High-Bandwidth Organic Light Emitting Diodes for Ultra-Low Cost Visible Light Communication Links

Visible light communications (VLC) have attracted considerable interest in recent years due to an increasing need for data communication links in home and enterprise environments. Organic light-emitting diodes (OLEDs) are widely used in display applications owing to their high brightness, high quality colour-rending capability and low cost. As a result, they are attractive candidates for the implementation of ultra-low cost visible light optical links in free-space and guided-wave communications. However, OLEDs need to exhibit a bandwidth of at least ~MHz to be able to support the modest data rates (~Mbps) required in these applications. Although fluorescent OLEDs typically exhibit shorter photon lifetimes than inorganic LEDs, the bandwidth performance of the large size OLEDs used in display applications are limited by their electrical characteristics. In this work, we present a detailed physical simulation that describes well the performance of fast OLED devices that exhibit significant -3 dB bandwidths (f-3dB) of 44 MHz obtained for a 0.12 mm2 device. It is demonstrated that the reduction of the device size results in a significant bandwidth improvement due primarily to a reduction in parasitic capacitance of the devices, though this is counteracted by carrier dynamic effects. The model provides an insight into the basic physical properties of the OLED and may be used for optimisation of future generations of OLED devices.EPSRC EP/K00042X/1 EPSRC Studentship 146672

Triple-cation perovskite solar cells for visible light communications

Funding: UK EPSRC under grant codes EP/L015110/1 and EP/L017008/1. Dr. L. K. Jagadamma acknowledges support from a Marie Skłodowska-Curie Individual Fellowship (European Commission) (MCIF: No. 745776). Harald Haas acknowledges financial support from the Wolfson Foundation and Royal Society. He also acknowledges financial support by the Engineering and Physical Sciences Research Council (EPSRC) under an Established Career Fellowship grant EP/R007101/1.Hybrid perovskite materials are widely researched due to their high absorptivity, inexpensive synthesis, and promise in photovoltaic devices. These materials are also of interest as highly sensitive photodetectors. In this study, their potential for use in visible light communication is explored in a configuration that allows for simultaneous energy and data harvesting. Using a triple-cation material and appropriate device design, a new record data rate for perovskite photodetectors of 56 Mbps and power conversion efficiencies above 20% under white LED illumination are achieved. With this device design, the −3  dB bandwidth is increased by minimizing the dominating time constant of the system. This correlation between the bandwidth and time constant is proved using measurements of time-resolved photoluminescence, transient photovoltage, and device resistance.Publisher PDFPeer reviewe

245 MHz bandwidth organic light-emitting diodes used in a gigabit optical wireless data link

Funding: UK EPSRC (EP/K00042X/I, EP/R005281/1, EP/R007101/1 and EP/R035164/1); Marie Skłodowska Curie Individual Fellowship (703387).Organic optoelectronic devices combine high-performance, simple fabrication and distinctive form factors. They are widely integrated in smart devices and wearables as flexible, high pixel density organic light emitting diode (OLED) displays, and may be scaled to large area by roll-to-roll printing for lightweight solar power systems. Exceptionally thin and flexible organic devices may enable future integrated bioelectronics and security features. However, as a result of their low charge mobility, these are generally thought to be slow devices with microsecond response times, thereby limiting their full scope of potential applications. By investigating the factors limiting their bandwidth and overcoming them, we demonstrate here exceptionally fast OLEDs with bandwidths in the hundreds of MHz range. This opens up a wide range of potential applications in spectroscopy, communications, sensing and optical ranging. As an illustration of this, we have demonstrated visible light communication using OLEDs with data rates exceeding 1 gigabit per second.Publisher PDFPeer reviewe

Synthesis and properties of novel star-shaped oligofluorene conjugated systems with BODIPY cores

Star-shaped conjugated systems with varying oligofluorene arm length and substitution patterns of the central BODIPY core have been synthesised, leading to two families of compounds, T-B1–T-B4 and Y-B1–Y-B4, with T- and Y-shaped motifs, respectively. Thermal stability, cyclic voltammetry, absorption and photoluminescence spectroscopy of each member of these two families were studied in order to determine their suitability as emissive materials in photonic applications

Optical antennas for wavelength division multiplexing in visible light communications beyond the étendue limit

Funding: UK EPSRC via “Ultra-parallel visible light communications (UP-VLC)” (Programme Grant EP/K00042X/1) and “Super Receivers for Visible Light Communications” (related grants EP/R00689X/1 and EP/R005281/1).This paper presents a novel fluorescent receiver for visible light communications that combines a broad field of view (FoV) with a significant gain, and simultaneously enables wavelength division multiplexing (WDM) for data communications. These characteristics are achieved by creating a receiver that includes a stack of two fluorescent optical antennas, each designed to receive a different part of the spectrum and create two separable communication channels. The device outperforms the theoretical étendue limit in terms of the combination of FoV and gain it can achieve. Furthermore, the demonstrated de‐multiplexing of blue and green spectral components enables the parallel streaming of data by WDM. Since these devices are inexpensive, simple, and compact, they can easily be integrated into portable electronic devices such as phones, tablets, and laptops.PostprintPeer reviewe

High-speed MIMO communication and simultaneous energy harvesting using novel organic photovoltaics

A data rate of 363-Mb/s is achieved in a multiple-input-multiple-output experiment using 4 organic photovoltaics as receivers. The same system simultaneously extracted 10.9-mW. The resulting system model predicts 133-Gb/s using a 1000-cell organic solar panel

Synthesis and properties of novel star-shaped oligofluorene conjugated systems with BODIPY cores

Star-shaped conjugated systems with varying oligofluorene arm length and substitution patterns of the central BODIPY core have been synthesised, leading to two families of compounds, T-B1-T-B4 and Y-B1-Y-B4, with T- and Y-shaped motifs, respectively. Thermal stability, cyclic voltammetry, absorption and photoluminescence spectroscopy of each member of these two families were studied in order to determine their suitability as emissive materials in photonic applications

Ultra-wide coverage VLC system with alignment-free receiver

In this work we present an ultra-wide coverage visible light communication (VLC) system based on a mechanical beam steering system at the transmitter terminal and a fluorescent optical concentrator at the receiver. The transmitter and receiver full field of view (FFOV) are 100° and 120°, respectively. The coverage area of the transmitter system at 2m is 12.5m2 and a data link of 300Mbps has been achieved at this range.Postprin

Lateral electrical transport, optical properties and photocurrent measurements in two-dimensional arrays of silicon nanocrystals embedded in SiO2

In this study we investigate the electronic transport, the optical properties, and photocurrent in two-dimensional arrays of silicon nanocrystals (Si NCs) embedded in silicon dioxide, grown on quartz and having sizes in the range between less than 2 and 20 nm. Electronic transport is determined by the collective effect of Coulomb blockade gaps in the Si NCs. Absorption spectra show the well-known upshift of the energy bandgap with decreasing NC size. Photocurrent follows the absorption spectra confirming that it is composed of photo-generated carriers within the Si NCs. In films containing Si NCs with sizes less than 2 nm, strong quantum confinement and exciton localization are observed, resulting in light emission and absence of photocurrent. Our results show that Si NCs are useful building blocks of photovoltaic devices for use as better absorbers than bulk Si in the visible and ultraviolet spectral range. However, when strong quantum confinement effects come into play, carrier transport is significantly reduced due to strong exciton localization and Coulomb blockade effects, thus leading to limited photocurrent

Study of silicon nanostructures for photovoltaic applications

The present thesis was devoted to the investigation of the different properties of Si nanocrystals that are of interest for their use in solar cells. The properties that were studied involve structural, optical and optoelectronic properties, as well as electrical transport phenomena in single- and multilayer arrays of Si nanocrystals. Finally, a novel Si-based photovoltaic cell was studied and fabricated, with the objective of its future use in combination with Si nanocrystals. The first chapter of the present thesis is devoted to an introduction to photovoltaic systems and to a bibliographic presentation of the use of Si nanocrystals in efficient photovoltaic devices.The second chapter is devoted to sample preparation. We first give an overview of the different techniques used to form Si nanocrystals and oxidise them. We then present the characterisation techniques used in the present thesis for the structural and morphological characterisation of the nanocrystal arrays, together with the obtained results for the sample structure and morphology.Chapter three is devoted to the absorption in semiconductors in general and more specifically in Si nanocrystals. The main results from this study are as follows: •Observation of the quantum confinement phenomenon.•It was found that the nanocrystalline silicon shows increased absorption coefficient compared with the crystalline silicon.•The refractive indexes of the samples were calculated.•The phenomenon of the photoluminescence was observed in some samples. Chapter four is devoted to the extended study of electrical transport phenomena in arrays of Si nanocrystals arranged in single and multilayered structures. This study is very relevant for the use of Si nanocrystals in solar cell devices. We present the different mechanisms involved and conclusions are drawn for the specific nanocrystal layers studied. The main results are as follows:•Interpretation of the conductivity in nanocrystalline silicon.•Observation of the space charge limited current.•Observation of Coulomb blockade in the smaller Si nanocrystals for temperatures lower than 200 K.In chapter five the optoelectronic properties of Si nanocrystals are presented. More specifically, the theory of photo-transport is reviewed, together with the main results obtained within the thesis on the mechanism of phototransport and the different parameters that are relevant for photovoltaics. The main conclusions from this study are as follows:•Interpretation of the photoconductivity mechanisms.•Observation of the dependence between the photoconductivity and the dimensions of the nanocrystals.•The life time of the photo-excited carriers was found to be few μsec and the recombination was of linear type. The last part of the thesis is devoted to the fabrication of a novel Si-based solar cell and the optimization of its performance. The specific solar cell can be used in combination with Si nanocrystals for optimization of its performance through "down shifting".Ο στόχος της παρούσας διατριβής ήταν η μελέτη των ιδιοτήτων που παρουσιάζουν οι νανοκρυσταλλίτες πυριτίου με στόχο την εφαρμογή τους σε φωτοβολταϊκές διατάξεις. Το φάσμα των ιδιοτήτων οι οποίες εξετάστηκαν στο πλαίσιο της διατριβής περιλαμβάνει τις δομικές, οπτικές, οπτικοηλεκτρονικές ιδιότητες, καθώς και φαινόμενα μεταφοράς σε συστοιχίες νανοκρυσταλλιτών σε μονοστρωματική ή πολυστρωματική διάταξη. Επίσης κατασκευάσθηκε και μελετήθηκε φωτοβολταϊκή διάταξη Si με στόχο τη μελλοντική ενσωμάτωση σε αυτή νανοκρυσταλλιτών Si. Στο πρώτο κεφάλαιο της παρούσας διατριβής γίνεται αρχικά μια εισαγωγή στα φωτοβολταϊκά συστήματα και στοιχεία, και στη συνέχεια γίνεται βιβλιογραφική παρουσίαση των εφαρμογών των νανοκρυσταλλιτών Si για κατασκευή φθηνότερων και πιο αποδοτικών φωτοβολταϊκών στοιχείων.Το δεύτερο κεφάλαιο επικεντρώνεται στην παρασκευή των προς μελέτη δειγμάτων. Παρουσιάζεται αρχικά μια σύνοψη των διαφόρων τεχνικών παρασκευής και οξείδωσης νανοκρυσταλλιτών Si. Ακολουθεί επίσης η παρουσίαση των τεχνικών χαρακτηρισμού των δομικών και μορφολογικών ιδιοτήτων των υμενίων νανοκρυσταλλικού Si, ενώ στο πειραματικό μέρος γίνεται η παρουσίαση των λεπτομερειών κατασκευής και των αποτελεσμάτων της μορφολογικής μελέτης.Στο τρίτο κεφάλαιο γίνεται η παρουσίαση του φαινομένου της απορρόφησης στους ημιαγωγούς γενικότερα και στους νανοκρυσταλλίτες ημιαγωγών ειδικότερα. Στο πειραματικό μέρος γίνεται η παρουσίαση των οργάνων με τη βοήθεια των οποίων μελετήθηκαν οι οπτικές τους ιδιότητες και των αποτελεσμάτων. Τα κύρια αποτελέσματα από τις οπτικές μετρήσεις έχουν ως εξής: •Η παρατήρηση του φαινομένου του κβαντικού εντοπισμού στις δομές.•Βρέθηκε ότι το νανοκρυσταλλικό Si παρουσιάζει αυξημένο συντελεστή απορρόφησης σε σχέση με αυτόν του κρυσταλλικού Si.•Υπολογίστηκαν οι συντελεστές διάθλασης των δειγμάτων.•Παρατηρήθηκε το φαινόμενο της φωτοφωταύγειας σε μερικά από τα δείγματα. Στο τέταρτο κεφάλαιο γίνεται αναλυτική μελέτη των βασικών φαινομένων ηλεκτρικής μεταφοράς φορέων σε συστοιχίες νανοκρυσταλλιτών σε μονοστρωματικά και πολυστρωματικά λεπτά υμένια. Η μελέτη αυτή αγωγιμότητας των υμενίων είναι πολύ σημαντική εν όψει των εφαρμογών τους σε φωτοβολταϊκές διατάξεις. Εγινε συστηματική μελέτη των μηχανισμών αγωγιμότητας στους νανοκρυσταλλίτες Si, παρουσιάζονται δε η θεωρία, τα αποτελέσματα και τα συμπεράσματα του ηλεκτρικού χαρακτηρισμού. Τα κύρια αποτελέσματα από τη μελέτη αυτή έχουν ως εξής:•Ερμηνεία της αγωγιμότητας στο νανοκρυσταλλικό Si.•Παρατήρηση του φαινομένου περιορισμού ρεύματος λόγω φορτίων χώρου.•Παρατήρηση του φαινομένου φραγής Coulomb για τους μικρότερους νανοκρυσταλλίτες, για θερμοκρασίες μικρότερες των 200 Κ.Στο πέμπτο κεφάλαιο γίνεται η παρουσίαση των οπτικοηλεκρονικών τους ιδιοτήτων. Συγκεκριμένα γίνεται η περιγραφή της θεωρίας της φωτοαγωγιμότητας και των πειραματικών αποτελεσμάτων μελέτης στους νανοκρυσταλλίτες Si. Τα κύρια αποτελέσματα έχουν ως εξής: •Ερμηνεία των μηχανισμών της φωτοαγωγιμότητας.•Παρατηρήθηκε η εξάρτηση μεταξύ του φωτορεύματος και του μεγέθους των νανοκρυσταλλιτών.•Ο χρόνος ζωής των φωτο-διεγερμένων φορέων είναι μερικές δεκάδες μsec και η επανασύνδεση είναι γραμμικού τύπου. Η διατριβή κλείνει με την περιγραφή της κατασκευής πρωτότυπης πειραματικής φωτοβολταϊκής διάταξης Si και τη μελέτη μεθόδων μεγιστοποίησης της απόδοσής της. Η διάταξη αυτή μπορεί να χρησιμοποιηθεί στη συνέχεια για την ενσωμάτωση νανοκρυσταλλιτών Si με στόχο την περαιτέρω βελτίωση των χαρακτηριστικών της