Jan Genoe

Highly Crystalline C8-BTBT Thin-Film Transistors by Lateral Homo-Epitaxial Growth on Printed Templates

Highly crystalline thin films of organic semiconductors offer great potential for fundamental material studies as well as for realizing high-performance, low-cost flexible electronics. The fabrication of these films directly on inert substrates is typically done by meniscus-guided coating techniques. The resulting layers show morphological defects that hinder charge transport and induce large device-to-device variability. Here, a double-step method for organic semiconductor layers combining a solution-processed templating layer and a lateral homo-epitaxial growth by a thermal evaporation step is reported. The epitaxial regrowth repairs most of the morphological defects inherent to meniscus-guided coatings. The resulting film is highly crystalline and features a mobility increased by a factor of three and a relative spread in device characteristics improved by almost half an order of magnitude. This method is easily adaptable to other coating techniques and offers a route toward the fabrication of high-performance, large-area electronics based on highly crystalline thin films of organic semiconductors.

Effects of hole self-trapping by polarons on transport and negative bias illumination stress in amorphous-IGZO

The effects of hole injection in amorphous indium-gallium-zinc-oxide (a-IGZO) are analyzed by means of first-principles calculations. The injection of holes in the valence band tail states leads to their capture as a polaron, with high self-trapping energies (from 0.44 to 1.15u2009eV). Once formed, they mediate the formation of peroxides and remain localized close to the hole injection source due to the presence of a large diffusion energy barrier (of at least 0.6u2009eV). Their diffusion mechanism can be mediated by the presence of hydrogen. The capture of these holes is correlated with the low off-current observed for a-IGZO transistors, as well as with the difficulty to obtain a p-type conductivity. The results further support the formation of peroxides as being the root cause of Negative Bias Illumination Stress (NBIS). The strong self-trapping substantially reduces the injection of holes from the contact and limits the creation of peroxides from a direct hole injection. In the presence of light, the concentr...

Off-current reduction in p-type SnO thin film transistors

SnO is one of the few candidates for p-type oxide thin film transistors (TFTs) because it retains a reasonable high hole mobility in a nanocrystalline film. However, the high off-current of SnO TFT limits its usefulness. In this work, SnO TFTs were fabricated using thermal evaporation under ultra-high vacuum. In order to decrease the off-current in p-type SnO thin film transistors (TFTs), we used yttrium to reduce n-type minority charges in the channel. The on/off ratio of the TFT increases from 102 to 5u2009×u2009104 and the mobility of the TFT in the saturated regime reduces from 1.6 to 1.4u2009cm2/V s when the SnO channel is doped with 1u2009wt. % of Y. Grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy reveal that the reduction of SnO2 in the Y-doped SnO TFT channel is the main reason for the improvement in the TFT characteristics.SnO is one of the few candidates for p-type oxide thin film transistors (TFTs) because it retains a reasonable high hole mobility in a nanocrystalline film. However, the high off-current of SnO TFT limits its usefulness. In this work, SnO TFTs were fabricated using thermal evaporation under ultra-high vacuum. In order to decrease the off-current in p-type SnO thin film transistors (TFTs), we used yttrium to reduce n-type minority charges in the channel. The on/off ratio of the TFT increases from 102 to 5u2009×u2009104 and the mobility of the TFT in the saturated regime reduces from 1.6 to 1.4u2009cm2/V s when the SnO channel is doped with 1u2009wt. % of Y. Grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy reveal that the reduction of SnO2 in the Y-doped SnO TFT channel is the main reason for the improvement in the TFT characteristics.

Flexible selfbiased 66.7nJ/c.s. 6bit 26S/s successive-approximation C-2C ADC with offset cancellation using unipolar Metal-Oxide TFTs

Metal-Oxide thin-film transistors (TFTs) present unique opportunities to develop robust, low-cost and transparent electronics that can mechanically endure on flexible and stretchable substrates over large area in an industry compatible technology. Analog to digital converters (ADC) are an essential part of the Internet-of-Everything, where a multitude of sensing applications are envisaged, such as temperature and pressure sensor tags on human skin. In this work, dual-gate InGaZnO TFTs (IGZO) are demonstrated to achieve a 6-bit successive approximation (SAR) C-2C ADC operated at a clock of up to 400Hz and a power dissipation of 52.2µW at a power supply of Vdd=15V. The ADC achieves a differential nonlinearity (DNL) of 0.7LSB and an integral nonlinearity (INL) of 0.58 LSB using only n-type TFTs. A figure of merit (FoM) of 66.7nJ/c.s. is achieved from an ADC on flexible substrate.

Contact resistance characterization in organic thin film transistors (Conference Presentation)

Proper thin film transistor (TFT) operation requires that its contact resistance Rc remains only a fraction of its channel resistance Rch. The integration of thin films based on latest generation organic semiconductors into downscaled TFTs with short channel length and high capacitance dielectric results in devices with very low Rch. Matching this with a low enough Rc is very challenging, due to the notoriously poor charge injection into organic semiconductors. The viability of integrated circuit technologies based on organic TFTs hinges on solving this critical contact resistance issue.nnTo properly address this, it is important to use a common metric based on simple, comparable contact resistance measurements. Rc is commonly measured using the Transfer Length Method (TLM) that involves the characterization of TFTs of different channel lengths in the linear regime. We find, however, that the precision and the absolute value of the extracted Rc is greatly influenced by the conditions used to characterize each TFT. This seriously complicates the comparison to other literature values. In this talk, we present an in-depth study of the TLM technique aimed at solving these particular problems. nnOur TLM structures are based on high mobility organic TFTs, fabricated with different technologies and topologies. We conduct a systematic comparison of voltage- and current-controlled measurements with constant lateral electric field and charge density. As a result, we delineate the conditions to conduct TLM characterization and data treatment for clean Rc extraction. We also identify the measurement parameters that count in establishing a good Rc benchmark.

Organic light-emitting transistors with overlapping gate structure: towards high efficiency at high current density (Conference Presentation)

Organic light-emitting transistors (OLETs) combining the dual functions of electrical switching and light emission are promising devices to push large amounts of charge carriers into an isolated recombination area. Most OLETs proposed so far, however, suffer from hole-electron current imbalance when driven at high currents. This results in a light emission zone very close to one of electrodes. The proximity of the electrode is detrimental to efficient light emission as the metal significantly quenches excitons. In addition, in single gate OLETs, the emission zone can unpredictably switch from one contact to the other upon small bias changes since electron and hole currents are not controlled individually. Dual-gate architectures have been proposed to independently control the transport of both types of charge carriers towards the recombination zone. In the split-gate OLET, where both gates lie side by side in the same plane, light is exclusively emitted from the center of the channel, far from the electrodes. But the unavoidable horizonal gap between the gates creates a highly resistive region in the vicinity of the recombination zone that lowers the electrical efficiency of the device.nnHere, we introduce the overlapping-gate OLET, a novel dual-gate architecture in which one gate partially covers the other, leaving no horizontal gap between the gates. By accumulating charge carriers in the transport layer, each gate independently opens a unipolar gapless channel to transport and inject charge directly into the recombination zone, thereby avoiding transport through highly resistive ungated regions. For the active layer, we propose a vacuum-evaporated multi-layered structure of organic semiconductors: The transport layers are based on high mobility p- and n-type materials for efficient lateral transport. The recombination zone is made of a fluorescent host-guest emissive layer. nnThanks to this architecture, the red light emitted by the overlapping-gate OLET is precisely located along the edge of the top gate that overlaps with the bottom gate. Therefore, light emission stays localized in the center of the channel, isolated from the quenching electrodes. Besides, the independent control over the supply of both types of charge carriers enables balanced transport up to high current densities. As a result, high performance red-emitting OLETs are demonstrated with an external quantum efficiency of 5.6% at a high luminance over 2000 cd m−2. Furthermore, the device shows no efficiency roll-off up to a current density of 30 mA/cm2. The conditions for balanced transport are rationalized through the development of an equivalent-circuit model. This shows that balanced transport is achieved when both transport channels are biased in the linear regime and that charge densities at the boundaries of the emissive layer are equal.nnThe overlapping gate light emitting transistor is a promising step towards the development of bright organic light emitting devices that combine high efficiency at high current densities.

Integrated Tin Monoxide P-Channel Thin-Film Transistors for Digital Circuit Applications

High-performance integrated tin monoxide bottom-gate staggered p-channel thin-film transistors (TFTs) are realized and reported. The active layer has been formed by thermal vacuum evaporation and rapid thermal annealing under a continuous nitrogen flow, resulting in field-effect mobilities up to 1.6 cm²/(<inline-formula> <tex-math notation=LaTeX>

Negative field‐dependent charge mobility in crystalline organic semiconductors with delocalized transport

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Growth Of Organic Semiconductor Thin Films with Multi-Micron Domain Size and Fabrication of Organic Transistors Using a Stencil Nanosieve

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Optimization of light emitting diodes based on bipolar double-barrier resonant-tunneling structures

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