Excitonic and Polaronic Properties of 2D Hybrid Organic–Inorganic Perovskites

We theoretically characterize the unusual white-light emission properties of two-dimensional (2D) hybrid organic–inorganic perovskites with an APbX₄ structure (where A is a bidentate organic cation and X = Cl, Br). In addition to band structure calculations including corrections due to spin–orbit couplings and electron–hole interactions, a computationally intensive molecular cluster approach is exploited to describe the excitonic and polaronic properties of these 2D perovskites at the atomistic level. Upon adding or removing an electron from the neutral systems, we find that strongly localized small polarons form in the 2D clusters. The polaron charge density is distributed over just ∼1.5 lattice sites, which is consistent with the calculated large polaron binding energies, on the order of ∼0.4–1.2 eV

Interfacial Charge Transfer Anisotropy in Polycrystalline Lead Iodide Perovskite Films

Solar cells based on organic–inorganic lead iodide perovskite (CH₃NH₃PbI₃) exhibit remarkably high power conversion efficiency (PCE). One of the key issues in solution-processed films is that often the polycrystalline domain orientation is not well-defined, which makes it difficult to predict energy alignment and charge transfer efficiency. Here we combine ab initio calculations and photoelectron spectroscopy to unravel the electronic structure and charge redistribution at the interface between different surfaces of CH₃NH₃PbI₃ and typical organic hole acceptor Spiro-OMeTAD and electron acceptor PCBM. We find that both hole and electron interfacial transfer depend strongly on the CH₃NH₃PbI₃ surface orientation: while the (001) and (110) surfaces tend to favor hole injection to Spiro-OMeTAD, the (100) surface facilitates electron transfer to PCBM due to surface delocalized charges and hole/electron accumulation at the CH₃NH₃PbI₃/organic interfaces. Molecular dynamic simulations indicate that this is due to strong orbital interactions under thermal fluctuations at room temperature, suggesting the possibility to further improve charge separation and extraction in perovskite-based solar cells by controlling perovskite film crystallization and surface orientation

Ambipolar Charge Photogeneration and Transfer at GaAs/P3HT Heterointerfaces

Recent work on hybrid photovoltaic systems based on conjugated polymers and III–V compound semiconductors with relatively high power conversion efficiency revived fundamental questions regarding the nature of charge separation and transfer at the interface between organic and inorganic semiconductors with different degrees of delocalization. In this work, we studied photoinduced charge generation and interfacial transfer dynamics in a prototypical photovoltaic <i>n-</i>type GaAs (111)B and poly­(3-hexyl-thiophene) (P3HT) bilayer system. Ultrafast spectroscopy and density functional theory calculations indicate the coexistence of electron and hole transfer at the GaAs/P3HT interface, leading to the generation of long-lived species and photoinduced absorption upon creation of hybrid interfacial states. This opens up new avenues for the use of low-dimensional III–V compounds (e.g., nanowires or quantum dots) in hybrid organic/inorganic photovoltaics, where advanced bandgap and density of states engineering may also be exploited as design parameters

Plasmonic Nanoclocks

Plasmonic spectra of “nanoclock” metamaterials can be topologically mapped on a torus. We manufactured arrays of such a metamaterial with different “time” shown on the clocks and demonstrated that the near-infrared spectra of the nanostructures can be predictably tuned exhibiting a rich series of high-order plasmon modes, from the electric dipole to exotic electric triakontadipole that could be engaged in chemo/biosensor applications

Unique Reversible Crystal-to-Crystal Phase TransitionStructural and Functional Properties of Fused Ladder Thienoarenes

Donor–acceptor type molecules based on fused ladder thienoarenes, indacenodithiophene (IDT), and dithienocyclopenta–thienothiophene (DTCTT), coupled with benzothiadiazole, are prepared, and their solid-state structures are investigated. They display a rich variety of solid phases ranging from amorphous glass states to crystalline states, upon changes in the central aromatic core and side group structures. Most notably, the DTCTT-based derivatives showed reversible crystal-to-crystal phase transitions in heating and cooling cycles. Unlike what has been seen in π-conjugated molecules, variable temperature XRD revealed that structural change occurs continuously during the transition. A columnar self-assembled structure with slip-stacked π–π interaction is proposed to be involved in the solid state. This research provides the evidence of unique structural behavior of the DTCTT-based molecules through the detailed structural analysis. This unique structural transition paves the way for these materials to have self-healing of crystal defects, leading to improved optoelectronic properties

Broadband Emission in Two-Dimensional Hybrid Perovskites: The Role of Structural Deformation

Only a selected group of two-dimensional (2D) lead–halide perovskites shows a peculiar broad-band photoluminescence. Here we show that the structural distortions of the perovskite lattice can determine the defectivity of the material by modulating the defect formation energies. By selecting and comparing two archetype systems, namely, (NBT)₂PbI₄ and (EDBE)­PbI₄ perovskites (NBT = <i>n</i>-butylammonium and EDBE = 2,2-(ethylenedioxy)­bis­(ethylammonium)), we find that only the latter, subject to larger deformation of the Pb–X bond length and X–Pb–X bond angles, sees the formation of V_F color centers whose radiative decay ultimately leads to broadened PL. These findings highlight the importance of structural engineering to control the optoelectronic properties of this class of soft materials

Label-Free Vapor Selectivity in Poly(<i>p</i>‑Phenylene Oxide) Photonic Crystal Sensors

The lack of sensors for low cost, extensive, and continuous detection of vapor pollutants is a serious concern for health and safety in industrialized urban areas. Colorimetric sensors, such as distributed Bragg reflectors made of polymers, could achieve this task thanks to their low cost and easy signal transduction but are typically affected by low vapor permeability and lack of selectivity without chemical labeling. Here we demonstrate all-polymer Bragg multilayers for label-free selective detection of organic volatile compounds. The system exploits the ability of amorphous poly­(<i>p</i>-phenylene oxide), PPO, to uptake large amount of guest molecules and to form cocrystalline phases with distinct optical properties. Bragg stacks embedding PPO active layers show selective colorimetric response to vapors of carbon tetrachloride and aromatic homologues, which can be revealed by the naked eye

Resonant Enhancement of Polymer–Cell Optostimulation by a Plasmonic Metasurface

Organic semiconductors have shown great potential as efficient bioelectronic materials. Specifically, photovoltaic polymers such as the workhorse poly(thiophene) derivatives, when stimulated with visible light, can depolarize neurons and generate action potentials, an effect that has been also employed for rescuing vision in blind rats. In this context, however, the coupling of such materials with optically resonant structures to enhance those photodriven biological effects is still in its infancy. Here, we employ the optical coupling between a nanostructured metasurface and poly(3-hexylthiophene) (P3HT) to improve the bioelectronic effects occurring upon photostimulation at the abiotic–biotic interface. In particular, we designed a spectrally tuned aluminum metasurface that can resonate with P3HT, hence augmenting the effective field experienced by the polymer. In turn, this leads to an 8-fold increase in invoked inward current in cells. This enhanced activation strategy could be useful to increase the effectiveness of P3HT-based prosthetic implants for degenerative retinal disorders

Lead-Free MA₂CuCl_<i>x</i>Br_4–<i>x</i> Hybrid Perovskites

Despite their extremely good performance in solar cells with efficiencies approaching 20% and the emerging application for light-emitting devices, organic–inorganic lead halide perovskites suffer from high content of toxic, polluting, and bioaccumulative Pb, which may eventually hamper their commercialization. Here, we present the synthesis of two-dimensional (2D) Cu-based hybrid perovskites and study their optoelectronic properties to investigate their potential application in solar cells and light-emitting devices, providing a new environmental-friendly alternative to Pb. The series (CH₃NH₃)₂­CuCl_<i>x</i>Br_4–<i>x</i> was studied in detail, with the role of Cl found to be essential for stabilization. By exploiting the additional Cu d–d transitions and appropriately tuning the Br/Cl ratio, which affects ligand-to-metal charge transfer transitions, the optical absorption in this series of compounds can be extended to the near-infrared for optimal spectral overlap with the solar irradiance. In situ formation of Cu⁺ ions was found to be responsible for the green photoluminescence of this material set. Processing conditions for integrating Cu-based perovskites into photovoltaic device architectures, as well as the factors currently limiting photovoltaic performance, are discussed: among them, we identified the combination of low absorption coefficient and heavy mass of the holes as main limitations for the solar cell efficiency. To the best of our knowledge, this is the first demonstration of the potential of 2D copper perovskite as light harvesters and lays the foundation for further development of perovskite based on transition metals as alternative lead-free materials. Appropriate molecular design will be necessary to improve the material’s properties and solar cell performance filling the gap with the state-of-the-art Pb-based perovskite devices

High Density Individually Addressable Nanowire Arrays Record Intracellular Activity from Primary Rodent and Human Stem Cell Derived Neurons

We report a new hybrid integration scheme that offers for the first time a nanowire-on-lead approach, which enables independent electrical addressability, is scalable, and has superior spatial resolution in vertical nanowire arrays. The fabrication of these nanowire arrays is demonstrated to be scalable down to submicrometer site-to-site spacing and can be combined with standard integrated circuit fabrication technologies. We utilize these arrays to perform electrophysiological recordings from mouse and rat primary neurons and human induced pluripotent stem cell (hiPSC)-derived neurons, which revealed high signal-to-noise ratios and sensitivity to subthreshold postsynaptic potentials (PSPs). We measured electrical activity from rodent neurons from 8 days in vitro (DIV) to 14 DIV and from hiPSC-derived neurons at 6 weeks in vitro post culture with signal amplitudes up to 99 mV. Overall, our platform paves the way for longitudinal electrophysiological experiments on synaptic activity in human iPSC based disease models of neuronal networks, critical for understanding the mechanisms of neurological diseases and for developing drugs to treat them