Organic Semiconductors

Type of physical science: Chemistry

Field of study: Chemical compounds

Organic solid substances possess an electronic conductivity which is much higher than insulators (whose conductivity is below 10^-14 per ohm per centimeter) but much lower than metals such as copper, aluminum, and nickel (whose conductivity ranges from 10 to the power of 2 to 10⁸ per ohm per centimeter). The conductivity of organic semiconductor materials generally ranges from 10^-2 to 10^-14 per ohm per centimeter.

Overview

Metals have a very high electronic conductivity, which means that electrons can easily move through them. Solid materials that have much less electronic conductivity compared to metals but higher electronic conductivity compared to insulators (through which electrons cannot move) are called the semiconductors. Semiconductors have two bands: the valence band, which has lower energy, and the conduction band, which has higher energy. These bands are separated by a difference of energy which is called an "energy gap" or "band gap." The lower is the band gap, the higher is the conductivity of semiconductors. Semiconductors are mainly inorganic solids.

Organic solids that exhibit electronic conductivity are called organic semiconductors.

Organic solids are generally nonconductors of electricity (that is, insulators), but some organic molecular solids that are formed out of unsaturated organic molecules exhibit semiconductivity.

In organic semiconductors, the band gaps are generally very high and, consequently, the electronic conductivity is much lower than that of inorganic semiconductors.

Most organic solids are insulators and do not exhibit electronic conductivity, which results from the fact that it is extremely difficult to purify organic solids so that they will exhibit the electronic conductivity of semiconductors. For example, a single crystal of anthracene organic solid obtained by chromatographic purification (the process of passing a substance through layers of impurity absorbers) plus sublimation (the direct transformation of solid to gas) and final zone refining (controlled melting) prior to crystallization exhibits a final impurity concentration on the order of one in one million molecules. The spectroscopic analysis of single crystals of pthalocyanine purified by sublimation in a vacuum four times reveals inorganic impurities on the order of one in one-tenth of a million molecules.

The only impurities that can be expected to affect the electronic conduction properties of an organic solid to a major degree are those that are easily transformed to ions, such as metal atoms to metal ions--for example, a copper atom to a copper ion (which has two fewer electrons than the copper atom). Such easily ionizable impurities can contribute to an increase in the conductivity of organic semiconductors. One can also easily understand conductivity from the point of view of the band structure in molecular crystals (for example, organic semiconductors).

The electrical properties of organic semiconductors are affected by light; therefore, it is often necessary to carry out the purification processes in the darkness or in subdued light. The electronic conductivity of organic semiconductors is thought to arise by various mechanisms such as photoconductivity (electronic conduction under illumination of light), polaron transfer, and the exciton transfer of photoexcited conductivity.

Although objections were raised to the application of band theory (the consideration of conduction and valence bands) to molecular crystals (organic semiconductors), band theory has been used to make predictions concerning the mobility of charge carriers (hence, the conductivity) in such crystals that comply with the experimental results. According to band theory, the electrons are thermally excited to the conduction band of the crystal and generate "holes" in the lower-energy band, that is, the valence bands. There are excess electrons in the conduction band of n-type organic semiconductors and excess holes in the valence band of p-type organic semiconductors. In the former, electrons act as the majority carriers of charge and holes act as the minority carriers of charge. Thus, electrons and holes carry the charge in the organic semiconductor.

Polaron theory was originally developed mainly for inorganic crystals, but it has also been applied to organic semiconductors. The difference between the ordinary band theory and the polaron theory lies in the fact that, in the ordinary band theory, the interactions between the electrons and the vibrations in the nuclei are treated as perturbations of the motion of the electrons in the bands, whereas in the polaron theory, the bands of energy are formed only after the electron has interacted with the local vibrations.

Another mechanism, called electron hopping, is also operative in organic semiconductors. In this mechanism, electrons hop from one molecular site to another and thus conduct electricity.

When organic crystals are exposed to a light source, the crystal starts to exhibit conductivity or semiconductivity. Such conductivity is termed "photoconduction." The photoconduction of organic semiconductors is generally explained in terms of exciton motion.

Excitons are the loosely bound electron-hole pairs that move in the crystal at the excitation energy and then dissociate at the interface or in crystal dislocations.

Organic semiconductors can be fabricated easily as thin films on conducting substrates by using various methods, such as the dip-spinning method and the chemical vapor deposition method. The conductivity of the film can be improved easily by proper doping (the addition of a small amount of guest atoms or molecules in the solids) with a suitable dopant such as iodine. If uniform crystallites are formed in the film, then the conductivity of the organic semiconducting film improves. The conductivity of organic semiconductors can be greatly improved by proper doping with a guest element. P-type organic semiconductors can be improved by doping with highly electronegative elements such as halogens like chlorine, bromine, and iodine.

The properties of organic semiconductors--band gaps, conductivity, the ability to absorb and reflect light, and the refractive index--can be tailored by the proper derivatization (the addition of a specific organic group or groups) of organic molecules. There are also many organic molecules that could be derivatized or doped to semiconducting molecules with optimum conductivity, band gaps, and carrier concentration.

Various types of organic semiconductor molecules exist, such as the organic-dye molecules (phthalocyanine, indigo, rudamin B, pyrelene), the organic polymers (polythiophene, polyaniline, polyindole, polyfuran), and organometallic compounds (ruthenium bipyridines).

Applications

Organic semiconductors are a new type of semiconductor material. They have not been used widely as yet, but many possibilities are opening up for their application. The application of most inorganic semiconductors could be easily replaced by organic semiconductors; as a result, the cost and weight of electronic devices will be reduced tremendously. The possible applications are in transistors, radios, televisions, audio-video systems, electronic devices, solar cells, and solar calculators, among other things.

The semiconducting organic dyes are potential candidates for use in thin-film solar cells. Organic-dye semiconductors can be used in the form of thin films because of their high degree of light absorbability in the small distance from the surface of the film to its interior. The advantage of the use of thin film is that it will be inexpensive and lightweight. The devices that will be fabricated using organic semiconductors will be much lighter than those made of inorganic semiconductors. Thus, great possibilities exist for organic semiconductors, or even organic conductors, to be used in future spaceships, space shuttles, and space stations, which need lightweight materials.

The conductivity of some organic molecules can be improved to metallic conductivity or even superconductivity by proper π-bond type conjugation. Thus, long-distance wiring could be carried out by using organic conducting wires. If wiring that is composed of organic conductors is manufactured in space ships, then the weight of wiring will be reduced by an even greater extent.

The concept of molecular electronics is linked to the use of organic semiconductors, which produce mainly molecular solids and liquid crystals. Such molecular solids will be widely used to construct miniature and lightweight devices.

Solar cells can be constructed by the vacuum deposition of organic semiconductors such as phthalocyanine and pyrelene, and these cells show a high photocurrent density.

"Electrochromism" can be defined as the change of color of the film after the application of electrical potential from an external source. As a result, electrochromic organic semiconductors will have potential applications in car windows, where the windows will be covered by a thin film of transparent organic-dye semiconductors. By adjusting the external voltage of the thin film of the organic semiconductor, the tint of the windows can be changed when needed.

Some interesting organic photoconductive systems have been prepared in the form of a solution of an organic semiconductor in an inert matrix. The photoconductor films contained less than 1 percent by weight of triphenyl methane or benzolphenone dye in an air-soluble plastic matrix--for example, a vinyl acetate-vinyl chloride copolymer. Photoconductors have immense applications in electronic devices and specifically in some light-sensitive sensor devices.

Some organic semiconducting dyes (for example, polythiophene) also exhibit an electrochromic effect. Since some organic dyes are highly photoconducting, they are of considerable practical importance as sensitizers for photographic emulsions.

Organic semiconductors may offer attractive possibilities in the wide field of power conversion. Remarkably high values of photovoltage have been attained in silver iodide and cadmium sulfide dyestuff photoelements.

The process of spectral sensitization is perhaps the oldest practical application of organic semiconductors. Spectral sensitization, usually by dyes, extends the spectral response of photoeffects to wavelengths of light that are substantially longer than the wavelength limit obtained in the absence of a sensitizer.

Another important application of organic semiconductors is in the solid-state electrochemical battery system, where an organic semiconducting solid-state charge transfer complex can be used as an electrolyte. High energy densities and conversion efficiencies of 72 percent have been attained for solid-state power sources.

Thin films of organic semiconductors may be applied to modify the contact potential between the mating surface. Also, the electrolytic decomposition of water by the immersion of a photovoltaic semiconductor in the liquid under illumination by solar light has been demonstrated. This process offers interesting chemical possibilities.

Context

Studies on organic semiconductors appeared as early as 1888, when the existence of the photovoltaic effect in dye films irradiated with ultraviolet light was observed. In 1906, the photoconductivity of anthracene was reported. In 1919, the photoconductivity of dyed collagen films was studied, and such studies have continued in numerous publications since 1941, particularly after Albert Szent-Gyorgyi's studies on their biological implications.

Early electrical work in the field was done on phthalocyanine in 1948. The first studies on organic semiconductor single-crystal phthalocyanines were conducted by Felix Gutmann and an associate in 1957. Lawrence E. Lyons concentrated on the study of optical spectroscopy and later on the electrical measurement of organic molecular crystals.

Research on organic semiconductors represents a meeting place of physical methods, organic chemicals, and solid-state theory. Solid-state concepts, developed primarily for the understanding of inorganic semiconductors (bonded covalently or ionically), are needed for the understanding of organic semiconductors. The field is still in a state of flux, both theoretically and experimentally; however, much progress was made in the 1980's in an attempt to understand the solid state and the origin of electrical properties.

The future of the utilization of organic semiconductors lies in the field of biochemistry and biology, since so many biochemical problems are linked to charge and energy transfer over long distances through relatively weak bonds, but highly organized systems. The energy transfer through the cytochrome chain in the respiratory process and the transmission of nerve impulses and photosynthesis are a few examples. The behavior of protein in the human body may be explained using the concept of organic semiconductors. The role of organic semiconductors in carcinogenesis is intriguing. Charge transfer related to organic semiconductors also enters into the problem of muscular contraction.

Another future use of organic semiconductors may be in the field of catalysis. Studies in this field may lead to significant advances in the field of fuel cells, and of biochemical fuel cells in particular. Both metal-free and metal phthalocyanine organic semiconductors may be the active catalyst for the hydrogen evolution reaction, which is an important reaction for energy conversion. Other future possibilities of organic semiconductors would be in the field of photophenomena, drug design, and power conversion and storage.

Principal terms

ELECTRONS and HOLES: charges in a solid can be carried either by negatively charged particles (electrons) or by regions of empty space in the solid from which a negative charge has been removed; the latter are called (positive) holes

ENERGY GAP: the difference between the energy of an electron in the bottom of the conduction band (upper energy level) and the top of the valence band (lower energy level) in a semiconducting solid; also known as band gap

EXCITONS: the loosely bound, excited electrons in a molecular crystal that are capable of moving in the crystal at the excitation energy

MOLECULAR CRYSTALS: solid crystalline substances that are neither covalently bonded (sharing electrons) nor ionically bonded (attracting opposite charges) between the oppositely charged ions; they are generally formed by attraction between the opposite dipoles among molecules

PHOTOCONDUCTIVITY: the intrinsic conductivity of a semiconductor that arises because of the thermal excitation of an electron from valence band to conduction band; photoconductivity occurs if the electron excitation is done by the absorption of photons (light particles) by the solid

POLARON: the combination of an electron and its interaction with the quantized lattice vibrations in a solid crystal (phonons); this entity can move through the crystal

Bibliography

Bube, Richard H. PHOTOCONDUCTIVITY OF SOLIDS. New York: John Wiley & Sons, 1960. A good discussion on photoconductivity.

Gutmann, Felix, and Lawrence E. Lyons. ORGANIC SEMICONDUCTORS. New York: John Wiley & Sons, 1967. This book could be considered as the encyclopedia for the simple and elaborate discussions of organic semiconductors.

Robertson, J. Monteath. ORGANIC CRYSTALS AND MOLECULES. Ithaca, N.Y.: Cornell University Press, 1953. For the general reader. Includes a discussion of organic molecules.

Szent-Gyorgyi, Albert. BIOENERGETICS. New York: Academic Press, 1957. An intriguing discussion on electrical conductivity in biological systems.

Szent-Gyorgyi, Albert. INTRODUCTION TO SUBMOLECULAR BIOLOGY. New York: Academic Press, 1960. An interesting book on the discussion of the use of the concepts of organic semiconduction to biological systems.