Protecting hot carriers by tuning hybrid perovskite structures with alkali cations.

Successful implementation of hot carrier solar cells requires preserving high carrier temperature as carriers migrate through the active layer. Here, we demonstrated that addition of alkali cations in hybrid organic-inorganic lead halide perovskites led to substantially elevated carrier temperature, reduced threshold for phonon bottleneck, and enhanced hot carrier transport. The synergetic effects from the Rb, Cs, and K cations result in ~900 K increase in the effective carrier temperature at a carrier density around 1018 cm-3 with an excitation 1.45 eV above the bandgap. In the doped thin films, the protected hot carriers migrate 100 s of nanometers longer than the undoped sample as imaged by ultrafast microscopy. We attributed these improvements to the relaxation of lattice strain and passivation of halide vacancies by alkali cations based on x-ray structural characterizations and first principles calculations.

Neuron Activity Dependent Redox Compartmentation Revealed with a Second Generation Red-Shifted Ratiometric Sensor.

Oxidative stress is a hallmark of several aging and trauma related neurological disorders, but the precise details of how altered neuronal activity elicits subcellular redox changes have remained difficult to resolve. Current redox sensitive dyes and fluorescent proteins can quantify spatially distinct changes in reactive oxygen species levels, but multicolor probes are needed to accurately analyze compartment-specific redox dynamics in single cells that can be masked by population averaging. We previously engineered genetically encoded red-shifted redox-sensitive fluorescent protein sensors using a Förster resonance energy transfer relay strategy. Here, we developed a second-generation excitation ratiometric sensor called rogRFP2 with improved red emission for quantitative live-cell imaging. Using this sensor to measure activity-dependent redox changes in individual cultured neurons, we observed an anticorrelation in which mitochondrial oxidation was accompanied by a concurrent reduction in the cytosol. This behavior was dependent on the activity of Complex I of the mitochondrial electron transport chain and could be modulated by the presence of cocultured astrocytes. We also demonstrated that the red fluorescent rogRFP2 facilitates ratiometric one- and two-photon redox imaging in rat brain slices and Drosophila retinas. Overall, the proof-of-concept studies reported here demonstrate that this new rogRFP2 redox sensor can be a powerful tool for understanding redox biology both in vitro and in vivo across model organisms.

Long-lived charge separation in two-dimensional ligand-perovskite heterostructures.

Rational design of heterojunctions using nanostructured materials is a useful strategy for achieving efficient interfacial charge separation in photovoltaics. Heterojunctions can be constructed between the organic ligands and the inorganic layers in two-dimensional perovskites, taking advantage of their highly programmable structures. Here, we investigate charge transfer and recombination at the interface between the thiophene-based semiconducting ligands and the lead halide inorganic sublattices using time-resolved photoluminescence and transient reflection spectroscopy in single two-dimensional perovskite crystals. These measurements demonstrate the charge transfer time around 10 ps and long-lived charge-separated state over the nanosecond time scale in two-dimensional ligand-perovskite heterostructures. The efficient charge transfer processes coupled with slow charge recombination suggest the potential for improving exciton dissociation and charge transport in two-dimensional perovskite solar cells.

Molecular engineering of organic-inorganic hybrid perovskites quantum wells.

Semiconductor quantum-well structures and superlattices are key building blocks in modern optoelectronics, but it is difficult to simultaneously realize defect-free epitaxial growth while fine tuning the chemical composition, layer thickness and band structure of each layer to achieve the desired performance. Here we demonstrate the modulation of the electronic structure-and consequently the optical properties-of organic semiconducting building blocks that are incorporated between the layers of perovskites through a facile solution processing step. Self-aggregation of the conjugated organic molecules is suppressed by functionalization with sterically demanding groups and single crystalline organic-perovskite hybrid quantum wells (down to one-unit-cell thick) are obtained. The energy and charge transfers between adjacent organic and inorganic layers are shown to be fast and efficient, owing to the atomically flat interface and ultrasmall interlayer distance of the perovskite materials. The resulting two-dimensional hybrid perovskites are very stable due to protection given by the bulky hydrophobic organic groups.

Ultrafast Dynamic Microscopy of Carrier and Exciton Transport.

We highlight the recent progress in ultrafast dynamic microscopy that combines ultrafast optical spectroscopy with microscopy approaches, focusing on the application transient absorption microscopy (TAM) to directly image energy and charge transport in solar energy harvesting and conversion systems. We discuss the principles, instrumentation, and resolutions of TAM. The simultaneous spatial, temporal, and excited-state-specific resolutions of TAM unraveled exciton and charge transport mechanisms that were previously obscured in conventional ultrafast spectroscopy measurements for systems such as organic solar cells, hybrid perovskite thin films, and molecular aggregates. We also discuss future directions to improve resolutions and to develop other ultrafast imaging contrasts beyond transient absorption.

Homogenized halides and alkali cation segregation in alloyed organic-inorganic perovskites.

The role of the alkali metal cations in halide perovskite solar cells is not well understood. Using synchrotron-based nano-x-ray fluorescence and complementary measurements, we found that the halide distribution becomes homogenized upon addition of cesium iodide, either alone or with rubidium iodide, for substoichiometric, stoichiometric, and overstoichiometric preparations, where the lead halide is varied with respect to organic halide precursors. Halide homogenization coincides with long-lived charge carrier decays, spatially homogeneous carrier dynamics (as visualized by ultrafast microscopy), and improved photovoltaic device performance. We found that rubidium and potassium phase-segregate in highly concentrated clusters. Alkali metals are beneficial at low concentrations, where they homogenize the halide distribution, but at higher concentrations, they form recombination-active second-phase clusters.

Tuning the interfacial and energetic interactions between a photoexcited conjugated polymer and open-shell small molecules.

Design rules and application spaces for closed-shell conjugated polymers have been well established in the field of organic electronics, but the emerging class of open-shell stable radicals has not been evaluated in such detail. Thus, establishing the underlying physical phenomena associated with the interactions between both classes of molecules is imperative for the effective utilization of these soft materials. Here, we establish that Förster Resonance Energy Transfer (FRET) is the dominant mechanism by which energy transfer occurs from a common conjugated polymer to various radical species using a combination of experimental and computational approaches. Specifically, we determined this fact by monitoring the fluorescence quenching of poly(3-hexylthiophene) (P3HT) in the presence of three radical species: (1) the galvinoxyl; (2) the 2-phenyl-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (PTIO); and (3) the 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radicals. Both in solution and in the solid-state, the galvinoxyl and PTIO radicals showed quenching that was on par with that of a common fullerene electron-accepting derivative, due to the considerable overlap of their absorbance spectrum with the fluorescence spectrum of the P3HT species, which indicated that isoenergetic electronic transitions existed for both species. Conversely, TEMPO showed minimal quenching at similar concentrations due to the lack of such an overlap. Furthermore, computational studies demonstrated that FRET would occur at a significantly faster rate than other competing processes. These findings suggest that long-range energy transfer can be accomplished in applications when radicals that can act as FRET acceptors are utilized, forming a new design paradigm for future applications involving both closed- and open-shell soft materials.

Extending roGFP Emission via Förster-Type Resonance Energy Transfer Relay Enables Simultaneous Dual Compartment Ratiometric Redox Imaging in Live Cells.

Reactive oxygen species (ROS) mediate both intercellular and intraorganellar signaling, and ROS propagate oxidative stress between cellular compartments such as mitochondria and the cytosol. Each cellular compartment contains its own sources of ROS as well as antioxidant mechanisms, which contribute to dynamic fluctuations in ROS levels that occur during signaling, metabolism, and stress. However, the coupling of redox dynamics between cellular compartments has not been well studied because of the lack of available sensors to simultaneously measure more than one subcellular compartment in the same cell. Currently, the redox-sensitive green fluorescent protein, roGFP, has been used extensively to study compartment-specific redox dynamics because it provides a quantitative ratiometric readout and it is amenable to subcellular targeting as a genetically encoded sensor. Here, we report a new family of genetically encoded fluorescent protein sensors that extend the fluorescence emission of roGFP via Förster-type resonance energy transfer to an acceptor red fluorescent protein for dual-color live-cell microscopy. We characterize the redox and optical properties of the sensor proteins, and we demonstrate that they can be used to simultaneously measure cytosolic and mitochondrial ROS in living cells. Furthermore, we use these sensors to reveal cell-to-cell heterogeneity in redox coupling between the cytosol and mitochondria when neuroblastoma cells are exposed to reductive and metabolic stresses.

Long-range hot-carrier transport in hybrid perovskites visualized by ultrafast microscopy.

The Shockley-Queisser limit for solar cell efficiency can be overcome if hot carriers can be harvested before they thermalize. Recently, carrier cooling time up to 100 picoseconds was observed in hybrid perovskites, but it is unclear whether these long-lived hot carriers can migrate long distance for efficient collection. We report direct visualization of hot-carrier migration in methylammonium lead iodide (CH3NH3PbI3) thin films by ultrafast transient absorption microscopy, demonstrating three distinct transport regimes. Quasiballistic transport was observed to correlate with excess kinetic energy, resulting in up to 230 nanometers transport distance that could overcome grain boundaries. The nonequilibrium transport persisted over tens of picoseconds and ~600 nanometers before reaching the diffusive transport limit. These results suggest potential applications of hot-carrier devices based on hybrid perovskites.