Seelam Prasanthkumar

Self-Assembled Gelators for Organic Electronics

Nature excels at engineering materials by using the principles of chemical synthesis and molecular self-assembly with the help of noncovalent forces. Learning from these phenomena, scientists have been able to create a variety of self-assembled artificial materials of different size, shapes, and properties for wide ranging applications. An area of great interest in this regard is solvent-assisted gel formation with functional organic molecules, thus leading to one-dimensional fibers. Such fibers have improved electronic properties and are potential soft materials for organic electronic devices, particularly in bulk heterojunction solar cells. Described herein is how molecular self-assembly, which was originally proposed as a simple laboratory curiosity, has helped the evolution of a variety of soft functional materials useful for advanced electronic devices such as organic field-effect transistors and organic solar cells. Highlights on some of the recent developments are discussed.

Solution phase epitaxial self-assembly and high charge-carrier mobility nanofibers of semiconducting molecular gelators

Trithienylenevinylenes having amide end functional groups form supramolecular gels in nonpolar solvents, comprised of self-assembled nanowires. These gels exhibit the unique property of solution phase epitaxy leading to the alignment of fibers on mica surface. FP-TRMC studies revealed high charge carrier mobility for xerogels from decane-chloroform whereas films obtained from chloroform solutions showed less mobility, highlighting the role of self-assembly and gelation on the electronic properties of semiconducting molecular gelators. This study opens the window for a new class of conducting gelators which may find wide application in organic electronic devices.

Self-Assembly of Thienylenevinylene Molecular Wires to Semiconducting Gels with Doped Metallic Conductivity

Oligo(thienylenevinylene) (OTV) based gelators with high conductivity are reported. When compared to OTV1, OTV2 having an increased conjugation length forms relatively strong gels with a metallic conductivity of 4.8 S/cm upon doping which is the highest value reported for an organogelator. This new class of conducting gels is expected to be useful for organic electronics and photonics application, particularly for bulk heterojunction devices.

Self-Assembly of Oligo(para-phenylenevinylene)s through Arene—Perfluoroarene Interactions: π Gels with Longitudinally Controlled Fiber Growth and Supramolecular Exciplex-Mediated Enhanced Emission

The arene-perfluoroarene (ArH-ArF) interaction, which has been extensively studied in the field of solid-state chemistry, is exploited in the hierarchical self-assembly of oligo(para-phenylenevinylene)s (OPVs) with controlled longitudinal fiber growth that leads to gelation. The size of the self-assembled fibers of a pentafluorophenyl-functionalized OPV 5 could be controlled through C-FH--C hydrogen bonding and pi stacking. The ability of fluoroaromatic compounds to form excited-state complexes with aromatic amines has been utilized to form a supramolecular exciplex, exclusively in the gel state, that exhibits enhanced emission. Thus, the commonly encountered fluorescence quenching during the self-assembly of OPVs could be considerably prevented by exciplex formation with N,N-dimethylaniline (DMA), which only occurred for the fluorinated OPV and not for the non-fluorinated analogue 4. In the former case, a threefold enhancement in the emission intensity could be observed in the gel state, whereas no change in emission occurred in solution. Thus, the major limitations of spontaneous fiber growth and fluorescence self-quenching encountered in the self-assembly of OPVs could be controlled to a great extent by using the versatile ArH-ArF interaction.

Organic Donor–Acceptor Assemblies form Coaxial p–n Heterojunctions with High Photoconductivity†

The formation of coaxial p-n heterojunctions by mesoscale alignment of self-sorted donor and acceptor molecules, important to achieve high photocurrent generation in organic semiconductor-based assemblies, remains a challenging topic. Herein, we show that mixing a p-type πu2005gelator (TTV) with an n-type semiconductor (PBI) results in the formation of self-sorted fibers which are coaxially aligned to form interfacial p-n heterojunctions. UV/Vis absorption spectroscopy, powder X-ray diffraction studies, atomic force microscopy, and Kelvin-probe force microscopy revealed an initial self-sorting at the molecular level and a subsequent mesoscale self-assembly of the resulted supramolecular fibers leading to coaxially aligned p-n heterojunctions. A flash photolysis time-resolved microwave conductivity (FP-TRMC) study revealed a 12-fold enhancement in the anisotropic photoconductivity of TTV/PBI coaxial fibers when compared to the individual assemblies of the donor/acceptor molecules.

Emerging of Inorganic Hole Transporting Materials For Perovskite Solar Cells

Hole transporting material (HTM) is a significant component to achieve the high performance perovskite solar cells (PSCs). Over the years, inorganic, organic and hybrid (organic-inorganic) material based HTMs have been developed and investigated successfully. Today, perovskite solar cells achieved the efficiency of 22.1u2009% with with 2,2,7,7-tetrakis(N,N-di-p-methoxyphenyl-amine) 9,9-spirobifluorene (spiro-OMeTAD) as HTM. Nevertheless, synthesis and cost of organic HTMs is a major challenging issue and therefore alternative materials are required. From the past few years, inorganic HTMs showed large improvement in power conversion efficiency (PCE) and stability. Recently CuOx reached the PCE of 19.0% with better stability. These developments affirms that inorganic HTMs are better alternativesto the organic HTMs for next generation PSCs. In this report, we mainly focussed on the recent advances of inorganic and hybrid HTMs for PSCs and highlighted the efficiency and stability of PSCs improved by changing metal oxides as HTMs. Consequently, we expect that energy levels of these inorganic HTMs matches very well with the valence band of perovskites and improved efficiency helps in future practical deployment of low cost PSCs.

Selective Synthesis of Single‐ and Multi‐Walled Supramolecular Nanotubes by Using Solvophobic/Solvophilic Controls: Stepwise Radial Growth via “Coil‐on‐Tube” Intermediates

Novel hexa-peri-hexabenzocoronene (HBC) derivatives, (F) HBC and (F) HBC*, which carry perfluoroalkyl segments on one side of the HBC core and long alkyl tails on the other, were synthesized. Their perfluoroalkyl segments are highly solvated in C6 F6 (solvophilic effect) and do not assemble, whereas in CH2 Cl2 , they are excluded (solvophobic effect) and assemble together consequently. For example, the use of C6 F6 and CH2 Cl2 as assembling media for (F) HBC leads to the selective formation of single- and multi-walled nanotubes, respectively. When a higher monomer concentration is applied in CH2 Cl2 , multi-walled nanotubes with a larger number of walls result. (F) HBC in CH2 Cl2 self-assembles rather slowly, thereby allowing for the observation of coil-on-tube structures, which are possible intermediates for the stepwise radial growth of the nanotubular wall. Casting of the multi-walled nanotubes onto a quartz plate yields a superhydrophobic thin film with a water contact angle of 161±2°.

Effect of spacers and anchoring groups of extended π-conjugated tetrathiafulvalene based sensitizers on the performance of dye sensitized solar cells

Four new extended π-conjugated tetrathiafulvalene (ex-TTF) based dyes featuring a donor–π–acceptor (D–π–A) configuration with varying π-spacers and anchoring groups were synthesized and characterized. The sensitizer having the 4-ethynyl phenyl π-spacer (G4) shows red shifted absorption maxima in comparison with the sensitizer having only the phenyl π-spacer (G2). All four sensitizers undergo reversible oxidations to form stable radical cations. TDDFT calculations highlighted that the LUMO of the G4 sensitizer is more stabilized by the incorporation of the ethynyl group between the π-spacer and the cyanoacrylic acid anchoring group that aid to inject electrons efficiently into TiO2 thereby resulting in an enhanced power conversion efficiency of 6.36% when compared to the other derivatives, which is also confirmed by the intensity-modulated photovoltage spectroscopy (IMVS) method. Kinetic studies demonstrated that fast regeneration of the oxidized dye by the redox couple is an important factor behind enhanced efficiencies in solar cells. Finally, the performance of the present sensitizers is compared to that of previously reported sensitizers.

Unprecedented Charge‐Transfer Complex of Fused Diporphyrin as Near‐Infrared Absorption Induced High Aspect Ratio Nanorods

Charge-transfer (CT) complexes of near-infrared absorbing systems have been unknown until now. Consequently, structural similarities between donor and acceptor are rather important to achieve this phenomenon. Herein, we report electron donors such as non-fused diporphyrin-anthracene (DP), zinc diporphyrin-anthracene (ZnDP) and fused zinc diporphyrin-anthracene (FZnDP) in which FZnDP absorbs in NIR region and permits a CT complex with the electron acceptor, perylene diimide (PDI) in CHCl3 exclusively. UV/Vis-NIR absorption, 1 Hu2005NMR, NOESY and powder X-ray diffraction analysis demonstrated that the CT complex formation occurs by π-π stacking between perylene units in FZnDP and PDI upon mixing together in a 1:1 molar concentration in CHCl3 , unlike non-fused ZnDP and DP. TEM and AFM images revealed that the CT complex initially forms nanospheres leading to nanorods by diffusion of CH3 OH vapors into the CHCl3 solution of FZnDP/PDI (1:1 molar ratio). Therefore, these CT nanorods could lead to significant advances in optical, biological and ferroelectric applications.

Stipulating Low Production Cost Solar Cells All Set to Retail

Todays solar cells are exceptionally in demand whilst excess exploitation of natural fossil fuels. In this context, the first and second generation solar cells commercially available in market for more than decades however limitations in production cost and large-scale applications insist to generate inexpensive materials for fabrication. Thereby, organic materials based solar cells explored and emerging as third generation solar cells which possess flexibility, low cost and large-scale applications. For example, organic photovoltaics, dye sensitized solar cells and perovskite (organic-inorganic) solar cells (PSCs) are considered third generation solar cells wherein PSCs reached the record power conversion efficiency (PCE ∼23u2009%) and durability assists great advantages for commercialization in near future. Moreover, we reported various global renowned companies involved producing the modules and materials for three generation solar cells, hence, majority of companies considered commercialization of perovskite based solar cells assist low cost photovoltaics to meet the current energy necessities and environmental safety.