We synthesized lithium quinolate complexes, 8- hydroxyquinolinolato lithium (Liq) and 2-methyl-8- hydroxyquinolinolato lithium (LiMeq) as emitter and electron injection/transport materials to be used in conventional two layer organic light emitting diodes in combination with N,N'-bis(p-methoxyphenyl)-N,N'-diphenyl benzidine (DMeOTPD) as hole transport material (HTL). The lithium complexes were also examined as interface materials in combination with 8- hydroxyquinolinolato Al(III) as emitter material. The device efficiency using these complexes were optimized using combinatorial methods. We also compared the electron injection, transport and emission properties of Li-complexes with the well known emitter Alq<SUB>3</SUB> in the same experiment taking advantage of the combinatorial approach. The Li- quinolates are found to be efficient emitter molecules. But the efficiencies of lithium quinolate devices are lower than that of Alq<SUB>3</SUB> devices. Contrary to the Alq<SUB>3</SUB> emission, the Li-quinolates exhibit bathochromic shift of emission compared to the respective photoluminescence spectra. No clear evidence for exciplex formation was seen by comparing the photoluminescence spectrum of an equimolar mixture of Li-quinolate and DMeOTPD with the observed EL spectrum. However, the lithium complexes increase the efficiency of an optimized ITO/DMeOTPD/Alq<SUB>3</SUB>/Al device considerably when used as a thin interface layer between Alq<SUB>3</SUB> and aluminum. The improvement of device characteristics using lithium quinolates is similar to that obtained using LiF salt. The mechanism of improvement of efficiency using a lithium complex interface layer seems to be of chemical doping nature.
The synthesis and properties of two semiconducting donor-acceptor (D-A) block copolymers and their application in nanostructured bulk heterojunction solar cells are reported. The donor segments were obtained via nitroxide mediated polymerization of either bis(4-methoxyphenyl)-4'-vinylphenylamine or N,N'-bis(4-methoxyphenyl)-N-phenyl-N'-4-vinylphenyl-[1,1'biphenyl]-4,4'-diamine. Narrow-distributed macroinitiators, poly{bis(4-methoxyphenyl)-4'-vinylphenylamine} (PvDMTPA) and poly{N,N'-bis(4-methoxyphenyl)-N-phenyl-N'-4-vinylphenyl-[1,1'biphenyl]-4,4'-diamine} (PvDMTPD) were used to polymerize the acceptor monomer perylene diimide acrylate (PerAcr) to yield block copolymers with well-defined molecular weights. Different diblock copolymers, PvDMTPA-block-PPerAcr and PvDMTPD-block-PPerAcr, with high perylene diimide weight fractions were prepared. The block copolymers exhibited efficient fluorescence quenching. Transmission electron microscopy revealed wire-like and worm-like nanostructures throughout bulk samples. Thin film photovoltaic devices showed short circuit currents of up to 1.21 mA/cm2 and power conversion efficiencies η of 0.32% under AM 1.5 illumination conditions.
A combinatorial approach combining vapor deposition of organic molecules and a movable mask technique was used to screen and optimize materials and organic light emitting device configurations fast and efficiently. Some low molecular weight triphenyldiamine derivatives with different electronic and thermal properties were compared in two layer, ITO/TPD/Alq3/Al device configurations. The optimum thickness for Alq3 layer was obtained by evaporating a linear gradient of Alq3 on top of various TPD layers. Further, a landscape library with two orthogonal linear gradients of TPD and Alq3 was prepared to investigate the dependence of efficiency on thickness of both layers simultaneously. The necessity and the efficiency of an additional spiro-qunioxaline compound as electron transporting/hole blocking layer was also investigated using a landscape library of Alq3 versus spiro-quinoxaline on top of TPD. The efficiency of the two layer device depends not only on the Alq3 layer thickness, but also on the TPD layer thickness. The photometric efficiency of a TPD/Alq3 device can be improved by replacing the optimum Alq3 layer thickness by certain combinations of Alq3/spiro-quinoxaline layers.
Oligomeric LED systems have received little attention compared to that devoted to polymeric LED systems. This paper focuses on the electroluminescent behavior of a homologous series of six phenylenevinylene oligomers—their molecular structure is illustrated in the Figure—in LEDs in combination with a comparative study of related cyclic voltammetry data.
Abstract Summary: An alkoxyamine initiator carrying a perylene bisimide unit suitable for the nitroxide mediated controlled radical polymerization of different monomers was synthesized. The synthesis, characterization, and properties of a series of polymers obtained from monomers such as 4‐vinyltriphenylamine, styrene, and different acrylates using this initiator are described. The controlled nature of the polymerization was demonstrated by time‐dependent measurements of the conversion and the molecular weight. The incorporation of the single fluorescent unit to the polymer chain end was verified by MALDI‐TOF MS. The perylene bisimide acts as an electron acceptor with a strong fluorescence. Since 4‐vinyltriphenylamine is a donor monomer, the resulting polymers exhibit photoluminescence quenching due to electron transfer between the donor polymer chain and the acceptor moiety. The perylene bisimide moiety shows aggregation via π‐π stacking which was studied using UV‐vis and fluorescence spectroscopy. By controlling the polymer chain length, the stacking of the perylene bisimide can be controlled. The LUMO and HOMO levels of the perylene bisimide initiator and the dye‐labeled polymer were determined by cyclic voltammetry as −3.7 and −6.0 eV, respectively. With this approach tailor‐made fluorescent dye‐labeled polymers with desired architecture, low polydispersity, and controlled molecular weight can be obtained as model systems for electron and energy transfer studies. Schematic representation of electron transfer in dye‐labeled poly(vinyltriphenylamine). magnified image Schematic representation of electron transfer in dye‐labeled poly(vinyltriphenylamine).
The synthesis, characterization and thermotropic properties of novel asymmetrically substituted discotic molecules, perylene diester benzimidazoles (PDBIs), are presented. PDBIs were designed with an imidazole unit at 3,4 positions and a bisester moiety at 9,10 positions of the perylene tetracarboxylic acid core. By attaching linear or branched aliphatic substituents at the ester moiety and two alkyl or alkoxy substituents at the benzimidazole unit, sufficient solubility and the flexibility to obtain mesophases was guaranteed. Thermotropic behaviour, which is strongly influenced by the nature of the respective substituents at the diester and benzimidazole moiety, was investigated using differential scanning calorimetry (DSC), polarization optical microscopy (POM) and X-ray diffraction measurements (XRD). All PDBIs under investigation self-organize into liquid crystalline columnar hexagonal phases (Colh), among them PDBI-3 even at room temperature. Also the formation of a room temperature columnar plastic phase (Colhp) and the formation of a lamellar phase was observed. Due to extension of the π-conjugation system, the absorption of these well soluble discogens is significantly extended to longer wavelengths in the visible regime up to 680 nm.
It has been a long-term goal to understand the molecular orientation in films of conjugated polymers, which is crucial to their efficient exploitation. Here, we show that the surface energies determine the crystal orientation in films of model conjugated polymers, substituted polythiophenes crystallized on substrates. We systematically increase the surface energy of edge-on crystals formed at the vacuum interface by attaching polar groups to the ends of the polymer side chains. This suppresses crystallization at the vacuum interface, resulting in a uniform face-on crystal orientation induced by the graphene substrate in polythiophene films as thick as 200 nm, which is relevant for devices. Surprisingly, face-on crystal orientation is attained in the modified polythiophenes crystallized even on amorphous surfaces. Furthermore, for the samples with still competing interfacial interactions, the crystal orientation can be switched in the same sample, depending on the crystallization conditions. Thus, we report a fundamental understanding and control of the equilibrium crystal orientation in films of conjugated polymers.
