Solid-state µ-MAS and thin-film NMR spectroscopy on hybrid organic/inorganic perovskite photovoltaic materials
Analytical Chemistry
Solid State and Surface Chemistry, Material Synthesis
Final Report Abstract
During my research fellowship I developed NMR crystallographic approaches, i.e., the combination of NMR and NQR spectroscopy with X-ray diffraction and computational modelling, in a suit of sub-projects to study different structural aspects of hybrid lead halide perovskites to gain an atomic-level characterization of its defect chemistry, ion disorder and ion dynamics. The research was focused on the quaternary phase diagram MAPbI3-MAPbBr3-FAPbBr3-FAPbI3, where the mixing of cations in MA1-xFAxPbI3 systems and of halides in MAPbI3-xBrx systems was systematically studied with respect to disorder and dynamics of the A cations (MA+ and FA+) and halide anions (I- and Br-). As such, it could be shown that throughout the compositional space of mixed cation MA1-xFAxPbI3 systems, a slight tendency for MA and FA clustering occurs which results in FA-rich and MA-rich domains, i.e., local compositional fluctuations, within the perovskite particles. In contrast, the mechanochemical synthesis of MAPbI3-xBrx results in homogenous random halide distributions except for Br-rich compositions, where Br-rich nano-domains are formed. Interestingly, both, A cation and halide mixing, significantly influence the reorientational A cation dynamics. In both cases, the isotropic reorientation in the cubic phases becomes anisotropic upon mixing ions in the perovskite crystal structure. We are currently working on the implementation of machinelearning force-field molecular dynamics (MLFF MD) simulations (in collaboration with Dr. Bokdam, University of Twente) to model the MA and FA dynamics in the disordered MAPbI3-xBrx and MA1-xFAxPbI3 systems, requiring large supercells and long MD simulation times (t>100 ps) to cover both the disorder and dynamics. The simulations will allow us to connect the experimental N NMR and 1H DQ NMR data with structural motifs and pinpoint the key interaction between the lattice and the cation dynamics. Furthermore, thermally, and light-induced halide dynamics in mixed MAPbI3-xBrx were investigated in-situ to develop a better understanding of halide segregation upon illumination. The NMR spectroscopic investigations of thermal halide mixing revealed complex halide kinetics, which seem to be influenced by different halide migration pathways, presumably mainly along and over particle interfaces and within the bulk crystalline parts. The current results represent the basis for future systematic studies of thermal halide mixing as a function of perovskite morphology (particle size, interface modifications, defect chemistry and density), which represent the counterpart for halide segregation processes upon light irradiation of mixed MAPbI3-xBrx perovskites. The latter was tackled through in-situ illumination XRD studies, which point towards a directed halide migration in the particles, with I-ions diffusing towards the surfaces, while Brmigrates opposite towards the centre of the particles. This might be induced through structural strain, as light irradiation leads to a lattice expansion, which presumably is attenuated throughout the particles due to the light penetration depth following the Lambert-Beer law. Currently, the XRD setup is being adapted to further strengthen this hypothesis. To also allow for in-situ NMR investigations upon illuminating the perovskites to gain information about local halide pathways, in-situ NMR setups were developed in wideline and high-resolution MAS conditions. Unfortunately, current limitations are a homogenous illumination of the sample, as well as the light penetration depths within perovskite particles. Thus, future work will focus on maximising the light intensity and irradiation radius. The research fellowship allowed me to develop expertise on the structural and dynamical characterization of hybrid lead halide perovskites applying NMR crystallographic approaches. Furthermore, fruitful, and on-going collaborations were built, especially with the junior group of Dr. Menno Bokdam (University of Twente) focusing on MLFF MD simulations, and with the junior group of Dr. Fabian Panzer (University of Bayreuth) focusing on the preparation of high-quality perovskite powders and their structure-optics relationships using various optical and electrical spectroscopy characterization techniques. Both cooperations have been proven powerful to complement the experimental NMR findings and correlate them to the perovskite’s morphologies and opto-electronic properties. The results for a suit of mixed systems revealed a complex interplay of defects, disorder and dynamics in the perovskite crystal lattice impacting crucially on the opto-electronic properties and stabilities of the perovskites. The studies highlight the necessity for a detailed understanding of structure-property relations in these complex systems to allow for deriving design rules for highly stable and efficient perovskite compositions and morphologies.
Publications
- Microscopic (Dis)Order and Dynamics of Cations in Mixed FA/MA Lead Halide Perovskites. J. Phys. Chem. C 2021, 125 (3), 1742–1753
Grüninger, H.; Bokdam, M.; Leupold, N.; Tinnemans, P.; Moos, R.; De Wijs, G. A.; Panzer, F.; Kentgens, A. P. M.
(See online at https://doi.org/10.1021/acs.jpcc.0c10042) - Suppressed Ion Migration in Powder-Based Perovskite Thick Films Using an Ionic Liquid. J. Mater. Chem. C 2021, 9 (35), 11827–11837
Ramming, P.; Leupold, N.; Schötz, K.; Köhler, A.; Moos, R.; Grüninger, H.; Panzer, F.
(See online at https://doi.org/10.1039/d1tc01554k)
