The Rise of Organic Electronics: - Organic Semiconductors

One is passing through a period of Micro Electronics revolution. For the common man, the word “Electronics” creates a “virtual” image of audiovisual gadgets like radio and T.V. in his mind. But the actual fact or truth is that today the growth of any industry like communication, instrumentation, control and even Information Technology is dependent upon electronics to a great extent.

Engineering, as such is not limited within its own shells. In 1994, US Army Col. Richard M Satava, General Surgeon at the Walter Reed Army Medical Center in Washington DC made a bold declaration about the future of medicine. “ We are in the midst of a fundamental change in the field of medicine which is enabled by the information revolution.” Of the many disciplines arising from this new Information era, “virtual reality” (of which we might have heard of) holds the greatest promise. The medicine is practiced the way it was, computer based simulator (virtual reality simulator) would have the surgeon’s skills even before the patient is touched and it would also enable medical students to train on virtual cadavers nearly indistinguishable from a real person.

When I think of PCs, its performance, what comes to my mind is the ICs and when I think of ICs, I think of semiconductors. So, we see that the wonderful combination of Group III-V compounds is like an “atom” of the electronic products.

The modern era of semiconductor electronics was ushered in by the invention of the bipolar transistor in 1947 by John Bardeen, Watter Brattain, and William Shockley at the Bell Telephone Laboratories. Just as the transistor revolutionized electronics by offering flexibility, convenience and reliability, the “Integrated Circuits” (ICs) consisting of many thousands and thousands of transistors, diodes, resistors and capacitors in a chip of a semiconductor enabled new applications for electronics that were not possible with discrete devices.

Let me take you to a world of semiconductor, particularly, organic semiconductors.

Dawn of Organic Electronics :

Organic semiconductors are strong candidates for creating flexible, full-color displays and circuits on plastic. Organic materials are poised as never before to transform the world of circuit and display technology. The future holds tremendous opportunity for the low cost and sometimes surprisingly high performance offered by organic electronic and opto- electronic devices. Using organic light-emitting devices (OLEDs), organic full-color displays may eventually replace the liquid crystal LCDs for use with laptop and even desktop computers. Such displays can be deposited on flexible plastic foils, eliminating the fragile and heavy glass substrates used in LCDs and can emit bright light, all with efficiencies higher than that can be obtained with incandescent light bulbs.

Organic semiconductors have been subject of intense scientific investigation for the past 50 years. These materials primarily consisting of carbon, hydrogen and oxygen, were considered to be merely a scientific curiosity. The weak intermolecular bonds in the solid state give them properties of both semiconductor and insulators and thus their study has deepened the fundamental understanding of the electronic and optical properties of solids. But organic semiconductors attracted industrial interest when it was recognized that many of them are photoconductive under visible light- used in electro photography (or xerography).

Both polymeric and small molecule OLEDs operate by accepting charge carriers of opposite polarities, electrons and holes from the cathodes and anode contacts respectively. An externally applied voltage driver drives these carriers into the recombination region where they form a neutral bound state or exciton.

On an average, one singlet and three triplets are joined for each four electron hole pairs injected into the exciton formation region of OLEDs. Quantum mechanics allows de-excitation or recombination of the singlets within a few nanoseconds of formation leading to photon emission called fluorescence. Recombination of triplet exciton is low, i.e. 1ms to 1 sec and usually results in heat rather than light. But if a heavy metal atom like Iridium or platinum is placed in an otherwise organic molecule, the characteristics of singlet and triplet excitons makes speeding the emission of light to within 100 nanoseconds to 100 microseconds. This kind of emission is called phosphorescence.

From the figure, substrate can be glass or plastic. A low work function is necessary to ensure efficient, low resistance injection of electrons from the cathode to ETL.

One of the principle reasons that OLED technology has attracted such intense interest is its potential for use in full color display that might eventually replace active matrix LCDs. The organic display consists of organic light emitting element or pixel. OLEDs have the potential to achieve operational lifetimes of many hundreds of thousands of hours, possibility of roll-up displays on curved surfaces. Organic semiconductors are also being applied to thin film transistors.

Organic electronics are already entering the commercial world while it is always difficult to predict when and what future product will be introduced, many manufacturers are now working to introduce cell phones and personal digital assistants with OLED displays. It is possible that soon, portable and lightweight roll-up OLED displays will replace the bulky and power hungry CRT(Cathode Ray Tube). That has been the television standard or 50 years. Use of high efficiency phosphorescent, flexible OLED displays in laptop computers and even

for home video applications may be a striking feature in no more than a few years into the future.

But there remains much to be done - Achieving higher efficiencies, lower operating voltages, larger device lifetime, are some of the challenges the engineers are faced. Organic thin films have an excellent chance of becoming the technology of choice for the next generation of high resolution, high efficiency flat panel displays. In addition to displays, many opportunities for application of organic thin film semiconductors are largely untapped. So, the exploration of an emerging organic electronics technology continues.