Damien Garrot

Photoluminescence excitation (PLE) and polarization spectroscopy are used to investigate the origin of the ultrasharp defect-center luminescence of MAPbI3 (MA = CH3NH3) single crystals at low temperature. We find that the excitation of the defect states through the free-exciton (FE) resonance is inefficient. Instead, the defect luminescence is strongly enhanced under sub-band-gap excitation, approximately 20 meV below the band gap. This channel enhances the sharp lines and the broad photoluminescence background nonselectively and is attributed to a bound-to-free (BF) transition. Polarization spectroscopy across multiple crystals reveals a degree of linear polarization as high as 40% for the ultrasharp defect lines, while a maximum of 16% is reached for the near-band-edge (NBE) emission. Distinct polarization axes are observed for the two emissions. This anisotropy suggests a preferential orientation of the defect dipoles, governed by the local symmetry of the emitting centers rather than by the crystal symmetry. The excitation and polarization results are consistent with a donor–acceptor pair (DAP) origin of the ultrasharp emission and open the way to a deeper understanding of the electronic properties of defects in halide perovskites (HPs).

Solution-processed halide perovskites (HPs) have demonstrated exceptional optoelectronic performance and a range of unusual physical properties. Many of their intriguing characteristics can be attributed to the interactions among the dynamic polar lattice, charge carriers, and point defects. Here, microphotoluminescence (μPL) spectroscopy is used to investigate the spectral diffusion (SD) of ultrasharp emission lines (3. Temporal spectral fluctuations are typically considered a hallmark of single nano-object emission and are absent in ensemble-averaged luminescence. Contrary to this expectation, the absence of photon antibunching demonstrates that the observed spectral diffusion arises from the collective behavior of an ensemble of emitters. The magnitude of the spectral fluctuations varies from hundreds of μeV to the meV range. The time traces show continuous spectral jitter and discrete spectral jumps. The frequency of spectral jumps increases with excitation power, and statistical analysis shows a transition from Gaussian to Lorentzian statistics with an increasing excitation density. Furthermore, correlated spectral fluctuations are observed among multiple sharp emission lines. The energy spacing and excitation power dependence demonstrate that the synchronous lines are not phonon sidebands but instead arise from distinct defect states. The temporal correlations indicate that multiple defect states are coupled to the same local fluctuations of their nanoenvironment. These results suggest the existence of domains of slow, correlated lattice deformations that induce a shared perturbation of the optical transitions. Overall, these findings provide new insights into the defect-lattice interactions in HPs.

Double perovskites have emerged as promising alternatives to lead halide perovskites, aiming to mitigate challenges related to toxicity and chemical instability. Among them, mixed‐valence gold halides such as Cs ₂ Au ⁺ Au ^{3 +} Cl ₆ , which contain only a single type of metal cation in two oxidation states, stand out due to their unique structural and electronic properties. These materials exhibit strong absorption in the near‐infrared range, making them attractive candidates for optoelectronic applications such as photovoltaics. In this work, temperature‐dependent optical spectroscopy techniques are employed to demonstrate that these compounds exhibit particularly strong polar electron‐phonon coupling, which has a profound impact on their optoelectronic properties. In particular, this coupling induces a temperature‐dependent absorption tail that progressively reshapes the absorption spectrum. It is shown that this tail leads to a forbidden band‐egde recombination, which explains the reported difficulties in detecting a photoluminescence signal from this class of double perovskites.

Achieving a better understanding of the physics of defects in halide perovskites (HPs) is a key challenge for improving the efficiency of the devices. Herein, a comprehensive study of the defect emission of CH₃NH₃PbI₃ perovskite single crystals is presented. The emission of the pristine surface of cleaved crystals is systematically investigated based on steady‐state and time‐resolved micro‐photoluminescence (micro‐PL) spectroscopy. Donor–acceptor pair (DAP) recombination is observed due to the presence of native shallow defects. The DAP spectra present an important variability depending on the location on the surface of the crystals due to inhomogeneous defect distribution. A strong blueshift of the emission is measured as a function of excitation power and is explained by fluctuating potential caused by compensated defects. With increasing photocarrier density, a transition from a structureless to a structured DAP emission with several longitudinal optical phonon replicas is observed. The DAP transition is characterized by the redshift of the emission with time and a slow, non‐exponential PL decay. Sharp discrete lines with sub‐meV widths, the most recognizable spectral signature of DAP transition, are revealed. Herein, the apparently contradictory previous observations on defect emission of CH₃NH₃PbI₃ are reconciled by the results and new insights are provided into the properties of defects in HPs.

We report on a series of low dimensional 1D enantiomerically pure and racemic lead bromide perovskite compounds of the formula BuA-PbBr₃, MBA-PbBr₃ and EBA-PbBr₃ (BuA, cation derived from 2-Butylamine, MBA from Methylbenzylamine and EBA from α-Ethylbenzylamine) incorporating monovalent chiral ammonium derivative cations. A series of 6 enantiomerically pure compounds is described. The compounds are fully characterized from a structural point of view using X-Ray diffraction on single crystals and XRPD. Their photoluminescence emission properties have been carefully investigated. The use of cations of different sizes allows tuning the distances between the inorganic one-dimensional (1D) chains. All compounds show broadband below-bandgap emission, attributed to Self-Trapped Exciton (STE) recombination. The photoluminescence is characterized by a significant degree of linear polarization at room temperature, which can be explained by the structural anisotropy of the transition dipole moments and quantum confinement of the excitons in the 1D material. Our results indicate that the described compounds are of high interest for linearly-polarized broadband light-emitting devices and demonstrate the possibility of engineering the crystalline structure by the appropriate choice of the organic cation.

The large tolerance of hybrid perovksites to the trapping of electrons by defects is a key asset in photovoltaic applications. Here, the ionic surface terminations of CH₃NH₃PbI₃ are employed as a testbed to study the effect of electrostatic fields on the dynamics of excited carriers. We characterize the transition across the tetragonal to orthorhombic phase. The observed type II band offset and drift of the excited electrons highlight the important role that organic cations have on the screening of local electrostatic fields. When the orientation of organic cations is frozen in the orthorhombic phase, the positively charged termination induces a massive accumulation of excited electrons at the surface of the sample. Conversely, no electron accumulation is observed in the tetragonal phase. We conclude that the local fields cannot penetrate in the sample when the polarizability of freely moving cations boosts the dielectric constant up to ε = 120.

We investigated the coherent evolution of the electronic spin at low temperature in high-quality CH₃NH₃PbI₃ polycrystalline films by picosecond-resolved photoinduced Faraday rotation. We show that this coherent evolution can be tuned by choosing the pump–probe energy within the lowest optical-absorption band, and we explain it as the result of two main contributions: the localized electron and the localized hole. Their corresponding amplitude ratios are not constant across the lowest absorption band—an observation which disqualifies a free exciton from being at the origin of the electronic spin coherent evolution. We measured a spin coherence time of localized electrons (holes) of 4.4 ns (3.7 ns) at 1.635 eV, which evolves to about 7 ns at 1.612 eV (the hole coherence time remains almost constant at lower energies). Finally, we provide a global image of the spin coherent evolution in bulk metal halide perovskite, which overcomes recent controversies.

The family of 2D Ruddlesden-Popper perovskites is currently attracting great interest of the scientific community as highly promising materials for energy harvesting and light emission applications. Despite the fact that these materials are known for decades, only recently has it become apparent that their optical properties are driven by the exciton-phonon coupling which is controlled by the organic spacers. However, the detailed mechanism of this coupling, which gives rise to complex absorption and emission spectra, is the subject of ongoing controversy. In this work we show that the particularly rich, absorption spectra of \mbox{(PE)₂(CH₃NH₃)_n-1Pb_nI_3n+1, (where PE stands for phenylethylammonium and n=1,2,3), are related to a vibronic progression of excitonic transition. In contrast to other two dimensional perovskites we observe a coupling to a high energy (40\,meV) phonon mode probably related to the torsional motion of the NH₃⁺ head of the organic spacer.