The ability to manipulate quantum dot (QD) surfaces is foundational to their technological deployment. Surface manipulation of metal halide perovskite (MHP) QDs has proven particularly challenging in comparison to that of more established inorganic materials due to dynamic surface species and low material formation energy; most conventional methods of chemical manipulation targeted at the MHP QD surface will result in transformation or dissolution of the MHP crystal. In previous work, we have demonstrated record-efficiency QD solar cells (QDSCs) based on ligand-exchange procedures that electronically couple MHP QDs yet maintain their nanocrystalline size, which stabilizes the corner-sharing structure of the constituent PbI octahedra with optoelectronic properties optimal for solar energy conversion. In this work, we employ a variety of spectroscopic techniques to develop a molecular-level understanding of the MHP QD surface chemistry in this system. We individually target both the anionic (oleate) and cationic (oleylammonium) ligands. We find that atmospheric moisture aids the process by hydrolysis of methyl acetate to generate acetic acid and methanol. Acetic acid then replaces native oleate ligands to yield QD surface-bound acetate and free oleic acid. The native oleylammonium ligands remain throughout this film deposition process and are exchanged during a final treatment step employing smaller cations-namely, formamidinium. This final treatment has a narrow processing window; initial treatment at this stage leads to a more strongly coupled QD regime followed by transformation into a bulk MHP film after longer treatment. These insights provide chemical understanding to the deposition of high-quality, electronically coupled MHP QD films that maintain both quantum confinement and their crystalline phase and attain high photovoltaic performance.

We report a detailed study on APbX3 (A = formamidinium (FA+), Cs+; X = I–, Br–) perovskite quantum dots (PQDs) with combined A- and X-site alloying that exhibits both a wide bandgap and high open-circuit voltage (V oc) for the application of a potential top cell in tandem junction photovoltaic (PV) devices. The nanocrystal alloying affords control over the optical bandgap and is readily achieved by solution-phase cation and anion exchange between previously synthesized FAPbI3 and CsPbBr3 PQDs. Increasing only the Br– content of the PQDs widens the bandgap but results in shorter carrier lifetimes and associated V oc losses in devices. These deleterious effects can be mitigated by replacing Cs+ with FA+, resulting in wide-bandgap PQD absorbers with improved charge-carrier mobility and PVs with higher V oc. Although further device optimization is required, these results demonstrate the potential of FA1–x Cs x Pb­(I1–x Br x )3 PQDs for wide-bandgap perovskite PVs with high V oc.

Light-induced changes in photophysical and electronic properties in metal halide perovskites can affect their performance in photovoltaic devices, light-emitting diodes, and other applications. Here we reveal that light induces a slow, reversible enhancement in photoluminescence (PL) lifetime and intensity in films of perovskite-phase CsPbI3 nanocrystals. When films of CsPbI3 nanocrystals stored in air are photoexcited, their PL lifetime and intensity increase by as much as a factor of 5 over the course of 20–30 min. Several hours later, without additional light excitation, the initial PL lifetime and intensity return. Placing the films under vacuum or nitrogen for several minutes was also found to increase the PL lifetime and intensity. We propose a model of slow, humidity- and light-sensitive surface states in perovskite-phase CsPbI3 nanocrystals.

Color-changing materials have a variety of applications, ranging from smart windows to sensors. Here, we report deliquescent chromism of thin, color neutral films of nickel­(II) iodide (NiI2) that are less than 10 μm thick. This behavior does not occur in the bulk material. Dark brown thin films of crystalline NiI2 turn clear when exposed to humidity and can be switched back to the dark state when mildly heated (>35 °C). This optical transition between dark and clear states of an NiI2 thin film is reversible with thermal cycling.