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 1,1dicyanomethylene-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.
Brominated A–D–A-type small-molecule acceptor ITIC-2Br-γ with certain molecular structure was designed and synthesized. Compared to the mixture of three isomers (ITIC-2Br-m), ITIC-2Br-γ shows stronger absorption in the region of 600–800 nm, which is beneficial to afford higher J SC. Furthermore, single-crystal structure analysis of ITIC-2Br-γ indicates that although the bromine atom has a larger size, the end groups of adjacent molecules still exhibit strong interactions with short π–π distance of 3.28 Å. Because of the Br···S and O···S interactions, all molecules form an interpenetrated three-dimensional network, which is beneficial for the charge to transport along multidirections. The organic solar cells (OSCs) based on the PBDB-T-2F:ITIC-2Br-γ blend film exhibit a higher power conversion efficiency (PCE) of 12.05% due to its better film morphology and higher charge mobilities, whereas a PBDB-T-2F:ITIC-2Br-m-based device only shows a moderate PCE of 10.88%. These results indicate that separation and purification of the brominated A–D–A-type small molecules are an effective way to further improve their photovoltaic performances. Furthermore, bromination is easily synthesized and is of low-cost, which exhibits great potential for the preparation of large-scale OSCs.
Two small molecule acceptors with chlorinated IC as end groups and 10-ring- and 12-ring-fused cores as central units, named R10-4Cl and R12-4Cl, were designed and synthesized, which exhibit low optical band gaps of 1.43 and 1.35 eV, respectively. X-ray crystallographic analysis of R10-4Cl shows that the end groups of adjacent molecules are parallel and partially overlap with a short π–π distance of 3.32 Å, which is helpful for electron transport in this direction. At the same time, there is another type of molecular orientation that lies in these two molecules with an angle about 64.7° because of the close contact of S···O with a distance of 3.15 Å. The two types of molecular arrangements result in an interpenetrated network structure in R10-4Cl films, which is helpful for the rapid charge transfer either along the horizontal direction or the sloping direction. After blending with a PBDB-T polymer donor, the R10-4Cl-based device shows wide photocurrent response from the visible to near-infrared regions, resulting in the better usage of the sunlight source. Benefited from this comprehensive solar energy absorption and the interpenetrated charge transfer, the R10-4Cl-based devices show a power conversion up to 10.7% with an improved J SC of 18.9 mA cm–2.
Poly-and perfluoroalkyl substances (PFAS) are known as "forever chemicals" due to their ubiquitous persistence in the environment, and their negative human health effects. Among them, short-chain PFAS are of increasing concern due to their high solubility and mobility in water, while possessing persistency and toxicity nature like their longer-chain analogs. The most common method for PFAS removal from water is by sorption with activated carbons or ion exchange resins, but these adsorbents only exhibit limited removal efficiency against short-chain PFAS, and they require frequent replacement leading to high operational cost. Here we review and discuss the potential of using bio-adsorbents, which can be derived from common biomass feedstocks, as low-cost alternatives to traditional adsorbents, while these materials can also possess good removal efficiency against short-chain PFAS. We further provide the perspective on the designs of low-cost, activated bio-adsorbent systems that can be implemented for effective removal of short-chain PFAS.
Although most manufacturers stopped using long-chain per- and polyfluoroalkyl substances (PFASs), including perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), short-chain PFASs are still widely employed. Short-chain PFASs are less known...
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