Colloidal nanocrystals (NCs) of lead halide perovskites are considered highly promising materials that combine the exceptional optoelectronic properties of lead halide perovskites with tunability from quantum confinement. But can we assume that these materials are in the strong confinement regime? Here, we report an ultrafast transient absorption study of cubic CsPbBr 3 NCs as a function of size, compared with the bulk material. For NCs above ∼7 nm edge length, spectral signatures are similar to the bulk material−characterized by state-filling with uncorrelated charges−but discrete new kinetic components emerge at high fluence due to bimolecular recombination occurring in a discrete volume. Only for the smallest NCs (∼4 nm edge length) are strong quantum confinement effects manifest in TA spectral dynamics; focusing toward discrete energy states, enhanced bandgap renormalization energy, and departure from a Boltzmann statistical carrier cooling. At high fluence, we find that a hot-phonon bottleneck effect slows carrier cooling, but this appears to be intrinsic to the material, rather than size dependent. Overall, we find that the smallest NCs are understood in the framework of quantum confinement, however for the widely used NCs with edge lengths >7 nm the photophysics of bulk lead halide perovskites are a better point of reference.

Five polymer donors with distinct chemical structures and different electronic properties are surveyed in a planar and narrow-bandgap fused-ring electron acceptor (IDIC)-based organic solar cells, which exhibit power conversion efficiencies of up to 11%.

Modest exciton diffusion lengths dictate the need for nanostructured bulk heterojunctions in organic photovoltaic (OPV) cells; however, this morphology compromises charge collection. Here, we reveal rapid exciton diffusion in films of a fused-ring electron acceptor that, when blended with a donor, already outperforms fullerene-based OPV cells. Temperature-dependent ultrafast exciton annihilation measurements are used to resolve a quasi-activationless exciton diffusion coefficient of at least 2 × 10–2 cm2/s, substantially exceeding typical organic semiconductors and consistent with the 20–50 nm domain sizes in optimized blends. Enhanced three-dimensional diffusion is shown to arise from molecular and packing factors; the rigid planar molecular structure is associated with low reorganization energy, good transition dipole moment alignment, high chromophore density, and low disorder, all enhancing long-range resonant energy transfer. Relieving exciton diffusion constraints has important implications for OPVs; large, ordered, and pure domains enhance charge separation and transport, and suppress recombination, thereby boosting fill factors. Further enhancements to diffusion lengths may even obviate the need for the bulk heterojunction morphology.