The production of transparent displays requires depositing a 50-100 nm thick ITO transparent conductive film (Indium Tin Oxide) on a glass/plastic substrate, which ensures both conductivity and maintains a transparency rate of over 85%. This is followed by bonding a micro-structurally adjusted display layer (such as LCD with reduced backlight obstruction or self-emissive OLED) with optical adhesive. Some products also incorporate a transparent touch film with a 2-5 μm pitch. Through processes like vacuum evaporation and photolithography, interface reflection is eliminated, ultimately achieving a clear and transparent display effect.
Substrate and Conductive Layer
The foundation of a transparent display consists of the substrate and the conductive layer. The substrate commonly uses 500 μm thick float glass or 125 μm PET plastic (Polyethylene Terephthalate); the former is scratch-resistant, while the latter is bendable. The conductive layer is an Indium Tin Oxide (ITO) thin film, coated via vacuum evaporation or magnetron sputtering, with a thickness controlled between 50-100 nm (1/1000th of a human hair). It must meet a transparency rate of 85%-90% and a sheet resistance of 10-30 ohms/square, balancing conductivity and light transmittance.
The “Skeleton” of the Screen
A transparent display can transmit light and show images, and the first step relies on the substrate to support the subsequent conductive and display layers, and even determine if the screen is rigid or flexible. Choosing the wrong substrate messes everything up: either the transparency is too low, making the screen look grey, or it’s too brittle, cracking upon touch, or creasing after several bends. Common substrates on the market are divided into two categories: glass and plastic, with thicknesses ranging from hundreds of micrometers to a fraction of a millimeter. The surface must be polished smoother than a mirror, with every process step strictly adhering to precise data.
How to Choose Materials
There are two main substrate materials, and the choice depends entirely on what the screen will be used for.
Float glass is the most common, typically 500 μm (0.5 mm) thick, and more uniform than standard window glass. Its inherent transparency is 91% (without coating), and its surface hardness is Mohs 6-7, leaving only a faint white mark when scratched with a key. It is the preferred choice for commercial transparent displays.
For instance, transparent advertising screens in shopping malls use 800 μm float glass substrates, which can withstand thousands of people viewing them daily, with surface scratches barely discernible by touch after three years.
PET plastic is lighter and softer, with a thickness of 125-250 μm (0.125-0.25 mm), weighing less than 1/3 of glass. Its transparency is 89% (slightly lower but adequate), and it can be bent into a 180-degree curve, making it suitable for electronic shelf labels or foldable devices.
However, it is sensitive to heat; processing temperatures above 100℃ cause deformation. Therefore, PET substrates require a layer of silicon dioxide buffer coating first, and the conductive film is deposited with the temperature controlled below 80℃, otherwise the film will wrinkle.
How to Determine the Thickness
Commercial fixed screens often use 500-700 μm glass. For example, the flight information display at an airport uses a 600 μm thick substrate. If a 1kg steel ball is dropped from a height of 1 meter, the screen does not crack.
If 400 μm thin glass is used, one out of three pieces will crack in the same test, and it is more prone to breakage during transport vibration.
Flexible displays require 125-175 μm PET. A certain foldable transparent tablet with a 150 μm thick substrate showed only a 2% drop in transparency after 100,000 bends (5 mm radius), with virtually no visible blurring.
Using 200 μm thickness results in high internal stress during bending, leading to wrinkling and a wavy pattern in the displayed image.
Surface Treatment
The substrate surface must be smooth down to the nanometer level, otherwise light scattering will cause the screen to look hazy. The treatment is divided into two steps:
Polishing: Float glass uses chemical mechanical polishing, where a chemical solution and abrasive particles “grind” the surface. After treatment, the roughness is below 0.5 nm (one millionth of a human hair), and the transparency is increased from 91% to 92%. Don’t underestimate the 1%; this slight increase makes the image on a mall screen more transparent, and customers see product colors more accurately.
Applying a Buffer Layer: PET substrates require a 50 nm thick coating of silicon dioxide. This layer not only reduces surface roughness from 2 nm to 0.8 nm but also ensures the subsequent ITO film adheres more firmly.
Environmental Reliability Test
Finished substrates must pass environmental checks. The lab simulates high temperature and humidity, as well as thermal cycling:
- 85℃, 85% humidity for 1000 hours: Glass substrates show no change; PET substrates absorb a little moisture, and transparency drops by 0.5%, still within the acceptable range.
- -40℃ to 85℃ switching every hour, 500 cycles: Glass is fine; PET shrinks slightly, but the dimensional change is less than 0.1%, which does not affect the bonding of other layers.
A certain brand conducted an outdoor test, hanging a glass substrate transparent screen by the seaside for two years. The substrate did not yellow, the transparency remained at 89%, and salt fog did not corrode small pits.
Conductive Layer
A transparent display can light up and show images thanks to an invisible conductive film that must conduct current to make the pixels work without blocking the light. This film is called Indium Tin Oxide (ITO), and its thickness is only one thousandth of a human hair.
How to “Draw” it onto the Substrate
There are two main methods for film deposition:
- Vacuum Evaporation: ITO material is heated to over 800℃, causing it to evaporate into gas, which then condenses into a film on the substrate surface. The equipment is cheap and suitable for small-batch production, but the film thickness can be uneven—for example, on a 500 μm thick glass substrate, the film thickness at the corners and the center might differ by 5 nm, leading to uneven transparency.
- Magnetron Sputtering: Argon ions bombard an ITO target (a metal piece about the size of a fingernail), “knocking out” target atoms, which then deposit on the substrate to form the film. The film thickness can be controlled to within ±2 nm, making it as uniform as a perfectly spread pancake. However, the equipment is expensive and the process must be done in a vacuum chamber, making it suitable for mass production.
A panel manufacturer tested that ITO film made by magnetron sputtering has a 1.5% higher transparency than that made by evaporation because the film is more uniform and scatters less light.
Thickness of the ITO Film
The thickness of the ITO film is strictly controlled. If it is too thin (e.g., 40 nm), the film is prone to cracking; a 40 nm film will break into several pieces after two bends. If it is too thick (120 nm), the transparency will drop from 88% to below 80%, making the screen look hazy.
The industry standard is 70-80 nm. A certain brand’s transparent screen uses 80 nm ITO film, with 88% transparency and a sheet resistance (a measure of conductivity) of 15 ohms/square. The low resistance means current flows quickly, the screen responds fast, and touch latency can be controlled to within 10 milliseconds, similar to a regular phone screen.
Conductive Layer Metrics
The conductive layer has two key metrics that directly determine the screen’s performance:
- Transparency Rate: ITO itself absorbs some light. High-quality films can achieve 85%-90% transparency. For example, a 500 μm glass substrate has 91% transparency without coating. After being coated with 80 nm of ITO, the overall transparency is 88%—a 3% reduction, but enough to make the screen look “transparent.”
- Sheet Resistance Uniformity: The sheet resistance of the entire film must have an error of less than 5%. For example, if the target is 15 ohms/square, the thinnest area should not exceed 15.75, and the thickest should not be less than 14.25.
ITO’s Flaws
While ITO is useful, it has a major drawback: the film is brittle. Repeated bending causes cracks. After 100,000 bends (5 mm radius), the ITO film develops micro-cracks, transparency drops by 1%, and sheet resistance increases by 2 ohms.
One manufacturer compared: a flexible screen using ITO showed normal display after 100,000 bends; one using silver nanowires started showing bright spots (broken silver lines shorting) after 50,000 cycles. Therefore, most current transparent displays still rely on ITO to “carry the load.”
Balancing Transparency and Conductivity
A transparent display can simultaneously be “see-through” and “display a clear image” by balancing transparency and conductivity. These two metrics are like a seesaw; if one is high, the other is low. The challenge is finding the optimal point where both are just good enough for use.
Transparency Rate
Ordinary glass has a transparency of 91%. A transparent display needs to achieve 85%-90% to be practical—below 85%, the screen appears grey, and the exhibited items or scene behind it become unclear.
For example: in a museum, a transparent screen with artifacts behind it requires a transparency of at least 88% for the audience to see the artifact details clearly.
If the transparency is 85%, the artifact’s color will darken by 10%, and details will be blurred; at 90%, color reproduction is close to viewing with the naked eye.
Sheet Resistance
Sheet resistance is a metric for measuring conductivity, measured in ohms/square (Ω/□). The lower the value, the smoother the current flows.
The conductive layer of a transparent screen (such as ITO film) requires 10-30Ω/□—if too low (e.g., 5Ω/□), material costs are high; if too high (over 40Ω/□), the current cannot flow effectively, and the screen will “respond slowly,” leading to touch latency or display ghosting.
For instance, a transparent advertising screen using 15Ω/□ ITO film takes 0.1 milliseconds for current to travel from one end to the other, with a touch response of 0.05 seconds. Switching to a 30Ω/□ film, the travel time is 0.2 milliseconds, and touch latency is 0.1 seconds, which is acceptable but provides a slightly inferior experience.
Thickness and Process
The balance between transparency and conductivity is mainly achieved by adjusting the thickness and manufacturing process of the ITO film.
Thickness is key: The thinner the ITO film, the higher the transparency, but the conductivity decreases. A 50 nm thick ITO film has 90% transparency and 40Ω/□ sheet resistance; 100 nm thick has 88% transparency and 15Ω/□; 150 nm thick has 85% transparency and 8Ω/□.
The industry chooses 80-100 nm because at this thickness, transparency is 88% or more, and sheet resistance is around 15Ω/□, which meets most application requirements.
Process affects uniformity: Magnetron sputtering is more uniform than vacuum evaporation. A manufacturer test showed that ITO film made by magnetron sputtering has a thickness error of ±2 nm and transparency fluctuation of less than 0.5%; vacuum evaporation has an error of ±5 nm and transparency fluctuation of 1%-2%.
Three Tests
The effectiveness of the balance must be verified through testing.
Transparency Rate Test: Measured with a spectrophotometer in the 400-700 nm wavelength range (the light segment sensitive to the human eye), requiring transparency ≥85%. A certain brand’s transparent screen tested at 88%, which meets the standard.
Sheet Resistance Uniformity Test: A four-point probe is used to scan the entire film, requiring the sheet resistance error in 95% of the area to be <5%. For example, if the target is 15Ω/□, at most 5% of the area can exceed 15.75Ω/□.
Aging Test: The screen is placed in high temperature and high humidity (85℃/85% humidity) for 1000 hours, requiring transparency to decrease by no more than 1%, and sheet resistance to increase by no more than 2Ω/□. A certain screen tested showed a drop in transparency from 88% to 87% and an increase in sheet resistance from 15Ω/□ to 17Ω/□, still within the acceptable range.
Silver Nanowires and Graphene
Although ITO offers a good balance, it is brittle and Indium is expensive. Silver Nanowires (AgNWs) have 89% transparency (better than ITO), but a sheet resistance of 20Ω/□ (similar to ITO). The problem is that silver lines are easily oxidized; after six months, transparency drops to 85%, and the screen appears yellow.
Graphene is more ideal: 90% transparency, 10Ω/□ sheet resistance (better conductivity), but mass production is difficult—large-area film formation is non-uniform, and the cost is 5 times that of ITO.
Replacing Electrodes with Transparent Material
Traditional displays use metal electrodes (such as aluminum, about 100 nm thick) that completely block light. Transparent displays must replace these with a transparent conductive layer. The mainstream choice is Indium Tin Oxide (ITO), which has a visible light transparency of 90%-95%, a thickness of 50-150 nm, and a sheet resistance of 10-30 ohms/square (sufficient conductivity to drive pixels). This retains the ability to transmit current while allowing approximately 40%-70% of ambient light to pass through the screen, achieving the effect of “seeing the image and the background.”
Why Choose ITO
The first step for a traditional screen to become transparent is replacing the metal electrodes with a transparent conductive layer. Several materials have been studied, including Fluorine-doped Tin Oxide (FTO), graphene, and silver nanowires, but the industry mostly settled on Indium Tin Oxide (ITO).
Sufficient Transparency
ITO’s average transparency in the visible light spectrum (400-700 nm wavelength, the light most sensitive to the human eye) can reach 90%-95%, close to the transparency level of ordinary window glass (about 92%).
This means only 5%-10% of the light is absorbed or reflected as it passes through the ITO layer; most of it goes through smoothly.
In comparison: FTO has a similar transparency of around 90%, but its infrared transparency is higher (which can lead to more heat when infrared light is dominant); Graphene’s theoretical transparency can reach 97%, but in actual mass production, due to non-uniform film thickness, transparency is only 92%-94%, and it is prone to light scattering due to surface defects; Silver nanowires are even more exaggerated; laboratory transparency can reach 98%, but the connection points between lines are easily oxidized, and over time, transparency drops below 90%, showing poor stability.
Stable Conductivity
Transparency alone is not enough; conductive capability must keep up. The metric for conductivity is “sheet resistance” (resistance per unit area, lower value means better conductivity).
ITO’s sheet resistance is typically 10-30 ohms/square, which is just right for display needs: for example, on a 55-inch transparent OLED screen using ITO electrodes, pixel response speed can be maintained at the microsecond level, resulting in a smooth image without motion blur.
Looking at other materials: FTO’s sheet resistance is similar to ITO, 10-25 ohms/square, but it requires a higher deposition temperature (350-400℃), which is not suitable for flexible substrates (like plastic) as it easily deforms them; Graphene’s sheet resistance can be as low as 1-5 ohms/square, better conductivity than ITO, but it’s difficult to achieve large-area uniformity in mass production. A 1-square-meter graphene film’s sheet resistance might jump from 1 ohm/square to 10 ohms/square, causing local dimming or failure to light up on the screen; Silver nanowires have a sheet resistance of about 5-15 ohms/square, seemingly good, but the contact points between lines are prone to a sharp increase in resistance, leading to poor overall conductive stability.
Mass Production and Cost
ITO’s production process has been mature for decades, with “magnetron sputtering” being the mainstream method: in a vacuum chamber, argon ions bombard an Indium Tin alloy target (90% Indium, 10% Tin), and the target atoms deposit on the substrate to form the thin film.
This equipment has many global suppliers (such as Applied Materials, Tokyo Electron), and used equipment is not expensive. A production line investment is about 50 million USD, capable of producing 3 million 55-inch transparent screens annually.
Other materials are more problematic: FTO requires Chemical Vapor Deposition (CVD), which has 30% higher equipment investment, and the target contains Fluorine, increasing the cost of waste gas treatment;
Graphene requires CVD or mechanical exfoliation. The former has a low yield rate of only 60%-70% (ITO is over 90%), and the latter can only produce small-area samples; Silver nanowires require solution coating, which is prone to agglomeration during drying, resulting in a yield rate of less than 50%. Furthermore, silver raw material prices fluctuate significantly (silver price per ounce rising from 15 USD to 30 USD, directly doubling the cost).
Overall, ITO’s comprehensive manufacturing cost is 15%-20% lower than FTO and over 30% lower than graphene.
Practical Performance
Beyond performance and cost, actual usage effectiveness is the most telling. A certain brand of transparent OLED TV uses ITO electrodes, with measured data:
- Overall transparency 58% (in ambient light, the spine of books on the shelf behind can be clearly seen);
- Pixel brightness uniformity 95% (brightness difference between the top-left and bottom-right corners of the same image is less than 5%);
- After 1000 hours of continuous operation, the sheet resistance change is less than 2% (good conductive stability, the image does not become dimmer over time).
If graphene electrodes were used, the same test showed:
- Initial transparency 92%, dropped to 88% after 100 hours (due to oxidation);
- Pixel brightness uniformity 85% (local sheet resistance fluctuation is high);
- After 500 hours of continuous operation, noticeable dark spots appeared (resistance surge at contact points).
“Attaching” the ITO
After replacing the screen’s electrodes with transparent ITO, the next step is “attaching” this nanometer-level transparent conductive film to the substrate. This is not ordinary “taping” but involves using high-energy particles to “bombard” Indium and Tin atoms onto the glass or flexible film in a vacuum environment, forming a uniform and flawless thin film. The most common factory method is magnetron sputtering. The entire process is like “plating a layer of invisible conductive mist” onto the screen, with all details hidden within the parameters.
Indium Tin Alloy Target
The raw material for the ITO film is the Indium Tin alloy target, with a critical composition ratio: 90% Indium and 10% Tin.
The target must be highly pure (purity 99.99% or more), otherwise impurities will mix into the thin film, leading to a drop in transparency or localized non-conductivity.
One target weighs about 5-20 kg and can sputter-coat 50-100 square meters of substrate. It must be replaced when used up, and its cost accounts for about 30% of the entire process.
Entering the Vacuum Chamber
Before sputtering, the substrate is placed into the vacuum chamber of the magnetron sputtering machine. The chamber must be pumped down to a vacuum level below 10⁻³ Pa (equivalent to one ten-billionth of Earth’s atmospheric pressure), 100 times cleaner than a hospital operating room.
This step is to expel oxygen, water vapor, and dust from the air; even a single dust particle with a diameter of 0.1 μm can create a “pit” in the thin film, causing a local short circuit or uneven transparency.
After achieving a vacuum, a small amount of Argon gas (99.999% purity) is introduced into the chamber, with the pressure controlled at 0.1-1 Pa.
The Sputtering Process
When argon ions bombard the target, they “knock out” the Indium and Tin atoms from the target surface. These energized atoms fly towards the substrate and deposit to form the ITO thin film. The parameters of the entire process directly affect the film quality:
- Sputtering Power: If the power is too low, the atoms fly too slowly, and the film grows slowly and is not dense; if the power is too high, the target can easily be “burned through” (known as “target poisoning”). Factories usually set the power at 1-3 kW (for a single-target machine), which ensures a deposition rate (50-100 nm per hour) without damaging the target.
- Substrate Temperature: The substrate must be heated to 200-300℃. At low temperatures, Indium and Tin atoms do not “settle well” on the substrate and easily form a disordered structure, causing the film’s transparency to drop below 85%; at high temperatures, the atoms are arranged more tightly, and transparency can stabilize above 90%, but temperatures too high (over 350℃) will deform flexible substrates (such as PET plastic).
- Target to Substrate Distance: This distance must be maintained at 5-10 cm.
The Thin Film Must Not Have Pinholes
After sputtering is complete, the quality of the ITO thin film must be checked. The most critical factor is the absence of pinholes. Even a pinhole with a diameter of 1 μm (1/50th of a human hair) can cause current to “leak out,” resulting in a local dark spot on the screen. Factories use an optical microscope (1000x magnification) or an ellipsometer to scan the entire film. A pass rate of over 95% is required for shipment.
Other Methods
Besides magnetron sputtering, some factories have tried solution coating: ITO nanoparticles are made into an ink, coated onto the substrate, and baked to form the film. This method has lower costs (30% less equipment investment) but has significant drawbacks:
- Poor film uniformity, with thickness varying by up to 10 nm in different locations of the same batch, causing sheet resistance fluctuations;
- Solvent evaporation during drying tends to leave voids, with transparency only reaching 88%-90% (2%-4% lower than magnetron sputtering).
There is also electron beam evaporation: an electron beam bombards the target, causing Indium and Tin atoms to evaporate and then deposit. However, electron beam equipment is expensive (one unit costs 20 million USD), and the evaporation rate is slow (only 20 nm per hour), making it unsuitable for large-scale production.
Actual Production Example
A Japanese panel manufacturer’s transparent OLED production line uses magnetron sputtering for ITO electrodes:
- Single substrate size: G10.5 generation line (2940×3370 mm);
- Sputtering time: about 45 minutes per side (depositing 100 nm film);
- Yield rate: 92% (main defects are pinholes and non-uniform thickness);
- Final ITO film performance: Thickness 95±5 nm, sheet resistance 18±2 ohms/square, visible light transparency 92%.
Precision Coating Process
After replacing the transparent screen’s electrodes with ITO, the real challenge is “printing” this nanometer-level transparent conductive film onto the substrate. This is not like ink-jet printing; it’s about making Indium and Tin atoms land precisely on the glass or flexible film to form a conductive film that is both transparent and uniform. The most common factory method is magnetron sputtering, where every step, from raw material preparation to final film formation, is precisely controlled with millimeter or even nanometer-level parameters.
Indium and Tin Ratio
The “raw material” for the ITO film is the Indium Tin alloy target. This item is like a large coin, 20-30 cm in diameter, 5-8 cm thick, and weighs 5-20 kg.
Its composition ratio directly determines the film performance—90% Indium and 10% Tin.
A little less Tin, and the film resistance will surge; a little more Tin, and the transparency will drop by 3%-5%.
Target purity is even more critical; it must be 99.99% or more (4N grade). If even 0.01% of impurities (like iron or copper) are mixed in, they will form “dark spots” in the thin film, causing the local transparency to drop by over 10%.
How many substrates can one target sputter-coat? Taking the G8.5 generation line (2200×2500 mm) as an example, one target can coat about 300-400 substrates and must be replaced afterwards.
The cost of the target accounts for 30%-40% of the entire coating process, which is one of the main expenses of the ITO process.
Vacuum Level
Before sputtering, the substrate is sent into the vacuum chamber of the magnetron sputtering machine. The chamber must be pumped down to a vacuum level below 10⁻³ Pa (equivalent to one ten-billionth of Earth’s atmospheric pressure), 1 million times cleaner than a hospital Class 100 operating room (10⁰ Pa).
This step takes 15-30 minutes, using molecular and mechanical pumps to remove oxygen, water vapor, and dust from the air.
Why so “clean”? Because oxygen in the air reacts with Indium and Tin atoms to form Indium Oxide or Tin Oxide, making the film brittle and reducing transparency; dust is more troublesome—a dust particle with a diameter of 0.1 μm (1/500th of a human hair) will create a pit in the thin film, causing current to “leak out” and a local black spot on the screen.
After achieving a vacuum, high-purity Argon gas (99.999%) is introduced into the chamber, with the flow rate controlled at 50-100 standard cubic centimeters per minute (sccm), maintaining a low pressure of 0.1-1 Pa in the chamber.
Checking the Thin Film
After sputtering, the quality of the ITO thin film is inspected. The most critical checks are for pinhole absence and thickness uniformity:
- Pinhole detection uses an ellipsometer, scanning the entire film with a resolution of 0.1 μm. The acceptance standard is fewer than 10 pinholes per square meter—even a pinhole with a diameter of 1 μm will increase local current density by 10%, causing the pixel to dim.
- Thickness uniformity is required to be within ±5%. For example, if the target thickness is 100 nm, the entire film thickness must be between 95-105 nm. Factories use a profilometer to measure 5 points at the four corners and the center of the substrate; deviations exceeding 5% require rework.
Other Processes
Some factories have also tried solution coating: ITO nanoparticles are ground into powder, mixed with water/alcohol to form an ink, coated onto the substrate using a roll-to-roll coater, and then baked in an oven (150℃, 30 minutes). This method has cheaper equipment (30% less investment), but many problems:
- Poor film uniformity, with thickness varying by up to 20 nm in different locations of the same batch, causing a 15% sheet resistance fluctuation;
- Solvent evaporation during drying leaves voids, with transparency only reaching 88%-90% (2%-4% lower than magnetron sputtering);
- Low yield rate, only 70%-80% (magnetron sputtering yield is over 90%).
The electron beam evaporation method is even more expensive: an electron beam bombards the target, causing Indium and Tin atoms to evaporate and then deposit. However, one unit costs 20 million USD, the evaporation rate is slow (20 nm per hour), and the evaporated atoms have low energy, resulting in poor film adhesion and easy peeling.
Reference Data for Actual Production
A South Korean panel manufacturer’s transparent OLED production line uses magnetron sputtering for ITO electrodes:
- Substrate size: G10.5 generation line (2940×3370 mm);
- Sputtering time: 45 minutes per side (depositing 100 nm film);
- Yield rate: 92% (main defects are pinholes and non-uniform thickness);
- Film performance: Thickness 98±4 nm, sheet resistance 17±1 ohms/square, visible light transparency 92%.
Screen Assembly and Display Adjustment
The process requires first cleaning the substrate (e.g., 50 μm PET panel), then sequentially stacking the transparent electrode layer (ITO, 150 nm thick), organic light-emitting layer (2-3 μm), and encapsulation layer (5 μm). These are bonded using optical adhesive (25 μm thick) with an alignment accuracy of ±5 μm. After completion, the transparency is measured (target ≥80%), and pixels are adjusted using a laser calibration instrument to ensure uniform brightness in the light-emitting area (error <10%) and no afterimages in the transparent area.
Stacking Materials
First, a 50-100 μm thick PET plastic substrate is laid down (a supermarket shopping bag is about 12 μm thick; this substrate is 4-8 times thinner). Then, a 100-200 nm thick Indium Tin Oxide (ITO) transparent electrode is deposited using a vacuum evaporation machine. Next, a 2-3 μm organic light-emitting layer is spread (the material is stacked like fine sand), followed by a 3-5 μm encapsulation layer (for moisture protection) and a 20-30 μm OCA optical adhesive. The alignment accuracy for each layer is ±5 μm (about 1/20th of a human hair); misalignment can cause uneven light emission or blurring in the transparent area.
Substrate Selection
Two materials are commonly used: one is a 50-100 μm thick PET plastic (this substrate is 4-8 times thinner than a shopping bag, which is about 12 μm thick), and the other is Soda-lime glass (ordinary window glass is about 3 mm thick; here, only 1/30th of that thickness is used).
Applying Electrodes
The next layer is the Indium Tin Oxide (ITO) transparent electrode, which acts as the “highway” for the current. The thickness must be controlled at 100-200 nm (1 nm is 1/50,000th of a human hair):
If too thin (e.g., 90 nm), resistance increases, current cannot flow, and the light-emitting area will appear dim; if too thick (210 nm), the material itself becomes opaque, blocking the light behind it.
ITO is coated using a “magnetron sputtering machine.” The principle is like using high-speed metal particles to “smash” a target (an Indium Tin Oxide block), causing the particles to sputter onto the substrate and form a thin film.
The machine’s gas pressure (0.5 Pa) and power (3 kW) must be adjusted to control the particle speed; otherwise, the film will be uneven and pitted.
After coating, an ellipsometer measures the thickness; an error exceeding 5 nm requires recoating—this is equivalent to finding a grain of millet on a running track.
Spreading the Light-Emitting Layer
The light-emitting layer is the “bulb” of the screen, using organic small molecule materials (such as Alq3) or Quantum Dots (nanometer-sized semiconductor particles).
This layer is 2-3 μm thick and must be spread as uniformly as a thin pancake: if too thick (4 μm), more heat is generated as current passes through, shortening the lifespan; if too thin (1.5 μm), the light output is insufficient, and the screen looks grey.
The light-emitting layer is coated using the “solution spin coating method”: the material is dissolved in chlorobenzene solvent, dropped onto the ITO electrode, and then spun evenly by a turntable rotating at 2000 revolutions/minute.
If the rotation speed is too slow (1500 rpm), the material piles up into small mounds; if too fast (2500 rpm), the edges are too thin, and the center is thick. After spinning, it is baked in a vacuum oven for 10 minutes to evaporate the solvent, leaving a uniform thin film.
Ultra-thin Waterproof Coating
The light-emitting layer is susceptible to moisture, so it must be immediately covered with an encapsulation layer. Atomic Layer Deposition (ALD) technology is used to coat the surface with a 3-5 μm composite film of Aluminum Oxide (Al₂O₃) + Epoxy Resin.
This layer acts like plastic wrap, completely encasing the light-emitting layer: if a 0.1 μm gap is left, moisture will seep in, causing the organic material to blacken and fail within 3 months.
The ALD machine can only deposit 0.1 nm at a time, requiring 30-50 repetitions to achieve the 3 μm thickness.
Thickness must be monitored during the process using a “Quartz Crystal Microbalance” to measure how much weight each layer adds and convert it to thickness. This is like weighing 100 sheets of A4 paper with an electronic scale, where the weight difference of each sheet cannot exceed 0.01 grams.
Final Bonding of the Protective Layer
The top layer is covered with OCA optical adhesive, 20-30 μm thick, which serves to bond the layers firmly and reduce light refraction.
After bonding, it is inspected with a microscope; if the number of bubbles within 1 square centimeter exceeds 3, it requires rework.
The entire stacking process takes place in a Class 10,000 cleanroom (air containing ≤10,000 dust particles with a diameter >0.5 μm per cubic meter). Workers wear full dustproof suits and move slowly like defusing a bomb.
After each piece is stacked, an “optical alignment instrument” checks the position of each layer: if the alignment error between the ITO electrode and the light-emitting layer exceeds ±5 μm (1/20th of a human hair), the screen will exhibit a “Yin-Yang face” (uneven brightness).
Example: A certain brand’s 55-inch transparent screen had a first-pass yield rate of only 70% in stacking. The main issues were non-uniform thickness of the light-emitting layer (30%) and leakage in the encapsulation layer (25%).
Before leaving the factory, the transparency rate must be tested. It must be ≥80% in the normal state; otherwise, the user will see the background as if it were covered with frosted glass.
Measuring Transparency and Pixels
After stacking, the transparent screen requires the adjustment of two key parameters: first, measuring the transparency rate using a spectrophotometer to shine white light and calculate the overall light transmission ratio of the screen. The target is ≥80%; otherwise, it requires rework, polishing, or re-bonding. Second, calibrating the pixels using a calibration instrument to scan the screen and adjust the current of each pixel, ensuring the brightness deviation in the light-emitting area is <10% and there are no afterimages in the transparent area.
Measuring Transparency Rate
The transparency rate is measured using a spectrophotometer, a machine that emits white light and then receives the light transmitted through the screen to calculate the ratio.
The instrument’s lens is 20 cm away from the screen, and the light is shone at a 45-degree angle (simulating normal viewing angle).
The acceptance standard is transparency rate ≥80% in the normal state (black screen, not displaying anything).
If the measurement is only 75%, where might the problem lie?
It could be that the encapsulation layer is too thick—the designed 3 μm aluminum oxide film was sputtered 0.5 μm thicker, blocking some light;
It could also be that the OCA adhesive was not pressed evenly, containing 1 μm small air bubbles that scatter light upon impact, preventing transmission.
In this case, the screen must be disassembled, and either the encapsulation layer is laser-ground 0.5 μm thinner (with precision controlled at ±0.1 μm), or the adhesive is recoated and pressed twice more with a roller to squeeze out the bubbles.
During the debugging of a 43-inch transparent screen, the transparency rate was stuck at 78% for 3 consecutive batches.
It was later found that the PET substrate was not thoroughly cleaned before entering the workshop, and a layer of invisible grease had adhered to the surface, preventing a tight bond between the OCA adhesive and the substrate, leaving micron-sized gaps in between.
The problem was solved by using a plasma cleaner to blow argon gas for an extra 5 minutes to decompose the grease, which immediately raised the transparency rate to 82%.
Pixel Calibration
The first step is adjusting brightness uniformity, using a Konica Minolta CS-2000 Spectroradiometer to scan close to the screen and measure the brightness of each pixel.
The target is a brightness deviation of <10% in the light-emitting area—for example, if the center pixel is 300 nits, the dimmest edge cannot be below 270 nits.
If a pixel is dim, its current is increased from 15 microamperes to 16 microamperes until the brightness meets the standard.
This requires patience; a 55-inch screen has 1920×1080 pixels. Engineers must visually inspect the screen or use software to generate a grayscale image (a gradient bar from black to white) to check for any sudden bright or dark bands.
The second step is eliminating afterimages, using a “checkerboard pattern”: black and white squares alternate, displayed for 1 hour. Then, the screen is checked for faint grey afterimages next to the white squares. The industry standard requires the afterimage area to be <0.1 square millimeters (about the size of a pinhead).
Iterative Adjustment
Measuring transparency and calibrating pixels are interconnected. Thinning the encapsulation layer to increase transparency might make the light-emitting layer more susceptible to moisture, speeding up brightness decay; adding a reverse current to calibrate pixels might reduce transparency by 0.5%.
Therefore, debugging is typically a “measure-adjust-re-measure” cycle. The debugging record of a certain brand’s 55-inch transparent screen showed: 12 rounds of adjustments in the first 3 days, increasing transparency from 79% to 81%, then dropping to 80.5% due to minor encapsulation layer adjustments; pixel brightness deviation was reduced from 15% to 8%, but the afterimage area increased from 0.08 square millimeters to 0.12 square millimeters.
Before shipment, “scene simulation testing” must be conducted: the screen is placed in a dark box, and 1000 lux of white light (equivalent to a sunny view outside the window) is turned on to measure if the transparency is still ≥70% (the basic requirement for users to see the background clearly). Then, it is placed in a 50 lux dark room, the screen brightness is adjusted to 300 nits, and the text clarity is checked.
Simulating Real-World Scenarios
After parameter debugging, the transparent screen is tested in a simulation box for three conditions: under strong light (1000 lux, like a sunny view outside the window), transparency must be ≥70% to check if the background street scene is clear; under weak light (50 lux, like indoors in the evening), the brightness of the light-emitting area must be over 300 nits, and the text must not appear grey. It is then placed in an 85℃ + 85% humidity box for 72 hours, where no water mist or afterimages are allowed. Finally, 1000 hours of continuous video playback must not result in a brightness decay exceeding 5%.
Clarity under Strong Light
The first step of the test is “strong light exposure”: the screen is placed in a dark box, and a 1000 lux white light simulator (equivalent to midday outdoor brightness) is turned on.
The acceptance threshold is ≥70%. If the measurement is only 65%, it means the encapsulation layer or OCA adhesive is blocking too much light. The problem might be non-uniform sputtering of the aluminum oxide film during encapsulation, with local thickness exceeding 5 μm, or air bubbles not fully pressed out of the OCA adhesive.
One brand once had a batch of outdoor screens where the strong-light transparency was stuck at 68%. It was later discovered that the PET substrate chosen was too thin (50 μm); it slightly deformed under high temperatures, causing the ITO electrode and light-emitting layer to be misaligned by 0.1 mm, blocking some light.
Switching to a 100 μm substrate raised the transparency to 73%, passing the test.
Hot and Cold Stress
The screen might be moved from an air-conditioned room to direct sunlight, or placed outdoors in winter. The test involves “fire and ice”: placing it in a temperature and humidity chamber, first raising the temperature to 60℃ and holding it for 24 hours to check for deformation or delamination; then dropping the temperature to -20℃ and freezing it for 24 hours, followed by powering it on to measure display performance.
The focus is on the moisture risk under high temperatures. The light-emitting layer is sensitive to moisture. If there are micro-cracks in the encapsulation layer, at 60℃ with 90% humidity in the chamber, moisture will penetrate.
After the test, the screen is disassembled and inspected. An infrared microscope checks the encapsulation layer; cracks wider than 0.1 μm are considered a failure.
During one test, a certain transparent screen showed “snowflakes” on the display when powered on at -20℃.
Sealing in Humid Environments
The test uses a 90% RH humidity chamber at 30℃ for 72 hours. After completion, the screen is disassembled, and an electron microscope is used to check the surface of the light-emitting layer for water marks larger than 0.5 μm in diameter.
Moisture protection relies on the coordination of the encapsulation layer and the adhesive; the aluminum oxide film must be dense enough (porosity <1%), and the OCA adhesive must be made of hydrophobic material (contact angle >90 degrees).
A batch of screens once showed a grey edge in the light-emitting area after the humidity test. The microscope revealed connected lines of water marks, indicating non-uniform coating at the encapsulation layer’s edge, leaving a 0.2 μm gap. The problem was resolved by adjusting the angle of the sputtering machine’s nozzle, increasing the edge film thickness from 2 μm to 3 μm.
Continuous Operation
The screen continuously plays video for 1000 hours (looping pure color images and high-contrast images) to measure three metrics:
- Brightness decay in the light-emitting area must not be >5% (e.g., initial 300 nits, ≥285 nits after 1000 hours);
- Transparency drop in the transparent area must be <3% (from 80% to ≥77.6%);
- There must be no permanent afterimages (after a checkerboard pattern is displayed for 1000 hours, the afterimage area must be <0.05 square millimeters).
The Alq3 material used in the light-emitting layer decomposes after long-term electrical excitation. The peak current was reduced, for example, from 20 microamperes to 18 microamperes, to extend the material’s lifespan. During one screen test, brightness decayed by 6% after 1000 hours. Engineers adjusted the pixel driving waveform to reduce instantaneous current impact, lowering the decay to 4%, which met the standard.
Shipment Only After All Tests Pass
The final verification record for a certain brand’s 55-inch transparent screen showed: out of the first batch of 100 units, 15 units failed the strong-light transparency test (substrate replaced), 8 units showed water marks at high temperatures (encapsulation adhesive replaced), and 5 units had brightness decay exceeding 5% (driving current adjusted).
