Varun Vohra

Toward White Light Emission through Efficient Two-Step Energy Transfer in Hybrid Nanofibers

Nanosized zeolite L crystals containing about 550 strongly luminescent acceptor molecules have been modified by grafting a conjugated oligomer on their external surface. The 25 nm sized crystals have consequently been embedded in polymeric nanofibers obtained by electrospinning. The fluorescent molecule grafted on the external surface allows addressing the guests in the zeolite nanochannels through an efficient two-step energy transfer from the polymer nanofiber. The so obtained hybrid nanofibers exhibit intense emissions from the three fundamental colors using a single excitation wavelength. The molecule grafted on the external surface of the nanocrystal also induces a higher compatibility of the hybrid organic/inorganic nanomaterials in the conjugated polymer and therefore high concentrations of zeolites embedded in the nanofibers are obtained. Playing on this concentration, the emission of the nanofiber can be tuned and eventually be used for fabricating white-light emitting nanofibers. This hybrid nanomaterial opens new perspectives for low-cost nano organic light emitting diodes fabrication with considerable impact on the lighting and display technologies.

Highly Emissive Nanostructured Thin Films of Organic Host–Guests for Energy Conversion

All-organic nanostructured host-guest systems, based on dyes inserted in the nanochannels of perhydrotriphenylene (PHTP) and deoxycholic acid (DCA), show enhanced fluorescence properties with quantum yields even higher than those of the dyes in solution, thanks to the high concentration of emissive molecules with controlled spatial and geometrical organization that prevents aggregation quenching. Both host molecules crystallize, growing with the long axis oriented along the direction of the nanochannels where the linear-chain dyes are inserted, to yield crystals emitting well-polarized light. For the DCA-based host-guests, homogeneous thin films suitable for several applications are obtained. Colour emission in such films can be tuned by co-inclusion of two or three dyes due to resonant energy-transfer processes. We show that films obtained by low-cost techniques, such as solution casting and spin-coating, convert UV light into visible light with an efficiency much higher than that of the standard polymeric blends.

Multilevel organization in hybrid thin films for optoelectronic applications.

In this work we report two simple approaches to prepare hybrid thin films displaying a high concentration of zeolite crystals that could be used as active layers in optoelectronic devices. In the first approach, in order to organize nanodimensional zeolite crystals of 40 nm diameter in an electroactive environment, we chemically modify their external surface and play on the hydrophilic/hydrophobic forces. We obtain inorganic nanocrystals that self-organize in honeycomb electroluminescent polymer structures obtained by breath figure formation. The different functionalizations of the zeolite surface result in different organizations inside the cavities of the polymeric structure. The second approach involving soft-litography techniques allows one to arrange single dye-loaded zeolite L crystals of 800 nm of length by mechanical loading into the nanocavities of a conjugated polymer. Both techniques result in the formation of thin hybrid films displaying three levels of organization: organization of the dye molecules inside the zeolite nanochannels, organization of the zeolite crystals inside the polymer cavities, and micro- or nanostructuration of the polymer.

Organic solar cells based on nanoporous P3HT obtained from self-assembled P3HT:PS templates

We demonstrate a solution-based method to create vertical nanoporous structures of semiconducting polymer poly(3-hexylthiophene) over large areas by taking advantage of the spontaneous phase segregation between poly(3-hexylthiophene) and polystyrene deposited from a single solution and, in a second step, removing polystyrene by selectively dissolving it. Nanoporous films with pore diameters which can be tuned down to 120 nm are produced by varying the composition. The potential of the method is further demonstrated by fabricating fully operational solar cells after deposition of [6,6]-phenyl-C61-butyric acid methyl ester from an adequate solvent. Under optimized conditions, the devices based on nanostructured thin films exhibit enhanced efficiencies with respect to graded bilayers and bulk heterojunction organic photovoltaic devices. We relate the increase in fill factor observed in the nanostructured devices to changes in the orientation of poly(3-hexylthiophene) chains induced by nanoconfinement and self-assembly with polystyrene resulting from this simple solution process without the use of any elaborate chemistry or soft lithography.

Bifunctional microstructured films and surfaces obtained by soft lithography from breath figure arrays

Breath figure films of polystyrene and polyalkylthiophene presenting regular patterns of 600 nm and 5 µm are used as templates to prepare negative polydimethylsiloxane stamps containing an entrapped quantity of the primary polymer. These breath figure replica are used to generate single or double layers of the entrapped conjugated polymer in the form of networks on any substrates. Besides this, polystyrene printed in this way can be used as a resist mask that allows the polymerization of aniline in a regular micrometric arrangement on a platinum electrode or to generate a regular acid/base patterning on a plain polyaniline film.

Addition of regiorandom poly(3-hexylthiophene) to solution processed poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester graded bilayers to tune the vertical concentration gradient

Donor-acceptor vertical concentration gradient in the active layer is of crucial importance in graded bilayer poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cells. We demonstrate that upon addition of regiorandom P3HT to graded regioregular P3HT:PCBM bilayers, we are able to tune the vertical concentration gradient. With the help of energy-dispersive x-ray spectroscopy elemental mapping of the device cross-sections, we find a strong relationship between the concentration gradient profile and the device performances. Upon addition of regiorandom P3HT, the devices exhibit power conversion efficiencies up to 3.83% (compare to 3.09% for regioregular P3HT devices).

Self-assembled nanofibers of fluorescent zeolite L crystals and conjugated polymer.

Through this work, we present self-assembled structures which can be obtained by mixing surface modified dye loaded zeolite L crystals and cationic precursors of a conjugated polymer. The zeolite crystals are modified with anionic end groups which give the former a polyanionic character and allow a polyelectrolytic assembly. Microfluidic forces, introduced during the drying of a drop of water containing both polyelectrolytes casted on a clean glass substrate, and localized adsorption on the single zeolite crystal lead to the formation of micro- and nanofibers of highly ordered zeolite nanocrystals. As a result, the fibers display a very well polarized emission from the organic dye included into the nanochannels of the inorganic crystal. Playing on the relative concentrations of polycation and zeolites, and after thermal conversion of the polymer precursor, rigid and insoluble fibers with diameters ranging from less than 200 nm to a few micrometers are obtained.

Uniaxial macroscopic alignment of conjugated polymer systems by directional crystallization during blade coating

We have developed a one step method for the deposition of uniaxially aligned conjugated polymer layers by directional epitaxial crystallization directly from solution. Oriented, square centimeter sized samples of poly(3-hexylthiophene) (P3HT) deposited from a chlorobenzene (CB) solution containing the additional crystallizable solvent 1,3,5-trichlorobenzene (TCB) are obtained by blade coating. Moreover, we show that the developed technique is not restricted to this specific material combination, but is instead applicable to a range of conjugated polymers and crystallizable solvents. Finally, we demonstrate the potential of this technique by realizing an organic photovoltaic device that exhibits a polarization dependent photoresponse.

Low-Cost and Green Fabrication of Polymer Electronic Devices by Push-Coating of the Polymer Active Layers

Because of both its easy processability and compatibility with roll-to-roll processes, polymer electronics is considered to be the most promising technology for the future generation of low-cost electronic devices such as light-emitting diodes and solar cells. However, the state-of-the-art deposition technique for polymer electronics (spin-coating) generates a high volume of chlorinated solution wastes during the active layer fabrication. Here, we demonstrate that devices with similar or higher performances can be manufactured using the push-coating technique in which a poly(dimethylsiloxane) (PDMS) layer is simply laid over a very small amount of solution (less than 1μL/covered cm2), which is then left for drying. Using mm thick PDMS provides a means to control the solvent diffusion kinetics (sorption/retention) and removes the necessity for additional applied pressure to generate the desired active layer thickness. Unlike spin-coating, push-coating is a slow drying process that induces a higher degree of crystallinity in the polymer thin film without the necessity for a post-annealing step. The polymer light-emitting diodes and solar cells prepared by push-coating exhibit slightly higher performances with respect to the reference spin-coated devices, whereas at the same time reduce the amounts of active layer materials and chlorinated solvents by 50 and 20 times, respectively. These increased performances can be correlated to the higher polymer crystallinities obtained without applying a post-annealing treatment. As push-coating is a roll-to-roll compatible method, the results presented here open the path to low-cost and eco-friendly fabrication of a wide range of emerging devices based on conjugated polymer materials.

Transfer-printing of active layers to achieve high quality interfaces in sequentially deposited multilayer inverted polymer solar cells fabricated in air

Abstract Polymer solar cells (PSCs) are greatly influenced by both the vertical concentration gradient in the active layer and the quality of the various interfaces. To achieve vertical concentration gradients in inverted PSCs, a sequential deposition approach is necessary. However, a direct approach to sequential deposition by spin-coating results in partial dissolution of the underlying layers which decreases the control over the process and results in not well-defined interfaces. Here, we demonstrate that by using a transfer-printing process based on polydimethylsiloxane (PDMS) stamps we can obtain increased control over the thickness of the various layers while at the same time increasing the quality of the interfaces and the overall concentration gradient within the active layer of PSCs prepared in air. To optimize the process and understand the influence of various interlayers, our approach is based on surface free energy, spreading parameters and work of adhesion calculations. The key parameter presented here is the insertion of high quality hole transporting and electron transporting layers, respectively above and underneath the active layer of the inverted structure PSC which not only facilitates the transfer process but also induces the adequate vertical concentration gradient in the device to facilitate charge extraction. The resulting non-encapsulated devices (active layer prepared in air) demonstrate over 40% increase in power conversion efficiency with respect to the reference spin-coated inverted PSCs.