We study the effects of a series of post-deposition ligand treatments on the photoluminescence (PL) of polycrystalline methylammonium lead triiodide perovskite thin films. We show that a variety of Lewis bases can improve the bulk PL quantum efficiency (PLQE) and extend the average PL lifetime, ⟨τ⟩, with large enhancements concentrated at grain boundaries. Notably, we demonstrate thin-film PLQE as high as 35 ± 1% and ⟨τ⟩ as long as 8.82 ± 0.03 μs at solar equivalent carrier densities using tri-n-octylphosphine oxide-treated films. Using glow discharge optical emission spectroscopy and nuclear magnetic resonance spectroscopy, we show that the ligands are incorporated primarily at the film surface and are acting as electron donors. These results indicate it is possible to obtain thin-film PL lifetime and PLQE values that are comparable to those from single crystals by control over surface chemistry.

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We use correlated confocal and wide-field fluorescence microscopy to probe the interplay between local variations in charge carrier recombination and charge carrier transport in methylammonium lead triiodide perovskite thin films. We find that local photoluminescence variations present in confocal imaging are also observed in wide-field imaging, while intensity-dependent confocal measurements show that the heterogeneity in nonradiative losses observed at low excitation powers becomes less pronounced at higher excitation powers. Both confocal and wide-field images show that carriers undergo anisotropic diffusion due to differences in intergrain connectivity. These data are all qualitatively consistent with trap-dominated variations in local photoluminescence intensity and with grain boundaries that exhibit varying degrees of opacity to carrier transport. We use a two-dimensional kinetic model to simulate and compare confocal time-resolved photoluminescence decay traces with experimental data. The simulations further support the assignment of local variations in nonradiative recombination as the primary cause of photoluminescence heterogeneity in the films studied herein. These results point to surface passivation and intergrain connectivity as areas that could yield improvements in perovskite solar cells and optoelectronic device performance.

New battery technology will be crucial to the electrification of transportation and aviation 1, 2 , but battery innovations can take years to deliver. For battery electrolytes, the many design variables present in selecting multiple solvents, salts, and their relative ratios [3][4][5][6][7] mean that optimization studies are slow and laborious, even those restricted to small search spaces. The key challenge is to lower the number and time-cost of experiments needed to formulate an electrolyte for a given objective.