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Chang H.-W.,Institute For Angewandte Photophysik | Chang H.-W.,National Taiwan University | Kim Y.H.,Institute For Angewandte Photophysik | Kim Y.H.,Pukyong National University | And 9 more authors.
Organic Electronics: physics, materials, applications | Year: 2014

In this work, we demonstrate color-stable, ITO-free white organic light-emitting diodes (WOLEDs) with enhanced efficiencies by combining the high-conductivity conducting polymer PEDOT:PSS as transparent electrode and a nanoparticle-based scattering layer (NPSL) as the effective optical out-coupling layer. In addition to efficiency enhancement, the NPSL is also beneficial to the stabilization of electroluminescent spectra/colors over viewing angles. Both the PEDOT:PSS and the NPSL can be fabricated by simple, low-temperature solution processing. The integration of both solution-processable transparent electrodes and light extraction structures into OLEDs is particularly attractive for applications since they simultaneously provide manufacturing, cost and efficiency advantages. © 2014 Elsevier B.V. All rights reserved.

Gohri V.,Institute For Angewandte Photophysik | Hofmann S.,Institute For Angewandte Photophysik | Reineke S.,Institute For Angewandte Photophysik | Rosenow T.,Institute For Angewandte Photophysik | And 6 more authors.
Organic Electronics: physics, materials, applications | Year: 2011

We investigate white top-emitting organic light-emitting diodes (OLEDs) based on a heterostructure of down-conversion (DC) layers. The white DCOLED comprises consecutive organic conversion layers of 4-dicyanomethylene-2-methyl- 6-p-dimethylaminostyryl-4H-pyran (DCM) doped in a matrix of tris(8-hydroxy- quinolinato)aluminum (Alq3), and N4,N41′-bis-(4-tert-butyl- phenyl)-N4,N4′-di-fluoranthen-3-yl-diphenylether-4,4′-diamine (OYSE). The DC layers also function as capping layers to enhance the light outcoupling and optical modification of the underlying blue OLED. White light emission with CIE color coordinates of (0.27, 0.26) and a color rendering index of 60 is achieved. Furthermore, the spectral angular dependence of the white device is examined. © 2011 Elsevier B.V. All rights reserved.

Poelking C.,Max Planck Institute for Polymer Research | Tietze M.,Institute For Angewandte Photophysik | Elschner C.,Institute For Angewandte Photophysik | Olthof S.,University of Cologne | And 6 more authors.
Nature Materials | Year: 2015

Structural order in organic solar cells is paramount: it reduces energetic disorder, boosts charge and exciton mobilities, and assists exciton splitting. Owing to spatial localization of electronic states, microscopic descriptions of photovoltaic processes tend to overlook the influence of structural features at the mesoscale. Long-range electrostatic interactions nevertheless probe this ordering, making local properties depend on the mesoscopic order. Using a technique developed to address spatially aperiodic excitations in thin films and in bulk, we show how inclusion of mesoscale order resolves the controversy between experimental and theoretical results for the energy-level profile and alignment in a variety of photovoltaic systems, with direct experimental validation. Optimal use of long-range ordering also rationalizes the acceptor-donor-acceptor paradigm for molecular design of donor dyes. We predict open-circuit voltages of planar heterojunction solar cells in excellent agreement with experimental data, based only on crystal structures and interfacial orientation. © 2015 Macmillan Publishers Limited. All rights reserved.

Park Y.,Institute For Angewandte Photophysik | Muller-Meskamp L.,Institute For Angewandte Photophysik | Vandewal K.,Institute For Angewandte Photophysik | Leo K.,Institute For Angewandte Photophysik | Leo K.,Canadian Institute for Advanced Research CIFAR
Applied Physics Letters | Year: 2016

The performance of organic optoelectronic devices can be improved by employing a suitable optical cavity design beyond the standard plane layer approach, e.g., by the inclusion of periodically or randomly textured structures which increase light incoupling or extraction. One of the simplest approaches is to add an additional layer containing light scattering particles into the device stack. Solution processed poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin films are promising for replacing the brittle and expensive indium tin oxide transparent electrode. We use a blend of 100 nm TiO2 scattering particles in PEDOT:PSS solution to fabricate transparent electrode films which also functions as a scattering layer. When utilized in an organic photovoltaic device, a power conversion efficiency of 7.92% is achieved, which is an 8.6% relative improvement compared to a device with a neat PEDOT:PSS electrode without the nanoparticles. This improvement is caused by an increase in short-circuit current due to an improved photon harvesting in the 320 nm-700 nm spectral wavelength range. © 2016 Author(s).

Gupta S.,TU Dresden | Agrawal M.,Leibniz Institute fur Polymerforschung Dresden E.v. Hohe | Conrad M.,TU Dresden | Hutter N.A.,TU Munich | And 6 more authors.
Advanced Functional Materials | Year: 2010

A simple, fast, and versatile approach to the fabrication of outstanding surface enhanced Raman spectroscopy (SERS) substrates by exploiting the optical properties of the Ag nanoparticles and functional as well as organizational characteristics of the polymer brushes is reported. First, poly(2- (dimethylamino)ethyl methacrylate) brushes are synthesized directly on glassy carbon by self-initiated photografting and photopolymerization and thoroughly characterized in terms of their thickness, wettability, morphology, and chemical structure by means of ellipsometry, contact angle, AFM, and XPS, respectively. Second, Ag nanoparticles are homogeneously immobilized into the brush layer, resulting in a sensor platform for the detection of organic molecules by SERS. The surface enhancement factor (SEF) as determined by the detection of Rhodamine 6G is calculated as 6 × 10 6. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Park Y.,Institute For Angewandte Photophysik | Nehm F.,Institute For Angewandte Photophysik | Muller-Meskamp L.,Institute For Angewandte Photophysik | Vandewal K.,Institute For Angewandte Photophysik | And 2 more authors.
Optics Express | Year: 2016

We demonstrate flexible small molecular solar cells on periodically patterned plastic substrate (LCD display film) using a highly transparent poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) electrode with flexible thin atomic layer deposited (ALD) AlOx top and bottom encapsulation. The organic photovoltaic device (OPV) on this display film shows a power conversion efficiency of 7.48%, which is a 13.0% improvement as compared to a device fabricated on a planar poly-ethylen-terephtalate (PET) substrate (6.62%) and even higher than the efficiency of a device using planar glass substrate (7.15%). The improvement is mainly due to an enhanced harvesting of photons with wavelengths shorter than 500 nm. Moreover, the fully encapsulated device is sufficiently flexible to withstand a bending with a 10 mm radius for more than 50 cycles at ambient condition. These results indicate that the use of standard optical display films is a cheap, simple and efficient way to increase the photocurrent and overall efficiency of organic photovoltaic devices. © 2016 Optical Society of America.

Wunsche J.,Institute For Angewandte Photophysik | Wunsche J.,Ecole Polytechnique de Montréal | Reineke S.,Institute For Angewandte Photophysik | Lussem B.,Institute For Angewandte Photophysik | Leo K.,Institute For Angewandte Photophysik
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

A detailed investigation of the diffusion of triplet excitons in a layer of N, N′ -di-1-naphthalenyl- N, N′ -diphenyl- [1, 1′: 4′, 1″: 4″, 1‴ -quaterphenyl]- 4, 4‴ -diamine (4P-NPD) incorporated in organic light-emitting diodes is presented. An appropriate method to measure the triplet diffusion length in fluorescent host materials is the spatial separation of the site of exciton generation from the site of radiative triplet decay by inserting a host spacer layer of varying thickness. However, cavity effects, the quenching and blocking of excitons at the boundaries of the spacer layer, and direct charge-carrier recombination in the sensing layer need to be taken into account. We use a specially designed layer stack, which excludes the influence of cavity effects on the measurements and a strongly quenching sensing layer, which ensures well-defined boundary conditions. The quenching of excitons by the sensing layer, the generation zone, and direct charge-carrier recombination are investigated experimentally and their influence on the extracted diffusion length are discussed. The significance of triplet-triplet annihilation in this analysis is estimated by a current-dependent evaluation. An analytic model for the dependence of the sensing layer emission on the spacer thickness is presented, which includes the important effects. By this means, we find a triplet diffusion length of 11±3 nm in 4P-NPD. © 2010 The American Physical Society.

Kim Y.H.,Institute For Angewandte Photophysik | Kim Y.H.,Pukyong National University | Muller-Meskamp L.,Institute For Angewandte Photophysik | Leo K.,Institute For Angewandte Photophysik
Advanced Energy Materials | Year: 2015

A hybrid ultratransparent poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/metal grid thin-film is demonstrated as a transparent electrode for organic solar cells. The transmittance of the PEDOT:PSS thin-films on glass reaches values as high as 91.5%, a nearly 100% transmittance ratio. The device with the hybrid electrode shows an efficiency of 2.8%, which is comparable to that of an indium tin oxide based reference device. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Kasemann D.,Institute For Angewandte Photophysik | Bruckner R.,Institute For Angewandte Photophysik | Frob H.,Institute For Angewandte Photophysik | Leo K.,Institute For Angewandte Photophysik
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We investigate organic light-emitting diodes (OLEDs) comprising the singlet emitter system 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) doped into aluminium tris(8-hydro-xyquinoline) (Alq3) at high excitation densities. With the OLED active area reduced to 100×100μm2, current densities up to 800 A/cm2 are achieved in pulsed operation. These devices exhibit an intense electroluminescence (EL) turn-on peak on the nanosecond time scale. With the help of streak camera measurements, we prove that the steady state EL of the fluorescent OLEDs is reduced due to singlet-triplet quenching. We demonstrate that short electrical pulses with a rise time of 10 ns make the separation of singlet emission and singlet-triplet quenching in time domain possible. By modeling the singlet and triplet population dynamics in the emission layer, we find that the triplet-triplet annihilation-rate coefficient in doped fluorescent materials is triplet-density dependent at high excitation density. The increased triplet lifetime usually observed in host:guest systems due to triplet trapping on guest molecules vanishes at high current densities. An increase in current density leads to an increased triplet-triplet annihilation rate, while the triplet density in the emission layer stays constant. © 2011 American Physical Society.

PubMed | Institute For Angewandte Photophysik
Type: Journal Article | Journal: Nano letters | Year: 2014

Organic Zener diodes with a precisely adjustable reverse breakdown from -3 to -15 V without any influence on the forward current-voltage curve are realized. This is accomplished by controlling the width of the charge depletion zone in a pin-diode with an accuracy of one nanometer independently of the doping concentration and the thickness of the intrinsic layer. The breakdown effect with its exponential current voltage behavior and a weak temperature dependence is explained by a tunneling mechanism across the highest occupied molecular orbital-lowest unoccupied molecular orbital gap of neighboring molecules. The experimental data are confirmed by a minimal Hamiltonian model approach, including coherent tunneling and incoherent hopping processes as possible charge transport pathways through the effective device region.

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