Structural and Electronic Properties of Two-Dimensional Lead Halide Perovskites
Creators: name: REDING, JOSHUA DEAN
Publisher: name: University of Surrey
Year: 2022

description: Over the last decade, lead halide perovskites have attracted a strong interest due to their low cost, ability to be solution processed for use in solar cells, LEDs, and other optoelectronic devices; however, they suffer from atmospheric degradation which prevents widespread commercial adoption. By reducing the perovskite dimensionality to a two-dimensional material by increasing the size of the A site cation, the long term stability can be increased, while maintaining many of the appealing optoelectronic properties of 3D perovskites. While some preliminary research into the fundamental optoelectronic and structural properties of 2D lead halide perovskites has occurred, there remain substantial gaps in the body of knowledge. In this thesis, we report on the optical, electronic, and structural properties of 2D BA2Pb(I_{1-x}Br_x)4, (BA+, C4H9NH3) and PEA2Pb(I_{1-x}Br_x)4 (PEA+, (C8H9NH3), x = 0.0, 0.25, 0.50, 0.75, 1.0) perovskites as studied using density functional theory. We show that the optoelectronic properties of these materials is dependent not only on the composition of halides but on their arrangement within the lead halide octahedra. By preferentially locating atoms with higher atomic number in the in-plane locations, a band gap reduction of 40 and 80 meV is demonstrated for PEA2PbI3Br and BA2PbI3Br as compared to PEA2PbI4 and BA2PbI4 respectively. This is the first time such a band gap bowing has been reported as a result of halide mixing, and arises due to a combination of the energy level offset of the electronic states and distortions in the lead halide octahedra as a result of the differing halide size. This band gap reduction, if able to be consistently reproduced in experimental thin films, would increase the usefulness of 2D perovskites as a solar cell capping layer on a 3D perovskite absorbing layer, and increase the range of wavelengths available for LEDs. Using this work as a theoretical foundation, we then report on the optical and electronic properties of 2D PEA2Pb(I_{1-x}Br_x)4 (x = 0, 0.37, 0.53, 0.76, 1.0) thin films. We report for the first time a series of phonon replicas resolvable using photoluminescence measurements at temperatures below 225 K. These phonon replicas are indicative of the coupling of LO phonons originating in the organic molecule to the electronic states. These high energy phonons, E = 35 - 70 meV, are significantly higher energy than those previously reported and these show distinctly non-polaronic character unlike their 3D counterparts. This key outcome shows that the vibronically coupled band structure is responsible for the broadened Stokes-shifted luminescence peaks seen in mixed halide 2D perovskites as the ratio of bromine to iodine is increased. This provides a solution to the controversial nature of the radiative recombination in 2D perovskites that has long been present. These replica peaks show broadband photoemission across the visible spectrum and highlights their usage as broadband emitters for LED applications. Finally, we report carrier dynamics including showing the self-trapping of free excitons into bound states which occurs on sub-ps timescales. This trapping is a substantial barrier to using 2D perovskites as a material for applications that requires a high degree of carrier mobility. We show resolvable coupling between a low energy lead halide bending mode and the electronic states that de-phases in the first ten picoseconds. We then report nanosecond radiative lifetimes for bound excitons coupled to the vibronic energy states, and sub-ns lifetimes for bound excitons at the zero-phonon line. This thesis fills key gaps in the understanding of the fundamental optical, structural, and electronic properties of 2D lead halide perovskites and shows how the inclusion of small amounts of bromine into the lattice can result in positive effects for future optoelectronic applications.