Materials

Intense interest has arisen over the last few years in halide perovskite materials, which exhibit remarkable performances across a wide range of optoelectronic applications such as photovoltaics and light-emitting diodes to name a few.  These hybrid perovskite materials are composed of an organic cation, a metal cation, and a halogen anion.  Our efforts have focused on the heavily studied methylammonium (MA) lead iodide, CH3NH3PbI3, structure.  A combination of factors appears to be at play in propelling MAPbI3 and other halide derivatives to the extremely high power-conversion efficiencies reported, some in excess of 20%.  These factors include large optical absorption cross sections, a low exciton binding energy at room temperature which insures easy dissociation to free carriers, and high charge carrier motilities combined with low charge-carrier recombination rates, which provides an efficient route to carrier extraction.   While these bulk properties over these materials have been well-studied, the role of local structure and morphology are far less understood.  In fact, it is alarming that many different values of the basic physical properties have been reported in nominally identical materials and syntheses.   This hints that such values may vary greatly at the single particle level, where spatial heterogeneities and disorder may dominate.    In collaboration with the Kanatzidis group, we have used a home-built transient absorption microscope to study carrier dynamics in MAPbI3 thin films.  Our first effort was to use our understanding of how free carriers and excitons behave differently to map the population of each in single polycrystallite domains.  Using a multi-dimensional approach and analysis that included extensive carrier density studies, we discovered that both free-carrier and exciton populations co-exist and are spatially segregated on the 100s of nanometer length scale. Properties of the material optimal for entirely different applications such as photocapture and transport or lasing are present in the same sample prepared under nominally identical conditions.  These studies addressed a long-standing debate on the nature of carriers and the effects of morphology.

Top Left: Cross-sectional SEM of lead halide perovskite solar cell (courtesy of Kanatzidis group). After light absorption, exciton readily dissociates into electron and hole which then migrate towards their respective electrode. Top Right: crystal structure of hybrid perovskite showing methylammonium cation in the center.  Bottom Left: Distribution of free carriers (R) and excitons (B) as determined by transient absorption microscopy. High-energy carriers cause either a blue- or red-shift depending on carrier identity near the band edge.  The dependence of this shift on pump fluence (Bottom Right) supports these assignments.  Different carrier types are separated by only a few 100s of nanometers, highlighting the importance of local morphology on material properties.

 

S. Nah, B. Spokoyny, C. Stoumpos, C. M. M. Soe, M. Kanatzidis, and E. Harel, Spatially segregated free-carrier and exciton populations in individual lead halide perovskite grains, Nat. Photonics., DOI: 10.1038/NPHOTON.2017.36 (2017). PDF

Team Members: Xinyi Jiang, Sunhong Jun