Synthesis and characterization of mixed-metal hybrid perovskites

Abstract

Metal halide perovskites have attracted a lot of attention over the last decade as a potential low cost alternative to traditional silicon based photovoltaics. Solar cells based on these materials have already achieved power conversion efficiency (PCE) of 22%. However, these high performing compositions are lead containing which is regarded as a potential risk for humans as well as the environment. A lot of research effort has been put into completely replacing lead with other group-14 elements such as tin and germanium, due to their similar sizes and electronic configuration. These kinds of perovskites have shown promising optoelectronic properties but are highly unstable due to the easy oxidation of tin and germanium.
In this thesis an alternative approach of mixing lead with other smaller divalent metal cations is explored. MAPbI3 is synthesized using lead acetate due to the facile removal of byproducts and its tolerance for mixing with other metal salts. The alternate metal salts were selected on the basis of their solubility in commonly used solvents and the suitability of the crystal structure of the precursor compound for perovskite structure formation. We found manganese to be a suitable substituent of lead and the upper limit for these mixed metal perovskites after geometrical calculations as well as experimental verification is found to be around 30%. Though the mixed metal compositions maintain the tetragonal crystal structure of lead based perovskites, a secondary crystalline phase is observed with increased lead substitution. Efforts are made to identify its composition and to remove it by optimizing the thermal treatment as well as the ratio between the other precursors. The optimized recipe for 30% lead substituted showed phase purity as well as good optical and electronic properties. Detailed compositional analysis revealed that, unlike MAPbI3 synthesized using chloride based precursors, in these mixed metal compositions chlorine is also incorporated in the films containing manganese especially near the substrate interface. This suggests that the smaller metal cation has an affinity for the smaller halide anion and that it plays a key role in the initiation of nucleation in such mixed metal (Pb:Mn) compositions. Finally, solar cells were made as a proof of concept incorporating these mixed metal perovskites and devices with up to 1.45% PCE were obtained.