Photophysics of Hybrid Perovskites with Electroactive Molecules

Abstract

Incorporating π-conjugated organic cations into hybrid organic–inorganic perovskites offers a compelling strategy to unlock new optoelectronic functionalities by mediating the interaction between molecular orbitals and the inorganic lattice bands. Whereas conventional perovskites rely on electronically inert organic spacers, this thesis explores systems where electroactive cations actively participate in excited-state processes through inorganic–organic coupling. Central to this exploration is how structural parameters, such as linker length, molecular orientation, and dimensionality, dictate the nature and strength of this hybrid coupling. Photoinduced charge transfer and a sub-gap charge transfer state are demonstrated in the 2D perovskite (Cz-Ci)2PbI4, and these properties are shown to be tunable through the chain length of the alkylammonium spacer, Ci. In (Pyr-Ci)2PbI4, the linker length controls the photophysical properties in an intriguing way; significant charge transfer hybridisation is observed in (Pyr-C4)2PbI4, which through interlayer exciton delocalisation controls out-of-plane absorption and transport, but not in (Pyr-C2)2PbI4 due to an orthogonal molecular orientation. Despite the presence of a π-conjugated molecule in the 2D perovskite lattice, (Pyr-C2)2PbI4 behaves like a traditional quantum well perovskite. Beyond conventional 2D perovskites, perovskite analogues with different octahedral connectivity and lattice dimensionality are investigated. The use of 7H-dibenzo[c,g]carbazole (DBCz) as a cation gives rise to two distinct structures: the layered 2D perovskite (DBCz)2PbI4 and an edge-sharing analogue, (DBCz)PbI3. Both exhibit photoinduced hole transfer, but only the edge-sharing phase exhibits charge-transfer-mediated triplet formation and coherent phonon generation at room temperature. Finally, we show that removing the alkyl spacer entirely can boost inorganic–organic interactions. In (dmpz)Pb2I6, strong vibronic coupling accompanies excitation of a bright charge-transfer state, with ultrafast vibrational coherences observed of both molecular and inorganic lattice modes. Together, these findings represent a systematic investigation of coupling regimes in hybrid perovskites, from weakly interacting quantum wells to highly mixed electronic–vibrational charge transfer states. This thesis establishes design principles for tuning inorganic–organic interactions through synthetic molecular engineering. These principles may be extended to real-world optoelectronic architectures that demand precise control over interlayer charge transfer as well as transport.