity of the ditopic cations and the lead halide octahedral distortions. Density functional theory calculations reveal, in turn, the pivotal role of octahedral distortions in the formation of self-trapped excitons, which are responsible for the insurgence of broad emission and large Stokes shifts together with a contribution of halide vacancies. For the considered MHP series, the use of conventional octahedral distortion parameters allows us to nicely describe the trend of emission properties, thus providing a solid guide for further materials design.


Synthesis
Single crystals of ( the stoichiometric amount of the solid diamine (liquid for the 1,3-XDA) was added.The crystals formation was obtained by a slow cooling down to room temperature at 2°C h -1 .

Single crystal and powder X-ray diffraction
Single crystal data collections (λ = 0.71073 Å) were performed using a Bruker D8 Venture with Cu and Mo microfocus X-ray sources and PHOTON II detector with Bruker APEX3 program.The Bruker SAINT software 1 was used for integration and data reduction, while absorption correction was performed using SADABS-2016/2 2 .Crystal structures (CCDC) were solved and refined using SHELXT 2014/5 and SHELXL 2018/3 3,4 .

Photoluminescence and Absorption measurements
Photoluminescence: PL measurements were performed by means of a NANOLOG FL3-2iHR spectrofluorometer, Horiba Scientific, equipped with a 450W Xenon lamp as excitation source and an iHR320 triple-grating turret spectrometer as excitation monochromator.A single channel photomultiplier tube was used as detector.
Absorption: RS spectra were acquired in the wavelength range 300-800 nm directly on the powders by using a Jasco V-750 spectrophotometer, equipped with an integrating sphere (Jasco ISV-922).
Defect formation energies (DFE) and thermodynamic ionization levels (TIL) were calculated as follows: where is the energy of the defective supercell with defect X in charge state q, is the energy of the pristine supercell, and are the number and chemical potential of the added and     subtracted species, respectively, and are the valence band energy and the Fermi energy, respectively, and are electrostatic potential corrections due to the finite size of the supercell. 13,14   Electrostatic finite-size effects have been accounted for using the Freysoldt-Neugebauer-Van de Walle approach as implemented in the sxdefectalign code. 15[18][19] Table S1.Static dielectric constants for the bromide and chloride perovskite compounds from DFPT based on the PBE level of theory.

1, 3 -
Figure S1.Chemical structure of the diamines used in for the synthesis of Pb-Cl perovskites.

Figure S3 .
Figure S3.Projected density of states (pDOS) for investigate bromine-based 2D perovskites in the electronic ground state at PBE0 level of theory with α=0.25.The color code for all curves is given in the legend of panel a.

Figure S4 .
Figure S4.Projected density of states (pDOS) for investigate chlorine-based perovskites in the electronic ground state at PBE0 level of theory with α=0.25.The color code for all curves is given in the legend of panel a.

Table S2 .
Experimental band gap and theoretical band gap energies calculated on the PBE0 level of theory in units of eV.