Understanding the mechanisms of charge carrier generation and transport in photovoltaic systems is a key to get to efficient devices. The primary step towards such an understanding is a thorough analysis of the electronic structure and properties of those systems, yielding more targeted designs.    

This is even more important in the context of hybrid lead-halide perovskite solar cells : Further improvement of their power conversion efficiency and stability requires now a clearer vision of the electron transfer properties occurring at their interface. Indeed, the main limitation towards reaching the Schockley-Queisser efficiency limit in those perovskite systems arises from phenomena detrimental to sustained charge separation. Besides, such phenomena, such as carrier trapping and accumulation at interfaces, are also believed to concoure to the well-known hysteresis problem observed in i-V curves. As such, the understanding of electron and hole transfer processes and the related interfacial phenomena is essential for further progresses.     

Ideal systems for the study of such processes are colloidal suspensions : Nanoparticles actually exhibit a large surface/volume ratio, which creates a favorable environment for the observation of interfacial phenomena. Herein, we aim at structurally characterizing colloidal methylammonium lead bromide perovskites (MAPbBr3) nanoparticles suspended in an organic solvent and studying the electron transfer at their surface. The dynamics of charge transfer has been scrutinized in chlorobenzene solution with three different electron donors : spiro-MeOTAD, MeO-TPD and 1,4-bis(diphenylamino)benzene, as well as one acceptor, benzoquinone.    

Ultrafast transient absorption spectroscopy along with nanosecond flash photolysis experiments have been performed,  a necessary combination to get insights into two very different timescales of interest. Furthermore, Stern-Volmer analyses and time-resolved fluorescence measurements have shed light on the nature of the donor-acceptor interaction and the long-time behaviour of the photoluminescence. We show that the obtained nanoparticles consist of a highly heterogeneous mixture of 3D and multi-layered perovskites that exhibit highly efficient energy transfer processes towards bulk (3D) perovskite domains. In addition, we observed that electron transfer at their interface occurs within unusually long timescales.