Daniel Kasemann

Band structure engineering in organic semiconductors

Organic solar cells tuned by blending Electrical engineers can finetune the energetics of rigid photovoltaics and transistors by blending different semiconducting materials. However, its hard to apply this tuning protocol to the flexible class of carbon-based semiconductors. Schwarze et al. now show that continuous band energy tuning is indeed possible by varying the blend ratios of certain organic phthalocyanines and their fluorinated or chlorinated derivatives (see the Perspective by Ueno). They demonstrated the effect, which they attribute to quadrupolar interactions, in model solar cells. Science, this issue p. 1446; see also p. 1395 Quadrupolar interactions enable continuous energetic tuning of organic semiconductor blends. A key breakthrough in modern electronics was the introduction of band structure engineering, the design of almost arbitrary electronic potential structures by alloying different semiconductors to continuously tune the band gap and band-edge energies. Implementation of this approach in organic semiconductors has been hindered by strong localization of the electronic states in these materials. We show that the influence of so far largely ignored long-range Coulomb interactions provides a workaround. Photoelectron spectroscopy confirms that the ionization energies of crystalline organic semiconductors can be continuously tuned over a wide range by blending them with their halogenated derivatives. Correspondingly, the photovoltaic gap and open-circuit voltage of organic solar cells can be continuously tuned by the blending ratio of these donors.

Vertical organic transistors.

Organic switching devices such as field effect transistors (OFETs) are a key element of future flexible electronic devices. So far, however, a commercial breakthrough has not been achieved because these devices usually lack in switching speed (e.g. for logic applications) and current density (e.g. for display pixel driving). The limited performance is caused by a combination of comparatively low charge carrier mobilities and the large channel length caused by the need for low-cost structuring. Vertical Organic Transistors are a novel technology that has the potential to overcome these limitations of OFETs. Vertical Organic Transistors allow to scale the channel length of organic transistors into the 100 nm regime without cost intensive structuring techniques. Several different approaches have been proposed in literature, which show high output currents, low operation voltages, and comparatively high speed even without sub-μm structuring technologies. In this review, these different approaches are compared and recent progress is highlighted.

Adjustable white-light emission from a photo-structured micro-OLED array

White organic light-emitting diodes (OLEDs) are promising candidates for future solid-state lighting applications and backplane illumination in large-area displays. One very specific feature of OLEDs, which is currently gaining momentum, is that they can enable tunable white light emission. This feature is conventionally realized either through the vertical stacking of independent OLEDs emitting different colors or in lateral arrangement of OLEDs. The vertical design is optically difficult to optimize and often results in efficiency compromises between the units. In contrast, the lateral concept introduces severe area losses to dark regions between the subunits, which requires a significantly larger overall device area to achieve equal brightness. Here we demonstrate a color-tunable, two-color OLED device realized by side-by-side alignment of yellow and blue p-i-n OLEDs structured down to 20 μm by a simple and up-scalable orthogonal photolithography technique. This layout eliminates the problems of conventional lateral approaches by utilizing all area for light emission. The corresponding emission of the photo-patterned two-unit OLED can be tuned over a wide range from yellow to white to blue colors. The independent control of the different units allows the desired overall spectrum to be set at any given brightness level. Operated as a white light source, the microstructured OLED reaches a luminous efficacy of 13 lm W−1 at 1000 cd m−2 without an additional light outcoupling enhancement and reaches a color rendering index of 68 when operated near the color point E. Finally, we demonstrate an improved device lifetime by means of size variation of the subunits.

Line-on-Line Organic-Organic Heteroepitaxy of Quaterrylene on Hexa-peri-hexabenzocoronene on Au(111)

In a recent paper, we discussed the optical properties of a heterostructure consisting of a highly ordered monolayer of quaterrylene (QT), electronically decoupled from the gold substrate by a predeposited epitaxial monolayer of hexa-peri-hexabenzocoronene (HBC). Here we now present the detailed structural investigation of this organic double-layer system. We show that the structure of the heterosystem can be identified as line-on-line coincidence (lol), a new type of epitaxy discovered by us previously for the system 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on HBC on highly oriented pyrolytic graphite (HOPG). Additionally, we provide evidence on the basis of advanced potential energy calculations that indeed energetic gain drives this lol growth mode.

Advanced Organic Permeable‐Base Transistor with Superior Performance

An optimized vertical organic permeable-base transistor (OPBT) competing with the best organic field-effect transistors in performance, while employing low-cost fabrication techniques, is presented. The OPBT stands out by its excellent power efficiency at the highest frequencies.

Top-gate organic depletion and inversion transistors with doped channel and injection contact

Organic field-effect transistors constitute a vibrant research field and open application perspectives in flexible electronics. For a commercial breakthrough, however, significant performance improvements are still needed, e.g., stable and high charge carrier mobility and on-off ratio, tunable threshold voltage, as well as integrability criteria such as n- and p-channel operation and top-gate architecture. Here, we show pentacene-based top-gate organic transistors operated in depletion and inversion regimes, realized by doping source and drain contacts as well as a thin layer of the transistor channel. By varying the doping concentration and the thickness of the doped channel, we control the position of the threshold voltage without degrading on-off ratio or mobility. Capacitance-voltage measurements show that an inversion channel can indeed be formed, e.g., an n-doped channel can be inverted to a p-type inversion channel with highly p-doped contacts. The Cytop polymer dielectric minimizes hysteresis, and the transistors can be biased for prolonged cycles without a shift of threshold voltage, indicating excellent operation stability.

Hole mobility in thermally evaporated pentacene: Morphological and directional dependence

Pentacene has been extensively studied as an active material for organic field-effect transistors as it shows very good charge carrier mobility along its preferred transport direction. In this contribution, we investigate the hole transport in pentacene thin films by measurement in conventional lateral organic field-effect transistors (OFETs), which yields the hole mobility along the a-b plane of pentacene, and by the recently published potential mapping (POEM) approach, which allows for direct extraction of the charge carrier mobility perpendicular to the substrate, in this case perpendicular to the a-b plane, without the assumption of a specific transport model. While the mobility along the a-b plane—determined from OFET measurements—is found to be in the region of 0.45 cm2/Vs, transport perpendicular to this plane shows an average mobility at least one order of magnitude lower. Investigating also how these effective mobility values depend on the deposition rate of the pentacene films, we find that the ...

Controlling morphology: A vertical organic transistor with a self-structured permeable base using the bottom electrode as seed layer

Flexible organic permeable base transistors are a promising transistor technology, enabling high transconductance without the need for cost-intensive structuring techniques. Here, we present a simple approach to enhance the transmission and thus the current gain of a permeable base transistor. By adding a morphology modifying gold layer beneath the organic semiconductor, the interface to the base electrode is adjusted, resulting in a self-structured permeable base. Furthermore, we show that doping is essential not only for charge injection at the emitter, but is also required at the collector for a good performance. We show that the transmission can be increased to 98% by tuning the built-in field at the collector to actively gather charge carriers. The built-in field also leads to a very low minimum operation voltage <0.5 V, resulting in a low power consumption.

Operation mechanism of high performance organic permeable base transistors with an insulated and perforated base electrode

The organic permeable base transistor is a vertical transistor architecture that enables high performance while maintaining a simple low-resolution fabrication. It has been argued that the charge transport through the nano-sized openings of the central base electrode limits the performance. Here, we demonstrate by using 3D drift-diffusion simulations that this is not the case in the relevant operation range. At low current densities, the applied base potential controls the number of charges that can pass through an opening and the opening is the current limiting factor. However, at higher current densities, charges accumulate within the openings and in front of the base insulation, allowing for an efficient lateral transport of charges towards the next opening. The on-state in the current-voltage characteristics reaches the maximum possible current given by space charge limited current transport through the intrinsic semiconductor layers. Thus, even a small effective area of the openings can drive huge cu...

The impact of molecular weight, air exposure and molecular doping on the charge transport properties and electronic defects in dithienyl-diketopyrrolopyrrole-thieno[3,2-b]thiophene copolymers

We performed an in-depth study of high molecular weight poly[3,6-(dithiophene-2-yl)-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-thieno[3,2-b]thiophene] P(DPP2OD-TT) synthesized through the Stille coupling polycondensation in order to understand the correlation between molecular weight, processing conditions and charge transport. We observed a rapid increase in its aggregation in solution with increasing molecular weight which strongly limits the solubility and processability for weight average molecular weights beyond 200 kg mol−1. This results in severe limitation in the charge transport properties of the polymer. We further observe the presence of bulk electronic defects in all different polymer batches that severely limit the current flow and manifest themselves in organic field effect transistors as apparent charge density dependence of the mobility. These defects are passivated by exposure to an ambient atmosphere, as confirmed by an increase in current and mobility that is no more charge density dependent. This is further confirmed by the result of chemical doping using 2,2-(perfluoronaphthalene-2,6-diylidene)dimalononitrile, F6TCNNQ, which leads to the filling of the trap states and a higher charge density independent mobility of up to 1 cm2 V−1 s−1.