Organic Field Effect Transistors (OFETs), while showing a lot of promise, currently suffer from a number of limitations. Organic doping can help to overcome these limitations. It opens up a number of new possibilities by offering a way to define majority charge carriers, control the charge carrier density, threshold voltage etc. precisely and produce devices with better performance, stability, and reproducibility. The doping techniques explored in OFETs thus far have been in the range of a few wt.%, which has limited the use of doping to contact doping or a thin doped layer at the gate dielectric interface. Furthermore, the high doping concentrations used place serious limitations on the doping efficiency that can be achieved. Here we demonstrate the successful use of low doping in the 100ppm range throughout the bulk of the organic semiconductor layer of an OFET with the use of a rotating shutter.
Organic field-effect transistors (OFETs) have found a wide range of uses due to their attractive properties. A great deal of effort has been expended on boosting their mobilities, which tend to be low. Given this, accurate estimation of the mobility is crucial. We have developed a web application that automates or simplifies several of the steps required to estimate the mobility from experimental data. The app can be accessed at ofetanalysisapp.shinyapps.io/ofetanalysisapp. The app takes as inputs a file with the data and pieces of information like the number of OFETs and their channel lengths. The app has features that enable the user to mark OFETs as outliers, which are excluded from subsequent calculations. It fits nonlinear regression models to compute estimates of the mobility as well as the threshold voltage. The app provides several visualizations that give the user insight into the nature of the data. The estimates computed by the app can be downloaded in an Excel file so the user can perform further analysis. The use of the app is illustrated with a dataset from one of our OFET experiments.
Organic field-effect transistors (OFET) are important elements in thin-film electronics, being considered for
flat-panel or flexible displays, radio frequency identification systems, and sensor arrays. To optimize the
devices for high-frequency operation, the channel length, defined as the horizontal distance between the
source and the drain contact, can be scaled down. Here, an architecture with a vertical current flow, in particular the
Organic Permeable-Base Transistors (OPBT), opens up new opportunities, because the effective transit
length in vertical direction is precisely tunable in the nanometer range by the thickness of the semiconductor
layer. We present an advanced OPBT, competing with best OFETs while a low-cost, OLED-like fabrication
with low-resolution shadow masks is used (Klinger et al., Adv. Mater. 27, 2015). Its design consists of a stack
of three parallel electrodes separated by two semiconductor layers of C60 . The vertical current flow is
controlled by the middle base electrode with nano-sized openings passivated by an native oxide.
Using insulated layers to structure the active area, devices show an on/off ratio of 10⁶ , drive 11 A/cm² at an
operation voltage of 1 V, and have a low subthreshold slope of 102 mV/decade. These OPBTs show a unity
current-gain transit frequency of 2.2 MHz and off-state break-down fields above 1 MV/cm. Thus, our
optimized setup does not only set a benchmark for vertical organic transistors, but also outperforms best
lateral OFETs using similar low-cost structuring techniques in terms of power efficiency at high frequencies.
Organic p-i-n diodes enable the development of highly efficient organic devices such as organic light-emitting diodes. Understanding charge carrier trapping in these diodes is essential to comprehensively describe their electrical behaviors and increase their efficiency further. Here, a new bias stress protocol is developed to study charge trapping and the influence of trapping on molecular doping in organic p-i-n diodes. The results are discussed with the help of a novel analytical model, which is capable of quantifying the density of trapped charges and the doping efficiency from capacitance spectroscopy. We propose that this combined experimental/modeling approach is versatile and can lead to an advanced understanding of trapping in organic electronic devices.
Axel Fischer, Thomas Koprucki, Annegret Glitzky, Matthias Liero, Klaus Gärtner, Jacqueline Hauptmann, Sebastian Reineke, Daniel Kasemann, Björn Lüssem, Karl Leo, Reinhard Scholz
Large area OLEDs show pronounced Joule self-heating at high brightness. This heating induces brightness inhomogeneities, drastically increasing beyond a certain current level. We discuss this behavior considering 'S'-shaped negative differential resistance upon self-heating, even allowing for 'switched-back' regions where the luminance finally decreases (Fischer et al., Adv. Funct. Mater. 2014, 24, 3367). By using a multi-physics simulation the device characteristics can be modeled, resulting in a comprehensive understanding of the problem. Here, we present results for an OLED lighting panel considered for commercial application. It turns out that the strong electrothermal feedback in OLEDs prevents high luminance combined with a high degree of homogeneity unless new optimization strategies are considered.
Organic electronics hold the promise of enabling the field of flexible electronics. Several novel organic transistor concepts based on the technology of molecular doping are presented that open new directions to improve the performance of OFETs. The realization of doped organic transistors as well as organic junction field-effect transistors is demonstrated. Furthermore, vertical transistor concepts with channel lengths in the sub-micrometer
regime are discussed.
Excessive charge carrier densities in the emission layer of organic light-emitting diodes (OLEDs) can lead to significant
quenching by triplet-polaron-annihilation [1] or field-induced quenching [2]. Thus, to increase the efficiency of OLEDs
further, a technique for the reliable determination of charge carrier densities in OLEDs is most desirable.
Time-resolved spectroscopy is a powerful tool to investigate electronic and excitonic transfer processes [3]. By
application of a voltage pulse to a phosphorescent state-of-the-art OLED we find a transient overshoot after voltage turn-off.
This has primarily been found in phosphorescent OLEDs and has typically been attributed to delayed recombination
of trapped charge carriers [4-7].
In this contribution we investigate charge carrier accumulation within the emission layer (EML) and provide a method to
quantify the density of stored electrons.
We report on monochrome top emitting organic light emitting diodes (OLEDs) with inverted layer structure and discuss the optical and electrical optimization of OLED devices comprising the orange phosphorescent emitter Ir(MDQ)2(acac). We show that first, charge balance within the emitting layer is an important factor for efficient generation of light and second, optical outcoupling is a critical issue in top-emitting devices. We demonstrate the use of doped charge transport layers for efficient injection of charge carriers and optical modeling to improve outcoupling. The latter one is done via optimized cavity tuning and application of a dielectric capping layer. Finally, driving voltages of 4.2V at 1000 cd/m2, 19 lm/W and 17% external quantum efficiency (EQE) from devices made on metal substrates are reached.
The extraction of the luminous power internally generated by organic light emitting diodes (OLEDs) is still the
most severe limitation in the overall efficiency of these devices. We present a joint theoretical and experimental
study aimed to quantitatively evaluate the light outcoupling limitations of planar p-i-n type small-molecule
OLEDs, both in bottom and top emitting configuration. We discuss the physical origin of these limitations by
analyzing internal optical losses and overall light conversion efficiency in OLEDs.
Starting from a lab-curiosity, organic light emitting diodes have matured into a promising technology that has entered commercial markets. In particular for lighting applications, OLEDs can take advantage of their outstanding properties such as a high luminous efficacy, good color quality, and new design possibilities such
as illumination by
at light sources. In this contribution, new results on two approaches for highly efficient white OLEDs are presented: the all-phosphorescent concept and the triplet-harvesting approach.
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