The efficiency of solar cells with hybrid perovskites as an active semiconductor has reached 22% efficiency, which is competitive with the industry standard of 25% in single junction silicon-based devices. The competitiveness of hybrid perovskites with highly ordered semiconductors is inconsistent with the current understanding of semiconductors, as crystalline disorder is thought to decrease device performance, but the organic cation in hybrid perovskites increases crystalline and dynamic disorder. Perovskites have the general formula AMX3, where halide anions (X) and the metal (M) form a network of corner-sharing octahedra around the A-site cation (A), an organic cation in hybrid perovskites which sits in the voids between octahedra. The A-site cation, which in this case is formamidinium, introduces vibrational degrees of freedom that are not present in traditional inorganic semiconductors. Even though it is not understood how the A-site cation contributes to the electronic properties of the semiconductor, it appears to affect electronic properties and structural behavior, which most likely occurs from organic-inorganic coupling mediated by organic cation dynamics. I have prepared the perovskite formamidinium lead bromide (FaPbBr3) and diluted the formamidinium with cesium to prepare several members of the series Fa(1-x)Cs(x)PbBr3. These compounds have been structurally analyzed through temperature-dependent X-ray diffraction at Argonne National Laboratory, through inelastic neutron scattering at Oak Ridge National Laboratory and through heat capacity, in order to determine the effects of the organic cation on the structure of the octahedral network. While the temperature dependent X-ray diffraction shows the suppression of the lower temperature phase transition with the introduction of cesium, the inelastic neutron scattering shows little change across the spectrum with the introduction of cesium. Through analysis, we find that the structural changes of the octahedra are intimately coupled with the dynamics of the cation. Therefore, the presence of an organic cation adds complexity to the system.

Understanding how dipole dilution in hybrid perovskites affects photovoltaic properties

2019 Spring National Meeting

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