Post-Growth Dynamics and Growth Modeling of Organic Semiconductor Thin Films.

The ability to control the properties of organic thin films is crucial for obtaining highly performant thin-film devices. However, thin films may experience post-growth processes, even when the most sophisticated and controlled growth techniques such as organic molecular beam epitaxy (OMBE) are used. Such processes can modify the film structure and morphology and, thus, the film properties ultimately affecting device performances. For this reason, probing the occurrence of post-growth evolution is essential. Equally importantly, the processes responsible for this evolution should be addressed in view of finding a strategy to control and, possibly, leverage them for driving film properties. Here, nickel-tetraphenylporphyrin (NiTPP) thin films grown by OMBE on highly oriented pyrolytic graphite (HOPG) are selected as an exemplary system exhibiting a remarkable post-growth morphology evolution consistent with Ostwald-like ripening. To quantitatively describe the growth, the height-height correlation function (HHCF) analysis of the atomic force microscopy (AFM) images is carried out, clarifying the role of the post-growth evolution as an integral part of the whole growth process. The set of scaling exponents obtained confirms that the growth is mainly driven by diffusion combined with the presence of step-edge barriers, in agreement with the observed ripening phenomenon. Finally, the results together with the overall approach adopted demonstrate the reliability of the HHCF analysis in systems displaying post-growth evolution.

A porphyrin pentamer as a bright emitter for NIR OLEDs.

The luminescence and electroluminescence of an ethyne-linked zinc(ii) porphyrin pentamer have been investigated, by testing blends in two different conjugated polymer matrices, at a range of concentrations. The best results were obtained for blends with the conjugated polymer PIDT-2TPD, at a porphyrin loading of 1 wt%. This host matrix was selected because the excellent overlap between its emission spectrum and the low-energy region of the absorption spectrum of the porphyrin oligomer leads to efficient energy transfer. Thin films of this blend exhibit intense fluorescence in the near-infrared (NIR), with a peak emission wavelength of 886 nm and a photoluminescent quantum yield (PLQY) of 27% in the solid state. Light-emitting diodes (LEDs) fabricated with this blend as the emissive layer achieve average external quantum efficiencies (EQE) of 2.0% with peak emission at 830 nm and a turn-on voltage of 1.6 V. This performance is remarkable for a singlet NIR-emitter; 93% of the photons are emitted in the NIR (λ > 700 nm), indicating that conjugated porphyrin oligomers are promising emitters for non-toxic NIR OLEDs.

Molecular Encapsulation of Naphthalene Diimide (NDI) Based π-Conjugated Polymers: A Tool for Understanding Photoluminescence.

Conjugated polymers are an important class of chromophores for optoelectronic devices. Understanding and controlling their excited state properties, in particular, radiative and non-radiative recombination processes are among the greatest challenges that must be overcome. We report the synthesis and characterization of a molecularly encapsulated naphthalene diimide-based polymer, one of the most successfully used motifs, and explore its structural and optical properties. The molecular encapsulation enables a detailed understanding of the effect of interpolymer interactions. We reveal that the non-encapsulated analogue P(NDI-2OD-T) undergoes aggregation enhanced emission; an effect that is suppressed upon encapsulation due to an increasing π-interchain stacking distance. This suggests that decreasing π-stacking distances may be an attractive method to enhance the radiative properties of conjugated polymers in contrast to the current paradigm where it is viewed as a source of optical quenching.

Intrinsic photogeneration of long-lived charges in a donor-orthogonal acceptor conjugated polymer.

Efficient charge photogeneration in conjugated polymers typically requires the presence of a second component to act as electron acceptor. Here, we report a novel low band-gap conjugated polymer with a donor/orthogonal acceptor motif: poly-2,6-(4,4-dihexadecyl-4H-cyclopenta [2,1-b:3,4-b']dithiophene)-alt-2,6-spiro [cyclopenta[2,1-b:3,4-b']dithiophene-4,9'-fluorene]-2',7'-dicarbonitrile, referred to as PCPDT-sFCN. The role of the orthogonal acceptor is to spatially isolate the LUMO from the HOMO, allowing for negligible exchange energy between electrons in these orbitals and minimising the energy gap between singlet and triplet charge transfer states. We employ ultrafast and microsecond transient absorption spectroscopy to demonstrate that, even in the absence of a separate electron acceptor, PCPDT-sFCN shows efficient charge photogeneration in both pristine solution and film. This efficient charge generation is a result of an isoenergetic singlet/triplet charge transfer state equilibrium acting as a reservoir for charge carrier formation. Furthermore, clear evidence of enhanced triplet populations, which form in less than 1 ps, is observed. Using group theory, we show that this ultrafast triplet formation is due to highly efficient, quantum mechanically allowed intersystem crossing between the bright, initially photoexcited local singlet state and the triplet charge transfer state. Remarkably, the free charges that form via the charge transfer state are extraordinarily long-lived with millisecond lifetimes, possibly due to the stabilisation imparted by the spatial separation of PCPDT-sFCN's donor and orthogonal acceptor motifs. The efficient generation of long-lived charge carriers in a pristine polymer paves the way for single-material applications such as organic photovoltaics and photodetectors.

Non-toxic near-infrared light-emitting diodes.

Harnessing cost-efficient printable semiconductor materials as near-infrared (NIR) emitters in light-emitting diodes (LEDs) is extremely attractive for sensing and diagnostics, telecommunications, and biomedical sciences. However, the most efficient NIR LEDs suitable for printable electronics rely on emissive materials containing precious transition metal ions (such as platinum), which have triggered concerns about their poor biocompatibility and sustainability. Here, we review and highlight the latest progress in NIR LEDs based on non-toxic and low-cost functional materials suitable for solution-processing deposition. Different approaches to achieve NIR emission from organic and hybrid materials are discussed, with particular focus on fluorescent and exciplex-forming host-guest systems, thermally activated delayed fluorescent molecules, aggregation-induced emission fluorophores, as well as lead-free perovskites. Alternative strategies leveraging photonic microcavity effects and surface plasmon resonances to enhance the emission of such materials in the NIR are also presented. Finally, an outlook for critical challenges and opportunities of non-toxic NIR LEDs is provided.

Towards efficient near-infrared fluorescent organic light-emitting diodes.

The energy gap law (EG-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 EG -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. EG 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.

Chiral Oligothiophenes with Remarkable Circularly Polarized Luminescence and Electroluminescence in Thin Films.

This work reports the first observation of circularly polarized electroluminescence (CPEL) in thin films of self-organized oligothiophenes. Four new 1,4-phenylene and 9H-carbazole-based oligothiophenes were ad hoc designed to ensure efficient spontaneous formation of chiral supramolecular order. They were easily synthesized and their chiroptical properties in thin films were measured. Circularly polarized luminescence (CPL) spectra revealed glum in the order of 10-2 on a wide wavelengths range, originating from their self-organized chiral supramolecular organization. These molecules have reasonable properties as organic semiconductors and for this reason they can constitute the active layer of circularly-polarized organic light-emitting diodes (CP-OLEDs). Thus, we could investigate directly their electroluminescence (EL) and CPEL, without resorting to blends, but rather in a simple multilayer device with basic architecture. This is the first example of a CP-OLED with active layer made only of a small organic compound.

Visible light communication with efficient far-red/near-infrared polymer light-emitting diodes.

Visible light communication (VLC) is a wireless technology that relies on optical intensity modulation and is potentially a game changer for internet-of-things (IoT) connectivity. However, VLC is hindered by the low penetration depth of visible light in non-transparent media. One solution is to extend operation into the "nearly (in)visible" near-infrared (NIR, 700-1000 nm) region, thus also enabling VLC in photonic bio-applications, considering the biological tissue NIR semitransparency, while conveniently retaining vestigial red emission to help check the link operativity by simple eye inspection. Here, we report new far-red/NIR organic light-emitting diodes (OLEDs) with a 650-800 nm emission range and external quantum efficiencies among the highest reported in this spectral range (>2.7%, with maximum radiance and luminance of 3.5 mW/cm2 and 260 cd/m2, respectively). With these OLEDs, we then demonstrate a "real-time" VLC setup achieving a data rate of 2.2 Mb/s, which satisfies the requirements for IoT and biosensing applications. These are the highest rates ever reported for an online unequalised VLC link based on solution-processed OLEDs.

Doubly Encapsulated Perylene Diimides: Effect of Molecular Encapsulation on Photophysical Properties.

Intermolecular interactions play a fundamental role on the performance of conjugated materials in organic electronic devices, as they heavily influence their optoelectronic properties. Synthetic control over the solid state properties of organic optoelectronic materials is crucial to access real life applications. Perylene diimides (PDIs) are one of the most highly studied classes of organic fluorescent dyes. In the solid state, π-π stacking suppresses their emission, limiting their use in a variety of applications. Here, we report the synthesis of a novel PDI dye that is encapsulated by four alkylene straps. X-ray crystallography indicates that intermolecular π-π stacking is completely suppressed in the crystalline state. This is further validated by the photophysical properties of the dye in both solution and solid state and supported by theoretical calculations. However, we find that the introduction of the encapsulating "arms" results in the creation of charge-transfer states which modify the excited state properties. This article demonstrates that molecular encapsulation can be used as a powerful tool to tune intermolecular interactions and thereby gain an extra level of control over the solid state properties of organic optoelectronic materials.

Efficient Near-Infrared Electroluminescence at 840 nm with "Metal-Free" Small-Molecule:Polymer Blends.

Due to the so-called energy-gap law and aggregation quenching, the efficiency of organic light-emitting diodes (OLEDs) emitting above 800 nm is significantly lower than that of visible ones. Successful exploitation of triplet emission in phosphorescent materials containing heavy metals has been reported, with OLEDs achieving remarkable external quantum efficiencies (EQEs) up to 3.8% (peak wavelength > 800 nm). For OLEDs incorporating fluorescent materials free from heavy or toxic metals, however, we are not aware of any report of EQEs over 1% (again for emission peaking at wavelengths > 800 nm), even for devices leveraging thermally activated delayed fluorescence (TADF). Here, the development of polymer light-emitting diodes (PLEDs) peaking at 840 nm and exhibiting unprecedented EQEs (in excess of 1.15%) and turn-on voltages as low as 1.7 V is reported. These incorporate a novel triazolobenzothiadiazole-based emitter and a novel indacenodithiophene-based transport polymer matrix, affording excellent spectral and transport properties. To the best of knowledge, such values are the best ever reported for electroluminescence at 840 nm with a purely organic and solution-processed active layer, not leveraging triplet-assisted emission.

Highly Efficient Solid-State Near-infrared Organic Light-Emitting Diodes incorporating A-D-A Dyes based on α,ß-unsubstituted "BODIPY" Moieties.

We take advantage of a recent breakthrough in the synthesis of α,ß-unfunctionalised 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) moieties, which we symmetrically conjugate with oligothienyls in an unexpectedly stable form, and produce a "metal-free" A-D-A (acceptor-donor-acceptor) oligomer emitting in the near-infrared (NIR) thanks to delocalisation of the BODIPY low-lying lowest unoccupied molecular orbital (LUMO) over the oligothienyl moieties, as confirmed by density functional theory (DFT). We are able to retain a PL efficiency of 20% in the solid state (vs. 30% in dilute solutions) by incorporating such a dye in a wider gap polyfluorene matrix and demonstrate organic light-emitting diodes (OLEDs) emitting at 720 nm. We achieve external quantum efficiencies (EQEs) up to 1.1%, the highest value achieved so far by a "metal-free" NIR-OLED not intentionally benefitting from triplet-triplet annihilation. Our work demonstrates for the first time the promise of A-D-A type dyes for NIR OLEDs applications thereby paving the way for further optimisation.

Engineering of Semiconductor Nanocrystals for Light Emitting Applications.

Semiconductor nanocrystals are rapidly spreading into the display and lighting markets. Compared with liquid crystal and organic LED displays, nanocrystalline quantum dots (QDs) provide highly saturated colors, wide color gamut, resolution, rapid response time, optical efficiency, durability and low cost. This remarkable progress has been made possible by the rapid advances in the synthesis of colloidal QDs and by the progress in understanding the intriguing new physics exhibited by these nanoparticles. In this review, we provide support to the idea that suitably engineered core/graded-shell QDs exhibit exceptionally favorable optical properties, photoluminescence and optical gain, while keeping the synthesis facile and producing QDs well suited for light emitting applications. Solid-state laser emitters can greatly profit from QDs as efficient gain materials. Progress towards fabricating low threshold, solution processed DFB lasers that are optically pumped using one- and two-photon absorption is reviewed. In the field of display technologies, the exploitation of the exceptional photoluminescence properties of QDs for LCD backlighting has already advanced to commercial levels. The next big challenge is to develop the electroluminescence properties of QD to a similar state. We present an overview of QLED devices and of the great perspectives for next generation display and lighting technologies.

Investigation into the heterostructure interface of CdSe-based core-shell quantum dots using surface-enhanced Raman spectroscopy.

The structural nature of heterointerfaces in core-shell semiconductor quantum dots (QDs) plays a crucial role in tailoring their optical properties. In this work we have focused on using surface-enhanced Raman spectroscopy as a nondestructive tool to investigate the structural evolution of such interfaces in CdSe/CdS and CdSe/Cd0.5Zn0.5S colloidal QDs. A comparison between the two systems shows significant structural variation across the core-shell interfaces for the two different materials: a smooth interface for the former and an abrupt interface for the latter. This structural difference modifies the electronic structure within the QDs, which directly dictates the confinement behavior of the electrons and holes. The implications of this translate to a better understanding of why graded CdSe/CdS/Cd0.5Zn0.5S/ZnS QDs are so lucrative for linear and nonlinear fluorescence-based applications.