Introduction to Organic Electronics
Remember the Tom Cruise sci-fi movie that depicted fantastic technology such as video
display on a milk carton, eye scanners for consumer billboards, spider-like robots, and
“automatic” cars? Interestingly, the movie Minority Report borrowed heavily on active
research currently being conducted at universities, major corporations, and government
laboratories. Although most of the technologies depicted in the movie are being explored
by science, one of those technologies is closer than you think. This immerging
technology has the potential to lower costs of integrated electronics, to jump start the
flexible electronics industry, and may one day drive video displays on a milk carton.
In this article we describe this technology, organic electronics, and the impact on the
future of the electronics industry. First, a brief description about the operation of an
organic light emitting device (OLED) is provided from a quantum viewpoint. The
molecular models for describing an OLED are presented. Then, we describe the
electronic components currently possible with organic systems and the advantages and
disadvantages of them versus semi-conductor systems. Finally, we end by discussing the
future of organic electronics and the potential impact on tomorrow’s technology.
Quantum chemistry 101
What happens when you apply a high voltage between two ends of a cucumber?
Undoubtedly, you will fry and burn the cucumber, but you may also notice that the
cucumber seems to glow before burning. If the cucumber is thinner, the same
phenomenon occurs, only with less applied voltage. If the cucumber is a few microns
thick, something unusual happens. The cucumber emits a soft green light without burning
instantly. In fact, the cucumber has become an LED!
Cucumbers are composed of organic materials, i.e. molecules with carbon (C) bonds.
Since carbon is an alkali metal in the same group as silicon (Si), certain organic materials
have similar electronic transport properties to that of silicon; in fact, the cucumber
behaves like a poor semiconductor. The energy processes that describe semiconductors
can be applied to organic molecules, particularly the electronic transitions between
energy levels and electron/hole transport. We describe briefly how these two processes
can be used to predict the behavior of OLEDs in heterojunction devices.
OLEDs are described via electronic transitions between energy levels formed by the
molecular bonding structure. For example, in the simplest case, a diatomic molecule,
such as an H 2 molecule, forms two energy levels, the ground state E0 and first excited
state E1, as shown in Figure 1a and 1b. These energy levels, which are often called the
electronic states, are determined by the intermolecular distance of the nuclei. The
addition of vibrational and rotational interactions within the molecule form additional
states as indicated by the dashed lines in Figure 1c.
Typically, electrons occupy the lowest energy levels in the vibrational states near the
ground state E0. The electrons may gain energy via the absorption of electromagnetic
waves. When absorption occurs, the addition energy causes the electrons to occupy the
vibrational states in the first excited state E1, where the additional energy is Eabs = habs.
Since electrons in molecules prefer to be in the lowest energy configuration, any
electrons occupying the excited state E1 will relax back down to E0. The relaxation of
energy produces an emission energy Eemis = hemis. The entire process of absorption and
emission in organics is known as Franck-Condon Principle and is illustrated in Figure 1d.
This principle extends to larger organic molecular systems. The Jablonski diagram,
shown in Figure 2, shows the general electronic processes associated in many organic
materials used for OLEDs. The specific energy levels are often determined with
spectroscopy techniques such as nuclear magnetic resonance (NMR). Again, absorption
of energy excites electrons from the ground state S0 to the singlet state S1. The electrons
may occupy the state S1, or the energy may transfer to the triplet states T1, which are
allowed due to spin-orbit interactions with the electrons and nuclei. Relaxation of energy
causes emission of fluorescence or phosphorescence because the wavelength emis is
within the visible light regime.
So if you direct a laser at the appropriate wavelength to an organic material like perylene3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA), it will phosphoresce and thus
glows. A simple OLED can be constructed using a laser and some organic material, but
the use of lasers is extremely impractical. A practical approach is to design heterojunction
devices with organic materials. By layering different organic compounds together, we
can still exploit the electronic transitions and utilize electron/hole transport in molecules.
Electron/hole transport in organic materials is similar to semiconductors. In
semiconductors, the properties of a crystal lattice and well-ordered periodic energy
barriers yield valence and conduction bands for electron/hole transport. In organic
materials, the atoms that constitute typical organic systems such as H, C, N, O, F, P, S,
Cl have valence electrons in the s- and p- orbitals. When molecules are formed, the s- and
p-orbitals form - and - bonds. By overlapping - bonds between molecules and other
interactions such as Van der Waal bonds, two energy bands are formed called the HOMO
(highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital)
band, which are analogous to the valence and conduction band in semiconductors.
Conduction can be achieved by simply applying a voltage difference on an organic
material, which causes either the injection of holes into the HOMO band or electrons into
LUMO band. In other words, an organic compound behaves as either an n-type or p-type
material.
Figure 3 shows a heterojunction device for an OLED. Two organic layers make up the
OLED. One layer is an electron transporting layer (e.g. 8-tris-hydroxyquinoline
aluminum [Alq3]), while the other layer is a hole transporting layer (e.g. 4’-diamine
[TPD]). An applied voltage causes electrons to flow to the anode, while holes flow to the
cathode. Since there is an energy barrier between the two organic layers, electrons and
holes conduct via tunneling processes. However, excitons (the combination of holes and
electrons) also form and cause energy to be transferred via Forrester Transfer to the
excited states in the luminescent layer, which can be either the HTL or ETL depending on
the organic materials used. When relaxation occurs, the organic material phosphoresces
or fluoresces.
From organics to electronics
Now that we know how OLEDs generally work, we come to the exciting section of this
article, the electronic devices, as this is a magazine for everything electronics. Using
many of the principles presented in the previous section, organic LEDs and transistors
can be created. We describe these two electronic devices, focusing on device
characteristics and advantages and disadvantages over conventional electronics.
OLEDs potentially offer unique advantages over semiconductor LEDs. First, they emit a
wider color range, i.e. more visible wavelengths. Second, they have scalable emissive
areas from a few microns to a few centimeters in size. Since OLEDs are made of thinfilms of organic materials and have scalable area, they can be integrated into flexible
electronics such as color e-paper and active wallpaper displays. Furthermore the material
cost of OLEDs is cheaper than silicon, which leads to cheaper flat screen monitors. In
fact, companies have already demonstrated 13 inch working flat screen monitors using
OLEDs.
Although OLED monitors work wonderfully, there is one disadvantage preventing
commercialization of OLEDs: differential aging. When organic molecules are exposed to
oxygen, the molecules degrade over time – the same process that causes us to age. Long
exposure will cause OLEDs to decrease in power output and intensity for a fixed input
current. For large area OLEDs, black blots will form over time and eventually lead to the
failure of the device. Differential aging is a problem that affects all organic electronics.
As with modern digital electronics, organic transistors are the most fundamental device in
organic electronics. Thin-film transistors based on organic materials and polymers have
been used to develop organic CMOS driven microelectronics, including flexible displays,
radio-frequency identification tags, ring oscillators, shift registers, and simple arithmetic
logical unit (ALU) circuits. In terms of circuit design theory, organic transistors can be
used in the same way as modern transistors.
However in terms of practice, organic transistors are limited in application due to one
significant drawback: speed. A well-ordered organic transistor has mobility in excess of
10-2 cm2V-1s-1. No, this is not a misprint, the exponent is -2. So don’t expect organic
transistors to be used in any microwave applications anytime soon.
Challenges for the future
The message here is that overcoming the two major problems, aging and mobility, is the
key to the future development of organic electronics. These challenges are current active
research problems with many approaches to finding the solution. Here, we examine the
some approaches to solving these problems.
Differential aging is perhaps the largest obstacle facing organic electronics today. The
most ideal solution is to operate the organic device in a vacuum sealed environment, but
unfortunately this is practically impossible. The next best solution is to grow the organic
devices on plastic substrates, because plastic is an inexpensive way to integrate flexible
materials with organic devices. Plastics increase the lifetime of organic devices to several
thousand hours. However, the lifetime is still not good enough for commercial usage. To
increase the lifetime of organic devices, research into developing better plastics has
become a central focus. Surprisingly, the best plastics for organic devices at this time are
the same plastics used for potato chip bags.
The mobility of organic devices, particularly transistors, will limit the type of
applications for organic transistors. Researchers have discovered many ways of
increasing mobility, including utilizing Van der Waal bonds, increasing - bonding
energies, and investigating new materials and polymers. Unfortunately organic devices
will never have the high mobility that semiconductors have. So another approach is to
find better materials with higher mobility. For example, material scientists are using
amorphous silicon-organic hybrid materials rather than pure organic materials. Hybrid
materials offer high mobility and still offer the advantage of flexibility.
Once these two major obstacles are tackled, we may soon see organic electronics in many
consumer electronic products. But how will organic electronics affect the hobbyists? Will
we be able to get our hands on OLED or transistors devices and build interesting circuits
with them? Perhaps, but don’t be surprised if you can build organic circuits by simply
printing organic circuits on paper!
Since many of the organic materials used for organic electronics are dyes, specialized
ink-jet printers can be developed for organic circuit printing. Engineers and hobbyists can
quickly produce flexible paper-circuits and debug their designs. This type of rapid
prototyping is unheard of in the electronics industry and has the potential to save precious
man-hours in development time and lower circuit design and development costs.
Organic electronics will not replace today’s semiconductors, but with the proper
development and the right applications, organic electronics could be become
commonplace as paper.
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Sidebars/Figures
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Figure 1. (a) An example of the simple diatomic molecule H 2 . (b) This molecule forms two energy levels,
the ground state E0 and the first excited state E1. Notice that energy is dependent on the intermolecular
distance between nuclei. (c) Rotational and vibration energy levels are formed via electron-nuclei
interactions. The dashed lines indicate these energy levels. (d) Absorption of energy E abs = habs excited
electrons in E0 to E1. The electrons temporary occupy E1 until relaxing back to E0. Relaxation causes the
release of emission energy Eemis = hemis.
Figure 2. The Jablonski diagram depicts the general electronic structure of molecules. Aborption of the
appropriate electro-magnetic wave excites electrons from the singlet state S0 to the first singlet state S1.
Relaxation from S1 to S0 causes the molecule to Fluoresce. The energy of electron may also transfer to the
triplet state T1. The transfer of energy is known as intermolecular crossing. Relaxation from T 1 to S0 causes
the molecule to Phosphoresce.
Figure 3. (a) Organic heterojunction structure for an LED. An electron transporting layer (ETL) is grown
on top of a hole transporting layer (HTL). The anode and cathode are usually made of metallic material
such as magnesium-silver (Mg:Ag). (b) The electronic energy structure formed by the organic molecules.
The HOMO band allows electron transport and the LUMO band allows hole transport. A difference of
voltage injects holes and electrons into the organic materials. (c) The holes and electrons meet at the barrier
forming an exciton region. When the holes and electrons combine, energy is released and transferred via
the Forrester Transfer process to the luminescent organic material. Relaxation of electrons causes emission
of visible light.