Organic Electronics II

More Materials and Applications

Hagen Klauk

• Publisher: Wiley-VCH
• Year: 2012
• Print ISBN:
  9783527326471
• Edition: 1
• eBook ISBN:
  9783527640225
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Like its predecessor this book is devoted to the materials, manufacturing and applications aspects of organic thin-film transistors. Once
again authored by the most renowned experts from this fascinating and fast-moving area of research, it offers a joint perspective both
broad and in-depth on the latest developments in the areas of materials chemistry, transport physics, materials characterization, manufacturing technology, and circuit integration of organic transistors. With its many figures and detailed index, this book once again also serves as a ready reference.

Part I

Materials

Chapter 1

Organic Semiconductor Materials for Transistors

David Ian James, Jeremy Smith, Martin Heeney, Thomas D. Anthopoulos, Alberto Salleo, and Iain McCulloch

1.1 General Considerations

Recent advances in the electrical performance of organic semiconductor materials position organic electronics as a viable alternative to technologies based on amorphous silicon (a-Si). Traditionally a-Si-based transistors, which are used as the switching and amplifying components in modern electronics [1], require energy intensive batch manufacturing techniques. These include material deposition and patterning using a number of high-vacuum and high-temperature processing steps in addition to several subtractive lithographic patterning and mask steps, limiting throughput. Although this allows for the cost of individual transistors to be extremely low because of the high circuit density that can be obtained, the actual cost per unit area is very high. Alternatively, organic semiconductors can be formulated into inks and processed using solution-based printing processes [2–5]. This allows for large-area, high-throughput, low-temperature fabrication of organic field-effect transistors (OFETs), enabling not only a reduction in cost but also the migration to flexible circuitry, as lower temperatures enable the use of plastic substrates. The potential applications for these OFETs are numerous, ranging from flexible backplanes in active matrix displays to item-level radiofrequency identification tags.

OFETs are typically p-type (hole transporting) devices that are composed of a source and drain electrode connected by an organic semiconductor, with a gate electrode, insulated from the organic semiconductor via a dielectric material, as shown in Figure 1.1b. Holes are injected into the highest occupied molecular orbital (HOMO) of the organic semiconductor upon application of a negative gate voltage. The holes migrate to the accumulation layer, which forms at the semiconductor interface with the dielectric, and are transported between the source and drain upon application of an electric field between the two. Modulation of the gate voltage is used to turn the transistor ON and OFF, with the ON current and voltage required to turn the device on being figures of merit for the electrical performance of the device. The performance of the transistor is also governed by the charge carrier mobility of the semiconductor, which should be high to ensure fast charging speeds.

Figure 1.1 (a) Simple diagram of active matrix backplane circuitry and (b) cross section of corresponding TFT and pixel architecture.

In displays, OFETs can act as individual pixel switches in the backplane active matrix circuitry, as shown in Figure 1.1a. This technology is currently being used commercially in small-sized electrophoretic displays (EPDs), marketed as e-paper [6], to charge both the pixel and the storage capacitor. Active matrix backplanes are found in both liquid crystal displays (LCDs) and organic light emitting diode (OLED) displays, where a transistor also provides current to the emitting diode element. An advantage of the EPD effect is that the pixels are reflective to ambient light, which allows the pixel transistor to occupy the majority of the area underneath the pixel. This maximizes the transistor width, enabling more current to be delivered to the pixel, resulting in lower mobility specifications being required from the semiconductor. For small-sized devices (

10 cm diagonally) with low resolutions and low refresh rates, the mobility required is in the region of 0.01 cm² V⁻¹ s⁻¹, which is well within the capabilities of both polymer and small molecule semiconductors. In comparison, medium- to large-sized LCDs commonly used for monitor and television displays require semiconductor mobilities in excess of 0.5 cm² V⁻¹ s⁻¹, and currently employ a-Si or polysilicon for higher-resolution displays. EPDs are also bistable, as once the pixel and the storage capacitor are charged, no additional power is needed to maintain the image. This minimizes the duty cycle load of the transistor, thus extending the lifetime. One problem with EPDs is that it is possible for ionic impurities within the liquid EPD cell to facilitate current leakage from the capacitor, which means that higher charge carrier mobilities are required than would be expected and thus high-purity electrophoretic inks are required to reduce the current demands of the display effect.

The function of the transistor in a LCD is to apply an electric field across the pixel, thus switching the direction of the optical axis of the liquid crystals, which therefore generates the image. As the display operates in the transmissive mode, the transistor is directly in the path of the light source, and so must be small to maximize the aperture ratio of the pixel and thus increase the efficiency of light output. However, the use of smaller transistors means that the organic semiconducting material within the transistor needs to have a higher charge carrier mobility than that of the materials used in EPDs.

OLED displays have the potential to be fabricated using high-throughput printing techniques such as gravure or ink jet. Using organic transistors will allow for the complete integration of both front- and backplane fabrication processes. Top emitting devices, in which the OLED frontplane cathode is transparent, can be fabricated, allowing the OFET to be positioned underneath the emissive layer. Thus, the OFETs can be larger per pixel than the transistors used in LCDs with equivalent-sized pixels. However, as the current output from the transistor dictates the brightness of the pixel, the transistor to transistor uniformity must be very tight. Additionally, multiple OFETs are needed per OLED pixel, requiring OLED OFETs to be smaller than the OFETs used in EPDs, where only one OFET is required per pixel. This leads to the need for higher-mobility organic semiconductors as well as reduced transistor to transistor anisotropy to avoid issues of color shifts from differential pixel aging effects and nonuniform pixel brightness.

Higher-mobility semiconductor materials are also required, as future demand for larger screen sizes, better resolution, and faster refresh rates for video rate displays will lead to the need for higher ON currents as a result of the larger number of rows and columns, as well as the requirement for faster pixel charging speeds. The development of these materials is discussed in the next section.

1.2 Materials Properties of Organic Semiconductors

Organic semiconductors are based on the fact that the sp² hybridization of carbon in a double bond leaves a p_z orbital available for π bonding. The electrons in the π bond can be delocalized via conjugation with neighboring π bonds, thus giving rise to charge carrier mobility. For this reason, the majority of organic semiconductors are composed of aromatic units linked together, allowing π orbital conjugation along the length of the molecule. Charge transport within both small molecule and polymeric organic semiconductors generally occurs via a thermally activated hopping mechanism, and in an OFET, this occurs along the plane of the substrate, propagating within a thin layer of semiconductor only a few molecules thick at the dielectric interface. Thus, the semiconductor at this interface must be highly ordered into closely packed organized π stacks with correctly oriented and interconnected domains as illustrated in Figure 1.2. This can be achieved by utilizing coplanar aromatic molecules, which form a highly crystalline thin film microstructure domain leading to high charge carrier mobility.

Figure 1.2 Schematic representation of (a) misaligned and poorly connected lamellar domains and (b) coaligned domains. Morphology in (b) leads to more optimal charge transport.

Most high-performing semiconducting polymers exhibit a crystalline phase, melt transition, and amorphous phase on heating. The temperature at which the phase transition appears is dependent on the Gibbs free energy of each phase with respect to temperature, with the lowest free energy phase prevailing. Aromatic planar extended rigid-rod-type polymers have a predisposition to exhibit a liquid crystalline phase, due in part to their calamitic conformation. This phase is often masked by the lower free energy of the crystalline or amorphous phase. However, for some polymers, a liquid crystalline phase occurs between the crystalline and the isotropic melt phases (Figure 1.3). Annealing of the polymer within the liquid crystalline phase produces highly ordered and aligned crystalline thin films, which is desirable for high charge carrier mobilities. So in order to design polymers that incorporate a liquid crystalline phase, the entropy (dg/dT ) of the isotropic phase must be decreased. This can be achieved by increasing the stiffness of the polymer backbone by the use of coplanar aromatic molecules, which decreases the disorder of the melt (decreased slope of dg/dT ), allowing the liquid crystalline phase to appear.

Figure 1.3 Effect of stiffening polymer backbone on the polymer-phase free energy.

The molecular weight of polymers also has an influence on the charge carrier mobility [7]. Increasing the molecular weight has been shown [8, 9] to be beneficial up to a plateau region, a...

Table of contents

• Cover
• Related Titles
• Title Page
• Copyright
• Preface
• List of Contributors
• Part I: Materials
• Part II: Manufacturing
• Part III: Applications
• Index