# OLED (Organic Light Emitting Diode) screen
## physics behind OLEDs

OLEDs are emitting light thanks to the electron-hole recombination principle. 

OLEDs are made of: 
  * an organic emissive layer in which electron and hole recombine: electrons being excited by an electrical field unexcite themselves by emitting a photon
  * (optional: only for multilayer OLEDs) a hole transport and electron transport layer above and below the emissive layer, respectively: to improve the movment of holes and electrons towards the emissive layer 
  * (optional: only for multilayer OLEDs) a hole injection layer to improve the movement of hole from the anode towards the hole transport layer
  * two electrodes: an anode under the hole injection layer and a cathode on top of the electron transport layer
  * a subtrate - usually glass - under the anode

{{:capture_d_ecran_2026-04-02_160915.png?direct&400 |}} (([Joëlle Rault-Berthelot, Cyril Poriel, Diodes électroluminescentes organiques : quatre technologies différentes, 2021](https://univ-rennes.hal.science/hal-03388539/file/Rault-Berthelot-2021-%20OLED%204%20technologies%20diff%C3%A9rentes.pdf)))
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## Types of rigidity: flexible vs. rigid OLED screens 

Flexible OLED based products aren’t always bendable or foldable((P. Samorì and V. Palermo, [Flexible Carbon-based Electronics](https://books.google.fr/books?id=c_x0DwAAQBAJ). John Wiley & Sons, 2018)). Flexibility is used to provide the display with a non-traditional form factor, but it is then bonded to a rigid glass cover in the product. e.g.: some smartphones (Samsung Galaxy Note, LG G Flex), some smartwatches (Apple Watch, LG watch urbane). 1st smartphone using AMOLED flexible tehcnology = 2013. 

The difference in the manufacturing of flexible and rigid OLEDs remains in two processes steps: substrate and encapsulation ([[screen_manufacturing|check this page]] for more information). 
## Types of matrix: active vs. passive
### AMOLED (Active-Matrix OLED) 
AMOLEDs include complete layers of cathode, organic components and anode. The layers of anode consist of TFT (thin film transistors) in parallel to form a matrix, which helps in switching each pixel to its on or off state as required hence, forming an image. When the pixels are not needed, they turn off or a black image on display occurs. This is least power consuming type and has quick refresh rates. They are best used in computer monitors, electronic signs or big TV screens. 
(([Q. Liu and T. Zhang, ‘Comparison between AMOLED and Traditional Display Technology and Application of AMOLED’, SID Symp. Dig. Tech. Pap., vol. 53, no. S1, pp. 1018–1021, 2022, doi: 10.1002/sdtp.16179.](https://www.researchgate.net/publication/364399984_P-1314_Comparison_between_AMOLED_and_Traditional_Display_Technology_and_Application_of_AMOLED)))

### PMOLED (Passive-Matrix OLED)

Passive matrix organic light emitting display is restricted to its passive driving method, so it is hard to make the screen size and resolution larger because the brightness is reduced with the row numbers and it needs a driver Integrated Circuit (IC) with high current supply. In order to make high resolution and large size Organic Light Emitting Display (OLED), companies invest much money to produce Active Matrix Organic Light Emitting Display (AMOLED). But compared with AMOLED, Passive Matrix Organic Light Emitting Display (PMOLED) has simple fabrication process, which does not need very complex Thin Film Transistor (TFT) backplane and also has less material cost.
They are limited in size and resolution. They have wide applications in MP3 player, cell phones displays. 
(([Nikhil Sain and Deepesh Sharma, Priya Choudhary, ‘A Review Paper On: Organic Light-Emitting Diode (Oled) Technology and Applications’, International Journal of Engineering Applied Sciences and Technology, Vol. 4, Issue 11, ISSN No. 2455-2143, pp. 587–591, 202](https://www.ijeast.com/papers/587-591%2CTesma411%2CIJEAST.pdf))) 

## Type of emission colour

### RGB side-by-side

### white OLED with colour filters

### blue OLED with colour converting materials
## Type of stack structures: top vs. bottom emission
### top emission OLED
  * most used technology because less constraints on materials (no need for backplane to be transparent)
  * cathode must be transparent (Ag material)
{{ :capture_d_ecran_2026-04-02_092931.png?direct |}}

### bottom emission OLED
  * this technology emerged first, 
  * the main inconvenient is that the backplane (substrate, TFT matrix if AMOLED) must be transparent, 
  * anode must be transparent (ITO is the main material used because it is both conductive and transparent), 
  * used widly for larger screen sizes

{{ :capture_d_ecran_2026-04-02_093224.png?direct |}}
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## Other OLED technologies

### QD-OLED (Quantum-Dot OLED)
QD-OLED displays, which use blue OLED panels with QD color filters, started coming to market in 2023. 
(( ["Sony Bravia XR A95L QD-OLED TV review". 9 November 2023.](https://www.tomsguide.com/reviews/sony-bravia-xr-a95l-qd-oled-tv)))