Mayer Bond Order as a Metric of Complexation Effectiveness in Lead Halide Perovskite Solutions

One of the most appealing features of solar cells made from hybrid organic–inorganic perovskites is that they can be processed directly from solution, leading to low cost, energy-efficient processing. Numerous studies have shown that the composition of these solutions and the choice of solvent (or solvent blend) affects the efficiency of the resulting solar cell. Despite the importance of this correlation for performance, the choice of solvent(s) used to deposit the perovskite precursors has been largely a matter of experimental trial-and-error. In this work, we present a coherent theory explaining the molecular origin of the efficacy of solvent choice, which lends itself to the creation of a fast quantum mechanical-based screening process that facilitates the design of effective new solvents. We also provide the first theoretical confirmation of complexation of HOIP precursors in solution, including their structure and relative stability. We show that the Mayer bond order of a solvent's polar atoms predicts the solubility of the perovskite lead halide precursors in the solvent much more reliably than the relative polarity and Hansen polar solubility parameter suggested in the literature as being figures of merit.