Direct Observation of Structure and Dynamics of Photogenerated Charge Carriers in Poly(3-hexylthiophene) Films by Femtosecond Time-Resolved Near-IR Inverse Raman Spectroscopy

Green industry work: production of FeCl3 from iron and steel industry waste (mill scale) and its use in wastewater treatment

Mill scale (MS) is considered to be a significant metallurgical waste, but there is no economical method yet to utilize its metal content. In this study, which covers various processes in several stages, the solution of iron in MS, which is the Iron and Steel Industry (I&SI) waste, as FeCl3 (MS-FeCl3) in the thermoreactor in the presence of HCl, was investigated. In the next step, the conditions for using this solution as a coagulant in the treatment of I&SI wastewater were investigated using the jar test. The results of the treated water sample were compared by chemical oxygen demand (COD), total suspended solids (TSS), color, and turbidity analyses using commercial aluminum sulfate (Al2(SO4)3) and FeCl3 (C-FeCl3). Additionally, heavy metal analyses were conducted, and the treatment performance of three coagulants was presented. Accordingly, while 2.0 mg/L anionic polyelectrolyte was consumed at a dosage of 4.05 mg/L Al2(SO4)3 at pH 7.0, 0.25 mg/L anionic polyelectrolyte was consumed at a dosage of 1.29 mg/L at pH 5.0 in the C-FeCl3 and MS-FeCl3 studies. Also, Fe, Cr, Mn, Ni, Zn, Cd, Hg, and Pb removal efficiencies were over 93.56% for all three coagulant usage cases. The results showed that the wastewater treatment performance of MS-FeCl3 by the recycling of MS, which is an I&SI waste, was at the same level as C-FeCl3. Thus, thanks to recycling, waste scale can be used as an alternative to commercial products for green production.

Blade Coating Poly(3-hexylthiophene): The Importance of Molecular Weight on Thin-Film Microstructures

Poly(3-hexylthiophene) is one of the most prevalent and promising conjugated polymers for use in organic electronics. However, the deposition of this material in thin films is highly dependent on the process, such as blade coating versus spin coating and material properties such as molecular weight. Typically, large polymer dispersity makes it difficult to isolate the effect of molecular weight without considering a distribution. In this study, we characterize oligothiophenes of exactly 8, 11, and 14 repeat units, which were deposited into thin films by varying blade coating conditions and postdeposition annealing. From synchrotron-based grazing incidence wide-angle X-ray scattering (GIWAXS), scanning transmission X-ray microscopy (STXM) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS), Raman microscopy, optical microscopy, and X-ray diffraction (XRD), it was suggested that higher molecular weight polymers exhibit a fast-forming crystalline polymorph (form-1) while low molecular weight polymers exhibit a slow forming polymorph (form-2) with large domain boundaries. As molecular weight is gradually increased, the polymorph formed transitions from form-1 and form-2, where 11 repeat unit oligomers display both polymorphs. We also found that processing conditions can increase the formation of the form-2 polymorph. We also report improved organic thin film transistor (OTFT) performance when form-1 is present. Overall, oligothiophene polymorph formation is highly dependent on the molecular weight and processing conditions, providing critical insight into the importance of polymer weight control in the development of thin-film electronics based on conjugated polymers.

Unraveling Excited State Dynamics of a Single-Stranded DNA-Assembled Conjugated Polyelectrolyte

Conformational templating of conjugated polyelectrolytes with single-stranded DNAs (ssDNAs) has the prospect of tailoring excited state dynamics for specific optoelectronic applications. We use ultrafast time-resolved infrared spectroscopy to study the photophysics of a cationic polythiophene assembled with different ssDNAs, inducing distinct conformations (flexible disordered structures vs more rigid complexes with increased backbone planarity). Intrachain polarons are always produced upon selective excitation of the polymer, the extent being dependent on backbone torsional order. Polaron formation and decay were monitored through evolution of IR-active vibrational modes that interfere with mid-IR polaron electronic absorption giving rise to Fano-antiresonances. Selective UV excitation of ssDNAs revealed that stacking interactions between thiophene rings and nucleic acid bases can promote the formation of an intermolecular charge transfer complex. The findings inform designers of functional conjugated polymers by identifying that involvement of the scaffold in the photophysics needs to be considered when developing such structures for optoelectronic applications.

Time-Resolved Raman Spectroscopy of Polaron Formation in a Polymer Photocatalyst

Polymer photocatalysts are a synthetically diverse class of materials that can be used for the production of solar fuels such as H₂, but the underlying mechanisms by which they operate are poorly understood. Time-resolved vibrational spectroscopy provides a powerful structure-specific probe of photogenerated species. Here we report the use of time-resolved resonance Raman (TR³) spectroscopy to study the formation of polaron pairs and electron polarons in one of the most active linear polymer photocatalysts for H₂ production, poly(dibenzo[b,d]thiophene sulfone), P10. We identify that polaron-pair formation prior to thermalization of the initially generated excited states is an important pathway for the generation of long-lived photoelectrons.

Mode-Specific Vibrational Analysis of Exciton Delocalization and Structural Dynamics in Conjugated Oligomers

Exciton delocalization in organic semiconducting polymers, affected by structures at a molecular level, plays a crucial role in modulating relaxation pathways, such as charge generation and singlet fission, which can boost device efficiency. However, the structural diversity of polymers and broad signals from typical electronic spectroscopy have their limits when it comes to revealing the interplay between local structures and exciton delocalization. To tackle these problems, we apply femtosecond stimulated Raman spectroscopy in archetypical conjugated oligothiophenes with different chain lengths. We observed Raman frequency dispersions of symmetric bond stretching modes and mode-specific kinetics in the S₁ Raman spectra, which underpins the subtle and complex interplay between exciton delocalization and bond length alternation along the conjugation coordinate. Our results provide a more general picture of exciton delocalization in the context of molecular structures for conjugated materials.

Identification of the local electrical properties of crystalline and amorphous domains in electrochemically doped conjugated polymer thin films

Doped polymer thin films have several applications in electronic, optoelectronic and thermoelectric devices. Often the electrical properties of doped conjugated polymer thin films are affected by their local physical and mechanical characteristics. However, investigations into the effects of doping on local domain properties have not been carried out. Here, we study the physical, mechanical and optical properties of electrochemically doped P3HT thin films at the nanoscale and establish a relation between doping level and the physical properties of P3HT thin films. Bulk crystallinity of both pristine and doped P3HT thin films, characterized using grazing incidence X-ray diffraction, shows a clear loss in crystallinity upon doping. Nanoscale crystalline and amorphous domains in the films are visualized by multimode atomic force microscopy (AFM). It is apparent that the crystalline domains are most affected by doping and have a higher degree of doping compared to amorphous domains. This results in crystalline domains exhibiting superior electrical conductivity at a local level. These results are further supported by Raman mapping and elemental analysis of doped films. A direct relation is established between the physical, mechanical and electrical properties of doped P3HT thin films based on the AFM data. The findings demonstrate that higher dopant concentrations are found in crystalline domains compared to amorphous domains, which has not been shown before to the best of our knowledge. This study can be used to optimize the electronic properties of doped P3HT thin films for use in electronic and optoelectronic device applications.