Chang-Min Keum

Contact Resistance Effects in Highly Doped Organic Electrochemical Transistors

Injection at the source contact critically determines the behavior of depletion-type organic electrochemical transistors (OETs). The contact resistance of OETs increases exponentially with the gate voltage and strongly influences the modulation of the drain current by the gate voltage over a wide voltage range. A modified standard model accounting contact resistance can explain the particular shape of the transconductance.

Reaching saturation in patterned source vertical organic field effect transistors

Like most of the vertical transistors, the Patterned Source Vertical Organic Field Effect Transistor (PS-VOFET) does not exhibit saturation in the output characteristics. The importance of achieving a good saturation is demonstrated in a vertical organic light emitting transistor; however, this is critical for any application requiring the transistor to act as a current source. Thereafter, a 2D simulation tool was used to explain the physical mechanisms that prevent saturation as well as to suggest ways to overcome them. We found that by isolating the source facet from the drain-source electric field, the PS-VOFET architecture exhibits saturation. The process used for fabricating such saturation-enhancing structure is then described. The new device demonstrated close to an ideal saturation with only 1% change in the drain-source current over a 10 V change in the drain-source voltage.

Tuning charge carrier transport and optical birefringence in liquid-crystalline thin films : a new design space for organic light-emitting diodes

Liquid-crystalline organic semiconductors exhibit unique properties that make them highly interesting for organic optoelectronic applications. Their optical and electrical anisotropies and the possibility to control the alignment of the liquid-crystalline semiconductor allow not only to optimize charge carrier transport, but to tune the optical property of organic thin-film devices as well. In this study, the molecular orientation in a liquid-crystalline semiconductor film is tuned by a novel blading process as well as by different annealing protocols. The altered alignment is verified by cross-polarized optical microscopy and spectroscopic ellipsometry. It is shown that a change in alignment of the liquid-crystalline semiconductor improves charge transport in single charge carrier devices profoundly. Comparing the current-voltage characteristics of single charge carrier devices with simulations shows an excellent agreement and from this an in-depth understanding of single charge carrier transport in two-terminal devices is obtained. Finally, p-i-n type organic light-emitting diodes (OLEDs) compatible with vacuum processing techniques used in state-of-the-art OLEDs are demonstrated employing liquid-crystalline host matrix in the emission layer.

Improving the thermal stability of OLEDs by doping the electron transport layer with a reactive metal (Conference Presentation)

Organic light-emitting diodes (OLEDs) have reached a huge market as technology for small displays, e.g. in smartphones, and are entering the larger display and solid-state lighting markets as well. In parallel to these commercial successes, the OLED technology is adapted to a multitude of promising new applications, such as in optogenetics and medical therapy. However, it is still challenging to ensure good stability in applications that require high brightness (or high optical power density), in part due to the resulting resistive heating. Increased temperature can lead to a change in morphology of one or several organic layers, e.g. via crystallization of organic molecules, which then reduces electrical and optical performance and likely results in rapid device failure. Aside from an intrinsic resistive heating, heating can also be due to environmental effects during operation or fabrication and encapsulation of devices. For instance, atomic layer deposition (ALD) is a promising technique to form thin, yet highly protective encapsulation layers. However, state-of-the-art ALD processes require relatively high temperature during deposition (< 80 °C). 4,7-diphenyl-1,10-phenanthroline (BPhen) has been widely used as electron transporting layer (ETL) due its high electron mobility, particularly in an organic matrix-dopant system. However, it is well known that thin films of BPhen tend to recrystallize spontaneously. Annealing accelerates crystallization even further due to the relatively low glass transition temperature (Tg) of BPhen (62 °C). A straightforward way to enhance the device thermal stability is to make use of a high Tg material, yet materials have to be carefully adopted to provide appropriate functionality in OLEDs. In this contribution, we report the improvement of the thermal stability of OLEDs with BPhen based electron transport layers (ETLs) by cesium (Cs) doping. To verify the role of the Cs dopant in the BPhen matrix, recrystallization features of Cs-doped BPhen films with different doping concentrations were investigated using optical microscopy and atomic force microscopy. We also examined the photophysical properties of the films, i.e. photoluminescence (PL) and absorption. PL spectra exhibit monotonic red-shifts and broadening as the Cs doping concentration increases. This presumably indicates formation of metal complexes via interaction between the 1,10-phenanthroline group of BPhen molecules and the Cs ions. It was found that Cs plays a critical role, not only in inhibiting undesired recrystallization of BPhen molecules in a thin-film, but also in allowing BPhen layers to be thermally stable beyond the Tg of neat BPhen. Next, the electrical and optical properties of blue and red OLEDs that contain BPhen layers with different Cs-doping concentrations as ETL were characterized after annealing at temperatures between 60 and 100 °C. We find that higher doping concentrations lead to a marked increase in thermal device stability (quantified by current density and luminance at a fixed voltage). Making use of this observation, we successfully encapsulated BPhen based OLEDs with thin-film oxide layers using ALD. The results shown in this work may be transferable to other material systems and can thus provide a useful guideline to enhance the intrinsic thermal durability of organic devices and to render them compatible with processes involving thermal treatment.

Quantifying charge trapping and molecular doping in organic p-i-n diodes

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.