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Organic Electronics

Organic electronics is a field of material science concerning the design, synthesis, characterization, and application of small molecules or polymers that show desirable electronic properties such as conductivity. Unlike conventional inorganic conductors and semiconductors, organic electronic materials are constructed from organic (carbon-based) small molecules or polymers using synthetic strategies developed in the context of organic and polymer chemistry. One of the benefits of organic electronics is their low cost compared to traditional inorganic electronics.

History

Conductive materials are substances that can transmit electrical charges. Traditionally, most known conductive materials have been inorganic. Metals such as copper and aluminum are the most familiar conductive materials, and have high electrical conductivity due to their abundance of delocalized electrons that move freely throughout the inter-atomic spaces. Some metallic conductors are alloys of two or more metal elements, common examples of such alloys include steel, brass, bronze, and pewter.

In the eighteenth and early nineteenth centuries, people began to study the electrical conduction in metals. In his experiments with lightning, Benjamin Franklin proved that an electrical charge travels along a metallic rod. Later, Georg Simon Ohm discovered that the current passing through a substance is directly proportional to the potential difference, known as Ohm's law. This relationship between potential difference and current became a widely used measure of the ability of various materials to conduct electricity. Since the discovery of conductivity, studies have focused primarily on inorganic conductive materials with only a few exceptions.

Henry Letheby discovered the earliest known organic conductive material in 1862. Using anodic oxidation of aniline in sulfuric acid, he produced a partly conductive material, that was later identified as [polyaniline]]. In the 1950s, the phenomenon that polycyclic aromatic compounds formed semi-conducting charge-transfer complex salts with halogens was discovered, showing that some organic compounds could be conductive as well.

More recent work has expanded the range of known organic conductive materials. A high conductivity of 1 S/cm (S = Siemens)was reported in 1963 for a derivative of tetraiodopyrrole. In 1972, researchers found metallic conductivity(conductivity comparable to a metal) in the charge-transfer complex TTF-TCNQ.

In 1977, it was discovered that polyacetylene can be oxidized with halogens to produce conducting materials from either insulating or semiconducting materials. In recent decades, research on conductive polymers has prospered, and the 2000 Nobel Prize in Chemistry was awarded to Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa jointly for their work on conductive polymers.

Conductive plastics have recently undergone development for applications in industry. In 1987, the first organic diode device of was produced at Eastman Kodak by Ching W. Tang and Steven Van Slyke. spawning the field of organic light-emitting diodes (OLED) research and device production. For his work, Ching W. Tang is widely considered as the father of organic electronics for this work.

Technology forplastic electronics constructed on thin and flexible plastic substrates was developed in the 1990s. In 2000, the company Plastic Logic was founded as a spin-off of Cavendish Laboratory to develop a broad range of products using the plastic electronics technology.

Conductive Organic Materials

Typical Semiconducting Small Molecules

Attractive properties of polymer conductors include a wide range of electrical conductivity that, can be tuned by varying the concentrations of chemical dopants, mechanical flexibility, and high thermal stability. Organic conductive materials can be grouped into two main classes: conductive polymers and conductive small molecules.

Conductive small molecules are usually used in the construction of organic semiconductors, which exhibit degrees of electrical conductivity between those of insulators and metals. Semiconducting small molecules include polycyclic aromatic compounds such as pentacene, anthracene and rubrene.

Conductive polymers are typically intrinsically conductive. Their conductivity can be comparable to metals or semiconductors. Most conductive polymers are not thermoformable, during production. However they can provide very high electrical conductivity without showing similar mechanical properties to other commercially available polymers. Both organic synthesis and advanced dispersion techniques can be used to tune the electrical properties of conductive polymers, unlike typical inorganic conductors. The most well-studied class of conductive polymers is the so-called linear-backbone “polymer blacks” including polyacetylene, polypyrrole, polyaniline, and their copolymers. Poly(p-phenylene vinylene) and its derivatives are used for electroluminescent semiconducting polymers. Poly(3-alkythiophenes) are also a typical material for use in solar cells and transistors.

Organic Light-Emitting Diode

Main article: Organic Light Emitting Diode

An OLED (organic light-emitting diode) consists of a thin film of organic material that emits light under stimulation by an electric current. A typical OLED consists of an anode, a cathode, OLED organic material and a conductive layer.

Br6A, a next generation pure organic light emitting crystal family, original picture given by author

Schematic of a bilayer OLED: 1. Cathode (−), 2. Emissive Layer, 3. Emission of radiation, 4. Conductive Layer, 5. Anode (+)

Discovery of OLED

André Bernaose was the first to observe electroluminescence in organic materials, and Ching W. Tang, reported fabrication of an OLED device in 1987. The OLED device incorporated a double-layer structure motif consisting of separate hole transporting and electron-transporting layers, with light emission taking place in between the two layers. Their discovery opened a new era of current OLED research and device design.

Classification and Current research progress

OLED organic materials can be divided into two major families: small-molecule-based and polymer-based. Small molecule OLEDs (SM-OLEDs) include organometallic chelates(Alq3), fluorescent and phosphorescent dyes, and conjugated dendrimers. Fluorescent dyes can be selected according to the desired range of emission wavelengths; compounds like perylene and rubrene are often used. Very recently, Dr. Kim J. et al. at University of Michigan reported a pure organic light emitting crystal, Br6A, by modifying its halogen bonding, they succeeded in tuning the phosphorescence to different wavelengths including green, blue and red. By modifying the structure of Br6A, scientists are attempting to achieve a next generation organic light emitting diode. Devices based on small molecules are usually fabricated by thermal evaporation under vacuum. While this method enables the formation of well-controlled homogeneous film; is hampered by high cost and limited scalability.

Polymer light-emitting diodes (PLEDs), similar to SM-OLED, emit light under an applied electrical current. Polymer-based OLEDs are generally more efficient than SM-OLEDs requiring a comparatively lower amount of energy to produce the same luminescence. Common polymers used in PLEDs include derivatives of poly(p-phenylene vinylene) and polyfluorene. The emitted color can be tuned by substitution of different side chains onto the polymer backbone or modifying the stability of the polymer. In contrast to SM-OLEDs, polymer-based OLEDs cannot be fabricated through vacuum evaporation, and must instead be processed using solution-based techniques. Compared to thermal evaporation, solution based methods are more suited to creating films with large dimensions. Zhenan Bao. et al. at Stanford University reported a novel way to construct large-area organic semiconductor thin films using aligned single crystalline domains.

Organic Field-Effect Transistor

Main article: Organic Field-Effect Transistor

Rubrene-OFET with the highest charge mobility currently

Organic field-effect transistor is field-effect transistor made of organic molecules. Field-effect Transistor(FET) is any semiconductor material that utilizes electric field to control the shape of a channel of one type of charge carrier, thereby changing its conductivity. Two major classes of FET are n-type and p-type semiconductor, classified according to the charge type carried. In the case of organic FETs (OFETs), p-type OFET compounds are generally more stable than n-type due to the susceptibility of the latter to oxidative damage.

Discover of OFET

J.E. Lilienfeld first proposed the field-effect transistor in 1930, but the first OFET was not reported until 1987, when Koezuka et al. constructed one using Polythiophene which shows extremely high conductivity. Other conductive polymers have been shown to act as semiconductors, and newly synthesized and characterized compounds are reported weekly in prominent research journals. Many review articles exist documenting the development of these materials.

Classification of OFET and current research progress

Like OLEDs, OFETs can be classified into small-molecule and polymer-based system. Charge transport in OFETs can be quantified using a measure called carrier mobility; currently, rubrene-based OFETs show the highest carrier mobility of 20–40 cm²/(V·s). Another popular OFET material is Pentacene. Due to its low solubility in most organic solvents, it's difficult to fabricate thin film transistors (TFTs) from pentacene itself using conventional spin-cast or, dip coating methods, but this obstacle can be overcome by using the derivative TIPS-pentacene. Current research focuses more on thin-film transistor (TFT) model, which eliminates the usage of conductive materials. Very recently, two studies conducted by Dr. Bao Z. et al. and Dr. Kim J. et al. demonstrated control over the formation of designed thin-film transistors. By controlling the formation of crystalline TFT,it is possible to create an aligned (as opposed to randomly ordered) charge transport pathway, resulting in enhanced charge mobility.

Organic electronic devices

Five structures of organic photovoltaic materials

Electronic devices based on organic compounds are now widely used, with many new products under development. Sony reported the first full-color, video-rate, flexible, plastic display made purely of organic materials; television screen based on OLED materials; biodegradable electronics based on organic compound and low-cost organic solar cell are also available.

Fabrication Methods

There are important differences between the processing of small molecule organic semiconductors and semiconducting polymers. Small molecule semiconductors are quite often insoluble and typically require deposition via vacuum sublimation. While usually thin films of soluble conjugated polymers. Devices based on conductive polymers can be prepared by solution processing methods. Both solution processing and vacuum based methods produce amorphous and polycrystalline films with variable degree of disorder. “Wet” coating techniques require polymers to be dissolved in a volatile solvent, filtered and deposited onto a substrate. Common examples of solvent-based coating techniques include drop casting, spin-coating, doctor-blading, inkjet printing and screen printing. Spin-coating is a widely used technique for small area thin film production. It may result in a high degree of material loss. The doctor-blade technique results in a minimal material loss and was primarily developed for large area thin film production. Vacuum based thermal deposition of small molecules requires evaporation of molecules from a hot source. The molecules are then transported through vacuum onto a substrate. The process of condensing these molecules on the substrate surface results in thin film formation. Wet coating techniques can in some cases be applied to small molecules depending on their solubility.

Organic solar cell

Bilayer organic photovoltaic cell

Compared to conventional inorganic solar cell, organic solar cells have the advantage of lower fabrication cost. An organic solar cell is a device that uses organic electronics to convert light into electricity. Organic solar cells utilize organic photovoltaic materials, organic semiconductor diodes that convert light into electricity. Figure to the right shows five commonly used organic photovoltaic materials. Electrons in these organic molecules can be delocalized in a delocalized π orbital with a corresponding π* antibonding orbital. The difference in energy between thhe π orbital, or highest occupied molecular orbital(HOMO), and π* orbital, or lowest unoccupied molecular orbital(LUMO) is called the band gap of organic photovoltaic materials. Typically, the band gap lies in the range of 1-4eV.

The difference in the band gap of organic photovoltaic materials leads to different chemical structures and forms of organic solar cells. Different forms of solar cells includes single-layer organic photovoltaic cells, bilayer organic photovoltaic cells and heterojunction photovoltaic cells. However, all three of these types of solar cells share the approach of sandwiching the organic electronic layer between two metallic conductors, typically indium tin oxide.

Illustration of thin film transistor device

Device of Organic Field-effect Transistors

An organic field-effect transistor device consists of three major components: the source, the drain and the gate. Generally, a field-effect transistor has two plates, source in contact with drain and the gate respectively, working as conducting channel. The electrons move from source to the drain, and the gate serves to control the electrons’ movement from source to drain. Different types of FETs are designed based on carrier properties. Thin film transistor (TFT), among them, is an easy fabricating one. In a thin film transistor, the source and drain are made by directly depositing a thin layer of semiconductor followed by a thin film of insulator between semiconductor and the metal gate contact. Such a thin film is made by either thermal evaporation, or simply spins coating. In a TFT device, there is no carrier movement between the source and drain. After applying a positive charge, accumulation of electrons on the interface cause bending of the semiconductor and ultimately lowers the conduction band with regards to the Fermi-level of the semiconductor. Finally, a highly conductive channel is formed at the interface.