Thomas Winkler

Efficient semitransparent inverted organic solar cells with indium tin oxide top electrode

We reported on highly efficient semitransparent polymer solar cells comprising a transparent sputtered indium tin oxide (ITO) top electrode. We used an inverted cell structure with titanium dioxide prepared by atomic layer deposition as electron selective layer and molybdenum oxide (MoO3) as hole extraction layer. Moreover, the MoO3 layer prevents damage to the organic active materials due to the ITO sputtering process. For the semitransparent device, power conversion efficiencies of 1.9% were achieved with a high transmittance of 80% in the red region of the visible spectrum.

p-type doping efficiency of MoO3 in organic hole transport materials

We report on the p-type doping efficiency of molybdenum trioxide (MoO3) in the ambipolar organic charge transport material 4,4′-Bis(carbazol-9-yl)-biphenyl (CBP). Kelvin probe analysis is used to study the work function with increasing thickness of doped CBP layers with varied MoO3 concentration deposited on indium tin oxide (ITO). Based on the model of a one-sided abrupt (n+p) junction between ITO and the MoO3 doped CBP layer, the density of free holes has been determined. A surprisingly low p-type doping efficiency of less than 2% has been derived. Segregation and clustering of the MoO3 dopant could explain these results.

Reliable thin film encapsulation for organic light emitting diodes grown by low-temperature atomic layer deposition

We report on highly efficient gas diffusion barriers for organic light emitting diodes (OLEDs). Nanolaminate (NL) structures composed of alternating Al2O3 and ZrO2 sublayers grown by atomic layer deposition at 80 °C are used to realize long-term stable OLED devices. While the brightness of phosphorescent p-i-n OLEDs sealed by a single Al2O3 layer drops to 85% of the initial luminance of 1000 cd/m2 after 1000 h of continuous operation, OLEDs encapsulated with the NL retain more than 95% of their brightness. An extrapolated device lifetime substantially in excess of 10 000 h can be achieved, clearly proving the suitability of the NLs as highly dense and reliable thin film encapsulation of sensitive organic electronic devices.

A strategy towards p-type doping of organic materials with HOMO levels beyond 6 eV using tungsten oxide

The authors present a concept to p-dope organic hole transport materials with highest occupied molecular orbital (HOMO) levels on the order of 6 eV (e.g.4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA) or 4,4′-bis(carbazol-9-yl)biphenyl (CBP)) by using WO3 as acceptor. Owing to its large work function, WO3 allows for efficient electron transfer from organic semiconductors with HOMO levels in this region. The effect of p-type doping is evidenced by optical absorption spectroscopy, electrical transport characteristics and Kelvin probe analysis. Moreover, OLED devices have been realized which exhibit 30% enhanced power efficiency due to the p-type doping. The results are expected to have a significant impact on the development of organic (opto-)electronic devices.

Indium-free bottom electrodes for inverted organic solar cells with simplified cell architectures

Inverted organic bulk heterojunction solar cells employing a multilayer electrode comprising of a thin Ag layer embedded between layers of zinc tin oxide (ZTO) are compared to cells using an indium tin oxide electrode. The In-free ZTO/Ag/ZTO (ZAZ) electrodes exhibit a favorable work function of 4.3 eV and are shown to allow for excellent electron extraction even without a further interlayer. As a result, issues like transient cell characteristics known from cells comprising titania can be readily avoided. This renders ZAZ a perfectly suited bottom electrode for inverted organic solar cells with a simplified cell architecture.

Semi-transparent inverted organic solar cells

We will present efficient semi-transparent bulk-heterojunction [regioregular of poly(3-hexylthiophene): (6,6)-phenyl C61 butyric acid methyl ester] solar cells with an inverted device architecture. Highly transparent ZnO and TiO2 films prepared by Atomic Layer Deposition are used as cathode interlayers on top of ITO. The topanode consists of a RF-sputtered ITO layer. To avoid damage due to the plasma deposition of this layer, a sputtering buffer layer of MoO3 is used as protection. This concept allows for devices with a transmissivity higher than 60 % for wavelengths 650 nm. The thickness of the MoO3 buffer has been varied in order to study its effect on the electrical properties of the solar cell and its ability to prevent possible damage to the organic active layers upon ITO deposition. Without this buffer or for thin buffers it has been found that device performance is very poor concerning the leakage current, the fill factor, the short circuit current and the power conversion efficiencies. As a reference inverted solar cells with a metal electrode (Al) instead of the ITO-top contact are used. The variation between the PCE of top versus conventional illumination of the semi-transparent cells was also examined and will be interpreted in view of the results of the optical simulation of the dielectric device stack with and without reflection top electrode. Power conversion efficiencies of 2-3 % for the opaque inverted solar cells and 1.5-2.5 % for the semi-transparent devices were obtained under an AM1.5G illumination.

Thin Film Encapsulation of Top-Emitting OLEDs Using Atomic Layer Deposition

Multi-layer barriers prepared by atomic layer deposition are used to encapsulate organic light emitting diodes (OLEDs). This approach allows for both low water vapor transmission rates (5×10−7 g/(m2 day)) and outstanding chemical robustness. OLED lifetimes in excess of 20,000 h are achieved. For top-emitting OLEDs, the encapsulation layer concomitantly can be tailored to increase the external quantum efficiency by more than 40%. In addition, the prospects of ALD as encapsulation technique for the high volume production of organic electronic systems are discussed.

Reliability aspects of organic light emitting diodes

Various functional elements in organic optoelectronics devices are sensitive to oxygen an moisture. Thus, without an encapsulation the lifetime of organic light emitting diodes or organic solar cells does not meet the requirements of a serious application. We will discuss the particular challenges to form reliable, dense, pin-hole free thin-film barriers on top of organic devices. Specifically, atomic layer deposition (ALD) will be shown to be a highly attractive technique, that allows to operate at temperatures below 100 °C. Particularly, the use of nanolaminates - multilayer structures of two alternating oxide materials - have evolved as a promising candidate to seal organic devices with a powerful moisture barrier. We will discuss the preparation technology of these barrier layers, the characterization of their gas permeation rate as well as their performance in real OLED devices. With a proper ALD barrier, the lifetime of an OLED operated at 1 000 cd/m2 can be increased to be well in excess of 20 000 h. For top-emitting or transparent OLEDs, the optical properties of the thin-film encapsulation layer can be used to concomitantly tune the light extraction efficiency of the devices.

Highly efficient fully transparent inverted OLEDs

One of the unique selling propositions of OLEDs is their potential to realize highly transparent devices over the visible spectrum. This is because organic semiconductors provide a large Stokes-Shift and low intrinsic absorption losses. Hence, new areas of applications for displays and ambient lighting become accessible, for instance, the integration of OLEDs into the windshield or the ceiling of automobiles. The main challenge in the realization of fully transparent devices is the deposition of the top electrode. ITO is commonly used as transparent bottom anode in a conventional OLED. To obtain uniform light emission over the entire viewing angle and a low series resistance, a TCO such as ITO is desirable as top contact as well. However, sputter deposition of ITO on top of organic layers causes damage induced by high energetic particles and UV radiation. We have found an efficient process to protect the organic layers against the ITO rf magnetron deposition process of ITO for an inverted OLED (IOLED). The inverted structure allows the integration of OLEDs in more powerful n-channel transistors used in active matrix backplanes. Employing the green electrophosphorescent material Ir(ppy)3 lead to IOLED with a current efficiency of 50 cd/A and power efficiency of 24 lm/W at 100 cd/m2. The average transmittance exceeds 80 % in the visible region. The on-set voltage for light emission is lower than 3 V. In addition, by vertical stacking we achieved a very high current efficiency of more than 70 cd/A for transparent IOLED.

P‐157: Highly‐Efficient Gas Diffusion Barriers Based on Nanolaminates Prepared by Low‐Temperature ALD

Gas diffusion barriers based on nanolaminates of alternating Al2O3/ZrO2 layers prepared by atomic layer deposition at 80 °C are presented. Water vapor permeation rates as low as 4.7×10−5g/(m2 day) are determined (at 70 °C and 70 % rh). Compared to single Al2O3 encapsulation layers, a reduced density of statistical defects is found. The reliable nanolaminate encapsulation is demonstrated by OLED lifetime measurements.