Alejandro R. Goñi – Institut de Ciència de Materials de Barcelona (ICMAB)
Beyond their excellent photovoltaic performance, hybrid organic–inorganic metal halide perovskites exhibit unique physical phenomena and emerging functionalities prompted by the interplay between organic and inorganic components. The discovery of intrinsic quantum confinement effects in the form of oscillations in the optical absorption of formamidinium lead triiodide (FAPbI3) thin films (see Fig. 1) is a vivid example of the surprising physical properties of these hybrid materials. The relevance of this work resides in that the discrete features can be interpreted as manifestation of intrinsic quantum confinement effects, unintentionally occurring in FAPbI3 thin films. As illustrated in the inset to Fig. 1, quantum confinement acts on the electronic band structure either by discretization of the energy spectrum due to full restriction of the movement of a particle confined in deep wells (infinite potential barrier model), or by leading to formation of mini-bands, in case the (finite) confining potential exhibits periodicity. These changes in the electronic landscape lead to peaks in the joint density of states, as probed in absorption.
Writing in Nature ‘News & Views’, I make an important contribution to the interpretation of the observed quantum oscillations. I provide a simple but strong argument against ferroelectricity as the origin of such phenomenon. The oscillations are still apparent in the temperature range of the cubic phase of FAPbI3, for which ferroelectric order is strictly forbidden by symmetry. I also reinforce the interpretation based on phase polymorphism. I propose that a combination of strain build-up, changes in the surface energy and chemical bonding between perovskite and substrate can lead to quantum confinement by unintentional formation of inclusions of the perovskite phase surrounded by thin layers of a wide-gap, non-perovskitic phase. Understanding the mechanisms that lead to the quantum oscillations may suggest new routes for manipulating the electronic band structure of hybrid perovskites at the nanoscale to enhance optoelectronic performance by exploiting the confinementinduced discretization of the energy spectrum.