Hybrid lead halide perovskites exhibit carrier properties that resemble those of pristine nonpolar semiconductors despite static and dynamic disorder, but how carriers are protected from efficient scattering with charged defects and optical phonons is unknown. Here, we reveal the carrier protection mechanism by comparing three single-crystal lead bromide perovskites: CH ₃ NH ₃ PbBr ₃ , CH(NH ₂ ) ₂ PbBr ₃ , and CsPbBr ₃ . We observed hot fluorescence emission from energetic carriers with ~10 ² -picosecond lifetimes in CH ₃ NH ₃ PbBr ₃ or CH(NH ₂ ) ₂ PbBr ₃ , but not in CsPbBr ₃ . The hot fluorescence is correlated with liquid-like molecular reorientational motions, suggesting that dynamic screening protects energetic carriers via solvation or large polaron formation on time scales competitive with that of ultrafast cooling. Similar protections likely exist for band-edge carriers. The long-lived energetic carriers may enable hot-carrier solar cells with efficiencies exceeding the Shockley-Queisser limit.

Lead halide perovskites show marked defect tolerance responsible for their excellent optoelectronic properties. These properties might be explained by the formation of large polarons, but how they are formed and whether organic cations are essential remain open questions. We provide a direct time domain view of large polaron formation in single-crystal lead bromide perovskites CH₃NH₃PbBr₃ and CsPbBr₃. We found that large polaron forms predominantly from the deformation of the PbBr₃⁻ frameworks, irrespective of the cation type. The difference lies in the polaron formation time, which, in CH₃NH₃PbBr₃ (0.3 ps), is less than half of that in CsPbBr₃ (0.7 ps). First-principles calculations confirm large polaron formation, identify the Pb-Br-Pb deformation modes as responsible, and explain quantitatively the rate difference between CH₃NH₃PbBr₃ and CsPbBr₃. The findings reveal the general advantage of the soft [PbX₃]⁻ sublattice in charge carrier protection and suggest that there is likely no mechanistic limitations in using all-inorganic or mixed-cation lead halide perovskites to overcome instability problems and to tune the balance between charge carrier protection and mobility.

Despite numerous organic semiconducting materials synthesized for organic photovoltaics in the past decade, fullerenes are widely used as electron acceptors in highly efficient bulk-heterojunction solar cells. None of the non-fullerene bulk heterojunction solar cells have achieved efficiencies as high as fullerene-based solar cells. Design principles for fullerene-free acceptors remain unclear in the field. Here we report examples of helical molecular semiconductors as electron acceptors that are on par with fullerene derivatives in efficient solar cells. We achieved an 8.3% power conversion efficiency in a solar cell, which is a record high for non-fullerene bulk heterojunctions. Femtosecond transient absorption spectroscopy revealed both electron and hole transfer processes at the donor−acceptor interfaces. Atomic force microscopy reveals a mesh-like network of acceptors with pores that are tens of nanometres in diameter for efficient exciton separation and charge transport. This study describes a new motif for designing highly efficient acceptors for organic solar cells.

Larger monolayers with gold tapes: Although the exfoliation of monolayers of materials such as transition metal dichalcogenides produces high-quality electronic materials with low defect densities, the size of the monolayers is limited to the micrometer scale. Liu et al. modified this method by creating atomically flat gold layers on polymer supports. The strong van der Waals adhesion of the gold layer allowed monolayers to be exfoliated on the centimeter scale. Multilayers could be reassembled to artificial structures, such as a MoSe ₂ /WSe ₂ single-crystal bilayer with a twist angle chosen to quench intralayer exciton formation. Science , this issue p. 903

Efficient dynamic screening is key to the success of lead halide perovskites in solar cells and light emitting devices.