How you make perovskite materials matters. Lead (Pb) halide (e.g. chloride) perovskites are promising semiconducting materials for photovoltaic solar cells, because of their low cost and their high efficiency for converting sunlight into electricity. Semi-crystalline phases and compositional imperfections were found to permeate these materials when made by solution phase synthesis, even after heat treatment. In contrast, a new solvent-free, solid phase synthesis avoids the formation of semi-crystalline phases, and enables a better understanding of the origin and nature of those imperfections. Through optical techniques, X-rays and, for the first time, 207Pb solid-state nuclear magnetic resonance (NMR), a scientist’s equivalent of MRI, researchers probed the extent of alloying and phase segregation in these materials. 207Pb solid-state NMR is highly sensitive to how Pb is coordinated locally in materials and to electronic structure. This work demonstrates how different synthetic procedures impact the composition of perovskites and that control of the additional phases formed is needed to further improve the efficiency and stability of solar cells made from these materials.
Using 207Pb solid-state NMR, UV-Vis, and XRD, researchers showed that semi-crystalline phases and non-stoichiometric impurities permeate samples made in solution. Solid-phase synthesis avoids the former, but not the latter.
Persistent Dopants and Phase Segregation in Organolead Mixed-Halide Perovskites