Mass-production of flexible displays is possible on plastic substrates without needing metal or ITO layers – by either sheet-to-sheet or roll-to-roll printing.

The underlying technology involves organic materials that change colour under the influence of electricity.

Segments in the display are turned on or off at will by +3V or -3V applied for
Truly bistable displays, like electrophoretic types from Eink, permanently stay in the state they are set, without refresh, for years if necessary.

The timing above is for Rdot’s standard material choice – longer time-to-fade durations can be obtained at the expense of a longer on/off switching times. “The bistability time can range from only a few minutes up to 24 hours,” company CEO Felix Karlsson told Electronics Weekly.

It sees its electrochromic display being useful in devices that only need to portray information while they are being operated – a room thermostat, for example.

If the display needs to be read at any time, for the standard materials, a 25ms refresh every 15 minutes will maintain full contrast, said Rdot.

Because response is slow, intended applications are simple alpha-numeric and other segmented displays with one electrical connection per segment. Simple matrix displays are possible, said the firm, but it is concentrating on the lowest cost segmented applications for now.

Colour

Its standard colour change is from light grey to dark blue and vice versa – these two states are the natural colours of the basic electrochromic material. Red-to-black and green-to-black displays have also been made, as has a blue-to-black (see table). Black and white displays are in the lab, and custom colours can be developed to match corporate or brand colours.

“We can do other colours as well, but these are the colours that we have developed so far,” said Karlsson, “Typically we do not include the blue/black version since the blue ‘bright state’ is very dark. Light grey/dark blue is our standard colour, and we are about to initiate development projects with clients to develop additional colours.

Power

Power consumption is extremely low – see table below – with smaller segment areas consume less energy per cm² than large segment areas, so segment size needs to be taken into account when making energy consumption calculations. Driving voltages can be between 2V and 5V, with lower voltages consuming less power but switching slower.

As a rough guide, changing 1cm² of segment area, then sustaining it continuously, requires:

• 0.6μW/cm² for one full switch per day
• 1μW/cm² for one full switch per hour
• 33.3μW/cm² for one full switch per minute
• 2mW/cm² for one full switch per second

Explaining this guide, CEO Karlsson said: “For a constant ‘on’ display, the driving includes one switch pulse to change the segment area and one refresh pulse to maintain the segment area to keep a high contrast. The refresh pulse requires much less energy than the switch pulse and is applied every 15th minute in this example. If you switch every second or every minute, there will be no need to add a refresh pulse.”

He added that if only infrequent switching is required, such as once per day, energy consumption can be significantly reduced by optimising the display stack for longer bistability time – up to the 24 hours mentioned above.

There is more power information here.

Durability

The displays are not sensitive to UV, but without encapsulation there is a humidity dependence influences switching speed and fade time. A barrier with water vapour transmission rate of 10^-2g/m²/day is usually sufficient to stabilise operation, said Rdot. “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.”

Operation is over -5 to +40°C, with no permanent damage observed over -20 to +80°C, “but the display driver needs to be modified”, said Rdot.

Lifetime depends mostly on the number of switching cycles undergone,  with close to no degradation if the display is constantly in one state or the other. “For applications demanding 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,” said Rdot. “With accurate driving, the lifetime will exceed 100,000 switching cycles.”

An in-house sheet-to-sheet production line is available for prototyping and pilot production up to several thousand display units per month, according to Rdot. Typical project lead time from sketch to delivered display is usually <3months.

A preferred partner is available for roll-to-roll mass production, and licensing can be arranged for very large volumes.

Is this technology ready for use, could someone order a display tomorrow?

Yes, said Karlsson, the technology is ready for use, but the firm specialises in custom products, so it would be unusual for anyone to order a standard design. The process is almost always three-stage:

• Evaluation kit
• Custom design development
• Pilot production

“In stage 2 we make prototypes based on the customers’ specifications,” he said. “This usually includes the design and arrangement of the segments, and in more rare cases also the development of custom colours. Depending on environmental conditions and operating conditions, other aspects can also be modified, such as barrier layers and potential graphic overlays.”

Is the technology in the process of being designed into any end-user products?

Yes, said Karlsson, several development projects are underway and expect to reach pilot production soon. No products containing Rdot displays have yet reached the market, although they are built into products for evaluation purposes.

On the subject of evaluation kits, several are available:

• OK symbol (30 x 16mm²)
• Numerals (25 x 22mm²)
• One 7 segment number (30 x 40mm²)
• Five square spots from 10 x 10 to 1 x 1mm2 (35 x 37mm²)
• Seven bar-shaped spots (28 x 35mm²)