Arnab Shit

Graphene Quantum Dots from a Facile Sono-Fenton Reaction and Its Hybrid with a Polythiophene Graft Copolymer toward Photovoltaic Application

A new and facile approach for synthesizing graphene quantum dots (GQDs) using sono-Fenton reaction in an aqueous dispersion of graphene oxide (GO) is reported. The transmission electron microscopy (TEM) micrographs of GQDs indicate its average diameter as ∼5.6 ± 1.4 nm having a lattice parameter of 0.24 nm. GQDs are used to fabricate composites (PG) with a water-soluble polymer, polythiophene-g-poly[(diethylene glycol methyl ether methacrylate)-co-poly(N,N-dimethylaminoethyl methacrylate)] [PT-g-P(MeO2MA-co-DMAEMA), P]. TEM micrographs indicate that both P and PG possess distinct core-shell morphology and the average particle size of P (0.16 ± 0.08 μm) increases in PG (0.95 ± 0.45 μm). Fourier transform infrared and X-ray photoelectron spectrometry spectra suggest an interaction between -OH and -COOH groups of GQDs and -NMe2 groups of P. A decrease of the intensity ratio of Raman D and G bands (ID/IG) is noticed during GQD and PG formation. In contrast to GO, GQDs do not exhibit any absorption peak for its smaller-sized sp(2) domain, and in PG, the π-π* absorption of polythiophene (430 nm) of P disappears. The photoluminescence (PL) peak of GQD shifts from 450 to 580 nm upon a change in excitation from 270 to 540 nm. PL emission of PG at 537 nm is quenched, and it shifts toward lower wavelength (∼430 nm) with increasing aging time for energy transfer from P to GQDs followed by up-converted emission of GQDs. Both P and PG exhibit semiconducting behavior, and PG produces an almost reproducible photocurrent. Dye-sensitized solar cells (DSSCs) fabricated with an indium-titanium oxide/PG/graphite device using the N719 dye exhibit a short-circuit current (Jsc) of 4.36 mA/cm(2), an open-circuit voltage (Voc) of 0.78 V, a fill factor of 0.52, and a power conversion efficiency (PCE, η) of 1.76%. Extending the use of GQDs to fabricate DSSCs with polypyrrole, both Voc and Jsc increase with increasing GQD concentration, showing a maximum PCE of 2.09%. The PG composite exhibits better cell viability than the components.

Optoelectronic and photovoltaic properties of graphene quantum dot–polyaniline nanostructures

In aqueous dispersions of graphene quantum dots (GQDs) produced by a sono-Fenton method, aniline is in situ polymerized to produce different polyaniline (PANI)–GQD hybrids (PAGD) without using external dopant. FTIR studies indicate that the carboxylic acid groups of the GQDs dope PANI well. The UV-Vis spectra exhibit a π to polaron band transition of the PAGD hybrids and show a gradual red shift with increasing intensity for increasing amounts of GQDs due to the gradual uncoiling and increase of polarons in the doped PANI chains. The fluorescence intensity of the GQDs is drastically quenched in the PAGD hybrids suggesting effective charge transfer between the GQDs and PANI chains. The X-ray diffraction study suggests the presence of a lamellar structure with a lamellar thickness of 13.57 A. The morphologies of the PAGD hybrids studied using field emission scanning electron microscopy exhibit a change from flakes to rods with increasing GQD concentration, which has been attributed to the change from a flat to cylindrical lamella formation. The thermogravimetric analysis result indicates that, in comparison to HCl-doped PANI, the PAGD hybrids exhibit better thermal stability. In the PAGD composites the dc conductivity increases by three orders compared to that of the GQDs due to polaron formation in the PANI chains. The current–voltage (I–V) characteristics of the PAGD composites indicate semiconducting behaviour and on irradiation with light an almost reversible photoresponse occurs. Dye-sensitized solar cells (DSSCs) fabricated with the PAGD hybrids and N719 dye indicate a highest power conversion efficiency (PCE) of 3.12%. Impedance data of the PAGD hybrids exhibit semicircular Cole–Cole plots indicating the presence of a resistance (R)–capacitance (C) circuit where the capacitance is in parallel to the bulk resistance which increases with increasing GQD concentration. The Debye plot and the dielectric permittivity values also support the variation of the photovoltaic properties of the PAGD hybrids. The impedance spectra of the DSSCs indicate the presence of three semicircles exhibiting a complex equivalent circuit composed of three R–C circuits, and analysis of the data yields the lifetime values of photo-injected electrons supporting the PCE variation of the PAGD hybrids.

Dye-sensitized solar cell from a new organic n-type semiconductor/polyaniline composite: insight from impedance spectroscopy

Aniline was in situ polymerized in the presence of N,N-di((S)-1-carboxylethyl)-3,4 : 9,10 perylenetetracarboxyldiimide (PTCDA, n-type semiconductor) in dilute acetic acid medium at different concentrations of PTCDA to produce PANI–PTCDA composites (P10, P15 and P20) with PTCDA concentrations 9.8, 13.1 and 21.3% (w/w), respectively. Both FESEM and TEM images indicated that the nanotubular morphology of PANI was retained, with formation of new hairy surfaces in the composites. The FTIR and UV-Vis spectra suggested the presence of π-stacking interaction between PTCDA and PANI produced in the doped state. Blue shifts in the π to π* and π to polaron band transitions indicated H-aggregate formation and secondary de-doping, respectively. Among the composites, the DC-conductivity of P15 sample exhibited the highest value (1.1 × 10−2 S cm−1) and fully reversible nature of the photocurrent. The HOMO and LUMO levels for PTCDA (−5.85 eV and −3.90 eV), determined from cyclic voltammetry, were lower than those of PANI, which transfers its photo-excitons to LUMO of PTCDA generating the photocurrent. Dye-sensitized solar cells (DSSCs) fabricated with N719 dye exhibited the highest power conversion efficiency (PCE) of 2.88% for the P15 sample. The IPCE results indicated a maximum 52% conversion of incident photon in the range 360–660 nm and the longevity of the cell was found to be satisfactory. Nyquist plots of the impedance spectra of the DSSCs indicated the presence of three semicircles in each sample, representing a complex equivalent circuit diagram consisting of three resistance and capacitance circuits. The middle semicircle yields the resistance (Rrc) and chemical capacitance (Cμ) between PANI–PTCDA and electrolyte interface and these are highest for the P15 sample yielding higher lifetime and hence lesser back-reaction of the injected photoelectrons at the PANI–PTCDA/electrolyte interface, contributing to the highest PCE value in the composites.

Nanochannel morphology of polypyrrole-ZnO nanocomposites towards dye sensitized solar cell application†

Polypyrrole (PPy)–zinc oxide (ZnO) nanocomposites (PZ1, PZ2 and PZ3) are prepared by polymerizing pyrrole (1.3 × 10−6 mol) in the presence of various amounts of ZnO nanoparticles (NPs) (25, 100, 250 mg) in a lauric acid (LA)–cetyl trimethyl ammonium bromide (CTAB) coacervate gel template. PPy formed in the gel template has a nanorod morphology, but it gradually changes exhibiting an open nanotube (nanochannel) morphology in the PZ3 sample, although ZnO has a nanosphere morphology. PPy nanorod formation is attributed to the coalescence of PPy nanospheres and the nanochannel morphology of PZ3 is attributed to the restricted growth of PPy in the gel pores due to the steric hindrance of the ZnO NPs. The ZnO NPs have a hexagonal wurtzite structure, which is also retained in the PZ samples. FTIR and UV-vis spectra indicate the presence of an interaction between the PPy and ZnO NPs. The ZnO NPs exhibit only UV band gap fluorescence at 390 nm, and in the PZ3 sample there is a reduction in the peak intensity with a red shift of 8 nm, supporting a strong electronic interaction between PPy and the ZnO NPs. The dc conductivity of the PZ3 composite is the highest (7.9 × 10−5 S cm−1) among the other composites, and also among the components. A dye sensitized solar cell (DSC) fabricated with the PZ3 composite using N719 dye as the photosensitizer has the cell characteristics Voc = 0.56 V, Jsc = 8.54 mA cm−2 and the fill factor (FF) = 0.53 giving an overall power conversion efficiency η = 2.53%, which is higher than those fabricated with pure ZnO NPs (η = 1.87%) and the PZ1 (1.98%) samples.

Robust hybrid hydrogels with good rectification properties and their application as active materials for dye-sensitized solar cells: insights from AC impedance spectroscopy

Dihybrid (GP) and trihybrid (GPPS) hydrogels constructed by 5,5′-(1,3,5,7-tetraoxopyrrolo[3,4-f]isoindole-2,6-diyl)diisophthalic acid (P), graphene oxide (GO) and P, GO, and PEDOT:PSS are studied. The gelation occurs through H-bonding and π–π stacking interactions. A magnificent improvement occurs in the mechanical properties of the hybrid gels compared to the P gel. The dc-conductivity of GPPS xerogels is 4–5 orders of magnitude higher compared to the GP and P xerogels. The current–voltage (I–V) characteristic curves of the GP and GPPS gels exhibit rectification properties with the highest rectification ratio of 61; however, the xerogels exhibit semiconducting nature. The GP and GPPS xerogels exhibit photoresponse behavior, and the on–off cycles display a stable photocurrent for the latter system. Dye-sensitized solar cells (DSSCs) are fabricated taking the GPPS gels as active materials and the power conversion efficiency (PCE) increases with the increase of PEDOT:PSS concentration showing a maximum PCE of 4.5%. The IPCE curve shows an absorption range of 360–700 nm with the maximum absorbance of ∼57%. Impedance spectroscopy indicates a considerable difference in the Nyquist plots between the xerogels and gels; the latter exhibit a semicircle with an additional spike corresponding to the Warburg impedance. The Cole–Cole plot of the DSSC consists of three semicircles and the equivalent resistance–capacitance circuits elucidate the PCE values.

Enhancement of Energy Storage and Photoresponse Properties of Folic Acid–Polyaniline Hybrid Hydrogel by in Situ Growth of Ag Nanoparticles

Electrically conductive hydrogels are a fascinating class of materials that exhibit multifarious applications such as photoresponse, energy storage, etc., and the three-dimensional micro- and nanofibrillar structures of the gels are the key to those applications. Herein, we have synthesized a hybrid hydrogel based on folic acid (F) and polyaniline (PANI) in which F acts as a supramolecular cross-linker of PANI chains. The gels are mechanically robust and are characterized by field-emission scanning electron microscopy, transmission electron microscopy, and spectroscopic, rheological, and universal testing measurements. The hybrid xerogel exhibit a BET surface area 238 m2 g-1, conductivity of 0.04 S/cm, specific capacitance of 295 F/g at a current density of 1A/g, and photocurrent of ∼2 mA under white-light illumination. Silver nanoparticles (AgNPs) are in situ grown to elegantly improve the conductivity, energy storage, and photoresponse capability of the gels. The formation of AgNPs drastically improves the specific capacitances up to 646 F/g (at current density 1A/g), excellent rate capability (403 F/g at 20 A/g), and stable cycling performance with a retention ratio of 74% after 5000 cycles. The AgNPs embedded gel exhibits dramatic enhancement of photocurrent to 56 mA, and its time-dependent photoillumination corroborates faster rise and decay of current compared to those of folic acid-polyaniline hydrogel.

Engineering of modular organic photovoltaic devices with dye sensitized architecture

In the last two decades, organic photovoltaic devices, primarily bulk heterojunction solar cells, have progressed much in device performances but the requirement of a crystalline nanoscale phase separated domain has limited the modular characteristic of the approach and very specific donor:acceptor blends are preferred. Here, we show an alternative organic photovoltaic platform with dye sensitized architecture with excellent modular characteristics. Aniline in situ polymerized in the presence of different amounts (10, 12, 16, and 20 mg) of N,N-di((S)-1-carboxylphenylalanine)-3,4:9,10-perylenetetracarboxyldiimide (PPA) in citric acid medium results in composites designated as PP10, PP12, PP16 and PP20, respectively. Dye-sensitized organic solar cells fabricated with PP16 and N719 dye exhibit the highest power conversion efficiency (PCE) of 4.32%. SEM study reveals dense rod-like nanosphere morphology; FTIR, UV-vis, and photoluminescence spectra suggest the presence of a π-stacking interaction between PPA and polyaniline (PANI). Incident photon to current conversion efficiency (IPCE) results exhibit a maximum of 62% conversion of incident photons absorbing in the range of 360–680 nm. The lifetime of photo-injected electrons measured from impedance spectra confirms that the PP16 device has the highest lifetime (7.7 ms) accounting for the highest PCE. Comparison with the PCE of a reference device clearly indicates the influence of dopant acid of PANI and influence of interfacial interactions through a π-stacking process towards photovoltaic performance.

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.