Lithium-ion battery devices as a method of doping metal halide perovskites

Hybrid perovskite semiconductors are arguably one of the most promising material systems for the next generation of energy conversion devices studied today. Reports of record-breaking energy conversion efficiencies are disclosed on an almost yearly basis. However, unlike traditional inorganic semiconductors such as silicon and GaAs, the impact of inserting extrinsic species into these materials remains quite poorly understood, be it from an energy storage, or doping perspective. This thesis combines two fields of research - lithium-ion batteries and perovskite optoelectronics. The first study in Chapter 4 considers Ruddlesden-Popper 2D/3D perovskites from an energy storage point of view. In doing so, the macroscopic lithiation processes that occur when a perovskite electrode is subjected to galvanostatic cycling are discussed. It is shown that a mixed 2D/3D phase optimises the density of electrochemically active lead-halide layers, relative to the electrochemically inactive lithium diffusion pathways through the structure provided by butylammonium organic cations. However, the necessity for aprotic solvents in typical lithium-ion batteries engenders an issue of dissolution at the perovskite-electrolyte interface. This problem is addressed in two stages; first, using high-molarity electrolytes that reduce the amount of free solvent present and second, by using a polymeric solid state alternative. Chapter 5 shows how the process of lithiating a perovskite using a battery-inspired architecture can result in an n-type doped material and uses the Burstein-Moss bandgap shift effect to quantify this effect. The optical and electrochemical measurements of the doping concentration agree to within 96 %. Chapter 6 presents a mechanistic framework with which to understand the phase conversion processes that take place within the perovskite's ionic lattice while varying the dopant concentration over three orders of magnitude from 10¹⁸ cm⁻³ to 10²¹ cm⁻³. It does so by utilising bespoke lithium-ion battery designs that facilitate a suite of operando analysis techniques such as synchrotron XRD and optical microscopy. Three doping regimes, depending on the relative phase fraction of perovskite, are quantified. The overall conclusions suggest methods of optimising both perovskite-based energy conversion and energy storage devices in the future.