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Author(s): Lokesh Kumar Sahu*, Vaishali Soni*

Email(s): sahulucky14@gmail.com , vaishalisoni2310@gmail.com

Address: School of Studies in Electronics and Photonics, Pt. Ravishankar Shukla University Raipur, Chhattisgarh

Published In:   Volume - 35,      Issue - 1,     Year - 2022


Cite this article:
Sahu and Soni (2022). OLED: New Generation Display Technology. Journal of Ravishankar University (Part-B: Science), 35(1), pp. 01-08.




Journal of Ravishankar University–B, 35 (1), 1-8 (2022)

 
               

OLED: New Generation Display Technology

           Lokesh Kumar Sahua* and Vaishali Sonia*

                aSchool of Studies in Electronics and Photonics, Pt. Ravishankar Shukla University Raipur, Chhattisgarh

             *Corresponding Author: sahulucky14@gmail.com; vaishalisoni2310@gmail.com

[Received: 06 August 2021; Revised: 26 October 2021; Accepted: 15 February 2022]

 

Abstract: As time gets advanced multiple progressions have happened in the field of display devices. In this field first came a small LED then after CRT (Cathode Ray Tube) which is used in present days but due to its heaviness, we do not carry it from one place to another. Then after came LCD (Liquid Crystal Display), the problem of bulkiness and required large area overcome by LCD, the only problem with LCD is that it cannot see a clear picture from different angles. LCD is a lightweight and flexible plastic substrate. After all these innovations OLED came and OLED beat all issues of LCD and CRT. OLED is light and slim in design, it does not require any kind of backlight, OLEDs are self-luminous. Polymers are using to fabricate OLEDs or unlike LCDs small molecules in the flat panel zone. It has low power consumption (only 2-10 volts) also OLEDs has a wide viewing angle (up to 160 degrees). The applications of OLEDs in electronics are on the increase on daily basis from camera to cell phones to OLED televisions.

Keywords: Light Emitting Diode, Cathode Ray Tube and Liquid Crystal Display

Introduction

Organic Light Emitting Diode another innovation that depends on electroluminescence. OLED has received a lot of attention over the world because this is the only display device which is overcome all the problems related to past display devices. Basically, OLED is an energy conversion device i.e., converts electrical energy to light energy (Yan and Gao 2002, 101-106). OLED based on electroluminescence. Electroluminescence is light emission from a solid through which an electric current is passed. Unlike CRT (Cathode Ray Tube), the OLED fabrication process is easy and devices are thinner and lighter. Also, unlike LCD (Liquid Crystal Display), OLED has a different viewing angle and we don’t need any type of backlight. So, the power consumption and drive voltage are low. The first commercial OLED display was introduced by Pioneer Electronics as the front panel of car stereo in 1997. In OLEDs, the layers are a type of flat panel display created by sandwiching a layer of electroluminescent material such as GaAs between two layers of electrons. OLEDs are used to create digital displays in devices such as television screens, computers, portable systems such as mobile phones etc. OLED displays can use either passive-matrix OLED (PMOLED) or active-matrix (AMOLED) addressing schemes.

Organic Light Emitting Diode

The ‘OLED’ stands for Organic Light Emitting Diode. An OLED is a solid-state device or electronic device, a technology that uses LEDs in which the light is produced by organic molecules. These Organic Light Emitting Diode are considered to be the world’s best display panels. OLEDs displays are made by placing a series of organic thin films between two conductors. When an electric current is applied, a bright light is emitted. This is called electro phosphoresce, even with the layered system, these systems are thin. In comparison to human hair, it is less the 200 times or usually less than 500 nm. OLED doesn’t need any backlight and it has more energy efficient. The substance-using in OLED emits Red, Green, Blue or White light.  In OLED we can see any type of image from different angles(Shrinar 2005)

Figure 1 indicates how a junction diode allows current to float when electrons (black dots) and holes (white dots) move across the boundary between n-type (red) and p-kind (blue) semiconductor material.

LED is an assembly diode with an added feature: it emits light. Every time electrons fall. To assemble, they jump into holes in the other side, release more energy than the residue, and then emit a faster light bulb. All those flashes produce a dull, continuous light that is illuminated by LEDs.

OLEDs work in the same way on conventional diodes and LEDs, but instead of using n-type and p-type semiconductors, they use organic molecules to produce electrons and holes.

Construction and structure of an OLED

Simple OLED is built in six different layers. Above and below are layers of protective glass or plastic. Top layer is called the seal and the bottom layer is the substrate. Between those layers, there is a negative terminal (sometimes called a cathode) and a positive terminal (called anode). Finally, between the anode and the cathode is a two-layer layer made of organic molecules called the emissive layer (where light is produced, near the cathode) and the conductor layer (near the anode). Here's what it all looks like:(Placeholder11-5)


How does this sandwich of layers make light?                                                                                                                

·     To make OLED light, we simply attach the voltage (potential difference) across the anode and cathode.

·     As electricity begins to flow, the cathode receives electrons from the power source and anode loses them (or "finds holes," if you choose to view it that way).

·     We now have a situation where additional electrons cause the emissive layer to be badly charged (similar to the n-type in the junction diode), while the conduction layer is positive called (similar to type p items).

·     Permitting holes go much farther than bad electrons so cross the border from the transit layer to the delivery layer. When a hole (lack of electrons) meets an electron, both

·     objects erase and release small explosions of energy in the form of a particle of light - photon, in other words. This process is called recombination, and because it occurs several times per second OLED emits continuous light as long as the gas continues to flow.

Working of an OLED

Working of OLED is similar to an LED, it's important that you first understand how a basic LED works. An consists of two semiconductor materials, it is usually germanium and silicon, one of which is n-type (negatively charged) and the other are p-type. When you join these together you get an area where the electricity produced on their junction. OLED works on the same principle but instead of n-type and p-type they use organic molecules to release holes and electrons. During operation, we applied voltage to the OLED such that the anode is positive with respect to the cathode, where the anode is picked based upon the quality of their optical transparency, electrical conductivity and chemical stability. When a current electron flows, the cathode starts receiving electrons from the source and the anode starts losing them. As a result of this, the emissive layer starts becoming more negatively charged and the conductive layer becomes more positively charged. This process may also be described as the injection of electron holes into the HOMO (Highest Occupied Molecule’s orbit). Electrostatic forces bring the electrons and the holes towards each other and they recombine forming an exaction, bound state of electrons and holes.  Recombination energy is released and the molecule and the polymer segment in which the recombination occurs reaches an excited state. When a DC bias is applied to the electrodes, the injected electrons and holes can recombine in the organic layer and emit light of a certain colour depending on the properties of the organic materials.(Patel, Patel and Vatalia 2018)


Types of OLEDs

There are different types of OLED available at the moment and all of them are designed for a different aim: types are below.

Passive Matrix OLEDs (PMOLEDs)

PMOLEDs have strips of cathode, organic layers and strips of anode. Anode and cathode strips are placed perpendicular to each other. The intersection of cathode and anode make up the pixels where light is emitted. A current is applied to some strips of cathode and anode to determine pixels whether on or off. Again, the brightness of each pixel is proportional to the amount of applied current. PMOLEDs are easy to produce, but they consume more power than other types of OLEDs. However, power consumption is still less then LCDs and they are suitable for text and icons for small screens (2 to 3 inches diagonal) such as those you find in cell phones and MP3 players (Patel and Prajapati 2014).

 

Active-Matrix OLEDs (AMOLEDs)

AMOLEDs have full layers of cathode, organic molecules and anode. The anode layer forms a matrix in thin film transistor (TFT). This array sets pixels on or off to generate an image. AMOLEDs consumes less power than PMOLEDs because the TFT array requires less power than external circuitry, so they are efficient for large displays. Large screen TVs, monitors and billboards are some products that this type is used (Burroughes, et al. 1990).


Transparent OLEDs

Transparent OLEDs have only substrate, cathode and anode that are transparent components and when a transparent display is turned on it allows light to pass in both direction and when a transparent display is turned off are up to 85 percent as transparent as their substrate. This type of OLED can be either active or passive matrix.

Top-Emitting OLEDs

They have opaque or reflective substrate. They have mainly used for active-matrix design. This type is used in smart cards.

Foldable OLEDs

The flexible metallic foils or plastics are used as substrate for foldable OLEDs. They are very light and strong. Foldable OLEDs are used in cell phones. This reduces the breakage issue.

White OLEDs

In this type, white light is emitted and produce white light. It is more energy efficient than regular fluorescent lights that are currently used in homes and buildings. Their use could potentially reduce energy cost for lighting.

Recent trends in Fabrication and design technologies

The development of organic printed electronics has been growing exponentially with a variety of applications and is expected to bring new things to our future lives. In line with this practice, high efficiency of organisms with low production processes and certain factors such as thin body, low weight, bending and low energy consumption is required. Biodiversity has been investigated in the construction of this field. Basic guidelines for material construction and the latest advances in polymer emitting diodes (OLEDs) and organic photovoltaic cells (OPVs) have been reported.

Polymer OLEDs, as well as small OLED molecules, have features of:

(a) a very high resolution from an independent device (luminance-on/-off)

(b) Wide viewing angle

(c) Colours bright

(d) Small devices

(e) High speed conversion

(f) Low power consumption

A notable feature of polymer OLEDs is the use of cost-effectiveness in mass production. As shown in Table 2, small standard OLEDs, consisting of complex structures (multilayer structure), are named in the process of mechanical evaporation. On the other hand, polymer OLEDs have a simple device design, which can be done with a printer like an inkjet printer.

Comparison

OLED v/s LCD

LCDs are used in calculator screen, mobile display, computers and a lot more applications. Unlike LCDs, OLED produce their own light where LCDs require a backlight. LCDs has a great amount of power consumption where OLED has low power consumption. We can see the display in OLED from different angles (up to 160 degree) but in LCD we cannot see the display in wider way. OLED is slimmer than LCD.(Goyal and Goyal 2019).

OLED v/s LED

OLED display can be lighter and thinner than LED display. With OLED the colours do not get washed out when you watch from different angles. OLED screens are more energy efficient when compared to their LED counterparts (Ijeaku, et al. 2015).

Merits

·       COST: OLED can be printed onto any fitting substrate by an inkjet printer or even by screen printing, theoretically making them cheaper to produce than LCD or plasma display.

·       WEIGHT AND SUBSTRATE: OLED is light weighted and can be created on plastic substrate, the possibility of flexible light emitting diodes being fabricated or new applications such as roll up displays embedded in clothing.

·       POWER EFFECTIVENESS AND THICKNESS: OLEDs are thinner than LED, LCD or plasma displays and OLED does not create light.

·       REACTION TIME: OLEDs can faster response time than slandered LCD screens.

·       OLEDs have wider viewing angles compared to LCDs

Demerits

·       LIFETIME: Lifetime of an OLED is lesser than LCD. Red and Green OLED films have longer lifetime (46,000 to 2,30,000 hours), Blue OLEDs currently have much shorter lifetimes (up to around 14,000 hours).

·       WATER HURT: Water can easily hurt OLEDs.

·       POWER CONSUPTION:  OLED will consume around 40 percent of the power of an LCD displaying an image.

·       OUTSIDE EXECUTION: As an emissive display technology, OLEDs 100 percent converting electricity to light, unlike most LCDs which are to some extent reflective.

Applications of OLEDs

OLEDs are currently being used in small screens devices such as cell phones, DVD players, digital camera etc. It can be foldable and flexible and this makes it weight and space saving technology. Research and development in the field of OLEDs is proceeding rapidly and may lead to future applications in heads up displays, automobile, dashboards, billboard type display, home and office lighting and flexible displays. Video images could be much more realistic and constantly updated. The future newspaper might be an OLED display that updated with breaking news and like a normal newspaper you could fold it up when you are done reading it.

Conclusion

OLED technology is getting advanced in recent years, their higher efficiency and lower weight will make them different from other display technology. If we talk about the performance, the performance of OLEDs meets many of the targets necessary for applications in display. The most attractive feature of an OLED is it can be foldable and transparent. We can fold the screen and carry with us anywhere. Because of foldable we can use it in different ways like foldable newspaper so that we save papers. We can use OLED in automotive dashboards, billboards types display, home and office lighting etc.

 

References

 Burroughes, J. H., D.D. C. Bradley, A. R. Brown, R. N. Marks, K. Macky, and P. L. Burns. Nature, 1990: 347,539-541.

Goyal, Kavita, and Mamta Goyal. "Study of Light Emitting Diode." International Research Journal of Engineering and Technology (IRJET), 2019: 157-160.

Ijeaku, Aririguzo Marvis, Madu Hilary Chidubem, Emerole Kelechi chukwunonyerem, and Nwogu Uchenna Obioma. “Organic Light Emitting Diode (OLED).” American Journal of Engineering Research (AJER), 2015: 153-159.

Patel, Bhrijesh N., and Mrugesh M. Prajapati. "OLED: A Modern Display Technology." International Journal of Scientific and Research Publication, 2014: 1-5.

Patel, Kaushika D., Nilesh Patel, and Ravi Vatalia. "A New Generation of Organic Light Emitting Displays - OLED." International Journal of Scientific Research and Reviews, 2018: 697-706.

Shrinar, J. Organic Light Emitting Device. NewYork: AIP press, 2005.

Yan, L., and Y. Gao. Thin Solid Films. 2002, 101-106.



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