Organic Thin-Film Transistor Applications

Materials to Circuits

Brajesh Kumar Kaushik,Brijesh Kumar,Sanjay Prajapati,Poornima Mittal

• Publisher: CRC Press
• Year: 2016
• ISBN:
  9781315352596
• Edition: 1
• Topic: Technologie et ingénierie
• Subtopic: Microélectronique

Text provides information about advanced OTFT (Organic thin film transistor) structures, their modeling and extraction of performance parameters, materials of individual layers, their molecular structures, basics of pi-conjugated semiconducting materials and their properties, OTFT charge transport phenomena and fabrication techniques. It includes applications of OTFTs such as single and dual gate OTFT based inverter circuits along with bootstrap techniques, SRAM cell designs based on different material and circuit configurations, light emitting diodes (LEDs). Besides this, application of dual gate OTFT in the logic gate, shift register, Flip-Flop, counter circuits will be included as well.

I

Organic Device Physics and Modeling

Introduction

1

1.1 INTRODUCTION

Over the last decade, there has been a significant increase in the efforts to implement and develop electronic components on flexible and stretchable substrates. With the advent of soluble organic semiconductors (OSCs) and conductors, printed electronics became feasible. Printed electronics is usually linked with organic electronics where its characteristic feature is the usage of organic materials for realizing devices and circuits. The devices, incorporating organic materials are on the verge of commercialization. Organic devices that have emerged over several generations demonstrate advantages in terms of mechanical flexibility, lightweight, low cost, and straightforward fabrication at low temperature that allow cost-efficient production. Cost-effective fabrication of these devices on flexible substrates can eventually lead to huge benefits on various fronts.

Organic thin-film transistors (OTFTs) are predicted to have a range of imperative and high-end applications, such as flat panel display, radio-frequency identification (RFID) tag [1], sensors [2], static random access memory (SRAM) [3], e-paper [4], solar cell [5], differential amplifier [6], ring oscillator [7], and flexible integrated circuits [8]. Moreover, these transistors have turned out to be a promising backplane driver in organic light-emitting diode (OLED)-based large area flexible displays [9]. Cost-effective fabrication of OTFTs on the flexible substrate can ultimately benefit the RFID domain to replace the bar code technology intended for product and inventory identification. Recently, researchers have found them capable enough in realizing the circuits for smart textiles due to their good bending stability. Perhaps in the near future, technology may be developed through combined efforts in the areas of electronics, chemistry, physics, and material science leading to some modern and high-speed applications in graphics, animation, and the video games.

FIGURE 1.1 Cost versus performance of organic and inorganic semiconductors.

The performance of organic transistors is lower in comparison to the conventional transistors due to inferior mobility (μ); however, the production cost and flexibility of silicon-based devices are obvious constraints. Organic transistors provide an ideal solution as they are inexpensive, flexible, and promising enough to realize large-area electronic circuits. The OTFTs fabricated on flexible substrate have demonstrated comparable or even improved characteristics in comparison to hydrogenated amorphous silicon (a-Si:H)-based TFTs. On steady improvement, the mobility of organic transistors has been augmented by several orders, now in excess of 15 cm²/Vs [10] for single crystal and 3.2 cm²/Vs [11] for thin film.

With optimization of fabrication methodology and synthesis of novel materials, the mobility can be undoubtedly further increased. For a comparative study of organic and inorganic semiconductors applications, Figure 1.1 compares their performance and cost characteristics. Though, the performance of the organic transistor is not comparable to the silicon transistor, it still finds utilization in certain innovative applications that are not possible with conventional semiconductors, or if feasible they are too expensive to be realized commercially.

1.2 ORGANIC SEMICONDUCTOR MATERIALS FOR ORGANIC DEVICES

Initially organic polymers with carbon backbones were insulators and used for encapsulation. In 1977, discovery of highly conductive polymers marked the beginning of a new field of electronics called organic electronics. Organic semiconductors can be classified into two categories: polymers and small molecules. The mobility of solution-processed polymer semiconductors is lower than that of small molecules deposited by vacuum evaporation technique [12].

1.2.1 POLYMERS

During the 1960s, all organic polymers were considered to be nonconducting. They were used for packaging and insulation. Later, in 1977 Shirakawa et al. discovered the highly conductive polymer polyacetylene using oxidative doping with iodine [13]. Improvement in electrical characteristics was observed after the discovery of polyphenylene-vinylene (PPV) and polythiophene (PT) group conducting polymers [14]. The highest mobility of 0.1 cm²/Vs was reported for the solution-processed regioregular material poly(3-hexylthiophene) (P3HT) in comparison to poly(3-alkylthiophene) (P3AT) and poly(3-octylthiophene) (P3OT) conducting polymers [15,16].

Between 1998 and 2001, improvement in the mobility of P3HT was achieved by selecting appropriate solvents, thermal annealing, optimization of chain length, and substrate surface treatment before deposition of polymer films [17–19]. In 2000, Sirringhaus et al. demonstrated an air-stable polymer with mobility of 0.01–0.02 cm²/Vs called poly(9,9′-dioctyl-fluorene-co-bithiophene) (F8T2), using interface preparation and high-temperature annealing [20].

In 2004, Ong et al. developed poly(3,3′″-dialkyl-quaterthiophene)s (PQTs) with increased oxygen resistance, solution processability, and self assembly [21]. In 2006, McCulloch et al. analyzed poly(2,5-bis(3-alkylthiophen-2-yl)) thieno[3,2-b]thiophene) (PBTTT) displaying higher mobility and stability [22]. In 2010, Zschieschang et al. fabricat...