Monika M. Voigt

Memory performance and retention of an all-organic ferroelectric-like memory transistor

We have built a nonvolatile memory field-effect transistor (FET)-based on organic compounds. The gate-insulating polymer features ferroelectric-like characteristics when spun from solution into an amorphous phase. Thus, the memory transistor is built using techniques developed for organic transistors without requiring high temperature annealing steps. The memory exhibits channel resistance modulations and retention times close in performance to inorganic ferroelectric FETs (FEFETs), yet at a fraction of the cost.

Surface segregation and self-stratification in blends of spin-cast polyfluorene derivatives

We have used helium-3 nuclear reaction analysis to study the morphology of spin-cast blends of poly(9,-dioctylfluorene) (F8) and poly(9,-dioctylfluorene-alt-benzothiadiazole) as a function of composition, casting solvent, and initial solvent concentration. In blends cast from toluene, a surface segregated layer is observed for a broad range of F8 compositions. The surface excess has a maximum of ?nm at an F8 volume fraction = 0.5, dropping down to ?2?nm for = 0.2 and?0.8. The existence of such surface segregated layers could play an important role in charge transport in optoelectronic devices made from blends of conjugated polymers. Films cast from chloroform show negligible surface segregation. Scanning probe microscopy experiments show that films cast from toluene exhibit significant in-plane structure, in contrast to those cast from chloroform, which show minimal lateral structure.

Air-processed organic tandem solar cells on glass: toward competitive operating lifetimes

Photovoltaic devices based on organic semiconductors (OPVs) hold great promise as a cost-effective renewable energy platform because they can be processed from solution and deposited on flexible plastics using roll-to-roll processing. Despite important progress and reported power conversion efficiencies of more than 10% the rather limited stability of this type of devices raises concerns towards future commercialization. The tandem concept allows for both absorbing a broader range of the solar spectrum and reducing thermalization losses. We designed an organic tandem solar cell with an inverted device geometry comprising environmentally stable active and charge-selecting layers. Under continuous white light irradiation, we demonstrate an extrapolated, operating lifetime in excess of one decade. We elucidate that for the current generation of organic tandem cells one critical requirement for long operating lifetimes consists of periodic UV light treatment. These results suggest that new material approaches towards UV-resilient active and interfacial layers may enable efficient organic tandem solar cells with lifetimes competitive with traditional inorganic photovoltaics.

Flexible organic tandem solar modules with 6% efficiency: combining roll-to-roll compatible processing with high geometric fill factors

Organic solar cell technology bears the potential for high photovoltaic performance combined with truly low-cost, high-volume processing. Here we demonstrate organic tandem solar modules on flexible substrates fabricated by fully roll-to-roll compatible processing at temperatures <70 °C. By using ultrafast laser patterning we considerably reduced the “dead area” of the modules and achieved geometric fill factors beyond 90%. The modules revealed very low interconnection-resistance compared to the single tandem cells and exhibited a power conversion efficiency of up to 5.7%. Bending tests performed on the modules suggest high mechanical resilience for this type of device. Our findings inform concrete steps towards high efficiency photovoltaic applications on curved, foldable and moving surfaces.

High performance organic transistors on cheap, commercial substrates

We present here our latest results on high quality gate insulators for organic electronics. Ultra-thin films of n-octadecyltrichlorosilane-treated, anodized aluminium (Al) grown onto flexible, Al-sputtered polyester substrates combine low cost and manufacture under ambient conditions with excellent performance characteristics (negligible leakage and hysteresis, 400 nF cm −2 capacitance). With pentacene as organic semiconductor, we present organic transistors with inverse subthreshold slope of approximately 200 mV dec −1 , threshold of the order of −2 V and >10 5 on/off ratio. The subthreshold behaviour is significantly better than for transistors with pentacene deposited onto a gate insulator optimized for high mobility. Above the threshold, the higher capacitance compensates for the somewhat lower mobility. Crucially, the cheap, sputtered Al-on-polyester films resulted in transistors that were as good as transistors fabricated on the same substrate with evaporated Al. (Some figures in this article are in colour only in the electronic version)

Imaging Joule heating in a conjugated-polymer light-emitting diode using a scanning thermal microscope

We have used a scanning thermal microscope to image Joule heating in a conjugated-polymer light-emitting diode (LED). Our LEDs had an active diameter of 100 μm, which was defined using an insulating layer of silicon nitride patterned onto the LED anode. At an average power input of 0.2 mW into the LED, we find that the center of the cathode is some 0.2 K warmer than its periphery. The observed temperature distribution across the pixel is slightly asymmetric, an effect which may be correlated with spatial inhomogeneity in the local current density across the device. We present a finite element analysis thermal model which is able to accurately describe the observed temperature distribution across the LED cathode.

Structure of films of poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) crosslinked with glycerol

We show that films of poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) crosslinked with glycerol function well as hole injection layers for light-emitting diodes even after immersion in water. Neutron reflectometry, X-ray photoelectron spectroscopy, and nuclear reaction analysis are used to characterise the structure of these layers. The glycerol at a moderate concentration of 5% (w/v) provides the best performance when used as a part of a light-emitting diode, but larger quantities impact adversely on device quality. Glycerol is shown to be present in considerable quantities in the film, at ∼13% by volume at the surface. No significant surface segregation of any component is observed in these films, although poly(3,4-ethylene dioxythiophene) is absent from the surface.

Material Properties of Laser-Welded Thin Silicon Foils

An extended monocrystalline silicon base foil offers a great opportunity to combine low-cost production with high efficiency silicon solar cells on a large scale. By overcoming the area restriction of ingot-based monocrystalline silicon wafer production, costs could be decreased to thin film solar cell range. The extended monocrystalline silicon base foil consists of several individual thin silicon wafers which are welded together. A comparison of three different approaches to weld 50 μm thin silicon foils is investigated here: (1) laser spot welding with low constant feed speed, (2) laser line welding, and (3) keyhole welding. Cross-sections are prepared and analyzed by electron backscatter diffraction (EBSD) to reveal changes in the crystal structure at the welding side after laser irradiation. The treatment leads to the appearance of new grains and boundaries. The induced internal stress, using the three different laser welding processes, was investigated by micro-Raman analysis. We conclude that the keyhole welding process is the most favorable to produce thin silicon foils.

All-organic single-transistor permanent memory device

We present an all-organic permanent memory transistor using an amorphous spin-cast gate insulator. This gate insulator exhibits a remanent polarisation in its amorphous state, a unique property, which is best described as “ferroelectric-like”. The memory transistor thus built perform extremely well, even when compared to inorganic ferroelectric memory transistors; the memory “on” to memory “off” current ratio is close to 3x10 4 , while time-dependent studies show

Flexible organic tandem solar modules: a story of up-scaling

The competition in the field of solar energy between Organic Photovoltaics (OPVs) and several Inorganic Photovoltaic technologies is continuously increasing to reach the ultimate purpose of energy supply from inexpensive and easily manufactured solar cell units. Solution-processed printing techniques on flexible substrates attach a tremendous opportunity to the OPVs for the accomplishment of low-cost and large area applications. Furthermore, tandem architectures came to boost up even more OPVs by increasing the photon-harvesting properties of the device. In this work, we demonstrate the road of realizing flexible organic tandem solar modules constructed by a fully roll-to-roll compatible processing. The modules exhibit an efficiency of 5.4% with geometrical fill factors beyond 80% and minimized interconnection-resistance losses. The processing involves low temperature (<70 °C), coating methods compatible with slot die coating and high speed and precision laser patterning.