Aurora Rizzo

White organic light-emitting devices with CdSe/ZnS quantum dots as a red emitter

White hybrid organic/inorganic light-emitting devices (LEDs) have been fabricated by using stable red-emitting CdSe/ZnS core-shell quantum dots (QDs) covered with a trioctylphosphine oxide organic ligand. The device-active structure consists in a host/guest system with a blue-emitting poly[(9,9-dihexyloxyfluoren-2,7-diyl)-alt-co-(2-methoxy-5-{2-ethylhexyloxy}phenylen-1,4-diyl)] (PFH-MEH) polymer doped with red-emitting QDs and a green emitting metal chelate complex Alq3, with improved electron injection and transfer properties. A fairly pure white OLED with Commission Internationale de l’Eclairage coordinates of (0.30,0.33) is fabricated by accurate control of the Forster energy and charge-transfer mechanisms between the different device constituents obtained by tuning the concentration ratio of the QDs/PFH-MEH blend. In particular, charge-transfer processes to CdSe/ZnS core-shell quantum dots are found to be the key element for well-balanced white emission. Maximum external quantum efficiency up to 0.24%...

Investigating charge dynamics in halide perovskite-sensitized mesostructured solar cells

Charge generation and transport in (CH3NH3)PbI3−xClx sensitized mesostructured solar cells are investigated. A highly efficient charge generation is directly proven by time correlated single photon counting analysis. Photoinduced absorption and transient photovoltage investigations depict double charge recombination dynamics. To explain the high device performances according to those spectroscopic observations, we suggest the existence of two complementary paths for electron transport, involving either TiO2 or perovskite matrixes.

Polarized light emitting diode by long-range nanorod self-assembling on a water surface.

We demonstrate a straightforward strategy to fabricate a multilayer inorganic/organic polarized light-emitting diode device based on highly ordered arrays of rod-shaped nanocrystals as the active species. We have developed a simple and effective method that allows colloidal CdSe/CdS core/shell nanorods to be laterally aligned in smectic or nematic phases on the surface of water. A floating film of such ordered nanorods has been collected by a poly(dimethylsiloxane) (PDMS) stamp pad and transferred by contact printing onto previously evaporated organic layers. Thanks to the lateral nanorod alignment the as-prepared film exhibited strong polarized photoluminescence and it has been used as emissive layer in the polarized electroluminescent device.

Blue light emitting diodes based on fluorescent CdSe∕ZnS nanocrystals

The authors report on the blue electroluminescence from CdSe∕ZnS core/shell nanocrystals prepared from ultrasmall, magic size CdSe clusters that have a diameter of less than 2nm. The light emitting device consists of an active layer of nanocrystals blended with 4,4′,N,N′- diphenylcarbazole and an evaporated electron transporting/hole blocking layer made of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline. A blue, stable electroluminescence at 485nm from the hybrid device was observed, in good agreement with the photoluminescence spectra of a solid film of the same nanocrystals used for the device.

Effect of Mesostructured Layer upon Crystalline Properties and Device Performance on Perovskite Solar Cells

One of the most fascinating characteristics of perovskite solar cells (PSCs) is the retrieved obtainment of outstanding photovoltaic (PV) performances withstanding important device configuration variations. Here we have analyzed CH3NH3PbI3-xClx in planar or in mesostructured (MS) configurations, employing both titania and alumina scaffolds, fully infiltrated with perovskite material or presenting an overstanding layer. The use of the MS scaffold induces to the perovskite different structural properties, in terms of grain size, preferential orientation, and unit cell volume, in comparison to the ones of the material grown with no constraints, as we have found out by X-ray diffraction analyses. We have studied the effect of the PSC configuration on photoinduced absorption and time-resolved photoluminescence, complementary techniques that allow studying charge photogeneration and recombination. We have estimated electron diffusion length in the considered configurations observing a decrease when the material is confined in the MS scaffold with respect to a planar architecture. However, the presence of perovskite overlayer allows an overall recovering of long diffusion lengths explaining the record PV performances obtained with a device configuration bearing both the mesostructure and a perovskite overlayer. Our results suggest that performance in devices with perovskite overlayer is mainly ruled by the overlayer, whereas the mesoporous layer influences the contact properties.

White Light with Phosphorescent Protein Fibrils in OLEDs

Red and yellow phosphorescent insulin amyloid fibrils are used as guest-emitting species within a blue-emitting polyfluorene matrix in light-emitting diodes. The integration of the phosphorescent Ir-complex into the amyloid structures strongly improves the triplet exciton confinement and allows the fabrication of white-emitting device with a very low loading of phosphorescent complex. The overall performances of the devices are improved in comparison with the corresponding bare Ir-complexes. This approach opens a way to explore novel device architectures and to understand the exciton/charge transfer dynamics in phosphorescent light emitting diodes.

Growing perovskite into polymers for easy-processable optoelectronic devices

Here we conceive an innovative nanocomposite to endow hybrid perovskites with the easy processability of polymers, providing a tool to control film quality and material crystallinity. We verify that the employed semiconducting polymer, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), controls the self-assembly of CH3NH3PbI3 (MAPbI3) crystalline domains and favors the deposition of a very smooth and homogenous layer in one straightforward step. This idea offers a new paradigm for the implementation of polymer/perovskite nanocomposites towards versatile optoelectronic devices combined with the feasibility of mass production. As a proof-of-concept we propose the application of such nanocomposite in polymer solar cell architecture, demonstrating a power conversion efficiency up to 3%, to date the highest reported for MEH-PPV. On-purpose designed polymers are expected to suit the nanocomposite properties for the integration in diverse optoelectronic devices via facile processing condition.

Poly-(3-hexylthiophene)/[6,6]-phenyl-C61-butyric-acid-methyl-ester bilayer deposition by matrix-assisted pulsed laser evaporation for organic photovoltaic applications

A poly-(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) bilayer structure has been realized by single step matrix-assisted pulsed laser evaporation (ss-MAPLE) technique using the same solvent for both the polymers under vacuum conditions. Our ss-MAPLE procedure allows the fabrication of polymeric multilayer device stacks, which are very difficult to realize with the conventional solvent assisted deposition methods. A proof of concept bilayer P3HT/PCBM solar cell based on ss-MAPLE deposition has been realized and characterized. This demonstration qualifies ss-MAPLE as a general and alternative technique for the implementation of polymeric materials in hetero-structure device technology.

Bulk Heterojunction versus Diffused Bilayer: The Role of Device Geometry in Solution p-Doped Polymer-Based Solar Cells.

We exploit the effect of molecular p-type doping of P3HT in diffused bilayer (DB) polymer solar cells. In this alternative device geometry, the p-doping is accomplished in solution by blending the F4-TCNQ with P3HT. The p-doping both increases the film conductivity and reduces the potential barrier at the interface with the electrode. This results in an excellent power conversion efficiency of 4.02%, which is an improvement of ∼48% over the p-doped standard bulk heterojunction (BHJ) device. Combined VOC-light intensity dependence measurements and Kelvin probe force microscopy reveal that the DB device configuration is particularly advantageous, if compared to the conventional BHJ, because it enables optimization of the donor and acceptor layers independently to minimize the effect of trapping and to fully exploit the improved transport properties.

Multifunctional platinum porphyrin dendrimers as emitters in undoped phosphorescent based light emitting devices

The authors report on a class of platinum porphyrin based phosphorescent multifunctional first generation dendrimers (Pt-8Cn-TPP) incorporating a platinum porphyrin core as the emissive center and carbazole side groups as the hole, as well as energy transport fragments. Furthermore, the alkyl groups prevent aggregation and allow for the formation of transparent homogeneous films directly from solution. The intra- and intermolecular energy transfers by Forster and Dexter mechanisms from the carbazole side groups to the platinum porphyrin core have been investigated. Light emitting devices were fabricated and bright red electrophosphorescence was achieved from a single undoped emitting layer of Pt-8C4-TPP. The results indicate that platinum porphyrin dendrimers are very promising to make bright, red saturated and spin-coated phosphorescent devices with a simplified undoped emissive layer.