Conductive Hydrogels with Ultrastretchability and Adhesiveness for Flame- and Cold-Tolerant Strain Sensors.

Hydrogel strain sensors with extreme temperature tolerance have recently gained great attention. However, the sensing ability of these hydrogel strain sensors changes with temperature, resulting in the variety of output signals that causes signal distortion. In this study, double-network hydrogels comprising SiO2 nanoparticles composed of polyacrylamide and phytic acid-doped polypyrrole were prepared and applied on strain sensors with a wide sensing range, high adhesiveness, and invariable strain sensitivity under flame and cold environments. The hydrogels had stable conductivity, excellent adhesive strength of up to 79.7 kPa on various substrates, and high elongation of up to 1896% at subzero temperature and after heating. They also exhibited effective flame retardancy with low surface temperature (71.2 °C) after 1200 s of heating (200 °C) and antifreezing properties at a low temperature of -20 °C. Remarkably, even under cold temperature and heat treatment, the hydrogel-based strain sensor displayed consistent sensing behaviors in detecting human motions with a broad strain range (up to 500%) and steady gauge factor (GF, ∼2.90). Therefore, this work paves the way for the applications of hydrogel sensors in robotic skin, human-mechanical interfaces, and health monitoring devices under harsh operating environments.

Enhancement of All-Polymer Solar Cells by Addition of a Chlorinated Polymer and Formation of an Energy Cascade in a Nonhalogenated Solvent.

Ternary organic solar cells (OSCs) containing a three-component photoactive layer with cascading energy alignments could benefit the charge transfer and improve the open-circuit voltage and power conversion efficiency. Herein, we report the incorporation of a derived chlorinated polymer, J52-Cl, as a guest donor into the donor J52 and acceptor N2200 blend film. The lowest unoccupied molecular orbital and the highest occupied molecular orbital levels of J52-Cl are between the corresponding energy levels of J52 and N2200, and this leads to generation of a cascade of energy levels. Photoluminescence measurements and the J-V of devices containing the donors indicated that this incorporation of J52-Cl could promote the charge transfer of the solar cells. The contribution from J52-Cl reduced the energy loss of J52-based binary devices significantly from 0.932 to 0.797 eV and the nonradiative energy loss from 0.399 to 0.269 eV, leading to an enhancement of Voc from 0.79 to 0.93 V. This introduction of chlorinated polymers also improves the intermolecular interactions and leads to a favorable morphology with appropriate phase separation and interpenetrating networks. As expected, the power conversion efficiency of ternary all-polymer solar cells (all-PSCs) processed in o-xylene solvent was increased from 8.55 to 11.02%. These results indicate that the ternary devices with the appropriate cascade of energy levels can fine tune the device's open-circuit voltage and finally improve the photovoltaic performance of OSCs.

Chlorinated Benzo[1,2-b:4,5-c']dithiophene-4,8-dione Polymer Donor: A Small Atom Makes a Big Difference.

The position of a chlorine atom in a charge carrier of polymer solar cells (PSCs) is important to boost their photovoltaic performance. Herein, two chlorinated D-A conjugated polymers PBBD-Cl-α and PBBD-Cl-ß are synthesized based on two new building blocks (TTO-Cl-α and TTO-Cl-ß) respectively by introducing the chlorine atom into α or ß position of the upper thiophene of the highly electron-deficient benzo[1,2-b:4,5-c']dithiophene-4,8-dione moiety. Single-crystal analysis demonstrates that the chlorine-free TTO shows a π-π stacking distance (d π-π) of 3.55 Å. When H atom at the α position of thiophene of TTO is replaced by Cl, both π-π stacking distance (d π-π = 3.48 Å) and Cl···S distance (d Cl-S = 4.4 Å) are simultaneously reduced for TTO-Cl-α compared with TTO. TTO-Cl-ß then showed the Cl···S non-covalent interaction can further shorten the intermolecular π-π stacking separation to 3.23 Å, much smaller than that of TTO-Cl-α and TTO. After blending with BTP-eC9, PBBD-Cl-ß:BTP-eC9-based PSCs achieved an outstanding power conversion efficiency (PCE) of 16.20%, much higher than PBBD:BTP-eC9 (10.06%) and PBBD-Cl-α:BTP-eC9 (13.35%) based devices. These results provide an effective strategy for design and synthesis of highly efficient donor polymers by precise positioning of the chlorine substitution.

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.

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.

A Benzo[1,2-b:4,5-c']Dithiophene-4,8-Dione-Based Polymer Donor Achieving an Efficiency Over 16.

It is of great significance to develop efficient donor polymers during the rapid development of acceptor materials for nonfullerene bulk-heterojunction (BHJ) polymer solar cells. Herein, a new donor polymer, named PBTT-F, based on a strongly electron-deficient core (5,7-dibromo-2,3-bis(2-ethylhexyl)benzo[1,2-b:4,5-c']dithiophene-4,8-dione, TTDO), is developed through the design of cyclohexane-1,4-dione embedded into a thieno[3,4-b]thiophene (TT) unit. When blended with the acceptor Y6, the PBTT-F-based photovoltaic device exhibits an outstanding power conversion efficiency (PCE) of 16.1% with a very high fill factor (FF) of 77.1%. This polymer also shows high efficiency for a thick-film device, with a PCE of ≈14.2% being realized for an active layer thickness of 190 nm. In addition, the PBTT-F-based polymer solar cells also show good stability after storage for ≈700 h in a glove box, with a high PCE of ≈14.8%, which obviously shows that this kind of polymer is very promising for future commercial applications. This work provides a unique strategy for the molecular synthesis of donor polymers, and these results demonstrate that PBTT-F is a very promising donor polymer for use in polymer solar cells, providing an alternative choice for a variety of fullerene-free acceptor materials for the research community.

Isomer-free: Precise Positioning of Chlorine-Induced Interpenetrating Charge Transfer for Elevated Solar Conversion.

The influence caused by the position of the chlorine atom on end groups of two non-fullerene acceptors (ITIC-2Cl-δ and ITIC-2Cl-γ) was intensely investigated. The single-crystal structures show that ITIC-2Cl-γ has a better molecular planarity and closer π-π interaction distance. More importantly, a 3D rectangle-like interpenetrating network is formed in ITIC-2Cl-γ and is beneficial to rapid charge transfer along multiple directions, whereas only a linear stacked structure could be observed in ITIC-2Cl-δ. The two acceptor-based solar cells show power conversion efficiencies (PCEs) over 11%, higher than that of the ITIC-2Cl-m-based device (10.85%). An excellent PCE of 13.03% is obtained by the ITIC-2Cl-γ-based device. In addition, the ITIC-2Cl-γ-based device also shows the best device stability. This study indicates that chlorine positioning has a great impact on the acceptors; more importantly, the 3D network structure may be a promising strategy for non-fullerene acceptors to improve the PCE and stability of organic solar cells.

Chlorine Atom-Induced Molecular Interlocked Network in a Non-Fullerene Acceptor.

The differences between the introduction of chlorine and fluorine atoms to small-molecule acceptors were deeply investigated. From the single-crystal structures of three molecules, the Cl-substitution intervention into the molecular configuration and packing mainly lies in three aspects as follows: single molecule configuration, one direction of the intermolecular arrangement, and three-dimensional (3D) molecular packing. First, the introduction of the chlorine atom in IDIC-4Cl leads to a more planar molecular configuration than IDIC-4H and IDIC-4F because of the formation of a molecular interlocked network induced by the strong Cl···S intermolecular interactions. Second, IDIC-4Cl shows the closest π-π stacking distance and the smallest dihedral angle (0°) between adjacent molecules to form ideal J-aggregation, which should be beneficial for charge transportation between different connected molecules in this direction. Finally, the interlocked interactions between Cl and S atoms lead to a highly ordered 3D molecular packing, in which the end groups will form an ideal overlapped packing among different molecules, whereas the other two analogues with H or F show less ordered packing of their 1,1-dicyanomethylene-3-indanone ending groups. Organic solar cells based on IDIC-4Cl show the highest power conversion efficiency (PCE) of 9.24%, whereas the PCEs of IDIC-4H- and IDIC-4F-based devices are 4.57 and 7.10%, respectively.

Hydroxyl-Terminated CuInS2-Based Quantum Dots: Potential Cathode Interfacial Modifiers for Efficient Inverted Polymer Solar Cells.

The use of interfacial modifiers on cathode or anode layers can effectively reduce the recombination loss and thus have potential to enhance the device performance of polymer solar cells. In this work, we demonstrated that hydroxyl-terminated CuInS2-based quantum dots could be potential cathode interfacial modifiers on ZnO layer for inverted polymer solar cells. By casting of a thin film of CuInS2-based quantum dots onto ZnO layer, the controlled devices show obvious enhancements of open-circuit voltage, short-circuit current, and fill factor. With an optimized interfacial layer with ∼7 nm thickness, an improvement of power conversion efficiency up to 16% is obtained and the optimized power conversion efficiency of PTB7-based (PTB7: poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl) carbonyl] thieno[3,4-b] thiophenediyl]]) polymer solar cells approaches 8.51%. Detailed analysis shows that the performance enhancement can be explained to the improved light absorption, modified work function, reduced surface roughness, and the increased electron transfer of ZnO cathode interlayer.