Suresh Chand

Polymer–Polymer Förster Resonance Energy Transfer Significantly Boosts the Power Conversion Efficiency of Bulk‐Heterojunction Solar Cells

Optically resonant donor polymers can exploit a wider range of the solar spectrum effectively without a complicated tandem design in an organic solar cell. Ultrafast Förster resonance energy transfer (FRET) in a polymer-polymer system that significantly improves the power conversion efficiency in bulk heterojunction polymer solar cells from 6.8% to 8.9% is demonstrated, thus paving the way to achieving 15% efficient solar cells.

Defect induced photoluminescence and ferromagnetic properties of bio-compatible SWCNT/Ni hybrid bundles.

Designing of bio-compatible nanomagnets with multiple functionalities receives immense scientific attention due to their potential applications in bio-labeling, medical diagnosis and treatment. Here we report the synthesis of Nickel (Ni) incorporated single-walled carbon nanotube (SWCNT) hybrid and bio-compatible bundles having interesting magnetic and photoluminescence (PL) properties. The SWCNT exhibits a high-crystallinity and it has an average diameter of ∼1.7 nm. Ni particles of 10-20 nm were incorporated within the SWCNT bundles. These hybrid bundles exhibit PL and it is attributed to the presence of delocalized π electrons and their recombination at the defective sites of SWCNT. Magnetic characterization revealed that the SWCNT/Ni hybrid bundle possesses a high (50 Oe) coercivity compared to bulk Ni and a long range ferromagnetic ordering at room temperature. MTT-assay has been conducted to study the cytotoxicity of these hybrid nanostructures.

Polymeric stabilization of hybrid nanocomposites: a comparison between in situ and ex situ-grown CuInS2 in poly(3-hexylthiophene) polymer

CuInS2 (CIS) particles were directly synthesized in P3HT matrix with different concentrations ratio of P3HT and CIS (1:2, 1:4, and 1:8) by decomposition of copper indium xanthate (CIX). Here, copper indium xanthate and P3HT were mixed homogeneously in o-dichlorobenzene (DCB), which induced the formation of the CIS nanoparticles by the thermal decomposition of the precursor compound in situ at temperatures as low as 110xa0°C. The effects of the precursor concentration on the size of the CIS nanoparticles was studied by microstructure investigations (TEM, AFM, XRD) and UV–vis measurements show that these CIS composites possess a direct bandgap energy higher than 1.45xa0eV depending on the concentration of P3HT. PL quenching of P3HT polymer (i.e., higher accessible fraction of fluorophores) was found to be more for in situ rather than ex situ conditions for comparable CIX concentrations or particle size. This can be attributed to the fact that in in situ synthesis, P3HT act as surface directing template for CIS nanoparticles which is not so in the case of ex situ synthesis. Due to this, the polymeric stabilization of the CIS nanocomposites is better realized for in situ synthesis as compared to ex situ synthesis.

Dipolar alignment and consequent enhanced charge transport in poly (9, 9′ di octyl fluorene)-2, 7-ylene ethylnylene

Current density–voltage characteristics of poly (9, 9′ di octyl fluorene)-2,7-ylene ethylnylene thin films (∼120 nm) have been studied in hole only device configuration at different temperatures (290–100u2009K) in unpolarized and polarized samples. The hole mobility has been found to be enhanced as a result of dipolar alignment by exposure to a dc electric field via cooling at all elevated temperatures. At higher field, current density has been found to be governed by trapped charge limited currents (TCLC) with hole mobility strongly dependent on electric field and their respective charge transport parameters have been obtained for both samples. The density of trap states has been found to be decreased on polarization from 1.1u2009×u20091018 to 7.6u2009×u20091017 cm−3 and trap energy has correspondingly decreased from 43 to 35 meV. The TCLC model with Poole–Frenkel-type field-dependent mobility has been fitted into the data and found to be in excellent agreement. Temperature dependence of zero field mobility (µ0) and disorde...

Photophysical and charge transport properties of pyrazolines

Pyrazoline, an intense green emitting molecule both in solution and solid state, with extended π-conjugation has been synthesized via simple two-step reactions in high yields. Having the electron rich pyrazoline moiety with good redox behavior, pyrazolines can be potential candidates for charge transport material in organic electronic devices. UV-Visible absorption spectra of pyrazolines exhibit peaks below 400 nm, which is a desired feature for charge transport materials because it avoids interference with donor absorption that falls in the visible to NIR region. Electrochemical and theoretical studies show that the HOMO energy level lies at around −4.8 to 5.2 eV depending on the substituents, which is in fact compatible with the PEDOT:PSS/P3HT and work function of the ITO electrode. The experimental hole transport value, measured using the hole only device and space charge limited current (SCLC) method, was found to be in the range of 10−5 to 10−6 cm2 V−1 s−1, depending on the substituents. The maximum hole mobility calculated by theoretical methods for the pyrazolines is 0.75 cm2 V−1 s−1.

Plasmon induced ultrafast injection of hot electrons in Au nanoislands grown on a CdS film

Metal nanoparticle semiconductor heterostructures exhibit unique optoelectronic properties and have potential applications in energy harvesting, photodetectors, photocatalysts, and optoelectronic devices. The hot carriers are formed at the metal nanostructure semiconductor interface, and their efficient injection into the surrounding media (semiconductor) is a great challenge and understanding the physics behind the charge transfer is currently a topic of global research. Herein, we report the investigation on the hot electron injection in the Au nanoislands formed on a CdS film using ultrafast femtosecond spectroscopy. When the Au–CdS film is excited with visible light (above the band gap), a complete bleaching effect is observed. However, when it is excited with the photon energies below the band gap of the CdS film, an absorption signal is observed over a broad spectral range. The ultrafast charge transfer dynamics studied herein indicate the possibility of the plasmons formed in the Au nanoislands, which directly decay nonradiatively by injecting electrons in the conduction band of the CdS film and charge recombination with the Au nanoislands. Finally, we demonstrate the charge transfer in the metal semiconductor hybrid, which exhibits a significant alteration in the ultrafast optical properties compared to its individual components and is dependent on the excitation energy and can thus be exploited in the light harvesting devices.

p-Type doping of tetrafluorotetracynoquinodimethane (F4TCNQ) in poly(para-phenylene vinylene) (PPV) derivative “Super Yellow” (SY)

In this paper, we report a case of hole transport in tetrafluorotetracynoquinodimethane doped poly(para-phenylene vinylene) derivative “Super Yellow”. The hole mobility of pristine and doped polymer thin films was determined by impedance spectroscopy. The increase in hole mobility upon doping was also verified by current density–voltage measurements. It was found to be increased by two orders of magnitude upon doping. The increase in hole mobility upon p-type doping was explained on the basis of extended Gaussian disorder model by measuring current density–voltage characteristics in the same devices. At low bias, J–V characteristics exhibit clear space charge limited conduction in pristine state but ohmic behaviour when doped. Further, at higher voltages the current density increases non-linearly due to the field-dependent mobility and carrier concentration by filling of tail states of highest occupied molecular orbital of the host material. The room temperature J–V characteristics were well described with the single value of free hole density at low fields. At higher fields it becomes field-dependent followed by a field enhancement factor γ.

Charge transport study of polymer light emitting diodes by impedance spectroscopy

The transport properties of polymer blend have been investigated by using impedance spectroscopy (IS) and current density –voltage (J-V) measurements based on a single-layer structure of ITO/polymer/Al. Impedance spectroscopy measurements showed that the device can be simulated by an equivalent parallel R-C network with a contact resistance Rs in series, the resistance R decreases along with bias voltage. The variation of the whole conductance and capacitance of the device with frequency and bias can be understood by the carrier response and the transit time dependence on the bias in the polymer film.

Enhancement in charge transfer by non-ligand exchange process for colloidal hybrid organic(MEH-PPV):inorganic(CdSe) nanocomposites

In this work, we demonstrate the effect of surface modification of as-synthesized oleylamine-capped spherical CdSe QDs of size (5-7 nm). The as-prepared CdSe QDs are highly luminescent, monodispersive and exhibit energy transfer effects upon their dispersion in MEH-PPV polymer matrix. However, repetitive washing of CdSe QDs upon suitable chemical treatment leads to enhancement in charge transfer process as observed in their corresponding MEH-PPV: CdSe nanocomposites. Here, no evidence of agglomeration effects and surface states were found. This enhancement in charge transfer is mainly due to the partial removal of oleylamine capping ligand, which acts as a hindrance in the interaction between polymer and CdSe QDs. The importance of this study is that as-synthesized CdSe QDs show effective energy transfer whereas after chemical treatment, it shows enhanced charge transfer mechanism which makes their corresponding nanocomposites useful for different applications in organic electronic devices such as efficient electroluminescent (OLED) and photovoltaic (OPV) devices respectively.

Hybrid organic (P3HT and MEH-PPV)-inorganic (CuInGaSe2) nanocomposites: charge transfer and photostability studies

In our work, we demonstrate the use of Copper Indium Gallium Diselenide(CIGSe) nanocrystals as electron acceptor in conducting polymer POLY(3-HEXYLTHIOPHENE)(P3HT) AND POLY(P-PHENYLENE VINYLENE) (PPV). We study the photo-induced charge separation and photostability of CIGSe-conducting polymer nanocomposites for the application of solar cells. CIGSe with P3HT ensures efficient charge transfer process across polymer-CIGSe interface as evident from higher quenching of Photoluminescence(PL) emission and higher value of rates of PL decays(PL/PLi), Stern-Volmer quenching constant Kvs and number of accessible fluorophores( Fa), as compared to MEH-PPV polymer. P3HT polymer owing to its crystalline lamellar structure and ordered morphology impart stability to CIGSe composite. Hence superior morphology and photostability as compared to MEH-PPV:CIGSe nanocomposites. P3HT:CIGSe nanocomposites may be exploited as potential active layer in photovoltaic and photoelectrochemical devices.