Lead Halide Perovskite Nanocrystals in the Research Spotlight: Stability and Defect Tolerance

Segregation of Native Defects to the Grain Boundaries in Methylammonium Lead Iodide Perovskite

Native point and grain boundary (GB) defects are ubiquitous in methylammonium lead iodide (MAPbI₃) sensitizers employed in solar cells that are polycrystalline in nature. Here we use density functional theory (DFT) in conjunction with a thermodynamic approach to determine the stability and electronic properties of all native point defects and their interplays with Σ5-(210) GB in MAPbI₃. The transition levels of charged defects are investigated with inclusion of electrostatic charge corrections and spin-orbit coupling. We find that the GB region is a sink for most of the native point defects under different synthesis conditions. For the crystalline and bicrystalline MAPbI₃ with Σ5-(210) GB, we find respectively that the p-type antisite defects MA_I and Pb_I, where I substitutes for MA or Pb, introduce deep levels, and both are relatively stable under I-rich conditions. Hence, I-poor conditions are more preferable for synthesis of MAPbI₃ to have defects with electronically benign character.

High Photoluminescence Quantum Yields in Organic Semiconductor-Perovskite Composite Thin Films

One of the obstacles towards efficient radiative recombination in hybrid perovskites is a low exciton binding energy, typically in the orders of tens of meV. It has been shown that the use of electron-donor additives can lead to a substantial reduction of the non-radiative recombination in perovskite films. Herein, the approach using small molecules with semiconducting properties, which are candidates to be implemented in future optoelectronic devices, is presented. In particular, highly luminescent perovskite-organic semiconductor composite thin films have been developed, which can be processed from solution in a simple coating step. By tuning the relative concentration of methylammonium lead bromide (MAPbBr₃ ) and 9,9spirobifluoren-2-yl-diphenyl-phosphine oxide (SPPO1), it is possible to achieve photoluminescent quantum yields (PLQYs) as high as 85 %. This is attributed to the dual functions of SPPO1 that limit the grain growth while passivating the perovskite surface. The electroluminescence of these materials was investigated by fabricating multilayer LEDs, where charge injection and transport was found to be severely hindered for the perovskite/SPPO1 material. This was alleviated by partially substituting SPPO1 with a hole-transporting material, 1,3-bis(N-carbazolyl)benzene (mCP), leading to bright electroluminescence. The potential of combining perovskite and organic semiconductors to prepare materials with improved properties opens new avenues for the preparation of simple lightemitting devices using perovskites as the emitter.

Fluorescent Alloy CsPb x Mn1-x I3 Perovskite Nanocrystals with High Structural and Optical Stability

CsPbI₃ nanocrystals are still limited in their use because of their phase instability as they degrade into the yellow nonemitting δ-CsPbI₃ phase within a few days. We show that alloyed CsPb _x Mn_1-x I₃ nanocrystals have essentially the same optical features and crystal structure as the parent α-CsPbI₃ system, but they are stable in films and in solution for periods over a month. The stabilization stems from a small decrease in the lattice parameters slightly increasing the Goldsmith tolerance factor, combined with an increase in the cohesive energy. Finally, hybrid density functional calculations confirm that the Mn²⁺ levels fall within the conduction band, thus not strongly altering the optical properties.