B. Kraabel

Blue electroluminescent diodes utilizing blends of poly(p-phenylphenylene vinylene) in poly(9-vinylcarbazole)

Abstract We report blue light emission from diodes made from polymer blends composed of poly( p -phenylphenylene vinylene) (PPPV) in a hole-transporting polymer, poly(9-vinylcarbazole) (PVK). The soluble PPPV and PVK allow fabrication of light-emitting diodes (LEDs) by spin-casting the electroluminescent polymer blend from solution at room temperature with no subsequent processing or heat treatment required. The initial devices utilized calcium as the electron-injecting (rectifying) contact on the front surface of a PPPV/PVK film spin-cast onto a glass substrate partially coated with a layer of indium/tin oxide (ITO) as the hole-injecting contact. The LEDs turn on at ∼30 V and have a peak emission wavelength in the blue at 495 nm (at room temperature). The quantum efficiency is measured as a function of PPPV content in the blend; the maximum efficiency is approximately 0.16% photons/electron at a concentration of only 2% PPPV in PVK.

Subpicosecond photoinduced electron transfer from conjugated polymers to functionalized fullerenes

We report time‐resolved excited state absorption measurements which demonstrate subpicosecond photoinduced electron transfer using soluble derivatives of poly(p‐phenylene vinylene) as donors blended with a functionalized fullerene (methanofullerene) as acceptor. The subpicosecond photoinduced absorption spectra of the polymer/methanofullerene blends show that electron transfer from the donor to the acceptor occurs within a picosecond of photoexcitation of the conjugated polymer. Precise determination of the electron transfer dynamics was obtained by monitoring the dichroic ratio. The charge separated state is metastable and persists into the millisecond time domain, yielding an asymmetry of 10 orders of magnitude between the forward and reverse electron transfer times. The increased miscibility of the functionalized methanofullerene with the conjugated polymer is important for preparation of films with sufficiently high acceptor concentrations for practical devices based on photoinduced charge separation.

Direct observation of the intersystem crossing in poly(3‐octylthiophene)

Using subpicosecond photoinduced absorption, we monitor the evolution of singlet excitons in solutions of regiorandom poly(3‐octylthiophene) (P3OT), regioregular poly(3‐hexylthiophene) (P3HT), and a well‐defined α‐oligothiophene with 12 repeat units (T12). We find a luminescence lifetime of 500 ps, and we observe the intersystem crossing from the singlet manifold to the triplet manifold, with a rate constant of k−1ISC=1.2 ns. By measuring the quantum efficiency of luminescence, we estimate an intrinsic lifetime of 2 ns for the singlet excitons, and a nonradiative decay rate of k−1nr=1.5 ns. We find no difference in the intersystem crossing time for the different thiophene derivatives, implying the intersystem crossing rate is determined primarily by the relatively large spin–orbit interaction due to the sulfur heteroatom and not by chain defects, chain ends, or effects due to the side groups. In addition, we find that for t<50 ps a fraction of the photoexcitations undergo one‐dimensional diffusion limited...

Blue emission from polymer light-emitting diodes using non-conjugated polymer blends with air-stable electrodes

Abstract By dispersing a non-conjugated luminescent polymer and an electron-transporting material, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), into a hole-transporting polymer, poly(9-vinylcarbazole) (PVK), we have achieved improved efficiencies for blue light-emitting diodes (LEDs) fabricated with a copolyester (CPE) containing isolated 1,2-dinaphthylethene units. Quantum efficiencies of the LEDs with the solid dispersion of CPE and PBD in PVK as electroluminescent layer and indium as the electron injection electrodes are about 0.1% photons per electron, better by factor of 102 than similar devices made with only the CPE. The LEDs turn on at about 30 V and have a peak emission wavelength in the blue at 485 nm.

A 160-femtosecond optical image processor based on a conjugated polymer

Degenerate ground-state conjugated polymers exhibit large third-order nonlinear optical susceptibilities, including substantial two-photon absorption. With the use of a machine architecture suited to these material properties, ultrafast optical processors are possible. A four-wave mixing optical correlator was built with an air-stable, processable, degenerate ground-state conjugated polymer, poly(1,6-heptadiester). The continuously updatable processor correlates two 5000-pixel images in less than 160 femtoseconds, achieving peak processing rates of 3 x 1016 operations per second.

Signatures of excitons and polaron pairs in the femtosecond excited-state absorption spectra of phenylene-based conjugated polymers and oligomers

Abstract We present results of broadband femtosecond transient absorption measurements in thin films of several phenylenebased onjugated materials. We compare results for unsubstituted poly(phenylene vinylene) (PPV), polyflourene, and a substituted five-ring oligomer of PPV. We observe universal spectral features in all materials, implying a common origin for the photoexcited species in all members of this family. In each material, we observe two photoinduced absorption (PA) features in the visible region in addition to a stimulated emission (SE) feature. The first PA feature shows strong dependence of the dynamics on excitation intensity, and correlates precisely with the SE feature, proving that this feature is due to the primary emissive exciton. The second PA feature shows dynamics independent of intensity. In the oligomers, this feature grows quadratically with respect to the first PA feature, as a result of nonlinear interactions of the primary excitons. In the polymers, however, the second feature grows linearly with intensity, and indicates an ultrafast decay channel to nonemissive interchain species due to defects.

Subpicosecond Photoinduced Electron Transfer in Semiconducting Polymer - C60 Composites

Abstract We present the results of ultrafast photoinduced absorption studies of poly(3-octylthiophene), (P3OT) poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene)] (MEH-PPV) and poly(2,5-bis(cholestanoxy)-1,4-phenylene vinylenel, (BCHA-PPV) in their pure form and mixed with C60. The results showa subpicosecond photoinduced electron transfer from the polymer host onto the C60 molecule.

Highly efficient energy and charge transfer in thin self-assembled multilayered polymer films

We report the synthesis and characterization of multilayer self-assembled polymer films made from a water-soluble conjugated polymer, poly(2,5-methoxy-propyloxy sulfonate phenylene vinylene) (MPS-PPV). We observe a red shift of both the absorption and fluorescence spectra with increasing numbers of active MPS-PPV layers. We attribute this red shift to changing polymer conformation and efficient energy transfer. Upon adding a water-soluble C 60 or C 60 -VBA copolymer top layer, the luminescence spectrum is strongly quenched due to charge transfer. The estimated charge transfer quantum efficiency from PL quenching is ∼95%. We discuss in detail the unidirectional energy transfer followed by charge transfer in the self-assembled multilayered films.

Ultrafast dynamic holography in nanocrystal solids

We report efficient dynamic gratings in close-packed solids of CdSe nanocrystals. These gratings are formed on the subpicosecond time scale and have diffraction efficiencies up to 0.5% for film thicknesses of ∼0.5 μm. Nanocrystal solids combine the best features of inorganic semiconductors (large resonant nonlinearities and high photostability) and organic semiconducting polymers (chemical flexibility and tunability of optical properties by simple synthetic means). Additionally, nanocrystal solids allow precise control over the spectral position of the nonlinear optical response by simply varying the size of the nanocrystals used in fabricating the solid (quantum confinement effect).

Subpicosecond study of excitons in a phenylenevinylene conjugated polymer: exciton–exciton interactions and infrared photoinduced absorption features

Abstract We present a study of the fundamental photoexcitations in a phenylenevinylene conjugated polymer from the visible to the mid-infrared (IR) using polarized subpicosecond transient absorption (TA) spectroscopy. From 0.5 to roughly 1.3 eV we find that features due to singlet intrachain excitons dominate the photoinduced absorption. Two peaks are apparent in the spectrum: one near 0.5 eV and the other at ∼1.1 eV. In the regime of high excitation densities (>10 19 cm −3 ) , we find evidence of exciton–exciton interactions leading to exciton hopping between different chains or segments of the polymer.