Eike Becker

Large Area Electronics Using Printing Methods

After the demonstration of the first organic FET in 1986, a new era in the field of electronic began: the era of organic electronics. Although the reported performance of organic transistors is still considerably lower compared to that of silicon transistors, a new market is open for organic devices, where the excellent performance of silicon technology is not required. Several commercial applications for organic electronics have been suggested: organic RFID tags, electronic papers, imagers, sensors, organic LED drivers, etc. The main advantage of organic technologies over silicon technologies is the possibility of making low-cost, large area electronics. The main processes which allow patterning with suitable resolution on a large areas are printing methods. Here we will provide an overview of methods that can be useful in the low-cost production of large area electronics.

Inverted top-emitting organic light-emitting diodes using sputter-deposited anodes

We demonstrate vacuum-sublimed topside-emitting inverted organic light-emitting diodes (IOLEDs) employing low-power radio-frequency magnetron sputter-deposited indium tin oxide (ITO) anodes. The device introduces a two-step sputtering sequence to reduce damage incurred by the sputtering process, paired with a buffer- and hole-transporting material Pentacene. Systematic optimization of the organic growth sequence focused on device performance characterized by current and luminous efficiencies, suggest the incorporation of rather thick Pentacene layers. The optimized thickness is obtained as a trade-off between light absorption and protective properties of Pentacene. The optically and electrically undoped organic multilayer devices capped with 90-nm ITO exhibit high current efficiencies of 3.9 cd/A at a raised luminance level of 1.500 cd/m2, combined with luminous efficiencies of 0.7 lm/W. The inverted configuration allows for integration of organic light-emitting diodes (OLEDs) with preferentially used n-c...

All-organic thin-film transistors made of poly(3-butylthiophene) semiconducting and various polymeric insulating layers

We have fabricated fully patterned all-organic thin-film transistors with a variety of organic polymer insulators. Poly(3-butylthiophene) deposited by spin coating was used as the active organic layer. We have built top-gate structures with gates printed on top of the gate dielectric layer. The field enhanced current is weak with poly(4-vinyl phenol), but much stronger with polyvinyl alcohol and cyanoethylpullulan. Carrier mobilities as large as 0.04 cm2/V s were measured in the case of cyanoethylpullulan. A strong correlation is found between the solvents used for the dielectrics, dielectric constant of the insulator, and the field-effect mobility.

Inverted hybrid organic light-emitting device with polyethylene dioxythiophene-polystyrene sulfonate as an anode buffer layer

In this study we demonstrate inverted organic light-emitting diodes (OLEDs) utilizing highly conductive polyethylene dioxythiophene–polystyrene sulfonate as a buffer layer to a radio-frequency sputter-deposited indium–tin–oxide as the anode. In comparison to an entirely small-molecule-based reference, a reduction in operation voltage of 8.4 V at superior efficiencies of 4.2 cd/A and 1 lm/W obtained at 1.000 cd/m2 was achieved. The inverted cell configuration is desirable for next-generation active-matrix OLED displays.

Deep blue widely tunable organic solid-state laser based on a spirobifluorene derivative

We report on amplified spontaneous emission and optically pumped deep blue lasing in the organic spirobifluorene derivative 2,7-bis(biphenyl-4-yl)-2′,7′-di-tert-butyl-9,9′-spirobifluorene. Solid-state lasing is observed in thin films of this material deposited on a distributed-feedback (DFB) grating substrate. The laser wavelength can be tuned from 401.5 to 434.2 nm depending on the grating period of the Bragg reflector. The blue edge of this interval at 401.5 nm makes this laser an extremely short wavelength organic DFB laser. When pumping with a pulsed nitrogen laser at 337 nm, we observe a laser threshold energy density of 83 μJ/cm2. These results render this spiro compound an excellent candidate for blue-emitting diode lasers.

Ultrawide tuning range in doped organic solid-state lasers

We report on the tunability of 4-(Dicyanomethylene)-2-methyl-6-(julolidin-4-yl-vinyl)-4H-pyran (DCM2)-doped guest-host organic lasers. As host materials Tris-(8-hydroxy-quinoline)aluminum (Alq3), 4,4′‐N,N′-dicarbazole-bipheny1 (CBP), and N′N-di(1-naphthyl)-N,N′-diphenyl-1,1′-diphenyl-4,4′-diamine (NPD) are used. The largest tuning range was observed in the Alq3:DCM2 film with 115.3nm between 597.8 and 713.1nm. In CBP:DCM2, a tuning range of 85nm was measured, whereas in NPD:DCM2 only one laser wavelength at 624.4nm could be observed. When comparing the pump energies, we observed considerably lower threshold energy densities in Alq3:DCM2 and CBP:DCM2 compared to NPD:DCM2.

Vertical channel all-organic thin-film transistors

Technologically simple and cost-effective processes are essential for the fabrication of organic electronic devices. In this letter, we present a concept for making vertical channel all-organic thin-film transistors on glass substrate. This concept avoids the need for patterning processes with high lateral resolution by defining the channel length through the thickness of an insulating layer. Our devices are based on commercially available poly(ethylene dioxythiophene)/poly(styrene sulfonate) dispersion for source, drain, and gate electrodes, photoresist as the insulating layer and photosensitized poly(vinyl alcohol) as the gate insulator. Pentacene was used as the organic semiconductor. Functional devices with channel length of 2.4 μm and width of 1 mm have been realized, and we report electrical characteristics of these devices.

Laser threshold reduction in an all-spiro guest–host system

We report on stimulated emission in an all-spiro guest–host (G–H) system. Different doping concentrations of the guest molecule 2,2′,7,7′-tetrakis(2,2-diphenylvinyl)spiro-9,9′-bifluorene in the host material 2,7-bis(biphenyl-4-yl)-2′, 7′-di-tert-butyl-9,9′-spirobifluorene were investigated for amplified spontaneous emission (ASE) and distributed feedback (DFB) lasing. The ASE maximum could be shifted over 20nm by variation of the doping concentration. DFB lasing is observed in the pure host, the pure guest material, and in the G–H system. The laser wavelength can be tuned from 401.5 to 529.3nm by changing the grating period of the Bragg reflector and the doping concentration. A minimum threshold energy density of 6μJ∕cm2 was observed in second-order DFB structures for a doping concentration of 1.1%. In first-order DFB operation the threshold value could be further lowered to 320nJ∕cm2. These results render this material system an excellent candidate for stable and widely tunable lasers in the visible spec...

All-organic thin-film transistors patterned by means of selective electropolymerization

We have fabricated fully patterned all-organic thin-film transistors on polyimide substrates using selectively electropolymerized poly (3,4-ethylenedioxythiophene) doped with poly (styrene sulfonate) (PEDOT:PSS) for the source and drain contacts, PEDOT:PSS Baytron P dispersion for the gate electrodes, poly (4-vinyl phenol) or polyvinyl alcohol for the gate dielectric layers, and pentacene or poly (3-butylthiophene) for the organic active layers. We have built top-gate structures with gates printed on top of the gate dielectric layer. Carrier mobilities as large as 0.01 cm2/V s were measured. Functional all-organic transistors have been realized using a simple and potentially inexpensive technology that does not depend on photolithographical processes and that allows the preparation of feature sizes on the micrometer scale.

Low-voltage organic electroluminescence device with an ultrathin, hybrid structure

We have prepared organic light-emitting diodes with a narrow recombination zone confined by an organic double-heterojunction structure using both polymer and small molecules (a hybrid structure). In these light-emitting diodes, we used very thin small molecule layers, down to a total thickness of 40 nm, to achieve an exponential forward characteristic. These layers were evaporated on a highly conductive layer of PEDT:PSS for a high-yield process and for good charge injection at the anode. Although no doping processes were applied during device fabrication, either at the injecting electrodes or in the Alq3 layer, the diodes attained high brightness at very low voltage, for instance, 10.000 cd/m2 at voltage of 4.7 V.