Perpendicular crossing chains enable high mobility in a noncrystalline conjugated polymer.

The nature of interchain π-system contacts, and their relationship to hole transport, are elucidated for the high-mobility, noncrystalline conjugated polymer C16-IDTBT by the application of scanning tunneling microscopy, molecular dynamics, and quantum chemical calculations. The microstructure is shown to favor an unusual packing motif in which paired chains cross-over one another at near-perpendicular angles. By linking to mesoscale microstructural features, revealed by coarse-grained molecular dynamics and previous studies, and performing simulations of charge transport, it is demonstrated that the high mobility of C16-IDTBT can be explained by the promotion of a highly interconnected transport network, stemming from the adoption of perpendicular contacts at the nanoscale, in combination with fast intrachain transport.

Synthesis of model heterojunction interfaces reveals molecular-configuration-dependent photoinduced charge transfer.

Control of the molecular configuration at the interface of an organic heterojunction is key to the development of efficient optoelectronic devices. Due to the difficulty in characterizing these buried and (probably) disordered heterointerfaces, the interfacial structure in most systems remains a mystery. Here we demonstrate a synthetic strategy to design and control model interfaces, enabling their detailed study in isolation from the bulk material.

Anisotropic Electron-Phonon Interactions in 2D Lead-Halide Perovskites.

Two-dimensional (2D) hybrid organic-inorganic metal halide perovskites offer enhanced stability for perovskite-based applications. Their crystal structure's soft and ionic nature gives rise to strong interaction between charge carriers and ionic rearrangements. Here, we investigate the interaction of photogenerated electrons and ionic polarizations in single-crystal 2D perovskite butylammonium lead iodide (BAPI), varying the inorganic lamellae thickness in the 2D single crystals.

Suppression of Dexter transfer by covalent encapsulation for efficient matrix-free narrowband deep blue hyperfluorescent OLEDs.

Hyperfluorescence shows great promise for the next generation of commercially feasible blue organic light-emitting diodes, for which eliminating the Dexter transfer to terminal emitter triplet states is key to efficiency and stability. Current devices rely on high-gap matrices to prevent Dexter transfer, which unfortunately leads to overly complex devices from a fabrication standpoint. Here we introduce a molecular design where ultranarrowband blue emitters are covalently encapsulated by insulating alkylene straps.

Structure-Property Relationships for the Electronic Applications of Bis-Adduct Isomers of Phenyl-C Butyric Acid Methyl Ester.

Higher adducts of a fullerene, such as the bis-adduct of PCBM (bis-PCBM), can be used to achieve shallower molecular orbital energy levels than, for example, PCBM or C Substituting the bis-adduct for the parent fullerene is useful to increase the open-circuit voltage of organic solar cells or achieve better energy alignment as electron transport layers in, for example, perovskite solar cells. However, bis-PCBM is usually synthesized as a mixture of structural isomers, which can lead to both energetic and morphological disorder, negatively affecting device performance. Here, we present a comprehensive study on the molecular properties of 19 pure bis-isomers of PCBM using a variety of characterization methods, including ultraviolet photoelectron spectroscopy, thermal gravimetric analysis, differential scanning calorimetry, single crystal structure, and (time-dependent) density functional theory calculation.

Controlling swelling in mixed transport polymers through alkyl side-chain physical cross-linking.

Semiconducting conjugated polymers bearing glycol side chains can simultaneously transport both electronic and ionic charges with high charge mobilities, making them ideal electrode materials for a range of bioelectronic devices. However, heavily glycolated conjugated polymer films have been observed to swell irreversibly when subjected to an electrochemical bias in an aqueous electrolyte. The excessive swelling can lead to the degradation of their microstructure, and subsequently reduced device performance.

Confinement and Exciton Binding Energy Effects on Hot Carrier Cooling in Lead Halide Perovskite Nanomaterials.

The relaxation of the above-gap ("hot") carriers in lead halide perovskites (LHPs) is important for applications in photovoltaics and offers insights into carrier-carrier and carrier-phonon interactions. However, the role of quantum confinement in the hot carrier dynamics of nanosystems is still disputed. Here, we devise a single approach, ultrafast pump-push-probe spectroscopy, to study carrier cooling in six different size-controlled LHP nanomaterials.

The Effect of Glycol Side Chains on the Assembly and Microstructure of Conjugated Polymers.

Conjugated polymers with glycol-based chains, are emerging as a material class with promising applications as organic mixed ionic-electronic conductors, particularly in bioelectronics and thermoelectrics. However, little is still known about their microstructure and the role of the side chains in determining intermolecular interactions and polymer packing. Here, we use the combination of electrospray deposition and scanning tunneling microscopy to determine the microstructure of prototypical glycolated conjugated polymers (pgBTTT and p(g2T-TT)) with submonomer resolution.

Impact of Side-Chain Hydrophilicity on Packing, Swelling, and Ion Interactions in Oxy-Bithiophene Semiconductors.

Exchanging hydrophobic alkyl-based side chains to hydrophilic glycol-based side chains is a widely adopted method for improving mixed-transport device performance, despite the impact on solid-state packing and polymer-electrolyte interactions being poorly understood. Presented here is a molecular dynamics (MD) force field for modeling alkoxylated and glycolated polythiophenes. The force field is validated against known packing motifs for their monomer crystals.

High Power Irradiance Dependence of Charge Species Dynamics in Hybrid Perovskites and Kinetic Evidence for Transient Vibrational Stark Effect in Formamidinium.

Hybrid halide perovskites materials have the potential for both photovoltaic and light-emitting devices. Relatively little has been reported on the kinetics of charge relaxation upon intense excitation. In order to evaluate the illumination power density dependence on the charge recombination mechanism, we have applied a femtosecond transient mid-IR absorption spectroscopy with strong excitation to directly measure the charge kinetics via electron absorption.

The Role of Long-Alkyl-Group Spacers in Glycolated Copolymers for High-Performance Organic Electrochemical Transistors.

Semiconducting polymers with oligoethylene glycol (OEG) sidechains have attracted strong research interest for organic electrochemical transistor (OECT) applications. However, key molecular design rules for high-performance OECTs via efficient mixed electronic/ionic charge transport are still unclear. In this work, new glycolated copolymers (gDPP-TTT and gDPP-TTVTT) with diketopyrrolopyrrole (DPP) acceptor and thiophene (T) and vinylene (V) thiophene-based donor units are synthesized and characterized for accumulation mode OECTs, where a long-alkyl-group (C ) attached to the DPP unit acts as a spacer distancing the OEG groups from the polymer backbone.

Multipulse Terahertz Spectroscopy Unveils Hot Polaron Photoconductivity Dynamics in Metal-Halide Perovskites.

Hot carriers in metal-halide perovskites (MHPs) present a foundation for understanding carrier-phonon coupling in the materials as well as the prospective development of high-performance hot carrier photovoltaics. While the carrier population dynamics during cooling have been scrutinized, the evolution of the hot carrier properties, namely mobility, remains largely unexplored. Here we introduce novel ultrafast visible pump-infrared push-terahertz probe spectroscopy to monitor the real-time conductivity dynamics of cooling carriers in methylammonium lead iodide.

Giant Huang-Rhys Factor for Electron Capture by the Iodine Intersitial in Perovskite Solar Cells.

Improvement in the optoelectronic performance of halide perovskite semiconductors requires the identification and suppression of nonradiative carrier trapping processes. The iodine interstitial has been established as a deep level defect and implicated as an active recombination center. We analyze the quantum mechanics of carrier trapping.

Accelerated Hot-Carrier Cooling in MAPbI Perovskite by Pressure-Induced Lattice Compression.

Hot-carrier cooling (HCC) in metal halide perovskites above the Mott transition is significantly slower than in conventional semiconductors. This effect is commonly attributed to a hot-phonon bottleneck, but the influence of the lattice properties on the HCC behavior is poorly understood. Using pressure-dependent transient absorption spectroscopy, we find that at an excitation density below the Mott transition, pressure does not affect the HCC.

Assessment of dynamic structural instabilities across 24 cubic inorganic halide perovskites.

Metal halide perovskites are promising candidates for next-generation photovoltaic and optoelectronic applications. The flexible nature of the octahedral network introduces complexity when understanding their physical behavior. It has been shown that these materials are prone to decomposition and phase competition, and the local crystal structure often deviates from the average space group symmetry.

Descriptors for Electron and Hole Charge Carriers in Metal Oxides.

Metal oxides can act as insulators, semiconductors, or metals depending on their chemical composition and crystal structure. Metal oxide semiconductors, which support equilibrium populations of electron and hole charge carriers, have widespread applications including batteries, solar cells, and display technologies. It is often difficult to predict in advance whether these materials will exhibit localized or delocalized charge carriers upon oxidation or reduction.

Ultrafast Intraband Spectroscopy of Hot-Carrier Cooling in Lead-Halide Perovskites.

The rapid relaxation of above-band-gap "hot" carriers (HCs) imposes the key efficiency limit in lead-halide perovskite (LHP) solar cells. Recent studies have indicated that HC cooling in these systems may be sensitive to materials composition, as well as the energy and density of excited states. However, the key parameters underpinning the cooling mechanism are currently under debate.

Acoustic phonon lifetimes limit thermal transport in methylammonium lead iodide.

Hybrid organic-inorganic perovskites (HOIPs) have become an important class of semiconductors for solar cells and other optoelectronic applications. Electron-phonon coupling plays a critical role in all optoelectronic devices, and although the lattice dynamics and phonon frequencies of HOIPs have been well studied, little attention has been given to phonon lifetimes. We report high-precision momentum-resolved measurements of acoustic phonon lifetimes in the hybrid perovskite methylammonium lead iodide (MAPI), using inelastic neutron spectroscopy to provide high-energy resolution and fully deuterated single crystals to reduce incoherent scattering from hydrogen.

Polaron States in Fullerene Adducts Modeled by Coarse-Grained Molecular Dynamics and Tight Binding.

Strong electron-phonon coupling leads to polaron localization in molecular semiconductor materials and influences charge transport, but it is expensive to calculate atomistically. Here, we propose a simple and efficient model to determine the energy and spatial extent of polaron states within a coarse-grained representation of a disordered molecular film. We calculate the electronic structure of the molecular assembly using a tight-binding Hamiltonian and determine the polaron state self-consistently by perturbing the site energies by the dielectric response of the surrounding medium to the charge.

Rotational Cation Dynamics in Metal Halide Perovskites: Effect on Phonons and Material Properties.

The dynamics of organic cations in metal halide hybrid perovskites (MHPs) have been investigated using numerous experimental and computational techniques because of their suspected effects on the properties of MHPs. In this Perspective, we summarize and reconcile key findings and present new data to synthesize a unified understanding of the dynamics of the cations. We conclude that theory and experiment collectively paint a relatively complete picture of rotational dynamics within MHPs.

Highly Luminescent Encapsulated Narrow Bandgap Polymers Based on Diketopyrrolopyrrole.

We present the synthesis and characterization of a series of encapsulated diketopyrrolopyrrole red-emitting conjugated polymers. The novel materials display extremely high fluorescence quantum yields in both solution (>70%) and thin film (>20%). Both the absorption and emission spectra show clearer, more defined features compared to their naked counterparts demonstrating the suppression of inter and intramolecular aggregation.

Slow Cooling of Hot Polarons in Halide Perovskite Solar Cells.

Halide perovskites show unusual thermalization kinetics for above-bandgap photoexcitation. We explain this as a consequence of excess energy being deposited into discrete large polaron states. The crossover between low-fluence and high-fluence "phonon bottleneck" cooling is due to a Mott transition where the polarons overlap ( ≥ 10 cm) and the phonon subpopulations are shared.

Perspective: Theory and simulation of hybrid halide perovskites.

Organic-inorganic halide perovskites present a number of challenges for first-principles atomistic materials modeling. Such "plastic crystals" feature dynamic processes across multiple length and time scales. These include the following: (i) transport of slow ions and fast electrons; (ii) highly anharmonic lattice dynamics with short phonon lifetimes; (iii) local symmetry breaking of the average crystallographic space group; (iv) strong relativistic (spin-orbit coupling) effects on the electronic band structure; and (v) thermodynamic metastability and rapid chemical breakdown.