The efficiency and stability of blue organic light emitting devices (OLEDs) continue to be a primary roadblock to
developing organic solid state white lighting. For OLEDs to meet the high power conversion efficiency goal, they
will require both close to 100% internal quantum efficiency and low operating voltage in a white light emitting
device.1 It is generally accepted that such high quantum efficiency, can only be achieved with the use of
organometallic phosphor doped OLEDs. Blue OLEDs are particularly important for solid state lighting. The simplest
(and therefore likely the lowest cost) method of generating white light is to down convert part of the emission from a
blue light source with a system of external phosphors.2 A second method of generating white light requires the
superposition of the light from red, green and blue OLEDs in the correct ratio. Either of these two methods (and
indeed any method of generating white light with a high color rendering index) critically depends on a high efficiency
blue light component.3
Conductivity doping of charge transporting layers is becoming increasingly attractive for improving power efficiency in
OLEDs. However, the number of commercially available organic molecular p-dopants is limited. The electron
acceptor 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (F4-TCNQ) is the most utilized p-dopant. F4-TCNQ can
be used as a dopant for most hole transporting materials (HTM), but it is very volatile, which makes it difficult for
vacuum processing, and has a low sticking coefficient. Here we present the design of novel anchored molecular dopants
based on the TCNQ core. We first review how the reduction potential of TCNQ core is affected by substitution with
alkyl groups of different electronic properties. Electron donating groups have negative effect on the reduction potential
of the acceptor. However, attaching electron withdrawing groups such as halogens counteracts the effect of electron
donating groups. Using gas phase theoretical calculations we determined that trifluorinated TCNQ can be anchored
through a σ-coupled alkyl chain to an inert molecular anchor without sacrificing the electron affinity.
Phosphine oxide substitution of small molecules with high triplet exciton energies allows development of vacuum
sublimable, electron transporting host materials for blue OLEDs. Heteroaromatic building blocks (carbazole,
dibenzofuran and dibenzothiophene) with ET ~ 3 eV were incorporated into phosphine oxide (PO) structures. External
quantum efficiencies (EQEs) at lighting brightness (i.e., 800 cd/m2) reached as high as 9.8% at 5.2V for OLEDs using
the heteroaromatic PO hosts doped with the sky blue phosphor, iridium(III)bis(4,6-(di-fluorophenyl)-pyridinato-N,C2,)
picolinate (FIrpic). Comparing device properties at a similar current density (i.e., J = 13 mA/cm2) showed the
dibenzothiophene-bridged PO compound exhibits the highest EQEs and lowest operating voltages at all phosphor dopant
levels. These results are explained with respect to the effects of the inductive phosphine oxide substituents on
electrochemical, photophysical and electroluminescence properties of the substituted heteroaromatic building blocks.
High efficiency small molecule organic light emitting devices (OLEDs) based on light emission from an
electrophosphorescent dopant dispersed in an organic host matrix are well known. Achieving blue phosphorescent
OLEDs is particularly challenging because the host triplet energy should ideally be > 2.8 eV to prevent back-transfer of
energy from the dopant to the host matrix resulting in loss of efficiency. A design strategy for developing new host
materials with high triplet energies by using phosphine oxide (P=O) moieties as points of saturation in order to build
sublimable, electron transporting host materials starting from small, wide bandgap molecular building blocks (i.e.,
biphenyl, phenyl, naphthalene, octafluorobiphenyl, and N-ethylcarbazole) is described. Electrophosphorescent OLEDs
using the organic phosphine oxide compounds as host materials for the sky blue organometallic phosphor,
iridium(III)bis(4,6-(di-fluorophenyl)-pyridinato-N,C2,) picolinate (FIrpic) give maximum external quantum efficiencies
of ~ 8% and maximum luminance power efficiencies up to 25 lm/W.
We describe a flexible, transparent plastic substrate for OLED display applications. A flexible, composite thin film barrier is deposited under vacuum onto commercially available polymers, restricting moisture and oxygen permeation rates to undetectable levels using conventional permeation test equipment. The barrier is deposited under vacuum in a process compatible with conventional roll- coating technology. The film is capped with a thin film of transparent conductive oxide yielding an engineered substrate (BarixTM) for next generation, rugged, lightweight or flexible OLED displays. Preliminary tests indicate that the substrate is sufficiently impermeable to moisture and oxygen for application to moisture-sensitive display applications, such as organic light emitting displays, and is stable in pure oxygen to 200 degrees Celsius.
Optical nonlinearities can arise as the result of intense electromagnetic radiation fields inducing either changes in electron or nuclear configurations. Indeed, as is discussed in this paper, several mechanisms, including mechanisms depending upon electron-phonon coupling, may be elicited from the same material. The precise contribution that a given mechanism makes to observed optical nonlinearity is often dependent upon pulse conditions employed in transient nonlinear optical experiments. The ability to control optical nonlinearity by pulse conditions is demonstrated and analyzed for a high symmetry ladder polymer where contributions from coherent parametric mixing, excitons, and bipolarons are observed. The different timescales associated with various mechanisms for index of refraction and absorption changes are discussed. The utilization of photo-induced changes occurring on widely different timescales is demonstrated in the realization of efficient second harmonic generation by quasi- phase matching. The role of chemical synthesis in engineering multi-functional materials is discussed.
Multifunctional properties of nonlinear optical chromophores are discussed both in terms of a given chromophore exhibiting more than one type or mechanism of optical nonlinearity and in terms of a chromophore exhibiting useful auxiliary properties. For materials exhibiting more than one type of mechanism of optical nonlinearity, the concept of pulse-controlled optical nonlinearity is introduced and discussed. An analogy is drawn to multidimensional nuclear magnetic resonance studies which are useful in systematically elucidating excited state dynamics. Practically, pulsed control of optical nonlinearity provides a means of enhancing and modulating nonlinear optical phenomena. The photochemical reactivity of nonlinear optical chromophores is discussed in terms of fabricating ordered lattices appropriate for the development of integrated circuits and the realization of specific effects such as quasi-phase matching in second harmonic generation.
Several electroactive polymers, such as polyacetylene, polythiophene, poly [p-phenylene vinylene] and poly [2,5-thienylene vinylene] have shown promise as NLO-active materials over the past few years. However, as several theoretical and experimental research groups have pointed out in recent publications and symposia, it is not evident that long conjugation lengths are necessary for enhanced (chi) (3) activity. As recently demonstrated, copolyamides which incorporate polyenylic or PTV oligomeric repeat units show (chi) (3)/(alpha) values of ca. 10-13 esu-cm at 532 nm(band-edge). In this paper, the authors discuss how ladder subunits related to the electroactive polymers POL and PTL can be incorporated into polymer films as (a) copolymer repeat units, (b) pendant groups attached to poly [p-hydroxystyrene] and (c) guest-host composites in polycarbonate. Sharp optical absorptions are found in all cases as well as promising (chi) (3) properties.
During the past three years it has become more evident that long conjugation sequences in electroactive materials may
not be a stringent requirement for high third order nonlinear optical (NLO) activity. Since long conjugation lengths in these
materials often make them difficult to process, the resulting insolubility often precludes the formation of optical quality films
for device applications. The incorporation of shorter electroactive segments alternating with flexible non-active spacers may
allow high NLO activity coupled with good optical film forming capability. In this paper we would like to present several
approaches to copolymer design which incorporate various electroactive oligomer segments with well-defined conjugation
lengths. The control one obtains in this appmach allows the design of sharp optical windows, and the ability to tailor
absorption characteristics to particular frequencies.
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