Organic electronics

Semiconducting small molecules include polycyclic aromatic compounds such as pentacene and rubrene.

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. The most well-studied class of conductive polymers include 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.

André Bernanose[10][11] was the first person to observe electroluminescence in organic materials, and Ching W. Tang,[12] 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.

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),[12] 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.[13] 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.[14] [15]

Polymer light-emitting diodes (PLEDs), similar to SM-OLED, emit light under an applied electric 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)[16] 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.[17] et al. at Stanford University reported a novel way to construct large-area organic semiconductor thin films using aligned single crystalline domains.

J.E. Lilienfeld[18] 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[19] 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.[20][21][22][23][24]

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.[17] et al. and Dr. Kim J.[25] 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.

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.

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 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.[31][32][33]

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.[34]

Illustration of thin film transistor device

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 spin 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.[35]