Mohammed Al-Hashimi

Activated Singlet Exciton Fission in a Semiconducting Polymer

Singlet exciton fission is a spin-allowed process to generate two triplet excitons from a single absorbed photon. This phenomenon offers great potential in organic photovoltaics, but the mechanism remains poorly understood. Most reports to date have addressed intermolecular fission within small-molecular crystals. However, through appropriate chemical design chromophores capable of intramolecular fission can also be produced. Here we directly observe sub-100 fs activated singlet fission in a semiconducting poly(thienylenevinylene). We demonstrate that fission proceeds directly from the initial 1Bu exciton, contrary to current models that involve the lower-lying 2Ag exciton. In solution, the generated triplet pairs rapidly recombine and decay through the 2Ag state. In films, exciton diffusion breaks this symmetry and we observe long-lived triplets which form charge-transfer states in photovoltaic blends.

Facile infiltration of semiconducting polymer into mesoporous electrodes for hybrid solar cells

Hybrid composites of semiconducting polymers and metal oxides are promising combinations for solar cells. However, forming a well-controlled nanostructure with bicontinuous interpenetrating networks throughout the photoactive film is difficult to achieve. Pre-structured “mesoporous” metal oxide electrodes can act as a well-defined template for latter polymer infiltration. However, the long range infiltration of polymer chains into contorted porous channels has appeared to elude the scientific community, limiting the advancement of this technology. Here we present a structural and electronic characterisation of poly(3-hexylthiophene) (P3HT) infiltrated into mesoporous dye-sensitized TiO2. Through a combination of techniques we achieve uniform pore filling of P3HT up to depths of over 4 μm, but the volumetric fraction of the pores filled with polymer is less than 24%. Despite this low pore-filling, exceptionally efficient charge collection is demonstrated, illustrating that pore filling is not the critical issue for mesoporous hybrid solar cells.

Controlled synthesis of conjugated random copolymers in a droplet-based microreactor

We report the highly controlled synthesis of conjugated random copolymers in a droplet-based microfluidic reactor. Using two optically distinct polymers, poly(3-hexylthiophene) (P3HT) and poly(3-hexylselenophene) (P3HS), a series of highly regioregular random copolymers is generated with physical properties intermediate to those of the parent homopolymers. Analysis by 1H nuclear magnetic resonance spectroscopy reveals the co-polymerisation process to follow ideal Bernoullian behavior.

Synthesis of Polypentenamer and Poly(Vinyl Alcohol) with a Phase-Separable Polyisobutylene-Supported Second-Generation Hoveyda-Grubbs Catalyst

Equilibrium ring‐opening metathesis polymerization (ROMP) of cyclic olefins using a soluble supported second‐generation Ru complex has been investigated. Cycloolefin homo‐ and copolymers are of great academic and industrial importance owing to their interesting applications as packaging materials, adhesives in coatings, and optoelectronics. The supported complex exhibits good chemical stability and was effective in ROMP of strained cyclic olefins. In addition, the complex is easily phase separated from the product, resulting in lower residual ruthenium in the final polymer product compared with the homogeneous complex.

Ring-opening metathesis polymerization using polyisobutylene supported Grubbs second-generation catalyst

Olefin metathesis is among the most powerful tools for the formation of regio- and steroselective carbon–carbon double bonds. Applying the principles of Green Chemistry to the syntheses of polymers by developing useful strategies to facilitate catalyst and polymer product separation after a polymerization is vital. In the present study, a phase selectively soluble polymer bound second generation Grubbs catalyst was successfully used to carry out ring-opening metathesis polymerization (ROMP) on norbornene and a variety of different exo-norbornene derivatives. Polymers with low ruthenium contamination levels were achieved in comparison to the non-supported Grubbs catalyst which required multiple precipitations. Furthermore, the bound catalyst exhibits similar catalytic activity to its homogenous counterpart.

Ruthenium-Catalyzed Metathesis of Conjugated Polyenes

In the past decade, numerous examples of chemical technologies based on olefin metathesis have been developed to make olefin metathesis increasingly dominant in several sustainable and green chemical processes. In spite of the wide application profile, conjugated olefin metathesis, especially conjugated polyene metathesis, is an area of great interest with little exploration. The metathesis of conjugated polyenes is often cumbersome and requires a high catalyst loading, most probably because of the formation of poorly active or inactive ruthenium η3‐vinylcarbene intermediates. A mechanistic understanding and the development of a new highly active catalytic system for olefin metathesis will open new areas for exploration, such as the utilisation of cyclopentadiene and other petrochemical by‐products or a new way to use butadiene, isoprene and conjugated electron systems that contain natural products such as terpenes and polyunsaturated fatty acids. An understanding of the mechanism of ruthenium η1–η3‐vinylcarbene interconversion may open the way to the development of a new generation of Ru‐based latent metathesis catalyst systems. This review summarises the most relevant pioneering work focused on the metathesis of conjugated polyenes to open new ideas for the development of forthcoming latent metathesis catalysts and to explore different applications.

Bithiazole: An Intriguing Electron-Deficient Building for Plastic Electronic Applications

The heterocyclic thiazole unit has been extensively used as electron-deficient building block in π-conjugated materials over the last decade. Its incorporation into organic semiconducting materials is particularly interesting due to its structural resemblance to the more commonly used thiophene building block, thus allowing the optoelectronic properties of a material to be tuned without significantly perturbing its molecular structure. Here, we discuss the structural differences between thiazole- and thiophene-based organic semiconductors, and the effects on the physical properties of the materials. An overview of thiazole-based polymers is provided, which have emerged over the past decade for organic electronic applications and it is discussed how the incorporation of thiazole has affected the device performance of organic solar cells and organic field-effect transistors. Finally, in conclusion, an outlook is presented on how thiazole-based polymers can be incorporated into all-electron deficient polymers in order to obtain high-performance acceptor polymers for use in bulk-heterojunction solar cells and as organic field-effect transistors. Computational methods are used to discuss some newly designed acceptor building blocks that have the potential to be polymerized with a fused bithiazole moiety, hence propelling the advancement of air-stable n-type organic semiconductors.

Donor–acceptor conjugated ladder polymer via aromatization-driven thermodynamic annulation

Composed of alternating electron-rich and electron-deficient repeating units in a fused rigid backbone, donor–acceptor conjugated ladder polymers represent a class of promising material candidates possessing a host of intriguing properties. These polymers are challenging to synthesize because of the typically low reaction efficiency of electron-deficient acceptor units and the possibility of defect formation during the postpolymerization annulation. Herein we report the synthesis of a well-defined donor–acceptor ladder polymer, which overcame these challenges by utilizing aromatization-driven thermodynamic ring-closing olefin metathesis. The good solubility of the polymer allowed for the comprehensive investigation of its optical properties, intramolecular charge transfer, complex formation with dopant, and thin film processing. These results provide a foundation for future applications of donor–acceptor ladder polymers in electronic and optoelectronic devices.

High-k Gate Dielectrics for Emerging Flexible and Stretchable Electronics

Recent advances in flexible and stretchable electronics (FSE), a technology diverging from the conventional rigid silicon technology, have stimulated fundamental scientific and technological research efforts. FSE aims at enabling disruptive applications such as flexible displays, wearable sensors, printed RFID tags on packaging, electronics on skin/organs, and Internet-of-things as well as possibly reducing the cost of electronic device fabrication. Thus, the key materials components of electronics, the semiconductor, the dielectric, and the conductor as well as the passive (substrate, planarization, passivation, and encapsulation layers) must exhibit electrical performance and mechanical properties compatible with FSE components and products. In this review, we summarize and analyze recent advances in materials concepts as well as in thin-film fabrication techniques for high- k (or high-capacitance) gate dielectrics when integrated with FSE-compatible semiconductors such as organics, metal oxides, quantum dot arrays, carbon nanotubes, graphene, and other 2D semiconductors. Since thin-film transistors (TFTs) are the key enablers of FSE devices, we discuss TFT structures and operation mechanisms after a discussion on the needs and general requirements of gate dielectrics. Also, the advantages of high- k dielectrics over low- k ones in TFT applications were elaborated. Next, after presenting the design and properties of high- k polymers and inorganic, electrolyte, and hybrid dielectric families, we focus on the most important fabrication methodologies for their deposition as TFT gate dielectric thin films. Furthermore, we provide a detailed summary of recent progress in performance of FSE TFTs based on these high- k dielectrics, focusing primarily on emerging semiconductor types. Finally, we conclude with an outlook and challenges section.

Synthesis of low band gap polymers based on pyrrolo[3,2-d: 4,5-d′]bisthiazole (PBTz) and thienylenevinylene (TV) for organic thin-film transistors (OTFTs)

New low band gap copolymers P1–P4, based on thienylenevinylene (TV) and pyrrolo[3,2-d:4,5-d′]bisthiazole (PBTz) units composed of different alkyl side chains, such as 2-octyldodecyl (OD), n-hexadecyl (HD), 2-ethylhexyl (EH), and 9-heptadecyl (HD) groups, respectively, have been synthesized and characterized. Electrochemical and optical studies of the copolymers indicated low energy band gaps in the range of 1.40–1.47 eV. Moreover, theoretical calculation with density functional theory (DFT) and time-dependent DFT calculations demonstrated that the energy band gaps, HOMO energy levels and maximum absorption values in the copolymers were in good agreement with the experimental results. The decomposition temperature of all copolymers was measured to be above 340 °C by thermogravimetric analysis (TGA), which indicates high thermal stability. Thermally annealed OTFT devices based on P1–P4 thin films demonstrated a range of hole mobilities; thus, the P2 based OTFT device exhibited the highest hole mobility of 0.062 cm2 V−1 s−1.