Improving the intrinsic film quality of metal halide perovskites is very critical to increase the power conversion efficiency and long-term stability of perovskite solar cells. Here we report a multifunctional, non-volatile additive that can be used to modulate the kinetics of perovskite film growth through a hydrogen-bond-bridged intermediate phase. The additive enables the formation of large perovskite grains and coherent grain growth from bottom to the surface of the film. The enhanced film morphology results in significantly reduced non-radiative recombinations, thus boosting the power conversion efficiency of inverted (p–i–n) solar cells to 24.8% (24.5% certified) with a low energy loss of 0.36 eV. The unencapsulated devices exhibit improved thermal stability with a T 98 lifetime beyond 1,000 h under continuous heating at 65 ± 5 °C in a nitrogen-filled glovebox. This effective approach can also be applied to wide-bandgap perovskites and large-area devices to show reduced voltage loss and high efficiency.

The rapid development of non-fullerene acceptors (NFAs) with strong near-infrared absorption has led to remarkably enhanced short-circuit current density (Jsc) values in organic solar cells (OSCs). NFAs based on the benzotriazole (Bz) fused-ring π-core have great potential in delivering both high Jsc and decent open-circuit voltage values due to their strong intramolecular charge transfer with reasonably low energy loss. In this work, we have designed and synthesized a series of Bz-based NFAs, PN6SBO-4F, AN6SBO-4F and EHN6SEH-4F, via regiospecific N-alkyl engineering based on the high-performance NFA mBzS-4F that was reported previously. The molecular packing of mBzS-4F, AN6SBO-4F, and EHN6SEH-4F single crystals was analyzed using X-ray crystallography in order to provide a comprehensive understanding of the correlation between the molecular structure, the charge-transporting properties, and the solar cell performance. Compared with the typical honeycomb single-crystal structure of Y6 derivatives, these NFAs exhibit distinctly different molecular packing patterns. The strong interactions of terminal indanone groups in mBzS-4F and the J-aggregate-like packing in EHN6SEH-4F lead to the formation of ordered 3D networks in single-crystals with channels for efficient charge transport. Consequently, OSCs based on mBzS-4F and EHN6SEH-4F show efficient photon-to-current conversions, achieving the highest power conversion efficiency of 17.48% with a Jsc of 28.83 mA cm-2.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

At present, high-performance organic photovoltaics mostly adopt a bulk-heterojunction architecture, in which exciton dissociation is facilitated by charge-transfer states formed at numerous donor–acceptor (D-A) heterojunctions. However, the spin character of charge-transfer states originated from recombination of photocarriers allows relaxation to the lowest-energy triplet exciton (T 1 ) at these heterojunctions, causing photocurrent loss. Here we find that this loss pathway can be alleviated in sequentially processed planar–mixed heterojunction (PMHJ) devices, employing donor and acceptor with intrinsically weaker exciton binding strengths. The reduced D-A intermixing in PMHJ alleviates non-geminate recombination at D-A contacts, limiting the chance of relaxation, thus suppressing T 1 formation without sacrificing exciton dissociation efficiency. This resulted in devices with high power conversion efficiencies of >19%. We elucidate the working mechanisms for PMHJs and discuss the implications for material design, device engineering and photophysics, thus providing a comprehensive grounding for future organic photovoltaics to reach their full promise.

Passivating surface and bulk defects of perovskite films has been proven to be an effective way to minimize nonradiative recombination losses in perovskite solar cells (PVSCs). The lattice interference and perturbation of atomic periodicity at the perovskite surfaces often significantly affect the material properties and device efficiencies. By tailoring the terminal groups on the perovskite surface and modifying the surface chemical environment, the defects can be reduced to enhance the photovoltaic performance and stability of derived PVSCs. Here, we report a rationally designed bifunctional molecule, piperazinium iodide (PI), containing both R2NH and R2NH2+ groups on the same six-membered ring, behaving both as an electron donor and an electron acceptor to react with different surface-terminating ends on perovskite films. The resulting perovskite films after defect passivation show released surface residual stress, suppressed nonradiative recombination loss, and more n-type characteristics for sufficient energy transfer. Consequently, charge recombination is significantly suppressed to result in a high open-circuit voltage (VOC) of 1.17 V and a reduced VOC loss of 0.33 V. A very high power conversion efficiency (PCE) of 23.37% (with 22.75% certified) could be achieved, which is the highest value reported for inverted PVSCs. Our work reveals a very effective way of using rationally designed bifunctional molecules to simultaneously enhance the device performance and stability.

Stiff polymers with tunable self-healing and mechanical properties show promising potential in electrochemical devices.

In this work, a new phosphonium-containing cationic polyelectrolyte (PE1) has been rationally designed and developed via a facile click-chemistry type postfunctionalization, which can form complexes with highly polarizable anionic cyanines to significantly reduce the strong and random cyanine-cyanine interactions (i.e., aggregation) in the solid-state. This material design strategy enables an efficient translation of the favorable molecular properties of cyanines into macroscopic material properties. One of such complexes exhibits a very large third-order susceptibility over 10-10 esu with low nonlinear optical loss suitable for all optical signal processing.

Li–S batteries hold great promise as the utility of a thiol-based interface facilitates LiPS-trapping and improves the electrolyte wetting.

We extend the recently developed dual-arm Z-scan to increase the signal-to-noise ratio (SNR) for measuring the nonlinear refraction (NLR) of thin films on thick substrates. Similar to the case of solutes in solution, the phase shift due to NLR in a thin film can often be dominated by the phase shift due to NLR in the much thicker substrate. SNR enhancement is accomplished by simultaneously scanning a bare substrate and the film plus substrate in two separate but identical Z-scan arms. The subtraction of these signals taken simultaneously effectively cancels the nonlinear signal from the substrate, leaving only the signal from the film. More importantly, the SNR is increased since the correlated noise from effects such as beam-pointing instabilities cancels. To show the versatility of the dual-arm Z-scan method, we perform measurements on semiconductor and organic thin films, some less than 100 nm thick and with thicknesses up to 4 orders of magnitude less than the substrate.

Mechanochemical changes in absorption or fluorescence in solid polymers are important for the development of new methods of damage sensing, and must be demonstrated in structural polymers such as epoxies for widespread use. With this in mind, we have observed that diamine-cured epoxies containing 4,4′-diaminodiphenyl methane (DDM) framework display mechanochemical changes in both absorption and fluorescence. Samples change from original “blue” to mechanochemically activated “red” fluorescence in response to uniaxial compressive deformation beginning in the early stages of strain hardening. Accompanying the “red” fluorescence is an increase in radical concentration; both are impermanent in time, suggesting the reactive intermediate as the fluorophore. Orange and green chromophores are generated by compression as well; the orange chromophore is the red-emitting fluorophore while the green chromophore is non-fluorescent at ambient conditions. Our work indicates that the DDM structure is the origin of the mechanochromic responses, and stoichiometric variation and degree of cure are strong determinants for whether orange or green chromophores will form. Based on these results the red-emitting orange chromophore is proposed as a reactive radical intermediate of core DDM structure, generated by bond scission reactions on the epoxy network. The green chromophore is a quinoidal methine resulting from the intermediates.

Lithium-Sulfur (LiS) batteries are a front runner for next-generation secondary battery systems. This is partially due to the low cost, relative abundance, and environmentally benign nature of their active materials, but arguably most important is the order of magnitude increase in theoretical specific capacity of LiS cells compared to current state of the art lithium-metal oxide systems (~1,600 mAhg -1 vs ~160mAhg -1 , respectively). Despite these inherent benefits, the commercialization of LiS systems has been impeded by a rapid loss of capacity upon repeated cycling due to the complexities of lithiation and delithiation at the carbon/sulfur composite cathode. During discharge, elemental sulfur is sequentially reduced to lithium sulfide following the reaction S 8 + 16Li + + 16e - → 8Li 2 S , where intermediates Li 2 S (8-2) are formed, their length depending on depth of discharge. Long-chain lithium polysulfides (LiPS), Li 2 S (8-4) are highly soluble in organic electrolytes and, once solvated, will migrate to the lithium metal anode. This leads to permanent capacity loss and internal shorting via lithium polysulfide redox shuttling. To combat this limitation, extensive efforts have been focused on trapping long-chain LiPS by both: 1. physically confining them in mesoporous carbons, and 2. chemically inducing electrostatic attractions with electron rich heteroatoms on the cathode. In this work we not only aim to find synergy between these two approaches, but also investigate the potential for doubling the binding energy of electrostatic interactions by covalent tethering of long chain LiPS to the cathode surface via reversible di-sulfide bonding. Through a highly tunable one-step reaction, we functionalize mesoporous carbon surfaces with aromatic small molecules using in-situ generation of diazonium radicals. The flexibility of our approach allows for the introduction of a wide variety of surface functionality, and thus a platform to broaden our understanding of cathode/electrolyte interactions. Our current progress focuses on the spectroscopic, physical, and electrochemical characterization of a thiophenol terminated carbon. Through this work we have demonstrated not only successful modification of carbon particles, but an ability to control the density of modifier groups on the surface. In conjunction with finding an optimal concentration of surface modifiers, we investigate the role of modifiers and carbon pore size in modulating the kinetics and capacity retention of LiS cells. By optimizing pore-size and modifier concentration, we have drastically increased the cycle life of our devices, which maintain >900 mAhg -1 over 150+ cycles at 0.1C. We hypothesize that the mechanism responsible for this improvement is a reversible covalent interaction based on electrochemical measurements, which indicate a concentration-dependent shift toward solid-phase reaction pathways.

Carbon fiber–epoxy composites have become prevalent in the aerospace industry where mechanical properties and light weight are at a premium. The significant non-destructive evaluation challenges of composites require new solutions, especially in detecting early-stage, or incipient, thermal damage. The initial stages of thermal damage are chemical rather than physical, and can cause significant reduction in mechanical properties well before physical damage becomes detectable in ultrasonic testing. Thermochromic fluorescent probe molecules have the potential to sense incipient thermal damage more accurately than traditional inspection methods. We have designed a molecule which transitions from a colorless, non-fluorescent state to a colorful, highly fluorescent state when exposed to temperature–time combinations that can cause damage in composites. Moreover, this molecule can be dispersed in a polymer film and attached to composite parts as a removable sensor. This work presents an evaluation of the sensor performance of this thermochromic film in comparison to ultrasonic C-scan as a method to detect incipient thermal damage in one of the most widely used carbon fiber–epoxy composite systems. Composite samples exposed to varying thermal exposures were used to evaluate the fluorescent thermal sensor films, and the results are compared to the results of ultrasonic imaging and short-beam shear tests for interlaminar shear strength.

Freestanding gel electrolytes based on Li(G4)TFSI/PEG are demonstrated with enhanced lithium transport and stripping/plating performance due to unique chemical interactions.

The high chemo-selectivity of bis-aldehydes for condensation with both basic and acidic active methylene compounds facilitates a one-pot, three-component reaction to produce dipolar polyene chromophores as the major products for electro-optics.

Carbon fiber epoxy composites have become prevalent in a variety of industries, especially in aerospace. The significant non-destructive evaluation (NDE) challenges of composites require new solutions, especially in detecting the onset of thermal damage. This work proposes the use of thermochromic fluorescent molecules dispersed in the composites as sensors for such detection. A molecule has been developed which transitions from a colorless, non-fluorescent state to a colorful, highly fluorescent state when exposed to temperature-time combinations that can cause damage in composites. This molecule dispersed in polymer composites of epoxy and PDMS matrices shows unique activation kinetics that can be used to quantitatively simulate the degradation kinetics of aerospace epoxies. The novel sensor materials based on the thermochromic activation of fluorescence can provide highly efficient and widely applicable NDE materials and techniques for carbon fiber epoxy composites.

Compounds with polarizable π systems that are susceptible to attack with nucleophiles at C-Hal (Hal = Cl, Br) bonds react with Pd(PPh3)4 to yield net oxidative addition. X-ray structures show that the resulting Pd(PPh3)2Hal groups greatly reduce intermolecular π-π interactions. The Pd-functionalized dyes generally exhibit solution-like absorption spectra in films, whereas their Hal analogues exhibit features attributable to aggregation.