Over the last ten years, conjugated oligothiophenes have emerged as one of the largest families of organic semiconductors with potential applications in electronics devices. Thin film transistors (TFTs), photovoltaic solar cells, light-emitting diodes (LEDs), light modulators, photochromic switches and laser microcavities are some examples of devices that have been fabricated with oligothiophenes as the active materials. The key advantage of well-defined oligomers over their parent polymers is the high degree of molecular and crystalline ordering they can achieve. Polycrystalline and highly oriented thin films can be easily prepared from solution or by vacuum deposition. In many cases it has even been possible to grow single crystals and elucidate their X-ray structure. The aim of this review is to describe the structure of oligothiophene crystals and thin films and to explore its implications on the performances of various electronic devices.

Structural order in conjugated oligothiophenes and its implications on opto-electronic devices

Field-effect transistor with polythiophene thin film

The use of discrete organic compounds as active materials in
transistors is described, beginning with α-sexithiophene
(α-6T) and progressing to other thiophene oligomers and nonthiophene
semiconductors. Device operation, molecular design, synthesis, film
morphology and transport of holes and electrons are covered.

Organic molecular solids as thin film transistor semiconductors

Electrically conducting and thermally stable π-conjugated poly(arylene)s prepared by organometallic processes

Various π-conjugated copolymers constituted of π-excessive thiophene, selenophene, or furan units (Ar) and π-deficient pyridine or quinoxaline (Ar‘) units have been prepared in high yields by the following organometallic polycondensation methods:  (i) n X−Ar−Ar‘−X + n Ni(0)Lm → (-Ar−Ar‘)-n (X = halogen, Ni(0)Lm = zerovalent nickel complex), (ii) n X−Ar−X + n Me3Sn−Ar‘−SnMe3 → (-Ar−Ar‘)-n (palladium catalyzed), and (iii) a X−Ar−X + b X−Ar‘−X + (a + b)Ni(0)Lm → (-Ar)x(Ar‘)-y. Powder X-ray diffraction analysis confirms an alternative structure of a polymer prepared by the method ii. The copolymers have a molecular weight of 5.4 × 103 to 3.3 × 105 and an [η] value of 0.37 to 4.4 dL g-1. π−π* absorption bands of the copolymers generally show red shifts from those of the corresponding homopolymers, (-Ar)-n and (-Ar‘)-n, and the red shifts are accounted for by charge-transferred CT structures of the copolymers. For example, an alternative copolymer of thiophene and 2,3-diphenylquinoxaline gives rise to an absorp...

π-Conjugated Donor−Acceptor Copolymers Constituted of π-Excessive and π-Deficient Arylene Units. Optical and Electrochemical Properties in Relation to CT Structure of the Polymer

: X-ray scattering has been used to investigate the crystallinity and crystal structure of chemically coupled polythiophene. Heat treatment at elevated temperatures leads to significant increases in crystallinity (from approx. 35% as-synthesized up to approx. 56% after annealing at 380 C for 30 minutes) and coherence length indicative of chain growth and extension. Chemical analysis of the chain extended PT shows a major reduction in residual iodine content consistent with growth of the polymer chains to approximately 1200 thiophene rings. An initial model of the crystal structure of polythiophene is presented. (Author)

A. J. Heeger

Papers

X-ray scattering from poly(thiophene): crystallinity and crystallographic structure