The fast growing market of organic electronics, including organic photovoltaics (OPV), stimulates the development of
versatile technologies for structuring thin-film materials. Ultraviolet lasers have proven their full potential for patterning
single organic layers, but in a multilayer organic device the obtained layer selectivity is limited as all organic layers show
high UV absorption. In this paper, we introduce mid-infrared (IR) resonant ablation as an alternative approach, in which
a short pulse mid-infrared laser can be wavelength tuned to one of the molecular vibrational transitions of the organic
material to be ablated. As a result, the technique is selective in respect of processing a diversity of organics, which
usually have different infrared absorption bands. Mid-IR resonant ablation is demonstrated for a variety of organic thin
films, employing both nanosecond (15 ns) and picosecond (250 ps) laser pulses tunable between 3 and 4 microns. The
nanosecond experimental set-up is based on a commercial laser at 1064 nm pumping a singly resonant Optical
Parametric Oscillator (OPO) built around a Periodically-Poled Lithium Niobate (PPLN) crystal with several Quasi-Phase
Matching (QPM) periods, delivering more than 0.3 W of mid-IR power, corresponding to 15 μJ pulses. The picosecond
laser set-up is based on Optical Parametric Amplification (OPA) in a similar crystal, allowing for a comparison between
both pulse length regimes.
The wavelength of the mid-infrared laser can be tuned to one of the molecular vibrational transitions of the organic
material to be ablated. For that reason, the IR absorption spectra of the organic materials used in a typical OPV device
were characterized in the wavelength region that can be reached by the laser setups. Focus was on OPV substrate
materials, transparent conductive materials, hole transport materials, and absorber materials. The process has been
successfully demonstrated for selective thin film patterning, and the influence of the various laser parameters is
We report on the latest progress in the field of organic p-i-n tandem solar cells. The results of tandem solar cells with an efficiency of 9.8% are shown (certified by Fraunhofer ISE) with an active area of about 1.1 cm2. These solar cells show a promising intrinsic stability: a relative reduction of 5.7% of its initial power conversion efficiency was measured when stored at 85°C for 2400 hours. Additionally, we present a small OPV module with an active area of 122cm2 showing an efficiency of with 9%, and an excellent low light behavior. Furthermore, we present the latest results on optimized tandem solar cells showing a power conversion efficiency of 10.7 % (measured by SGS, accredited and independent testing facility, active area of 1.1cm2).
We report on the latest progress in the field of organic p-i-n tandem solar cells. The external quantum efficiencies of a tandem solar cell with two complementary absorbing bulk heterojunctions with an efficiency of 6.07% (certified by Fraunhofer ISE) with an active area of about 2cm2 is analyzed. These solar cells are extremely stable: a reduction of only 3% of its initial power conversion efficiency was measured when stored at 85°C for 5000 hours. The solar cell does not show any reduction in efficiency when stored under continuous illumination of a tungsten lamp corresponding to 1.5 suns for 5000 hours. Furthermore, we present the latest results on optimized tandem solar cells showing a power
conversion efficiency of 7.66 % (certified by Fraunhofer ISE, active area of 1.1cm2).
We report on latest progress in the field of p-i-n type tandem solar cells. An optimized tandem cell architecture with two
complementary absorbing bulk heterojunctions leads to a certified power conversion efficiency of 5.9% on 2 cm2 active
area. Moreover, we show that p-i-n type tandem solar cells can be extremely stable: Extrapolated lifetimes corresponding
to more than 30 years of sun illumination have been achieved. Furthermore, we show that efficiency and stability only
slightly decrease when transferring the cell architecture to large serially interconnected modules of more than 100 cm²
The currently starting technical exploitation of organic electronic devices requires a deep understanding of ageing and
degradation mechanisms. In addition to extrinsically caused ageing processes, such as the penetration of oxygen and
water in organic layers and subsequent (electro)chemical reactions, further degradation channels exist in such devices,
which are based on intrinsic chemical reactions of the materials used in the devices. At this time, we know the
degradation mechanisms of only few organic materials applied in organic light emitting devices (OLEDs). To detect
specific reaction products, we introduced laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS), a
method which allows to distinguish between desired and undesired compounds in thin film organic devices. We use LDITOF-
MS to detect the degradation products of different Iridium based emitter materials like Ir(MDQ)2acac (red emitter)
and FIrpic (light blue) in dc driven OLEDs and adapted test sample structures. Due to the dissociation behaviour of some
Ir complexes and the ability of their fragments to form complexes with several hole blocking materials, the degradation
mechanisms of the devices can be understood in terms of such chemical complex formation between the emitter
molecules and neighbouring materials. On the other hand, the knowledge about these mechanisms can be used to select
the right combination of materials for the benefit of long-living devices as we will show at the end of this work.
A way to reach highly efficient and stable red bottom emission organic light emitting diodes (OLEDs) is the use
of doped transport layers, charge and exciton blockers, and phosphorescent emitter materials to combine low
operating voltage and high quantum yield. We will show how efficiency and lifetime of such devices can be
In our contribution, we report on highly efficient red p-i-n type organic light emitting diodes using an iridium-based
electrophosphorescent dye, Ir(MDQ)2(acac), doped in α-NPD as host material. By proper adjustment of
the hole blocking layer, the device performance may be enhanced to 20 % external quantum efficiency at an
operation voltage of 2.4 V and a brightness of 100 cd/m2. At the same time, a power efficiency of 37.5 lm/W is
reached. The quantum efficiency is well above previously reported values for this emitter. We attribute this high
efficiency to a combination of a well-adjusted charge carrier balance in the emission layer and a low current
density needed to reach a certain luminance due to the use of doped transport layers. High chemical stability of
the blocker material assures a long device lifetime of 32.000 hours at 1.000 cd/m2 initial luminance.
Organic light-emitting diodes (OLEDs) based on small-molecule materials are currently developed for applications
in flat panel displays and general lighting sources. However, a large number of efficient deep blue emitters
still suffer from rather fast degradation and thus, requires further improvement.
The aim of the present work is to gain a fundamental understanding of the intrinsic degradation processes causing
the low stability of blue OLED emitters. For this purpose we study the photoluminescence (PL) degradation
instead of the most often investigated electroluminescence (EL) degradation to separate electrically and optically
We show a newly developed PL lifetime measurement system which allows the study of degradation processes
under the influence of either electron or hole currents. Using this set-up we demonstrate the very high PL stability
of the highly efficient blue singlet emitter 2,2',7,7'-tetrakis(2,2-diphenylvinyl)spiro-9,9'-bifluorene (Spiro-
DPVBi) under electron and hole currents and compare this to the lifetime of OLEDs using the same emitter material.
We present a novel organic light emitting device concept for white light generation with the potential for 100%
internal quantum efficiency, which employs fluorescent blue and phosphorescent green and orange emitters. Due
to its high triplet energy, the intrinsically non-radiative triplet excitons of the fluorescent blue emitter can still
be harvested for light emission by letting them diffuse to the phosphor-containing emission layers. Thus, all
electrically generated excitons can be used for light emission without the need for phosphorescent blue emitters,
which suffer from stability problems. We demonstrate the high potential of this concept in a device achieving
57.6 lmW-1 total external power efficiency at 100 cd m-2 (20.3% external quantum efficiency) and 37.5 lmW-1
(14.4%) at an illumination relevant brightness of 1,000 cd m-2, and a high color rendering index of 86.
Standard organic light emitting diodes (OLEDs) are usually bottom-emitting, i.e. they emit light through a transparent
and electrically conductive substrate. Usually, indium tin oxide (ITO) is used for this purpose. However, as indium is a
very expensive metal, replacing it is of vital interest for cheap OLED mass production, especially when it comes to
lighting applications. We suggest the use of a polymer instead of ITO, carrying out both hole transport and injection. In
contrast to conventional approaches, which use a conductive polymer on top of ITO as smoothening and hole injection
layer, we employ solely a highly conductive polymer in combination with an OLED comprising doped charge transport
layers. This allows us to renounce the ITO layer underneath.
We use a new, highly conductive formulation of PEDOT:PSS, called Baytron® PH 500, with a conductivity of typically
500 S/cm, providing a smooth and electrically well-conductive substrate for the OLED stack. The use of such a
polymeric injection layer and of a doped small-molecule OLED stack results in a low operating voltage of the devices.
The charge transport layers of the OLED consist of MeO-TPD (N,N,N',N'-tetrakis(4-methoxyphenyl)-benzidine) doped
with a low percentage of F4-TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane) for the hole transport layer
and of Bphen (4,7-diphenyl-1,10-phenanthroline) co-evaporated with Caesium for the electron transport layer. We
demonstrate both fluorescent and phosphorescent monochromic OLEDs based on Baytron® PH 500 which achieve good
efficiencies. The OLEDs made on Baytron® PH 500 are compared with devices made on an ITO anode. Although the
polymer possesses a somewhat lower conductivity than ITO, efficient devices can be fabricated. For example, using the
blue emitter Spiro-DPVBi (2,2',7,7'-tetrakis(2,2-diphenylvinyl)spiro-9,9'-bifluorene), we achieve an efficiency of up to
5.1 cd/A. As another example, we discuss green OLEDs based on the triplet emitter Ir(ppy)3 (fac tris(2-phenylpyridine)
iridium) doped in a wide gap material. In this case, even a higher efficiency than on ITO is reached: 62 cd/A at a
luminance of 100 cd/m2, corresponding to an increase in external quantum efficiency by 15% as compared to ITO.
In this contribution, we discuss several research results which have contributed
to the vision of a broad application of organic light emitting diodes in displays and
lighting. We discuss the factors which determine the efficiency of OLEDs. First, we
briefly discuss work on the controlled molecular doping of organic semiconductors,
having the same advantageous influence on the device properties than in inorganic
semiconductor devices, in particular in reducing the operating voltage. By comparison
with a simple theory, we show that the voltages achieved with doped OLEDs are
already very close to the thermodynamic limit. We then show, with the example of
red phosphorescent OLEDs, how many properties of the OLED are improved by
We report on white organic light emitting diodes with three stacked emitter layers comprising the fluorescent
blue emitter Spiro-DPVBi, the phosphorescent green emitter system TCTA:Ir(ppy)3 and the phosphorescent red
emitter system NPB:Ir(MDQ)2(acac). A thin additional layer of mixed TCTA and TPBi separates the fluorescent
and phosphorescent emitting regions, simultaneously confining excitons efficiently and letting electrons and holes
easily pass. Furthermore, phosphorescence quenching by Dexter transfer to the non-radiative triplet state of
Spiro-DPVBi is suppressed. Devices were optimized to get color coordinates very close to the warm white
standard illuminant A. Best devices have a current efficiency of 13.8 cd/A, CIE color coordinates of (0.45, 0.42),
and a color rendering index of 91 at a brightness of 1000 cd/m2. Due to the use of electrically doped charge
transport layers, the voltage needed for 1000 cd/m2 was only 3.0 V, which leads to a power efficiency of 14.4 lm/W
assuming Lambertian emission.
We present highly efficient, low voltage top emitting organic light-emitting diodes (OLEDs) employing the same material (Ag) for both anode and cathode. Benefiting from doped charge carrier injection and transport layers (p-i-n structure) the diodes show comparable or even better electric characteristics than similar bottom emission OLEDs although the work function of the electrodes in the top emission OLEDs and the HOMO/LUMO location of the transport materials do not coincide. A green top emitting OLED with an Ir(ppy)3 doped double emission layer (D-EML) is demonstrated showing an efficiency of 50 cd/A at a brightness of 1000 cd/m2 and at the same time needing a very low driving voltage of only 2.85 V for 1000 cd/m2. By putting an additional organic capping layer on top of the cathode, the optical structure of the device can be tuned and the efficiency of the diodes can be further improved to a maximum efficiency of 78 cd/A at a brightness of 1000 cd/m2. Using the same capping strategy, efficient phosphorescent red and fluorescent blue top emitting OLEDs are demonstrated with efficiencies at 1000 cd/m2 as high as 13.6 cd/A and 8.6 cd/A, respectively.
We present highly efficient, low-voltage multilayer organic light emitting diodes based on the phosphorescent emitter tris(2-phenylpyridine) iridium (Ir(ppy)3). The phosphor is doped into various wide gap electron-transport or hole-transport host materials embedded in between doped transport layers. We use 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) doped N,N,N',N'-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) as p-type hole-injection and transport layer, while cesium (Cs) and 4,7-diphenyl-1,10-phenanthroline (Bphen) are co-evaporated for the n-type doped electron transport layer. Sandwiched between these two transport layers, we insert one or two emission layers. This p-i-n structure results in efficient carrier injection from both contacts into the doped transport layers and low ohmic losses. Thus, lower operating voltages are obtained compared to conventional undoped OLEDs.
By doping Ir(ppy)3 into a double layer structure of predominantly electron and hole transporting hosts, a power efficiency of 70 lm/W and an external quantum efficiency of 19.5% is achieved at 100 cd/m2 (2.95V). Besides this, the efficiency decays only weakly with increasing current density (or brightness). A quantum efficiency of 13.5% is still obtained at a current density of 100 mA/m2 with a luminance around 50,000 cd/m2. This improvement can be attributed mainly to the fact that we prevent any charge accumulation at hole or electron blocking layers and spread the generation region to both sides of the interface between the two parts of the emission layer. Moreover, losses due to non-radiative decay of triplet excitons diffusing into regions without the phosphorescent dye are avoided.