Andreas Bacher

Novel hybrid solar cells consisting of inorganic nanoparticles and an organic hole transport material

A novel three-layer concept for efficient solid-state solar cells is presented. The hybrid devices consist of an inorganic nanocrystalline titanium dioxide layer for electron conduction, a surface-adsorbed ruthenium dye complex for light absorption, and an organic triphenyldiamine layer for the transport of holes. External quantum efficiencies of up to 0.2% have been achieved. Time-resolved current measurements with pulsed excitation of the dye layer reveal the charge transport to be totally controlled by the nanoparticles in the long-time regime. The high charge storage capability of the porous titanium dioxide layers is demonstrated by junction-recovery measurements.

Electroluminescence and electron transport in a perylene dye

Charge carrier transport in vapor-deposited films of 1,6,7,12-tetraphenoxy-N,N′-bis-(2,6-diisopropylphenyl)-perylene-3,4,9,10-bis(dicarboximide) was investigated using two different methods, the time-of-flight (TOF) technique and time-resolved electroluminescence. Electron mobilities of 10−5 cm2/V s were measured in the bulk using a time-of-flight technique. Hole transport was found to be dispersive and, thus, a transit time for holes could not be obtained. The above dye was also used to fabricate single layer light emitting diodes showing clearly visible red electroluminescence under ambient conditions. Our experiments on transit electroluminescence confirmed the measured electron mobility and ruled out the possibility that the transit time of holes is shorter than the time range investigated in our time-of-flight experiments.

Electroluminescent Behavior of a Homologous Series of Phenylenevinylene Oligomers

Finally, the film emission spectra of both the DEHF/PER and the DEHF/CNSt copolymers are unaffected bythermal annealing at 200˚C for 48 h, suggesting that noemissive excimer is formed in the annealed copolymers.The dramatic effect of annealing on the DEHF homopoly-mer is shown in Figure 3b. Although excimer formation isless pervasive in the branched DEHF derivative relative tothe di-n-hexylfluorene (DHF) homopolymer, a broad andunstructured longer-wavelength excimer emission (500–550 nm) that increases with annealing time and tempera-ture is observed.In summary, the synthesis of thermally stable, easily pro-cessable random copolymers of 9,9-di-2¢-ethylhexylfluor-ene and either perylene or a-cyanostilbene can be accom-plished by nickel-mediated coupling reaction. Here, thebranched DEHF units provide adequate solubility whenunsubstituted perylene and a-cyanostilbene units are incor-porated into the conjugated main chain. We have demon-strated that formation of excimers in thin films of poly-(fluorene) derivatives upon thermal annealing can be sup-pressed by incorporation of lower-bandgap chromophoricsegments, which serve as efficient excitonic energy trapsupon excitation of the poly(fluorene) chromophoric seg-ments. This study provides indirect evidence of the possiblerole of the anthracene subunits in DHF/ANT copolymersin suppressing excimer formation.

Relationship between structure and electroluminescence of oligo(p-phenylenevinylene)s

The preparation of LEDs with poly(p-phenylenevinylene) (PPV) as emitting material is well established, However, due to the presence of a distribution of conjugated chain lengths in the polymer, systematic investigations of the electroluminescence with polymeric materials are difficult, as far as the optical emission is concerned. We are studying the relationship between structural variation of substituted oligo(p-phenylenevinylene)s and their electroluminescent behaviour using a series of distyrylbenzenes with a variety of substituents in order to investigate their influence on the electroluminescence (EL). Furthermore, we synthesized a homologous series of monodisperse oligo(2,5-dipropoxy-1,4-phenylenevinylene)s with up to 11 repeating units. This series covers the spectrum from monomer to polymer. The influence on the EL can be investigated by preparing single layer LEDs using vapor deposition or spincoating of the oligomers in a polystyrene (PS) matrix. The comparison of photoluminescence (PL)- and EL-spectra shows that the photophysical properties of the oligomers are strongly altered by aggregation phenomena

Spectral tuning of light emitting diodes with phenyl-thiophenes

Abstract We report electroluminescence data on single and double layer devices based on well-defined phenyl-thiophenes (PTs) (PPPP T , PPP T P, PP T PP), wherein 2,5-thiophene ( T ) is systematically translated through a p -quinquephenyl (PPPPP) framework. Single layer light emitting diodes (LEDs) were prepared by vacuum deposition of the PTs and an aluminum top electrode onto an ITO substrate; double layer devices were prepared using Alq 3 as the electron-transport and emitter layer. The single layer LEDs exhibited light emission that could be tuned across the spectral range from blue to green depending upon the location of the thiophene moiety. All of the double layer devices emit in the 510-nm range which suggests that this emission originates from Alq 3 and that the PTs function as hole-transport materials. For both types of devices we present spectroscopic data, the wavelength dependence of the electroluminescence, and the current–voltage characteristics. Cyclic voltammetry was used to determine the redox behavior and the pertinent electronic energy levels of the PTs. The experimental UV λ max values and HOMO values of the PTs were contrasted with ab initio calculations and found to be in good agreement.

Light-emitting diodes based on phenylenevinylene oligomers with defined chain lengths

Abstract We present characteristic electroluminescence data on single-layer devices based on three different oligo phenylenevinylene compounds with defined chain lengths. Light-emitting diodes (LEDs) were prepared by subsequent vacuum evaporation of the oligomer film and the A1 top electrode onto an indium—tin oxide (ITO) substrate. The LEDs show light emission in the spectral range of yellow to orange depending on the conjugation length of the compound used. We report photoluminescence and optical absorption data of the thin sublimed films, together with the wavelength dependence of the electroluminescence and with the current—voltage characteristics of the LED devices. The redox behavior and, thus, the pertinent electronic energy levels of the phenylenevinylene oligomers were characterized using cyclic voltammetry. The experimental data can be interpreted within a consistent model description.

Synthesis and properties of new hole transport materials for organic light emitting devices

We synthesized low molecular weight triphenyldiamines (TPDs), novel 1,3,5-tris(diarylamino)benzenes (TDABs), polymeric triphenyldiamines and insoluble triphenylamine networks based on tris(4-ethynylphenyl)amine as hole transport materials for electroluminescent displays. The HOMO energy values as determined from cyclic voltammetry measurements for TPDs and TDABs are between -4.97 and -5.16 eV. By using a polymeric TPD as hole transport layer and tris(8-quinolinolato) aluminium as emitter, LEDs with an onset voltage of 3V and a luminance up to 900 cs?m2 were obtained under ambient conditions, using airstable Al-electrode as cathode and ITO as anode.

Triphenylenes: a new class of hole transporting material in organic light-emitting diodes

Electroluminescent devices consisting of triphenylene derivatives as hole transport layer (HTL) and 8- hydroxyquinoline aluminium complex (Alq3) as emitting layer with ITO as anode and aluminium as cathode are presented. Triphenylene compounds were evaporated or spin coated from solution. In addition side chain polymers with triphenylenes are used. Finally we prepared a polymeric HTL by photopolymerization of an acrylate functionalized triphenylene monomer. Some of the devices, based on ITO/Triphenylene/Alq3/Al showed brightness values up to 1000 cd/m2 and low turn on voltages from 4-5 V.