Origins of the long-range exciton diffusion in perovskite nanocrystal films: photon recycling vs exciton hopping

The outstanding optoelectronic performance of lead halide perovskites lies in their exceptional carrier diffusion properties. As the perovskite material dimensionality is reduced to exploit the quantum confinement effects, the disruption to the perovskite lattice, often with insulating organic ligands, raises new questions on the charge diffusion properties. Herein, we report direct imaging of >1 μm exciton diffusion lengths in CH 3 NH 3 PbBr 3 perovskite nanocrystal (PNC) films. Surprisingly, the resulting exciton mobilities in these PNC films can reach 10 ± 2 cm 2 V −1 s −1 , which is counterintuitively several times higher than the carrier mobility in 3D perovskite films. We show that this ultralong exciton diffusion originates from both efficient inter-NC exciton hopping (via Förster energy transfer) and the photon recycling process with a smaller yet significant contribution. Importantly, our study not only sheds new light on the highly debated origins of the excellent exciton diffusion in PNC films but also highlights the potential of PNCs for optoelectronic applications. Exciton hopping: Energy travels far in perovskite nanocrystal films Excitons quasi-particles in methylammonium lead bromide perovskite nanocrystal films can travel over a surprisingly long distance. Tze Chien Sum and colleagues at Singapore’s Nanyang Technological University uncovered that the long diffusion lengths of more than one micrometer are largely due to excitons hopping from one nanocrystal to another. They are also caused, albeit to a lesser degree, by photon recycling, in which absorbed photons are re-emitted within the material, generating more charge carriers. The team found that perovskite nanocrystals connected by octylamine ligands showed the longest exciton hopping range compared to those connected by hexylamine or oleylamine ligands. The findings demonstrate the potential of lead halide perovskite nanocrystal films for fabricating smaller, faster, and more energy-efficient electronic devices. They also improve the basic understanding of the mechanisms behind long-range energy transport in these films.