AbstractPerovskite based photovoltaics have developed from a low performing offshoot of dye sensitized solar cells into its own field of solar cell research within a decade, with thousands of publications per year and record efficiencies greater than 23 %. Perovskite solar cell devices suffer from several important issues, toxicity and instability being the main two. Ion migration has been shown to cause material instability and hysteresis in devices. Substituting different cations and anions into the perovskite ABX3 structure reduces hysteresis. Understanding the impact of substitution is important towards improved device design in the future.
In this thesis, muon spin relaxation (μSR) is presented as a novel probe for iodide diffusion. The migration of iodide is initially observed in methyl ammonium lead iodide and a deuterated analogue. The measured activation energy of 0.17 eV matches well with recent literature values for iodide migration showing, for the first time, that it is possible to detect iodide using μSR. The use of μSR to study iodide diffusion in perovskite was then taken further by investigating perovskites with 5 mol% cation substitution. The results here show that the substitution of 5% guanidinium causes complete inhibition of iodide diffusion due to small distortions of the perovskite lattice, making migration energetically expensive. A small decrease in diffusion coefficient was also observed for Cs+ and formamidinium substitution.
The diffusion of bromide in pure methylammonium lead bromide and mixed anion perovskites was also investigated using μSR. Detecting bromide was not possible despite its similar properties to iodide. The diffusion of iodide in mixed anion perovskites was shown to be restricted as no iodide diffusion was detected in MAPb(Br0.17I0.83)3. Finally, the diffusion of iodide in 2D/3D hybrid perovskites was investigated using electrochemical impedance spectroscopy. It was shown that diffusion is once again strongly inhibited by the addition of larger A-site cations with just 1 mol% phenylethylammonium iodide causing inhibition of iodide diffusion in the measured temperature range. It was also shown that vertically
aligned 2D/3D perovskites also show inhibited iodide diffusion.
|Date of Award||4 Sept 2019|
|Supervisor||Andrew Johnson (Supervisor), Simon Lewis (Supervisor) & Petra Cameron (Supervisor)|