Ultrafast spectroscopy of charge-carrier dynamics and stability in lead-halide perovskites for thin-film photovoltaics

Hybrid metal halide perovskites have emerged as an important new class of materials for photovoltaics. For their integration in tandem solar cells, optimised photocurrent matching between top and bottom cells requires careful control over bandgap energy and charge transport properties, for which tunable mixed bromide-iodide lead perovskites are the most prominent contenders. This thesis investigates photophysical properties of highly tunable mixed-halide and mixed-cation perovskite films with the aim of correlating their optoelectronic parameters with crystal phase changes induced by compositional variations. Firstly, the compositional variation of mixed-halide FAPb(BrxI(1-x))3 perovskites is explored. Bimolecular and Auger charge-carrier recombination rate constants strongly correlate with the bromide content, x, suggesting a link with electronic structure. With rising x, the constants increase by up to an order of magnitude, yet still remain significantly below the Langevin limit. FAPbBr3 and FAPbI3 exhibit charge-carrier mobilities of 14 and 27 cm2 V-1 s-1 and diffusion lengths exceeding 1 μm, while mobilities across the mixed bromide-iodide system depend on crystalline phase disorder. Particularly, the mid-bandgap and bromide rich regions are found to be disordered and prone to photo-induced halide segregation. Subsequently, a highly crystalline mid-bandgap mixed-cation lead mixed-halide FA0.83Cs0.17Pb(Br0.4I0.6)3 perovskite is developed and investigated with focus on understanding the impact of adding caesium to the A-cation site in the lattice. Compared to single-cation FAPb(Br0.4I0.6)3, the crystal phase is stable and the chargecarrier mobility measured from THz spectroscopy reaches 21 cm2 V-1 s-1 resulting in a diffusion length of 3 μm. With photoinduced halide segregation being inherently suppressed, another benefit of controlled A-cation mixing could be revealed. In an aging-study, the correlation between the colloids concentration present in the precursor solution of FA0.83Cs0.17Pb(I0.8Br0.2)3 and nucleation stages for the crystallization of perovskite is explored. Having direct impact on morphology and photophysical properties, charge-carrier mobilities are reported to correlate with colloid concentration reaching > 20 cm2 V-1 s-1 for an optimum concentration. Ultimately, the full parameter space of mixed-cation (0 < y < 1) lead mixedhalide (0 < x < 1) CsyFA(1-y)Pb(BrxI(1-x))3 perovskites is examined, where crucial links between crystal phase stability, photo-stability and optoelectronic properties are highlighted. Finely tuning the caesium cation in the lattice yields excellent structural stability and charge-carrier transport properties for the parameter space between 0.1 < y < 0.3. For an optimized caesium content, the orthogonal halide-variation parameter space for Cs0.17FA0.83Pb(BrxI(1-x))3 achieves charge-carrier mobilities of 11 -40 cm2 V-1 s-1 and diffusion lengths of 0.8-4.4 μm.