Benjia Dou1 2 Lance Wheeler1 Jeffrey Christians1 David Moore1 Steve Harvey1 Joseph Berry1 Frank Barnes2 Sean Shaheen2 3 Maikel van Hest1

1, National Renewable Energy Laboratory, Lakewood, Colorado, United States
2, University of Colorado Boulder, Boulder, Colorado, United States
3, University of Colorado Boulder, Boulder, Colorado, United States

With the advancement in metal halide perovskite (MHP) research, it is now increasingly manifested that mixed cations and halides have many advantages over the single cation/halide MHPs. However, to push the commercialization of mixed cation/halide based MHP, myriad of stability properties, such as ink shelf life, must be understood. Here, significant stoichiometric changes are observed in highly alloyed formamidinium (FA), methylammonium (MA), cesium (Cs), lead (Pb), bromide (Br) and iodide (I) containing perovskite films that were fabricated from perovskite ink with different aging in the dark under N2. This stoichiometric instability is caused by a change in ink composition over time where the ability of MA+, Cs+ and I- to incorporate into the perovskite film is compromised with ink aging. Such change is initiated by the hydrolysis of dimethylformamide and resulted formation of dimethylamine. Dimethylamine then induce Lewis base-acid reaction with Cs+ and Pb2+. As a result, the quantity of reactive Pb2+ present in the ink is reduced over time, which leads to less MA+ and I- incorporation into the perovskite film as they are energetically not favored to coordinate with Pb2+ compared to their counterparts (FA+, Br-). To address this ink aging induced film stoichiometry change, solid precursor salts were ball milled into a stable powder, which was proven, via various optoelectronic characterizations, to be a very effective way of storing large quantities of highly alloyed perovskites precursor materials.