Tuning polymer-backbone coplanarity and conformational order to achieve high-performance printed all-polymer solar cells.

All-polymer solar cells (all-PSCs) offer improved morphological and mechanical stability compared with those containing small-molecule-acceptors (SMAs). They can be processed with a broader range of conditions, making them desirable for printing techniques. In this study, we report a high-performance polymer acceptor design based on bithiazole linker (PY-BTz) that are on par with SMAs. We demonstrate that bithiazole induces a more coplanar and ordered conformation compared to bithiophene due to the synergistic effect of non-covalent backbone planarization and reduced steric encumbrances. As a result, PY-BTz shows a significantly higher efficiency of 16.4% in comparison to the polymer acceptors based on commonly used thiophene-based linkers (i.e., PY-2T, 9.8%). Detailed analyses reveal that this improvement is associated with enhanced conjugation along the backbone and closer interchain π-stacking, resulting in higher charge mobilities, suppressed charge recombination, and reduced energetic disorder. Remarkably, an efficiency of 14.7% is realized for all-PSCs that are solution-sheared in ambient conditions, which is among the highest for devices prepared under conditions relevant to scalable printing techniques. This work uncovers a strategy for promoting backbone conjugation and planarization in emerging polymer acceptors that can lead to superior all-PSCs.

Reducing delamination of an electron-transporting polymer from a metal oxide for electrochemical applications.

Delamination of the electron-transporting polymer N2200 from indium tin oxide (ITO) in aqueous electrolytes is mitigated by modifying ITO with an azide-functionalized phosphonic acid (PA) which, upon UV irradiation, reacts with the polymer. The optical, electrochemical, and spectroelectrochemical properties of N2200 thin films are retained in aqueous and non-aqueous media.

Effect of Antisolvent Additives in Aqueous Zinc Sulfate Electrolytes for Zinc Metal Anodes: The Case of Acetonitrile.

Aqueous zinc-ion batteries (ZIBs) employing zinc metal anodes are gaining traction as batteries for moderate to long duration energy storage at scale. However, corrosion of the zinc metal anode through reaction with water limits battery efficiency. Much research in the past few years has focused on additives that decrease hydrogen evolution, but the precise mechanisms by which this takes place are often understudied and remain unclear. In this work, we study the role of an acetonitrile antisolvent additive in improving the performance of aqueous ZnSO4 electrolytes using experimental and computational techniques. We demonstrate that acetonitrile actively modifies the interfacial chemistry during Zn metal plating, which results in improved performance of acetonitrile-containing electrolytes. Collectively, this work demonstrates the effectiveness of solvent additive systems in battery performance and durability and provides a new framework for future efforts to optimize ion transport and performance in ZIBs.

Ligand Steric Profile Tunes the Reactivity of Indium Phosphide Clusters.

Indium phosphide quantum dots have become an industrially relevant material for solid-state lighting and wide color gamut displays. The synthesis of indium phosphide quantum dots from indium carboxylates and tris(trimethylsilyl)phosphine (P(SiMe3)3) is understood to proceed through the formation of magic-sized clusters, with In37P20(O2CR)51 being the key isolable intermediate. The reactivity of the In37P20(O2CR)51 cluster is a vital parameter in controlling the conversion to quantum dots. Herein, we report structural perturbations of In37P20(O2CR)51 clusters induced by tuning the steric properties of a series of substituted phenylacetate ligands. This approach allows for control over reactivity with P(SiMe3)3, where meta-substituents enhance the susceptibility to ligand displacement, and para-substituents hinder phosphine diffusion to the core. Thermolysis studies show that with complete cluster dissolution, steric profile can modulate the nucleation period, resulting in a nanocrystal size dependence on ligand steric profile. The enhanced stability from ligand engineering also allows for the isolation and structural characterization by single-crystal X-ray diffraction of a new III-V magic-sized cluster with the formula In26P13(O2CR)39. This intermediate precedes the In37P20(O2CR)51 cluster on the InP QD reaction coordinate. The physical and electronic structure of this cluster are analyzed, providing new insight into previously unrecognized relationships between II-VI and III-V materials and the discrete growth of III-V cluster intermediates.

Navigating the Potential Energy Surface of CdSe Magic-Sized Clusters: Synthesis and Interconversion of Atomically Precise Nanocrystal Polymorphs.

Magic-sized clusters (MSCs) are kinetically stable, atomically precise intermediates along the quantum dot (QD) reaction potential energy surface. Literature precedent establishes two classes of cadmium selenide MSCs with QD-like inorganic cores: one class is proposed to be cation-rich with a zincblende crystal structure, while the other is proposed to be stoichiometric with a "wurtzite-like" core. However, the wide range of synthetic protocols used to access MSCs has made direct comparisons of their structure and surface chemistry difficult. Furthermore, the physical and chemical relationships between MSC polymorphs are yet to be established. Here, we demonstrate that both cation-rich and stoichiometric CdSe MSCs can be synthesized from identical reagents and can be interconverted through the addition of either excess cadmium or selenium precursor. The structural and compositional differences between these two polymorphs are contrasted using a combination of 1H NMR spectroscopy, X-ray diffraction (XRD), pair distribution function (PDF) analysis, inductively coupled plasma optical emission spectroscopy, and UV-vis transient absorption spectroscopy. The subsequent polymorph interconversion reactions are monitored by UV-vis absorption spectroscopy, with evidence for an altered cluster atomic structure observed by powder XRD and PDF analysis. This work helps to simplify the complex picture of the CdSe nanocrystal landscape and provides a method to explore structure-property relationships in colloidal semiconductors through atomically precise synthesis.

Soft Materials for Photoelectrochemical Fuel Production.

Polymer semiconductors are fascinating materials that could enable delivery of chemical fuels from water and sunlight, offering several potential advantages over their inorganic counterparts. These include extensive synthetic tunability of optoelectronic and redox properties and unique opportunities to tailor catalytic sites via chemical control over the nanoenvironment. Added to this is proven functionality of polymer semiconductors in solar cells, low-cost processability, and potential for large-area scalability. Herein we discuss recent progress on soft photoelectrochemical systems and define three critical knowledge gaps that must be closed for these materials to reach their full potential. We must (1) understand the influence of electrolyte penetration on photoinduced charge separation, transport, and recombination, (2) learn to exploit the swollen polymer/electrolyte interphase to drive selective fuel formation, and (3) establish co-design criteria for soft materials that sustain function in the face of harsh chemical challenges. Achieving these formidable goals would enable tailorable systems for driving photoelectrochemical fuel production at scale.

Highly stretchable polymer semiconductor thin films with multi-modal energy dissipation and high relative stretchability.

Stretchable polymer semiconductors (PSCs) have seen great advancements alongside the development of soft electronics. But it remains a challenge to simultaneously achieve high charge carrier mobility and stretchability. Herein, we report the finding that stretchable PSC thin films (<100-nm-thick) with high stretchability tend to exhibit multi-modal energy dissipation mechanisms and have a large relative stretchability (rS) defined by the ratio of the entropic energy dissipation to the enthalpic energy dissipation under strain. They effectively recovered the original molecular ordering, as well as electrical performance, after strain was released. The highest rS value with a model polymer (P4) exhibited an average charge carrier mobility of 0.2 cm2V-1s-1 under 100% biaxial strain, while PSCs with low rS values showed irreversible morphology changes and rapid degradation of electrical performance under strain. These results suggest rS can be used as a parameter to compare the reliability and reversibility of stretchable PSC thin films.

Design Rules for Obtaining Narrow Luminescence from Semiconductors Made in Solution.

Solution-processed semiconductors are in demand for present and next-generation optoelectronic technologies ranging from displays to quantum light sources because of their scalability and ease of integration into devices with diverse form factors. One of the central requirements for semiconductors used in these applications is a narrow photoluminescence (PL) line width. Narrow emission line widths are needed to ensure both color and single-photon purity, raising the question of what design rules are needed to obtain narrow emission from semiconductors made in solution. In this review, we first examine the requirements for colloidal emitters for a variety of applications including light-emitting diodes, photodetectors, lasers, and quantum information science. Next, we will delve into the sources of spectral broadening, including "homogeneous" broadening from dynamical broadening mechanisms in single-particle spectra, heterogeneous broadening from static structural differences in ensemble spectra, and spectral diffusion. Then, we compare the current state of the art in terms of emission line width for a variety of colloidal materials including II-VI quantum dots (QDs) and nanoplatelets, III-V QDs, alloyed QDs, metal-halide perovskites including nanocrystals and 2D structures, doped nanocrystals, and, finally, as a point of comparison, organic molecules. We end with some conclusions and connections, including an outline of promising paths forward.

Colloidal, Room-Temperature Growth of Metal Oxide Shells on InP Quantum Dots.

We demonstrate colloidal, layer-by-layer growth of metal oxide shells on InP quantum dots (QDs) at room temperature. We show with computational modeling that native InP QD surface oxides give rise to nonradiative pathways due to the presence of surface-localized dark states near the band edges. Replacing surface indium with zinc to form a ZnO shell results in reduced nonradiative decay and a density of states at the valence band edge that resembles defect-free, stoichiometric InP. We then developed a synthetic strategy using stoichiometric amounts of common atomic layer deposition precursors in alternating cycles to achieve layer-by-layer growth. Metal-oxide-shelled InP QDs show bulk and local structural perturbations as determined by X-ray diffraction and extended X-ray absorption fine structure spectroscopy. Upon growing ZnSe shells of varying thickness on the oxide-shelled QDs, we observe increased photoluminescence (PL) quantum yields and narrowing of the emission linewidths that we attribute to decreased ion diffusion to the shell, as supported by phosphorus X-ray emission spectroscopy. These results present a versatile strategy to control QD interfaces for novel heterostructure design by leveraging surface oxides. This work also contributes to our understanding of the connections between structural complexity and PL properties in technologically relevant colloidal optoelectronic materials.

Grazing-incidence diffraction reveals cellulose and pectin organization in hydrated plant primary cell wall.

The primary cell wall is highly hydrated in its native state, yet many structural studies have been conducted on dried samples. Here, we use grazing-incidence wide-angle X-ray scattering (GIWAXS) with a humidity chamber, which enhances scattering and the signal-to-noise ratio while keeping outer onion epidermal peels hydrated, to examine cell wall properties. GIWAXS of hydrated and dried onion reveals that the cellulose ([Formula: see text]) lattice spacing decreases slightly upon drying, while the (200) lattice parameters are unchanged. Additionally, the ([Formula: see text]) diffraction intensity increases relative to (200). Density functional theory models of hydrated and dry cellulose microfibrils corroborate changes in crystalline properties upon drying. GIWAXS also reveals a peak that we attribute to pectin chain aggregation. We speculate that dehydration perturbs the hydrogen bonding network within cellulose crystals and collapses the pectin network without affecting the lateral distribution of pectin chain aggregates.

In Situ and Operando Characterizations of Metal Halide Perovskite and Solar Cells: Insights from Lab-Sized Devices to Upscaling Processes.

The performance and stability of metal halide perovskite solar cells strongly depend on precursor materials and deposition methods adopted during the perovskite layer preparation. There are often a number of different formation pathways available when preparing perovskite films. Since the precise pathway and intermediary mechanisms affect the resulting properties of the cells, in situ studies have been conducted to unravel the mechanisms involved in the formation and evolution of perovskite phases. These studies contributed to the development of procedures to improve the structural, morphological, and optoelectronic properties of the films and to move beyond spin-coating, with the use of scalable techniques. To explore the performance and degradation of devices, operando studies have been conducted on solar cells subjected to normal operating conditions, or stressed with humidity, high temperatures, and light radiation. This review presents an update of studies conducted in situ using a wide range of structural, imaging, and spectroscopic techniques, involving the formation/degradation of halide perovskites. Operando studies are also addressed, emphasizing the latest degradation results for perovskite solar cells. These works demonstrate the importance of in situ and operando studies to achieve the level of stability required for scale-up and consequent commercial deployment of these cells.

Unlocking the potential of polymeric desalination membranes by understanding molecular-level interactions and transport mechanisms.

Polyamide reverse osmosis (PA-RO) membranes achieve remarkably high water permeability and salt rejection, making them a key technology for addressing water shortages through processes including seawater desalination and wastewater reuse. However, current state-of-the-art membranes suffer from challenges related to inadequate selectivity, fouling, and a poor ability of existing models to predict performance. In this Perspective, we assert that a molecular understanding of the mechanisms that govern selectivity and transport of PA-RO and other polymer membranes is crucial to both guide future membrane development efforts and improve the predictive capability of transport models. We summarize the current understanding of ion, water, and polymer interactions in PA-RO membranes, drawing insights from nanofiltration and ion exchange membranes. Building on this knowledge, we explore how these interactions impact the transport properties of membranes, highlighting assumptions of transport models that warrant further investigation to improve predictive capabilities and elucidate underlying transport mechanisms. We then underscore recent advances in in situ characterization techniques that allow for direct measurements of previously difficult-to-obtain information on hydrated polymer membrane properties, hydrated ion properties, and ion-water-membrane interactions as well as powerful computational and electrochemical methods that facilitate systematic studies of transport phenomena.

Degree of Orientation in Liquid Crystalline Elastomers Defines the Magnitude and Rate of Actuation.

The anisotropy of liquid crystalline elastomers (LCEs) is derived from the interaction-facilitated orientation of the molecular constituents. Here, we correlate the thermomechanical response of a series of LCEs subjected to mechanical alignment to measurements of the Hermans orientation parameter. The LCEs were systematically prepared with varying concentrations of liquid crystalline mesogens, which affects the relative degree of achievable order. These compositions were subject to varying degrees of mechanical alignment to prepare LCEs with orientations that span a wide range of orientation parameters. The stimuli-response of the LCEs indicates that the liquid crystalline content defines the temperature of actuation, whereas the orientation parameter of the LCE is intricately correlated to both the total actuation strain of the LCE as well as the rate of thermomechanical response.

Use of a Multiple Hydride Donor To Achieve an n-Doped Polymer with High Solvent Resistance.

The ability to insolubilize doped semiconducting polymer layers can help enable the fabrication of efficient multilayer solution-processed electronic and optoelectronic devices. Here, we present a promising approach to simultaneously n-dope and largely insolubilize conjugated polymer films using tetrakis[{4-(1,3-dimethyl-2,3-dihydro-1H-benzo[d]imidazol-2-yl)phenoxy}methyl]methane (tetrakis-O-DMBI-H), which consists of four 2,3-dihydro-1H-benzoimidazole (DMBI-H) n-dopant moieties covalently linked to one another. Doping a thiophene-fused benzodifurandione-based oligo(p-phenylenevinylene)-co-thiophene polymer (TBDOPV-T) with tetrakis-O-DMBI-H results in a highly n-doped film with bulk conductivity of 15 S cm-1. Optical absorption spectra provide evidence for film retention of ∼93% after immersion in o-dichlorobenzene for 5 min. The optical absorption signature of the charge carriers in the n-doped polymer decreases only slightly more than that of the neutral polymer under these conditions, indicating that the exposure to solvent also results in negligible dedoping of the film. Moreover, thermal treatment studies on a tetrakis-O-DMBI-H-doped TBDOPV-T film in contact with another undoped polymer film indicate immobilization of the molecular dopant in TBDOPV-T. This is attributed to the multiple electrostatic interactions between each dopant tetracation and up to four nearby anionic doped polymer segments.

Vapor deposition rate modifies anisotropic glassy structure of an anthracene-based organic semiconductor.

We control the anisotropic molecular packing of vapor-deposited glasses of ABH113, a deuterated anthracene derivative with promise for future organic light emitting diode materials, by changing the deposition rate and substrate temperature at which they are prepared. We find that at substrate temperatures from 0.65 Tg to 0.92 Tg, the deposition rate significantly modifies the orientational order in the vapor-deposited glasses as characterized by x-ray scattering and birefringence. Both measures of anisotropic order can be described by a single deposition rate-substrate temperature superposition (RTS). This supports the applicability of the surface equilibration mechanism and generalizes the RTS principle from previous model systems with liquid crystalline order to non-mesogenic organic semiconductors. We find that vapor-deposited glasses of ABH113 have significantly enhanced density and thermal stability compared to their counterparts prepared by liquid-cooling. For organic semiconductors, the results of this study provide an efficient guide for using the deposition rate to prepare stable glasses with controlled molecular packing.

Scattering techniques for mixed donor-acceptor characterization in organic photovoltaics.

Precise control of the complex morphology of organic photovoltaic bulk heterojunction (BHJ) active layers remains an important yet challenging approach for improving power conversion efficiency. Of particular interest are the interfacial regions between electron donor and acceptor molecules where charge separation and charge recombination occur. Often, these interfaces feature a molecularly mixed donor-acceptor phase. This mixed phase has been extensively studied in polymer:fullerene systems but is poorly understood in state-of-the-art polymer:non-fullerene acceptor blends. Accurate, quantitative characterization of this mixed phase is critical to unraveling its importance for charge separation and recombination processes within the BHJ. Here, we detail X-ray and neutron scattering characterization techniques and analysis methods to quantify the mixed phase within BHJ active layers. We then review the existing literature where these techniques have been successfully used on several different material systems and correlated to device performance. Finally, future challenges for characterizing non-fullerene acceptor systems are addressed, and emerging strategies are discussed.

Bridging the thermodynamics and kinetics of temperature-induced morphology evolution in polymer/fullerene organic solar cell bulk heterojunction.

The performance of organic solar cells (OSC) critically depends on the morphology of the active layer. After deposition, the active layer is in a metastable state and prone to changes that lead to cell degradation. Here, a high efficiency fullerene:polymer blend is used as a model system to follow the temperature-induced morphology evolution through a series of thermal annealing treatments. Electron microscopy analysis of the nano-scale phase evolution during the early stages of thermal annealing revealed that spinodal decomposition, i.e. spontaneous phase separation with no nucleation stage, is possibly responsible for the formation of a fine scale bicontinuous structure. In the later evolution stages, large polycrystalline fullerene aggregates are formed. Optical microscopy and scattering revealed that aggregate-growth follows the Johnson-Mehl-Avrami-Kolmogorov equation indicating a heterogeneous transformation process, i.e., through nucleation and growth. These two mechanisms, spinodal decomposition vs. nucleation and growth, are mutually exclusive and their co-existence is surprising. This unexpected observation is resolved by introducing a metastable monotectic phase diagram and showing that the morphology evolution goes through two distinct and consecutive transformation processes where spinodal decomposition of the amorphous donor:acceptor blend is followed by nucleation and growth of crystalline acceptor aggregates. Finally, this unified thermodynamic and kinetic mechanism allows us to correlate the morphology evolution with OSC degradation during thermal annealing.

Surface equilibration mechanism controls the molecular packing of glassy molecular semiconductors at organic interfaces.

Glasses prepared by physical vapor deposition (PVD) are anisotropic, and the average molecular orientation can be varied significantly by controlling the deposition conditions. While previous work has characterized the average structure of thick PVD glasses, most experiments are not sensitive to the structure near an underlying substrate or interface. Given the profound influence of the substrate on the growth of crystalline or liquid crystalline materials, an underlying substrate might be expected to substantially alter the structure of a PVD glass, and this near-interface structure is important for the function of organic electronic devices prepared by PVD, such as organic light-emitting diodes. To study molecular packing near buried organic-organic interfaces, we prepare superlattice structures (stacks of 5- or 10-nm layers) of organic semiconductors, Alq3 (Tris-(8-hydroxyquinoline)aluminum) and DSA-Ph (1,4-di-[4-(N,N-diphenyl)amino]styrylbenzene), using PVD. Superlattice structures significantly increase the fraction of the films near buried interfaces, thereby allowing for quantitative characterization of interfacial packing. Remarkably, both X-ray scattering and spectroscopic ellipsometry indicate that the substrate exerts a negligible influence on PVD glass structure. Thus, the surface equilibration mechanism previously advanced for thick films can successfully describe PVD glass structure even within the first monolayer of deposition on an organic substrate.

Alloying a single and a double perovskite: a Cu+/2+ mixed-valence layered halide perovskite with strong optical absorption.

Introducing heterovalent cations at the octahedral sites of halide perovskites can substantially change their optoelectronic properties. Yet, in most cases, only small amounts of such metals can be incorporated as impurities into the three-dimensional lattice. Here, we exploit the greater structural flexibility of the two-dimensional (2D) perovskite framework to place three distinct stoichiometric cations in the octahedral sites. The new layered perovskites AI 4[CuII(CuIInIII)0.5Cl8] (1, A = organic cation) may be derived from a CuI-InIII double perovskite by replacing half of the octahedral metal sites with Cu2+. Electron paramagnetic resonance and X-ray absorption spectroscopy confirm the presence of Cu2+ in 1. Crystallographic studies demonstrate that 1 represents an averaging of the CuI-InIII double perovskite and CuII single perovskite structures. However, whereas the highly insulating CuI-InIII and CuII perovskites are colorless and yellow, respectively, 1 is black, with substantially higher electronic conductivity than that of either endmember. We trace these emergent properties in 1 to intervalence charge transfer between the mixed-valence Cu centers. We further propose a tiling model to describe how the Cu+, Cu2+, and In3+ coordination spheres can pack most favorably into a 2D perovskite lattice, which explains the unusual 1 : 2 : 1 ratio of these cations found in 1. Magnetic susceptibility data of 1 further corroborate this packing model. The emergence of enhanced visible light absorption and electronic conductivity in 1 demonstrates the importance of devising strategies for increasing the compositional complexity of halide perovskites.