11:10 - 11:35InvitedID: 124
PROBING STRUCTURE-EXCITED STATE PROPERTIES RELATIONSHIP
National Taiwan University, Taipei, Taiwan
I would like to present the exploitation of molecular design and spectroscopic technique, together with the theoretical approach, to probe several cutting-edge issues regarding the lighting materials and hence the OLEDs performance. The talk is in three folds, First, I will focus on the use of step-scan FT-Vis and FTIR to perform pump-probe experiment in an attempt to analyzed the structure and relaxation dynamics of exciplex and hence the efficiency of corresponding OLEDs efficiency. Secondly, the morphology dependent and the associated exciton coupling will be presented using both inorganic (the Pt(II) complexes) and organic (proton-transfer molecules) compounds as prototypes. For the former case, highly emissive NIR emission is achieved with record NIR OLED being achieved due to the breakdown of the emission energy gap law. For the latter, the correlation between molecular stacking and the emission properties have been established to give a semi-quantitative guideline for the aggregation induce emission. Finally, I would like to present a series of phenazine derivatives that undergo excited-state structural planarization. The corresponding time-resolved spectral evolution was resolved and discussed. Depending on the substituents and external stimulus such as viscosity, the emission covers whole panchromatic visible spectrum.
11:35 - 11:50ID: 150
SEARCHING FOR OLED EMITTERS WITH INVISIBLE LUMINESCENCE
1Physics Depertment, Durham University, Durham, United Kingdom; 2Chemistry Depertment, Durham University, Durham, United Kingdom
The near-infrared (NIR) refers to that region of the electromagnetic spectrum that is just beyond what the eye perceives as red light. A typical definition is the wavelength range 700-1400 nm. NIR radiation has lower energy than visible light, and though invisible to the eye, the NIR is a technologically important region of the spectrum. NIR light penetrates tissue more deeply and is less subject to scattering, and is also invisible to the eye. These are properties that may open up new opportunities in bio-imaging, medicine, and industry. Yet, there are very few OLEDs for the NIR region, and most investigated to date have low efficiency. The reason is that deep red / NIR luminescent molecules are typically less efficient than their blue and green counterparts. Thus, the development of design strategies for new, efficient-emitting, deep red / NIR molecular materials is very attractive.
A number of factors conspire to reduce the luminescence quantum yield of molecular materials that emit in the deep red and NIR regions. Non-radiative decay through coupling of the electronic excited state with higher vibrational levels of the ground state becomes more efficient as excited-state energy decreases, owing to greater Franck-Condon overlap of pertinent vibrational levels - the so-called "energy gap law". For organometallic emitters, in particular, this is compounded by typically slower phosphorescence rate constants in the red / NIR, since the amount of metal character in the excited state tends to decrease with increasing ligand conjugation, needed to shift the emission to the long-wavelength region.
Here, we discuss recent investigations in our group aiming to explore excimer formation and energy transfer mechanisms to promote efficient luminescence in the deep-red / NIR region. The work will focus on the discussion of the photophysics of novel compounds and their application in prototype OLEDs.
11:50 - 12:05ID: 251
EFFICIENT FAR-RED/NEAR-INFRARED POLYMER LIGHT-EMITTING DIODES INCORPORATING A DIKETOPYRROLOPYRROLE DERIVATIVE
1Department Physics and Astronomy and London Centre for Nanotechnology, University College London, London, United Kingdom; 2Communications and Information Systems, University College London, London, United Kingdom; 3School of Engineering, Newcastle University, Newcastle, United Kingdom; 4Institute of Organic Chemistry of the Polish Academy of Sciences, Warsaw, Poland; 5Institute for the Study of Nanostructured Materials (ISMN-CNR), Bologna, Italy
The spectral range between 650 nm and 800 nm presents significant challenges and massive opportunities for organic light-emitting diodes. These span from widening of the bandwidth for visible light communications (VLC) to biodetection (the prime example of which is pulse oximetry), including even apparently far-fetched applications such as horticulture.
Here, we present a full steady-state and transient optoelectronic characterisation of near-infrared (NIR) polymer light-emitting diodes (PLEDs) incorporating a newly synthesised π-expanded fluorescent diketopyrrolopyrrole dye (eDPP), doped into a polyfluorenic matrix to avoid concentration quenching. Such PLEDs exhibit electroluminescence peaked at 670 nm, with 43% of photons falling in the NIR spectral range (λ > 700 nm) and almost complete (99%) quenching of the host emission thanks to efficient energy/charge transfer.
Most remarkably, however, eDPP PLEDs show radiances reaching up to 4 mW/cm2 and external quantum efficiency exceeding 3%. Such values are among the highest reported so far for PLEDs based on dyes emitting above 650 nm, whose energy-gap is sufficiently narrow to cause a substantial vibrational overlap between the ground and the excited states, ultimately responsible for a more efficient non-radiative exciton recombination rate compared to wider-gap emitters (the so‑called “Energy‑gap law”).
In terms of optoelectronic signalling performance, we measured a maximum bandwidth of ~350 kHz and transmission speeds > 1 Mb/s, error free, by integrating the PLEDs in a real‑time VLC setup. Although such speeds are unprecedented for an online VLC link (without leveraging equalisation algorithms) based on organic LEDs, and already high enough to support an indoor point‑to‑point link, even faster speeds can be envisaged upon a synergistic optimisation of both the PLED architecture and the VLC system design.
12:05 - 12:20ID: 198
NEW BORON(III) BLUE EMITTERS FOR ALL-SOLUTION PROCESSED OLEDS: MOLECULAR DESIGN ASSISTED BY THEORETICAL MODELING
1Federal University of Santa Catarina, Florianopolis, Brazil; 2Federal University of Ouro Preto, Ouro Preto, Brazil
Luminescent boron (III) complexes have recently been employed as emitters in organic light emitting diodes (OLEDs) with reasonable success. Although emitters for this class with all colors have already been reported, highly efficient and stable blue emitters for applications in solution processed devices still pose a challenge. Here, we present the design, synthesis and characterization of a new series of boron complexes based on the 2-(benzothiazol-2-yl)phenol ligand (HBT), with different donor and acceptor groups responsible for modulating the emission properties, from blue to red. The molecular design was assisted by calculations using our newly developed formalism, where we demonstrate that the absorption and fluorescence spectra can be successfully predicted, being a powerful technique to evaluate molecular photophysical properties prior to synthesis. In addition, density functional theory (DFT) allows to understand the molecular and electronic structure of the molecules in greater detail. The molecules showed fluorescence efficiency as high as Φ = 0.88 and all-solution processed OLEDs were prepared and characterized under ambient atmosphere, after dispersion in the emitting layer. Surprisingly, even considering these rather simple experimental conditions, the blue emitters displayed superior properties compared to present literature, in particular with respect to the current efficiency stability.