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.

Singlet fission is a process whereby two triplet excitons can be produced from one photon, potentially increasing the efficiency of photovoltaic devices. Endothermic singlet fission is desired for a maximum energy-conversion efficiency, and such systems have been considered to form an excimer-like state with multiexcitonic character prior to the appearance of triplets. However, the role of the excimer as an intermediate has, until now, been unclear. Here we show, using 5,12-bis((triisopropylsilyl)ethynyl)tetracene in solution as a prototypical example, that, rather than acting as an intermediate, the excimer serves to trap excited states to the detriment of singlet-fission yield. We clearly demonstrate that singlet fission and its conjugate process, triplet-triplet annihilation, occur at a longer intermolecular distance than an excimer intermediate would impute. These results establish that an endothermic singlet-fission material must be designed to avoid excimer formation, thus allowing singlet fission to reach its full potential in enhancing photovoltaic energy conversion.

Low-energy photons, which are not used by a solar cell, can be converted to higher-energy photons by photon upconversion. The figure-of-merit of upconvertors, J UC , is given in mA cm −2 under 1 sun excitation conditions. A device-relevant J UC should not be less than 0.1 mA cm −2 . However, to date, the highest J UC reported is in the 10 −2 mA cm −2 range. In this Perspective, we analyze the shortcomings of previously reported devices and unfold a roadmap toward device-relevant, high-efficiency upconvertors.

Promising efficiencies and a high open-circuit voltage are demonstrated for a novel low band-gap all-polymer photovoltaic blend.