Printed Electrochromic Display

The rdot display is very simple. It is an entirely printed segmented display on a plastic substrate. On the other hand, it comes in many shapes and forms to match with various customer demands which make it slightly more complicated. Use the navigation or scroll down to read more about various technical aspects that are important to understanding if the rdot display matches your requirements and preferences.

Technology Overview
Driving and Energy Consumption
Manufacturing
Operating Conditions
System on Display™
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Technology Overview

The display stack consists of organic layers including a plastic substrate, an electrochromic material, and an electrode for each segment. Additional layers such as graphical overlays, circuits, and barrier layers may be added if required.

The electrochromic layer will change optical state when a voltage is applied. With a typical voltage of 3V, the response time for a small segment is approximately 100 ms. The display is also semi bistable. The bistability period can be modified from minutes to days. Shorter bistability results in faster switching speeds and vice versa.

electrochromic display stack
Example Specifications
White Reflectance 40%
Contrast Ratio 1:3
Angle Dependency No
Thickness 100 - 200 µm
Graphical layout Segments
Segment dimensions 1 mm - 100 mm
Response time 100-1000 ms

Color Availability

The optical properties, such as colors, reflectance, and contrast ratios, are tunable and may be optimized depending on the use case. Graphical overlays are also commonly used to combine the tunable electrochromic colors with traditional prints. Below are example specifications for tunable colors. There are no fundamental limitations regarding what colors that can be achieved. Please contact us if you have any specific requirements.

Example Specifications (Red)
Contrast Ratio (Y/Y) 1:8
Contrast Ratio (ΔE) 60.9
L*a*b* bright state [36.3, 55.8, 38.8]
L*a*b* dark state [10.6, 19.3, -3.6]
Example Specifications (Green)
Contrast Ratio (Y/Y) 1:11
Contrast Ratio (ΔE) 67.8
L*a*b* bright state [49.4, -54.1, 46.2]
L*a*b* dark state [13.4, -29, -5.5]
Example Specifications (Blue)
Contrast Ratio (Y/Y) 1:7
Contrast Ratio (ΔE) 47.5
L*a*b* bright state [57, −3.7, −10.5]
L*a*b* dark state [20.6, 10.2, −37.7]

Ultra-low Energy Consumption

Most energy is consumed when the display is updated, less energy is required for a static image.

The rdot display is semi-bistable, which means that it has an image memory. The display memory time is approximately 15 minutes, after that period a small refresh pulse is required to reach full contrast again. Almost all the energy is consumed when the display segments are updated and close to no energy is needed to show a static image.

Segment dimensions are important.

Smaller segment areas consume less energy per square centimeter than large segment areas. Consequently, segment sizes need to be taken into account when making energy consumption calculations.

Information about the display driving conditions and use case are crucial.

The display segments are turned on and off similarly to how a supercapacitor is charged. Apply a voltage for a short period (≈100 ms) to charge the display. After approximately 15 minutes the display will start to discharge, and the segment contrast will degrade. When the degradation has initiated, a refresh pulse with only a fraction of the full switch energy may be applied. Alternatively, if no refresh pulse is used, the display will gradually fade out over the next few hours.

Driving voltage influences energy and switching speed.

The driving voltages that can be applied shall be in the range of 2-5 volts. Using a lower voltage will consume less energy than a high voltage. The trade-off is longer switching speed with lower voltages.

Energy consumption data

Examples below will indicate the total energy consumption to keep a seven segment display with a total segment area of one square centimeter contineously on. A full switch is defined as turning all segments from OFF to ON.

One full switch per day: 0,0006 mW/cm2
One full switch per hour: 0,0010 mW/cm2
One full switch per minute: 0,0333 mW/cm2
One full switch per second: 2 mW/cm2

Example Energy Consumption
Average segment area 0.1 square centimeters
Number of segments 21 (3 x 7 segment)
Total display segments area 2.1 square centimeters
Driving voltage 3 V
Full switch energy for one segment 0.06 mJ
Full switch energy for all segments 1.26 mJ
Update speed < 100 ms
electrochromic display stack

Example display: 3x7 segment layout

Read more about energy consumption and driving methods.

A display driving scenario when the display content is allowed to fade out

For various applications, the display is only needed when the user is interacting with the device and altering the information that should be displayed. The illustration below shows a scenario where the screen is activated when a number changes. A voltage is applied for a brief period to update the display information. Once updated, the voltage is removed to let the display information fade out over the next few hours. This display driving scenario creates an extremely low-powered solution. See the table for example specifications and energy consumption.
Example Energy Consumption
Average segment area 0.1 square centimeters
Number of segments 21 (3 x 7 segment)
Total display segments area 2.1 square centimeters
Driving voltage 3 V
Full switch energy for all segments 1.26 mJ
#switches during product lifetime 10 000
Required battery capacity 1.2 mAh
Update display when needed Illustration

A display driving scenario when the display is always on and readable

Several applications require that the user can read the display at any time. The display memory is utilized to optimize the driving to achieve as low energy consumption as possible. After a full switch, e.g., when the display information is updated, the display driver can go to deep sleep for approximately 15 minutes. After the display memory time of 15 minutes, a small refresh pulse is needed to maintain full contrast. The refresh pulsing will continuously run every 15 minutes to keep the display powered continuously. A refresh pulse typically consumes 25% of the energy of a full segment switch (entirely off to on or vice versa). See table and the illustrations for example specifications.
Example Energy Consumption
Average segment area 0.1 square centimeters
Number of segments 21 (3 x 7 segment)
Total display segments area 2.1 square centimeters
Driving voltage 3 V
Full switch energy for all segments 1.26 mJ
Display active lifetime 5 years
Refresh pulse frequency Every 15th minutes
#switches during product lifetime 10 000
Refresh pulse energy (%of full switch energy) 25%
Required battery capacity 6 mAh
Display Always On Illustration

Manufacturing

The rdot display is manufactured using conventional and cost-effective printing processes. The production process is, in a wider sense, commonly referred as printed electronics. Both Sheet-to-Sheet (S2S) and Roll-to-Roll (R2R) production methods are available

Rapid Prototyping and Pilot Production

The in-house sheet-to-sheet production line is the preferred choice for rapid prototyping and custom design projects. The typical project lead time from sketch to delivered display is usually less than three months. The same S2S production line can be used for pilot production with capacity up to several thousand display units per month. Tooling and NRE costs are generally around 5 000-15 000 EUR without any MOQ requirements. The tools can later be reused during the pilot production phase.

Mass Production

A roll-to-roll production method is the most cost-effective option for mass production. Rdot has a preferred manufacturing partner for mass production, but are also open for licensing if very large volumes are demanded. The cost per produced display area is dependent on factors such as expected volume, display footprint, and environmental requirements. Contact us with your project-specific details for cost estimations.

Operating conditions

Environmental Stability

The environmental stability depends on the choice of encapsulation. A non-encapsulated display has a humidity dependence that will influence the switching speed and bistability. No encapsulation is satisfactory when the operating conditions are in well-known and uniform. If the environment is more demanding or shifting, a gas barrier layer is required. For most circumstances, a WVTR (water vapor transmission rate) of 10-2 g/m2/day is sufficient. Several alternative barrier materials are available for a suitable cost-benefit decision.

Display Lifetime

The lifetime of the display is directly correlated with the number of switching cycles that are performed. One switching cycle is counted when the display is switched from OFF -> ON -> OFF. The degradation of the display is caused when a voltage is applied to turn the display off (from colored to bright state). Close to no degradation is caused if the display is constantly on (colored) or when it is constantly off (bright state). For applications demanding extended lifetimes (more than 1000 switching cycles), it is recommended to keep the contrast approximately 20% below the maximum contrast by adjusting the switching time or voltage when switching from dark to bright state. With accurate driving, the lifetime will exceed 100 000 switching cycles.

Example Specifications (encapsulated display)
WVTR 1 x 10-2 g/m2/day
Humidity minimum 10% RH
Humidity maximum 85% RH
Temperature minimum -5°C validated, -20°C expected
Temperature maximum 40°C validated, 80°C expected
Shelf life >3 years
Storage temperature 0°C - 40°C
Storage humidity 0°C - 40°C
Number of switching cycles > 100 000 (with accurate driving)

System on Display™

We offer development of system solutions with printed electronics circuits and surface-mounted components on the back of the display substrate. Please contact us if you want to know more.

Single-Substrate Solution

The display on the front, printed circuits and components on the back, everything on one ultra-thin substrate. No need for external PCBs and connectors.

Roll-to-Roll Printed Display

Our standard printed and low-power display on the front side.

Surface-Mount Components

Sensors, MCU, power supply, and other components that are needed for a full system solution.

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