Light-Emitting Devices with Conjugated Polymers

This article introduces a previous study and tremendous progress in basic theoretical modeling, material developments and device engineering for polymer light-emitting devices (PLEDs)

Luminescent Materials in Lighting, Display, Solar Cell, Sensing, and Biomedical Applications

This chapter comprises a broader extent of the luminescence phenomenon with the mechanism involved therein as well as applications. Typically, the up and down conversion and downshifting behavior of the optical materials have been elucidated in brief. The fundamental understanding of these optical materials has been described by using schematic representations. It is well documented that the rare earth-based optical materials are known for their luminescent enrichment due to availability of the ladder-like energy levels. These energy levels can be utilized for the excitation of the luminescent materials by using a suitable excitation source. In the process of development of luminescent materials, choice of host matrices and dopant ions is very crucial. Strong correlation of these optical materials has been shown with the current scenario of our society and daily life. In view of the ongoing research, nanophosphor, glasses, and quantum dots with size- and shape-dependent optical behavior have been given in detail. The involved mechanism and the energy transfer phenomenon have been well elucidated by schematic and figures for the evident explanation to the readers. Our emphasis is to elucidate these optical materials in the development of innovative multifunctional applications such as lighting, display, sensing, LEDs, solar cell, and biological applications

Energy transfer and photodegradation of Perylene Orange:LDS821 system in poly(methl methacrylate)

ArticleApplied Optics. 45(21): 5385-5390 (2006)journal articl

Mechanisms of light energy harvesting in dendrimers and hyperbranched polymers

Since their earliest synthesis, much interest has arisen in the use of dendritic and structurally allied forms of polymer for light energy harvesting, especially as organic adjuncts for solar energy devices. With the facility to accommodate a proliferation of antenna chromophores, such materials can capture and channel light energy with a high degree of efficiency, each polymer unit potentially delivering the energy of one photon-or more, when optical nonlinearity is involved. To ensure the highest efficiency of operation, it is essential to understand the processes responsible for photon capture and channelling of the resulting electronic excitation. Highlighting the latest theoretical advances, this paper reviews the principal mechanisms, which prove to involve a complex interplay of structural, spectroscopic and electrodynamic properties. Designing materials with the capacity to capture and control light energy facilitates applications that now extend from solar energy to medical photonics. © 2011 by the authors; licensee MDPI, Basel, Switzerland

Less Is More: Dilution Enhances Optical and Electrical Performance of a TADF Exciplex

A surprising yet highly practical approach to improve the performance of a TADF exciplex blend is reported. Using the TSBPA donor and PO-T2T acceptor to form an exciplex, we are able to blue shift the emission, increase PLQY from 58 to 80%, and increase the device EQE from 14.8 to 19.2% by simply diluting the exciplex with an inert high triplet energy host material—here either UGH-3 or DPEPO. These effects are explained in terms of an increasing donor–acceptor distance and associated charge separation, while different behaviors observed in the different hosts are attributed to different energy barriers to electron transfer through the host. We expect that the observed performance-enhancing effects of dilution will be general to different exciplex blends and host materials and offer a new way to optimize the electrical properties of exciplex emission layers with narrow blue emission

Study of Thermally Activated Delayed Fluorescent Exciplexes and their Practical Applications in OLEDs

Exciplexes are intermolecular charge transfer (CT) complexes in which one electron donating (D) and one electron accepting (A) molecule interact in the excited state. The new bimolecular CT excited state species is the exciplex, a shortening of EXCIted state comPLEX. Until recently exciplexes were avoided in OLEDs structures since they constituted an efficiency loss pathway since they commonly possess low photoluminescence quantum yield (PLQY). Recently, the rise of thermally activated delayed fluorescence (TADF) applications for triplet harvesting in fluorescent OLEDs has resulted in renewed research interest in these bimolecular excited states. The TADF mechanism in fact, allows to upconvert triplet excited states (which are non-emissive in normal fluorescent emitters) into emissive singlet excited states thus boosting the efficiency of the emitter. To be efficient, the TADF mechanism needs to have minimal overlap between the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO). Exciplexes intrinsically possess this characteristic since the CT excited state is formed between two different molecules making exciplexes the perfect candidates as TADF emitters. For this reason, TADF exciplexes are attracting more and more attention although always in the shadow of their more successful intramolecular counterpart (covalently linked D-A fragments in a single molecule) since they could be more easily tailored to maximise their efficiency and modify their properties. The first part of this thesis demonstrates the surprising discovery that exciplex electronic energy and PLQY are not intrinsically fixed by the D/A couple forming the exciplex, and that these characteristics can be tuned and improved through solid state dilution. It is shown that the exciplex electronic energy can be controllably increased by varying average intermolecular distance between the D and A molecule within the exciplex blend by inserting a third inert molecule in the blend forming the film. The change in the exciplex electronic energy and PLQY is rationalised by a general reduction of the coulombic binding energy with D-A separation. In contrast, the PLQY enhancement is not general and determined to be related to the degree of flexibility of the exciplex forming molecules. The second part of this thesis showcases work that broadens the range of potential applications of TADF exciplex OLEDs, demonstrating their suitability as emitters for solution processed devices and how they can be used to confine the recombination zone of a standard phosphorescent OLED - leading to performance and stability improvements

Improved Performance of Organic Light-Emitting Transistors Enabled by Polyurethane Gate Dielectric

Organic light-emitting transistors (OLETs) are multifunctional optoelectronic devices that combine in a single structure the advantages of organic light emitting diodes (OLEDs) and organic field-effect transistors (OFETs). However, low charge mobility and high threshold voltage are critical hurdles to practical OLETs implementation. This work reports on the improvements obtained by using polyurethane films as dielectric layer material in place of the standard poly(methylmethacrylate) (PMMA) in OLET devices. It was found that polyurethane drastically reduces the number of traps in the device thereby improving electrical and optoelectronic device parameters. In addition, a model was developed to rationalize an anomalous behavior at the pinch-off voltage. Our findings represent a step forward to overcome the limiting factors of OLETs that prevent their use in commercial electronics by providing a simple route for low-bias device operation.Comment: 25 pages, 5 figures, 1 tabl