No abstract

No abstract

No abstract

In the past two decades a significant research effort has focused on the photophysical properties of advanced organic materials for optoelectronic devices. Examples include the development of organic white light emitting diodes (WLEDs) since they are at the heart of display technology [1] and offer potential applications as novel lighting sources that are less expensive and more efficient than conventional incandescent and fluorescent illumination sources. [2][3][4] The most impressive characteristics of organic WLEDs are those based on single emissive dopant, where molecular excitons are harnessed to form triplet excimers. [5][6][7][8] They combine molecular phosphorescence with the red-shifted excimer phosphorescence that yields the emission approaching white light. Here, we use electron donor-electron acceptor emitter layers, in which broad emission band of an exciplex mixes with excimer emission, enabling us to form an efficient white device with particularly high color rendering index of CRI = 90. One of the most stormily developing classes of organic photonic devices nowadays are organic light-emitting-diodes (LEDs).[1] Among them, white-light-emitting-diodes (WLEDs) are of particular interest because they offer low-cost alternatives for back-lights in flat panel displays and are considered as future illumination sources which are able to operate at low voltages with high luminance efficiency. [2][3][4] By definition, the emission spectrum of WLEDs must cover possibly uniformly the whole visible spectrum of electromagnetic radiation. Several routes have been employed to realize this goal, the fabrication of stacked [3] or multilayer [9] LED structures with separated molecular emitters, that is blue, green and red, was proposed initially. Recently, a high-performance organic WLED has been fabricated that exploits three different emitters mixed together in one emissive layer to get stable color balance at a high external electroluminescence (EL) quantum efficiency (QE) of 11 % photons/electron (ph/e) and color rendering index CRI = 85.[10] The blue fluorescence of a dye dopant was mixed with green and red emissions of two phosphorescent dopants. However, this approach requires a very careful adjustment of the concentration of each dye because energy transfers from the higher energy blue dye to the green dye and from the green dye to the red dye. A simplification of the device structure can be achieved by combining molecular (monomer) and excimer phosphorescence from one emitter doped in a single emissive layer. [5,7,8] Single dopant WLEDs give voltage independent white emission with external EL QE as high as 16 % ph/e [7,8] but, due to the excessive distinctiveness of individual components in the structured emission spectra, their CRIs do not exceed 75. [5,7,8] In efforts to improve the WLEDs based on a combination of monomer and excimer spectra, we now report the achievement of well balanced efficient white emission from a single emissive layer comprised of an electron donor (D) and an electron phosphorescen...

Luminescence quantum yield of molecular aggregates and excitons in α-sexithienyl thin films at variable temperature

A series of terdentate cyclometallated PtII complexes with remarkable luminescence properties are used as new phosphorescence‐emitting dopants in a blended host matrix as the emitting layer, resulting in very high electroluminescence efficiencies. Because of the high phosphorescence quantum yields of these Pt complexes and the efficient energy transfer from both singlet and triplet excited states of the host to the emitting guest, external electroluminescence quantum efficiencies as high as 4–16 % photons per carrier and luminous efficiencies of 15–40 cd A–1 are achieved. Moreover, these high efficiency values were maintained over a four‐decade current intensity span with no significant roll‐off. Tuning of the electroluminescence spectra from the yellow to the green‐bluish region of the chromaticity diagram is obtained simply by changing the substituents at the central 5‐position of the cyclometallating ligand.

vices the required Q S to obtain a specific m is slightly lower than in SiO 2 -and BZT-based devices assuming that there is a reduced concentration of traps in the pentacene channel of solution-processed BST-based TFTs relative to ones comprising BZT or SiO 2 gate insulators.The use of solution-processed BST films as the high e gate insulator in pentacene IGFETs, required annealing at 400 C. Hence, despite their excellent low voltage device characteristics and performance they are not compatible with transparent plastic substrates, in contrast to the sputtered mixed metal oxide BZT insulators reported in the literature, [8] which were deposited at room temperature. However, given their performance, which is very close to that of the widely used a-Si:H TFT, pentacene IGFETs comprising sol-gel deposited BST films could be good candidates for applications involving AMLCDs or AMOLEDs on glass substrates. The low operating voltage required to produce such performance and the very low subthreshold slope of pentacene/BST-based IGFETs make them very attractive for applications. Furthermore, due to the solutionbased deposition process used, such IGFETs can potentially reduce manufacturing costs, especially if other layers of the device are also deposited from solution. This can prove very important in the future, as cost is expected to become an increasingly important factor in the flat panel display industry.In conclusion, based on our understanding of the origins of the gate bias dependence of mobility in pentacene IGFETs, [8] we have designed and fabricated high-performance devices, comprising pentacene and a solution-processed, relatively high e BST gate insulator, that require operating voltages up to only 5 V.

A family of complexes (1a-3a and 1b-3b) was prepared, having the structure Ir(N^C^N)(N^C)Cl. Here, N^C(∧)N represents a terdentate, cyclometallating ligand derived from 1,3-di(2-pyridyl)benzene incorporating CH(3) (1a,b), F (2a,b), or CF(3) (3a,b) substituents at the 4 and 6 positions of the benzene ring, and N^C is 2-phenylpyridine (series a) or 2-(2,4-difluorophenyl)pyridine (series b). The complexes are formed using a stepwise procedure that relies on the initial introduction of the terdentate ligand to form a dichloro-bridged iridium dimer, followed by cleavage with the N^C ligand. (1)H NMR spectroscopy reveals that the isomer that is exclusively formed in each case is that in which the pyridyl ring of the N^C ligand is trans to the cyclometallating aryl ring of the N^C^N ligand. This conclusion is unequivocally confirmed by X-ray diffraction analysis for two of the complexes (1b and 3a). All of the complexes are highly luminescent in degassed solution at room temperature, emitting in the green (1a,b), blue-green (2a,b), and orange-red (3a,b) regions. The bidentate ligand offers independent fine-tuning of the emission energy: for each pair, the "b" complex is blue-shifted relative to the analogous "a" complex. These trends in the excited-state energies are rationalized in terms of the relative magnitudes of the effects of the substituents on the highest occupied and lowest unoccupied orbitals, convincingly supported by time-dependent density functional theory (TD-DFT) calculations. Luminescence quantum yields are high, up to 0.7 in solution and close to unity in a PMMA matrix for the green-emitting complexes. Organic light emitting devices (OLEDs) employing this family of complexes as phosphorescent emitters have been prepared. They display high efficiencies, at least comparable, and in some cases superior, to similar devices using the well-known tris-bidentate complexes such as fac-Ir(ppy)(3). The combination of terdentate and bidentate ligands is seen to offer a versatile approach to tuning of the photophysical properties of iridium-based emitters for such applications.