Fraunhofer Institute For Photonische Mikrosysteme

Dresden, Germany

Fraunhofer Institute For Photonische Mikrosysteme

Dresden, Germany
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Sinreich R.,University of Colorado at Boulder | Merten A.,University of Heidelberg | Merten A.,Fraunhofer Institute For Photonische Mikrosysteme | Molina L.,United Environment & Energy, Llc | And 2 more authors.
Atmospheric Measurement Techniques | Year: 2013

We present a novel parameterization method to convert multi-axis differential optical absorption spectroscopy (MAX-DOAS) differential slant column densities (dSCDs) into near-surface box-averaged volume mixing ratios. The approach is applicable inside the planetary boundary layer under conditions with significant aerosol load, and builds on the increased sensitivity of MAX-DOAS near the instrument altitude. It parameterizes radiative transfer model calculations and significantly reduces the computational effort, while retrieving ∼ 1 degree of freedom. The biggest benefit of this method is that the retrieval of an aerosol profile, which usually is necessary for deriving a trace gas concentration from MAX-DOAS dSCDs, is not needed. The method is applied to NO2 MAX-DOAS dSCDs recorded during the Mexico City Metropolitan Area 2006 (MCMA-2006) measurement campaign. The retrieved volume mixing ratios of two elevation angles (1° and 3°) are compared to volume mixing ratios measured by two longpath (LP)-DOAS instruments located at the same site. Measurements are found to agree well during times when vertical mixing is expected to be strong. However, inhomogeneities in the air mass above Mexico City can be detected by exploiting the different horizontal and vertical dimensions probed by the MAX-DOAS and LP-DOAS instruments. In particular, a vertical gradient in NO2 close to the ground can be observed in the afternoon, and is attributed to reduced mixing coupled with near-surface emission inside street canyons. The existence of a vertical gradient in the lower 250 m during parts of the day shows the general challenge of sampling the boundary layer in a representative way, and emphasizes the need of vertically resolved measurements. © Author(s) 2013.

De Moraes I.R.,TU Dresden | Scholz S.,Fraunhofer Institute For Photonische Mikrosysteme | Hermenau M.,TU Dresden | Tietze M.L.,TU Dresden | And 5 more authors.
Organic Electronics: physics, materials, applications | Year: 2015

Organic light emitting devices (OLEDs) are known to heat up when driven at high brightness levels required for lighting and bright display applications. This so called Joule heating can in the extreme case lead to a catastrophic failure (breakdown) of the device. In this work, we compare the effect of Joule heated and externally heated OLEDs by their electrical and optical response. A reduction in resistance is observed at elevated temperatures, both, for Joule heating, and for externally heated samples driven at low current density. In both cases, we attribute the change in resistance to a higher mobility of charge carriers at the elevated temperatures. Additionally, we observe a quenching of the emission efficiency in heated single layers as well as in OLEDs, treated with an external heat source as well as on Joule heated samples. © 2015 Published by Elsevier B.V.

Scholz S.,TU Dresden | Scholz S.,Fraunhofer Institute For Photonische Mikrosysteme | Kondakov D.,DuPont Company | Lussem B.,TU Dresden | And 2 more authors.
Chemical Reviews | Year: 2015

Degradation mechanisms and reactions in organic light emitting (OLED) devices were reviewed. Although many different degradation mechanisms have similar effects and appearances, they can be classified on the basis of external and internal causes of degradation. The degradation of OLED usually implies some undesirable internal processes such as chemical reactions, morphological (phase changes, crystallization, and delamination processes), and other physical changes. These processes result in various changes in device properties, most notably in the color-luminance-current-voltage characteristic of OLEDs. It should be stressed that the apparent separation of the decay curve into the long-term, short-term, and initial rise components might also be purely superficial and unrelated to the plurality of degradation mechanisms. The usage of highly purified materials and substrates, high vacuum conditions in the evaporation tool, and an appropriate device encapsulation generally results in better device stability.

Heera V.,Helmholtz Center Dresden | Fiedler J.,Helmholtz Center Dresden | Naumann M.,Helmholtz Center Dresden | Naumann M.,TU Dresden | And 10 more authors.
Superconductor Science and Technology | Year: 2014

Ge films with a mean Ga content of about 8 and 1 at.% hole concentration can be fabricated by ion implantation and subsequent flash-lamp annealing. The Ge:Ga films become superconducting below critical temperatures in the range between 1 and 2 K depending on the film resistance. The change in the macroscopic transport properties during step-wise surface etching can be described by a homogeneously doped layer model. However, the Ga distribution is extremely heterogeneous on the nanoscale. Atom-probe tomography analyses reveal the presence of Ga-rich precipitates with Ga clusters of up to 10 000 atoms. Since no percolating Ga clusters exist, it can be supposed that the heavy doping of Ge enables a coherent superconducting state via the proximity effect. © 2014 IOP Publishing Ltd.

Seifert R.,TU Dresden | Seifert R.,Von Ardenne Anlagentechnik GmbH | Rabelo De Moraes I.,TU Dresden | Scholz S.,Fraunhofer Institute For Photonische Mikrosysteme | And 4 more authors.
Organic Electronics: physics, materials, applications | Year: 2013

The stability and the degradation processes of two highly efficient blue-emitting phosphorescent materials, iridium(III) bis(4′,6′- difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6) and bis(2-(4,6-difluorophenyl) pyridyl-N,C2′)iridium(III)picolinate (FIrpic), which are commonly used as emitters in organic light emitting diodes (OLEDs), are investigated. Using single layers devices, the optical response and the half-lifetime behavior of the materials are investigated. Layers of FIr6 exposed to UV-light show the formation of a red emitting degradation product. We analyze the chemical reactions of the materials using laser desorption/ ionization time-of-flight mass spectrometry. Several products related to the chemical dissociation of the FIr6 molecule as well as charge complex formation between the emitter and the emitter dissociation products are detected. FIr6 and FIrpic are also compared by lifetime studies on commonly used OLED structures. We show that single layers and OLEDs based on FIrpic exhibit higher stability than those based on FIr6. An explanation for this behavior can be found by considering the chemical structure of the molecules. © 2012 Elsevier B.V. All rights reserved.

Hyun Kim Y.,TU Dresden | Sachse C.,TU Dresden | Hermenau M.,TU Dresden | Fehse K.,Fraunhofer Institute For Photonische Mikrosysteme | And 4 more authors.
Applied Physics Letters | Year: 2011

We report on efficient and stable ITO-free small molecule organic solar cells with conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) electrodes using a post-treatment process, causing selective removal of PSS. The solar cells with post-treated PEDOT:PSS electrodes show significantly improved short circuit current densities and efficiencies compared to untreated devices. Moreover, the removal of PSS by the post-treatment significantly improves the lifetime of devices, which are more resistant to loss of fill factor compared to untreated devices. © 2011 American Institute of Physics.

Gil T.H.,Fraunhofer Institute For Photonische Mikrosysteme | May C.,Fraunhofer Institute For Photonische Mikrosysteme | Scholz S.,TU Dresden | Franke S.,Fraunhofer Institute For Photonische Mikrosysteme | And 4 more authors.
Organic Electronics: physics, materials, applications | Year: 2010

In this study, we examine organic light emitting diodes (OLEDs) having Al top electrodes deposited on organic layers by direct-current magnetron sputtering. The OLEDs consisted of electronically doped transport layers and phosphorescent emission layer were characterized by typical current-voltage-luminance measurement. They showed higher leakage currents, decreased forward currents, and corresponding increases of driving voltage after the sputter deposition on the organic layers. The OLEDs exhibited randomly distributed bright spots on the active area, and the bright spots were investigated by scanning electron microscopy/energy-dispersive X-ray spectroscopy. In order to prove the origins of sputter damage, simple organic/Al layer samples were made and investigated by ellipsometry and laser-induced desorption/ionization time-of-flight mass spectrometry. The results are compared with previous works addressing the fundamental phenomena of magnetron sputtering. We conclude that the high leakage current originated from a penetration of sputtered metal atoms into the underlying organic layers, and the decrease of forward current resulted from an interface degradation caused by the radiation of plasma, which reduces charge carrier injection preferentially at the Al/organic layer interface. © 2009 Elsevier B.V. All rights reserved.

de Moraes I.R.,TU Dresden | Scholz S.,Fraunhofer Institute For Photonische Mikrosysteme | Leo K.,TU Dresden
Organic Electronics: physics, materials, applications | Year: 2016

The influence of the current density on the chemical degradation processes of a phosphorescent OLED based on Ir(ppy)3 emitter is investigated by laser-desorption/ionization time-of-flight mass spectrometry. Comparing the mass spectra collected for unaged and aged OLEDs, the formation of different chemical degradation products could be detected and are identified as dimer and trimer products of BPhen as well as Cs-adducts of these polymers and the well-known emitter-BPhen-adduct ([Ir(ppy)2BPhen]+). In this work, we will show that the formation of [Ir(ppy)2BPhen]+ depends strongly on the amount of the charge flowing through the device, where the other degradation products show a much different behaviour. © 2016 Elsevier B.V.

Kim Y.H.,TU Dresden | Sachse C.,TU Dresden | MacHala M.L.,TU Dresden | May C.,Fraunhofer Institute For Photonische Mikrosysteme | And 3 more authors.
Advanced Functional Materials | Year: 2011

Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films as stand-alone electrodes for organic solar cells have been optimized using a solvent post-treatment method. The treated PEDOT:PSS films show enhanced conductivities up to 1418 S cm-1, accompanied by structural and chemical changes. The effect of the solvent treatment on PEDOT:PSS has been investigated in detail and is shown to cause a reduction of insulating PSS in the conductive polymer layer. Using these optimized electrodes, ITO-free, small molecule organic solar cells with a zinc phthalocyanine (ZnPc):fullerene C60 bulk heterojunction have been produced on glass and PET substrates. The system was further improved by pre-heating the PEDOT:PSS electrodes, which enhanced the power conversion efficiency to the values obtained for solar cells on ITO electrodes. The results show that optimized PEDOT:PSS with solvent and thermal post-treatment can be a very promising electrode material for highly efficient flexible ITO-free organic solar cells. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Machala M.L.,Dresdner Innovationszentrum Energieeffizienz | Machala M.L.,TU Dresden | Mueller-Meskamp L.,Dresdner Innovationszentrum Energieeffizienz | Mueller-Meskamp L.,TU Dresden | And 6 more authors.
Organic Electronics: physics, materials, applications | Year: 2011

Thin conductive, hydrophobic films of poly(3,4-ethylenedioxythiophene) or PEDOT were synthesized on-substrate in the presence of the organic electron acceptor and dehydrogenating agent 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) using a versatile processing procedure. Significant polymerization in the processing solution was delayed by using common aprotic, ethereal solvents with low dielectric constants to prevent solvating the EDOT:DDQ charge transfer complex into radicals. Polymerization was initiated by an increase in concentration upon solvent evaporation during spin coating. A hydrophobic polymer matrix additive of polyvinyl acetate was used to aid in film formation, and a post-treatment rinse with acetonitrile was necessary to obtain a conductive film. Conductivities ranged from 17 to 59 S/cm, where the higher values were achieved at the cost of transparency. A work function of 4.62 eV was determined by UV photoelectron spectroscopy for one film recipe. When comparing conductive AFM results of PEDOT:DDQ to highly conductive, ethylene glycol-treated PEDOT: poly(styrenesulfonate) or PSS, surface currents were orders of magnitude higher for PEDOT:DDQ than for PEDOT:PSS. If optimized further, less acidic and hydrophobic PEDOT:DDQ films have the potential to replace PEDOT:PSS for use as a transparent electrode or charge transport layer in organic solar cells, organic light emitting diodes, touch screens, and other optoelectronic devices. © 2011 Elsevier B.V. All rights reserved.

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