Stephan Schmale

Visualizing a Homogeneous Blend in Bulk Heterojunction Polymer Solar Cells by Analytical Electron Microscopy

To increase efficiency of bulk heterojunctions for photovoltaic devices, the functional morphology of active layers has to be understood, requiring visualization and discrimination of materials with very similar characteristics. Here we combine high-resolution spectroscopic imaging using an analytical transmission electron microscope with nonlinear multivariate statistical analysis for classification of multispectral image data. We obtain a visual representation showing homogeneous phases of donor and acceptor, connected by a third composite phase, depending in its extent on the way the heterojunction is fabricated. For the first time we can correlate variations in nanoscale morphology determined by material contrast with measured solar cell efficiency. In particular we visualize a homogeneously blended phase, previously discussed to diminish charge separation in solar cell devices.

Transient characteristics of inverted polymer solar cells using titaniumoxide interlayers

Organic bulk heterojunction solar cells using titania interlayers as electron selective layers prepared by atomic layer deposition or wet processing are reported. Pristine devices show low filling factors (FFs) and consequently low efficiencies. Upon illumination with ultraviolet (UV) light, a significant increase in the FF is found. We study the impact of various ambient conditions (air, vacuum, and oxygen) on the dynamics of the decay of the FF after UV illumination. The interaction of oxygen and titania is evidenced as the dominant mechanism for the transient behavior of the polymer solar cells.

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.

Microwave annealing of polymer solar cells with various transparent anode materials

Efficient organic solar cells were produced through annealing with microwave radiation (2.45 GHz) within only 8 s. Efficiencies of up to 3% were obtained, similar to those of devices annealed with a hot plate for 300 s. We examined the effect of microwave irradiation on the individual layers of the solar cell architecture. Microwave absorption was found to be related exclusively to the sheet resistance of the layers. As a result, in a polymer solar cell comprising an electrode based on a transparent conducting oxide (TCO) the microwave annealing is evidenced to be directly linked to the microwave absorption in the TCO layer.

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.

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.