Paper TF-WeM5
The Reaction Between Pyridine and CH₃NH₃PbI₃ Surface-Confined Reaction or Bulk Transformation?

Wednesday, November 1, 2017, 9:20 am, Room 20

Abstract

The Methyl amine lead iodide perovskite (CH₃NH₃PbI₃) shows great potential in solar cells, light-emitting diodes, lasers, and chemical sensors. Base on the surface chemical properties of CH₃NH₃PbI₃, many chemicals with Lewis base group, for instance: pyridine, tetra-ethyl ammonium, were used to enhance the photovoltaic, optoelectronics performance and the stability in ambient environment. However, people found contradict results that Pyridine molecules not only passivate the surface Pb²⁺ sites of CH₃NH₃PbI₃, but also to bleach and recrystallize CH₃NH₃PbI₃. Surface passivation demands the confinement of the reaction at the surface region but recrystallizing and bleaching require the transformation of bulk CH₃NH₃PbI₃. The underlying mechanism for these seemly contradicting results are not well-understood. Our results show, at 25 ºC, partial pressure of pyridine vapor is a determining factor for its reaction behaviors with CH₃NH₃PbI₃: one can modify just the surface of CH₃NH₃PbI₃ by using pyridine vapor of the pressure less than 1.15 torr, but can transform the whole bulk CH₃NH₃PbI₃ film with a pyridine vapor of 1.3 torr or higher. The results indicate, for the first time, that a small change of free energy of pyridine vapor (~ 0.3 kJ/mol) can cause the transition from surface-confined reaction to bulk transformation. It is interesting that in all pressure ranges, pyridinium ions is the main product from the reaction between pyridine and CH₃NH₃PbI₃. The bulk transformation is probably due to the formation of a liquid-like film, which increases the mobility of species to catalyze the reaction between pyridine and CH₃NH₃PbI₃. These findings provide a guidance for designing experiments in applying pyridine and probably other amines to functionalize and transform CH₃NH₃PbI₃ and other hybrid halide perovskites.