Karl Leo

Three furan fused boron dipyrromethenes (BODIPYs) with a CF3 group on the meso-carbon are synthesized as near-infrared absorbing materials for vacuum processable organic solar cells. The best single junction device reaches a short-circuit current (jsc) of 13.3 mA cm–2 and a power conversion efficiency (PCE) of 6.1%. These values are highly promising for an electron donor material with an absorption onset beyond 900 nm. In a tandem solar cell comprising a NIR BODIPY subcell and a matching “green” absorber subcell, complementary absorption is achieved, resulting in PCE of ∼10%.

H-aggregated small molecular nanowires as near infrared absorbers for organic solar cells

Co-reporter:Felix Holzmüller, Nico Gräßler, Mona Sedighi, Eric Müller, Martin Knupfer, Olaf Zeika, Koen Vandewal, Christian Koerner, Karl Leo

Organic Electronics 2017 Volume 45(Volume 45) pp:

Publication Date(Web):1 June 2017

DOI:10.1016/j.orgel.2017.03.009

•Push-pull chromophore as near infrared absorber for organic solar cells.•Thin film absorption of up to 1100 nm by H-aggregation of the molecules.•Formation of micrometer long small molecular nanowires upon vacuum deposition.•External quantum efficiency of 18% at 1000 nm in organic solar cells with C60.Near infrared absorber materials for organic solar cells are needed to cover larger parts of the solar spectrum and, therefore, achieve higher short circuit current densities. We present a push-pull chromophore with a quinoid merocyanine structure (QM1) as donor material in vacuum deposited organic solar cells with a thin film absorption up to 1100 nm. A blue shift of the main absorption peaks of the thin film as compared to the solution indicates H-aggregation of the material. Morphology investigations using scanning electron microscopy and electron diffraction reveal a crystalline growth in nanowires, with lengths of hundreds of nanometers to a few micrometers, and diameters of a few tens of nanometers. Organic solar cells incorporating a blend of QM1 with C60 reach power conversion efficiencies up to 1.9% under 1 sun illumination with an external quantum efficiency of over 18% from 600 nm to 1000 nm.Download high-res image (299KB)Download full-size image

Microcavity-Enhanced Semitransparent Electrodes for Oligothiophene Small-Molecule Organic Solar Cells

Co-reporter:Dhriti Sundar Ghosh

Advanced Electronic Materials 2017 Volume 3(Issue 5) pp:

Publication Date(Web):2017/05/01

DOI:10.1002/aelm.201600518

Transparent electrodes (TEs) with high optical transparency (>85%) are usually considered to be prerequisite for realizing high-efficiency solar cells. On the contrary, here an indium-free TE is reported which is semitransparent with an average visible transparency of 66.7% only. The electrode consists of Ag 9 nm with TiO2 (30 nm) undercoat layer designed specifically for small molecule organic solar cell (SM-OSC) based on methylated oligothiophene derivatives. The electrode also contains a polyethyleneimine (PEI) wetting layer which promotes the growth of continuous, ultrasmooth, and highly conductive Ag films on the TiO2 layer even at very low thicknesses. The developed TiO2/PEI/Ag (TPA) electrode exhibits much better mechanical properties with only <8% increase in sheet resistance after 200 bending cycles when a tensile strain of 2.1% is applied. The role of TiO2 undercoat layer has been investigated in detail and is shown to enhance the microcavity effect, leading to increased light coupling especially in the lower wavelength regime. With comparable photocurrent, and high fill-factor values owing to much better electrical properties, the semitransparent TPA-TE based SM-OSC outperforms the state-of-art indium tin oxide (ITO) based reference device with photon conversion efficiency of 8.1% compared to 7.7% despite having lower transmittance (≈21%) relative to ITO.

Influence of Meso and Nanoscale Structure on the Properties of Highly Efficient Small Molecule Solar Cells

Co-reporter:Tobias Moench;Pascal Friederich;Felix Holzmueller;Bogdan Rutkowski;Johannes Benduhn;Timo Strunk;Christian Koerner;Koen Vewal;Aleksra Czyrska-Filemonowicz;Wolfgang Wenzel

Advanced Energy Materials 2016 Volume 6( Issue 4) pp:

Publication Date(Web):

DOI:10.1002/aenm.201501280

The nanoscale morphology of the bulk heterojunction absorber layer in an organic solar cell (OSC) is of key importance for its efficiency. The morphology of high performance vacuum-processed, small molecule OSCs based on oligothiophene derivatives (DCV5T-Me) blended with C₆₀ on various length scales is studied. The analytical electron microscopic techniques such as scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, highly sensitive external quantum efficiency measurements, and meso and nanoscale simulations are employed. Unique insights into the relation between processing, morphology, and efficiency of the final devices are obtained. It is shown that the connectivity of the oligothiophene-C₆₀ network is independent of the material domain size. The decisive quantity controlling the internal quantum efficiency is the energetic disorder induced by material mixing, strongly limiting charge and exciton transport in the OSCs.

Influence of the applied charge on the electro-chemical degradation in green phosphorescent organic light emitting diodes

Co-reporter:Inês Rabelo de Moraes, Sebastian Scholz, Karl Leo

Organic Electronics 2016 Volume 38() pp:164-171

Publication Date(Web):November 2016

DOI:10.1016/j.orgel.2016.07.026

•Influence of the applied charge on the chemical degradation processes of green OLEDs.•Electro-chemical degradation is investigated by LDI-TOF-MS technique.•BPhen polymeric species and the Ir(ppy)3-based reaction products are detected.•[Ir(ppy)2BPhen]+ formation depends strongly on the amount of the applied charge.•BPhen products formation show a much different behaviour.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.

Doped hole transport layers processed from solution: Planarization and bridging the voids in noncontinuous silver nanowire electrodes

Co-reporter:Ludwig Bormann, Franz Selzer, Nelli Weiß, David Kneppe, Karl Leo, Lars Müller-Meskamp

Organic Electronics 2016 Volume 28() pp:163-171

Publication Date(Web):January 2016

DOI:10.1016/j.orgel.2015.10.007

•A solution-processed small molecule hole transport layer (s-HTL) is introduced.•It exhibits similar conductivities to the standard vacuum deposition of this HTL.•This HTL sufficiently planarizes highly conductive transparent AgNW electrodes.•Organic solar cells on this AgNW/s-HTL electrode show efficiencies up to 4.4%.•On ITO, an efficiency of 4.1% was reached with the same s-HTL and organic device.We report about solution-processing of a doped small molecule hole transport layer (s-HTL) comprising of N,N′-((diphenyl-N,N′-bis)9,9,-dimethyl-fluoren-2-yl)-benzidine (BF-DPB) as matrix and the p-dopant “NDP9” in the non-halogenated solvent tetrahydrofuran (THF). We show that the doping process is already happening in solution and stays effective after coating. Conductivities achieved with this process are comparable to those reached by thermal co-evaporation under high vacuum, which is the usual deposition method for this material. Applied as planarization layer onto AgNW films with best performance values of 15Ω/□ and 83.5% total transmission including the substrate, the s-HTL is proving its ability to sufficiently smoothen the initially rough AgNW topography. We analyze the necessary lateral conductivity to bridge micrometer-large voids in the mesh, as they are inherent in nanowire network electrodes. In combination with zinc phtalocyanine:C60 organic solar cells, a s-HTL conductivity less than 1 × 10−4 S/cm can lead to decreased device performance with a loss greater than 10% for nanowires with 90 nm diameter and the associated mesh width. Furthermore, we demonstrate more efficient vacuum-deposited p-i-n solar cells with an oligothiophene (DCV5T-Me) as donor, C60 as acceptor. They exhibit power conversion efficiencies up to 4.4% on AgNW bottom electrodes with s-HTL, compared to 4.1% on ITO with s-HTL as reference device.

Efficient flexible organic photovoltaics using silver nanowires and polymer based transparent electrodes

Co-reporter:Yoonseok Park, Ludwig Bormann, Lars Müller-Meskamp, Koen Vandewal, Karl Leo

Organic Electronics 2016 Volume 36() pp:68-72

Publication Date(Web):September 2016

DOI:10.1016/j.orgel.2016.05.032

•We demonstrate efficient flexible organic photovoltaics using AgNW and PEDOT: PSS combined electrode (7.15%) which is the best performance of small molecule OPV using AgNW electrodes so far.•Water based thin PEDOT: PSS layer can help network formation between AgNW by leading to humidity assisted AgNW fusing, resulting in a maximum processing temperature of only 120 °C (which is stable processing temperature for flexible PET substrate).•OPV devices are demonstrated on PET substrates (6.91% PCE) with a flexible AgNW electrode, showing a largely retained performance when bent at radii down to 3 mm.Planarization and filling voids between wires are key issues when using nanowire electrodes in flexible solar cells such as organic photovoltaics (OPV). For this purpose, we use poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) which leads to an electrically well connected silver nanowire (AgNW) network. Furthermore, the use of water based PEDOT: PSS leads to humidity assisted AgNW fusing, resulting in a maximum processing temperature of only 120 °C. OPV cells using this AgNW/PEDOT: PSS transparent electrodes exhibit power conversion efficiencies up to 7.15%. Moreover, OPV devices on PET substrates with an alumina encapsulation and barrier adhesive show excellent mechanical flexibility.

New Electron Transport Materials for High Performance Organic Solar Cells: Synthesis and Properties of Symmetrical and Asymmetrical 1,4,5,8-Naphthalenetetracarboxylic Dianhydride Derivatives

Co-reporter:Nadzeya A. Kukhta;Olaf Zeika;Johannes Widmer;Christian Koerner;Rico Meerheim;Annette Petrich;Norwid-Rasmus Behrnd;Juozas Vidas Grazulevicius

Advanced Electronic Materials 2016 Volume 2( Issue 5) pp:

Publication Date(Web):

DOI:10.1002/aelm.201600047

Two series – symmetrical and asymmetrical – of electron-transporting 1,4,5,8-naphthalenetetracarboxylic dianhydride derivatives are designed and synthesized. The compounds show high thermal stability with initial destruction temperatures reaching 481 °C. The materials absorb light in UV–visible region, however, avoiding parasitic absorption that makes them suitable for the usage in high performance solar cells. Cyclic voltammetry deduces relatively high electron affinity (3.84–4.10 eV) indicating strong electron accepting nature of the molecules. The synthesized compounds have remarkable electrical properties such as high conductivity (up to 4 × 10⁻⁵ S cm⁻¹) and electron mobility (up to 10⁻³ cm² V⁻¹ s⁻¹). The highest solar cell efficiency is obtained for asymmetrical adduct of 1,4,5,8-naphthalenetetracarboxylic dianhydride, pyridine-3-amine, and pyridin-3,4-diamine, and exceeds the efficiency of the reference material C₆₀. A comparative study of the experimentally estimated and theoretically calculated by means of density functional theory method characteristics of the electron-transporting materials is presented. The control of the symmetry of the molecular structures is responsible for the high performance of compounds in the target devices.

Degradation Mechanisms and Reactions in Organic Light-Emitting Devices

Co-reporter:Sebastian Scholz, Denis Kondakov, Björn Lüssem, and Karl Leo

Chemical Reviews 2015 Volume 115(Issue 16) pp:8449

Publication Date(Web):July 31, 2015

DOI:10.1021/cr400704v

Impact of temperature on the efficiency of organic light emitting diodes

Co-reporter:Inês Rabelo de Moraes, Sebastian Scholz, Martin Hermenau, Max L. Tietze, Tobias Schwab, Simone Hofmann, Malte C. Gather, Karl Leo

Organic Electronics 2015 Volume 26() pp:158-163

Publication Date(Web):November 2015

DOI:10.1016/j.orgel.2015.07.032

•A study of the influence of the temperature on the electrical performance of red phosphorescent OLEDs.•Externally heated and Joule heated OLEDs are compared and investigated.•Decrease of efficiency and resistivity with increase of the device temperature observed.•Slight red shift of the CIE coordinates with increase of the device temperature observed.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.

Doped-carbazolocarbazoles as hole transporting materials in small molecule solar cells with different architectures

Co-reporter:David Curiel, Miriam Más Montoya, Markus Hummert, Moritz Riede, Karl Leo

Organic Electronics 2015 Volume 17() pp:28-32

Publication Date(Web):February 2015

DOI:10.1016/j.orgel.2014.11.013

•Doped carbazolocarbazole as hole transporting layer in organic solar cells.•Substituent effects on the performance of carbazolocarbazoles as HTL.•S-kink dependence on organic solar cell architecture.The adaptation of interfacial layers to the stacked architecture of organic solar cells represents a very useful strategy for improved device operation. In this context, heteroacenic structures such as carbazolocarbazoles have been doped and evaluated as hole transporting materials in small molecule solar cell with either inverted or conventional architecture. S-kinks in the IV-curve detected for the inverted solar cells could be remarkably corrected by reversing the deposition sequence, highlighting the importance of buffer layer adjustment. Some of the studied carbazolocarbazoles proved to be a suitable molecule to be used as hole transporting materials.

Co-evaporant induced crystallization of zinc phthalocyanine:C60 blends for solar cells

Co-reporter:Felix Holzmueller, Lutz Wilde, Florian Wölzl, Christian Koerner, Koen Vandewal, Karl Leo

Organic Electronics 2015 Volume 27() pp:133-136

Publication Date(Web):December 2015

DOI:10.1016/j.orgel.2015.08.031

•Deposition of ZnPc:C60 blend layers for organic solar cells.•Increase of blend crystallinity by co-evaporation of polydimethylsiloxane.•No further increase in substrate temperature or decrease in deposition rate.•Increase in power conversion efficiency upon co-evaporation of PDMS.Donor–acceptor small molecule blends are a key component of efficient organic solar cells. Their efficient function in terms of exciton transport, exciton separation, and carrier transport is critically dependent on morphology, which is especially difficult to control when the layers are prepared by vacuum evaporation. We investigate co-evaporant induced crystallization for a blend of zinc phthalocyanine (ZnPc) and C60 in single layers and organic solar cells. Polydimethylsiloxane (PDMS) is chosen as an additive and simultaneously evaporated during the deposition of the blend layer on a heated substrate. Grazing incidence X-ray diffraction measurements prove a strong increase in crystallinity upon co-evaporation. Laser desorption/ionization time-of-flight mass spectrometry measurements identify remainders of PDMS in the co-evaporated layers, demonstrating that PDMS is incorporated into the film. Nevertheless, co-evaporated ZnPc:C60 solar cells show improved short circuit current densities, fill factors, and power conversion efficiencies.

Coexisting localized and extended optical Bloch states in a periodic deep wire array microcavity

Co-reporter:Franz J.F. Löchner, Andreas Mischok, Robert Brückner, Vadim G. Lyssenko, Alexander A. Zakhidov, Hartmut Fröb, K. Leo

Superlattices and Microstructures 2015 Volume 85() pp:646-652

Publication Date(Web):September 2015

DOI:10.1016/j.spmi.2015.06.020

•We observe the photon dispersion in a 1D periodically patterned microcavity.•Discrete localized and extended optical Bloch states are obtained.•The effective mass of confined photons in our microcavity is modelled.•A modified Kronig–Penney model is presented considering polarization and detuning.•We find excellent agreement between experiment, numerical and analytical calculation.We embed periodic SiO2 wires in an organic microcavity, producing a rectangular potential by the different optical thicknesses of the active layer due to the additional SiO2 layer. By μμ-photoluminescence spectroscopy, we observe the energy dispersion of the photons and obtain discrete localized below and extended Bloch states above the potential barrier, respectively, showing that electro-magnetic waves can behave like massive particles, such as electrons, in crystal lattices. We investigate the dependencies on wire width and period and use the Kronig–Penney model to describe the photon energy dispersion, including an “effective mass” of a photon propagating through a microcavity implying polarization splitting. We obtain excellent agreement between experiment, simulation and analytical calculation.

Color-stable, ITO-free white organic light-emitting diodes with enhanced efficiency using solution-processed transparent electrodes and optical outcoupling layers

Co-reporter:Hong-Wei Chang, Yong Hyun Kim, Jonghee Lee, Simone Hofmann, Björn Lüssem, Lars Müller-Meskamp, Malte C. Gather, Karl Leo, Chung-Chih Wu

Organic Electronics 2014 Volume 15(Issue 5) pp:1028-1034

Publication Date(Web):May 2014

DOI:10.1016/j.orgel.2014.02.017

•Color-stable, ITO-free white OLEDs with enhanced efficiencies were demonstrated.•High-conductivity PEDOT:PSS was used as the transparent anode.•Nanoparticle-based scattering layers were used for light extraction.•Both can be fabricated by simple, low-temperature solution processing.•Such an approach simultaneously provides manufacturing, cost and efficiency advantages.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.

Direct Electrical Evidence of Plasmonic Near-Field Enhancement in Small Molecule Organic Solar Cells

Co-reporter:Till Jägeler-Hoheisel ; Franz Selzer ; Moritz Riede

The Journal of Physical Chemistry C 2014 Volume 118(Issue 28) pp:15128-15135

Publication Date(Web):July 8, 2014

DOI:10.1021/jp5025087

We present a simple and versatile technique to introduce plasmonic silver nanoparticles into organic thin film devices by in situ vacuum deposition. Silver particles with 80 nm diameter at the back of small molecule organic solar cells increase the power conversion efficiency (PCE). Doped organic transport layers allow one to separate electrical and optical effects. By a systematic variation of the position of the silver particles within the solar cell stack, we can thus clearly distinguish a near-field photocurrent gain in the IR that decays to one-half on length scales of around 4 nm, and a less distance-dependent selective mirror effect for short wavelength, which allows one to optimize devices for different wavelengths simultaneously. Device optimization reveals that plasmonic increased absorption can be used to significantly reduce the thickness of the absorber layers and gain efficiency through improved transport properties. A plasmonic zinc phthalocyanine fullerene-C60 solar cell that yields improved photocurrent, fill factor, and PCE of 2.6% includes one-half of the absorber material of an optimized reference device with PCE of 2.4%. The design priciples for plasmonic solar cells are general and were confirmed in thin devices containing zinc 1,8,15,22-tetrafluoro-phthalocyanine, improving the PCE from 2.7% to 3.4%.

Chemical degradation mechanisms of highly efficient blue phosphorescent emitters used for organic light emitting diodes

Co-reporter:Ruben Seifert, Inês Rabelo de Moraes, Sebastian Scholz, Malte C. Gather, Björn Lüssem, Karl Leo

Organic Electronics 2013 Volume 14(Issue 1) pp:115-123

Publication Date(Web):January 2013

DOI:10.1016/j.orgel.2012.10.003

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.Graphical abstractHighlights► Chemical and physical degradation mechanisms of FIr6 and FIrpic emitters for OLEDs. ► OLEDs based on FIrpic exhibit higher stability than those based on FIr6. ► Layers of FIr6 exposed to UV-light presented a red emitting degradation product. ► The chemical reactions of the materials are analyzed by using LDI-TOF-MS.

Influence of Hole-Transport Layers and Donor Materials on Open-Circuit Voltage and Shape of IV Curves of Organic Solar Cells

Co-reporter:Wolfgang Tress;Moritz Riede

Advanced Functional Materials 2011 Volume 21( Issue 11) pp:2140-2149

Publication Date(Web):

DOI:10.1002/adfm.201002669

Abstract

The effect of injection and extraction barriers on flat heterojunction (FHJ) and bulk heterojunction (BHJ) organic solar cells is analyzed. The barriers are realized by a combination of p-type materials with HOMOs varying between –5.0 and –5.6 eV as hole-transport layer (HTL) and as donor in vacuum-evaporated multilayer p-i-metal small-molecule solar cells. The HTL/donor interface can be seen as a model for the influence of contacts in organic solar cells in general. Using drift-diffusion simulations we are well able to reproduce and explain the experimental I–V curves qualitatively. In FHJ solar cells the open-circuit voltage (V_oc) is determined by the donor and is independent of the HTL. In BHJ solar cells, however, V_oc decreases if injection barriers are present. This different behavior is caused by a blocking of the charge carriers at a spatially localized donor/acceptor heterojunction, which is only present in the FHJ solar cells. The forward current is dominated by the choice of HTL. An energy mismatch in the HOMOs leads to kinks in the I–V curves in the cases for which V_oc is independent of the HTL.

Analysis of chemical degradation mechanism within sky blue phosphorescent organic light emitting diodes by laser-desorption/ionization time-of-flight mass spectrometry

Co-reporter:Ines Rabelo de Moraes, Sebastian Scholz, Björn Lüssem, Karl Leo

Organic Electronics 2011 Volume 12(Issue 2) pp:341-347

Publication Date(Web):February 2011

DOI:10.1016/j.orgel.2010.11.004

We present an investigation of the intrinsic chemical degradation mechanisms of phosphorescent OLEDs based on the common sky blue emitter bis(2-(4,6-difluorophenyl)pyridyl-N,C2′)iridium(III)picolinate (FIrpic). The OLEDs are investigated using the laser-desorption/ionization time-of-flight mass spectrometry. The comparison between the collected spectra for electrically aged and unaged OLEDs allows the identification of different reaction products, like fragments which are mainly related to the chemical dissociation of FIrpic molecules during the OLED operation. We present different reaction pathways of the blue emitter FIrpic. One proposed reaction indicates that the short lifetimes of the OLEDs may be related to the irreversible dissociation of the FIrpic molecule by the loss of carbon dioxide (CO2) from the picolinate ligand. Additionally, the formation of chemical complexes between different fragments of the FIrpic molecules with their neighbour materials is visible. The cesium adducts of FIrpic formation indicate a possible contribution of the dopant to the OLED degradation process. Finally, we show rearrangement effects in the Cs doped electron transporting layer. This rearrangement is indicated by the presence of m/z signals of the BPhen dimer and adducts of the dimer with Cs.Graphical abstractResearch highlights► Intrinsic degradation mechanisms within OLEDs based on the common phosphorescent emitter FIrpic. ► Discovering of different chemical dissociation pathways on the FIrpic molecules. ► The responsibility of the irreversible dissociation mechanism by CO2-elimination from the FIrpic molecule. ► Detection of different iridium complexes, and cesium adducts of FIrpic by the laser desorption time of flight mass spectrometry. ► Observation of rearrangement effects in the Cs doped electron transporting layer during the OLED operation.

White top-emitting organic light-emitting diodes with forward directed emission and high color quality

Co-reporter:Patricia Freitag, Sebastian Reineke, Selina Olthof, Mauro Furno, Björn Lüssem, Karl Leo

Organic Electronics 2010 Volume 11(Issue 10) pp:1676-1682

Publication Date(Web):October 2010

DOI:10.1016/j.orgel.2010.07.017

We demonstrate top-emitting organic light-emitting diodes (OLEDs) with white emission spectra, employing a three color hybrid cavity structure with two highly efficient phosphorescent orange-red and green emitters (iridium(III)bis(2-methyldibenzo-[f,h]chinoxalin)(acetylacetonate) (Ir(MDQ)2(acac)) and tris(2-phenylpyridin)iridium(III) (Ir(ppy)3)) and the stable blue fluorescent emitter 2,5,8,11-tetra-tert-butylperylene (TBPe). In contrast to Lambertian emission characteristics, our devices show enhanced emission in the forward direction up to angles of 50°. In order to optically adjust the balance of blue and red light in the emission spectra, the influence of two different anode mirror materials are studied. We further test varied electron blocking layer materials to improve the device performance. An optimized layer structure with a hybrid anode mirror yields luminous efficacies of 13.3 lm/W (4.9% external quantum efficiency (EQE)) for the blocker material 2,2′,7,7′-tetrakis-(N,N-diphenylamino)-9,9′-spirobifluoren (S-TAD) and 12.4 lm/W (5.3% EQE) for N,N′-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine (NPB), respectively, at approximate luminances of 1000 cd/m2. Using an aluminum anode, the emission shows Commission Internationale d’Eclairage chromaticity coordinates (CIE 1931) of (0.420, 0.407) at approximately 1000 cd/m2 and color rendering indices of up to 77.

Enhanced Photogeneration of Triplet Excitons in an Oligothiophene–Fullerene Blend

Co-reporter:C. Uhrich;R. Schueppel;K. Leo Dr.;M. Pfeiffer Dr.;E. Brier;E. Reinold Dr.;P. Baeuerle Dr.

ChemPhysChem 2007 Volume 8(Issue 10) pp:1497-1503

Publication Date(Web):13 JUN 2007

DOI:10.1002/cphc.200700306

Photoinduced and transient absorption spectroscopy is used to study triplet exciton dynamics in thin films of a new thiophene-based oligomer (DCV3T) and blends of DCV3T and fullerene C₆₀. We find enhanced DCV3T triplet exciton generation in the blend layer, which is explained as an excitonic ping–pong effect: singlet energy transfer from DCV3T to C₆₀, followed by immediate intersystem crossing to C₆₀, and triplet exciton back-transfer. Estimations of the rate constants involved show that the ping–pong effect has an overall efficiency close to unity. The singlet–singlet energy transfer from DCV3T to C₆₀ is demonstrated by efficient quenching of DCV3T luminescence in the blend, leading to sensitized emission of C₆₀. We discuss a promising new concept of solar cells with an enlarged active-layer thickness based on potentially long-ranged triplet exciton diffusion in combination with efficient intersystem crossing.