Optical Characterisation of Hybrid Perovskites for Photovoltaic Applications

Research interest in hybrid perovskites for photovoltaic applications has accelerated
rapidly over the past 10 years. Hybrid metal halide perovskites are a family of materials of the form ABX3, where A is either an organic cation or a mix of organic and inorganic cations, B is a divalent metal cation and X is a halide anion. Hybrid perovskites are promising candidates for absorbers for photovoltaic cells, as they exhibit many favourable properties such as long charge carrier lifetimes, long diffusion lengths
and high charge carrier mobilities. Furthermore, the ability to fabricate thin films of such materials via solution processing means that fabricating photovoltaic cells based on hybrid perovskite is relatively inexpensive, low temperature and facile, and can be adapted for deposition on flexible substrates and for roll-to-roll processing.

Hybrid perovskites which are composed of mixed A-site cations and mixed X-site halides are of particular interest due to the ability to tune the optical properties of the material through compositional engineering. The double cation, mixed halide perovskite (FAPbI3)0.85(MAPbBr3)0.15 and the triple cation, mixed halide perovskite Cs0.05FA0.76MA0.19PbI2.55Br0.45 are both excellent candidates for solar cell absorbers. In this thesis, the optical properties of these materials are investigated through a combination of spectroscopic techniques, and their crystal structure is probed through X-ray diffraction
measurements.

In Chapter 4, proof of concept is presented for a time-resolved PL mapping system,
designed and built during this PhD project for the correlation of surface morphology with
local fluorescence lifetimes in hybrid perovskite thin films. In Chapter 5, the TRPL mapping system is utilised to investigate the relationship between surface morphology and
fluorescence lifetimes in (FAPbI3) 0.85(MAPbBr3)0.15 and Cs0.05 FA0.76MA0.19PbI2.55Br0.45
thin films. It is found that both types of films exhibit wrinkled morphology as a result
of processing conditions, and these wrinkles correlate with local variations in PL intensity and PL lifetime. The effect of vacuum-assisted solution processing (VASP) on the morphology of spray-cast triple cation perovskites is also explored. The TRPL mapping system reveals a significant increase in surface uniformity as a result of the vacuum treatment, accompanied by lengthened fluorescence lifetimes and increased spatial ho�mogeneity of lifetimes. These findings correlated with enhanced device performance in PV cells based on VASP-treated films.

In Chapter 6, absorbance and photoluminescence spectroscopy are employed to investigate the temperature dependence of the optical properties of (FAPbI3)0.85(MAPbBr3)0.15,
and variable temperature X-ray diffraction measurements are taken to determine the
phase behaviour of the material. Two phase transitions were identified for this material:
a high temperature transition from a pseudo-cubic phase to a pseudo-tetragonal phase at
∼ 260 K (-13◦C), and a low temperature phase transition at ∼ 80 K (-193◦C) to a lower
symmetry variation on the tetragonal phase. This material exhibits phase-specific optoelectronic properties, such as the discontinuity in the temperature-dependent blueshift in
the optical band gap observed to correlate with the high temperature phase transition.
A correlation between the size and shape of the lattice unit cell and the resulting recombination rates in the material is found, speculated to be linked to polaron formation in
the material.
Chapter 7 extends these investigations into the Cs-containing triple cation perovskite
family of materials. In this chapter, varying amounts of Cs were added to (FAPbI3)
0.85(MAPbBr3)0.15 to determine the effect of Cs incorporation on the phase behaviour
and optoelectronic properties of mixed cation perovskites. It is shown that the phase behaviour of Cs-containing triple cation perovskites is largely the same as that of (FAPbI3)
0.85(MAPbBr3)0.15, but that the high temperature cubic-tetragonal phase transition shifts
to higher temperatures as Cs content is increased. The band gap of these Cs-containing
perovskites widens with increasing temperature, and it is speculated that this widening
is driven by the lengthening of the lattice parameter c with increasing temperature.