All-inorganic cesium lead iodide (CsPbI3) perovskite has improved thermal stability over the organic–inorganic hybrid perovskites and a suitable bandgap for optoelectronic and photovoltaic applications, but it is thermodynamically unstable at room temperature and has multiple structural polymorphs. Here, we show that the use of long-chain ammonium additives during thin film deposition as surface capping ligands results in the stabilization of metastable bulk CsPbI3 perovskite phases without alloying mixed cations or anions into the perovskite lattice. Moreover, two different metastable CsPbI3 perovskite polymorphs in the cubic (α-CsPbI3) and the much less common orthorhombic (β-CsPbI3) structures can be directly synthesized in a one-step spin coating film deposition by using oleylammonium or phenylethyl­ammonium additives, respectively, and both phases are stable at room temperature for months. Time-resolved photoluminescence and photoluminescence quenching experiments show that the photoexcited species in the stabilized orthorhombic CsPbI3 thin film are mainly free carriers under solar illumination with a carrier lifetime of ∼50 ns and carrier diffusion length on the order of ∼100 nm, which implies efficient carrier transport within the film despite the presence of surface ligands. Our results provide a new chemical strategy to synthesize metastable all-inorganic CsPbI3 perovskites, which, together with the good photophysical properties, will open them up for applications in photovoltaic and other optoelectronic devices.

Organic-inorganic lead iodide perovskites are efficient materials for photovoltaics and light-emitting diodes. We report carrier decay dynamics of nanorods of mixed cation formamidinium and methylammonium lead iodide perovskites [HC(NH)][CHNH]PbI (FAMAPbI) synthesized through a simple solution method. The structure and FA/MA composition ratio of the single-crystal FAMAPbI nanorods are fully characterized, which shows that the mixed cation FAMAPbI nanorods are stabilized in the perovskite structure. The photoluminescence (PL) emission from FAMAPbI nanorods continuously shifts from 821 to 782 nm as the MA ratio (x) increases from 0 to 1 and is shown to be inhomogeneously broadened. Time-resolved PL from individual FAMAPbI nanorods demonstrates that lifetimes of mixed cation FAMAPbI nanorods are longer than those of the pure FAPbI or MAPbI nanorods, and the FAMAPbI displays the longest average PL lifetime of about 2 μs. These results suggest that mixed cation FAMAPbI perovskites are promising for high-efficiency photovoltaics and other optoelectronic applications.

Hybrid organic–inorganic perovskites demonstrate desirable photophysical behaviors and promising applications from efficient photovoltaics to lasing, but the fundamental nature of excited state species is still under debate. We collected time-resolved photoluminescence of single-crystal nanoplates of methylammonium lead iodide perovskite (MAPbI3) with excitation over a range of fluences and repetition rates to provide a more complete photophysical picture. A fundamentally different way of simulating the photophysics is developed that relies on unnormalized decays, global analysis over a large array of conditions, and inclusion of steady-state behavior; these details are critical to capturing observed behaviors. These additional constraints require inclusion of spatially correlated pairs along with free carriers and traps, demonstrating the importance of our comprehensive analysis. Modeling geminate and nongeminate pathways shows that geminate processes are dominant at high carrier densities and early times and that geminate recombination is catalyzed by free holes. Our combination of data and simulation provides a detailed picture of perovskite photophysics across multiple excitation regimes that was not previously available.

Intrinsically disordered proteins, such as tau protein, adopt a variety of conformations in solution, complicating solution-phase structural studies. We employ an anti-Brownian electrokinetic (ABEL) trap to prolong measurements of single tau proteins in solution. Once trapped, we record the fluorescence anisotropy to investigate the diversity of conformations sampled by the single molecules. A distribution of anisotropy values obtained from trapped tau protein is conspicuously bimodal while those obtained by trapping a globular protein or individual fluorophores are not. Time-resolved fluorescence anisotropy measurements are used to provide an explanation of the bimodal distribution as originating from a shift in the compaction of the two different families of conformations.

Time-resolved fluorescence anisotropy reveals conformational variation of single tau proteins in solution.