Compositional Diversity and Structural Dynamics of Perovskite Solar Cells

: (Alternative format thesis).

Abstract

This thesis deals with perovskite materials for photovoltaic applications. Specifically the methylammonium lead iodide perovskite (MAPbI3) has experienced a recent surge of research interest, due to its promising performance as a photovoltaic absorber and its potential to provide cheap and abundant energy. The downside remains an insufficient stability as well as a poor understanding of the relationship between structure, property and photovoltaic performance. The aim of this thesis is to improve both shortcomings, stability and fundamental understanding, by optimising the fabrication process and an in-depth analysis of different MAPbI3 derivatives. For the optimisation of the fabrication process, the substrate treatment protocol was optimised regarding its influence on the adjacent TiO2 compact layer and its electro- chemical response. Secondly, the cell layout and measurement protocol was optimised. Finally, two new fabrication routes were introduced. One that allows a crystallisation of the perovskite thin-film at a lower temperature (about 84 ºC) by using a high air-flow rate; the other fabrication technique uses a vacuum assisted vapour conversion of PbI2, which allows for a controlled conversion process without environmental exposure and without the necessity of a glovebox. New perovskite materials were synthesised, which show an enhanced stability to- wards water, by replacing the organic A-site cation methylammonium (MA+). Firstly, cation mixtures with Cs+ (CsxMA1-xPbI3) show structural limitations with an esti- mated substitution limit of x > 0.13, that is not solely caused by steric factors, as expected, but indicates chemical bonding contributions. Secondly, the organic cation azetidinium (Az+) was employed, which does not form a perovskite structure on its own (δ-AzPbI3), like the δ-CsPbI3. However, solid-state solutions AzxMA1-xPbI3 formed for small Az-ratios (x ≤ 0.05) and showed an enhanced photovoltaic performance and reduced hysteresis. Notably, both systems, Cs+ and Az+ containing, improved the water-resistance of the resulting perovskite. An initial assessment of the instability of the AzPbI3 was undertaken with Raman spectroscopy and indicates a strong interaction between the amine group and the inorganic part, which could offer an alternative bonding scheme to the three-dimensional [PbX6]4– octahedra network and therefore result in the formation of a two-dimensional structure. Finally, different halide derivatives MAPbX3 (X = Cl, Br, I) were analysed regarding their structural dynamics, by measuring the spectroscopic response in resonance and off-resonance conditions. It was shown that the low-frequency modes are mainly caused by the inorganic lattice and matched the model of an harmonic oscillator, thus having negligible influence from the organic cation. Internal MA+ vibrations showed an continuous red-shift for smaller halide derivatives, which shows an increasing interaction with the inorganic scaffold. Temperature dependent measurements for the MAPbBr3 between 100 K and RT indicate an order-disorder transition with an activation energy of about Ea = 7.5 ± 1.0 kJ for the reorientation motion. The measurement of all MAPbX3 species at 100 K indicates long-range order between the MA+ cations in the orthorhombic phase.

Date of Award	7 Aug 2017
Original language	English
Awarding Institution	University of Bath
Sponsors	Destiny
Supervisor	Petra Cameron (Supervisor)