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intro_oled [2026/06/05 11:39] – [Working principle] sophieintro_oled [2026/06/05 16:45] (current) – [Other topics] yusufabdillah
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-An **OLED** (Organic Light-Emitting Diode) is a type of LED whose **emissive layer** consists of an organic compound film that produces light when an electric current flows through it. This organic layer is sandwiched between two **electrodes,** at least one of which is typically transparent. The technology is widely found in everyday screens, including televisions, computer monitors, smartphones, and handheld game consoles. Additionally, the development of white OLEDs for solid-state lighting applications remains an active area of research. This is imaged on {{ref>basic_structure_oled}}.+An **OLED** (Organic Light-Emitting Diode) is a type of LED whose **emissive layer** consists of an organic compound film that produces light when an electric current flows through it. This organic layer is sandwiched between two **electrodes,** at least one of which is typically transparent. The technology is widely found in everyday screens, including televisions, computer monitors, smartphones, and handheld game consoles. Additionally, the development of white OLEDs for solid-state lighting applications remains an active area of research. This is shown on {{ref>basic_structure_oled}}.
  
 <figure center blank|basic_structure_oled> <figure center blank|basic_structure_oled>
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 One notable area of development is the use of OLED in non-conventional display formats.** Transparent** OLED panels, which allow light to pass through when pixels are off, are used in applications such as retail signage and heads-up displays. **Curved and flexible** OLED screens have been integrated into various consumer devices, taking advantage of the technology's ability to be deposited on non-rigid substrates. **Foldable displays**, now present in a growing number of smartphones and tablets, also rely on flexible OLED panels to enable their form factor. \\ One notable area of development is the use of OLED in non-conventional display formats.** Transparent** OLED panels, which allow light to pass through when pixels are off, are used in applications such as retail signage and heads-up displays. **Curved and flexible** OLED screens have been integrated into various consumer devices, taking advantage of the technology's ability to be deposited on non-rigid substrates. **Foldable displays**, now present in a growing number of smartphones and tablets, also rely on flexible OLED panels to enable their form factor. \\
  
-To understand in more details what are the prevalent applications for screens, please check this page: [[screen_market|Screen market review]].+To understand in more detail what are the prevalent applications for screens, please check this page: [[screen_market|Screen market review]].
  
 <figure center|applications_OLED> <figure center|applications_OLED>
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 <figure right |PNjunction> <figure right |PNjunction>
 {{ :capture_d_ecran_2026-06-05_113442.png?direct&600|}} {{ :capture_d_ecran_2026-06-05_113442.png?direct&600|}}
-<caption> [[https://warwick.ac.uk/fac/sci/physics/current/postgraduate/regs/mpagswarwick/ex5/devices/led/|Electron-hole Recombination]]</caption>+<caption> PN junction working principle</caption>
 </figure> </figure>
-An **OLED** consists of a stack of organic layers sandwiched between an **anode** and a **cathode**. Usually there are at least 3 organic layers: a **hole transport layer (HTL)**, an **electron transport layer (ETL)**, and an **emission layer (EML)**. When applying a specific electrical voltage between the anode and the cathode, electrons (coming from the cathode through the ETL) and holes (coming from the anode through the HTL) are injected in the EML. Holes have a positive charge and are called **"p particules"**; electrons have a negative charge and are called **"n particules"**. Holes and electrons recombined together in the EML - which is the **p-n junction** - forming excited electron-hole pairs as imaging in {{ref>PNjunction}}. When a pair is deexciting, a photon is emitted: this is the **electroluminescence**. +An **OLED** consists of a stack of organic layers sandwiched between an **anode** and a **cathode**. Usually there are at least 3 organic layers: a **hole transport layer (HTL)**, an **electron transport layer (ETL)**, and an **emission layer (EML)**. When a specific electrical voltage is applied between the anode and the cathode, electrons (coming from the cathode through the ETL) and holes (coming from the anode through the HTL) are injected into the EML. Holes have a positive charge and are called **"p particles"**; electrons have a negative charge and are called **"n particles"**. Holes and electrons recombine in the EML - which is the **p-n junction** - forming excited electron-hole pairs as imaging in {{ref>PNjunction}}. When a pair returns to its ground state, a photon is emitted: this is the **electroluminescence**. 
  
  
  
-The choice of materials constituting the ETL, HTL, and EML is fundamental to determine the OLED properties. To optimise the electron-hole recombination, and thus the OLED efficiency, the number of holes and the number of electrons must be equal in the EML. This is quite a challenge in organic semi-conductors because hole mobility is higher than the electron one. The ETL contains often **luminophors** (especially in the small-molecule technology) which are molecules that emit light only in presence of an electron-hole pair in a particular state (singlet). To improve the efficiency of the device, some transition materials can be added in the EML (this is the phosphorescent OLED technology) to allow light emission even if pairs are not in the singlet state. +The choice of materials constituting the ETL, HTL, and EML is fundamental in determining the OLED properties. To optimise the electron-hole recombination, and thus the OLED efficiency, the number of holes and the number of electrons must be equal in the EML. This is quite a challenge in organic semi-conductors because hole mobility is higher than electron mobility. The ETL often contains **luminophores** (especially in the small-molecule technology) which are molecules that emit light only in the presence of an electron-hole pair in a particular state (singlet). To improve the efficiency of the device, some transition materials can be added in the EML (this is the phosphorescent OLED technology) to allow light emission even if pairs are not in the singlet state. 
  
 Usually, luminophors are based on **PPV (poly(p-phenylene vinylene)** and on **PFO (polyfluorene)**.  Usually, luminophors are based on **PPV (poly(p-phenylene vinylene)** and on **PFO (polyfluorene)**. 
-One of the electrodes must be reflective and the over transparent, depending on the [[intro_oled#structure_stacks|structure stack]]. The reflective one  is mostly made of ** Al** or **Ag** [(OLED_fundamentals > [[https://ia801606.us.archive.org/35/items/ebooks_201907/Daniel%20J.%20Gaspar%2C%20Evgueni%20Polikarpov%20-%20OLED%20Fundamentals_%20Materials%2C%20Devices%2C%20and%20Processing%20of%20Organic%20Light-Emitting%20Diodes-CRC%20Press%20%282015%29.pdf | Daniel J. Gaspar, Evgueni Polikarpov - OLED Fundamentals_ Materials, Devices, and Processing of Organic Light-Emitting Diodes-CRC Press (2015)]])] , and the semi-transparent one is mostly made of **Indium-Tin-Oxide (ITO)** or **Mg:Ag** [(OLED_fundamentals)].+One of the electrodes must be reflective and the other transparent, depending on the [[intro_oled#structure_stacks|structure stack]]. The reflective one  is mostly made of ** Al** or **Ag** [(OLED_fundamentals > [[https://ia801606.us.archive.org/35/items/ebooks_201907/Daniel%20J.%20Gaspar%2C%20Evgueni%20Polikarpov%20-%20OLED%20Fundamentals_%20Materials%2C%20Devices%2C%20and%20Processing%20of%20Organic%20Light-Emitting%20Diodes-CRC%20Press%20%282015%29.pdf | Daniel J. Gaspar, Evgueni Polikarpov - OLED Fundamentals_ Materials, Devices, and Processing of Organic Light-Emitting Diodes-CRC Press (2015)]])] , and the semi-transparent one is mostly made of **Indium-Tin-Oxide (ITO)** or **Mg:Ag** [(OLED_fundamentals)].
  
  
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 - **SMOLED (Small Molecules OLED)** - **SMOLED (Small Molecules OLED)**
  
-The term OLED refers by default to the small molecule technology as it is the prevalent OLED technology. The light emission is produced by **luminiphors** (in presence of electron-hole pairs), which are small molecule constituting the EML. The typical molecules used in SMOLEDs are organometallic chelates (e.g. Alq3 [([[https://www.ossila.com/products/alq3?_pos=1&_sid=41e6651ab&_ss=r]])]) or conjugated dendrimers. +The term OLED generally refers to small-molecule technology as it is the prevalent OLED technology. The light emission is produced by **luminophores** (in presence of electron-hole pairs), which are small molecules constituting the EML. The typical molecules used in SMOLEDs are organometallic chelates (e.g. Alq3 [([[https://www.ossila.com/products/alq3?_pos=1&_sid=41e6651ab&_ss=r]])]) or conjugated dendrimers. 
  
 - **PLED (Polymer OLED)** - **PLED (Polymer OLED)**
  
-In this technology, **polymers** are resposible for light emission (instead of luminophors in semi-conductors). +In this technology, **polymers** are responsible for light emission (instead of luminophores in semiconductors). 
 The PLED working principle is the same as in SMOLED, but the EML consists of polymers only. They are deposited by thin-film deposition.  The PLED working principle is the same as in SMOLED, but the EML consists of polymers only. They are deposited by thin-film deposition. 
  
 - **QD-OLED (Quantum-Dot OLED)** - **QD-OLED (Quantum-Dot OLED)**
  
-There are two types of QD displays: photo-emissive and electro-emissive. The main technology of QD display on the market is **photo-emissive** as the electro-emissive is only experimental to this date (2026). Hence, the term QD-OLED refers by default to photo-emissive QD-OLED, which its working principle is detailed bellow. +There are two types of QD displays: photo-emissive and electro-emissive. The main technology of QD display on the market is **photo-emissive** as the electro-emissive is only experimental to this date (2026). Hence, the term QD-OLED refers by default to photo-emissive QD-OLED, whose working principle is detailed below. 
  
-This technology is emerging in the market since 2023, developed mainly by Samsung[([[https://www.tomsguide.com/reviews/sony-bravia-xr-a95l-qd-oled-tv|"Sony Bravia XR A95L QD-OLED TV review". 9 November 2023.]])]. It combines the SMOLED principle with the quantum-dot principle. A layer of SMOLED are emitting a **monochromatic blue light**, that is then converted by **quantum dots** (a thin layer of crystals placed above the blue OLED) to obtain the RGB spectrum. The size of the quantum dots determines the wavelength of the filtered rays and is therefore a dimensionning parameter.+This technology is emerging in the market since 2023, developed mainly by Samsung[([[https://www.tomsguide.com/reviews/sony-bravia-xr-a95l-qd-oled-tv|"Sony Bravia XR A95L QD-OLED TV review". 9 November 2023.]])]. It combines the SMOLED principle with the quantum-dot principle. A layer of SMOLED emits a **monochromatic blue light**, that is then converted by **quantum dots** (a thin layer of crystals placed above the blue OLED) to obtain the RGB spectrum. The size of the quantum dots determines the wavelength of the filtered rays and is therefore a dimensioning parameter.
  
 - **PhOLED (Phosphorescent OLED)** - **PhOLED (Phosphorescent OLED)**
  
-Phosphorescent OLEDs emit light by both **fluorescence** and **phosphorescence** phenomenon. In classic OLEDs only fluorescence is observed thanks to electron-hole pairs (in a singlet state) deexcitation. By adding an organometallic complex into the EML, more electron-hole pairs are able to emit light (those in a triplet state) producing phosphorescence. The enhencement of the light emission efficiency by adding a material is called **doping** the EML. +Phosphorescent OLEDs emit light by both **fluorescence** and **phosphorescence** phenomena. In classic OLEDs only fluorescence is observed thanks to electron-hole pairs (in a singlet state) de-excitation. By adding an organometallic complex into the EML, more electron-hole pairs are able to emit light (those in a triplet state) producing phosphorescence. The enhancement of the light emission efficiency by adding a material is called **doping** the EML. 
  
 - **micro-OLED** - **micro-OLED**
  
-This technology is used mainly for screens **measuring in the order of a micrometer**. Hence, OLEDs have to be smaller: that is where the real technological challenge lies. To this date, this technology is still under development as **only some wavelengths can be produced** by micro-OLED+This technology is used mainly for screens **measuring in the order of a micrometer**. Hence, OLEDs have to be smaller: that is where the real technological challenge lies. To date, this technology is still under development as **only some wavelengths can be produced** by micro-OLED
  
  
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-OLED displays can be built using different architectures, each with its own approach to generating color.+OLED displays can be built using different architectures, each with its own approach to generating colour. 
  
  
 - **WOLED (white OLED) with colour filters** - **WOLED (white OLED) with colour filters**
  
-In this architecture, every sub-pixel emits **white light**. Color is then obtained by passing that light through dedicated color filters (red, green, blue). The intensity of each sub-pixel is individually controlled to achieve the desired color and brightness. This approach is commonly used in large-screen applications such as TV panels, as it simplifies the manufacturing process.  +In this architecture, every sub-pixel emits **white light**. Colour is then obtained by passing that light through dedicated colour filters (red, green, blue). The intensity of each sub-pixel is individually controlled to achieve the desired colour and brightness. This approach is commonly used in large-screen applications such as TV panels, as it simplifies the manufacturing process.  
-Several approaches exist to generate white light within a sub-pixel. **Single-stack** white OLEDs, which rely on a single emissive unit, tend to suffer from low efficiency and are therefore rarely used in practice. The most common solution is to **stack two or three** emissive units on top of each other, which significantly improves efficiency and brightness. The diagram below shows an example of a **two-stack white OLED** architecture, combining red, green, and blue emissive layers :+Several approaches exist to generate white light within a sub-pixel. **Single-stack** white OLEDs, which rely on a single emissive unit, tend to suffer from low efficiency and are therefore rarely used in practice. The most common solution is to **stack two or three** emissive units on top of each other, which significantly improves efficiency and brightness. The diagram below illustrates a **two-stack white OLED** architecture, combining red, green, and blue emissive layers :
  
 <figure center |white_oled_2_stack> <figure center |white_oled_2_stack>
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 - **RGB side-by-side OLED** - **RGB side-by-side OLED**
  
-In this structure, each sub-pixel is an independent OLED emitting its own color : **red, green, or blue**. By controlling the intensity of each sub-pixel individually, any color can be reproduced with high accuracy. This approach offers excellent color purity and energy efficiency, since only the required colors are actually lit. It is widely used in smartphones and high-end displays.+In this structure, each sub-pixel is an independent OLED emitting its own colour: **red, green, or blue**. By controlling the intensity of each sub-pixel individually, any colour can be reproduced with high accuracy. This approach offers excellent color purity and energy efficiency, since only the required colours are actually lit. It is widely used in smartphones and high-end displays.
  
 - **Blue OLED with colour converting materials** - **Blue OLED with colour converting materials**
  
-In this architecture, all sub-pixels start from a **blue OLED emitter. Color conversion materials** such as quantum dots, are then used to shift the blue light into red or green for the corresponding sub-pixels. This is typically based on QD-OLED technology. This approach combines the manufacturing simplicity of a single emitter type with the color quality benefits of per-color emission.+In this architecture, all sub-pixels start from a **blue OLED emitter. Colour conversion materials** such as quantum dots are then used to shift the blue light into red or green for the corresponding sub-pixels. This is typically based on QD-OLED technology. This approach combines the manufacturing simplicity of a single emitter type with the colour quality benefits of per-color emission.
  
 <figure center |different_structure_type_OLED> <figure center |different_structure_type_OLED>
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 <caption>Different structure types [(Flat_Panel_Display)]</caption> <caption>Different structure types [(Flat_Panel_Display)]</caption>
 </figure> </figure>
 +
 +- **number of stacks**
 +
 +There are various stacks possible, depending on the display size or even on the display performance (brightness, power consumption). A one-stack OLED consists of the superposition of anode, HIL, HTL, EML, ETL, EIL, cathode. A two-stacked OLED consists of two one-stack OLED in series connection. The {{ref>stacks of OLEDs}} below set out the material layouts for these different options. The interconnection layer is called a charge generation layer (CGL) and plays both roles of anode and cathode. {{ref>stacks of OLEDs}} illustrates examples of stacks. The materials indicated with the arrows are abreviations for their scientific full name, their chemical formulas can be easily found in the literature. A material M1 doped with a material M2 is denoted as M1:M2.
 +
 +<figure center|stacks of OLEDs>
 +{{ :capture_d_ecran_2026-06-05_103936.png?direct&200 |}}
 +{{ :capture_d_ecran_2026-06-05_103916.png?direct&300 |}}
 +{{ :capture_d_ecran_2026-06-05_103930.png?direct&300 |}}
 +<caption>: Examples of OLEDs stacks</caption>
 +</figure>
 +
 +[comment]: <> (
 +^ one-stack RGB side-by-side OLED ^ material ^
 +| cathode |  |
 +| ETL |  |
 +| EML - red&blue&green |  |
 +| HTL | |
 +| HIL | |
 +| anode | ITO |
 +
 +^ two-stack white RGB OLED ^ material ^
 +| cathode | Al |
 +| ETL | Mg:Al |
 +| EML - green| [[https://www.sigmaaldrich.com/FR/fr/product/aldrich/444561?srsltid=AfmBOopl7PLZmnmwH5S6QneZGv8splZD44KYLcU3r6u8flNNaEFILTK2|Alq3]] |
 +| CGL| [[https://www.lumtec.com.tw/products-view.php?ID=330|BCP]] |
 +| EML - blue&red:dopant | [[https://www.ossila.com/products/npb|NDP]]:[[https://lumtec.com.tw/products-view.php?ID=292|DCM2]] |
 +| HTL | [[https://www.ossila.com/products/npb|NDP]] |
 +| HIL | none |
 +| anode | ITO |
 +
 +^ one-stack white RGBY OLED ^ material ^
 +| cathode | Al |
 +| ETL | NET18:NDN26 |
 +| HBL | NET18 |
 +| EML - green:dopant | BH121:GD403 |
 +| EML - blue:dopant | BH121:EK9 |
 +| EML - yellow:dopant | HT1:YD3 |
 +| EML - red:dopant | HT1:RD3 |
 +| EBL | BH5 |
 +| HTL | BH5:NDP9 |
 +| HIL | none |
 +| anode | ITO |
 +)
 +
 +
 \\ \\
  
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-The characteristic of this structure is that light is directed from the emissive layer **towards the anode** and the substrate: they have to be **transparent** in the physics meaning if the term (i.e. not interact with a wave, and hence not absorb light rays). Usually, the transparent electrode is made of **ITO (indium-tin oxide)** and the substrate is made of **glass**.+The characteristic of this structure is that light is directed from the emissive layer **towards the anode** and the substrate: they have to be **transparent** in the physical sense of the term (i.e. not interact with a wave, and hence not absorb light rays). Usually, the transparent electrode is made of **ITO (indium-tin oxide)** and the substrate is made of **glass**.
  
-Because the emissive layer emits in all space direction, the cathode has to be made of a **reflective material** (e.g. silver) to redirect rays towards the anode. +Because the emissive layer emits light in all directions, the cathode has to be made of a **reflective material** (e.g. silver) to redirect rays towards the anode. 
  
-The main disadvantage of the bottom-emission structure is that light has to pass through the pixel control circuit (the TFT matrix in the case of an AMOLED) that cannot be fully transparent. This implies a lower quantity of light that can actually get out of the device. It is in this context that the top-emission structure emerged, to compensate for this drawback.+The main disadvantage of the bottom-emission structure is that light has to pass through the pixel control circuit (the TFT matrix in the case of an AMOLED) that cannot be fully transparent. This implies a lower amount of light that can actually get out of the device. It is in this context that the top-emission structure emerged, to compensate for this drawback.
  
 <figure center |bottom_emission_OLED> <figure center |bottom_emission_OLED>
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-In this structure, light is emitted from the emissive layer **towards the cathode**, it must therefore be transparent. Conversely, the **anode must be reflective** to redirect light towards the cathode. ITO is not a great fit for the cathode material due to technical constraints during the material deposition, and preferably a **thin-film silver** or **magnesium-silver alloys** is used. The cathode is however **semi-transparent** rather than transparent (some part of the incident rays are transmitted by the material and another part is reflected) which is not a problem if the material have **great transmittance** and **conductivity**. +In this structure, light is emitted from the emissive layer **towards the cathode**, it must therefore be transparent. Conversely, the **anode must be reflective** to redirect light towards the cathode. ITO is not a great fit for the cathode material due to technical constraints during the material deposition, and preferably a **thin-film silver** or **magnesium-silver alloys** is used. The cathode is however **semi-transparent** rather than transparent (some part of the incident rays are transmitted by the material and another part is reflected) which is not a problem if the material has **great transmittance** and **conductivity**. 
  
 <figure center |top_emission_OLED> <figure center |top_emission_OLED>
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-This structure has **two transparent electrodes**: it is a combination of both bottom- and top-emission structures. This helps obtaining **higher contrast levels** and make it particularly adapted for **outdoor devices**. +This structure has **two transparent electrodes**: it is a combination of both bottom- and top-emission structures. This helps obtaining **higher contrast levels** and makes it particularly suitable for **outdoor devices**. 
  
  
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 - **AMOLED (Active-Matrix OLED)** - **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. +**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 switch 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 the least power-consuming type and has quick refresh rates. They are commonly 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))] [([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))]
  
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 **Types of rigidity: flexible vs. rigid OLED screens ** **Types of rigidity: flexible vs. rigid OLED screens **
  
-Flexible OLED based products aren’t always bendable or foldable  +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. +[(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). The first smartphone using flexible AMOLED technology was introduced in 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).+The difference in the manufacturing of flexible and rigid OLEDs lies in two processes steps: substrate and encapsulation ([[screen_manufacturing|check this page]] for more information).