Selective Hole and Electron Transport in Efficient Quaternary Blend Organic Solar Cells
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Arunagiri, Lingeswaran↗
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Interlayer Triplet Energy Transfer in Dion-Jacobson 2D Perovskites Containing Naphthalene Diammonium Cations
The ability to tune the heterojunction energy level alignment in layered 2D perovskites is well documented, with both early works and more recent studies reporting interfacial triplet energy transfer in certain pairings of lead halide with small conjugated organic spacer cations. Such 2D perovskites are promising systems for sensitized molecular phosphorescence, which has varied applications in optoelectronics, solar energy conversion, and more. However, even for a given organic cation core, variations in the cation structure (e.g. location of the ammonium substituent, length of the alkyl linker, and valency of the cation) can dramatically alter the organic layer morphology and electronic coupling between the inorganic and organic layers. While interlayer triplet energy transfer has been studied in Ruddlesden-Popper 2D perovskites containing monovalent naphthalene cations, the photophysical properties of their Dion-Jacobson analogue, formed using divalent naphthalene cations, have not been reported. In this study, we examine interlayer energy transfer in a series of mixed-halide 2D perovskites formed by divalent naphthalene cations. We find that the sensitized phosphorescence in these compounds is dominated by naphthalene triplet excimer emission, but when the lead halide exciton level is tuned near resonance with the triplet level of naphthalene, emission from the naphthalene triplet monomer competes with triplet excimer formation. Interlayer energy transfer in these compounds is further supported by ultrafast transient absorption spectroscopy. Ultimately, the ability to gain control over interlayer interactions in layered 2D perovskites through cation design will help uncover new functions and applications for these materials.
2D↗
Anomalous Charge Transfer from Organic Ligands to Metal Halides in Zero-Dimensional [(C 6 H 5 ) 4 P] 2 SbCl 5 Enabled by Pressure-Induced Lone Pair-π Interaction
Low-dimensional (low-D) organic metal halide hybrids (OMHHs) have emerged as fascinating candidates for optoelectronics due to their integrated properties from both organic and inorganic components. However, for most of low-D OMHHs, especially the zero-D (0D) compounds, the inferior electronic coupling between organic ligands and inorganic metal halides prevents efficient charge transfer at the hybrid interfaces and thus limits their further tunability of optical and electronic properties. Here, using pressure to regulate the interfacial interactions, efficient charge transfer from organic ligands to metal halides is achieved, which leads to a near-unity photoluminescence quantum yield (PLQY) at around 6.0 GPa in a 0D OMHH, [(C 6 H 5 ) 4 P] 2 SbCl 5 . In situ experimental characterizations and theoretical simulations reveal that the pressure-induced electronic coupling between the lone-pair electrons of Sb 3+ and the π electrons of benzene ring (lp-π interaction) serves as an unexpected “bridge” for the charge transfer. Further, our work opens a versatile strategy for the new materials design by manipulating the lp-π interactions in organic–inorganic hybrid systems.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Comparative scintillation performance of EJ-309, EJ-276, and a novel organic glass
An organic glass scintillator developed by Sandia National Laboratories was characterized in terms of its light output and pulse shape discrimination (PSD) properties and compared to commercial liquid (EJ-309) and plastic (EJ-276) organic scintillators. The electron light output was determined through relative comparison of the 137 Cs Compton edge location. The proton light yield was measured using a double time-of-flight technique at the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory. Using a tunable broad-spectrum neutron source and an array of pulse-shape-discriminating observation scintillators, a continuous measurement of the proton light yield was performed for EJ-309 (200 keV–3.2 MeV), EJ-276 (170 keV–4.9 MeV), and the organic glass (50 keV–20 MeV) . Finally, the PSD properties of the organic glass, EJ-309, and EJ-276 were evaluated using an AmBe source and compared via a figure-of-merit metric. The organic glass exhibited a higher electron light output than both EJ-309 and EJ-276. Its proton light yield and PSD performance were comparable to EJ-309 and superior to that of EJ-276. Finally, with these performance characteristics, the organic glass scintillator is well poised to replace current state-of-the-art PSD-capable scintillators in a range of fast neutron detection applications.
47 OTHER INSTRUMENTATION↗
The UV Photoinduced Ring-Closing Reaction of Cyclopentadiene Probed with Ultrafast Electron Diffraction
Conjugated cyclic organic molecules are common across many fields such as pharmaceuticals, are naturally occurring in biological systems, and are used in synthetic materials. One particular area of interest from a photochemical point of view is the formation of highly strained cyclic organics. We investigate the photoinduced reaction of cyclopentadiene, a five-membered organic ring molecule which can form strained three and four carbon rings after photoexcitation with UV light, with the gas-phase ultrafast electron diffraction instrument at the SLAC MeV-UED facility. Electron diffraction offers a direct probe sensitive to the nuclear geometry during the reaction, allowing for the determination of the distribution of products formed following photoexcitation. We observe the simultaneous formation of the highly strained ring- closed bicyclo[2.1.0]pentene and vibrationally hot cyclopentadiene within the temporal resolution of the experiment, and determine the relative yield of all reaction products. Furthermore, the experimental results are in good agreement with the predictions of trajectory simulations.
Computational Chemistry↗
Improved Organic Electrochemical Transistors via Directed Crystallizable Small Molecule Templating
Organic electrochemical transistors (OECTs) are of great interest as biosensors and for flexible electronics applications. Historically, processing techniques have been widely used to improve the performance of polymers in organic electronics but have been underutilized in the field of OECTs. Here, in this work, we study the effects of a crystallizable small molecule template for improving the performance of poly(3-hexylthiophene-2,5-diyl) (P3HT)-based OECTs. We utilize 1,3,5-trichlorobenzene (TCB) as a crystallizable additive to induce directional crystallization during blade coating. This procedure results in an aligned, highly ordered, and moderately porous layer of the semiconducting polymer. We find that compared to neat P3HT solutions, films cast with TCB additive show a greater than 8× improvement in the steady state figure of merit for OECTs. Additionally, the optimum TCB loading produces anisotropic OECTs where the polymer chain aligned parallel to the current outperforms the perpendicular alignment by more than a factor of 10. We investigated the mechanism of this improvement and found that polymer alignment, polymer ordering, and film porosity all play a role in improving the device performance. Overall, these results demonstrate the importance of processing on the optimization of polymers for OECTs and suggest a route to greatly improve the performance of commercially available hydrophobic polymers.
36 MATERIALS SCIENCE↗
Franck-Condon Tuning of Optical Cycling Centers by Organic Functionalization
Laser induced electronic excitations that spontaneously emit photons and decay directly to the initial ground state (“optical cycling transitions”) are used in quantum information and precision measurement for state initialization and readout. To extend this primarily atomic technique to large, organic compounds, we theoretically investigate optical cycling of alkaline earth phenoxides and their functionalized derivatives. We find that optical cycle leakage due to wave function mismatch is low in these species, and can be further suppressed by using chemical substitution to boost the electron-withdrawing strength of the aromatic molecular ligand through resonance and induction effects. This provides a straightforward way to use chemical functional groups to construct optical cycling moieties for laser cooling, state preparation, and quantum measurement.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Photoinduced Electron-Transfer Distance Is Controlled by the Driving Force in Solid-State Organic Donor-Acceptor Systems
Understanding how short-range electron transfer generates photocurrent in organic solar cells and what photochemical driving force is needed to maximize efficiencies has been an immensely difficult problem. Here we show how these questions are intertwined: the driving force controls the average charge-transfer distance, skipping the shortest-range states in the optimal case. Using photoinduced absorption-detected magnetic resonance, we measure average charge-separation distances in dilute donor–acceptor blends as a function of driving force, and compare these with free charge yields measured by time-resolved microwave conductivity. We find that the largest driving forces show the shortest charge-separation distances, explaining their suppressed free-carrier generation. These results support a long-range electron-transfer model, wherein the driving force controls the initial delocalization of the electron–hole pair. The minimum driving force required for efficient charge separation (and minimal voltage losses) is ultimately set by the dielectric constant of the material.
14 SOLAR ENERGY↗
N‐Heterocyclic Carbene Functionalization of 2D Transition Metal Dichalcogenides via a Frustrated Lewis Pair Strategy
Molecular functionalization of 2D transition metal dichalcogenides (TMDs), which unites tailorability of organic molecules and robustness of inorganic solids, is important for tuning their surface properties. While it is common to leverage the electronic effect of organic ligands to functionalize TMDs, the corresponding steric hinderance has remained elusive and underexplored. Herein N‐heterocyclic carbene (NHC) functionalization of WS 2 is demonstrated via a frustrated Lewis pair (FLP) route, which exploits both the electronic and steric effect for TMD functionalization. The Lewis base, 1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene (IMes), is anchored to the sulfur vacancy sites (Lewis acid) of the WS 2 nanostructures with the hybrid‐interface‐mediated steric hinderance, mimicking the FLP chemistry and enabling dihydrogen cleavage at room temperature and atmospheric pressure. Solid‐state nuclear magnetic resonance (ssNMR) results reveal the local chemical environment of the activated hydrogen species, which can be transferred for room‐temperature hydrogenation reactions. Finally, the insights are useful for designing weak, non‐covalent bonds to modify the 2D surface of TMDs for a broad scope of applications.
2D materials↗
Emerging Two-Dimensional Organic Semiconductor-Incorporated Perovskites–A Fascinating Family of Hybrid Electronic Materials
Halide perovskites have attracted a great amount of attention owing to their unique materials chemistry, excellent electronic properties, and low-cost manufacturing. Two-dimensional (2D) halide perovskites, originating from three-dimensional (3D) perovskite structures, are structurally more diverse and therefore create functional possibilities beyond 3D perovskites. The much less restrictive size constraints on the organic component of these hybrid materials particularly provide an exciting platform for designing unprecedented materials and functionalities at the molecular level. In this Perspective, we discuss the concept and recent development of a sub-class of 2D perovskites, namely, organic semiconductor-incorporated perovskites (OSiPs). OSiPs combine the electronic functionality of organic semiconductors with the soft and dynamic halide perovskite lattice, offering opportunities for tailoring the energy landscape, lattice and carrier dynamics, and electron/ion transport properties for various fundamental studies, as well as device applications. Specifically, we summarize recent advances in the design, synthesis, and structural analysis of OSiPs with various organic conjugated moieties as well as the application of OSiPs in photovoltaics, light-emitting devices, and transistors. As a result, challenges and further opportunities for OSiPs in molecular design, integration of novel functionality, film quality, and stability issues are addressed.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Advanced Electron Microscopy of Nanophased Synthetic Polymers and Soft Complexes for Energy and Medicine Applications
After decades of developments, electron microscopy has become a powerful and irreplaceable tool in understanding the ionic, electrical, mechanical, chemical, and other functional performances of next-generation polymers and soft complexes. The recent progress in electron microscopy of nanostructured polymers and soft assemblies is important for applications in many different fields, including, but not limited to, mesoporous and nanoporous materials, absorbents, membranes, solid electrolytes, battery electrodes, ion- and electron-transporting materials, organic semiconductors, soft robotics, optoelectronic devices, biomass, soft magnetic materials, and pharmaceutical drug design. For synthetic polymers and soft complexes, there are four main characteristics that differentiate them from their inorganic or biomacromolecular counterparts in electron microscopy studies: (1) lower contrast, (2) abundance of light elements, (3) polydispersity or nanomorphological variations, and (4) large changes induced by electron beams. Since 2011, the Center for Nanophase Materials Sciences (CNMS) at Oak Ridge National Laboratory has been working with numerous facility users on nanostructured polymer composites, block copolymers, polymer brushes, conjugated molecules, organic–inorganic hybrid nanomaterials, organic–inorganic interfaces, organic crystals, and other soft complexes. This review crystalizes some of the essential challenges, successes, failures, and techniques during the process in the past ten years. It also presents some outlooks and future expectations on the basis of these works at the intersection of electron microscopy, soft matter, and artificial intelligence. Machine learning is expected to automate and facilitate image processing and information extraction of polymer and soft hybrid nanostructures in aspects such as dose-controlled imaging and structure analysis.
36 MATERIALS SCIENCE↗
Room-Temperature Quantitative Quantum Sensing of Lithium Ions with a Radical-Embedded Metal–Organic Framework
Recent advancements in quantum sensing have sparked transformative detection technologies with high sensitivity, precision, and spatial resolution. Owing to their atomic-level tunability, molecular qubits and ensembles thereof are promising candidates for sensing chemical analytes. Here, in this study, we show quantum sensing of lithium ions in solution at room temperature with an ensemble of organic radicals integrated in a microporous metal–organic framework (MOF). The organic radicals exhibit electron spin coherence and microwave addressability at room temperature, thus behaving as qubits. The high surface area of the MOF promotes accessibility of the guest analytes to the organic qubits, enabling unambiguous identification of lithium ions and quantitative measurement of their concentration through relaxometric and hyperfine spectroscopic methods based on electron paramagnetic resonance (EPR) spectroscopy. The sensing principle presented in this work is applicable to other metal ions with nonzero nuclear spin.
36 MATERIALS SCIENCE↗
A 3D nonfullerene electron acceptor with a 9,9'-bicarbazole backbone for high-efficiency organic solar cells
One-dimensional ladder-type nonfullerene electron acceptors (NFAs) with large fused ring cores have been typically used in highly efficient organic solar cells (OSCs). Prior studies have demonstrated that small molecule acceptors with three-dimensional (3D) structures may exhibit low energy loss, and hence can lead to improved OSC performance. In this study, a new 3D NFA ( 99CZ-8F ) with a 9,9'-bicarbazole backbone was designed, synthesized, and characterized, where two linear A-D-A architectures were linked by a single N–N bond. 99CZ-8F showed strong absorption in the range of 500–800 nm in the solid state, which is complementary to the absorption of the donor material PM6 . After regulating the morphology of the active layer via binary solvent mixture, the optimized device exhibited a maximum power conversion efficiency (PCE) of 6.6 ± 0.1%, which is among the best reported values for 3D nonfullerene electron acceptor based OSCs
14 SOLAR ENERGY↗
Determination of Vibrational Motions Driving Photoinduced Electron Transfer Reactions in Molecular Crystals and Organic Thin Films
The objective of this research is to develop an understanding of how nuclear motions can be used to drive and control photo-induced charge transfer and singlet fission reactions in molecular crystals and thin films. By following the structural evolution of reacting molecules and nanostructures on the timescale of their nuclear motion, we will determine what interplay of nuclear coordinates is most efficient in driving electron transfers, a necessary first step in most photovoltaic and photocatalytic processes. We will identify which nuclear coordinates effectively promote desirable electron transfer processes, or ineffectively dissipate photon energy into unwanted thermal energy, thus guiding the design and implementation of molecular systems tailored for high efficiency processes. Specifically, we aim to identify which nuclear coordinates are responsible for driving electron transfer and singlet fission reactions at different points along the reaction coordinate, particularly in regimes which deviate from Marcus theory predictions. We will do so by investigating systems with electron transfer and singlet fission occurring from non-thermalized states due to strong donor-acceptor electronic coupling. First, we will prove that specific vibrational modes are responsible for charge transfer processes in molecular crystals. Secondly, we will investigate how vibrational coherences and specific nuclear motions can be used to drive high yield singlet fission processes. Finally, we will determine how complex film morphology affects long-range charge transport, a key limiting factor in our ability to make gains in efficiency.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Tunable Electronic Energy Level Alignment and Exciton Diversity in Organic–Inorganic van der Waals Heterostructures
van der Waals stacking of two-dimensional (2D) materials offers a powerful platform for engineering material interfaces with tailored electronic and optical properties. While most van der Waals multilayers have featured inorganic monolayers, incorporating molecular monolayers introduces additional degrees of tunability and functionality. Here, in this study, we investigate hybrid bilayers composed of atomically thin perylene-based molecular crystals interfaced with monolayer transition metal dichalcogenides (TMDs), specifically MoS 2 and WS 2 . Using the ab initio many-body perturbation theory within the GW approximation and the Bethe-Salpeter equation approach, we predict emergent properties beyond those of the isolated constituent systems. Notably, we find substantial renormalization of monolayer molecular crystal band gap due to TMD-induced polarization. Furthermore, by varying the TMD monolayer, we demonstrate tuning of the energy level alignment of the bilayer and subsequent control over a diversity of lowest-energy excitons, which include strongly bound hybrid excitons and long-lived charge-transfer excitons. These findings establish organic-inorganic van der Waals heterostructures as a promising class of materials for tunable optoelectronic devices and quantum excitonic phenomena, expanding the design space for low-dimensional systems.
GW-BSE calculations↗
Revisiting the Role of Entropy for Charge Separation in 1D Pi-Conjugated Semiconductors
Free carrier generation in organic donor/acceptor heterojunctions and redox-doped organic semiconductors is poorly understood, since assumed tight electron-hole binding conflicts with observed high free carrier yields. Cornerstone analyses that have guided the field for over 15 years predict that entropy can stabilize free charges in 2D and 3D pi-conjugated semiconductors but not in 1D systems. Here, the impact of entropy on charge generation in 1D pi-conjugated semiconductors is revisited by exploiting a greatly simplified system where enthalpy considerations alone should not allow for free charge generation. Noncontact solution-phase microwave conductivity is used to investigate the carrier density-dependent conductivity and dielectric constant in isolated chemically doped semiconducting single-walled carbon nanotubes in a low-dielectric solvent. Dopant chemical structure dramatically influences the carrier density-dependent complex conductivity, with bulky dopants facilitating carrier escape even at carrier densities below one carrier per nanotube. Three distinct numerical calculations show that entropic stabilization dramatically lowers the Gibbs energy barrier for free charge generation, explaining the high yield of free carriers, even in 1D. This renewed understanding of entropy's role in carrier generation has important implications for designing organic electronic devices-such as solar cells and thermoelectric energy harvesters-for enhanced carrier yield, conductivity, and performance.
36 MATERIALS SCIENCE↗