Positive Feedback Mechanism of Probe Sonication for the Perovskite Films in Solar Cells.

The performance of perovskite solar cells (PSCs) is governed by the quality of perovskite films, whereby compact, pinhole-free perovskite films are desired, in addition to its composition. We have demonstrated probe sonication as a processing technique to provide positive feedback for enhancing the perovskite film quality and photovoltaic parameters, with two systems, CH3NH3PbI3 (MAPbI3) and Cs0.17FA0.83Pb(I0.83Br0.17)3. In probe sonication, the ultrasound results in the formation, growth, and collapse of the bubbles through shock wave inside the gas phase of the collapsing bubble. This phenomenon has a chemical impact on the nucleation of the perovskite phases and interconnectivity of the grains. The 60 min sonicated films with stronger hydrogen bonding network are devoid of unwanted Pb0, δ-FAPbI3, and PbI2 phases, having tightly packed homogeneous grains, minimum electron-hole recombination pathways, and improved light absorption. The surface potential remains mostly unaltered across the grains and grain boundaries, and the realignment of the Fermi energy (EF) favors facile carrier transport. The photoconversion efficiency (PCE) of the MAPbI3 and Cs0.17FA0.83Pb(I0.83Br0.17)3 devices is improved by 28.1 and 17.2% in comparison to the pristine perovskites, respectively. The 60 min sonicated Cs0.17FA0.83Pb(I0.83Br0.17)3 PSC has 20.20 ± 0.40% PCE with 1000 h ambient stability having >60% retention of the original PCE.

Multimodal Biofilm Inactivation Using a Photocatalytic Bismuth Perovskite-TiO2-Ru(II)polypyridyl-Based Multisite Heterojunction.

Infectious bacterial biofilms are recalcitrant to most antibiotics compared to their planktonic version, and the lack of appropriate therapeutic strategies for mitigating them poses a serious threat to clinical treatment. A ternary heterojunction material derived from a Bi-based perovskite-TiO2 hybrid and a [Ru(2,2'-bpy)2(4,4'-dicarboxy-2,2'-bpy)]2+ (2,2'-bpy, 2,2'-bipyridyl) as a photosensitizer (RuPS) is developed. This hybrid material is found to be capable of generating reactive oxygen species (ROS)/reactive nitrogen species (RNS) upon solar light irradiation. The aligned band edges and effective exciton dynamics between multisite heterojunctions are established by steady-state/time-resolved optical and other spectroscopic studies. Proposed mechanistic pathways for the photocatalytic generation of ROS/RNS are rationalized based on a cascade-redox processes arising from three catalytic centers. These ROS/RNS are utilized to demonstrate a proof-of-concept in treating two elusive bacterial biofilms while maintaining a high level of biocompatibility (IC50 > 1 mg/mL). The in situ generation of radical species (ROS/RNS) upon photoirradiation is established with EPR spectroscopic measurements and colorimetric assays. Experimental results showed improved efficacy toward biofilm inactivation of the ternary heterojunction material as compared to their individual/binary counterparts under solar light irradiation. The multisite heterojunction formation helped with better exciton delocalization for an efficient catalytic biofilm inactivation. This was rationalized based on the favorable exciton dissociation followed by the onset of multiple oxidation and reduction sites in the ternary heterojunction. This together with exceptional photoelectric features of lead-free halide perovskites outlines a proof-of-principle demonstration in biomedical optoelectronics addressing multimodal antibiofilm/antimicrobial modality.

Covalent Organic Framework Thin-Film Photodetectors from Solution-Processable Porous Nanospheres.

The synthesis of homogeneous covalent organic framework (COF) thin films on a desired substrate with decent crystallinity, porosity, and uniform thickness has great potential for optoelectronic applications. We have used a solution-processable sphere transmutation process to synthesize 300 ± 20 nm uniform COF thin films on a 2 × 2 cm2 TiO2-coated fluorine-doped tin oxide (FTO) surface. This process controls the nucleation of COF crystallites and molecular morphology that helps the nanospheres to arrange periodically to form homogeneous COF thin films. We have synthesized four COF thin films (TpDPP, TpEtBt, TpTab, and TpTta) with different functional backbones. In a close agreement between the experiment and density functional theory, the TpEtBr COF film showed the lowest optical band gap (2.26 eV) and highest excited-state lifetime (8.52 ns) among all four COF films. Hence, the TpEtBr COF film can participate in efficient charge generation and separation. We constructed optoelectronic devices having a glass/FTO/TiO2/COF-film/Au architecture, which serves as a model system to study the optoelectronic charge transport properties of COF thin films under dark and illuminated conditions. Visible light with a calibrated intensity of 100 mW cm-2 was used for the excitation of COF thin films. All of the COF thin films exhibit significant photocurrent after illumination with visible light in comparison to the dark. Hence, all of the COF films behave as good photoactive substrates with minimal pinhole defects. The fabricated out-of-plane photodetector device based on the TpEtBr COF thin film exhibits high photocurrent density (2.65 ± 0.24 mA cm-2 at 0.5 V) and hole mobility (8.15 ± 0.64 ×10-3 cm2 V-1 S-1) compared to other as-synthesized films, indicating the best photoactive characteristics.

Cs3Bi2I9 nanodiscs with phase and Bi(III) state stability under reductive potential or illumination for H2 generation from diluted aqueous HI.

The increasingly popular, lead-free perovskite, Cs3Bi2I9 has a vulnerable Bi3+ state under reductive potentials, due to the high standard reduction potential of Bi3+/Biδ+ (0 < δ < 3). Contrary to this fundamental understanding, herein, ligand-coated Cs3Bi2I9 nanodiscs (NDs) demonstrate outstanding electrochemical stability with up to -1 V versus a saturated calomel electrode in aqueous 0.63 M (5% v/v) and 6.34 M (50% v/v) hydroiodic acid (HI), with a minor BiI3 fraction due to the unavoidable partial aqueous disintegration of the perovskite phase after 8 and 16 h, respectively. A dynamic equilibrium of saturated 0.005 M NDs maintains the common ion effect of I-, and remarkably stabilizes ∼93% Bi3+ in 0.63 M HI under a strong reductive potential. In comparison, the hexagonal phase of bulk Cs3Bi2I9 disintegrates considerably in the semi-aqueous media. Lowering the concentration of synthetic HI from the commonly used ∼50% v/v by elevating the pH from -0.8 to 0.2 helps in reducing the cost per unit of H2 production. Our Cs3Bi2I9 NDs with a hexagonal lattice have 4-6 (002) planes stacked along the c-axis. With 0.005 M photostable NDs, 22.5 µmol h-1 H2 is photochemically obtained within 8 h in a 6.34 M HI solution. Electrocatalytic H2 evolution occurs with a turnover frequency of 11.7 H2 per s at -533 mV and outstanding operational stability for more than 20 h.

Magnetic diversity in three-dimensional two-fold-interpenetrated structures: a story of two compounds.

A magnetostructural correlation has been carried out for two newly synthesized two-fold-interpenetrated three-dimensional structures. Compound 1, denoted as [Ni(L1)(L2)]·2DMF (where L1 is 1,4-benzene-dicarboxylic acid (BDC), L2 is 4,4-oxybis-(N-(pyridine-4-yl)benzamide), and DMF is N,N'-dimethylformamide), was observed to have a three-dimensional structure with two-fold interpenetration. Compound 2, denoted as [Co(L3)(L2)]·2DMF (where L3 is 2,5-thiopehene-dicarboxylic acid (TDC)), was also observed to display a three-dimensional structure with an architecture identical to that of compound 1. Both compounds were well characterised using several techniques including single-crystal X-ray diffraction, powder X-ray diffraction, TGA, and IR. Magnetism and specific heat measurements of compound 1 revealed a canted-antiferromagnetic transition at TN ≈ 4 K and a field-induced spin-flop transition at a relatively low field strength. These exotic features were attributed to the low-symmetry space group P(1[combining macron]) and single-ion anisotropy of the Ni2+ sub-lattice. In contrast, compound 2 was found to be weakly antiferromagnetic in nature with a negligible interaction between the magnetic Co2+ ions.

Transformation of One-Dimensional Achiral Structure to Three-Dimensional Chiral Structure: Mechanistic Study and Catalytic Activities of Chiral Structure.

We have isolated two copper-based coordination polymers through solvent diffusion and solvothermal methods using copper salt, furan dicarboxylic acid (FDC), 4,4'-bipyridine (bpy) in MeOH/ethylene glycol, and water solvents. Compound 1 is adopting P21/c space group and adopts a one-dimensional wirelike structure with a free carboxylate anion. Compound 2 crystallizes in chiral space group P65. This is a three-dimensional structure with helical chains. This helicity might be the reason for chiral generation and symmetry breaking. We have converted compound 1 to compound 2 using grinding, followed by a solvothermal method. The circular dichroism data of 2 showed that it is an enantioenriched compound. We have shown that compound 2 is a very good catalyst for chemo- and regioselective enamination reaction and for azide-alkyne Huisgen cycloaddition, respectively.