Project

SYNCHRONICS - Supramolecular materials for optoelectronics and photonics

Goal: SYNCHRONICS (EU-funded MSCA ETN action, 2015-2018) is implementing a joint training and research programme for 15 Early Stage Researchers on the synthesis, characterisation and application to photonics of supramolecularly-engineered organic materials. Such materials are incorporated in state-of-the-art photonic devices fabricated using top-quality facilities and unique expertise available within the network.
www.synchronics-etn.eu

Date: 1 January 2015

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Project log

Franco Cacialli
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The energy gap law ( E G -law) and aggregation quenching are the main limitations to overcome in the design of near-infrared (NIR) organic emitters. Here, we achieve unprecedented results by synergistically addressing both of these limitations. First, we propose porphyrin oligomers with increasing length to attenuate the effects of the E G -law by suppressing the non-radiative rate growth, and to increase the radiative rate via enhancement of the oscillator strength. Second, we design side chains to suppress aggregation quenching. We find that the logarithmic rate of variation in the non-radiative rate vs. E G is suppressed by an order of magnitude with respect to previous studies, and we complement this breakthrough by demonstrating organic light-emitting diodes with an average external quantum efficiency of ~1.1%, which is very promising for a heavy-metal-free 850 nm emitter. We also present a novel quantitative model of the internal quantum efficiency for active layers supporting triplet-to-singlet conversion. These results provide a general strategy for designing high-luminance NIR emitters.
Giuseppe Maria Paternò
added a research item
The high power-per-weight ratio displayed by metal-halide perovskite PVs is a key advantage of these promising devices for applications that require low payload, such as in space and avionics. However, little is known on the effect of the outer space radiation environment on these devices. Here, we report the first in operando study on fast neutron irradiation of perovskite solar cells. We show a remarkable resilience of these devices against one of the most hazardous form of radiation that can be found at flight altitude and in space. In particular, our results highlight a comparable in operando degradation pattern between the light soaked and light + neutron irradiated devices. However, whereas light-induced degradation is fully reversible, neutrons lead to permanent effects likely originating from atomic displacement in the active material and possible neutron activation of the anode. We also preliminary speculate that such irreversible worsening is alleviated by the formation of neutron-induced shallow traps, which act as dopants and contribute to the increase of open circuit voltage and decrease of leakage current in the light + neutron irradiated devices. The high radiation dose that perovskite-based solar cells can potentially withstand renders these devices highly appealing for space and avionic applications.
Giuseppe Maria Paternò
added a research item
In the last two decades, the three‐beam pump–push–probe (PPP) technique has become a well‐established tool for investigating the multidimensional configurational space of a molecule, as it permits disclosure of precious information about the multiple and often complex deactivation pathways of the excited molecule. From the spectroscopic point of view, such a tool has revealed details about the efficiency of charge pair generation and conformational relaxation in π‐conjugated molecules and macromolecules. In addition, PPP is effectively utilized for modulating the gain signal in conjugated materials by taking advantage of the spectral overlap between stimulated emission and charge absorption in those systems. However, the relatively low stability of conjugated polymers under intense photoexcitation is a crucial limitation for their real employment in plastic optical fibers (POFs) and for signal control applications. Herein, the role of PPP for achieving ultrafast all‐optical switching in π‐conjugated systems is highlighted. Furthermore, new experimental data on optical switching of a newly synthesized nanographene molecule, namely dibenzo[hi,st]ovalene (DBOV), is reported. The superior environmental and photostability of DBOV and, in general, of graphene nanostructures can represent a great advantage for their effective applications in POFs and information and communications technology. Pump–push–probe (PPP) permits to modulate the gain signal in π‐conjugated materials. However, high photostability is necessary to achieve durable high‐frequency modulation. The role of PPP for attaining optical switching is highlighted herein. Furthermore, it is shown that a photostable graphene molecule can sustain effective ultrafast switching, a result that can open the way for possible operation in the THz regime.
Giuseppe Maria Paternò
added 2 research items
Excitonic 0D and 2D lead‐halide perovskites have been recently developed and investigated as new materials for light generation. Here broadband (>1 eV) emission from newly synthesized 0D lead‐free colloidal Cs3Bi2I9 nanocrystals (NCs) is reported. The nature of their emissive states as well as the relative dynamics which are currently hotly debated are investigated. In particular, it is found that the broadband emission is made by the coexistence of emissive excitons and sub‐bandgap emissive trap‐states. Remarkably, evidence of enhanced Raman scattering from the ligands is observed when attached to the NCs surface, an effect that is preliminarily attributed to strong exciton‐ligands electronic coupling in these systems. 0D and 2D lead‐halide perovskites are promising materials for light generation. However, toxicity of lead and instability limit their actual application. Here, we show that the broad emission of lead‐free Cs3Bi2I9 nanocrystals (NCs) arises from the coexistence of emissive excitons and sub‐bandgap trap‐states. Furthermore, evidence of enhanced Raman scattering from the ligands is observed when attached to the NCs surface.
Dibenzo[hi,st]ovalene (DBOV) is a nanographene molecule with quasi-zero dimensional electronic confinement, which displays relatively high oscillator strength, remarkable photostability and optical gain property. For these reasons, DBOV has been proposed as gain medium and active material for achieving strong exciton-photon coupling in microcavity. Here, we study the stimulated emission properties of three DBOV derivatives with different substitution patterns. We found that these molecules likely undergo ultrafast intermolecular charge transfer processes occurring within their π-aggregates, which ultimately leads to quenching of stimulated emission and increase of the amplified spontaneous emission threshold. These effects can be minimized by installing bulky substituents on the peripheries that prevent π-π stacking. We can thus selectively favor either the luminescence/gain properties or the charge transport features by engineering the side groups.
Valentina Robbiano
added 2 research items
Silicon photonics would strongly benefit from monolithically integrated low-threshold silicon-based laser operating at room-temperature, representing today the main bottleneck towards low-cost and power-efficient electronic−photonic integrated circuits. Here we demonstrate low-threshold lasing from fully-transparent nanostructured porous silicon (PSi) monolithic microcavities (MCs) infiltrated with a polyfluorene derivative, namely poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO). The PFO-infiltrated PSiMCs support single-mode blue lasing at the resonance wavelength of 466 nm, with line width of ~1.3 nm and lasing threshold of 5 nJ (15 μJ/cm²), a value that is at the state-of-art of PFO lasers. Furthermore, time-resolved photoluminescence shows a significant shortening (~57%) of PFO emission lifetime in the PSiMC, with respect to non-resonant PSi reference structures, confirming a dramatic variation of the radiative decay rate of the excitation due to Purcell effects. Our results, given also that blue lasing is a worst case for silicon photonics, are highly appealing for the development of low-cost, low-threshold silicon-based lasers with lasing wavelength tuneable from visible to near-infrared by simple infiltration of suitable emitting polymers in monolithically integrated nanostructured PSiMCs.
We report a novel multi-step method for the preparation of ordered mesoporous titania scaffolds with potential for application to perovskite-based solar cells. The first step is the preparation of a monolayer of polystyrene nanoparticles at a water-air interface and its subsequent transfer onto a solid substrate. A titania precursor solution (titanium(IV) isopropoxide in ethanol) is then drop-cast onto the monolayer and left to "incubate" overnight. Instead of the expected inverse monolayer-structure, a subsequent calcination step of the precursor yields an ordered monolayer of hollow titania nanospheres with a wall thickness of ~ 30-50 nm, and a slightly larger diameter than that of the starting spheres. X-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) characterization of such scaffolds confirm they consist of nanocrystalline anatase titania, and that any polystyrene/carbon residues in the scaffolds are below the XPS detection level. As an illustrative application we prepared perovskite solar cells incorporating the templated-nanoparticle scaffolds displaying a respectable power conversion efficiency of ~9 %, twice as large as that of unoptimized "reference" cells (i.e. incorporating conventional mesoporous or compact titania scaffolds), thereby also demonstrating that the process is more robust with respect to optimization of the process parameters.
Paola Lova
added a research item
Thanks to exciting chemical and optical features, perylene bisimide J-aggregates are ideal candidates to be employed for high-performance plastic photonic devices. However, they generally tend to form π–π stacked H-aggregates that are unsuitable for implementation in polymer resonant cavities. In this work, the efficient compatibilization of a tailored perylene bisimide forming robust J-aggregated supramolecular polymers into amorphous polypropylene is introduced. The new nanocomposite is then implemented into an all-polymer planar microcavity, which provides strong and directional spectral redistribution of the J-aggregate photoluminescence, due to a strong modification of the photonic states. A systematic analysis of the photoemitting processes, including photoluminescence decay and quantum yields, shows that the optical confinement in the polymeric microcavity does not introduce any additional nonradiative de-excitation pathways to those already found in the J-aggregate nanocomposite film and pave the way to PBI-based high-performance plastic photonic devices.
Giuseppe Maria Paternò
added a research item
Squaraine dyes (SQs) represent a versatile class of functional molecules with strong absorption and emission features, widely used as near-infrared sensitizers in organic and hybrid photovoltaic devices. In this context, the photodynamics of such molecules has been seen to influence dramatically the efficiency of the photogeneration process. The most accepted interpretation of excited state deactivation in SQs is represented by a trans-cis photoisomerization around a CC double bond of the polymethinic-like bridge, although such scenario does not explain satisfyingly the decay route of SQs dyes in conformational constrained systems or in highly viscous environments. Here we combine steady-state and time-resolved spectroscopic techniques with high level ab initio calculations to shed light into the photophysics of cis-locked indolenine-based SQs. Our results point towards alternative deactivation routes, possibly involving a dark state in molecules lacking central substitution and the rotation of the central substituent in the core-functionalized ones. These novel results can suggest a synthetic rationale to design dyes that permit quantitative and effective charge generation/diffusion and collection in photovoltaic diodes and, thus, enhance their efficiency.
Giuseppe Maria Paternò
added a research item
In this work, we report the first demonstration of a solution processable, optically switchable 1D photonic crystal by implementing phototunable doped metal oxide nanocrystals. The resulting device structure shows bi-photonic response with the photonic bandgap covering the visible spectral range and the plasmon resonance of the doped metal oxide the near infrared. By means of a facile photodoping process, we tuned the plasmonic response and switched effectively the optical properties of the photonic crystal, translating the effect from the near infrared to the visible. The ultrafast bandgap pumping induces a signal change in the region of the photonic stopband, with recovery times of several picoseconds, providing a step toward the ultrafast optical switching. Optical modeling uncovers the importance to understand largely the variations of the dielectric function of the photodoped material, and variations in the high frequency region of the Drude response are responsible for the strong switching in the visible after photodoping. Our device configuration offers unprecedented tunablility due to flexibility in device design, cover wavelength ranges from the visible to the near infrared. Our findings indicate a new protocol to modify the optical response of photonic devices by optical triggers only.
Giuseppe Maria Paternò
added a research item
The study of supramolecular interactions and aggregation behaviour of functional materials is of great importance to tune and extend their spectral sensitivity and, hence, improve the optoelectronic response of related devices. In this study, we resolve spatially and spectrally the absorption and emission features of a squaraine aggregate by means of confocal microscopy and absorption/photoluminescence spectroscopy. We observe that the aggregate affords both a broad absorption spectrum (centred at 670 nm), likely originated by a dyes configuration with allowed J- and H- arrangements, and a strong and relatively narrow emission in the near-infrared (NIR) part of the spectrum (centred at 780 nm), with a remarkable Stokes shift of 110 nm that is among the largest exhibited by squaraine dyes. These peculiarities would be beneficial for extending the spectral sensitivity of bot photovoltaic and light-emitting diodes, and extremely appealing for possible applications of these aggregates as NIR fluorescent probes in biomedical applications.
Piotr Cegielski
added a research item
Metal-halide perovskites are a class of solution processed materials with remarkable optoelectronic properties such as high photoluminescence quantum yields and long carrier lifetimes, which makes them promising for a wide range of efficient photonic devices. In this work, we demonstrate the first successful integration of a perovskite laser onto a silicon nitride photonic chip. High throughput, low cost optical lithography is used, followed by indirect structuring of the perovskite waveguide. We embed methylammonium lead tri-iodide (MAPbI3) in a pre-patterned race-track microresonator and couple the emitted light to an integrated photonic waveguide. We clearly observe the build-up of spectrally narrow lasing modes at room temperature upon a pump threshold fluence of 19.6 µJ/cm². Our results evidence the possibility of on-chip lasers based on metal-halide perovskites with industry relevance on a commercially available dielectric photonic platform, which is a step forward towards low-cost integrated photonic devices.
Franco Cacialli
added a research item
We studied the surface properties of indium-tin oxide (ITO) modified by wet (aquaregia, ultrasonication, RCA) and dry (oxygen- and argon-plasma) treatments. The surface modification was investigated by surface energy, surface morphology, sheet resistance, carrier concentration, carrier mobility, and workfunction measurements. We report that the studied oxygen-plasma treatment induces: the highest surface energy with the highest polarity, the smoothest surface, the highest carrier density but the lowest mobility, the lowest sheet resistance, and the highest workfunction (stable in air). Polymer light-emitting diodes fabricated with the oxygen plasma treated substrates give the best performance in terms of electroluminescence efficiency and device lifetime. This is attributed to a favorable surface modification of ITO anodes by oxygen-plasma.
Private Profile
added 2 research items
Low-Temperature Photoluminescence Spectroscopy of Solvent-Free PCBM Single-Crystals Experimental details. Fluorescence spectra as a function of temperature and X-ray diffraction patterns for amorphous and polycrystalline PCBM. Additional details concerning the simulated spectra.
We use steady-state and time-resolved photoluminescence (PL) spectroscopy to investigate the luminescent properties of a sulfonated poly(diphenylenevinylene) lithium salt (PDV.Li) in water/propanol solutions at different concentrations, with a view to assessing its aggregation behavior. In particular, we compare results from uninsulated PDV.Li and cyclodextrin-threaded PDV.Li polyrotaxane (PDV.Li⊂β-CD). We find that addition of 1-propanol (≥20 weight%) leads to a significant blue-shift (of ∼0.20 eV) of the PL spectra, that we assign to suppressed interchain aggregation in PDV.Li solutions, with a concomitant fourfold increase in the fluorescence quantum efficiency (i.e. from 14 to 60%). Surprisingly, a moderate concentration of propanol increases further the luminescence efficiency even for PDV.Li⊂β-CD, whose supramolecular encapsulation already provides a shield against aggregation. Indeed, addition of propanol reduces the solvent polarity, and therefore helps solubilizing these materials that are still largely aromatic in nature. Interestingly, however, both uninsulated PDV.Li and polyrotaxane solutions exhibit signs of aggregation at high propanol fraction (>70%) with a distinctively weaker coupling than that of interchain states in PDV.Li at high water concentration and in pure water in particular. While we ascribe such behavior to a poor solvation of the polar moieties, we also report a different strength of aggregation for PDV.Li and PDV.Li⊂β-CD that can be attributed to the presence of the cyclodextrin rings. In PDV.Li⊂β-CD hydrogen bonding between the cyclodextrin rings may lead to closer packing between the polymer chains. We therefore suggest that a content of propanol between 30 and 70% provides a good balance of hydrophobic and hydrophilic interactions both for PDV.Li and PDV.Li⊂β-CD.
Giuseppe Maria Paternò
added a research item
Aviation and space applications can benefit significantly from lightweight organic electronics, now spanning from displays to logics, because of the vital importance of minimising payload (size and mass). It is thus crucial to assess the damage caused to such materials by cosmic rays and neutrons, which pose a variety of hazards through atomic displacements following neutron-nucleus collisions. Here we report the first study of the neutron radiation tolerance of two poly(thiophene)s-based organic semiconductors: poly(3-hexylthiophene-2,5-diyl), P3HT, and the liquid-crystalline poly(2,5-bis (3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene), PBTTT. We combine spectroscopic investigations with characterisation of intrinsic charge mobility to show that PBTTT exhibits significantly higher tolerance than P3HT. We explain this in terms of a superior chemical, structural and conformational stability of PBTTT, which can be ascribed to its higher crystallinity, in turn induced by a combination of molecular design features. Our approach can be used to develop design strategies for better neutron radiation-tolerant materials, thus paving the way for organic semiconductors to enter avionics and space applications.
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SYNCHRONICS (EU-funded MSCA ETN action, 2015-2018) is implementing a joint training and research programme for 15 Early Stage Researchers on the synthesis, characterisation and application to photonics of supramolecularly-engineered organic materials. Such materials are incorporated in state-of-the-art photonic devices fabricated using top-quality facilities and unique expertise available within the network.