The project has resulted in six peer-reviewed publications by the end of the funding period, the results of which are described below. The results have also been disseminated at eight national and international conferences.
Studying surface coatings on hybrid perovskite thin films is limited by the extremely low sample mass. Dynamic nuclear polarization (DNP) is a technique to dramatically improve the sensitivity of NMR experiments. In this project, the challenges preventing application of DNP to perovskites were overcome, which enabled the NMR spectrum to be measured for a 20 nm surface coating on a single perovskite thin film (10.1021/jacs.2c05316). This showed that the additive adopted a layered perovskite structure, but exhibited significantly greater disorder than a bulk layered perovskite. This disorder is important to consider when developing structure–activity relationship for perovskite surface treatments. The researcher further applied these DNP methods in a collaborative project on another important class of materials, core–shell nanoparticles grown by chemical atomic layer deposition, to deduce the mechanism by which the shell nucleates (10.1021/jacs.1c12538). How this shell forms was an important unanswered question in the field and required the sensitivity and atomic-scale picture afforded by DNP NMR.
The cation dynamics in hybrid perovskites play a role in the excellent optoelectronic properties for solar cell applications. In this project, new methodology was developed to measure the rotation about each axis of the cation in various mixed cation perovskites (10.1021/jacs.7b04930). This provides the community with definitive experimental data to inform theory and calculations.
This project also involved collaboration on three application-focussed perovskite projects, where NMR was used to determine the local structure of cutting-edge perovskite systems with different treatments/additives, to reveal their method of operation. (1) A record open-circuit voltage was achieved for a perovskite solar cell by treating with neopentylammonium chloride. The researcher used NMR to show that this was achieved by the passivation of the surface by the treatment (10.1021/acsenergylett.1c02431). (2) The efficiency and stability of both pure iodide and mixed iodide–bromide perovskite solar cells were greatly improved by treatment with alkyldimethylammonium amphiphiles. The researcher used NMR to demonstrate the interaction of these species with the perovskite surface, supporting ab initio calculations to distinguish the effect of different analogues (10.1016/j.joule.2022.11.013). (3) Photo-induced halide segregation is one of the major issues for 3D mixed-halide perovskites. Greater stability against segregation was demonstrated for layered 2D mixed-halide perovskites, which the researcher demonstrated arises from intrinsic halide heterogeneity in the pristine materials (10.1021/acsenergylett.3c00160).
