Anharmonic crystal lattice dynamics have been observed in lead halide perovskites on picosecond timescales. Here, we report that the soft nature of the perovskite crystal lattice gives rise to dynamic fluctuations in the electronic properties of excited states. We use linear polarization selective transient absorption spectroscopy to study the charge carrier relaxation dynamics in lead-halide perovskite films and nanocrystals. We find that photo-excited charge carriers maintain an initial polarization anisotropy for several picoseconds, independent of crystallite size and composition, and well beyond the reported timescales of carrier scattering. First-principles calculations find intrinsic anisotropies in the transition dipole moment, which depend on the orientation of light polarization and the polar distortion of the local crystal lattice. Lattice dynamics are imprinted in the optical transitions and anisotropies arise on the time-scales of structural motion. The strong coupling between electronic states and structural dynamics requires a unique interpretation of recombination and transport mechanisms.

A tandem structure was invented to enhance photon utilization efficiency and reduce thermal loss. Considering the unique advantages of non-fullerene acceptors (NFAs), the combination of NFAs and the tandem concept shows great potential for organic photovoltaics (OPVs). Herein, a simple strategy to balance the voltage-current trade-off in tandem OPVs by employing mixed NFAs in rear sub-cells is reported. The V OC and J SC of rear sub-cells can be tuned by using the blend of donor/NFA/NFA. This strategy offers an easy approach to balance the V OC and J SC in tandem OPVs toward higher power conversion efficiency (PCE) without rational and careful synthesis of new materials. Tandem OPVs based on mixed NFAs in rear sub-cells exhibited the best PCE of 13.3% in the lab. Importantly, the tandem devices were certified by the National Renewable Energy Laboratory (NREL) under asymptotic scans, and a PCE of 11.52% was achieved and recognized on the most recent NREL chart. Organic photovoltaics (OPVs) have attracted considerable attention in the last decade due to some potential advantages, such as flexibility, light weight, possible semi-transparency, and fast large-area fabrication with low energy consumption. To achieve higher performing OPVs, a tandem structure was invented by stacking two or more sub-cells together. In tandem OPVs, the photon utilization efficiency can be improved and the thermal losses can be reduced. Herein, a simple strategy to balance the voltage-current trade-off in tandem OPVs by employing mixed non-fullerene acceptors in rear sub-cells is reported. By this strategy, tandem OPVs exhibited the best power conversion efficiency (PCE) of 13.3% in the lab. Importantly, the tandem devices were certified by the National Renewable Energy Laboratory (NREL) under the new protocols (asymptotic scans), and a PCE of 11.52% was achieved and recognized on the most recent NREL chart. A simple strategy to balance the voltage-current trade-off in tandem organic photovoltaics by employing mixed non-fullerene acceptors in rear sub-cells is reported. The tandem device with the best power conversion efficiency (PCE) of 13.3% was achieved in the lab. Importantly, the tandem devices were certified by the National Renewable Energy Laboratory (NREL) under the new protocols (asymptotic scans, which are much tougher than regular fast scans); a PCE of 11.52% was achieved and recognized on the most recent NREL chart.