Summary: Photoinduced electron transfer (PET) is a phenomenon wherein the absorption of light by a chemical species provides an energetic driving force for an electron transfer reaction. 1 – 4 This mechanism is relevant in many areas of chemistry, including the study of natural and artificial photosynthesis, photovoltaics, and photosensitive materials. In recent years, research in the area of photoredox catalysis has leveraged PET for the catalytic generation of both neutral and charged organic free radical species. These technologies have enabled a wide range of previously inaccessible chemical transformations and have seen widespread utilization in both academic and industrial settings. These reactions are often catalyzed by visible-light absorbing organic molecules or transition-metal complexes of ruthenium, iridium, chromium, or copper. 5 , 6 While a wide variety of closed shell organic molecules have been shown to behave as competent electron transfer catalysts in photoredox reactions, there are only limited reports of PET reactions involving neutral organic radicals as an excited state donor or acceptor. This is perhaps somewhat unsurprising in light of previously reported doublet excited state lifetimes for neutral organic radicals, which are typically several orders of magnitude shorter than singlet lifetimes for known transition metal photoredox catalysts. 7 – 11 Herein we document the discovery, characterization, and reactivity of a neutral acridine radical with a maximum excited state oxidation potential of −3.36 V vs. SCE: significantly more reducing than elemental lithium and marking it as one of the most potent chemical reductants reported. 12 Spectroscopic, computational, and chemical studies indicate that the formation of a twisted intramolecular charge transfer species enables the population of higher energy doublet excited states, leading to the observed potent photoreductant behavior. We demonstrate that this catalytically-generated PET catalyst facilitates several chemical reactions that typically require alkali metal reductants and bodes well for the adoption of this system in additional organic transformations requiring dissolving metal reductants.

Layered perovskites have been shown to improve the stability of perovskite solar cells while its operation mechanism remains unclear. Here we investigate the process for the conversion of light to electrical current in high performance layered perovskite solar cells by examining its real morphology. The layered perovskite films in this study are found to be a mixture of layered and three dimensional (3D)-like phases with phase separations at micrometer and nanometer scale in both vertical and lateral directions. This phase separation is explained by the surface initiated crystallization process and the competition of the crystallization between 3D-like and layered perovskites. We further propose that the working mechanisms of the layered perovskite solar cells involve energy transfer from layered to 3D-like perovskite network. The impact of morphology on efficiency and stability of the hot-cast layered perovskite solar cells are also discussed to provide guidelines for the future improvement.

The efficiencies of green and red perovskite light-emitting diodes (PeLEDs) have been increased close to their theoretical upper limit, while the efficiency of blue PeLEDs is lagging far behind. Here we report enhancing the efficiency of sky-blue PeLEDs by overcoming a major hurdle of low photoluminescence quantum efficiency in wide-bandgap perovskites. Blending phenylethylammonium chloride into cesium lead halide perovskites yields a mixture of two-dimensional and three-dimensional perovskites, which enhances photoluminescence quantum efficiency from 1.1% to 19.8%. Adding yttrium (III) chloride into the mixture further enhances photoluminescence quantum efficiency to 49.7%. Yttrium is found to incorporate into the three-dimensional perovskite grain, while it is still rich at grain boundaries and surfaces. The yttrium on grain surface increases the bandgap of grain shell, which confines the charge carriers inside grains for efficient radiative recombination. Record efficiencies of 11.0% and 4.8% were obtained in sky-blue and blue PeLEDs, respectively.

Two-dimensional perovskites have emerged as more intrinsically stable materials for solar cells. Chemical tuning of spacer organic cations has attracted great interest due to their additional functionalities. However, how the chemical nature of the organic cations affects the properties of two-dimensional perovskites and devices is rarely reported. Here we demonstrate that the selection of spacer cations (i.e., selective fluorination of phenethylammonium) affects the film properties of two-dimensional perovskites, leading to different device performance of two-dimensional perovskite solar cells (average n = 4). Structural analysis reveals that different packing arrangements and orientational disorder of the spacer cations result in orientational degeneracy and different formation energies, largely explaining the difference in film properties. This work provides key missing information on how spacer cations exert influence on desirable electronic properties and device performance of two-dimensional perovskites via the weak and cooperative interactions of these cations in the crystal lattice.

I. Technical aspects of transient grating experimentsFemtosecond spectroscopy experiments are based on a Quantronix Q-lite seeded Integra C Titanium Sapphire amplifier generating 130 fs, 800 nm, 2.0 mJ laser pulses at 1 kHz. The laser system pumps two home-built noncollinear optical parametric amplifiers (NOPA). 1,2 The NOPA used for pump pulse generation has a spectral bandwidth corresponding to transform limited 15 fs laser pulses, whereas the NOPA from which probe pulses are derived generates spectra spanning the full 500-750 nm wavelength range. Portions of the probe spectrum are filtered in a fused silica prism compressor for use in experiments. Pump and probe pulses are compressed to 15-20 fs with a time-bandwidth product of 0.5-0.6 where residual third-order dispersion prevents compression to the Fourier transform limit.Transient grating (TG) experiments are performed with the interferometer shown in Figure S1. The interferometer generates a trapezoidal laser beam geometry with diffractive-optics (DO) for passively phase-stabilized interferometric signal detection. [3][4][5][6][7][8][9] The TG interferometer uses a DO (Holoeye) producing an angle of 4.65 degrees between the +/-1 diffraction orders at 590 nm. The pump (pulses 1 and 2) and probe (pulses 3) beams are crossed at angle of approximately 4.6 degrees in the DO. Pulses 1 and 2 arrive at the sample at the same time, and are delayed with respect to pulses 3 and 4 with a motorized translation stage. The signal is phase-matched so that it is automatically collinear with the reference pulse after the sample.