Breaking the Boundaries of the Goldschmidt Tolerance Factor with Ethylammonium Lead Iodide Perovskite Nanocrystals

TL;DR

This study synthesizes ethylammonium lead iodide nanocrystals, demonstrating their instability, sensitivity to electron irradiation, and tunable photoluminescence, challenging traditional tolerance factor limits in perovskite design.

Contribution

It introduces a new lead iodide perovskite nanocrystal with a large A-cation, surpassing the Goldschmidt tolerance factor, and explores its stability, degradation, and optical properties.

Findings

EAPbI3 nanocrystals are highly unstable and degrade within one day.

Photoluminescence can be tuned between 664-690 nm by size control.

Partial alloying with Cs+ improves emission efficiency.

Abstract

We report the synthesis of ethylammonium lead iodide (EAPbI3) colloidal nanocrystals as another member of the lead halide perovskites family. The insertion of an unusually large A-cation (274 pm in diameter) in the perovskite structure, hitherto considered unlikely due to the unfavorable Goldschmidt tolerance factor, results in a significantly larger lattice parameter compared to the Cs-, methylammonium- and formamidinium-based lead halide perovskite homologues. As a consequence, EAPbI3 nanocrystals are highly unstable, evolving to a non-perovskite delta-EAPbI3 polymorph within one day. Also, EAPbI3 nanocrystals are very sensitive to electron irradiation and quickly degrade to PbI2 upon exposure to the electron beam, following a mechanism similar to that of other hybrid lead iodide perovskites (although degradation can be reduced by partially replacing the EA+ ions with Cs+ ions). Interestingly, in some cases during this degradation the formation of an epitaxial interface between (EAxCs1-x)PbI3 and PbI2 is observed. The photoluminescence emission of the EAPbI3 perovskite nanocrystals, albeit being characterized by a low quantum yield (around 1%), can be tuned in the 664-690 nm range by regulating their size during the synthesis. The emission efficiency can be improved upon partial alloying at the A site with Cs+ or formamidinium cations. Furthermore, the morphology of the EAPbI3 nanocrystals can be chosen to be either nanocube or nanoplatelet, depending on the synthesis conditions.