Organometal halide perovskites in the form of nanocrystals (NCs) have attracted enormous attention due to their unique optoelectronic and photoluminescence (PL) properties. Here, we examine the phase composition and the temperature dependence of emission line width broadening in formamidinium lead bromide (FAPbBr) perovskite nanocrystals (NCs) for light-emitting applications and identify different charge-carrier scattering mechanisms. Our results show most of the emission is from the orthorhombic phase. The PL line width broadening at high temperature is dominated by the Fröhlich interaction between the free charge carriers and the optical phonons. At low temperatures, the peak of the PL spectrum exhibits a continuous red shift indicating an increase of excitons contribution at lower temperatures, and concurrently the line width also narrows down due to the inhibition of the optical phonons. From the temperature-dependent measurements, the coupling strength of both the charge phonon interaction and the exciton phonon interaction have been determined. The obtained results indicate that the charge phonon coupling strengths are higher compared to the exciton phonon coupling.

A one-dimensional coordination polymer shows nonlinear properties of PL, SHG and THG.

The present work reports a new concept on how to diminish dark twist intramolecular charge transfer (TICT) states with the zero-twist D−A systems in order to design frameworks with dual solution and solid-state emission property. The study began with theoretical calculations to understand the structural needs followed by the chemical synthesis of conceptually new two molecular designs, 1 and 2, with zero-twist angle between electron donor and acceptor units linked through a covalent bond and finally their applications in OLED devices. Oxazole was used as an acceptor in combination with the phenothiazene core as the donor, and the effect of enhanced electron donation was studied using methyl and anisole donor groups. DFT studies indicated a partial segregation of HOMO−LUMO levels in molecular designs, and the photophysics of these planar charge transfer molecules have been investigated. Natural transition orbital (NTO) calculations were carried out to understand excited-state transition character in these D−A molecules. Molecular level studies through single-crystal analysis revealed the importance of steric factor in controlling other molecular parameters, particularly short-range molecular forces. The synthesized compounds were eventually utilized in green-emitting OLED devices as a pristine emitting layer. Compound 2 showed better device efficiency than 1 in unoptimized devices largely due to the presence of the anisole group which prevented stacking of molecules. Solution-state emission and electroluminescence data of fabricated devices using 1 and 2 pointed out that molecular modification helped to enhance emission efficiency of 2 without shifting the emission wavelength.

We have imaged the emissions from excitons and free charges in a methylammonium lead bromide perovskite (MAPbBr 3 ) crystal. In a direct band gap semiconductor, dynamics of excited electrons and holes in hybrid lead-halide perovskites is rather complex because of the formation of excitons and the presence of traps and structural inhomogeneities. A recent report by Nah et al.

Photoluminescence (PL) spectra from thin films (TFs) and bulk crystals (BCs) of hybrid organo-halide perovskites are significantly different, the origin of which and their impact on the efficiency of the perovskite-based photoactive devices have been debated. We have used two-photon PL to study the temperature-dependent changes in the spectra of the TFs and the BCs of methylammonium lead bromide (MAPbBr3) perovskites in order to clarify the origin of the differences. Our results show that the differences in the spectra are due to the variation in the phase composition. At room temperature, the tetragonal (TE) phase is dominant in the BCs, while the orthorhombic (OR) phase is dominant in the TFs. The PL spectra of the TFs also show discernible contributions from the TE and the cubic phases. At lower temperatures, the increase in excitonic recombination causes a red shift of the PL spectra from the TFs, while a phase transition from the TE to the OR phase results in a blue shift of the PL from the BCs. The temperature-dependent narrowing of the PL linewidths shows a stronger coupling between the longitudinal optical phonons and the free carriers in the OR phase as compared to the TE phase, implying a reduced carrier mobility. However, as the OR phase is metastable at the room temperature, the slow phase transition to the TE phase should improve the photocurrent yield in the TFs, provided that the sample is shielded from other types of degradation.