Органическая электроника: материалы, свойства и применение

Field of materials science

Organic electronics is a field of materials science concerning the design, synthesis, characterization, and application of organic 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) molecules or polymers using synthetic strategies developed in the context of organic chemistry and polymer chemistry. One of the promised benefits of organic electronics is their potential low cost compared to traditional electronics. Attractive properties of polymeric conductors include their electrical conductivity (which can be varied by the concentrations of dopants) and comparatively high mechanical flexibility. Challenges to the implementation of organic electronic materials are their inferior thermal stability, high cost, and diverse fabrication issues.

Organic conductive materials can be grouped into two main classes: polymers and conductive molecular solids and salts. Polycyclic aromatic compounds such as pentacene and rubrene often form semiconducting materials when partially oxidized. Conductive polymers are often typically intrinsically conductive or at least semiconductors. They sometimes show mechanical properties comparable to those of conventional organic polymers. Both organic synthesis and advanced dispersion techniques can be used to tune the electrical properties of conductive polymers, unlike typical inorganic conductors. Well studied class of conductive polymers include polyacetylene, polypyrrole, polythiophenes, and polyaniline. Poly(p phenylene vinylene) and its derivatives are electroluminescent semiconducting polymers. Poly(3 alkythiophenes) have been incorporated into prototypes of solar cells and transistors.

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. OLED organic materials can be divided into two major families: small molecule based and polymer based. Small molecule OLEDs (SM OLEDs) include tris(8 hydroxyquinolinato)aluminium 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. 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) are generally more efficient than SM OLEDs. Common polymers used in PLEDs include derivatives of poly(p phenylene vinylene) Silicon thin film solar cells on flexible substrates allow a significant cost reduction of large area photovoltaics for several reasons:

The so called 'roll to roll' deposition on flexible sheets is much easier to realize in terms of technological effort than deposition on fragile and heavy glass sheets. Transport and installation of lightweight flexible solar cells also saves cost as compared to cells on glass. Inexpensive polymeric substrates like polyethylene terephthalate (PET) or polycarbonate (PC) have the potential for further cost reduction in photovoltaics. Protomorphous solar cells prove to be a promising concept for efficient and low cost photovoltaics on cheap and flexible substrates for large area production as well as small and mobile applications. 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.

Small molecule semiconductors are often insoluble, necessitating deposition via vacuum sublimation. 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 semiconductor diodes 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 the 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. and n type (such phenyl C61 butyric acid methyl ester, PCBM). Certain organic semiconductors can also present both p type and n type (i. e., ambipolar) characteristics. Such technology allows for the fabrication of large area, flexible, low cost electronics. One of the main advantages is that being mainly a low temperature process compared to CMOS, different type of materials can be utilized. This makes them in turn great candidates for sensing.

Conductive polymers are lighter, more flexible, and less expensive than inorganic conductors. This makes them a desirable alternative in many applications. It also creates the possibility of new applications that would be impossible using copper or silicon. Organic electronics not only includes organic semiconductors, but also organic dielectrics, conductors and light emitters. New applications include smart windows and electronic paper. Conductive polymers are expected to play an important role in the emerging science of molecular computers.