Halide perovskites have rapidly become materials of great interest in the photovoltaic community, due largely to their high efficiencies and the rapid pace at which the efficiency of perovskite solar cells is increasing. The main focus of work has been on the organic-inorganic hybrid perovskite methylammonium lead iodide CH3NH3PbI3, often known as MAPI, which shows great promise for a new generation of photovoltaics. As the search for more efficient cells progresses, mixed ‘A’ site cation cells have come to the forefront of research, with efficiencies reaching over 20 %. The fully inorganic perovskite CsPbI3 has been proposed as an alternative to MAPI and also shows great promise, although the viability of CsPbI3 for use in solar cells remains to be conclusively demonstrated. Another alternative to MAPI is the formamidinium (CH(NH2)2+) analogue FAPI. The highest efficiency cells use FAPI as a dopant with small quantities of smaller methylammonium, caesium and rubidium cations doped onto the ‘A’ site. Computer modelling techniques have been used and a potential model has been developed which accurately reproduces all the observed structures in the Cs(Pb,Sn (I,Br,Cl)3 series. The developed potential model simulates all the phases in the examined series to a high degree of accuracy.The potential model was then used to examine the effect of varying the Cs cation size on the relative stabilities of both the perovskite and non perovskite phases of CsPbI3. In addition, ab initio molecular dynamics simulations have been carried out on the doped FAPI structures, revealing at the atomic scale the behaviour which drives their high efficiencies. The effect of cation doping on octahedral tilting and locking was studied, showing how incorporation of small cations can cause fundamental changes in the dynamics of the structure, boosting it’s stability and efficiency.
|Date of Award||5 Feb 2018|
|Supervisor||Saiful Islam (Supervisor) & Christopher Eames (Advisor)|