Entry 01: Charge Transporting Molecules for Organic Electronics
1. Introduction
One thing that distinguishes organic electronics from traditional inorganic electronics is the use of organic molecules as charge transporting materials. These molecules are often called organic semiconductors. The advantages of organic semiconductors include easy processibility, light weight, and compatibility with flexible (plastic) substrates. More importantly, their electrical properties and solid-state morphologies can be tuned through molecular design and engineering.
Organic semiconductors are typically based on π-electron systems, which means most of them contain benzene or thiophene unit. Throughout the development organic electronics over the past few decades, a huge variety of organic semiconductors have been synthesized by chemists. Most of these molecules are used in three major applications: organic light-emitting diodes (OLEDs), organic thin-film transistors (OTFTs), and organic solar cells (OSCs). This entry focuses on charge transporting materials for OLEDs and OTFTs. We will see that different requirements for film properties, i.e. amorphous films for OLEDs vs. polycrystalline films for OTFTs, result in quite different molecular designs.
2. Charge transporting molecules for organic light-emitting diodes
One key requirement for the molecules used in OLEDs is that they should form stable amorphous films. This is because amorphous films exhibit isotropic and homogeneous properties without the presence of grain boundaries and pinholes. Although small molecules tend to crystallize at room temperature, chemists have discovered many successful design strategies to make molecules form stable amorphous films. Figure 1 summarizes some of the hole-transporting molecules for OLEDs. Although they seem symmetrical and planer on the screen, almost all of them are nonplaner in 3D. Some of the molecules are fairly large. The reason behind this is to increase the glass transition temperature (Tg) to make the films more thermally stable.
Fig. 1. Charge transporting molecules for OLEDs. Hexagons represent benzene rings, blue dots represent nitrogen atoms. Some substitutes have been omitted.
3. Charge transporting molecules for organic thin-film transistors
3.1. Molecular designs
One of the most important requirements for charge transporting molecules used in OTFTs is high charge carrier mobility. To archive this, molecules need to from ordered, crystalline structure in thin films, so that charge can be transported easily from one molecule to another. This requirement leads to dramatically different molecular designs from those for OLED molecules, as shown in Figure 2. Many molecules contain benzene or thiophene rings either linearly linked by single bonds or fused together, giving a rod-like shape. All of these molecules are conjugated π-electron systems, which means they are planar or close to planar.
Fig. 2. Charge transporting molecules for OTFTs. Hexagons represent benzene rings, pentagons with orange dots represent thiophene rings, gray spheres represent alkyl substitutes.
3.2. Crystal Structures of molecules for OTFTs
The reason for studying crystal structures of organic semiconductors is to understand the relationship between charge transport and solid-state packing of organic molecules. Two kinds of packing motifs are often found in the single crystals of organic semiconductors. The first is herringbone packing with edge-to-face interaction between molecules. A good example of herringbone packing is pentacene. The second is π stacking with face-to-face interaction. Rubrene adopts this kind of packing.
In general, the interaction between molecules in π-stacking crystals is stronger than that in crystals with herringbone packing. This stronger interaction often leads to higher mobility. Rubrene single crystal, for example, has a mobility of as high as 15 cm2V-1s-1. Unfortunately, π-stacking materials usually cannot form good thin films, which makes them unsuitable for OTFT application. On the other hand, many materials with herringbone packing can form thin films with good morphology. Pentacene, for example, forms polycrystalline films with a mobility of ~1 cm2V-1s-1. Designing new π-stacking materials that can form good thin films is the current challenge for chemists.
Fig. 3. Crystal structures of pentacene and rubrene. Pentacene has a herringbone structure, while rubrene has a π-stacking structure.