Advances in solution-processed near-infrared light-emitting diodes

Near-infrared light-emitting diodes based on solution-processed semiconductors, such as organics, halide perovskites and colloidal quantum dots, have emerged as a viable technological platform for biomedical applications, night vision, surveillance and optical communications. The recently gained increased understanding of the relationship between materials structure and photophysical properties has enabled the design of efficient emitters leading to devices with external quantum efficiencies exceeding 20%. Despite considerable strides made, challenges remain in achieving high radiance, reducing efficiency roll-off and extending operating lifetime. This Review summarizes recent advances on emissive materials synthetic methods and device key attributes that collectively contribute to improved performance of the fabricated light-emitting devices

Roadmap on energy harvesting materials

Ambient energy harvesting has great potential to contribute to sustainable development and address growing environmental challenges. Converting waste energy from energy-intensive processes and systems (e.g. combustion engines and furnaces) is crucial to reducing their environmental impact and achieving net-zero emissions. Compact energy harvesters will also be key to powering the exponentially growing smart devices ecosystem that is part of the Internet of Things, thus enabling futuristic applications that can improve our quality of life (e.g. smart homes, smart cities, smart manufacturing, and smart healthcare). To achieve these goals, innovative materials are needed to efficiently convert ambient energy into electricity through various physical mechanisms, such as the photovoltaic effect, thermoelectricity, piezoelectricity, triboelectricity, and radiofrequency wireless power transfer. By bringing together the perspectives of experts in various types of energy harvesting materials, this Roadmap provides extensive insights into recent advances and present challenges in the field. Additionally, the Roadmap analyses the key performance metrics of these technologies in relation to their ultimate energy conversion limits. Building on these insights, the Roadmap outlines promising directions for future research to fully harness the potential of energy harvesting materials for green energy anytime, anywhere

Multiple narrowband bidirectional self-powered organic photodetector with fast response

Benefiting from the discovery of novel organic semiconductor materials bearing tunable absorption characteristics, narrowband organic photodetectors (OPDs) with improved performance are reported. Alongside new photoactive materials synthesis, several device engineering strategies are introduced to achieve narrowband OPDs, allowing less dependence of the device performance on the synthetic variation control, such as batch-to-batch differences in molecular weight. However, fabrication of multiband OPDs remains a challenge. Current solutions are usually based on vertical multi-stacking of photosensitive layers with different absorption spectra and voltage-modulated charge collection/injection, which renders the device fabrication too complex, while their response speed and band selectivity is limited. In this work, the concept of optical cavity is adopted to demonstrate self-powered and fast response speed single-junction bidirectional organic photodetectors with dual narrowband detection in the ultraviolet (UV

Modification in optoelectronic characteristics of organic light emitting diodes (OLEDs) by using triarylsulfonium salts

272 σ.Αντικείμενο της παρούσας Διδακτορικής Διατριβής είναι η διερεύνηση των δυνατοτήτων τροποποίησης των οπτοηλεκτρονικών χαρακτηριστικών πολυμερικών οργανικών διόδων εκπομπής φωτός (OLEDs) χρησιμοποιώντας άλατα τριαρυλοσουλφωνίου. Τα άλατα αυτά είναι γνωστοί φωτοπαραγωγοί οξέος και έχουν ήδη χρησιμοποιηθεί σε οργανικές ηλεκτρονικές διατάξεις για τη σχηματοποίηση των ημιαγώγιμων πολυμερικών υμενίων με την τεχνική της φωτολιθογραφίας. Αρχικά, μελετήθηκε η επίδραση της προσθήκης αλάτων τριφαινυλοσουλφωνίου (TPS) στο ενεργό υμένιο των OLEDs στις οπτοηλεκτρονικές ιδιότητες των διατάξεων για δύο διαφορετικά πολυμερή (πράσινης και μπλε εκπομπής). Βρέθηκε ότι τα άλατα TPS, τα οποία διαθέτουν εκτεταμένη π-συζυγία λόγω των βενζολικών δακτυλίων, ενώ είναι και ιοντικές ενώσεις, ενισχύουν την έγχυση και τη μεταφορά των φορέων από τα ηλεκτρόδια στο πολυμερικό υμένιο. Κατόπιν, εξετάστηκε η εναπόθεση των αλάτων TPS από διάλυμα πάνω από το ενεργό πολυμερικό υμένιο, όπου λειτουργούν ως στρώματα έγχυσης ηλεκτρονίων. Βρέθηκε ότι, η βελτίωση της έγχυσης των ηλεκτρονίων οφείλεται, κυρίως, στη μεταβολή της κατανομής του εσωτερικού πεδίου, λόγω της πολικότητας του άλατος. Τέλος, διερευνήθηκε η δυνατότητα φωτοχημικής τροποποίησης του φάσματος εκπομπής OLEDs, με σκοπό τη σχηματοποίηση διαφορετικών χρωματικών περιοχών στο ίδιο πολυμερικό υμένιο. Χρησιμοποιήθηκαν για πρώτη φορά ως φωτοεκπομποί φωσφορίζουσες ενώσεις, οργανομεταλλικά σύμπλοκα του Λευκοχρύσου και του Ιριδίου, τα οποία παρουσίασαν μεταβολή του χρώματος εκπομπής τους κατόπιν πρωτονίωσής τους από το φωτοχημικά παραγόμενο οξύ του άλατος TPS. Η μεθοδολογία που ακολουθήθηκε περιελάμβανε: (α) τη μελέτη της ηλεκτρονιακής δομής των αλάτων και των πολυμερών και των ηλεκτρονιακών μεταπτώσεων των σύνθετων πολυμερικών υμενίων με φασματοσκοπικές και βολταμετρικές τεχνικές, (β) τη μελέτη της μορφολογίας των υμενίων αυτών με τεχνικές μικροσκοπίας ηλεκτρονιακής σάρωσης και ατομικής δύναμης και (γ) την κατασκευή διατάξεων OLED και το λεπτομερή οπτοηλεκτρονικό χαρακτηρισμό αυτών. Συμπερασματικά, τα άλατα TPS αποτελούν μια πολύ ενδιαφέρουσα κατηγορία ιοντικών ενώσεων με ελκυστικές φωτοφυσικές και φωτοχημικές ιδιότητες που μπορούν να βρουν ποικίλες εφαρμογές σε οργανικές οπτοηλεκτρονικές διατάξεις με βελτιωμένα λειτουργικά χαρακτηριστικά. Η διατριβή αυτή ανοίγει το δρόμο για χρησιμοποίηση και πιθανώς σύνθεση άλλων αλάτων τριαρυλοσουλφωνίου με κατάλληλα ενεργειακά επίπεδα, έτσι ώστε να επιτευχθεί βελτιστοποίηση της απόδοσης των διατάξεων OLED.Objective of this dissertation is the investigation of modification in the optoelectronic characteristics of Organic Light Emitting Diodes (OLEDs) by using triphenylsulfonium (TPS) salts. TPS salts are well-known photoacid generators and they have been implemented in organic electronic devices in order to fabricate pixilated matrix displays or solution-deposited multilayered OLEDs by applying photolithography techniques. Firstly, the effect of TPS-salts addition in the active layer on the optoelectronic properties of OLEDs was studied for the cases of a yellow-emitting and a blue-emitting polymer matrix. It was found that, both the ionic nature of TPS-salts as well as the extended charge delocalisation over the benzene rings influences the injection of charges from the electrodes and their transport through the polymer layer. Next, the deposition of TPS-salts on top of the polymer layer from their solution in orthogonal solvents revealed the potential of these compounds to efficiently act as Electron Injecting Layers (EILs). It was found that the enhancement of the injection of electrons upon utilization of such polar interfacial layers was, mainly, due to the favourable redistribution of the internal field and the subsequent reduction of the cathode effective work function. Finally, the photochemically induced patterning of different colour areas in a single polymer layer was achieved. The colour tuning of phosphorescent emitters, in particular Platinum and Iridium complexes, was demonstrated for the first time. These complexes were able to change their emission colour following protonation of their basic site by the acid generated upon exposure of the TPS-salts in UV radiation. The methodological approach involved: (a) the spectroscopic and voltametric study of the electronic structure of TPS-salts and polymers and the electronic transitions of the complex polymeric films, (b) the films surface investigation with atomic force and scanning electron microscopies and (c) OLED fabrication and extensive optoelectronic characterisation. In conclusion, it was shown that triphenylsulfonium salts are ionic compounds with attractive photophysical and photochemical properties that can be easily implemented in high performance organic optoelectronic devices. The TPS-salts presented in this work can act as model compounds for future optimization of their functional properties through proper selection of both anions and cations and fine-tuning of their energy levels.Δήμητρα Γ. Γεωργιάδο

Adhesion lithography for fabrication of printed radio-frequency diodes

Radio-frequency (RF) diodes are quintessential elements of passive RF identification tags that are used on livestock, luxury objects, and healthcare products. They are also used in near-field communication applications that enable wireless data transfer between devices. An RF diode—when matched to a suitable antenna—picks up the alternating current (AC) signal that is emitted from an RF source and transforms it to a DC signal. The DC signal can then be used to decode the information stored in the tagged object, or to simply power another electronic or optoelectronic device (e.g., a sensor, battery, or LED). The high demand for RF-harvesting devices, however, can only fully be met if their fabrication costs are substantially reduced. To realize this reduction in cost, novel printing technologies that permit manufacturing on large substrates are required. With these technologies it should be possible to ascertain that an increased diode performance (in terms of a small voltage drop, minimum leakage current, and a large gamut of operating frequencies) is attained

High throughput fabrication of nanoscale optoelectronic devices on large area flexible substrates using adhesion lithography

Nanoscale optoelectronic devices based on coplanar nanogap electrodes, when compared with traditional vertical devices, exhibit attractive characteristics, such as high density of integration, high sensitivity, fast response and multifunctionality. Moreover, their low-cost high-throughput fabrication on flexible disposable substrates opens up several new applications in sectors ranging from telecommunications and consumer electronics to healthcare - to name a few. However, their commercial exploitation has been hitherto impeded by technological bottlenecks, owing to the incompatibility of currently available fabrication techniques, eg. e-beam lithography, with industrial upscaling. Adhesion lithography is a nanopatterning technique that allows the facile high yield fabrication of coplanar metal electrodes separated by a sub-15 nm gap on large area substrates of any type, including plastic. These electrodes, when combined with solution-processed and/or low-dimensional nanostructured materials deposited at low, plastic-compatible, temperatures give rise to nanoscale optoelectronic devices with intriguing properties. It will be shown that both nanoscale light-emitting and light-sensing devices can be fabricated upon using light-emitting polymers along with self-assembling surface modifiers, and lead halide perovskites and functionalised colloidal PbS quantum dots, respectively. Emphasis will be given in recent advances in flexible nanoscale photodetectors fabricated with nanogap coplanar electrodes, operating in DUV up to NIR part of the spectrum. These devices exhibit high responsivity, sensitivity and fast response speed (hundreds of nanoseconds) owing to the extreme downscaling of key device dimensions. These results demonstrate that adhesion lithography combined with advanced materials concepts constitutes a new fabrication paradigm enabling a plethora of advanced applications within the field of flexible electronics

Influence of the anion on the optoelectronic characteristics of triphenylsulfonium salts modified polymer light emitting devices

Triphenylsulfonium salts addition in the emitting layer of polymer light emitting diodes (PLEDs) has been shown to be beneficial for charge injection and transport due to both ionic effects and π-conjugation in the phenyl rings of the cation. In some cases the emission profile can be also modified through an electroplex formation. Herein we investigate the effect of four TPS-salts with different counter anions on the overall PLED performance upon blending each salt with the conjugated polymer poly[2-(6-cyano-6-methyl-heptyloxy)-1,4-phenylene] (CN-PPP). In particular, three perfluoroalkanesulfonate organic anions of increasing size (triflate, nonaflate, PFOS) and a perfluorinated inorganic anion (SbF6) are compared. It is shown that the anion size affects primarily the turn-on and operational voltage, whereas its chemical nature is crucial for achieving high luminance values. The counteranion exerts also a direct impact on the dispersion properties of the salt in the polymer matrix, and thus, the film morphology, which in turn influences the emission colour and efficiency of an electroplex that is proposed to be formed at the sulfonium salt/polymer interfaces in the bulk. This study highlights the importance of properly selecting the counterions of the salts added in the emitting layer of PLEDs, which, in addition to their various functionalities, significantly influence device performance.</p

Incorporating triphenyl sulfonium salts in polyfluorene PLEDs: an all-organic approach to improved charge injection

All-organic sulfonium salts are introduced as a class of ionic compounds that show high compatibility with conjugated polymers and may form blends with attractive luminescent properties leading to significant improvement in single-layer polymer light emitting diodes' (PLEDs') performance. We demonstrate that triphenylsulfonium (TPS) triflate:polyfluorene-co-benzothiadiazole (F8BT)-blend based PLEDs show a lower turn-on voltage, an increased luminous efficiency and higher peak luminance values. These results are being rationalized in terms of anionic accumulation and space charge formation at the anode side, which facilitates hole injection, leading to more balanced injection and subsequently to a higher recombination rate. Moreover, we find that the salt anion size plays a critical role in the device operating characteristics. The judicious choice of both the salt and the emitting polymer by considering relative energy level alignment, salt electrochemical stability and acquired thermodynamic stability of blend morphology is important for the achievement of high performance PLEDs without requiring elaborate device architectures.</p

Effect of triphenylsulfonium triflate addition in wide band-gap polymer light-emitting diodes: improved charge injection, transport and electroplex-induced emission tuning

The presence of mobile anions in the emitting layer of polymer-based OLEDs has been proven to influence substantially the injection characteristics of the diode. In this work we report on the improvement of both injection and transport of charge carriers in blue emitting poly[2-(6-cyano-6-methyl-heptyloxy)-1,4- phenylene] (CN-PPP) based OLEDs upon insertion of the all-organic triphenylsulfonium (TPS) triflate salt in the emitting layer. On one hand, the anion displacement influences the energetics at the polymer/anode interface facilitating hole injection, whereas, on the other hand, the triphenylsulfonium cations act as electron transporting sites. The OLEDs exhibit significantly reduced turn-on voltage to half their initial value and increased luminance at low operating voltage. Moreover, the large energetic mismatch of the polymer and the triphenylsulfonium salt as well as the polarity induced by the ions result in simultaneous dual emission originating from the polymer exciton and from an electroplex, which is proposed to be formed at the triphenylsulfonium salt/polymer interfaces in the bulk. These results show that triphenylsulfonium salts represent an attractive class of materials that can be blended with conjugated polymers and can modify their electrical and/or emissive characteristics.</p

Radio frequency diodes and circuits fabricated via adhesion lithography

The commercial interest in Radio Frequency Identification (RFID) tags keeps growing, as new application sectors, spanning from healthcare to electronic article surveillance (EAS) and personal identification, are constantly emerging for these types of electronic devices. The increasing demand for the so-called “smart labels” necessitates their high throughput manufacturing, and indeed on thin flexible substrates, that will reduce the cost and render them competitive to the currently widely employed barcodes. Adhesion Lithography (a-Lith) is a novel patterning technique that allows the facile high yield fabrication of co-planar large aspect ratio (&lt;100,000) metal electrodes separated by a sub-20 nm gap on large area substrates of any type. Deposition of high mobility semiconductors from their solution at low, compatible with plastic substrates, temperatures and application of specific processing protocols can dramatically improve the performance of the fabricated Schottky diodes. It will be shown that in this manner both organic and inorganic high speed diodes and rectifiers can be obtained, operating at frequencies much higher than the 13.56 MHz benchmark, currently employed in passive RFID tags and near filed communications (NFC). This showcases the universality of this method towards fabricating high speed p- and n-type diodes, irrespective of the substrate, simply based on the extreme downscaling of key device dimensions obtained in these nanoscale structures. The potential for scaling up this technique at low cost, combined with the significant performance optimisation and improved functionality that can be attained through intelligent material selection, render a-Lith unique within the field of plastic electronics