N-type Organic Semiconductors and Conductors


All-plastic electronic devices such as organic thin film transistors (TFT), organic light emitting diodes (OLEDs), printable circuits, organic capacitors, and organic photovoltaic devices have received much attention in the past few years and some device prototypes are being considered for market entry. Like their inorganic counterparts, organic devices operate because of the specific properties of, and interactions among p-type and n-type conducting and semiconducting materials. Most of the organic electronic materials used in these devices are highly conjugated molecules or polymers that support the injection and allow the mobility of the charge carriers (holes in the case of p-type materials and electrons in the case of n-type materials). To date the great majority of the electronic organic materials that have been investigated are p-type materials. In constrast, the selection of n-type organic materials is limited to a very small number of molecules and polymers. This is because, from a molecular design point of view, it is easier to design electron-rich conjugated polymers (p-type) than electron-poor ones (n-type). In addition, most of the current n-type materials have serious drawbacks including poor solubility, difficulty of synthesis and poor stability in air. Therefore, there is a need for new n-type organic materials to improve the performance and the durability of current devices and to allow the design of new more versatile device configurations.

Conductor vs. Semiconductor, N-type vs. N-doped

Intrinsically conducting polymers (ICPs) are polymers with extended pi conjugation along the molecular backbone, and their conductivity can be changed by several orders of magnitude by doping. Like inorganic semiconductors, ICPs can be either p-doped or n-doped. However, the doping process for organic materials is different than for inorganic materials. For example, p-doping is the partial oxidation of the polymer by a chemical oxidant or an electrode and causes depopulation of the bonding p orbital (HOMO) with the injection of “holes”. n-Doping is the partial reduction of the polymer by a chemical reducing agent or electrode with the injection of electrons in the antibonding p system (LUMO, MacDiarmid 2001). These conjugated polymers in the undoped state are typically semiconducting, the doping process incorporates charge carriers (either electrons or holes), and if these are stable, the polymer becomes electrically conducting to a level that is commensurate with its doping level.

Many p-type conducting and semi-conducting polymers have recently entered the market and are successfully competing with conventional inorganic semiconductors and conductors. Commercial p-type intrinsically conducting polymers (ICP) include polyaniline, polypyrrole, and several derivatives of polythiophenes. In contrast, it is more difficult to design electron-deficient systems. Most of the polymers currently used as the n-type material in photovoltaics and TFTs are hydrocarbon-based polymers with electron-withdrawing substituents such as cyano or nitro groups (see figure below right) and a few ladder polymers such as BBL ({poly(7-oxo,10H-benz[de]imidazo[4’,5:5,6]-benzimidazo [2,1-a]isoquinoline-3,4:10,11-tetrayl)-10-carbonyl}). Substitued fullerenes are also widely employed as n-type materials in organic electronics with the most popular shown in the figure at right. However, even these n-type materials can be difficult to reduce or n-dope, leading to redox properties that make them unstable in air. Furthermore, all the current n-type materials are difficult to process, and some of them are difficult to synthesize. As a result, the use of n-type conducting polymers has so far been of academic interest only. However, n-type conducting and semiconducting polymers are needed for the fabrication of organic light emitting diodes (OLED), thin film transistors (TFT), and in photovoltaic devices. N-doped conducting materials could be used for example as the cathode in OLEDs to replace the currently used low-work function metals (Ca, Al, and Mg), while n-type semi-conducting materials could be used either as the electron transporting layer (ETL) or the emitting layer (EL) in OLEDs. Furthermore, n-type semi-conducting materials are needed to build n-channel field effect transistors and for the active layer of solar cells.

N-type Organic Semiconductors and Solar Cells

There is increasing interest in a new type of PV technology that is based on organic semiconducting materials. This new technology has the potential to revolutionize the PV industry because the materials used to make devices are lightweight, flexible and can be coated/fabricated using inexpensive processes. For example, organic PVs could potentially be fabricated using a continuous reel-to-reel printing process, whereas high efficiency inorganic PVs are made using costly vacuum deposition techniques on rigid substrates. Unfortunately, the highest conversion efficiencies achieved using organic PVs are between 1-3% in highly optimized cells. Obviously, in order for organic PV technology to be competitive, higher efficiencies are needed. This can only be achieved by the development of organic semiconducting materials with specifically designed electronic properties.

Like inorganic PVs, organic devices are made by creating interfaces of an electron-rich material (p-type) with an electron-poor material (n-type). The detailed operational mechanism of organic PVs is beyond the scope of the present text; for now it is sufficient to note that two basic material types are required. The figure above right shows a schematic diagram of organic semiconductors sandwiched between a high work function electrode (ITO) and a low work function electrode (Al). For a single layer device, photogenerated exitons must overcome high energy barriers to produce a photocurrent so this type of device is not very efficient. A two layer device combines an n-type and a p-type semiconductor such that exitons generated at the p-n interface have a energetically favorable pathway to produce a photocurrent. However, a simple flat p-n interface will not produce a high-efficiency solar cell. Interpenetrating blends of n- and p-type materials must be made to present a high surface area of interface within the active layer of the cell.

TDA's new n-type materials

During previous NSF-funded work, TDA has developed a number of new n-type organic materials for use in organic light emitting devices (OLEDs). These materials are soluble, stable in light and air for extended periods of time, and highly fluorescent (see figure at right). In this initial research we sucessfully fabricated and tested prototype OLEDs. In the future we are planning to extend the application of these new materials by fabricating and testing organic solar cell prototypes.




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