Understanding advanced materials for solar panels: interplay of composition, structure and optoelectronic properties in new metal-halide semiconductors
Keywords: Physics
Spectroscopy
Semiconductors
Energy
Perovskite
Terahertz Spectroscopy
Solar PV

Interest in metal-halide semiconductors has risen rapidly over the past decade, notably due to the success of lead-halide perovskites in solar cells, whose efficiencies have risen to rival those of best-in-class commercial materials. Alongside this improvement in device-based performance, a detailed understanding of the underlying optoelectronic properties of such materials has developed, focusing on the most successful lead-halide perovskite materials. This thesis provides insights into how the composition and structure of such materials influence their optoelectronic properties, focusing especially on charge-carrier dynamics and transport. The materials investigated for this thesis all move beyond conventional lead-halide perovskites, pushing the boundaries of materials discovery and thus making a clear understanding of their fundamental properties crucial to their successful deployment in optoelectronic and energy applications.
Materials that combine the high charge-carrier lifetimes and mobilities, strong absorption, and good crystallinity of three-dimensional perovskites with the stability of two-dimensional perovskites are promising candidates for use in solar cells. To fully understand the optoelectronic properties of these 2D–3D hybrid perovskite systems, materials with general composition\linebreak BAx(FA0.83Cs0.17)1-xPb(I0.6Br0.4)3 are investigated. Small amounts of butylammonium (BA) are found to help to improve crystallinity and passivate grain boundaries, reducing trap-mediated charge-carrier recombination and enhancing charge-carrier mobilities. For low amounts of BA, the benevolent effects of reduced recombination and enhanced mobilities yield charge-carrier diffusion lengths up to 7.7 µm, making these 2D-3D hybrid materials highly promising for solar-cell applications.
Due to concerns around the toxicity of lead, an interest in lead-free alternatives has developed in recent years, with a particular focus on silver-bismuth-halide materials. The optoelectronic properties of the lead-free semiconductor Cu2AgBiI6 are studied using temperature-dependent spectroscopy. Ultrafast charge-carrier localization effects are observed, evident from picosecond-scale THz photoconductivity decays and a rise in intensity with decreasing temperature of long-lived, highly Stokes-shifted photoluminescence. Together, these observations point towards strong charge-lattice couplings and the formation of small polaron states in Cu2AgBiI6. Intriguingly, similar charge-carrier localisation and temperature-activated hopping transport is also observed in the closely related double perovksite material Cs2AgBiBr6.
Finally, the charge-carrier dynamics and transport properties of five compositions along the AgBiI4 -CuI solid solution line are investigated. Increasing copper content is found to enhance photoluminescence intensity and charge-carrier transport, and this is shown to derive from reduced cation disorder and improved electronic connectivity due to the presence of Cu+. Further, increased Cu+ content enhances the band curvature around the valence band maximum, resulting in lower charge-carrier effective masses, reduced exciton binding energies and higher mobilities. Ultrafast charge-carrier localisation is observed across all compositions investigated, lowering the charge-carrier mobility and leading to Langevin-like bimolecular recombination. Together with the above temperature-dependent results, these findings indicate that charge-carrier localisation is intrinsically linked to the presence of silver and bismuth in these materials, drawing a clear experimental link between the electronic dimension of a material and the likelihood of charge-carrier localisation taking place.