Present day smart phone display screens are almost made up of OLED (Organic Light Emitting Diode). Television screens that uses OLED are also available. Due to their high cost it is not ubiquitous in the TV market. OLED emerges as the promising display technology of near future.
Introduction to LED
Diode is an electronic device. Electronics is a branch of science that deals with study of flow of electrons in a semiconductor or vacuum or gas. Semi is a Latin prefix meaning 'half'. Group 4 elements like Silicon and Germanium has four electrons in the outer orbit and they behave as a perfect semiconductor. Diode is a two terminal device. When current passes through typical diode, small amount heat or infra red emissions occur. But in the case of Light Emitting Diode (LED) instead of heat, light is produced. At present organic materials like polymers are used to make LED.
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| Figure 1. (a) Typical LED (inorganic) (b) Symbol for diode (c) Symbol for LED Image Courtesy: Wikipedia |
Organic Polymers
General public associate the word “Organic” with a produce that is free from artificial chemicals. In science the word organic is associated with carbon compounds like carbonates, oxides, hydrocarbons and so on.
Polymers are made up of long chains of molecules. Almost all polymers are organic in nature. Few Inorganic polymers also do exist. Silicone is a best example for inorganic polymer.
General public associate the word “Organic” with a produce that is free from artificial chemicals. In science the word organic is associated with carbon compounds like carbonates, oxides, hydrocarbons and so on.
Polymers are made up of long chains of molecules. Almost all polymers are organic in nature. Few Inorganic polymers also do exist. Silicone is a best example for inorganic polymer.
Organic diode Vs inorganic diode
- Inorganic semiconductors are crystalline in nature. Organic semiconductors are amorphous in nature.
- Inorganic semiconductors have distinct Valence band (outer most orbit of an atom) and Conduction band. Energy level of conduction band is more than Valence band. In organic semiconductor HOMO (Highest Occupied Molecular Orbital) level can be compared with Valence band. Likewise LUMO (Lowest Unoccupied Molecular Orbital) can be compared with Conduction band [2] .
- Electrons in conduction band are free to move. In case of organic electronics, electrons are localized i.e. their movements are confined within molecules only.
- Electron mobility is highly restricted in polymers. So, they can never replace silicon or inorganic semiconductors.
Plastic is an insulator or conductor?
Plastics are malleable i.e. moldable polymers. Plastic is used to insulate electric wires. From that one will deduce plastic is an insulator. Three scientists namely Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa found a way to make polymer to behave like metal i.e. conductor in the year 1977. For their discovery they were awarded Noble Prize in Chemistry in the year 2000 [1]. They published their work in the journal of chemical society, chemical communications. Title of the research article is "Synthesis of electrically conducting organic polymers: Halogen derivatives of polyacetylene (CH)n".
Plastics are malleable i.e. moldable polymers. Plastic is used to insulate electric wires. From that one will deduce plastic is an insulator. Three scientists namely Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa found a way to make polymer to behave like metal i.e. conductor in the year 1977. For their discovery they were awarded Noble Prize in Chemistry in the year 2000 [1]. They published their work in the journal of chemical society, chemical communications. Title of the research article is "Synthesis of electrically conducting organic polymers: Halogen derivatives of polyacetylene (CH)n".
How to make conductive polymer?
Two conditions to be met if a polymer to be electrically conductive. First electrons need to be free from molecules to move. This is possible in a polymer that is made up of conjugated double bonds (alternating single and double bonds) like Polyacetylene [1]. Next plastic has to be doped. This can be done either by removing electrons from polymers or inserting electrons into the polymers. In the year 1977 Heeger, MacDiarmid and Shirakawa took a thin film of polyacetylene and oxidized with iodine vapour to improve the conductivity by billion times.
Organic Vs inorganic semiconductors: Bonding matters
In the inorganic semiconductor like silicon the bonding between silicon atoms are made up of covalent or coordinate bonding. Atom to atom bonding is called primary bonding. As we know the basic entity in organic semiconductor is a molecule. Molecules are made up of atoms. They are held together by primary bonding. The bonding among molecules are called secondary bonding and made up of Van der Waal bonds [2]. Unlike secondary bonding, primary bonding requires very high temperature to make changes into them.
Vander Waal bonding and Flexible (substrate) electronics
Little energy is required to form a material that is made up Van der Waal bonding. Thus fabrication of organic electronic devices can be carried out at low temperate. It means fabrication cost of organic electronics devices should be lesser than inorganic counterpart. Next, due to low temperature processing, organic electronic devices can be fabricated on substrates like plastic, paper, cloth and so on. It brings a new manufacturing paradigm called "flexible electronics" [2].
Thickness of the display screen depends upon the thickness of LEDs and panel circuitry. Thus if LEDs are manufactured by thin film, obviously thickness of the screen comes down and looks really sleek. Cost of making thin film silicon is high as it involves evaporation of silicon and deposition of silicon on the substrate. In case of organic electronics, the polymer is dissolved in a solution and applied on the substrate to make a thin film LED.
Research activities in India
Indian Institute of Technology (IIT) at Kanpur has research centre for large area flexible electronics named “Samtel Centre for Display Technologies [3]. Resources available in the centre's website is very limited.
References
[1] The 2000 Nobel Prize in Chemistry - Popular Information | https://www.nobelprize.org/nobel_prizes/chemistry/laureates/2000/popular.html
[2] Module No- 4 Optoelectronic Device Physics Lecture No. - 38 Organic Electronics, "Optoelectronic Materials and Devices, Prof. Dr. Monica Katiyar, Professor, Material Science and Engineering, IIT Kanpur | http://textofvideo.nptel.iitm.ac.in/113104012/lec38.pdf, 18 pages, 700 KB.
[3] Samtel Centre for Display Technologies - IIT Kanpur | http://www.iitk.ac.in/scdt/
Note
In well written article one can find a “smooth flow”as one read from start to end. This makes reading enjoyable. But this article is made up of collection of snippets. Each paragraph acts as a snippet. So, connection or link between paragraphs are very limited. From a purist point of view this article should be graded “poor”. Writing a snippets-article is easier and able to concentrate on content rather than article structure. I hope readers will get accustomed to this format.
