Structure-property relationships and mixed conductivity in layered hybrid perovskites based on phenyl-derived spacers

Hybrid halide perovskites are currently one of the most studied semiconductors. However, due to poor intrinsic and extrinsic stability, further developments to commercialize devices based on hybrid halide perovskites are limited. Many different strategies have been used to tackle this issue. Dimensionality reduction of hybrid halide perovskites has proven to be one of the most promising to improve stability or change material functionality. Thus, in my thesis, I focus on the phenyl-derived spacer molecules which form Dion-Jacobson (DJ) type layered hybrid perovskites.In Chapter 2, I investigated the stability properties of DJ layered hybrid perovskites based on 1,4-phenylenedimethanammonium (PDMA) spacers. I demonstrated its propensity to hydrate when exposed to a humid environment. Unlike MAPbI3, hydration is reversible as no PbI2 was detected after several hydration cycles. Furthermore, we found that the hydration occurs within minutes. It can be significantly slowed down by post-annealing samples in the air. Nonetheless, our results revealed that layered hybrid perovskites are susceptible to humidity.In Chapter 3, I have extended the study by looking at the spacer size effects on material properties. More specifically, I looked at the role of the alkyl chain length in bifunctional phenyl-derived spacers. We found that in para configuration, at least one methylene group, linking aromatic core and ammonium cation, is needed to form layered hybrid perovskites. Perovskites with the longest spacers show the best stability in humid environments.In Chapter 4, I have looked at the effects of spacer fluorination. We expected to control the π–π interactions between the spacer molecules by incorporating perfluorinated PDMA (F-PDMA) spacers, which have opposite quadrupole moments compared to their nonfluorinated counterpart. However, our study reveals nanosegregation in layered hybrid perovskites based on mixed spacers. Stability tests unveiled higher materials durability in a humid environment when using F-PDMA spacers, which are more hydrophobic than PDMA.In Chapter 5, I investigated the mechanical properties of DJ and Ruddlesden–Popper (RP) layered hybrid iodide and bromide perovskites. Materials were subject to hydrostatic pressure in the 0–0.35 GPa range to study structure-property relationships. Both RP and DJ perovskites show the most significant compressioniiialong the layer stacking direction leading to the reduced quantum confinement evidenced by the redshift in the optical bandgap. Among all the compositions, benzylammonium lead bromide ((BzA)2PbBr4) exhibited the most significant optical and structural property changes. It is attributed to the non-centrosymmetric crystal structure.In Chapter 6, I have described the electrical properties of both DJ and RP hybrid iodide perovskites based on PDMA and BzA spacers, respectively. Our results revealed that both perovskites are mixed conductors with very comparable conductivities, which are significantly lower than MAPbI3. Surprisingly, the iodine exchange rate in these samples is comparable to three-dimensional hybrid perovskites. The iodine doping experiments suggest that iodide vacancies are the main ionic carriers. Lastly, we obtained 0.6-0.9 eV activation energy for ionic carriers, larger than 3D perovskites. This implies that dimensionality reduction is an effective strategy to reduce ion migration.