Stretching of immersed polyelectrolyte brushes in shear flow.

The way that polymer brushes respond to shear flow has important implications in various applications, including antifouling, corrosion protection, and stimuli-responsive materials. However, there is still much to learn about the behaviours and mechanisms that govern these responses. To address this gap in knowledge, our study uses in situ X-ray reflectivity to investigate how poly(styrene sulfonate) (PSS) brushes stretch and change in different environments, such as isopropanol (a poor solvent), water (a good solvent), and aqueous solutions containing various cations (Cs+, Ba2+, La3+, and Y3+). We have designed a custom apparatus that exposes the PSS brushes to both tangential shear forces from the primary flow and upward drag forces from a secondary flow. Our experimental findings clearly show that shear forces have a significant impact on how the chains in PSS brushes are arranged. At low shear rates, the tangential shear force causes the chains to tilt, leading to brush contraction. In contrast, higher shear rates generate an upward shear force that stretches and expands the chains. By analysing electron density profiles obtained from X-ray reflectivity, we gain valuable insights into how the PSS brushes respond structurally, especially the role of the diffuse layer in this dynamic behaviour. Our results highlight the importance of the initial chain configuration, which is influenced by the solvent and cations present, in shaping how polymer brushes respond to shear flow. The strength of the salt bridge network also plays a crucial role in determining how easily the brushes can stretch, with stronger networks offering more resistance to stretching. Ultimately, our study aims to enhance our understanding of polymer physics at interfaces, with a particular focus on practical applications involving polymer brushes.

Improving Thermal and Photostability of Polymer Solar Cells by Robust Interface Engineering.

As the power conversion efficiency (PCE) of organic photovoltaics (OPVs) approaches 19%, increasing research attention is being paid to enhancing the device's long-term stability. In this study, a robust interface engineering of graphene oxide nanosheets (GNS) is expounded on improving the thermal and photostability of non-fullerene bulk-heterojunction (NFA BHJ) OPVs to a practical level. Three distinct GNSs (GNS, N-doped GNS (N-GNS), and N,S-doped GNS (NS-GNS)) synthesized through a pyrolysis method are applied as the ZnO modifier in inverted OPVs. The results reveal that the GNS modification introduces passivation and dipole effects to enable better energy-level alignment and to facilitate charge transfer across the ZnO/BHJ interface. Besides, it optimizes the BHJ morphology of the photoactive layer, and the N,S doping of GNS further enhances the interaction with the photoactive components to enable a more idea BHJ morphology. Consequently, the NS-GNS device delivers enhanced performance from 14.5% (control device) to 16.5%. Moreover, the thermally/chemically stable GNS is shown to stabilize the morphology of the ZnO electron transport layer (ETL) and to endow the BHJ morphology of the photoactive layer grown atop with a more stable thermodynamic property. This largely reduces the microstructure changes and the associated charge recombination in the BHJ layer under constant thermal/light stresses. Finally, the NS-GNS device is demonstrated to exhibit an impressive T80 lifetime (time at which PCE of the device decays to 80% of the initial PCE) of 2712 h under a constant thermal condition at 65 °C in a glovebox and an outstanding photostability with a T80 lifetime of 2000 h under constant AM1.5G 1-sun illumination in an N2 -controlled environment.

Cesium Lead Halide Perovskite Nanocrystals Assembled in Metal-Organic Frameworks for Stable Blue Light Emitting Diodes.

All inorganic cesium lead trihalide nanocrystals are promising light emitters for bright light emitting diodes (LEDs). Here, CsPb(BrCl)1.5 nanocrystals in metal-organic frameworks (MOF) thin films are demonstrated to achieve bright and stable blue LEDs. The lead metal nodes in the MOF thin film react with Cs-halide salts, resulting in 10-20 nm nanocrystals. This is revealed by X-ray scattering and transmission electron microscopy. Employing the CsPbX3 -MOF thin films as emission layers, bright deep blue and sky-blue LEDs are demonstrated that emit at 452 and 476 nm respectively. The maximum external quantum efficiencies of these devices are 0.72% for deep blue LEDs and 5.6% for sky blue LEDs. More importantly, the device can maintain 50% of its original electroluminescence (T50 ) for 2.23 h when driving at 4.2 V. Detailed optical spectroscopy and time-of-flight secondary ion mass spectroscopy suggest that the ion migration can be suppressed that maintains the emission brightness and spectra. The study provides a new route for fabricating stable blue light emitting diodes with all-inorganic perovskite nanocrystals.

Quasi-2D Perovskite Crystalline Layers for Printable Direct Conversion X-Ray Imaging.

Polycrystalline perovskite film-based X-ray detector is an appealing technology for assembling large scale imager by printing methods. However, thick crystalline layer without trap and solvent residual is challenging to fabricate. Here, the authors report a solution method to produce high quality quasi-2D perovskite crystalline layers and detectors that are suitable for X-ray imaging. By introducing n-butylamine iodide into methylammonium lead iodide precursor and coating at elevated temperatures, compact and crystalline layers with exceptional uniformity are obtained on both rigid and flexible substrates. Photodiodes built with the quasi-2D layers exhibit a low dark current and stable operation under constant electrical field over 96 h in dark, and over 15 h under X-ray irradiation. The detector responds sensitively under X-ray, delivering a high sensitivity of 1214 µC Gyair -1  cm-2 and a sensitivity gain is observed when operated under higher fields. Finally, high resolution images are demonstrated using a single pixel device that can resolve 80-200 µm features. This work paves the path for printable direct conversion X-ray imager development.

Facile Fabrication of Self-Assembly Functionalized Polythiophene Hole Transporting Layer for High Performance Perovskite Solar Cells.

Crystallinity and crystal orientation have a predominant impact on a materials' semiconducting properties, thus it is essential to manipulate the microstructure arrangements for desired semiconducting device performance. Here, ultra-uniform hole-transporting material (HTM) by self-assembling COOH-functionalized P3HT (P3HT-COOH) is fabricated, on which near single crystal quality perovskite thin film can be grown. In particular, the self-assembly approach facilitates the P3HT-COOH molecules to form an ordered and homogeneous monolayer on top of the indium tin oxide (ITO) electrode facilitate the perovskite crystalline film growth with high quality and preferred orientations. After detailed spectroscopy and device characterizations, it is found that the carboxylic acid anchoring groups can down-shift the work function and passivate the ITO surface, retarding the interface carrier recombination. As a result, the device made with the self-assembled HTM show high open-circuit voltage over 1.10 V and extend the lifetime over 4,300 h when storing at 30% relative humidity. Moreover, the cell works efficiently under much reduced light power, making it useful as power source under dim-light conditions. The demonstration suggests a new facile way of fabricating monolayer HTM for high efficiency perovskite devices, as well as the interconnecting layer needed for tandem cell.

Effects of Halogenated End Groups on the Performance of Nonfullerene Acceptors.

The end groups' halogenations among the nonfullerene acceptors (NFAs) were a very useful method to fabricate high-performance NFAs-based organic solar cells (OSCs). We report three high-performance NFAs, BTIC-4EO-4F, BTIC-4EO-4Cl, and BTIC-4EO-4Br. They all have a fused benzothiadiazole as the core unit and different dihalogenated end groups (IC-2F, IC-2Cl, and IC-2Br) as the terminal unit. Thanks to the improved intramolecular charge-transfer ability of the brominated NFAs, bromination is more effective than fluorination and chlorination in lowering the energy levels and red-shifting the absorption spectra of the resulting NFAs. When compared with the chlorinated and fluorinated counterparts, the BTIC-4EO-4Br blend films exhibit lower roughness, better phase separation size, and stronger face-on stacking. When blended with poly{[4,8-bis[5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5-b']-dithiophene-2,6-diyl]-alt-[2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c']dithiophene-1,3-diyl]]} (PBDB-TF) as the polymer donor material, the BTIC-4EO-4Br-based OSCs exhibit the highest power conversion efficiency (12.41%), with a higher current density and a higher open-circuit voltage than the BTIC-4EO-4Cl-based OSCs (11.29%) and BTIC-4EO-4F-based OSCs (10.64%). These results show that the bromination of the NFAs' electron-withdrawing end groups can also be very effective in constructing high-performance photovoltaic materials.

Crown Ether Modulation Enables over 23% Efficient Formamidinium-Based Perovskite Solar Cells.

The use of molecular modulators to reduce the defect density at the surface and grain boundaries of perovskite materials has been demonstrated to be an effective approach to enhance the photovoltaic performance and device stability of perovskite solar cells. Herein, we employ crown ethers to modulate perovskite films, affording passivation of undercoordinated surface defects. This interaction has been elucidated by solid-state nuclear magnetic resonance and density functional theory calculations. The crown ether hosts induce the formation of host-guest complexes on the surface of the perovskite films, which reduces the concentration of surface electronic defects and suppresses nonradiative recombination by 40%, while minimizing moisture permeation. As a result, we achieved substantially improved photovoltaic performance with power conversion efficiencies exceeding 23%, accompanied by enhanced stability under ambient and operational conditions. This work opens a new avenue to improve the performance and stability of perovskite-based optoelectronic devices through supramolecular chemistry.

Synergistic Effect of Alkyl Chain and Chlorination Engineering on High-Performance Nonfullerene Acceptors.

In this work, three new nonfullerene acceptors (BT6IC-BO-4Cl, BT6IC-HD-4Cl, and BT6IC-OD-4Cl), which comprise a central fused benzothiadiazole core and two dichlorinated end groups and substituted with different branched alkyl chains [2-butyloctyl (BO), longer 2-hexyldecyl (HD), and 2-octyldodecyl (OD)], are successfully designed and prepared. The influences of the branched alkyl chain with different lengths on the electronic/optoelectronic property, electrochemistry, and photovoltaic performance are systematically investigated. It has been revealed that BT6IC-HD-4Cl, which had the medium alkyl chain (2-hexyldecyl) length, has the best photovoltaic performance when using PDBT-TF as the electron donor. The BT6IC-HD-4Cl-based device shows an impressive power conversion efficiency of 14.90%, much higher than BT6IC-BO-4Cl (14.45%)- and BT6IC-OD-4Cl (9.60%)-based devices. All these evidence shows that the subtle changes in the alkyl substituent of these high-performance chlorinated acceptors can have a big impact on the structural order and molecular packing of the resultant nonfullerene acceptors and ultimately on the photovoltaic performance of the final solar devices.