Ralf Fink

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.

A comparison of hole blocking/electron transport polymers in organic LEDs

Three main-chain aromatic polyethers with different electroactive heterocyclic moieties, 1,4-quinoxaline, 1,3,4-oxadiazole and 1,3,5-triazine, have been synthesized. The polymers are amorphous with glass transition temperatures above 200 C. The polymers with these high electron affinity units were used as hole blocking/electron transport layers (HBETL) in light-emitting diodes (LEDs) having the HBETL casted on top of a hole transport/emitting PPV layer. In order to compare the influence of the different polyethers on the LED characteristics, three multilayer devices (ITO/PPV/HBETL/AI) with different HBETLs were investigated. Relative to the single layer PPV device, quantum efficiencies were improved by two orders of magnitude in all multilayer devices and power efficiency was increased using poly(quinoxaline ether) as HBETL. To investigate the electrochemical behavior of the three HBETLs, cyclic voltammetry measurements were carried out and the HOMO/LUMO energy values determined from redox potentials were used to understand the hole blocking property. Lowering the onset voltage using the poly(quinoxaline ether) as HBETL in two-layer devices is compatible with the high electron affinity of this polymer.

Aromatic polyethers with 1,3,5-triazine units as hole-blocking/electron-transport materials in LEDs

Various fluoro-functionalized aromatic 1,3,5-triazine monomers were prepared. A series low molar mass and poly- (1,3,5-triazine)-ethers were synthesized by a condensation reaction. The polymers as well as the low molar mass compounds have excellent thermal stability and are amorphous. In order to examine the potential to apply these compounds in organic electroluminescent devices, the redox properties were studied by cyclic voltammetry. It was found that the monomers have high electron affinity and reach LUMO values in the range of -2.7 to -3.1 eV. Additionally high oxidation stability with HOMO values lower than -6.4 eV follows hole blocking capabilities. This opens the possibility to utilize 1,3,5-triazine containing materials as electron injection/hole blocking layer in LEDs. First LED results are in agreement to these high electron affinities.

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.