OLED - Part II

​‌I. OLED device structure and operation
OLED is a multi-layered device. Emissive layer and conductive layer are the most important layers among them. Both layers contain HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) band. They are analogous to inorganic semiconductors' valence and conduction band.  Emissive layer is connected to cathode and conductive layer is connected to transparent anode. The transparent anode is made up of Indium Tin Oxide (ITO). Electrons are pumped from cathode to emissive layer's LUMO region. Likewise holes are pumped into conductive layer's HOMO region by anode. It reaches emissive layer's HOMO region. As the electron find hole in the emissive layer, it plunges into the hole leaving the excess energy as photon (light particle) [1]. This phenomenon is called electroluminance.  Photon colour depends upon the polymer used in the emissive layer. For example, PPV (polyphenylene vinylene) emits yellow-green light at 510 nm and Poly florene emits blue color. Above the cathode opaque substrate is placed. Below the transparent anode transparent glass is placed. Through the glass generated light passes and reach the viewers. Few more layers are added to improve the efficiency of the OLED. Discussing them is beyond the scope of this article. Human hair is 200 times bigger than OLED structure. So, one can imagine the size of OLED. 

II. OLED Types
OLED can be broadly classified into two categories. One is AMOLED (Active Matrix OLED) and another is PMOLED (Passive Matrix OLED). If faster refresh rates are required as in Television or Smart phone AMOLED is used. Display devices like 16 character display and low end cell phone displays go for PMOLED. As usual they are cheaper to fabricate but their life is limited. PMOLED has stripes of anode and cathode that criss-cross to form a matrix as in Fig 1.(a). In the AMOLED common cathode is used as in Fig. 1. (b). Anode is connected to TFT (Thin Film Transistor) layer. This TFT contains array of Transistors and capacitors. Transistor acts as on-and-off switch and capacitor stores the energy to supply the anode. Refreshing circuitry is used to refresh TFT in regular intervals. Thus each pixel in AMOLED is quickly and accurately energized. Fabrication of TFT into OLED structure is complex but it provides good quality display. Fig 1. (b) is not actual structure. It is author's rendition after studying text related to AMOLED.

It is possible to have transparent cathode instead of transparent anode. Having transparent cathode helps to overcome the restriction posed by TFT structure on the transparent anode. Thus more light is emitted and improvement in pixel brightness and higher resolution is achieved. This  technique is called top emission adaptive current drive technology [1].  

Fig 1. (a) Structure of PMOLED (b) Structure of AMOLED (author's rendition)

OLED - Part 1

     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. 

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. 

VLC media player

      VLC media player is freely downloadable software [1] having a size of 28 MB and two billion downloads. It works on 14 platforms like Windows, Linux, Android, Apple TV, ChromeOS and so on. As expected, VLC is regarded as the world's most preferred media player [2].  It is difficult to believe that VLC player developer team consists of 12 active members including five core developers [3]. The team thrive on the donations they collect. Building a very successful software using small band of developers with shoe string budget is really fascinating. Adequate documentation regarding the VLC player project and about the people involved are not available in Internet. This post tries to summarize the scrambled pieces of information about VLC player.

     VLC stands for VideoLAN Client. VLS i.e. VideoLAN Server existed in the initial stages of development. Later it was merged with VLC. Technically it is possible to stream a video over a network using VLC player. VLC is software developed by a non-profit organization named VideoLAN. As the name suggests the VLC project was developed to send video over Local Area Network (LAN). The software was developed by students of École Centrale, (a famous engineering college in France, located in Paris) to deliver videos from satellite across the computer network present in the campus. [4]. It was code named as "Network 2000". The project was started in the year 1996. The birth date of VLC is not 1996 instead it is 1st February 2001. On this day only the VLC project was made as an Open Source by École Centrale authorities. General public uses “free software” and  “Open software” interchangeably. Free software means the price of the software is zero. Open software permits the user to access the code (programs). Thus users can fine tune the software according to their wish. Invariably open software comes with zero price tag and general public think these two terms are synonymous.

 

                                              Photo of Mr. Jean Baptiste Kempf.             Image Courtesy:  ocsmag.com

Rise of Anti-science

I recently stumbled upon a promotional website for a book titled “Unscientific America” [1]. I read the material presented in the promotional website and the websites that reviewed the book “Unscientific America”[3-4]. In this post views of authors and reviewers are summarized and presented.

At the time of World War II (1939-1945), scientists were regarded as superstars. Their contribution was very vital for war machinery and in turn paved way to victory. Over the decades the disenchantment with science proliferated among the public. Scientists were unaware of this phenomenon and went on business as usual [3].

National Academy of Sciences (NAS) is a non-profit society having distinguished scientists as its members. It was created by act of Congress (American parliament) and the draft was signed by Abraham Lincoln in 1863. It functions as an advisory board to America in the matters of science and technology. NAS has produced a report titled “Rising Above the Gathering Storm” in 2005. In that it has acknowledged the erosion of America’s supremacy in science and Technology. Dramatic increase in science students is offered as a solution to stem the erosion.

Theme of Unscientific America’ revolves around decline in science and technology temper and its consequences. Authors of the book lament that the urgent problems of current century need science-based solutions. But science illiteracy or dislike is increasing as decades passes by. For example only one minute is devoted to science out of five hour news reporting [1]. Over the decades the size of science weekly sections in newspaper has shrunken by two-thirds. Around 46 percentage of Americans deny evolution. Above examples is tip of the iceberg in anti-science stand.

Benefits of Engaging with Public

Science helps us to understand the world around us. Next, science can be used for specific problems (like extending the shelf life of vegetables and meat, making smokeless stove etc). Science cannot solve problems that has significant human influence. Problems like malnutrition, corruption and poverty are classic examples.

Application of science has produced many useful things to human beings. One such thing is vaccination technique. Using vaccination dreaded diseases like diphtheria, small pox and polio can be prevented from occurring. Thereby improving the human being's living conditions. But there is resistance to use  vaccination due to misinformation (fraudulent research publications [1], public apathy towards government [2]). A research paper appeared in Lancet journal linking autism and vaccination use. Subsequent research finding proved that vaccination has no bearing on autism [1]. Anyway the misinformation reached the public well than the subsequent research finding.

Nowadays affluent people prefer 'Organic farm produce' rather than produce enriched by chemical fertilizer. The problem is not chemical fertilizers but over use of fertilizers by ill trained farmers. But, public blame science for spoiling the agricultural lands and food.  So, it becomes imperative for scientists to communicate with common people and debunk the misinformation they hold.