Indirect tail states formation by thermal-induced polar fluctuations in halide perovskites

Halide perovskites possess enormous potential for various optoelectronic applications. Presently, a clear understanding of the interplay between the lattice and electronic effects is still elusive. Specifically, the weakly absorbing tail states and dual emission from perovskites are not satisfactorily described by existing theories based on the Urbach tail and reabsorption effect. Herein, through temperature-dependent and time-resolved spectroscopy on metal halide perovskite single crystals with organic or inorganic A-site cations, we confirm the existence of indirect tail states below the direct transition edge to arise from a dynamical Rashba splitting effect, caused by the PbBr6 octahedral thermal polar distortions at elevated temperatures. This dynamic effect is distinct from the static Rashba splitting effect, caused by non-spherical A-site cations or surface induced lattice distortions. Our findings shed fresh perspectives on the electronic-lattice relations paramount for the design and optimization of emergent perovskites, revealing broad implications for light harvesting/photo-detection and light emission/lasing applications.

Origin of unusual bandgap shift and dual emission in organic-inorganic lead halide perovskites

Neha Arora, Ariadni Boziki, Mohammad Ibrahim Dar, Michael Graetzel, Gwénolé Jean Jacopin, Simone Meloni, Ursula Röthlisberger, Shaik Mohammed Zakeeruddin

Emission characteristics of metal halide perovskites play a key role in the current widespread investigations into their potential uses in optoelectronics and photonics. However, a fundamental understanding of the molecular origin of the unusual blueshift of the bandgap and dual emission in perovskites is still lacking. In this direction, we investigated the extraordinary photoluminescence behavior of three representatives of this important class of photonic materials, that is, CH3NH3PbI3, CH3NH3PbBr3, and CH(NH2)(2)PbBr3, which emerged from our thorough studies of the effects of temperature on their bandgap and emission decay dynamics using time-integrated and time-resolved photoluminescence spectroscopy. The low-temperature (

2016

Charge carrier and exciton dynamics within hybrid lead halide perovskites of mixed composition and dimensionality

Marine Eva Fedora Bouduban

Research is nowadays usually directed by its potentiality to birth technological applications of relevance in the present environmento-socio-economical context, which can be seen as fair. This can however appear frustrating to us scientists, as it sometimes embodies an obstacle to our curiosity and creative freedom. As a consequence, research topics exhibiting both high impact potential and strong scientific interest, in terms of novel ideas and self-questioning driving force, can be seen as some kind of Holy Grail. Hybrid lead halide perovskites (HOIPs) seem to fit this profile: demonstrating an extraordinary potential for optoelectronic applications, their high versatility in terms of composition and dimensionality allow the exploration of a large number of fascinating solid-state physics phenomena. This thesis faithfully reflects this double nature: each of the projects described is dedicated to a different type of perovskites, at the compositional of dimensional level, or to questions pertaining their embodiment in application-intended devices. Chapters 1 provides the scientific background necessary to the understanding of this work, from the fundamental concepts underlying light-matter interaction and semiconductor photophysics to the specificities of HOIPs and their optoelectronic devices. The second chapter presents the main concepts related to the experimental methods of use in this work. In particular, we discuss the interpretation of femtosecond time-resolved transient absorbance measurements and their analysis, as well as the nature of the electroabsorption signal and the information it holds about of the systems under study. In chapters 3 and 4, we discuss 3D HOIPs of mixed composition (MAyFA1âyPbI3âxBrx.), and address both the photophysics in their bulk and at their interfaces with electron-transporting and hole-transporting materials used in photovoltaic devices (SnO2 and spiro-OMeTAD). We show the formation of bulk charge transfer (CT) excitons between domains of different iodide/bromide proportion, and suggest this to be at the origin of a sustained charge separation, yielding better photovoltaic performance. Similarly, using a novel experimental strategy, we propose that charge injection at the SnO2/perovskite interface is mediated by interfacial CT excitons, and thus, slowed down. Furthermore, our experimental strategy also confirms the importance of additives in photovoltaic-intended spiro-OMeTAD layers, by demonstrating that injected charges accumulate in undoped spiro-OMeTAD. Chapter 5 constitute the transition towards perovskite systems of lower dimensionality: it presents a fundamental study of MAPbBr3 (MA = CH3NH3) nanoparticle aggregates, in solutions involving a distribution of 3D nanoparticles and quasi-2D nanoplatelets of different thicknesses. We highlight inter-structure interactions in the form of a cascade of energy and charge transfer, the latter being mediated by the formation of interparticle CT excitons. In chapter 6, we continue our exploration of the multi-dimensionality of HOIPs by focusing on 3D/2D perovskite bilayers, where the 2D layer involves the cations phenethylammonium (PEA) and 4-fluorophenethylammonium (FPEA), in the context of their photovoltaic performance. In particular, we demonstrate a strong relationship between the photophysics at 3D/2D interfaces and the crystal growth and orientation of the 2D layer, directly determined by the structure of its organic cation. We go one step

EPFL2019

Piezoelectric anisotropy-phase transition relations in perovskite single crystals

Marko Budimir, Dragan Damjanovic, Nava Setter

The orientation dependence of the longitudinal piezoelectric coefficient, d(33)(), is investigated as a function of temperature in BaTiO3 and PbTiO3 crystals using the Landau-Ginsburg-Devonshire theory. We show that a presence of the ferroelectric-ferroelectric phase transitions in BaTiO3 leads to enhanced d(33)() along nonpolar directions. The reason for this is that in the vicinity of a phase transition temperature at which a polarization vector changes its direction (tetragonal-orthorhombic/monoclinic, orthorhombic/monoclinic-rhombohedral), the shear piezoelectric coefficients become high. It is shown for all ferroelectric phases of BaTiO3 that the shear stress deforms the crystal cell and changes the polarization direction in a similar way as the corresponding temperature-induced phase transition. The influence of the piezoelectric shear effect on the anisotropy of d(33)() is particularly pronounced in the orthorhombic/monoclinic phase where the piezoelectric shear coefficients are determined by the presence of both the high-temperature tetragonal and the low-temperature rhombohedral phases. In PbTiO3, which does not exhibit ferroelectric-ferroelectric phase transitions, the shear piezoelectric effect is weak and d(33)() has its maximum along the polar axis at all temperatures. These results can be generalized to include phase transitions induced by electric-field and composition variations and are valid for all perovskite materials, including complex relaxor-ferroelectric perovskites that have recently attracted attention for their exceptionally large piezoelectric properties. (C) 2003 American Institute of Physics.

2003

Charge carrier and exciton dynamics within hybrid lead halide perovskites of mixed composition and dimensionality

Marine Eva Fedora Bouduban

EPFL2019