Vijayakumar C. Nair

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 π gelator (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.

Luminescent hybrid perovskite nanoparticles as a new platform for selective detection of 2,4,6-trinitrophenol

Luminescent organic–inorganic perovskite (CH3NH3PbBr3) nanoparticles are used for the detection of 2,4,6-trinitrophenol (TNP, picric acid) in the solution and vapour state. Unlike most fluorescence based sensors, hybrid perovskite nanoparticles showed high selectivity and good sensitivity towards TNP, particularly in the solution state. Hydrogen bonding ability and electron accepting strength of TNP were found to play key roles in the detection mechanism.

Heteroatom induced contrasting effects on the stimuli responsive properties of anthracene based donor–π–acceptor fluorophores

Two donor–π–acceptor type fluorophores consisting of anthracene as the donor and benzoxazole (ABO) or benzothiazole (ABT) as the acceptor are synthesised. Both fluorophores exhibit excellent solid state luminescence, and respond to external stimuli such as mechanical force (mechanochromism) and protons (acidochromism). Though the differences between the two molecules are marginal, their stimuli responsive behaviours are contrasting. ABO shows stable mechanochromism but the protonation induced colour change is unstable. On the other hand, ABT shows stable acidochromism but luminescence changes induced by mechanical stress are reverted back quickly. A detailed study reveals that the variance in the heteroatoms in benzoxazole/benzothiazole moieties (oxygen and sulphur, respectively) significantly affects both the intra- and intermolecular electronic interactions of the molecules resulting in such observations. In the case of ABO, the benzoxazole moieties interact through weak edge-to-edge π-stacking, whereas in ABT, the ring overlap is more significant making the π-stacking a stronger face-to-face type, which helps in the restoration of the molecular ordering in ABT, which is energetically more viable. Regarding the acidochromic properties, the presence of more electronegative oxygen (compared to sulphur) in ABO draws electrons efficiently towards it making the nitrogen less basic and the complex formed between ABO and TFA less stable. On the other hand, electron density on the nitrogen in ABT might be higher compared to ABO due to the presence of less electronegative sulphur leading to the formation of a stable complex between ABT and TFA.

Formation of Coaxial Nanocables with Amplified Supramolecular Chirality through an Interaction between Carbon Nanotubes and a Chiral π-Gelator

In an attempt to gather experimental evidence for the influence of carbon allotropes on supramolecular chirality, we found that carbon nanotubes (CNTs) facilitate amplification of the molecular chirality of a π-gelator (MC-OPV) to supramolecular helicity at a concentration much lower than that required for intermolecular interaction. For example, at a concentration 1.8×10(-4)  m, MC-OPV did not exhibit a CD signal; however, the addition of 0-0.6 mg of SWNTs resulted in amplified chirality as evident from the CD spectrum. Surprisingly, AFM analysis revealed the formation of thick helical fibers with a width of more than 100 nm. High-resolution TEM analysis and solid-state UV/Vis/NIR spectroscopy revealed that the thick helical fibers were cylindrical cables composed of individually wrapped and coaxially aligned SWNTs. Such an impressive effect of CNTs on supramolecular chirality and cylindrical-cable formation has not been reported previously.

Self-Assembled Organic Materials for Photovoltaic Application

Organic photovoltaic cells based on bulk-heterojunction architecture have been a topic of intense research for the past two decades. Recent reports on power conversion efficiency surpassing 10% suggest these devices are a viable low-cost choice for a range of applications where conventional silicon solar cells are not suitable. Further improvements in efficiency could be achieved with the enhanced interaction between the donor and acceptor components. Effective utilization of supramolecular interactions to tailor and manipulate the communication between the components in the blend is a good strategy towards this end. Literature reports suggest that the long-term stability of organic solar cells, a major hurdle for commercial applications, can also be partially addressed by generating stable supramolecular nanostructures. In this review, we have made an attempt to summarize advances in small molecule, oligomer and polymer based systems, wherein supramolecular interactions such as hydrogen-bonding, pi-pi stacking, and dipole-dipole are explored for realizing stable and efficient bulk-heterojunction solar cells.

Thiophene-bithiazole based metal-free dye as DSSC sensitizer: Effect of co-adsorbents on photovoltaic efficiency

AbstractA new molecule consisting of a bithiazole chromophore sandwiched between two thiophenes, functionalized with benzothiophene unit at one end and cyanoacrylic acid at the other end (BT1) was synthesized, photophysical properties were studied and employed as a photosensitizer in dye-sensitized solar cells (DSSCs). The molecule exhibited an intense absorption in the UV-visible region with absorption extending up to 500 nm. The ground and excited state potentials of BT1 were calculated to be 1.29 and -0.65 V, respectively vs. NHE using cyclic voltammetry. The ground state energy level is more positive than the triiodide electrolyte and excited state energy level is considerably more negative than the TiO2 satisfying the energetic requirements for a photosensitizer in DSSC. The solar cells fabricated from BT1 exhibited an efficiency of 1.13%. The effect of various co-adsorbents (CDCA,TP1 andTP2) on the DSSC performance was investigated in detail. In the presence of CDCA, the photovoltaic efficiency was enhanced to 1.25%, whereas, in the presence of TP1 and TP2, the efficiency lowered to 0.20% and 0.59%, respectively. The increased efficiency in the presence of CDCA could be attributed to the prevention of the aggregation of dye molecules induced by CDCA. On the other hand, TP1 and TP2 were found to be not as effective as CDCA to prevent aggregation leading to the lowering of photoconversion efficiency. Graphical AbstractA benzothiophene-bithiazole based metal-free sensitizer end-functionalized with cyanoacrylic acid acceptor was synthesized, photophysical properties were studied and used for dye sensitized solar cell (DSSC) application. The effect of various co-adsorbents on the DSSC performance was investigated.

Zero-Dimensional Methylammonium Bismuth Iodide-Based Lead-Free Perovskite Capacitor

Symmetrical electrochemical capacitors are attracting immense attention because of their fast charging–discharging ability, high energy density, and low cost of production. The current research in this area is mainly focused on exploring novel low-cost electrode materials with higher energy and power densities. In the present work, we fabricated an electrochemical double-layer capacitor using methylammonium bismuth iodide (CH3NH3)3Bi2I9, a lead-free, zero-dimensional hybrid perovskite material. A maximum areal capacitance of 5.5 mF/cm2 was obtained, and the device retained 84.8% of its initial maximum capacitance even after 10 000 charge–discharge cycles. Impedance spectroscopy measurements revealed that the active layer provides a high surface area for the electrolyte to access. As a result, the charge transport resistance is reasonably low, which is advantageous for delivering excellent performance.

Effect of Differential Self-Assembly on Mechanochromic Luminescence of Fluorene-Benzothiadiazole-Based Fluorophores

Supramolecular self-assembly is an excellent tool for controlling the optical and electronic properties of chromophore-based molecular systems. Herein, we demonstrate how differential self-assembly affects mechanoresponsive luminescence of fluorene-benzothiadiazole-based fluorophores. We have synthesized two donor–acceptor–donor-type conjugated oligomers consisting of fluorene as the donor and benzothiadiazole as the acceptor. For facile self-assembly, both molecules are end-functionalized with hydrogen-bonding amide groups. Differential self-assembly was induced by attaching alkyl chains of different lengths onto the fluorene moiety: hexyl (FB-C6) and dodecyl (FB-C12). The molecules self-assemble to form well-defined nanostructures in nonpolar solvents and solvent mixtures. Although their optical properties in solution are not affected by the alkyl chain length, significant effects were observed in the self-assembled state, particularly in the excitation energy migration properties. As a result, remarkable differences were observed in the mechanochromic luminescence properties of the molecules. A precise structure–property correlation is made using UV–visible absorption and fluorescence spectroscopy, time-correlated single-photon counting analysis, scanning electron microscopy, and X-ray diffraction spectroscopy.

Following the TRMC Trail: Optimization of Photovoltaic Efficiency and Structure-Property Correlation of Thiophene Oligomers.

Semiconducting conjugated oligomers having same end group (N-ethylrhodanine) but different central core (thiophene: OT-T, bithiophene: OT-BT, thienothiophene: OT-TT) connected through thiophene pi-linker (alkylated terthiophene) were synthesized for solution processable bulk-heterojunction solar cells. The effect of the incorporation of an extra thiophene to the central thiophene unit either through C-C bond linkage to form bithiophene or by fusing two thiophenes together to form thienothiophene on the optoelectronic properties and photovoltaic performances of the oligomers were studied in detail. Flash photolysis time-resolved microwave conductivity (FP-TRMC) technique shows OT-TT has significantly higher photoconductivity than OT-T and OT-BT implying that the former can outperform the latter two derivatives by a wide margin under identical conditions in a bulk-heterojunction solar cell device. However, the initial photovoltaic devices fabricated from all three oligomers (with PC71BM as the acceptor) gave power conversion efficiencies (PCEs) of about 0.7%, which was counterintuitive to the TRMC observation. By using TRMC results as a guiding tool, solution engineering was carried out; no remarkable changes were seen in the PCE of OT-T and OT-BT. On the other hand, 5-fold enhancement in the device efficiency was achieved in OT-TT (PCE: 3.52%, VOC: 0.80 V, JSC: 8.74 mA cm(-2), FF: 0.50), which was in correlation with the TRMC results. The structure-property correlation and the fundamental reasons for the improvement in device performance upon solvent engineering were deduced through UV-vis absorption, atomic force microscopy, bright-field transmission electron microscopy, photoluminescence quenching analysis and two-dimensional grazing incidence X-ray diffraction studies.

Reversible switching of solid-state luminescence by heat-induced interconversion of molecular packing

Dynamic switching of solid-sate luminescence with high contrast and reproducibility is a challenging task but important for several optoelectronic applications. One promising approach towards this end is to control the mode of solid-state packing of luminescent organic chromophores with an external stimulus. Herein, we present the heat-induced interconversion of molecular packing in acceptor–donor–acceptor type divinylbenzene derivatives. This interconversion is associated with the switching of solid-state luminescence from red to greenish-yellow (Δλ = 95 nm). A detailed investigation of the photophysical properties provided molecular and supramolecular level comprehension of the factors guiding the luminescence switching. The transition between different packing modes is associated with differential excited state coupling and excitation energy migration efficiencies due to the variance in the chromophore organization. Viewed more broadly, our findings illustrate that subtle control over energy migration processes in molecular assemblies by heating may result in functional organic materials with switchable luminescence.