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

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 (MAyFA1–yPbI3–xBrx.), 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