Critical Evaluation of Organic Thin-Film Transistor Models

Thin-film transistors (TFTs) represent a wide-spread tool to determine the charge-carrier mobility of materials. Mobilities and further transistor parameters like contact resistances are commonly extracted from the electrical characteristics. However, the trust in such extracted parameters is limited, because their values depend on the extraction technique and on the underlying transistor model. We propose a technique to establish whether a chosen model is adequate to represent the transistor operation. This two-step technique analyzes the electrical measurements of a series of TFTs with different channel lengths. The first step extracts the parameters for each individual transistor by fitting the full output and transfer characteristics to the transistor model. The second step checks whether the channel-length dependence of the extracted parameters is consistent with the model. We demonstrate the merit of the technique for distinct sets of organic TFTs that differ in the semiconductor, the contacts, and the geometry. Independent of the transistor set, our technique consistently reveals that state-of-the-art transistor models fail to reproduce the correct channel-length dependence. Our technique suggests that contemporary transistor models require improvements in terms of charge-carrier-density dependence of the mobility and/or the consideration of uncompensated charges in the transistor channel.Comment: 20 pages, 10 figure

High performance p-type organic thin film transistors with an intrinsically photopatternable, ultrathin polymer dielectric layer

AbstractA high-performing bottom-gate top-contact pentacene-based oTFT technology with an ultrathin (25–48nm) and electrically dense photopatternable polymeric gate dielectric layer is reported. The photosensitive polymer poly((±)endo,exo-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid, diphenylester) (PNDPE) is patterned directly by UV-exposure (λ=254nm) at a dose typical for conventionally used negative photoresists without the need for any additional photoinitiator. The polymer itself undergoes a photo-Fries rearrangement reaction under UV illumination, which is accompanied by a selective cross-linking of the macromolecules, leading to a change in solubility in organic solvents. This crosslinking reaction and the negative photoresist behavior are investigated by means of sol–gel analysis. The resulting transistors show a field-effect mobility up to 0.8cm2V−1s−1 at an operation voltage as low as −4.5V. The ultra-low subthreshold swing in the order of 0.1Vdec−1 as well as the completely hysteresis-free transistor characteristics are indicating a very low interface trap density. It can be shown that the device performance is completely stable upon UV-irradiation and development according to a very robust chemical rearrangement. The excellent interface properties, the high stability and the small thickness make the PNDPE gate dielectric a promising candidate for fast organic electronic circuits

Photo reactive and natural source based materials as functional layers in organic thin film transistors and organic complementary inverters

Das Ziel dieser Arbeit ist die Untersuchung neuartiger Materialien zur Herstellung organischer elektronischer Bauelemente. Der erste Abschnitt befasst sich mit der Integration eines photovernetzbaren Polymers (poly(()endo,exo-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid, diphenylester), PNDPE) als Gate-Dielektrikum in einem organischen Dünnschichttransistor (engl. organic thin film transistor, OTFT). Hinsichtlich seiner Eignung als Dielektrikum wurde das Material auf die dielektrischen Eigenschaften, die Grenzflächeneigenschaften zwischen Halbleiter und Dielektrikum, den Einfluss von Ladungsträgerhaftstellen (traps), das Halbleiterwachstum sowie die Strukturierbarkeit untersucht. Des Weiteren wurde das Material in einen selbstjustierenden, Rolle zu Rolle kompatiblen Prozess zur Herstellung großflächiger und kostengünstiger organischer elektronischer Elemente auf flexiblen Substraten integriert. Die weiteren Teile der Arbeit befassen sich mit dem Einsatz von natürlichen bzw. von der Natur inspirierten Materialien (Cellulose, trimethylsilylcellulose (TMSC), Indigoide) in organischen elektronischen Bauelementen. Diese Materialien wurden als dünne Isolator- (Cellulose, TMSC) oder halbleitende (Indigoide) Schichten in eine Inverter Schaltung integriert. Um das elektrische Verhalten der Inverter besser zu verstehen, wurden diese und die integrierten Transistoren hinsichtlich ihrer elektrischer Eigenschaften und Stabilität charakterisiert. Die Zugabe eines Photoinitiators zur TMSC ermöglichte sowohl eine positive als auch negative Photostrukturierung, welche in weiterer Folge erfolgreich in OTFTs angewendet wurde. Diese Arbeit leistet einen Beitrag zum besseren Verständnis der elektrischen Funktionsweise von OTFTs und Invertern. Die hier gezeigte hohe Leistungsfähigkeit der Bauelemente macht diese untersuchten Materialen zu aussichtsreichen Kandidaten für die Herstellung von hochwertiger, flexibler, nachhaltiger und kostengünstiger organischer Elektronik.The objective of this thesis is the evaluation of novel materials as functional layers in organic electronic devices. At the beginning of the thesis an intrinsically photocurable polymer poly(()endo,exo-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid, diphenylester) (PNDPE) is investigated on its suitability as polymer gate dielectric layer in organic thin film transistors (OTFTs). For that reason special attention was given to studies of the dielectric properties, dielectric-semiconductor interfacial properties, nature of charge carrier traps, semiconductor morphology and patterning behavior. Subsequently, the dielectric was integrated in a self-aligned, roll-to-roll compatible fabrication technique applicable for fast organic electronic devices on large area. The further parts of the thesis are dedicated to natural source based materials, namely cellulose, TMSC (trimethylsilylcellulose) and indigoides for the fabrication of organic electronic devices. For that reason these materials were integrated as thin insulating (cellulose/TMSC) or semiconducting (indigoids) layers in a complementary inverter technology. In order to understand the electrical behavior of the transistors and inverters, special attention was given to the investigation of the semiconductor-dielectric interface, the semiconductor morphology, the nature of traps, the influence of mobility levels on the inverter performance and its long-term stability. Finally, a positive as well as a negative tone photopatterning of cellulose is shown and successfully used in OTFTs. These findings will contribute to a deeper understanding of the functionality of thin film transistors, inverters, patternable dielectrics and the semiconductor/dielectric interface in OTFTs. The high device performance achieved in this work makes these materials promising candidates for future applications of high quality, flexible, fast processable, sustainable and cheap organic electronic devices and circuits.vorgelegt von Andreas PetritzZsfassungen in dt. und in engl. SpracheGraz, Univ., Diss., 2015OeBB(VLID)40892

Ultraflexible Organic Active Matrix Sensor Sheet for Tactile and Biosignal Monitoring

Abstract Flexible sensors are currently the subject of intensive research, as they allow cost‐effective and environmentally friendly production of large‐area, flexible, and when fabricated on ultrathin substrates, highly conformable devices. Among many intriguing applications, tactile and biosignal monitoring, where lightweight sensors with high wearing comfort are particularly interesting, is focused on here. The required spatiotemporal resolution of the signals is achieved by integrating the sensors in an active matrix configuration. Organic ferroelectric transducers of high uniformity, characterized, for example, by a sensitivity spread of only 1.5%, are combined with similarly uniform ultralow noise level organic thin film transistors operating below 5 V, showing, for example, a threshold voltage variation of just 0.13 V, in a 12 × 12 sensor array. The transistors transition frequency of up to 160 kHz (saturation range) and 17 kHz (linear range) allows for a high spatiotemporal resolution of ≈3 mm at a frame rate of 1400 fps. The thickness of only 2.8 µm renders the organic active matrix sensor sheet ultraflexible and therefore virtually imperceptible on the human skin. Real‐time monitoring of tactile modes in a subset of 8 × 3 pixels and of the pulse wave including heart rate and blood pressure using four sensors of the matrix is demonstrated

Critical Evaluation of Organic Thin-Film Transistor Models

The thin-film transistor (TFT) is a popular tool for determining the charge-carrier mobility in semiconductors, as the mobility (and other transistor parameters, such as the contact resistances) can be conveniently extracted from its measured current-voltage characteristics. However, the accuracy of the extracted parameters is quite limited, because their values depend on the extraction technique and on the validity of the underlying transistor model. We propose here a new approach for validating to what extent a chosen transistor model is able to predict correctly the transistor operation. In the two-step fitting approach we have developed, we analyze the measured current-voltage characteristics of a series of TFTs with different channel lengths. In the first step, the transistor parameters are extracted from each individual transistor by fitting the output and transfer characteristics to the transistor model. In the second step, we check whether the channel-length dependence of the extracted parameters is consistent with the underlying model. We present results obtained from organic TFTs fabricated in two different laboratories using two different device architectures, three different organic semiconductors and five different materials combinations for the source and drain contacts. For each set of TFTs, our approach reveals that the state-of-the-art transistor models fail to reproduce correctly the channel-length-dependence of the transistor parameters. Our approach suggests that conventional transistor models require improvements in terms of the charge-carrier-density dependence of the mobility and/or in terms of the consideration of uncompensated charges in the carrier-accumulation channel

Study of Pressure Distribution in Floor Tiles with Printed P(VDF:TrFE) Sensors for Smart Surface Applications

Pressure sensors integrated in surfaces, such as the floor, can enable movement, event, and object detection with relatively little effort and without raising privacy concerns, such as video surveillance. Usually, this requires a distributed array of sensor pixels, whose design must be optimized according to the expected use case to reduce implementation costs while providing sufficient sensitivity. In this work, we present an unobtrusive smart floor concept based on floor tiles equipped with a printed piezoelectric sensor matrix. The sensor element adds less than 130 µm in thickness to the floor tile and offers a pressure sensitivity of 36 pC/N for a 1 cm2 pixel size. A floor model was established to simulate how the localized pressure excitation acting on the floor spreads into the sensor layer, where the error is only 1.5%. The model is valuable for optimizing the pixel density and arrangement for event and object detection while considering the smart floor implementation in buildings. Finally, a demonstration, including wireless connection to the computer, is presented, showing the viability of the tile to detect finger touch or movement of a metallic rod

Fine-Tuning the Performance of Ultraflexible Organic Complementary Circuits on a Single Substrate via a Nanoscale Interfacial Photochemical Reaction

Flexible electronics has paved the way toward the development of next-generation wearable and implantable healthcare devices, including multimodal sensors. Integrating flexible circuits with transducers on a single substrate is desirable for processing vital signals. However, the trade-off between low power consumption and high operating speed is a major bottleneck. Organic thin-film transistors (OTFTs) are suitable for developing flexible circuits owing to their intrinsic flexibility and compatibility with the printing process. We used a photoreactive insulating polymer poly((±)endo,exo-bicyclo[2.2.1]hept-ene-2,3-dicarboxylic acid, diphenylester) (PNDPE) to modulate the power consumption and operating speed of ultraflexible organic circuits fabricated on a single substrate. The turn-on voltage (Von) of the p- and n-type OTFTs was controlled through a nanoscale interfacial photochemical reaction. The time-of-flight secondary ion mass spectrometry revealed the preferential occurrence of the PNDPE photochemical reaction in the vicinity of the semiconductor–dielectric interface. The power consumption and operating speed of the ultraflexible complementary inverters were tuned by a factor of 6 and 4, respectively. The minimum static power consumption was 30 ± 9 pW at transient and 4 ± 1 pW at standby. Furthermore, within the tuning range of the operating speed and at a supply voltage above 2.5 V, the minimum stage delay time was of the order of hundreds of microseconds. We demonstrated electromyogram measurements to emphasize the advantage of the nanoscale interfacial photochemical reaction. Our study suggests that a nanoscale interfacial photochemical reaction can be employed to develop imperceptible and wearable multimodal sensors with organic signal processing circuits that exhibit low power consumption