Designing Organic Semiconducting Materials: The Promise of Flexible Electronics

by Professor Seth C. Rasmussen

Introduction

Most organic polymers (i.e. plastics) are insulators and are often used to isolate metallic conductors from other conducting materials. However, studies beginning in the 1960s revealed that the electrical conductivity of conjugated organic polymers (Figure 1) can be controlled through oxidation or reduction¹. These initial discoveries have led to a modern class of organic materials with the conductivity of classical inorganic systems, but with many of the desirable properties of organic plastics, including mechanical flexibility and low production costs^2-6.

Figure 1. Commonly studied conjugated polymers.

Such organic materials, often called synthetic metals^7,8, are semiconductors in their neutral state and exhibit increased conductivity upon oxidation or reduction. As a result, they have received considerable fundamental and technological interest, leading to their current use in such applications (Figure 2) as sensors, organic field effect transistors (OFETs), organic photovoltaic (OPV) devices, electrochromic devices, and organic light emitting-diodes (OLEDs)^2-6. In addition, the flexible, plastic nature of the organic materials used as the active layers in such electronic devices has led to the realistic promise of flexible electronics in the near future^9-12.

Figure 2. Free standing conjugated polymer film (A) and examples of electrochromics (B), OPV devices (C)13 and OLED devices (D)14.

Molecular Design

One advantage of utilizing conjugated polymers for technological applications is the ability to tune the material properties at the molecular level⁴. This is typically accomplished through synthetic modification of the monomeric units, the combination of dissimilar units to make copolymeric systems, or in some cases through post-polymerization processes. The most promising organic semiconducting materials combine a number of critical properties, including processiblity, stability, conjugation length, band gap energy, and charge mobility. The ability to modify these properties is of the utmost importance for the efficient application of these materials. For example, soluble, stable systems are required for their physical processing and incorporation in technological devices, while the remaining properties determine the effectiveness of the polymer as an electronic material.

Physical Properties

The stability of conjugated polymers can be divided into electrochemical stability and stability of the polymer to chemical or photochemical reactions^15,16. For example, neutral polyacetylene is considered to be electrochemically stable, but is susceptible to triplet oxygen insertion, resulting in chemical decomposition. Due to both environmental stability and ease of synthetic modification, polythiophenes are thought to be one of the most versatile classes of conjugated polymers and thus exhibit perhaps the greatest amount of synthetic diversity^3-6.

Due to their rigid nature, conjugated materials exhibit limited solubility in comparison to saturated organic polymers. As a result, the majority of unfunctionalized systems are insoluble. Beginning in 1986, however, it was found that substitution of alkyl chains (butyl or longer) in the β-position of thiophenes resulted in soluble and fusible polymers^5,6. The application of such flexible side chains to conjugated polymers in general has since resulted in large numbers of soluble materials that can be easily solution processed. It should be noted, though, that while such methods can be used to enhance solubility, there are a number of unintended consequences, the most significant of which is the addition of steric effects that can twist the polymer backbone out of planarity^4,6.

Optical and Electronic Properties

The optical and electronic properties of these materials are determined by the extent of conjugation along the material backbone and thus a high degree of conjugation is essential. Conjugation length can be determined by several factors, of which the simplest are the overall length of the conjugation path and the molecule's planarity. As the conjugation length is dependent on the overlap of orbitals between neighboring π-bonds, it can be influenced by torsional strain along the backbone and significant deviations from planarity (i.e. > 40° between units)¹⁷ result in decreased conjugation^4,6. In addition, as the potential conjugation path increases with molecular length, small molecules generally exhibit shorter conjugation lengths than polymeric materials. Extension of the conjugation length with molecular length is not infinite, however, as delocalization limits result in a maximum effective conjugation length (~20-30 rings in polythiophene)^18,19. Further extending chain length can affect physical properties, but has little effect on electronic and optical properties.

Directly related to conjugation is the material's band gap (E_g), which is the energy between the filled valence and empty conduction bands and thus corresponds to the HOMO-LUMO gap of the solid state material^4,20,21. As a result, it determines the lowest energy absorbance and the energy of any emission. Bond length alternation is considered to have one of the largest effects on the E_g of organic materials (greater alternation gives larger E_g)^22,23. Materials based on aromatic rings, however, differ in that they have non-degenerate resonance structures (Figure 3). Here, the aromatic form has a larger E_g, but is more stable and thus represents the ground-state structure. For these systems, it is thought that increasing quinoidal character in the ground state has a greater effect than limiting bond length alternation^22,23.

Figure 3. A) Aromatic and quinoid resonance structures of polythiophene; B) Low band gap polymers due to enhanced quinoidal character4,24-26.

Additional factors include backbone planarity, the heteroatom in heterocyclic systems, and monomer aromaticity (Figure 4)^4,15,27-30. The effect of the heteroatom is thought to strongly correlate with its electron affinity, with higher affinities resulting in lower E_g values²⁹. It has also been proposed that monomer aromaticity determines the confinement potential of the π-electrons. As the confinement within the ring becomes stronger, the delocalization length along the backbone decreases, resulting in larger E_g values³⁰. The last factor to consider is interchain coupling that can occur via stacking in the solid state, resulting in increased electron delocalization and a reduction in E_g . Reduced molecular ordering can increase the spatial distance between polymer chains, reducing interchain coupling and increasing E_g. Such molecular ordering can also directly affect the materials charge mobility.

Figure 4. Illustrative examples of molecular effects on band gap.

In conclusion, the combination of all the above factors determines the resultant material properties and the key to designing next generation organic materials is the enhancement of one or more of these factors. However, as many of these factors are interrelated, it can be challenging to selectively modify one factor without affecting others.

References

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Copyright AZoM.com,Professor Seth C. Rasmussen (North Dakota State University)