A Review on Self-Assembly Driven Optoelectronic Properties of Polythiophene-Peptide and Polythiophene-Polymer Conjugates.

Polythiophene (PT) is an important conducting polymer for its outstanding optoelectronic properties. Here, we delineate the self-assembly-driven optoelectronic properties of PT-peptide and PT-polymer conjugates, taking examples from recent literature reports. PT-peptide conjugates made by both covalent and noncovalent approaches are discussed. Poly(3-thiophene acetic acid) (P3TAA) covalently coupled with Gly-Gly-His tripeptide, C-protected and deprotected tripeptide H2N-F-F-V-OMe, etc. exhibits self-assembly-driven absorbance, fluorescence, photocurrent, and electronic properties. Noncovalent PT-peptide conjugates produced via ionic, H-bonding, and π-stacking interactions show tunable morphology and optoelectronic properties by varying the composition of a component. PT conjugated with Alzheimer's disease peptide (KLVFFAE, Aß16-22) shows enhanced photocatalytic water splitting, cationic PT(CPT-I)-perylene bisimide-appended dipeptide (PBI-DY), and anionic PT-perylene diimide-appended cationic peptide (PBI-NH3+) conjugates and exhibits self-assembly-driven enhanced photoswitching and organic mixed electronic and ionic conductivity (OMEIC) properties. In the PT-polymer conjugates, self-assembly-driven optoelectronic properties of covalently produced PT-random copolymers, PT-block copolymers, PT-graft-random copolymers, and PT-graft-block copolymer conjugates are discussed. The HOMO-LUMO levels of hyperbranched polymers are optimized to obtain better power conversion efficiency (PCE) in the bulk heterojunction (BHJ) solar cell than in linear polymers, and P3TAA-ran-P3HT (43 mol % P3TAA) conjugated with MAPbI3 perovskite exhibits higher PCE (10%) than that with only P3TAA hole-transporting material. In the ampholytic polythiophene (APT), on increasing pH, the morphology changes from the vesicle to fibrillar network for the dethreading of the PT chain, resulting in a red shift of the absorbance peak, an enormous increase in PL intensity, lowering of the charge transfer resistance, and an induction of Warburg impedance for the release of quencher I- ions. The PT-g-(PDMAEMA-co-PGLU-HEM) graft copolymer self-assembles with Con-A lectin, causing fluorescence quenching, and acts as a sensor for Con-A with a LOD of 57 mg/L. Varying sequences of the block copolymer containing pH-responsive PDMAEMA and temperature-responsive PDEGMEM grafted to the PT backbone shows different self-assembly, optical, electronic, and photocurrent properties depending on the proximity and preponderance of the block sequence on the PT backbone.

Reversible Stimuli-Dependent Aggregation-Induced Emission from a "Nonfluorescent" Amphiphilic PVDF Graft Copolymer.

A poly(vinylidine fluoride) graft random copolymer of t-butyl aminoethyl methacrylate (tBAEMA) and oligo(ethylene glycol) methyl ether methacrylate (OEGMA, Mn = 300) [PVDF-g-P(tBAEMA-ran-OEGMA), PVBO] is synthesized by atom transfer radical polymerization (ATRP), and PVBO is fractionated to get a highly water-soluble fraction (PVBO-1) showing a reversible on/off fluorescence behavior with gradual increase and decrease in pH, respectively, achieving a maximum quantum yield of 0.18 at pH = 12. PVBO-1 dissolved in water shows large multimicellar aggregates (MMcA), but at pH 12, crumbling of larger aggregates to much smaller micelles occurs, forming nonconjugated polymer dots (NCPDs), as supported by transmission electron microscopy and dynamic light scattering study. The reversible fluorescence on/off behavior also occurs with the decrease and increase of temperature. Theoretical study indicates that, at high pH, most of the amino groups become neutral and exhibit a strong tendency to form aggregates from crowding of a large number of carbonyl and amine groups, minimizing the HOMO-LUMO gap, showing an absorption peak at the visible region, and generating aggregation-induced emission.

A review on recent advances in polymer and peptide hydrogels.

In this review, we focus on the very recent developments on the use of the stimuli responsive properties of polymer hydrogels for targeted drug delivery, tissue engineering, and biosensing utilizing their different optoelectronic properties. Besides, the stimuli-responsive hydrogels, the conducting polymer hydrogels are discussed, with specific attention to the energy generation and storage behavior of the xerogel derived from the hydrogel. The electronic and ionic conducting gels have been discussed that have applications in various electronic devices, e.g., organic field effect transistors, soft robotics, ionic skins, and sensors. The properties of polymer hybrid gels containing carbon nanomaterials have been exemplified here giving attention to applications in supercapacitors, dye sensitized solar cells, photocurrent switching, etc. Recent trends in the properties and applications of some natural polymer gels to produce thermal and acoustic insulating materials, drug delivery vehicles, self-healing material, tissue engineering, etc., are discussed. Besides the polymer gels, peptide gels of different dipeptides, tripeptides, oligopeptides, polypeptides, cyclic peptides, etc., are discussed, giving attention mainly to biosensing, bioimaging, and drug delivery applications. The properties of peptide-based hybrid hydrogels with polymers, nanoparticles, nucleotides, fullerene, etc., are discussed, giving specific attention to drug delivery, cell culture, bio-sensing, and bioimaging properties. Thus, the present review delineates, in short, the preparation, properties, and applications of different polymer and peptide hydrogels prepared in the past few years.

Zwitterionic Poly(vinylidene fluoride) Graft Copolymer with Unexpected Fluorescence Property.

Recently, there has been a growth of research on the nonconjugated polymer exhibiting fluorescence property and it would be exciting if fluorescence property is developed in zwitterionic polymers because of their good water solubility. Poly(vinylidene fluoride) (PVDF) grafted with poly(dimethyl amino ethyl methacrylate) (PDMAEMA) is fractionated and a highly water-soluble fraction (PVDM-1) is quaternized with 1,3-propane sultone, producing a zwitterionic polymer, PVDF- g-PDMAEMA-sultone (PVDMS). PVDM-1 shows the fluorescence property with very low quantum yield (1%) in water, but on quaternization, fluorescence quantum yield increases to 8%. Transmission electron microscopy results indicate that the PVDM-1 cast from water has vesicular morphology, whereas PVDMS exhibits aggregated vesicular morphology. The 1H NMR spectra indicate the presence of 72 mol % DMAEMA in PVDM-1 wherein 66% of -NMe2 groups is quaternized upon postpolymerization modification. PVDM-1 exhibits absorption peaks at 210, 276, and 457 nm with a hump at 430 nm, whereas PVDMS exhibits two absorption peaks at 203 and 297 nm. PVDM-1 exhibits a broad emission peak at 534 nm, whereas PVDMS exhibits a sharp emission peak at 438 nm. An attempt has been made from density functional theory calculations to shed light on the origin of fluorescence in both PVDM-1 and in the zwitterionic PVDMS. The excitonic decay occurs from the lowest unoccupied molecular orbital (LUMO) of carbonyl group to the highest occupied molecular orbital (HOMO) of tertiary amine group for PVDM-1, whereas in PVDMS, the excitonic transition occurs from the LUMO situated over the quaternary ammonium group to the HOMO located on the electron-rich terminal sulfonate group.

A New Facile Synthesis of Tungsten Oxide from Tungsten Disulfide: Structure Dependent Supercapacitor and Negative Differential Resistance Properties.

Tungsten oxide (WO3 ) is an emerging 2D nanomaterial possessing unique physicochemical properties extending a wide spectrum of novel applications which are limited due to lack of efficient synthesis of high-quality WO3 . Here, a facile new synthetic method of forming WO3 from tungsten sulfide, WS2 is reported. Spectroscopic, microscopic, and X-ray studies indicate formation of flower like aggregated nanosized WO3 plates of highly crystalline cubic phase via intermediate orthorhombic tungstite, WO3. H2 O phase. The charge storage ability of WO3 is extremely high (508 F g-1 at current density of 1 A g-1 ) at negative potential range compared to tungstite (194 F g-1 at 1 A g-1 ). Moreover, high (97%) capacity retention after 1000 cycles and capacitive charge storage nature of WO3 electrode suggest its supremacy as a negative electrode of supercapacitors. The asymmetric supercapacitor, based on the WO3 as a negative electrode and mildly reduced graphene oxide as a positive electrode, manifests high energy density of 218.3 mWhm-2 at power density 1750 mWm-2 , and exceptionally high power density, 17 500 mW m-2 , with energy density of 121.5 mWh m-2 . Furthermore, the negative differential resistance (NDR) property of both WO3 and WO3 .H2 O are reported for the first time and NDR is explained with density of state approach.

Triarylamine-Cored Dendritic Molecular Gel for Efficient Colorometric, Fluorometric, and Impedometeric Detection of Picric Acid.

Detection of nitroaromatics at ultralow concentration is a major security concern in defense, forensics, and environmental science. To this end, a new triarylamine-cored dendritic gelator (OGR) was synthesized, which produced thermoreversible, thixotropic, and fluorescent gels in n-octanol. On gelation, both π-π* transitions and the emission peak of the gelator show redshifts with a 4.5-fold increase of fluorescence intensity in the gel state indicating J-aggregation. The nitrogen lone-pair electrons of OGR make it a donor, and electron transfer occurs to acceptor nitroaromatics causing fluorescence quenching, which is further promoted due to its acidity. The Stern-Volmer rate constants measured for different nitroaromatics showed that it senses picric acid (PA) best. The contact-mode technique with OGR-treated paper strips can allow naked-eye detection of PA under UV light down to 10-11 m concentration within 30 s. Reusability of the gel is achieved by treating OGR@PAx with NaOH solution. Impedance spectroscopic results indicated a decrease of both charge-transport resistance and Warburg impedance on successive addition of PA. The limits of detection of PA determined from fluorescence and impedance measurements match well. Thus, the OGR gel is a reusable, low-cost, specific sensor for PA by naked-eye colorimetric, fluorescence, and impedance techniques.

Optoelectronic Properties of Self-Assembled Nanostructures of Polymer Functionalized Polythiophene and Graphene.

In this Feature Article, we discuss the variation of optoelectronic properties with the aggregation style of polythiophene (PT) graft copolymers and polymer-modified graphene systems. Grafting of flexible polymers on a PT chain exhibits several self-organized patterns under various conditions, causing different optical and electronic properties, arising from the different conformational states of the conjugated chain. Graphene, a zero band gap material, is functionalized with polymers both covalently and noncovalently to create a finite band gap importing new optoelectronic properties. The polymer-triggered self-assembled nanostructures of PT and graphene-based materials bring unique optical/electronic properties suitable for sensing toxic ions, nitroaromatics, and surfactants, for drug delivery, and also for fabricating molecular logic gates, electronic rectifiers, photocurrent devices, etc.

Light-Induced Conformational Change of Uracil-Anchored Polythiophene-Regulating Thermo-Responsiveness.

Tuning the electronic structure of a π-conjugated polymer from the responsive side chains is generally done to get desired optoelectronic properties, and it would be very fruitful when light is used as an exciting tool that can also affect the backbone chain conformation. For this purpose, polythiophene- g-poly-[ N-(6-methyluracilyl)- N, N-dimethylamino chloride]ethyl methacrylate (PTDU) is synthesized. On exposure to diffuse sunlight, the uracil moieties of the grafted chains cause the absorption maximum of PTDU solution to show gradual blue shift of 87 nm and a gradual blue shift of 46 nm in the emission maximum, quenching its fluorescence with time. These effects occur specifically at the absorption range of polythiophene (PT) chromophore on direct exposure of light of different wavelengths, and the optimum wavelength is found to be 420 nm. Impedance study suggests a decrease in charge transfer resistance upon exposure because of conformational change of PTDU. Theoretical study indicates that on exposure to visible light, uracil moieties move toward the backbone to facilitate photoinduced electron transfer between the PT and the uracil, attributing to the variation in optoelectronic properties. Morphological and light-scattering studies exhibit a decrease in particle size because of coiling of the PT backbone and squeezing of the grafted chain on light exposure. The transparent orange-colored PTDU solution becomes hazy with a hike in emission intensity on addition of sodium halides and becomes reversibly transparent or hazy on heating or cooling. The screening of cationic centers of PTDU by varying halide anion concentration tunes the phase transition temperature. Thus, the light-induced variation in the backbone conformation is responsible for tuning the optoelectronic properties and regulates the thermos-responsiveness of the PTDU solution in the presence of halide ions.

Copolymers of poly(3-thiopheneacetic acid) with poly(3-hexylthiophene) as hole-transporting material for interfacially engineered perovskite solar cell by modulating band positions for higher efficiency.

In order to tune the band positions of the hole-transporting material (HTM) in an interfacially engineered perovskite solar cell (PSC), random copolymers of poly(3-thiopheneacetic acid) and poly(3-hexylthiophene) (P3TAA-co-P3HT) with different compositions were produced by oxidative polymerization. The copolymers were characterized using 1H NMR, FTIR, and UV-vis spectroscopy and gel permeation chromatography. Here, ZnO nanoparticles were used as the electron-transporting material (ETM) and methylammonium lead iodide (MAPbI3) perovskite was used as the light-absorbing material to form an FTO/ZnO/MAPbI3/copolymer/Ag device, of which the power conversion efficiency (PCE) was found to be dependent on the copolymer composition and reached a maximum (∼10%) at a P3TAA content of 43 mol% in the copolymer (P3). The band gaps of the copolymers as determined from UV-vis spectroscopy and cyclic voltammetry exhibit a staggered-gap hetero-interface configuration in which the HOMO and LUMO of P3 closely match those of MAPbI3 and give rise to the maximum PCE. Time-resolved photoluminescence spectra of MAPbI3/HTM samples indicate that charge transfer across the perovskite/copolymer interface was faster with a reduced recombination rate for a P3 sample. The electrochemical impedance spectra (EIS) of the PSCs exhibit Nyquist plots with two semicircles, which correspond to an equivalent circuit consisting of two parallel R-C and R-CPE circuits connected in series. Analysis of the data indicates that the effective electron lifetime was longest for the P3 copolymer, which indicates that the charge recombination was lower than that in the components and other copolymers. The copolymers exhibited an intermediate stability with respect to their components, and amongst the copolymers P3 exhibited the highest stability.

Deciphering the Effect of Polymer-Assisted Doping on the Optoelectronic Properties of Block Copolymer-Anchored Graphene Oxide.

Doping facilitates the tuning of band gap, providing an opportunity to tailor the optoelectronic properties of graphene in a simple way, and polymer-assisted doping is a new route to combine the optoelectronic properties of graphene with the properties of a polymer. In this endeavor, a linear diblock copolymer, polycaprolactone-block-poly(dimethyl aminoethyl methacrylate) (PCL13-b-PDMAEMA117) (GPCLD) is grafted from the graphene oxide (GO) surface via consecutive ring opening and atom transfer radical polymerization. GPCLD is characterized using proton nuclear magnetic resonance (1H NMR), Fourier transform infrared spectroscopy, atomic force microscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, and Raman spectroscopy. The phase transition behavior of the GPCLD solution with varying temperature and pH is monitored using fluorescence spectroscopy and dynamic light scattering. Temperature-dependent 1H NMR spectra at pH 9.2 indicate the influence of temperature on the interaction between GPCLD and solvent (water) molecules causing the phase separation. Fluorescence spectra at pH 4 and 9.2 give the evidence of localized p- and n-type doping of graphene assisted by the pendent PDMAEMA chains. In the impedance spectra of GPCLD films, the Nyquist plots vary with pH; at pH 4, they exhibit a semicircle at higher frequencies and a spike at lower frequencies; at pH 7.0, the spike is replaced by an arc; and at pH 9.2, the semicircle at higher frequencies vanishes and only a spike is noticed, all of these suggesting different types of doping of graphene at different pH values. The dc-conductivity also varies with pH and temperature because of the different types of doping. The current (I)-voltage (V) property of GPCLD at different pH values is very unique: at pH 9.2, an interesting feature of negative differential resistance (NDR) is observed; at pH 7, the rectification property is observed; and at pH 4, again the NDR property is observed. The temperature-dependent I-V property at pH 7 and 9.2 clearly indicates a signature of doping, dedoping, and redoping because of the change in the interaction of GO with the grafted polymer arising from coiling and decoiling of polymer chains.

Influence of Chain Length on the Self-Assembly of Poly(ε-caprolactone)-Grafted Graphene Quantum Dots.

The multifarious applications of graphene quantum dots (GQDs) necessitate surface modifications to enhance their solution processability. Herein, we report the synthesis and self-assembly of GQDs grafted with poly(ε-caprolactone) (PCL) of different degrees of polymerization (3, 7, 15, and 21) produced from ring-opening polymerization. Optical and morphological studies unveil the transformation of the assemblies from J-aggregates to H-aggregates, accompanied by an alteration in morphology from toroid to spheroid to rodlike structures with increasing chain length of PCL. Functionalized GQDs with lower chain lengths of PCL at higher concentration also assemble into liquid-crystalline phases as observed from birefringent textures, which are later correlated to the formation of columnar hexagonal (Colh) mesophases. However, no such behavior is observed at higher chain lengths of PCL under identical conditions. Therefore, it is evident that the variation in the PCL chain length plays a crucial role in the self-assembly, which is primarily triggered by the van der Waals force between the polymer chains dictating the π stacking of GQDs, resulting in different self- aggregated behavior.

Influence of Hofmeister I- on Tuning Optoelectronic Properties of Ampholytic Polythiophene by Varying pH and Conjugating with RNA.

A significant tuning of optoelectronic properties of polythiophene (PT) chains due to Hofmeister iodide (I-) ion is demonstrated in ampholytic polythiophene [polythiophene-g-poly{(N,N,N-trimethylamino iodide)ethyl methacrylate-co-methacrylic acid}, APT] at different pHs. In acidic medium, the absorption and emission signals of PT chromophore exhibit appreciable blue shift in the presence of I- as counteranion only. The cooperative effect of undissociated -COOH and quaternary ammonium groups immobilize I- near the apolar PT chain causing threading of grafted chains and hence twisting of the backbone attributing to the blue shift. As medium pH is increased, dethreading of the PT backbone occurs due to ionization of -COOH group, releasing quencher iodide ions from the vicinity of the PT chains resulting in a red shift in absorption and a sharp hike in fluorescence intensity (390 times) for an increase of excitons lifetime. With an increase of pH, morphology changes from a multivesicular aggregate with vacuoles to smaller size vesicles and finally to nanofibrillar network structure. Dethreading is also found when APT interacts with RNA showing a significant hike of fluorescence (22 times) for displacing iodide ions forming a nanofibrillar network morphology. Threading and dethreading also affect the resistance, capacitance, and Warburg impedance values of APT. Molecular dynamics simulation of a model APT chain in a water box supports the threading at lower pH where the iodide ions pose nearer to the PT chain than that at higher pH causing dethreading. So the influence of Hofmeister I- ion is established for tuning the optoelectronic properties of a novel PT based polyampholyte by changing pH or by conjugating with RNA.

Surfactant-Triggered Fluorescence Turn "on/off" Behavior of a Polythiophene-graft-Polyampholyte.

Polythiophene-graft-polyampholyte (PTP) is synthesized using N,N-dimethylaminoethyl methacrylate and tert-butyl methacrylate monomers by grafting from polythiophene backbone, followed by hydrolysis. The resulting polymer exhibits aqueous solubility via formation of small-sized miceller aggregates with hydrophobic polythiophene at the center and radiating polyionic side chains (cationic or anionic depending on the pH of the medium) at the outer periphery. The critical micelle concentration of PTP in acidic solution (0.025 mg/mL, pH = 2.7) is determined from fluorescence spectroscopy. PTP exhibits reversible fluorescence on and off response in both acidic and basic medium with the sequential addition of differently charged ionic surfactants, repeatedly. The fluorescence intensity of PTP at pH 2.7 increases with the addition of an anionic surfactant, sodium dodecyl benzenesulfonate (SDBS), due to the self-aggregation forming compound micelles. The fluorescence intensity of these solutions again decreases on addition of a cationic surfactant, cetyltrimethylammonium bromide (CTAB), because of assembling of SDBS with CTAB, thus deassembling the PTP-SDBS aggregates. At pH 9.2, these turn on and turn off responses are also shown by PTP with the sequential addition of cationic surfactant (CTAB) and anionic surfactant (SDBS), respectively. This result shows that PTP has potential for surfactant-induced reversible fluorescence turn on and off using ionic surfactant (SDBS and CTAB) through self-assembling and deassembling of the ionic aggregates. The reversible aggregation and disaggregation process of PTP with the surfactants at both acidic and basic pH is supported from dynamic light scattering and Fourier transform infrared spectroscopy. The morphology of the above systems studied by transmission and scanning electron microscopy also supports the above aggregation and disaggregation process.

A Comparative Account of the Kinetics of Light-Induced E-Z Isomerization of an Anthracene-Based Organogelator in Sol, Gel, Xerogel, and Powder States: Fiber to Crystal Transformation.

The organogel of (E)-N'-(anthracene-10-ylmethylene)-3,4,5-tris(dodecyloxy)benzohydrazide (I) in methyl cyclohexane having a fibrillar network structure exhibits excellent fluorescence, which decreases sharply with time upon photoirradiation at λ = 365 nm. It has been attributed to the transformation of the E isomer of I to the Z isomer, and the kinetics of E-Z isomerization are compared for the sol, gel, xerogel, and powder states. The rate constants at different temperatures are measured from Avrami plots and its increase with an increase in temperature, indicating temperature acts as a promoter for photoirradiated E-Z isomeization along the imine (C═N) bond. In the powder form, the rate constant values are the lowest compared to those of other states for all temperatures and the xerogels exhibit the highest rate of E-Z isomerization. The rate constants of sol and gel states mostly lie between the two. The wide-angle X-ray scattering pattern changes after ultraviolet (UV) irradiation with the generation of new sharp peaks whose intensities increase with an increase in irradiation time. A polarized optical microscopic study indicates formation of small crystalline dots on the fibers in the gels, dendritic morphology on the xerogel fibers, and large needlelike morphology at the surface boundary of the solid. The dried I gel exhibits a melting peak at 96.7 °C, but upon irradiation, two peaks are observed at 98.5 and 152.7 °C; the latter has been attributed to the melting of crystals of Z isomers. Similar higher melting peaks are observed both for the xerogel and for powders after UV irradiation; the powders exhibit the highest meting peak at 159.4 °C. Possible reasons for the variation of rate constant values in the four different states and the difference in morphology and melting points of crystals of Z isomers of I are discussed.

Nanoengineering of a Supramolecular Gel by Copolymer Incorporation: Enhancement of Gelation Rate, Mechanical Property, Fluorescence, and Conductivity.

In the quest to engineer the nanofibrillar morphology of folic acid (F) gel, poly(4-vinylpyridine-co-styrene) (PVPS) is judiciously integrated as a polymeric additive because of its potential to form H-bonding and π-stacking with F. The hybrid gels are designated as F-PVPSx gels, where x denotes the amount of PVPS (mg) added in 2 mL of F gel (0.3%, w/v). The assistance of PVPS in the gelation of F is manifested from the drop in critical gelation concentration and increased fiber diameter and branching of F-PVPSx gels compared to that of F gel. PVPS induces a magnificent improvement of mechanical properties: a 500 times increase of storage modulus and ∼62 times increase of yield stress in the F-PVPS5 gel compared to the F gel. The complex modulus also increases with increasing PVPS concentration with a maximum in F-PVPS5 gel. Creep recovery experiments suggest PVPS induced elasticity in the otherwise viscous F gel. The fluorescence intensity of F-PVPSx gels at first increases with increasing PVPS concentration showing maxima at F-PVPS5 gel and then slowly decreases. Gelation is monitored by time-dependent fluorescence spectroscopy, and it is observed that F and F-PVPSx gels exhibit perfectly opposite trend; the former shows a sigmoidal decrease in fluorescence intensity during gelation, but the latter shows a sigmoidal increase. The gelation rate constants calculated from Avrami treatment on the time-dependent fluorescence data manifest that PVPS effectively enhances the gelation rate showing a maximum for F-PVPS5 gel. The hybrid gel exhibit 5 orders increase of dc conductivity than that of F-gel showing semiconducting nature in the current-voltage plot. The Nyquist plot in impedance spectra of F-PVPS5 xerogel exhibit a depressed semicircle with a spike at lower frequency region, and the equivalent circuit represents a complex combination of resistance-capacitance circuits attributed to the hybrid morphology of the gel fibers.

Interface engineering of hybrid perovskite solar cells with poly(3-thiophene acetic acid) under ambient conditions.

The properties of methyl ammonium lead iodide (MAPbI3) perovskite solar cells with poly(3-thiophene acetic acid) (P3TAA) as a hole transporting material (HTM) and a dense layer of ZnO nanoparticle film as an electron transporting material (ETM) are described using the conventional ZnO (n)/perovskite (i)/P3TAA (p) (n-i-p) architecture. The FT-IR spectra of a MAPbI3/P3TAA mixture indicate a shift of the N-H stretching and the abolition of the N-H bending peak indicating the interaction between the components. UV-Vis spectra of the mixture exhibit a large red shift of the π-π* transition peak of the conjugated chain arising from the interaction causing an increase of the conjugation length. The cross-sectional SEM image of the device shows the sequence of the individual layers of ZnO, MAPbI3, P3TAA and Ag, respectively. The current density (J)-voltage (V) curves obtained upon illumination with a light of 100 mW cm(-2) indicate the average PCE to be 7.38 ± 0.59% under ambient conditions. The IPCE values of these cells reach about 63% across a broad range of wavelength (300-800 nm). The HOMO and the LUMO of P3TAA are measured using cyclic voltammetry and the optical band gap and the relative energy level of the components explain the operation of photocurrent in the cell. For comparison purposes a device using poly(3-hexyl thiophene) (P3HT) as the HTM is fabricated under similar conditions and it exhibits a lower PCE (5.85 ± 0.51%) than that of the P3TAA based device. The longevity of the P3TAA based cell is also found to be better than that of the P3HT based cell for storing in air. The UV-Vis and impedance spectral results clearly explain the above results, signifying the influence of the interface on the performance of hybrid solar cells.

An insight into the hybrid dye-sensitized solar cell from polyaniline-CdS nanotubes through impedance spectroscopy.

Composites of polyaniline (PANI) and cadmium sulfide nanoparticles (CdS NPs) are in situ synthesized by polymerizing aniline in acetic acid medium for different concentrations of CdS NPs. The composites, characterized by scanning and transmission electron microscopy, exhibit nanotubular morphology of PANI decorated with nanospheres of CdS NPs at their outer surface. The FTIR and UV-vis spectra indicate strong interaction between PANI and the CdS NPs suggesting a good conjugate for photovoltaic applications. The dc conductivities of the composite increase with an increase of the CdS concentration showing a maximum of 1.17 × 10-2 S cm-1 for the C200 sample (obtained by polymerizing 182 µL aniline with 200 mg CdS). The current-voltage plots indicate that the photocurrent is higher from the dark current and the separation between the dark current and photocurrent is maximum in C200. The effectiveness of these composites in DSSCs is studied under illumination of 100 mW cm-2 and the C200 device exhibits a maximum open circuit voltage of 0.73 V, a short circuit current of 8.22 mA cm-2, and a power conversion efficiency of 3.96%. The impedance study of the cells indicates that the lifetime of the photo-injected electrons is highest (1.34 ms) for the C200 sample, explaining the maximum PCE among the composites.

A co-assembled gel of a pyromellitic dianhydride derivative and polyaniline with optoelectronic and photovoltaic properties.

5,5'-(1,3,5,7-Tetraoxopyrrolo[3,4-f]isoindole-2,6-diyl)diisophthalic acid (PMDIG) is used to produce a supramolecular hydrogel via acid-base treatment. The field emission scanning electron micrograph and atomic force microscopy micrographs exhibit a fibrillar network structure from intermolecular supramolecular interaction, supported from Fourier transform infrared (FTIR) and UV-vis spectra. The fluorescence intensity of the PMDIG gel is 16 times higher than that of the sodium salt of PMDIG with a 42 nm red shift of the emission peak. Upon addition of an anilinium chloride solution to the PMDIG gel, it transforms into the sol, and when a solid ammonium persulfate is spread over it, a stable hydrogel is produced. The co-assembled PMDIG-polyaniline (PANI) gel exhibits a fibrillar network morphology, and the co-assembly is formed by the supramolecular interaction between the polyaniline (donor) and the PMDIG (acceptor) molecules, which is evident from FTIR spectra and wide angle X-ray scattering results. The UV-vis spectrum of the PMDIG-PANI hydrogel exhibits the characteristic peaks of polaron band transitions of the doped PANI. The PMDIG-PANI co-assembled hydrogel has a 51-fold higher storage modulus, a 52-fold higher elasticity, a 1.4-fold increase in stiffness, and a 5-fold increase of fragility compared to the values of the PMDIG hydrogel. The PMDIG-PANI xerogel exhibits a 4 order of magnitude increase in dc conductivity compared to that of PMDIG, and the I-V characteristic curve exhibits a rectification property under white light illumination showing photocurrent rectification, a new phenomenon reported here for the supramolecular gel systems. A dye-sensitized solar cell fabricated with an ITO/PMDIG-PANI/graphite device shows a power conversion efficiency (η) of 0.1%. A discussion of the mechanism of gel formation and the sol state of the PMDIG-aniline system is included considering the contact angle values of the xerogels.

A thixotropic supramolecular hydrogel of adenine and riboflavin-5'-phosphate sodium salt showing enhanced fluorescence properties.

An equimolar mixture of riboflavin-5'-phosphate sodium salt (RP) and adenine (AD) dissolved in a phosphate buffer (pH 4.0, 1.0% w/v) produces a red coloured transparent thixotropic hydrogel at 30 °C. The gelation of the RPAD system occurs in the pH range of 2­5. FTIR spectra and WAXS patterns indicate self-assembly via H-bonding between the >C=O group of RP and the amino/imino group of AD followed by supramolecular organization through a π-stacking process producing a fibrillar network structure. FESEM images clearly indicate that the nanofibres are produced from the intertwining of helical fibrils. The dynamic frequency sweep experiment of the supramolecular gel at a constant strain of 1% exhibits a wide linear viscoelastic region and a considerably higher G' value (460 Pa) than that of G'' (21 Pa) confirming the gel nature of the RPAD system. The hydrogel shows high stiffness (G'/G'' = 3.3), a high yield stress (σ*) (79.5 Pa) and a moderate critical strain (γ = 17.5%). Time sweep experiments at both low (0.1%) and high strain (100%) indicate the thixotropic property of the gel. The RPAD hydrogel shows non-Newtonian viscosity in the shear rate region (0.1­158 s(−1)) and after that there is a sudden fall of viscosity. The gel melting point obtained by the falling ball method is 6° higher than that obtained by the DSC method probably due to the presence of the thixotropic property of the gel. The UV-vis spectra indicate a red shift of the π­π* transition band of RP in the RPAD xerogel. On excitation of the RPAD hydrogel at 373 nm it shows twelve times enhancement of emission intensity with a 7 nm red shift of the emission peak. This has been attributed to the enhancement of lifetime from 2.2 ns in RP to 3.4 ns in the RPAD hydrogel. With increase of temperature, the fluorescence intensity of the RPAD hydrogel at first increases till 40 °C, then decreases up to 55 °C and it again increases after 60 °C.

Dye-sensitized solar cell from polyaniline-ZnS nanotubes and its characterization through impedance spectroscopy.

Polyaniline (PANI)-zinc sulphide (ZnS) nanocomposites (PAZs) are synthesized by polymerizing aniline in the presence of acetic acid with different concentrations of ZnS nanoparticles (NPs). FESEM and TEM images indicate the nanotube morphology of PANI and ZnS NPs remain adhered to the nanotube surface, but at higher ZnS concentration the nanotube morphology is lost. UV-vis spectra indicate PANI is in the doped state and the doping increases with an increase in ZnS concentration. Fluorescence intensity passes through a minimum with ZnS content and the dc-conductivity of the composites gradually increases with an increase in ZnS NP concentration. The I-V plot of PAZ composites indicates that the photocurrent is higher than that of the dark current at each voltage, and the device exhibits reversible turning "on" and "off" by switching the white light illumination "on" and "off". Dye-sensitized solar cells fabricated with PAZ composites display a reasonably higher power conversion efficiency (η = 3.38%) than pure ZnS NPs. An attempt is made to shed light on the operating mechanism of the DSSC from the impedance data using a Cole-Cole plot by drawing an equivalent circuit illustrating the different electronic and ionic transport processes within the cell.