Compositional variation in hybrid organic-inorganic lead halide perovskites : kinetically- versus thermodynamically-controlled synthesis

Abstract

Authors thank the Chinese Scholarship Council and the University of St Andrews for Ph.D. Studentship support (to JT, CSC No. 201603780020).The formation and study of partial solid solutions in Az1-xFAxPbBr3, using reportedly similar sized cations azetidinium (Az+) and formamidinium (FA+), was explored via mechanosynthesis and precipitation synthesis. The composition and lattice parameters of samples from both syntheses were analyzed by 1H NMR and Rietveld refinement of powder X-ray diffraction. A clear mismatch in the composition of the perovskite was found between the precipitated samples and the corresponding solutions. Such a mismatch was not observed for samples obtained via mechanosynthesis. The discrepancy suggests products are kinetically-controlled during precipitation, compared to thermodynamically controlled mechanosynthesis. Furthermore, the cell volume as a function of composition in both hexagonal, 6H (Az-rich) and cubic, 3C (FA-rich) perovskite solid solutions suggests that FA+ is actually smaller than Az+, contradicting the literature. In the 3C (Az-poor) solid solutions, the extent of Az1-xFAxPbBr3 is unexpectedly smaller than in the corresponding methylammonium (MA+) system, Az1-xMAxPbBr3, which suggests that the extent of solid solution formation in these halide perovskites is predominantly dependent on the average A-cation size while the size mismatch plays a lesser role in comparison to oxides.PostprintPeer reviewe