Electron and ambipolar transport in organic field-effect transistors.

Acenes have long been the subject of intense study because of the unique electronic properties associated with their pi-bond topology. Recent reports of impressive semiconductor properties of larger homologues have reinvigorated research in this field, leading to new methods for their synthesis, functionalization, and purification, as well as for fabricating organic electronic components. Studies performed on high-purity acene single crystals revealed their intrinsic electronic properties and provide useful benchmarks for thin film device research. New approaches to add functionality were developed to improve the processability of these materials in solution. These new functionalization strategies have recently allowed the synthesis of acenes larger than pentacene, which have hitherto been largely unavailable and poorly studied, as well as investigation of their associated structure/property relationships.

The larger acenes: versatile organic semiconductors.

2.3. Medical Devices and Sensors 9 2.4. Radio Frequency Applications 10 3. Materials 12 3.1. Organic Electronics Materials 12 3.2. Semiconducting Polymer Design 13 3.3. Poly(3-alkylthiophenes) 14 3.4. Poly(thieno(3,2-b)thiophenes 15 3.5. Benchmark Polymer Semiconductors 15 3.6. High Performance Polymer Semiconductors 15 4. Device Stability 16 4.1. Bias Stress in Organic Transistors 17 4.1.1. Bias Stress Characterization 17 4.1.2. Bias Stress Mechanism 18 4.2. Short Channel Effects in Organic Transistors 19 5. Materials Patterning and Integration 20 6. Conclusions 22 7. Acknowledgments 22 8. References 22

Materials and applications for large area electronics: solution-based approaches.

Organic transistors and circuits show great promise for the realization of futuristic roll-up displays, adaptive sensors for humanoid robots and ubiquitous radio-frequency identification tags. But today's organic circuits require operating voltages of 15 to 30 volts (10 to 20 batteries' worth), and they draw enough power to drain those batteries in a day. To overcome this major hurdle, Hagen Klauk et al. have developed a method of fabricating organic circuits that run on a single 1.5-volt battery for several years. The key to the method is the use of a layer of an insulating organic material just one molecule thick; although the layer is very thin, it leaks only a small amount of current, while it provides for a large capacitance. Two different types of organic semiconductors are used to fabricate transistors, logic gates and ring oscillators. A report of the development of organic electronic circuits, which require only a single 1.5V battery and last for several years. The main ingredient is the use of a single layer of an insulating organic material. Although the layer is very thin, it leaks only small amount of current, while providing for a large capacitance. The prospect of using low-temperature processable organic semiconductors to implement transistors, circuits, displays and sensors on arbitrary substrates, such as glass or plastics, offers enormous potential for a wide range of electronic products1. Of particular interest are portable devices that can be powered by small batteries or by near-field radio-frequency coupling. The main problem with existing approaches is the large power consumption of conventional organic circuits, which makes battery-powered applications problematic, if not impossible. Here we demonstrate an organic circuit with very low power consumption that uses a self-assembled monolayer gate dielectric and two different air-stable molecular semiconductors (pentacene and hexadecafluorocopperphthalocyanine, F16CuPc). The monolayer dielectric is grown on patterned metal gates at room temperature and is optimized to provide a large gate capacitance and low gate leakage currents. By combining low-voltage p-channel and n-channel organic thin-film transistors in a complementary circuit design, the static currents are reduced to below 100 pA per logic gate. We have fabricated complementary inverters, NAND gates, and ring oscillators that operate with supply voltages between 1.5 and 3 V and have a static power consumption of less than 1 nW per logic gate. These organic circuits are thus well suited for battery-powered systems such as portable display devices2 and large-surface sensor networks3 as well as for radio-frequency identification tags with extended operating range4.

Ultralow-power organic complementary circuits

Over the past 20 years, organic transistors have developed from a laboratory curiosity to a commercially viable technology. This critical review provides a short summary of several important aspects of organic transistors, including materials, microstructure, carrier transport, manufacturing, electrical properties, and performance limitations (200 references).

Organic thin-film transistors.

The properties of the dielectric strongly influence the performance of organic thin-film transistors. In this letter, we show experimental results that quantify the influence of the roughness of the dielectric on the mobility of pentacene transistors and discuss the cause of it. We consider the movement of charge carriers out of the “roughness valleys” or across those valleys at the dielectric–semiconductor interface as the limiting step for the roughness-dependent mobility in the transistor channel.

/pdf/influence-of-the-dielectric-roughness-on-the-performance-of-hu95e9xwdz.pdf

Influence of the dielectric roughness on the performance of pentacene transistors

Amain focus of research on organic semiconductors is their potential application in passive organic radio-frequency identification (RF-ID) tags. First prototypes working at 125 kHz have been shown by industrial research groups1. However, to be commercially viable, the organic RF-ID tag would need to be compatible with the base-carrier frequency of 13.56 MHz (ref. 2). High-frequency operation has been out of reach for devices based on organic semiconducting material, because of the intrinsically low mobility of those materials. Here, we report on a rectifier based on a pentacene diode that can rectify an incoming a.c. signal at 50 MHz. At 14 MHz, a rectified voltage of 11 V for an a.c. voltage with a peak-to-peak amplitude of 36 V has been achieved. On the basis of those results, we estimate the frequency limits of an organic diode showing that even the ultra-high-frequency band at around 800 MHz is within reach.

50 MHz rectifier based on an organic diode

In this article, we compare the direct current (dc) and high-frequency performance of two different organic diode structures, a vertical diode and an organic field effect transistor (OTFT) with shorted drain-gate contact, regarding their application in a rectifying circuit. For this purpose, we fabricated both diode structures using the organic semiconductor pentacene. dc measurements were performed showing a space-charge-limited current mobility of more than 0.1cm2∕Vs for the vertical diode and a field effect mobility of 0.8cm2∕Vs for the OTFT with shorted source-drain. High-frequency measurements of those diode structures in a rectifier configuration show that both types of diodes are able to follow the base-carrier frequency of 13.56MHz which is essential for viable radio-frequency-identification (rf-ID) tags. Based on those results we evaluate the performance limits and advantages of each diode configuration regarding their application in an organic rf-ID tag.

/pdf/comparison-of-organic-diode-structures-regarding-high-4yuzm8ujoq.pdf

Comparison of organic diode structures regarding high-frequency rectification behavior in radio-frequency identification tags

We have developed a method for integrating n- and p-type organic thin-film transistors (OTFTs) on the same substrate. An integrated shadow mask was used for the n- and p-type semiconductor patterning. The integrated shadow mask can be aligned with submicron accuracy relative to the OTFT substrate. This allows for the integration at transistor level of n- and p-type OTFTs on the same substrate. A complementary inverter was fabricated, showing excellent performance while operating at a supply voltage of 2V.

/pdf/low-voltage-complementary-organic-inverters-24tu99xgb8.pdf

Low voltage complementary organic inverters

We have developed a simple and efficient method for patterning small molecule semiconductors for applications in the field of organic electronics. In our approach, a profile is created using a single layer of photoresist, defining the regions where the organic semiconductor is to be deposited. Subsequent deposition of a small molecule semiconductor results in a discontinuity of the semiconductor film at the photoresist edge. The resulting transistor characteristics have an off current that is systematically below 1pA. We demonstrate both p-type and n-type organic thin-film transistors using this method, using pentacene and copper hexadecafluorophthalocyanine (F16CuPc), respectively.

/pdf/integrated-shadow-mask-method-for-patterning-small-molecule-2z30y1dnb0.pdf

Stijn De Vusser

Papers

Influence of the dielectric roughness on the performance of pentacene transistors

50 MHz rectifier based on an organic diode

Comparison of organic diode structures regarding high-frequency rectification behavior in radio-frequency identification tags

Low voltage complementary organic inverters

Integrated shadow mask method for patterning small molecule organic semiconductors