Showing papers by "Franco Cacialli published in 2015"

Inorganic caesium lead iodide perovskite solar cells

Abstract: The vast majority of perovskite solar cell research has focused on organic–inorganic lead trihalide perovskites. Herein, we present working inorganic CsPbI3 perovskite solar cells for the first time. CsPbI3 normally resides in a yellow non-perovskite phase at room temperature, but by careful processing control and development of a low-temperature phase transition route we have stabilised the material in the black perovskite phase at room temperature. As such, we have fabricated solar cell devices in a variety of architectures, with current–voltage curve measured efficiency up to 2.9% for a planar heterojunction architecture, and stabilised power conversion efficiency of 1.7%. The well-functioning planar junction devices demonstrate long-range electron and hole transport in this material. Importantly, this work identifies that the organic cation is not essential, but simply a convenience for forming lead triiodide perovskites with good photovoltaic properties. We additionally observe significant rate-dependent current–voltage hysteresis in CsPbI3 devices, despite the absence of the organic polar molecule previously thought to be a candidate for inducing hysteresis via ferroelectric polarisation. Due to its space group, CsPbI3 cannot be a ferroelectric material, and thus we can conclude that ferroelectricity is not required to explain current–voltage hysteresis in perovskite solar cells. Our report of working inorganic perovskite solar cells paves the way for further developments likely to lead to much more thermally stable perovskite solar cells and other optoelectronic devices.

Wavelength-Multiplexed Polymer LEDs: Towards 55 Mb/s Organic Visible Light Communications

Abstract: We present recent progress on visible light communication systems using polymer light-emitting diodes as the transmitters and a commercial silicon photodetector as the receiver In this paper, we use transmitters at red, green, and blue wavelengths to investigate the maximum on-off keying link performance of each device type as the first steps toward a wavelength-division multiplexed link We show that a total transmission speed of 13 Mb/s is achievable when considering the raw bandwidth of each of the RGB PLEDs Such a rate represents a 30% gain over previously demonstrated systems Further capacity improvement can be achieved using high performance artificial neural network equalizer offering a realistic prospect for transmission speeds up to 549 Mb/s

Thia- and selena-diazole containing polymers for near-infrared light-emitting diodes

Abstract: We report the optical characterization of near-infrared (NIR) emitters for polymer light-emitting diode (PLEDs) applications based on the copolymerization of a phthalimide-thiophene host polymer with a low-gap emitter containing the bisthienyl(benzotriazolothiadiazole) unit. We investigate different loadings of the low-gap emitter (in the range 1-3% by weight) and the substitution of a sulphur atom with a selenium atom to further extend the emission in the NIR up to 1000 nm. PLEDs based on copolymers with 1% loading give the best efficiency (0.09%) and show an almost pure NIR EL (95% in the NIR) peaking at 895 nm.

Low-Temperature Photoluminescence Spectroscopy of Solvent-Free PCBM Single-Crystals

Abstract: PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) is a highly soluble C60 derivative that is extensively used in organic solar cells, enabling power conversion efficiencies above 10%. Here we report, for the first time to the best of our knowledge, the photoluminescence of high-quality solvent-free PCBM crystals between room temperature and 4 K. Interestingly, the PL spectra of these crystals become increasingly structured as the temperature is lowered, with extremely well-resolved emission lines (and a minimum line width of ∼1.3 meV at 1.73 eV). We are able to account for such a structured emission by means of a vibronic coupling model including Franck–Condon, Jahn–Teller and Herzberg–Teller effects. Although optical transitions are not formally forbidden from the low-lying excited states of PCBM, the high symmetry of the electronically active fullerene core limits the intensity of the 0–0 transition, such that Herzberg–Teller transitions which borrow intensity from higher-lying states represent a large ...

Luminescent Properties of a Water-Soluble Conjugated Polymer Incorporating Graphene-Oxide Quantum Dots

Abstract: We report the incorporation of graphene-oxide quantum dots (GOQDs) into films, diluted solutions, and light-emitting diodes (LEDs) as part of a water-soluble derivative of poly(p-phenylene vinylene), or PDV.Li, to investigate their impact on the light-emission properties of this model conjugated polymer. Despite the well-known ability of graphene and graphene oxide to quench the photoluminescence of nearby emitters, we find that the addition of GOQDs to diluted solutions of PDV.Li does not significantly affect the photoluminescence (PL) dynamics of PDV.Li, bringing about only a modest quenching of the PL. However, loading the polymer with GOQDs led to a substantial decrease in the turn-on voltage of LEDs based on GOQD–PDV.Li composites. This effect can be attributed to either the improved morphology of the host polymer, resulting in an increase in the charge mobility, or the enhanced injection through GOQDs near the electrodes.

Investigation Of The Bragg-Snell Law In Photonic Crystals

Abstract: We present an optical experiment on photonic crystals suitable for an advanced physics laboratory course or a senior capstone project. Photonic crystals are periodically ordered composite systems made of materials that have different dielectric constants, and can be arranged in one, two, or three dimensions. They are characterized by a bandgap that depends on the size, arrangement and dielectric constant of the microstructures that make up the crystal. In addition, the bandgap spectrally shifts with the angle of incident light. These observations are captured by the Bragg-Snell law. In this paper, we describe a vertical deposition method for growing photonic crystals from a water suspension of polystyrene microspheres, as well as a simple transmission experiment that students can perform using a USBspectrometer to explore the Bragg-Snell law.

Experimental and Computational Study on the Temperature Behavior of CNT Networks

Abstract: We report on the electrical properties of carbon nanotubes networks deposited using spray deposition onto interdigitated gold structures. The properties have been measured experimentally and analyzed using a computational method. In particular, we studied the variation of resistance as a function of the temperature in networks having different nanotubes density. We found a good agreement between the simulation and the experimentally obtained results for the networks with higher densities. Our results suggest that the change in the temperature behavior is due to the desorption of oxygen molecules at relatively high temperature. This effect is found to be the function of the network density. To have a better insight on the conduction properties of the networks, we also model the voltage drop and the current flows across the networks and these results have been found in good agreement with experimental one reported in the literature.

Geometric and Electronic Structures of Boron(III)-Cored Dyes Tailored by Incorporation of Heteroatoms into Ligands

Abstract: Complexation of a boron atom with a series of bi- dentate heterocyclic ligands successfully gives rise to corre- sponding BF2-chelated heteroarenes, which could be consid- ered as novel boron(III)-cored dyes. These dye molecules ex- hibit planar structures and expanded p-conjugated back- bones due to the locked conformation with a boron center. The geometric and electronic structures of these BF2 com- plexes can be tailored by embedding heteroatoms in the unique modes to form positional isomer and isoelectronic structures. The structure-property relationship is further elu- cidated by studying the photophysical properties, electro- chemical behavior and quantum-chemical calculations.

Synthesis and photophysical characteristics of polyfluorene polyrotaxanes

Abstract: Two alternating polyfluorene polyrotaxanes (3·TM-βCD and 3·TM-γCD) have been synthesized by the coupling of 2,7-dibromofluorene encapsulated into 2,3,6-tri-O-methyl-β- or γ-cyclodextrin (TM-βCD, TM-γCD) cavities with 9,9-dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester. Their optical, electrochemical and morphological properties have been evaluated and compared to those of the non-rotaxane counterpart 3. The influence of TM-βCD or TM-γCD encapsulation on the thermal stability, solubility in common organic solvents, film forming ability was also investigated. Polyrotaxane 3·TM-βCD exhibits a hypsochromic shift, while 3·TM-γCD displays a bathochromic with respect to the non-rotaxane 3 counterpart. For the diluted CHCl3 solutions the fluorescence lifetimes of all compounds follow a mono-exponential decay with a time constant of ≈0.6 ns. At higher concentration the fluorescence decay remains mono-exponential for 3·TM-βCD and polymers 3, with a lifetime τ = 0.7 ns and 0.8 ns, whereas the 3·TM-γCD polyrotaxane shows a bi-exponential decay consisting of a main component (with a weight of 98% of the total luminescence) with a relatively short decay constant of τ1 = 0.7 ns and a minor component with a longer lifetime of τ2 = 5.4 ns (2%). The electrochemical band gap (ΔE g ) of 3·TM-βCD polyrotaxane is smaller than that of 3·TM-γCD and 3, respectively. The lower ΔE g value for 3·TM-βCD suggests that the encapsulation has a greater effect on the reduction process, which affects the LUMO energy level value. Based on AFM analysis, 3·TM-βCD and 3·TM-γCD polyrotaxane compounds exhibit a granular morphology with lower dispersity and smaller roughness exponent of the film surfaces in comparison with those of the neat copolymer 3.

Hybrid-Organic Photonic Structures for Light Emission Modification

Abstract: We report the modification of the photoluminescence (PL) and decay rates of two different green-emitting conjugated polymers incorporated into photonic crystals with the stop-bands spectrally tuned on their emission. We observe both suppression (in the stop-band) and enhancement (at the high-energy band-edge) of the photoluminescence. Time-resolved measurements also reveal a concomitant modification of the emission lifetime that is enhanced at the band-edge and suppressed within the stop-band, thus confirming a variation of the radiative decay rate of the excitations in such photonic nanostructures. We propose two examples of fluorescent photonic composite systems. The first consists of a hybrid Si-organic system, obtained by infiltration of the polymer inside a rugate filter (a 1D photonic crystal). The second example is a fully organic system obtained by self-assembling of solvent-compatible microspheres-polymer system for obtaining a synthetic opal (a 3D photonic crystal) with a uniform distribution of the emitting material across the photonic structure.

Deep-red electrophosphorescence from a platinum(II)–porphyrin complex copolymerised with polyfluorene for efficient energy transfer and triplet harvesting

Abstract: A series of polyfluorene-based polymers with a range of weight percentages (w/w) of a platinum(II)-containing porphyrin, 5,15-dimesityl-10,20-diphenylporphyrinato platinum(II) (MPP(Pt)), were synthesised and incorporated into organic light-emitting diodes. All polymers showed emission predominantly in the red/NIR region with only those polymers with porphyrin w/w of less than 2% showing residual tails at wavelengths lower than 600 nm, indicating increased emission from the porphyrin as w/w increases. The 2% loading of MPP(Pt) gave the highest efficiency LED (0.48%) and light output (2630 mW/m2).

Electrostatic discharge sensitivity investigation on organic field-effect thin film transistors

Abstract: Conventional CMOS technologies require high production costs, which are largely due to masks production and high specification equipment. Interestingly, conjugated polymers and plastic semiconductors offer potential substitutes to lower the production expenses. Such materials can be used in applications such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic photovoltaics (OPVs). However, electrical reliability, and especially potential damage by electrostatic discharge (ESD), is a major issue in the production cycle and cost, and also during operation. Here we present investigation of the ESD sensitivity of OFETs based on poly (3-hexylthiophene-2,5-diyl) (P3HT), which has so far received little or no attention in the literature.

Synergic Integration of Conjugated Luminescent Polymers and Three-Dimensional Silicon Microstructures for the Effective Synthesis of Photoluminescent Light Source Arrays

Abstract: In this work, a novel and straightforward technology for the fabrication of two-dimensional (2D) photoluminescent light source arrays by selective infiltration of conjugated luminescent polymers into three-dimensional (3D) silicon microstructures is presented. Poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) integration into 3D silicon microstructures is investigated by means of three different deposition techniques, namely, spin-coating, dip-coating and drop-casting/slow solvent evaporation. The microstructure is fabricated by electrochemical micromachining (ECM) technology and integrates 2D arrays of square holes with different sizes (about 40 and 4 μm), spatial periods (about 70 and 10 μm), and aspect ratios (ARs) (about one and ten). Notably, square holes with higher AR can be selectively filled with polymer using spin-coating and drop-casting techniques, whereas dip-coating technique allows selective polymer filling of square holes with lower AR. Independently of size, period and AR, each polymer-infiltrated hole behaves as a single light source, thus enabling the effective synthesis of 2D photoluminescent light source arrays.

Low-gap polymers incorporating a dicarboxylic imide moiety for near-infrared polymer light-emitting diodes

Abstract: We report our investigations of the photophysical properties of a so-called “donor-acceptor” (D-A) electroluminescent polymer incorporating N-alkyl-4,7-di(thien-2-yl)-2,1,3-benzothiadiazole-5,6-dicarboxylic imide (DI) as the electron acceptor and benzo[1,2-b:4,5-b′]dithiophene (BDT) acceptor units. Polymer light-emitting diodes (PLEDs) based on this copolymer give the best efficiency (external quantum efficiency EQE∼0.04%) and show a pure near-infrared (NIR) electroluminescence (EL) with a peak at 870 nm.