In a photovoltaic perovskite, entropy-driven structural transition enables kinetic trapping of a desired photovoltaic phase.

Long carrier lifetime is what makes hybrid organic-inorganic perovskites high-performance photovoltaic materials. Several microscopic mechanisms behind the unusually long carrier lifetime have been proposed, such as formation of large polarons, Rashba effect, ferroelectric domains, and photon recycling. Here, we show that the screening of band-edge charge carriers by rotation of organic cation molecules can be a major contribution to the prolonged carrier lifetime. Our results reveal that the band-edge carrier lifetime increases when the system enters from a phase with lower rotational entropy to another phase with higher entropy. These results imply that the recombination of the photoexcited electrons and holes is suppressed by the screening, leading to the formation of polarons and thereby extending the lifetime. Thus, searching for organic-inorganic perovskites with high rotational entropy over a wide range of temperature may be a key to achieve superior solar cell performance.organic-inorganic hybrid perovskite | carrier lifetime | photoluminescence | polaron T he record efficiency of hybrid organic-inorganic perovskite (HOIP)-based solar cells has reached above 22% (1-4), which is comparable to that of silicon solar cells. The most dominant contribution to the high photovoltaic performance of HOIPs comes from their long carrier lifetimes (≥ 1 μs), which translates to large carrier diffusion lengths despite their modest charge mobilities (5). Several microscopic mechanisms behind the unusually long carrier lifetime have been proposed, such as formation of ferroelectric domains (6-9), Rashba effect (10-12), photon recycling (13), and large polarons (14-16). When the HOIPs are replaced with all inorganic perovskites in the solar cell architecture, the device can still function as a solar cell. This indicates that the photons excite electrons and holes out of the inorganic metal halide atoms, which is consistent with the density functional theory (DFT) calculations that the corner interstitial cations, whether organic or inorganic, do not directly contribute to the band-edge states (17). However, the efficiency of the purely inorganic perovskites is currently at ∼11% (18-20), which is far below 22% of HOIP-based solar cells. This suggests that the presence of organic cation may be the key for achieving high solar cell efficiency. It is, however, yet to be understood how the organic cations enhance the efficiency.Among the aforementioned microscopic mechanisms, three are based on the role of organic cations. First, in the ferroelectric domain theory, nanoscale ferroelectric domains are formed due to alignment of organic cations (6-9). Such domains can spatially separate the photoexcited electron and holes and thereby reduce their recombination. Second, in the Rashba effect theory (10-12), the spin and orbit degrees of freedom of the inorganic atoms are coupled with the electric field generated by the organic cations. This results in the electronic band splitting for different spins and leads to an effectively in...

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The Spallation Neutron Source at Oak Ridge National Laboratory, USA, ushered in a new era of neutron scattering experiments through the use of event‐based data. Tagging each neutron event allows pump–probe experiments, measurements with a parameter asynchronous to the source, measurements with continuously varying parameters and novel ways of testing instrument components. This contribution will focus on a few examples. A pulsed magnet has been used to study diffraction under extreme fields. Continuous ramping of temperature is becoming standard on the POWGEN diffractometer. Battery degradation and phase transformations under heat and stress are often studied on the VULCAN diffractometer. Supercooled Al2O3 was studied on NOMAD. A study of a metallic glass through its glass transition was performed on the ARCS spectrometer, and the effect of source variation on chopper stability was studied for the SEQUOIA spectrometer. Besides a summary of these examples, an overview is provided of the hardware and software advances to enable these and many other event‐based measurements.