Organic Electronics

from my PhD dissertation

Organic electronics is a branch of electronics that is based on conducting and semiconducting polymers and molecules.[1-3] It relies on the electronic properties of π-conjugated molecules that derive their electronic conduction from alternating sequences of single and double carbon-carbon bonds along their main chain.

The field of organic electronics has been the focus of fundamental research for several decades and is now to the point of commercial application. Intense research and development continues in the area with the goal of realizing novel, next generation electronics for a multitude of applications that utilize the intrinsic tunability, low cost, and ease of processing that the materials have to offer.

Background

Figure 1 displays diagrams of organic molecules illustrating how larger chains are built from smaller molecules. Figure 1(a) shows a thiophene ring with a chemical formula of C4H4S, while Figure 1(b) displays polythiophene, with a chemical formula of (C4H4S)n. Figure 1(c) shows poly(3-hexylthiophene) (P3HT), an organic semiconductor, built around the polythiophene chain, with C6H13 side chains attached. The figure displays an example of the P3HT molecule with 100% regioregularity (RR), an alignment of the top and bottom side chains. A general characteristic of the molecules is the presence of double bonds alternating with single bonds along the polymer chain, which results in delocalization of π-conjugated electrons along the polymer backbone.






Figure 1.
An example of a conjugated polymer chain built around an organic small molecule. (a) Thiophene ring (C4H4S), (b) polythiophene (C4H4S)n and (c) poly(3-hexylthiophene), a thiophene derivative (polymerized thiophene), with C6H13 side chains being attached to one of the carbon atoms of an individual thiophene ring. The 100% regioregularity is achieved by having all of the top and bottom side chains aligned.

The electron configuration of the six electrons in a carbon atom, in its ground state, is
1s22s22p2. The electrons in the core orbitals do not contribute to the chemical bonding. In a carbon atom, four valence electrons, in the 2s22p2, are used to form covalent bonds. The s and p orbitals combine to form hybrid orbitals, which give rise to single, double, or triple bonds. In conjugated polymers, one 2s orbital pairs with the two 2p orbitals to form 3 sp2 hybrid orbitals, leaving one p orbital unhybridized. Two of the sp2 orbitals on each carbon atom form covalent bonds with neighboring carbons, the third generally forms a covalent bond with a hydrogen or side group. This is called a σ-bond, which is any bond with cylindrical symmetry around the internuclear axis.[4] The unhybridized pz orbital overlaps with the unhybridized pz orbital on the neighboring carbon. This is called a π-bond which arises from electrons approaching side by side, off the internuclear axis. Figure 2 illustrates the concept.[2, 5]


Figure 2.
Bonding in conducting conjugated polymers. (a) The sp2 hybrid orbitals are shown in light and dark blue, and the unhybridized pz orbitals shown in gray. Electrons are represented by the dots. The two sp2 hybrid orbitals on the side extend in and out of the plane of the page. (b) An example of a backbone of conjugated polymers.

The electrons in the
π-bonds are weakly bound, and are relatively easily delocalized. These delocalized π electrons are the conduction electrons in these organic materials. The sp2 hybridization in conducting polymers is important because this leaves one p electron per atom to form its own bond. Under the assumption that the single and double bonds have the same length, Bloch theory[6] states that solids formed from atoms or molecules with half-filled shells have partially filled bands with metallic transport properties. However, the length of the single and double bonds are not identical and the Peierls instability (the CC bonds are longer than the C=C bonds)[7] splits this simple band into 2 sub-bands, a completely filled valence band (highest occupied molecular orbital or HOMO level) and an empty conduction band (lowest unoccupied molecular orbital or LUMO level), separated by an energy gap, making the material a semiconductor.

Through the process of doping, the conductivity of original (undoped)
π-conjugated polymers can be changed from insulating to conducting, with the conductivity increasing as the doping level increases.[4] Both n-type (electron donating) and p-type (electron accepting) dopants have been used to induce an insulator to semiconductor to conductor transition in electronic polymers. Similar to the inorganic semiconductors, these dopants remove or add charges to the polymers, but the details are very different. Unlike substitutional doping that occurs for inorganic semiconductors, the dopant atomic or molecular ions are positioned interstitially between chains in π-conjugated polymers, and donate charges to or accept charges from the polymer backbone. The counter ion is not covalently bound to the polymer, but only attracted to it by the Coulomb force. In some cases called self-doping, these dopants are covalently bound to the polymer backbone.[8]

The physical flexibility of organic electronics devices is largely due to the fact that polymeric and other organic materials are held together by relatively weak intermolecular bonds (
σ-bond), as opposed to crystalline structure in inorganic electronics, and bending does not critically affect their mechanical integrity or charge transport properties.

Organic Electronic Circuit Elements

Prior to 1970s, polymers were generally restricted to use as insulators. In was not until 1977 when scientists at the University of Pennsylvania demonstrated that treating polyacetylene with halogen compounds increased its electrical conductivity to values comparable to metals,[9] a work that has earned them the Nobel Prize in 2000. Subsequently, work on polymeric conductors and semiconductors evolved over the next several decades. Today, most electronic circuit components commonly fabricated with inorganic elements, such as silicon or germanium, have already been demonstrated at some level in the polymer electronics field. All-polymer capacitors have been reported as early as the 1970s[10] and are currently available commercially. The first organic thin film transistor was reported in 1983 [11], followed by the first all-polymer thin film transistor reported circa 1990.[12]

Entire electric circuits made from all-polymer components have been demonstrated, including D flip-flops,[13] shift registers,[14] and ring oscillators.[15] A 128-bit organic RFID transponder chip with Ethernet-style encoding and wireless anti-collision protocol was demonstrated.[16] Complete circuits that allow for hardware-based cryptography for the use with RFID tags were also shown.[17] In early 2011, researchers at IMEC in Belgium have reported a 4000-organic-transistor, 8-bit logic, physically flexible processor fabricated on a 25 m thin plastic foil. Capable of executing about six hard coded instructions per second, the processor was developed with the use in RFID tags in mind.[18]

Organic Light Emitting Diodes (OLEDs) were reported as early as 1990[19] and became commercially available around 2004. Currently, stand-alone OLED indicators are commercially available, as are consumer electronic products that incorporate larger format OLED technology such as smart phones, television sets, and portable games. Organic Photovoltaic devices (OPVs) offer the promise of low production cost in high volumes.[20] Niche commercial products, such OPV-based solar computer cases and backpacks, are also currently available.

Organic Memristive Random Access Memory (in physics literature commonly referred to as Organic Bistable Devices, or OBDs)[21] are not yet available commercially. Their most widely cited applications include their ability to function as a memory element and are intended as a replacement for either hard-drives or Random Access Memory;[22] logic operations, such as their ability to perform the implication logic often regarded as the foundation of the conventional approach to Artificial Intelligence;[23] and the use as artificial synapses in neuromorphic systems.[3, 24]

Advantages and Disadvantages

Manufacturing inorganic electronics requires high temperatures, typically 400 C to 1400 C, high-vacuum, and very clean environments. These can result in very high production costs. In the research and development phase, organic electronics are also typically fabricated in clean room facilities. However, organic electronics tend to be more tolerant to particulate contamination. It is therefore anticipated that the production stage will not require full clean room facilities, but rather that the local environment, in the vicinity of the substrate, be kept within acceptable particulate levels. This is expected to have a direct effect on lowering the cost. Additionally, the substrate temperatures required for fabrication of organic devices are much lower and thus more compatible with manufacturing processes of plastic substrates. This should enable new, low cost integration of devices with plastics used in everyday consumer items. Finally, the organic nature of the materials used makes them intrinsically more biologically compatible allowing for future integration with biological organs or organisms.[25]

Solution processing of polymer electronics in particular is a key enabler of both low costs and rapid prototyping and fabrication. For laboratory research and development, spin coating (
Figure 3(a)) is often the technique of choice due to its simplicity and the good uniformity of thin films that it can produce. For larger scale fabrication, direct write printing methods such as inkjet printing (Figure 3(b)), spray coating, slot die printing, or flexographic printing are possible.[26] Printing has been used to produce electronic elements such as capacitors,[27] transistors,[28] OPVs,[29] and OLEDs.[30] Other examples of solution processing include blade coating (Figure 3(c))[31] and laminating.[32]


Figure 3.
Example of manufacturing equipment used for deposition of organic electronics materials; (a) spin coater, (b) printing solution dispenser (courtesy of SonoPlot, Inc.), and (c) blade coater.

The use of flexible substrates for organic electronics is advantageous both for production and for application (
Figure 4(a) and (b)). Transistors,[33] capacitors,[34] OPVs,[35] OLEDs,[36] and OM-RAMs[37] have all been demonstrated on flexible substrates. Figure 5 shows an example of roll-to-roll[38] printing of organic transistor circuits on industrial scale.


Figure 4.
Physically flexible organic field effect transistor.


Figure 5.
Industrial scale roll-to-roll fabrication of physically flexible printed electronics (courtesy of PolyIC GmbH & Co.).

There are however several challenges and disadvantages that are characteristic of organic electronic materials and devices. With regards to organic circuits, the price paid for ease of processing comes in the form of the operating bandwidth of the devices. Since carrier mobilities in organic semiconductors are typically orders of magnitude lower than in inorganic semiconductors, transistors and other devices are limited to much slower speeds. Also, solution processing of nanoscale devices with length scales approaching those of todays IC devices (10 20 nm regime) is challenging and will require advancements in techniques such as nanoimprint lithography.[39]

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