How PMOLED is Made | Manufacturing Process, Materials

Manufacturing requires the use of vacuum thermal evaporation technology to deposit an organic material layer with a total thickness of approximately 150 nanometers on the ITO anode.

It achieves progressive scanning drive through an orthogonally arranged electrode matrix, eliminating the need for an active backplane.

Manufacturing Process

PMOLED manufacturing does not require complex TFT backplanes. Its core process lies in using negative photoresist to construct cathode isolation pillars with an "inverted trapezoid" cross-section.

This structural height is usually controlled at 2 to 3 microns.

In the subsequent 10^-7 Torr high-vacuum thermal evaporation (VTE) process, it utilizes the physical shadowing effect to automatically cut off the metal deposition layer, thereby forming mutually independent cathode lines.

The entire organic light-emitting stack thickness is only 200 to 400 nanometers, which is extremely sensitive to moisture and oxygen.

Therefore, it must ultimately be hermetically sealed (glass-to-glass) in a nitrogen-filled glove box, accompanied by a desiccant.

ITO Substrate Etching

The manufacturing of a PMOLED display screen begins with a precision-machined transparent conductive glass.

Typically, 0.5 mm or 0.7 mm thick alkali-free glass or soda-lime glass is used as the substrate, with a layer of Indium Tin Oxide (ITO) film, 1200 Angstroms to 1500 Angstroms (Å) thick, pre-deposited on its surface via magnetron sputtering processes.

The Sheet Resistance of this ITO film is typically controlled between 10 to 15 Ohms/square (Ω/□) to ensure that the voltage drop (IR Drop) is maintained within an acceptable range under high current drive.

Before entering the photolithography production line, the entire substrate must undergo an extremely rigorous cleaning process, usually consisting of multiple tanks with surfactants, deionized water (DI Water), and ultrasonic cleaning.

The goal is to control surface particle contaminants to below 0.3 microns and ensure the water contact angle is less than 10 degrees, indicating the glass surface has reached a hydrophilic state suitable for subsequent photoresist coating.

The cleaning recipe usually involves: alkaline detergents to remove organic residues, switching ultrasonic frequencies between 40kHz and 100kHz to shake off particles of different sizes, and finally rinsing with ultra-pure water with a resistivity of 18 MΩ·cm, followed by high-speed spin drying to prevent water marks.

The production line uses a Spin Coater to drip positive photoresist onto the center of the high-speed rotating ITO glass.

The substrate typically rotates at a speed of 1000 to 1500 RPM. The centrifugal force causes the photoresist to form a uniformly thick film on the ITO surface, with the target thickness usually set around 1.5 microns.

After coating, the substrate is sent to a Hot Plate for a Soft Bake, with the temperature set at 90°C to 100°C for about 60 to 90 seconds.

This heat treatment process removes solvent components from the photoresist, enhances the adhesion of the resist layer to the ITO surface, and prevents sticking to the mask plate during subsequent contact exposure.

The substrate enters the UV exposure machine, covered by a chrome Photomask with pre-drawn anode column patterns.

Under irradiation of i-line (365 nanometers) or mixed-band UV light, the photoresist areas hit by the UV rays undergo a photochemical reaction, breaking their molecular chains and becoming soluble in the developer solution.

The exposure energy is typically controlled at 40 to 60 mJ/cm².

Subsequently, the substrate passes through a spray or immersion developer tank, using a 2.38% Tetramethylammonium Hydroxide (TMAH) or Potassium Hydroxide (KOH) solution to wash away the photoresist in the exposed areas, revealing the ITO surface that needs to be etched away, while the remaining photoresist covers the data lines that will become the anodes.

Wet Etching is the standard process for removing excess ITO.

The substrate is conveyed to a tank containing a specific etchant.

The common chemical formula is a mixed solution of Hydrochloric Acid (HCl) and Nitric Acid (HNO3), sometimes with a small amount of Ferric Chloride added as an oxidizing agent.

The temperature of the etchant is strictly controlled between 40°C and 50°C, because a temperature fluctuation exceeding 1°C significantly changes the Etch Rate, leading to uncontrollable line widths.

When the etching reaction occurs, the exposed ITO is chemically dissolved, and the reaction time is usually completed within a few minutes.

In addition to completely removing the unwanted conductive layer, the etching process also needs to control the slope (Taper Angle) of the ITO line edges.

Vertical 90-degree edges lead to discontinuous coverage of the subsequently deposited organic layers.

Therefore, the process goal is usually to control the edge slope between 40 degrees and 60 degrees, which requires adjustment of the etchant ratio and stirring method.

Typical etching parameters: Etchant concentration HCl:HNO3:H2O = 50:3:47, etch rate approx. 1000 Å/min, lateral Undercut needs to be controlled within 0.5 microns to guarantee pixel aperture ratio.

After etching is complete, the remaining photoresist has fulfilled its protective mission and must be removed using a Stripper.

Typically, amine-based organic solvents are used to soak and spray at 60°C to 70°C, causing the photoresist to swell and fall off. This is followed by multiple rounds of deionized water cleaning.

At this point, only the patterned ITO anode strips remain on the substrate. To ensure the uniformity of luminescence and the lifespan of the PMOLED, roughness testing must be performed on the etched ITO surface.

Using an Atomic Force Microscope (AFM) to scan the surface, the Root Mean Square (RMS) roughness must be lower than 20 Å (2 nanometers).

Any spikes higher than this value could puncture the organic emissive layer, which is only a few hundred nanometers thick, causing a device short circuit.

Cathode Isolation Pillars

The process usually requires the height of the isolation pillars to be between 2.0 microns and 5.0 microns.

This height is far greater than the total thickness of the subsequently deposited organic light-emitting layer (approx. 200 nm) and metal cathode layer (approx. 150 nm), typically maintaining an aspect ratio of 10:1 to 20:1 to ensure sufficient physical shadowing range during metal evaporation.

This special inverted trapezoidal structure is not obtained through conventional vertical etching but is formed naturally by utilizing the light absorption characteristics of the photoresist during the Ultraviolet (UV) exposure process.

When UV light (usually i-line 365 nm wavelength) irradiates a negative resist layer several microns thick, the light is gradually absorbed by the photosensitizer as it penetrates the resist layer, causing the light energy reaching the bottom of the resist layer to be significantly lower than that at the top.

Typical exposure energy is set in the range of 100 to 300 mJ/cm², which results in extremely high cross-linking density at the top of the resist layer and lower cross-linking density at the bottom. In the subsequent development step (using 2.38% TMAH developer), the dissolution rate at the bottom is faster than at the top, thereby forming an inward concave cut (Undercut) on the sidewall.

For a standard PMOLED pixel array, the overhang length needs to be controlled between 0.5 microns and 1.0 microns.

• Sidewall Taper Angle: The ideal angle between the isolation pillar sidewall and the substrate plane should be a negative angle, usually defined as 95 degrees to 130 degrees (relative to the substrate plane), or -5 degrees to -40 degrees relative to the vertical line.
• Top and Bottom Width Difference: The width at the top of the isolation pillar is usually set at 10 microns to 20 microns, while the bottom width in contact with the substrate shrinks by 2 microns to 4 microns.

After the isolation pillars are formed and have undergone development and cleaning, a high-temperature Hard Bake treatment must be performed.

This is a thermal stabilization process aiming to thoroughly remove residual organic solvents and moisture from inside the photoresist and fully cross-link the polymer structure.

Since the subsequent organic material deposition is extremely sensitive to impurities, the isolation pillars themselves cannot become a source of contamination.

The hard bake process is typically conducted in a vacuum or nitrogen environment at 220°C to 240°C for more than 60 minutes.

After this treatment, the chemical properties of the photoresist become extremely inert, capable of withstanding thermal radiation in subsequent high-vacuum chambers, and the Outgassing Rate needs to be reduced to below 10^-9 Torr·L/s/cm² to prevent gas release causing Pixel Shrinkage during the panel's lifespan.

The subsequent metal cathode deposition utilizes the principle of Line-of-Sight Deposition.

In a high vacuum environment of 10^-7 Torr, the aluminum or magnesium-silver alloy atomic beam has a very long mean free path and moves almost in a straight line.

When the atomic beam strikes the substrate vertically, the "eaves" at the top of the inverted trapezoidal isolation pillar act like an umbrella, shielding the cathode gap area beneath it.

• Cathode Breaking Mechanism: Metal atoms are deposited on the top surface of the isolation pillars and on the pixel light-emitting area between two isolation pillars.
• Shadow Zone: Due to the inward slope and overhang of the isolation pillar sidewalls, metal atoms cannot deposit onto the sidewalls of the isolation pillars, nor can they cover the substrate area immediately adjacent to the bottom of the isolation pillars.
• Electrical Isolation: This results in the metal film deposited in the pixel area being completely spatially disconnected from the metal film deposited on top of the isolation pillars.

If the area of the evaporation source is too large or the distance between the substrate and the evaporation source (TS Distance) is too close, resulting in an incident angle deviation exceeding 5 degrees to 10 degrees, metal atoms may bypass the overhang structure and deposit onto the sidewalls, causing Short Circuits between adjacent cathode rows.

Therefore, production lines typically set the distance between the substrate and the source at 500 mm to 1000 mm and strictly limit the angular oscillation of the substrate during rotation to ensure the cathode separation yield reaches over 99.9%.

Vacuum Thermal Evaporation

To ensure that organic materials do not react with oxygen or water molecules in the air during sublimation, the base pressure in the chamber must be pumped down to below 10^-7 Torr.

This vacuum level ensures that the Mean Free Path of gas molecules exceeds 50 meters, far greater than the distance from the evaporation source to the substrate.

In this environment, heated organic molecules can fly towards the substrate in a collision-free ballistic trajectory, ensuring the purity and uniformity of the film deposition.

Glass substrates are transferred between various chambers via robotic arms, without breaking the vacuum environment throughout the process, until all organic layers and metal cathodes are deposited.

Evaporation sources for organic materials typically use crucibles made of Tantalum or ceramics, also known as Knudsen Cells.

Since most small-molecule organic materials decompose easily at high temperatures, the heating temperature control precision must be maintained at ±0.1°C or better.

Resistance heating wires wrap around the crucible, heating the organic powder to its sublimation point, usually between 200°C and 450°C.

Unlike metal melting and evaporation, organic materials sublime directly from solid to gas.

For Doping processes, such as doping 2% to 5% fluorescent or phosphorescent dyes into the emissive layer, the system activates two independent evaporation sources simultaneously, achieving precise molecular mixing by independently controlling the deposition rate ratio of the two.

Real-time monitoring of the deposition rate relies on Quartz Crystal Microbalance (QCM) sensors.

These sensors are placed near the substrate and work using the piezoelectric effect. When organic molecules deposit on the surface of the quartz crystal, the oscillation frequency of the crystal shifts.

Standard 6 MHz quartz crystals are extremely sensitive to mass changes and can detect rate fluctuations of 0.1 Angstroms (Å)/second.

In actual production, the deposition rate of organic layers is typically controlled between 0.5 and 2.0 Å/second.

Excessively fast rates lead to reduced film density and increased surface roughness, while excessively slow rates may cause thermal radiation damage to the substrate or introduce too much background impurity gas.

A closed-loop control system reads the frequency feedback from the QCM and dynamically adjusts the heating power of the evaporation source via a PID algorithm to lock in the set deposition rate.

Deposition Layer	Typical Material Example	Process Temp Range	Deposition Rate (Å/s)	Layer Thickness (nm)	Functional Parameter
HIL	MTDATA / 2-TNATA	280°C - 320°C	1.0 - 2.0	10 - 60	HOMO Energy Level Matching
HTL	NPB / TPD	240°C - 280°C	1.0 - 2.0	30 - 50	High Hole Mobility
EML	Alq3 (Host) + Dopant	260°C - 300°C	0.5 - 1.0	20 - 40	Luminous Efficiency/Color Coordinates
ETL	Alq3 / TPBi	260°C - 300°C	1.0 - 2.0	30 - 50	Electron Transport Balance
Cathode	Al / Mg:Ag	600°C - 1000°C	5.0 - 10.0	100 - 150	Low Work Function/High Reflection

For the manufacture of color PMOLEDs, in addition to this full-area Open Mask deposition, a Fine Metal Mask (FMM) is required.

This mask is usually made of Invar Alloy because it has an extremely low coefficient of thermal expansion (< 1.5 ppm/°C), capable of withstanding radiant heat during evaporation without deformation.

The FMM is only 20 to 30 microns thick, with micropores corresponding to pixels etched via photolithography.

Inside the vacuum chamber, the FMM must be aligned with the glass substrate with high precision, typically requiring an alignment error of less than 2.0 microns.

Encapsulation and Dicing

The organic emissive layer of a PMOLED is extremely sensitive to water vapor and oxygen in the atmosphere.

Any Water Vapor Transmission Rate (WVTR) exceeding 10^-6 g/m²/day will cause cathode oxidation, subsequently forming non-emissive areas known as "Dark Spots."

Therefore, the encapsulation process must be completed in an inert gas glove box directly connected to the evaporation chamber.

The interior of the glove box is filled with high-purity nitrogen (99.999%), and the oxygen and water vapor concentrations are strictly maintained below 1.0 ppm (parts per million) via a gas purification system cycling continuously, with a dew point temperature usually lower than -76°C.

Even the Cap Glass or Metal Can introduced into this environment must be pre-baked in a vacuum oven at 100°C to 150°C for several hours to thoroughly remove water molecules adsorbed on the surface.

Traditional PMOLED encapsulation uses an Edge Seal structure.

Core components include a glass cover plate with a groove and a Desiccant capable of chemically absorbing moisture.

The groove depth is typically processed to 0.3 mm to 0.5 mm via sandblasting or Hydrofluoric Acid (HF) etching to accommodate the desiccant tablet and prevent it from touching the fragile organic layer.

The desiccant is usually selected from Calcium Oxide (CaO) or Barium Oxide (BaO) powder.

These alkaline earth metal oxides react irreversibly with penetrating water molecules to form hydroxides, thereby maintaining extreme dryness inside the encapsulation cavity.

The desiccant is usually encapsulated in a breathable membrane patch or printed as a Paste inside the cover plate groove, followed by high-temperature activation.

Industrial standards require the desiccant's moisture absorption capacity to guarantee device survival for over 1000 hours under accelerated aging test conditions of 85°C / 85% RH.

A high-precision automatic Dispenser extrudes UV-curable epoxy resin (UV Epoxy) along the edge trajectory of the cover plate.

The inner diameter of the dispensing needle is typically only 0.15 mm to 0.25 mm, and the movement speed is set at 50 mm/s to 100 mm/s to ensure the glue strip width is uniformly controlled at 0.4 mm to 0.6 mm, with height consistency error less than 10 microns.

To prevent air bubbles from being trapped inside the sealing ring during lamination, the glue strip trajectory is usually designed with tiny gaps, or lamination is performed in a vacuum environment.

When the substrate and the glue-coated cover plate are aligned and contacted in a vacuum laminator, the chamber pressure is adjusted to 10^-1 Torr to 10 Torr, using the pressure difference to compress the glue strip, filling the gap between the two glass plates to form an airtight seal.

Subsequently, the glue strip is irradiated by high-intensity UV light (UV LED) with a wavelength of 365 nanometers. The cumulative exposure energy needs to reach 3000 mJ/cm² to 6000 mJ/cm² to cause instant cross-linking and curing of the resin.

Process Parameter	Typical Value Range	Control Purpose
Glove Box Dew Point	< -70°C	Prevent organic layer moisture absorption degradation before encapsulation
Desiccant Absorption	> 15 mg/cm²	Guarantee low humidity environment throughout lifecycle
UV Glue Viscosity	30,000 - 50,000 cps	Maintain strip shape, prevent collapse or overflow
Lamination Gap	10 - 30 µm	Ensure uniform glue thickness, enhance airtightness
Cutting Wheel Angle	105° - 115°	Control crack depth, reduce glass chipping
ACF Bonding Temp	180°C - 220°C	Cure conductive glue matrix, release conductive particles

The completed Mother Glass usually contains dozens or even hundreds of independent display units, which must be separated via a Scribing & Breaking process.

This process consists of two steps: first is scribing, using a high-hardness Diamond Wheel or high-power pulse laser to apply pressure on the glass surface, creating a vertical Median Crack with a depth of about 10% to 20% of the glass thickness.

Wheel pressure needs to be precisely controlled at 0.15 MPa to 0.35 MPa, and scribing speed can reach 300 mm/s.

To avoid damaging the precise internal electrode circuitry, the positioning accuracy of the cutting line must be better than ±5 microns.

Next is Breaking, where a mechanical arm or pressure bar applies bending moment on the back of the scribe line, causing the crack to instantly propagate downwards through the entire glass thickness.

Modern production lines increasingly adopt Stealth Dicing technology, where a focused laser beam creates a modified layer directly inside the glass, followed by applying tension to separate the glass naturally.

This method significantly reduces glass Chipping and micro-cracks, increasing edge strength to over 100 MPa.

The separated individual panel modules (Cells) also need to undergo cleaning and IC Bonding processes.

Since PMOLED uses passive driving, the Driver IC that controls current is usually mounted directly on the extended section of the glass substrate, known as the COG (Chip on Glass) process.

This connection relies on Anisotropic Conductive Film (ACF). ACF is a layer of thermosetting resin tape containing micron-level conductive particles (usually nickel-plated polymer spheres with a diameter of 3 to 5 microns).

Materials

The foundation is 0.5mm to 0.7mm thick alkali-free glass substrate, coated with 120-150 nanometers (nm) thick Indium Tin Oxide (ITO) as the transparent anode, with film resistance typically controlled at 10-20 Ohms/square.

The core organic layer has a total thickness of only 100-450 nanometers, containing hole injection materials like Copper Phthalocyanine (CuPc) and emissive materials like Alq3.

The top cathode typically uses 100-200 nanometers of Magnesium-Silver alloy (Mg:Ag) or Aluminum (LiF/Al), with a work function required to be lower than 3.7 eV to facilitate electron injection.

Calcium Oxide (CaO) or Barium Oxide (BaO) desiccant must be placed inside the encapsulation cavity to maintain internal water vapor content below 10 ppm.

Substrate and Anode Treatment

The physical foundation of a PMOLED display panel is built upon high-precision glass substrates.

Industrial standards typically select alkali-free borosilicate glass with a thickness of 0.5 mm or 0.7 mm.

In certain applications requiring extreme thinness, the glass is chemically thinned to 0.2 mm or even lower.

The choice of this glass material is not arbitrary; the main consideration is that its Coefficient of Thermal Expansion (CTE) needs to be controlled between 3 to 5 ppm/°C to closely match the subsequently deposited metal and organic materials, preventing substrate warping or film cracking due to thermal stress during high-temperature processing.

The microscopic flatness of the glass surface is the first threshold determining yield.

The Root Mean Square (RMS) roughness must be strictly controlled within the range of 1.0 nanometers to 2.0 nanometers.

Any surface protrusion exceeding 100 nanometers will become a conductive channel during the subsequent evaporation of organic layers only hundreds of nanometers thick, directly leading to pixel short circuits.

To block sodium ions from migrating from inside the glass to pollute the organic layer, ordinary soda-lime glass surfaces are usually pre-sputtered with a barrier layer of Silicon Dioxide (SiO2) about 200 nanometers thick.

Industrial-grade PMOLED substrates require a light transmittance of over 85% to 90% in the visible light spectrum (400-700nm). For the ITO (Indium Tin Oxide) anode film used for passive driving, its composition ratio is typically precisely locked at a weight ratio of 90% Indium Oxide (In2O3) to 10% Tin Oxide (SnO2).

Deposition of the ITO anode layer on the glass substrate typically employs a DC Magnetron Sputtering process.

The base vacuum of the process chamber needs to be pumped down to below 10^-6 Torr.

Argon gas is introduced as the bombardment gas during sputtering, while 0.5% to 1.0% oxygen is added to adjust the stoichiometry and lattice structure of the film.

The deposited ITO film thickness is usually set at 120 nanometers to 150 nanometers.

Freshly deposited ITO is usually in an amorphous or microcrystalline state and requires annealing treatment at 200°C to 250°C to convert it into a polycrystalline structure, thereby stabilizing the sheet resistance within the target range of 10 to 15 Ohms/square and improving its chemical corrosion resistance.

The photolithography process converts the full-surface ITO film into the strip anode patterns required for PMOLED.

First, a layer of positive photoresist about 1.5 microns thick is spin-coated onto the clean ITO surface.

After a soft bake at 90°C to 100°C to remove solvents, exposure is performed using a UV aligner through a mask, with exposure energy density controlled at 50 to 100 mJ/cm².

The development step typically uses a 2.38% Tetramethylammonium Hydroxide (TMAH) solution to strip the photoresist in the exposed areas.

The subsequent etching stage mostly uses wet etching, with an etchant mixed from Hydrochloric Acid (HCl) and Nitric Acid (HNO3) in specific proportions, or an oxalic acid-based etchant, carried out in a constant temperature bath at 40°C to 50°C.

Ideally, the etched side should not be a vertical 90 degrees but should be controlled between 45 degrees and 60 degrees.

This gentle slope helps the subsequently evaporated organic material layers and cathode metal layers to cover continuously, avoiding breakage at the steps (Step Coverage Issue).

After etching is complete, residual photoresist is removed via a Stripper, followed by the substrate entering a multi-stage ultrasonic cleaning line. Cleaning media sequentially include acetone, isopropyl alcohol, and Deionized Water (DI Water) with a resistivity reaching 18 Megohm·cm, with each cleaning step accompanied by megasonic oscillation at 40kHz to 1MHz to remove sub-micron particles.

The final step before the substrate enters the vacuum evaporation chamber is anode surface treatment, aimed at adjusting the Work Function of the ITO.

Untreated ITO surfaces usually adsorb large amounts of hydrocarbon contaminants, resulting in a work function of only 4.6 eV to 4.7 eV.

This creates a large potential barrier against the Highest Occupied Molecular Orbital (HOMO) energy level of common hole injection materials (such as CuPc or m-MTDATA), which is unfavorable for hole injection.

Industry universally adopts UV-Ozone cleaning or Oxygen Plasma bombardment (O2 Plasma) for treatment.

In oxygen plasma treatment, the substrate is exposed to an environment with RF power of 100W to 200W and oxygen flow of 50 to 100 sccm for 1 to 5 minutes.

This process not only physically bombards and removes residual organic contaminants but, more importantly, increases the oxygen content on the ITO surface through chemical reactions, forming an oxygen-rich layer, thereby raising the work function to 5.0 eV to 5.2 eV.

A qualified surface contact angle should be less than 10 degrees, presenting a super-hydrophilic state, indicating the surface is in a high-energy clean state, ready to accept organic molecule deposition.

Organic Functional Material Layer

The base vacuum of the entire chamber must be maintained between 1.0E-7 Torr and 1.0E-6 Torr to ensure the mean free path of organic molecules is greater than the distance from the evaporation source to the substrate, typically set at 300 mm to 500 mm.

Under such vacuum levels, organic powders placed in Tantalum (Ta) or ceramic crucibles sublime via resistance heating, flying in gaseous form in straight lines to deposit onto the rotating substrate.

To guarantee film thickness uniformity meeting the industrial standard of +/- 3%, the substrate typically rotates at a speed of 5 rpm to 10 rpm.

Film thickness control relies on Quartz Crystal Microbalance (QCM) sensors for real-time monitoring.

By measuring the decay of crystal oscillation frequency to calculate the deposition rate, feedback controls the heating current of the crucible, precisely locking the deposition rate within the range of 0.1 Å/second to 5.0 Å/second.

The first stage of the organic stack is the establishment of the hole injection and transport system, aimed at reducing the energy barrier between the anode and the organic layer.

Immediately adjacent to the ITO anode surface is the Hole Injection Layer (HIL). The classic choice in the industry is Copper Phthalocyanine (CuPc) or 4,4',4''-tris(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA).

CuPc is a planar macrocyclic conjugated molecule with a Highest Occupied Molecular Orbital (HOMO) level of about 5.1 eV, perfectly situated between ITO (approx. 4.8 eV) and the subsequent Hole Transport Layer (approx. 5.4 eV), forming a stepped energy level arrangement.

The thickness of this layer is typically controlled at 10 nanometers to 20 nanometers; excessive thickness increases the voltage drive burden.

Following this is the deposition of the Hole Transport Layer (HTL). Current mainstream material is N,N'-di(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4-4'-diamine (NPB or α-NPD).

Compared to earlier TPD materials, NPB possesses a higher Glass Transition Temperature (Tg), approximately 96°C to 98°C, which significantly enhances the thermal stability of the device.

The thickness of the HTL is typically thicker, around 40 nanometers to 60 nanometers, utilizing its hole mobility of around 1.0E-4 cm²/V·s to rapidly transport holes to the emission interface, while using its relatively high Lowest Unoccupied Molecular Orbital (LUMO) energy level to block electrons from passing through, confining electrons within the emission zone.

The Emissive Layer (EML) is the most process-complex part of the entire stack structure, usually employing a "Host-Guest" doping system to improve luminous efficiency and adjust color.

• Host Material: The most widely used is Tris(8-hydroxyquinolinato)aluminum (Alq3). This is a green fluorescent material with excellent electron transport capabilities and thermal stability. Its electron affinity is about 3.0 eV, and ionization potential is about 5.7 eV. Alq3 behaves stably during thermal sublimation, with a sublimation temperature usually around 300°C.
• Guest Doping (Dopant): To obtain red or blue light, or to improve the purity of green light, trace amounts of fluorescent dyes need to be doped into the host material. For example, to obtain red light, DCJTB dye is co-evaporated into Alq3 at a volume concentration of 0.5% to 2.0%. This co-evaporation process requires two independent crucibles working simultaneously, with extremely precise rate control. For instance, when the host material rate is 2.0 Å/second, the dopant material rate might be only 0.02 Å/second. If the doping concentration exceeds 2% to 5%, the distance between dye molecules becomes too close, leading to Concentration Quenching, which conversely lowers luminous efficiency.
• Energy Transfer Mechanism: In this system, electrons and holes primarily combine on the host material molecules to form excitons. Subsequently, energy is transferred non-radiatively to the narrower bandgap guest dye molecules via the Förster energy transfer mechanism, exciting the dye molecules to emit light.

The Electron Transport and Injection Layer (ETL/EIL) is located above the emissive layer, responsible for efficiently injecting electrons from the cathode and transporting them to the emission zone.

Alq3, due to its good electron transport characteristics, is often used directly as the Electron Transport Layer, with thickness set at 20 nanometers to 40 nanometers.

Between Alq3 and the metal cathode, depositing an extremely thin Electron Injection Layer (EIL) is standard practice to lower driving voltage.

Lithium Fluoride (LiF) is the preferred material for this layer, and its thickness must be precisely controlled between 0.5 nanometers and 1.0 nanometers.

Although LiF itself is an insulator, at this ultra-thin thickness, electrons can pass through via the quantum tunneling effect. Simultaneously, LiF chemically reacts with the upper aluminum cathode to release low-work-function lithium ions, drastically reducing the electron injection barrier.

If LiF thickness exceeds 3.0 nanometers, its insulating properties will dominate, causing the device to fail.

Besides this, there are solutions using Cesium Fluoride (CsF) or Lithium 8-quinolinolate (Liq) as alternative materials. These materials also require deposition at extremely low rates (about 0.1 Å/second) under high vacuum.

Cathode Metal Materials

Since the Lowest Unoccupied Molecular Orbital (LUMO) energy level of the bottom organic electron transport layer (usually Alq3) is around 3.0 eV, to achieve efficient electron injection, the work function of the cathode material must be as close to this value as possible.

The most commonly adopted industrial solution is Magnesium-Silver alloy (Mg:Ag) co-evaporation.

Magnesium (Mg) serves as the low-work-function metal with a work function of about 3.66 eV, responsible for injecting electrons into the organic layer;

Silver (Ag), with a work function as high as 4.26 eV, serves to improve the oxidation resistance of the film and reduce sheet resistance.

In a high vacuum environment of 10^-6 Torr, magnesium and silver are placed in two independent resistance heating boats, with evaporation rates adjusted by independently controlling heating currents.

The standard volume doping ratio is usually locked at 10:1 (Mg:Ag), meaning the deposition rate of magnesium is set at 10 times that of silver. For example, if the magnesium rate is 5.0 Å/second, the silver rate is controlled at 0.5 Å/second.

The total thickness of this alloy film is typically deposited to 150 nanometers to 200 nanometers to ensure the film is completely continuous and opaque, while controlling sheet resistance below 0.5 Ohms/square to prevent electrode heating under high current drive.

Besides Magnesium-Silver alloy, another widely applied cathode structure is the Lithium Fluoride/Aluminum (LiF/Al) bilayer cathode.

This structure does not directly use aluminum as the injection metal because aluminum's work function is about 4.28 eV, creating a large barrier when directly contacting Alq3.

The process first deposits an extremely thin layer of Lithium Fluoride (LiF), with thickness extremely precisely controlled between 0.5 nanometers and 1.0 nanometers.

Although LiF is an insulator (bandgap approx. 14 eV), at this atomic-level thickness, electrons can pass through via quantum mechanical tunneling effects.

Subsequently, a 100 nanometer to 150 nanometer thick aluminum layer is deposited.

During aluminum deposition, high-temperature aluminum atoms striking the LiF surface induce a chemical displacement reaction (3LiF + Al → AlF3 + 3Li), releasing free lithium atoms with an extremely low work function (about 2.9 eV). These lithium atoms dope the interface, forming an ohmic contact.

The advantage of this structure is that aluminum, as a single metal, has an evaporation process easier to control than dual-source co-evaporation.

Furthermore, aluminum's reflectivity in the visible spectrum exceeds 90%, effectively reflecting light generated by the emissive layer through the glass substrate (bottom emission structure), enhancing overall light extraction efficiency.

Parameter Indicator	Magnesium-Silver Alloy (Mg:Ag)	Lithium Fluoride/Aluminum (LiF/Al)	Remarks
Work Function	~3.7 eV (Depends on ratio)	~2.9 eV (Interface effective value)	Matches ETL LUMO level
Typical Thickness	150 - 200 nm	LiF: 0.5-1.0 nm / Al: >100 nm	Extremely low tolerance for LiF thickness deviation
Deposition Ratio/Method	10:1 (Volume ratio) Co-evaporation	Layered Sequential Evaporation	Mg oxidizes easily, needs Ag for stability
Sheet Resistance	< 0.5 Ω/sq	< 0.2 Ω/sq	Aluminum conductivity superior to Mg:Ag
Optical Reflectivity	80% - 90%	> 90%	Affects brightness of bottom-emission devices

In PMOLED manufacturing, cathode patterning is a specific technical challenge because one cannot use photolithography and wet etching like with anodes; developer and acid solutions would instantly dissolve or destroy the underlying organic layers.

The standard process to solve this problem is introducing "Cathode Separators." Before evaporating organic layers and metals, photoresist structures with an inverted trapezoidal (mushroom-like) cross-section are first created on the substrate via photolithography.

The height is about 2 microns to 4 microns, with sidewalls featuring a negative slope (Undercut).

When metal vapor is incident perpendicular to the substrate, since its mean free path is far greater than the separator height, metal atoms fly in straight lines.

The overhang structure at the top of the inverted trapezoid creates a Shadowing Effect, causing the metal deposited on top of the separator to naturally disconnect from the metal deposited at the bottom of the channel, thereby automatically forming mutually electrically insulated cathode strips in a single evaporation process.

This physical self-alignment process requires the metal evaporation source to have high directionality, and substrate temperature must be strictly controlled below 80°C to prevent the photoresist separators from deforming or outgassing contaminants into the organic layer under high-temperature radiation.

Encapsulation and Getters

Because low-work-function cathode materials used inside PMOLEDs, such as Magnesium (Mg) and Calcium (Ca), are extremely reactive, they oxidize to form hydroxides within milliseconds upon contact with water molecules in the air, leading to cathode interface delamination;

Simultaneously, organic functional materials like Alq3 undergo electrochemical degradation or crystallization in the presence of trace moisture. This failure manifests microscopically as non-emissive "Dark Spots."

To curb this corrosion, which diffuses at speeds of microns per hour, the encapsulation process must reduce the external Water Vapor Transmission Rate (WVTR) to below 1.0E-6 g/m²/day, which is a full six orders of magnitude higher than the barrier requirements for food packaging.

The entire encapsulation operation must be completed in a glove box workstation filled with inert gas (usually high-purity Nitrogen N2).

The water and oxygen content in this environment is strictly monitored below 1.0 ppm (parts per million), and even the dew point temperature needs to be maintained at -80°C to -90°C, ensuring no environmental moisture is sealed inside the device.

The encapsulation cover plate is typically made of soda-lime glass with a thermal expansion coefficient matching the substrate.

Through sandblasting or chemical etching, a cavity with a depth of 0.2 mm to 0.5 mm is excavated in the center of the glass.

The existence of this cavity is crucial. It not only provides mechanical clearance for the internal organic layers and cathode structures, preventing the cover plate from directly crushing the fragile nanoscale films, but also provides physical space for placing the desiccant.

Inside this cavity, a chemical Getter is mounted or coated. The classic materials are Calcium Oxide (CaO) or Barium Oxide (BaO).

Unlike physical adsorption by silica gel in food bags, the reaction of Calcium Oxide with water (CaO + H2O → Ca(OH)2) is an irreversible chemical adsorption; it will not release absorbed moisture even at high temperatures.

Industrial standards typically require the desiccant's absorption capacity design to have extremely high redundancy.

For example, for a 1.5-inch display screen, the theoretical maximum water vapor penetration over its lifespan might be only 0.1 mg, but engineers typically fill it with desiccant capable of absorbing 5 mg to 10 mg of moisture.

Modern processes tend to use sheet desiccants composited with adhesives or liquid desiccant pastes. They are firmly adhered to the bottom of the groove with thickness controlled around 0.1 mm.

It must be guaranteed that even after swelling from water absorption, they will not touch the cathode surface below, otherwise stress damage or Newton's rings optical interference will occur.

Permanently bonding the cover plate to the substrate is a specially made Ultraviolet (UV) curable epoxy resin sealant.

This glue must possess low gas permeability, high bonding strength, and extremely low curing shrinkage (typically less than 3%).

In the glue dispensing process, an automatic dispenser utilizes a visual alignment system to trace a continuous closed frame along the edge of the cover plate with a line width of 0.5 mm to 1.0 mm.

Glass microspheres (Spacers) with a diameter precisely of 5 microns to 10 microns are usually mixed into the sealant. These microspheres act like pillars in architecture, used to control the glue layer thickness (Cell Gap) after encapsulation, ensuring uniform sealing height around the entire screen.

Characteristic Parameter	Value Range/Description	Remarks
Environmental Water/Oxygen Conc.	< 1.0 ppm	Inside Nitrogen-protected glove box
Getter Material	CaO, BaO	Chemical irreversible reaction
Sealant Line Width	0.5 mm - 1.5 mm	Determines lateral permeation path length
UV Curing Energy	3000 - 6000 mJ/cm²	Wavelength 365nm
WVTR Requirement	< 10^-6 g/m²/day	Device lifespan benchmark

When the glue-coated cover plate is laminated with the substrate containing the evaporated organic layers in a vacuum or high-purity nitrogen environment, the assembly is sent to a UV curing machine for sealing.

Here lies a massive process contradiction: UV light cures the glue, but simultaneously, high-energy UV light (wavelength around 365 nanometers) breaks the molecular bonds of organic materials (like Alq3), leading to pixel aging.

To solve this problem, when performing high-intensity exposure of 3000 mJ/cm² to 6000 mJ/cm², a precision-aligned metal mask must be used to shield the screen's display area, allowing UV light to irradiate only the glue lines at the edges.

Alternatively, Side Curing technology is used, allowing light to enter the glue layer laterally.

After curing is complete, the glue not only achieves a physical seal but must also withstand subsequent high-temperature and high-humidity aging tests.

Any tiny bubbles or glue line breaks will manifest in tests as inward-growing edge dark spots caused by external moisture intrusion.

This rigorous encapsulation process ensures that the PMOLED can maintain its vivid brightness and pure display effect over a service life of tens of thousands of hours, unaffected by atmospheric erosion.