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Eco-friendly digital advertising display, based on novel printable electrochromic polymers

Final Report Summary - ECOPIX (Eco-friendly digital advertising display, based on novel printable electrochromic polymers)

Executive Summary:
ECOPIX was a thirty months applied research project, which commenced in July 2014 and finished in December 2016, and was funded under the “Research for SMEs” programme of the European Commission Seventh Framework Programme (FP7).
Advertising promotes goods, services and information to the general public, specific target groups and other enterprises. It maintains or raises awareness about an issue, event, person, product or brand and supports choice and competition. Market demand for advertisement products is fueled by various factors, such economic growth, increased consumer expense level, low price, wide areas of usage, and others.
The quality of an advertisement product is strongly affected by the display method, which may cause changes in reaching to the expected audience. Thus, the quality of the advertising of the products poses a major challenge in the sector.
To this end, the ECOPIX project aimed at developing a novel technology for the digital out-of-home display, based on the use of RGB electrochromic (EC) materials. Via an electrochemical reaction, the color of EC materials can be changed from a colored state to a transparent state (or dark state) in response to an applied voltage.
Developments such as ECOPIX offer enormous opportunity to create added value, novel offering and market differentiation, to contribute to increasing the competitiveness of SMEs, as well as European manufacturers and suppliers of equipment.

Project Context and Objectives:
Advertisement is one of the oldest methods of sales messages. It becomes popular and major force of commerce as of mid-19th century. Only in EU 210,000 enterprises are classified to advertising activities and spending on media advertising is expected to reach €542 billion worldwide by 2012 .
During the project duration, ECOPIX has focused in the development of a compatible solution with the advertising sector, especially to the out-of-home advertising (OOH). OOH advertising refers to advertising in public spaces, and is designed to reach an audience on the go. With the decrease in the cost and increase in the equality of flat screen displays, digital advertising is replacing the printed posters and billboards.
Digital OOH (DOOH) is a sub-category of OOH where the message is displayed electronically, via LED or LCD screens. Unlike traditional billboards, where the printed sheets must be changed manually, DOOH displays may be updated on demand from a remote location. This is highly cost effective and it is known that DOOH billboards could generate 3-5 times more income than static billboards. Over the past decade DOOH has grown into a multi-billion dollar industry and is the fastest growing OOH segment. It is expected that digital billboards will continue to attract clients, leaving traditional billboard agencies to accept lower margins.
However, there are some limitations of existing DOOH and OOH technologies. LED and LCD billboards consume too much energy. Although they are recyclable, their reuses are not always monitored and are a potential source of toxins. Traditional paper billboards have also environmental drawbacks, due to their use of non-recyclable paper, toxic inks and adhesives.
The project has developed an innovative digital out-of-home color display, based on the use of electrochromic (EC) materials. Via an electrochemical reaction, the color of EC materials can be changed from a colored state to a transparent (“bleached”) state (or dark state) in response to an applied low voltage. The solution uses conductive polymer electrochromic materials, which have three important advantages:
• They are fabricated as reflective (front-lit displays).
• They only require a small refresh charge to maintain a static (non-video) image.
• They are processed as solutions using established and emerging cost effective printing methods such as ink-jet printing, screen printing, spray coating and low cost / high volume roll to roll (R2R) methods.
The ECOPIX display is based on the use of red-transparent, green-transparent and blue-transparent conductive polymer electrochromic materials. These and other materials are formulated into printable inks, and are printed onto flexible plastic film substrates using low cost printing methods. ECOPIX exploits these advantages to develop a reflective DOOH color display (with passive illumination via ambient sunlight during the day) which can offer market leading low power consumption for a digital display. Front illumination will be added for viewing at night, in a similar manner to traditional billboards. The solution is completed with the development of display driver and player electronics and software to allow a new image to be uploaded remotely from a web browser, rendered for the properties of the display, and displayed.
These features will allow ECOPIX to compete with traditional printed billboards and digital billboards, and also open a range of new creative possibilities for the medium, which are all of great commercial interest to the SME participants. The ECOPIX DOOH technology will significantly reduce the recurrent printing and installation costs of traditional printed billboards and enhance returns by supporting value adding “day-parting”. The cost effective technology will also generate minimal electronic waste. ECOPIX will result in direct economic benefits for consortium partners via new opportunities for the manufacture, distribution, installation and service of the outdoor display.
While the European OOH industry is dominated by large enterprises (LEs) such as JC Decaux and Clear Channel, there are 280,000 SMEs in the sectors as potential end users of ECPOPIX. The initial specific objectives can be summarized as follows:
• To define the detailed specifications, including a system selling price of <€250/m2, lifetime >12 months, power consumption <1W/m2 , display size up to 2x3m, image changeover time in under 10 minutes To also define the EC pixel layout, materials and fabrication steps based on ink-jet, screen printing, rotogravure, and flexographic, printed electronics.
• To synthesize the R, G and B electrode material, electrolyte material, and counter electrode material; and use them to build simple R, G and B “pilot” electrochromic devices (ECDs), and to characterize these ECDs according to industry standards.
• To specify / design and purchase the upper and lower plastic film substrates with their electronically conductive electrode pattern. To formulate the printable inks, set up the printing processes, and fabricate and test 18 display sheets, each measuring 20x30cm2 approx.
• To design and build a modular and highly expandable display driver which can periodically address and bias each PECD within a multi-sheet display such that each PECD expresses the design color tone (from dark to bleached). To design and build the “player” software on an industrial PC platform, to receive display images and scheduling from a remote station, and will render these images for display, via the display driver.
• To integrate a 2x2 sheet EC display on a supporting surface, and provide the electrical connections between the sheets and from the outer edges of the display to the display driver. To then test the display system with the player and the display driver. To build and test two identical systems of this type.
• To demonstrate, characterize and validate a first display system in the laboratory, and to demonstrate and validate a second, identical display system in the field at LETRATEC and IDKLIC installations.
• To increase understanding and knowledge of:
o Synthesis of conductive polymer electrochromic (EC) materials and design and characterization of electrochromic devices based on those materials.
o Formulation of printable inks and the fabrication of EC displays using low cost printing methods.
o Building of large area displays using passive matrix addressing, and rendering of images for display
The overriding goal of this project is to ensure that the pre-competitive ECOPIX prototype resulting from this project fulfils the threshold requirements to ensure its further development post-project into a fully industrial system that is taken to market, where its beneficial impact will be felt at European level.

Project Results:
The first goal of the project was focused on understanding the technological needs of European advertising processors, as well as market needs and perceptions in terms of product quality, new technologies, etc. To this end, consortium partners rely on the inputs received mainly from 2 partners, IDKLIC and LETRATEC that have a reasonable experience and participate actively in the sector. The gained in-sight was further broadened by desk research on market and socioeconomic information, literature reviews and patent searches on the technology at hand. With this information, both the industry and research partners worked closely to define the industrial needs of the proposed ECOPIX system.
Early in the project different possibilities to generate variable pixels were discussed. The best solution for the lab-test pixel was found to be the use of RGB over CMY. This solution was proved to be very challenging at the technical level. Basic concepts were applied to the design of the pixel, although the synthesize of electrochromic materials significantly increased the complexity of the design. To have real red, green and blue was the main challenge of the project. The formulation of printable ink and the production of display driver were another considerable hurdle in the design of this equipment. Nevertheless, a complete ECOPIX test was built consisting in a RGB pixel scheme; a display and driver.
At the time of project submission, the proposed electrochromic displays consisted on EC pixels arrays, where each pixel was formed by six addressable red-transmissive, blue-transmissive and green-transmissive ECDs based on three EC conductive polymers. During the development stage of the project, the EC pixel layout was altered to consist of 9 addressable ECDs based on the three. This final layout possesses larger dimensions, than what was foreseen in the DOW, and the new pixel is now squared. This alteration was made with the objective of simplifying the display controller, since the initial rectangular shape, with 6 sub pixels, would have required the controller to compensate the anisotropic distortion caused by the “stretching” of the image leading to the need of creation of complex algorithms.

Figure 1: Scheme of the RGB-transmissive pixel layout.
In relation to pixel dimensions, the size of each individual ECD was, from the beginning, 10x10 mm2, but the gap between them suffered alterations along the project. Early gaps of 2.75 mm, defined by the separator for the pixels, corresponded to a fill-factor of 62% (Figure 3a) that reduced the quality of the displayed images. With subsequent changes to the matrix structure, it was possible to reach a final gap of 0.5 mm, which corresponds to a fill-factor of 90,25%, with square pixel dimension of 30x30 mm2 (Figure 3b).

Figure 2: a) Initial pixel layout; b) Final pixel layout
Relative to the ECDs structure, each has nine superimposed layers, composed by electrochromic materials, electrodes, electrolyte, and counter electrode.

Development of the electrochromic matrices
With respect to the structure of the electrochromic matrices, the initial matrix possessed two conductive layers patterned into lines and columns, continuous electrolyte and counter-electrode layers and an EC material layer patterned in squares. It also used a spacer to contain the gel electrolyte and control its thickness. In the initial, passive matrices, pixel addressing was achieved by controlling the voltage applied in each row/column present in the top and bottom transparent conductive layers. A common electrolyte was used but cross talking between the pixels was evident, requiring a different approach.
With the objective of achieving independent control of the ECDs, a modified matrix structure was created (Type II). Type II possessed two conductive layers patterned into lines and columns and square patterned electrolyte, counter-electrode and EC layers patterned in squares. A spacer was also used to contain the electrolyte and create the pattern. This approach was still based in passive matrices but made use of a patterned electrolyte layer, so that each pixel had an individual layer of electrolyte, reducing the charge transfer between neighboring pixels, resulting in a better control. Nonetheless, cross talking between the pixels was still noticeable and resulted in the degradation of the image reproduction. Another problem that arose was the electrical resistance of the ITO, that contributed to higher switching times and variable voltages had to be applied, to obtain a similar transition for all the pixels.
In response to the continuous occurrence of leaching between the ECDs, an active matrix structure was created (Type III) to assure an independent control of each pixel. The method applied for pixel control suffered important alterations, with the objective of improving image quality by preventing ionic leaching by surrounding pixels. To resolve the leaching, a new pixel addressing process was implemented by creating through holes in one of the electrodes, PET/ITO layers, allowing for an individual control of each of the pixels, while the other ITO electrode is grounded. The construction process of the layers is more complex due to the added steps that require a high precision, in the development of the electrode with vias, but the cross-talking issue is solved following this innovative approach. These active matrices are composed of three square patterned bottom layers, namely the bottom conductive layer, the ECD layer and silver pads, and four continuous layers, namely the electrolyte layer, the counter-electrode layer and top conductive layer. A spacer frame is also utilized to contain the electrolyte and control its thickness. Each of the layers present in the final matrix were thoroughly studied and extensive research was conducted with the objective of reaching the objectives propose in the project.
Development of the transparent conductive layers
Regarding the two transparent conductive layers, the construction of addressable pixels required the patterning of the bottom conductive electrodes to produce the addressable matrix, when the electrodes are assembled. PET/ITO sheets were selected as the basis for these layers, having the possibility being acquired without patterns and uniformly conductive on one side. Whilst it is possible to buy pre-patterned ITO from some suppliers, this has disadvantages such as higher price and may also induce delays as a result from the need of constantly changing the patterns during the development phase. So, in ECOPIX we focused on methods that can be performed in-house such as chemical etching and laser ablation/etching. Chemical etching requires the use of lithographic imaging, solvents and rinsing baths that may degrade the ITO layer, so laser etching methods have been deemed preferable, because it is a simpler process and facilitates the control of the depth of the etching and the required pattern. Considered all advantages and disadvantages identified for the different etching methods, it was opted to develop and optimize an in-house method of ITO etching using a CO2 laser, that can be industrialized.
Formulation of the electrochromic layer
As for three electrochromic pigments used to create the RGB colors, ECOPIX synthesized soluble red-transparent, green transparent and blue-transparent conductive polymers to provide the three legs of the color space. As one of the main aims of this project was to create an economical alternative for large area advertising, the electrochromic devices to be created were envisioned as cheap and reliable display units, that retained superior image quality. Towards generating a reasonable color gamut, polymers that have green, red and blue color in their reduced states were synthetized, and optimized during the project. These materials also possess two additional very critical properties: they can be switched to a highly transparent state upon oxidation and they can give different hues of their colors via a controlled applied potential. The structure of the red to transparent polymer (red-ECP) created is given below.

Figure 3: Red-ECP chemical structure

Even though polythiophene was promoted by many authors as a red to transparent polymer, the transparent state of polythiophene (or poly(3-hexylthiophene)) have significant blue hue which will hinder the application of this material. The Red-ECP shown above has a highly transparent oxidized state with high optical contrast values. Relative to the green electrochromic polymer, although a vast number of red and blue colored polymers are known due to the dependence of these colors on one wavelength absorption, green polymers were essentially, with the first truly green polymers with highly transparent oxidized states were developed by the Toppare Group. A soluble version of this material was also developed which opened the way for use of these materials in commercial device applications. The structure of the material is shown below and it was selected as the green leg of the RGB system in the ECOPIX project.

Figure 4: Green-ECP chemical structure
Following the synthesis of the RGB electrochromic polymers, one of the ECOPIX objectives was to formulate inks based on those polymers that could be applied using several deposition techniques. These polymers have similar chemical properties in terms of solubility, being typically soluble in halogenated solvents such as chloroform and 1,2 dichlorobenzene. As mentioned before, in the final display structure the electrochromic layer is patterned and composed of squares of different electrochromic polymers: red-ECP, green-ECP and blue-ECP arranged as showed in Figure 1. Thus, the deposition/printing technique selected for the electrochromic layer must allow the deposition in well-defined areas, since that directly influences the dimension of the pixels which can be achieved, the gap between the pixels and the quality of the final display. Taking into account all these considerations, three techniques were selected for the deposition of the different electroactive polymers: spray coating, a simple coating technique which does not require complex formulation processes, screen-printing, a versatile printing technique, widely used in printed electronics, and inkjet printing, a digital printing technique.
In general, all inks or pastes for screen-printing are composed by a pigment or other active elements and a vehicle to convey the pigment/active element throughout the process to its position on the printed surface. Usually, the vehicle is also responsible for the adhesion of the active element to the substrate and controls part of the dried ink film properties. In some cases, inks may contain a catalytic agent to accelerate a chemical change within the ink. Concerning to the solvent, in general its selection is based on their ability to dissolve the resin and disperse/dissolve the active element and other inks components. To choose a solvent for the ink vehicle, a thorough solubility study was conducted to identify a solvent suited for these polymers. The solubility of the different electrochromic polymers was tested in several organic solvents with high boiling points, most of them known to be used in screen-printing inks formulation. A detailed analysis of the results of the solubility tests indicated that the best solvent for all electrochromic polymers is anisole, as it showed the best capacity to dissolve them, not requiring heat to initiate their dissolution. It also showed no signs of causing an alteration in the polymers chemical properties since no color shift was visually observed. As for the selection of a binder, only by using ethyl cellulose was it possible to promote the creation of a uniform film. Different percentages of binder were tested, and the results showed that only by using 1% (m/m%) of ethyl cellulose was it possible to obtain a film with electrochromic properties. However, the film was degraded by the electrolyte. As such, it was not possible to create usable electrochromic screen-printing inks, and a decision to utilize sprayable inks in the production of the electrochromic sheets was reached.
Although, spray coating requires the use of masks to achieve patterned films and consequently higher losses of material during the fabrication process, the time needed for detailed formulation processes is substantially reduced when compared with other coating/printing techniques, and the dried films can be exclusively composed by the functional material, in this case the electroactive polymer. PEDOT:PSS inks were used to compare what results could be achieve by screen printing or spray coating electrochromic films. When comparing the absorbance of the PEDOT:PSS films for different applied potentials it was possible to conclude that the spray coated film has a lower performance when compared with the screen-printed films, and that, while having similar absorbance in the transparent state, the variation of the absorbance is quite smaller when compared with the screen-printed films. These differences are probably due to the more compact morphological structure of the screen-printed films when compared with the spray coated films, resulting in a higher conductivity due to a better dispersion of the polymer.
As for the inkjet printed films using P3HT, they presented a distinctive electrochromic behavior, but very low levels of uniformity. The deposition time for small films (rectangles of 2 x 1 cm2) was also very long when compared with screen-printed or spray techniques. For that reason, the inkjet printing technique was considered not suitable for the assemble large matrices and therefore no further optimization steps were done. Tests were also performed with RED-ECP in a XENIA inkjet printer, with not satisfying results due to the uniformity of the films and clogging of the printed head
Formulation of the electrolyte layer
One of the key layers of the ECD’s is the electrolyte layer that must be ionically conductive to assure the current flow between the working and the counter electrode, but electronically insulator to minimize self-bleaching, and also transparent in the visible region, so that it does not interfere with the color obtained from the electrochromic materials that are being used. During the project two approaches were followed focused in different types of electrolytes: gel electrolyte and a UV photopolymerizable electrolyte, with different compositions and different curing processes. The main differences between the two electrolytes types are in the curing process, thermal evaporation of the solvent or UV curing, and in the viscosity, while characteristics such as transparency and ionic conductivity are similar. In the first pilot ECDs, a gel electrolyte polymer was prepared and used. However due to the pixel structure design, there were problems associated to use this GEP, namely, the acetonitrile evaporation was too slow and the resulting electrolyte viscosity was too low. Hence a UV photocurable GEP was selected as the electrolyte material, where the polymer was replaced by a monomer and a photoinitiator.. After UV curing, the GEP becomes solid, enabling the lamination of the counter-electrode. When comparing the two types of electrolytes, gel electrolyte and UV photo curable, both have a similar chemical composition and both are electrochemical inert and transparent. The main difference between them is the curing process, which dictates the one that should be use depending on the device size. For medium and larger matrices (> 12 x 12 cm2) it would not be possible to control the evaporation of the solvent in a uniform process, resulting in different ionic conductive and uncontrollable pixels due to the different electrochemical properties. In this case, the best option is a photo curable electrolyte, which allows for a better and more uniform control in the curing process over large area devices.

Formulation of the counter electrode layer
As mentioned, the display unit of the ECOPIX project uses a side by side pixel array. In this arrangement, colored polymeric electrochromic materials can be oxidized to lighter hues of the same color or transparent to generate a viable color gamut. Hence for the double layer electrochromic device structure, it was necessary to introduce a charge balancing layer to improve the lifetime of the devices and refresh rate of the pixels. The material selected for the counter electrode layer had only to compensate the charges and could not have any visible color change. In other words, the counter electrode material should be transparent in both reduced and oxidized state. From the two promising, solution-processable polymers available, PTMA was selected for the charge compensation layer, since it possesses the simpler and cheaper synthesis pathway. Its chemical structure is displayed on Figure 5.

Figure 5: PTMA chemical structure
This additional layer was also deposited by spray coating techniques, mainly due to the low viscosity of the solution and due to the need of reducing the number of deposition techniques used to produce the ECD.

Development of the control software and hardware
Like other digital display, the screen needs both hardware and software driver to address and control the intensity of the pixels. However, to utilize the innovative screen, the characteristic of each pixel component has to be well studied. Afterwards, efficient control can be applied on it to display the required colors. To incorporate thousands of pixels to show a required picture is a challenging problem to be solved in this part of the project.
Research on the ECDs’ characteristics
According to the reports D2.1 D2.2 there are two basic characteristics of the ECD devices:
1) The color saturation and the intensity of these electrochemical materials will change according to the potential applied on them.
2) The color will sustain for a while after the applied potential is removed.
The recommended potential from the preliminary reports is -2.5 ~+1.5V which corresponds to the fully saturated color state to fully transmissive state. The change of the color can be continuous as the potential applied can change continuously. However, the color of the ECDs under different potentials needs to be quantified to make sure the color is reproducible. Besides, as it is reported, the devices will be damaged if they are exposed to a very high potential or to a relative high potential for a long time. Hence the limit of the devices should be tested as well.
To further study the color characteristic of the ECD devices, a portable spectrophotometer was used to measure the spectrum of the ECDs. There are built-in Xenon light source in the spectrophotometer which is used for measuring the reflective color. The ECD devices are placed on the pure white background. With the help of the open source software, an automatic test platform is developed for the study of the ECD's characteristic. The platform hardware consists of two parts, the spectrophotometer and one programmable power supply. The power source is composed of two Digital-Analog Converter (DAC) which is used for generating the potential within the range -5V and +5V.
In the test, the platform outputs a series of different potentials from 1.5V to -2.7V with the step of 0.3 V. However before each potential level is applied, the EC component is ‘refreshed’, i.e. reset to the transparent state. The spectrum of an EC component is measured at 5 seconds (s), 15s, 30s, 60s, 120s and 300s after the EC component is exposed to the potential.

Figure 6: a) Potential – Spectrum of Blue ECD b) The Colour in RGB Space

Through the tests, a series of potential – spectrum curves were obtained for each kind of the EC component. The measured curve is then transformed to the corresponding color in RGB color space and plotted on the computer screen for visualization. From this set of experiments, we have the following observations:
1. The color of the ECD changes gradually after they are exposed to a potential. Usually, it takes 30s – 100s for the parameters to be stable.
2. When the potential goes up from -2.85V to +1.57V the EC component changes from colored state to transparent state. Vice versa, when the potential changes from +1.57V to -2.85V the EC changes from its transparent state to full colored state. However, in these two processes, the color intensities achieved at the same voltage are not the same.
3. The spectrum changes in a nonlinear or hysteretic matter, i.e. the potentials that are needed to obtain a specific color from the full color state or from the transparent state are different. The color is reproducible if the increasing direction and the potential applied is the same. It is not safe to keep the potential applied on the EC component for more than 10 seconds once the component is fully activated.

Hardware design for efficient refreshing
The proposed maximum potential range for the large area ECDs is from -3.0V to +3.0V but to fully activate the potential of the device and for the future extension and upgrade, the range is designed to be from -5.0V to 5.0V. Normally, there are two ways to power the ECDs with the potential required. One solution is to use produced DAC chips to generate the required voltage. A reasonable resolution of voltages can be achieved using chips with 8 bits or 12 bits, which respectively lead to voltages precisions at 0.009V and 0.0006V with a reference input of 2.5V. The other solution is to use the Pulse Width Modulation (PWM) wave, which is a common method for the large LED screen. The main advantage of the PWM solution is that it enables adapting the existing built integrated circuits which are designed for LED displays for our needs. The main disadvantage of the PWM wave is that the output very noisy, and might reduce the lifetime of the ECDs. In comparison, the solution using DAC chips are more expensive and the output range is relatively small, but the accuracy and the stability is much better than the solution using PWM wave.
To keep the total cost in a reasonable range, we employed an octal 8-bit voltage output DAC, and the output range is from 0V to 5V. To get the targeting range, two amplifiers are used in the circuit. The first one is used to minus the output from the DAC by 2.5V and then double the result. By doing that, the output range would be from -5.0V to 5.0V. The second one is used as the voltage follower, which help to increase the load capacity of the circuit.
Module controller
The drivers are designed to be modularized in a highly expandable way so that they can be easily assembled to a big display. For one module, there are 24 x 24 EC components on it, actually, it is composed of 4 12 x 12 EC sheets. Each module is controlled by a single-chip, which is also the module controller. Together with other peripheral chips, they can provide a number of functionalities, including control signal preparation, pixel addressing, DA conversion for bias voltage, etc.
There are 3 ways to address the pixels in the module. The active matrix method is the most common one, the idea behind this is to allow multiple EC components to share the same control signal. A control signal ‘scans’ through several EC components in iterations, giving them a different bias voltage at required time slot to change or sustain their colors. This approach takes advantages of the persistence vision property of the human vision system, that human’s vision is not sensitive to high-frequency changes and the perceived images at one time usually lasts for a short period of time before they completely disappear. So for one EC component, the signal passes by in one iteration, giving it a specific bias voltage for a short moment, then it moves on to other EC components in a sequence. When the signal is on an EC component, the required voltage is given to it and it enters the expected color state. After the signal passes on, this EC component may sustain its color stage for a moment, but will eventually return to its natural state if no further signal is applied. The control signal is supposed to finish one scan and start the next in a suitable frequency, so that the time for each scan is short enough to sustain colors and long enough to cope with the response time of EC components for the three different colors. Another critical advantage that makes the dynamic approach more practical is that, such a design will greatly reduce the hardware size and cost.
With the static control, each pixel will receive its own control signal provided by an independent control voltage that is generated from a D/A converter or PWM wave. In this solution, the control of ECDs can be expected to be simple and robust. It can work without any requirement on the ECD response time. However, this implies high hardware size and cost when a large display board consisting of an enormous number of pixels is considered.
Passive matrix combines the advantages of the static control and active matrix solution by providing parallel signals to multiple EC components in the matrix at one time. Actually, this solution is a compromise between the performance and the cost. Drawing on the refresh time of the single EC component, this solution is finally adopted. For the 24 x 24 matrix in the module, it is divided into 36 4 x 4 sub-modules. Accordingly, there are 36 individual signal sources (DAC) outputs for each of the submodules, and they will address the 16 EC components one by one periodically.
Hardware design for addressing
For the 16 EC components in one 4x4 submodule, a 16-Channel High-Speed Analog Multiplexer is used to address each component. However, all the multiplexers share the same address signal, which means all the EC components with the same address in the submodules will update simultaneously. But the multiplexers do not need to work together, they can be switched on/off individually.
Another problem arises because of the input signal range of the multiplexers. The output of the amplifier (voltage follower) is passed on to the multiplexer, but the range of it is from -5.0 V to +5.0V. If the multiplexer uses the same ground as the single-chip, the multiplexer won’t work with the negative potential due to the specification of the chip. The solution is to make fully use of the multiplexer’s large input range, use the virtual ground (-5.0V) to fulfil the requirement. At the same time, we employed a few amplifiers to adjust the output address signal of the single chip to be with the same ground reference.
Colour space mapping
Most digital display in our daily life use RGB color space since most of them emissive which mean the screen can emitting light. However, the ECDs are reflective display, like the paper, they need external light to illuminate the content. Hence they can not use the value from the RGB space directly. Besides, the R, G, B components of the ECDs are not the standard colors, a color space mapping algorithm is required to map the color space which is defined by the ECDs to the normal RGB colour space.
To minimize the output error between the input RGB value and the actual output, the error was firstly analyzed theoretically through mathematical modelling. Assuming the same amount of red, green and blue components are placed in a small region, the perceived color of this region can be estimate through the spatial integration of the color components. Then, for a given weighting factor which indicates the relative importance of the chrominance and luminance errors, an optimized solution can be found from the target function. With the solution obtained, a new color space mapping algorithm is achieved to convert the value of the pixels in the RGB model to fit our ECD devices' model.
Simulation Platform
To verify the proposed algorithm and demonstrate the potential outcome of the display, a platform is developed to simulate the output. The idea of the simulation is to use the pixels of the normal computer screen (LCD or LED) to reconstruct the output of the ECD. In the simulation, 9 different intensities of each color of the ECDs are considered, which are converted from the sampled potential – spectrum curves. The pixel size is 1cm x 1 cm and the gap size is 0.1cm in the simulation, but they can be adjusted in the platform as well. The viewers distance from the display can be simulated by zooming in or out of the simulation output. As the pixels’ layout of the ECD is side by side, the arrangement of the pixels can affect the final performance. Via the simulation, the performance of several different arrangement of the pixels is tested. The results from the test shows that the diagonal layout is less likely to be affected the aliasing effect (Moiré pattern), comparing to the vertical layout which is commonly used in LED display.

Figure 7: Original Image and the Simulation Output
Adaptive control mechanism
Due to the large lag of the ECD's response time comparing to the LCD panel, a creative control approach is proposed to further enhance the ECD refreshing performance. A PID (proportional–integral–derivative) based adaptive method is employed to reduce the time needed for the ECD to be fully activated or deactivated. As the research on the characteristic of the EC components indicates the ECD devices will be totally destroyed or the lifetime will be reduced if the applied potential exceeds the safe range. Hence, the designed control mechanism will be responsible for the improving the lifetime as well.
Even though the display is designed for the static image and don’t require too much on the refresh rate, we still need to refresh the display with a certain frequency, because the color of the EC components will not stay the same after activated. For each iteration of refreshing, the potential applied on the pixel will be adjusted according to the value of the original value of the pixel and the previous value applied on the pixels.
Task Scheduler
To develop a player software for the display, there is a need for the player to incorporate both the hardware and software task together. However, to accomplish this, a preliminary integration is needed, which requires to connect the module controller in the lower level with the central controller on the higher level. In this aspect, a task scheduler is designed as the core of the software to allocate the recourses on the central controller and coordinate the tasks. This task scheduler is responsible for defining the priorities of the most important tasks among the three subtasks: the image pre-processing unit, the user interface with a server back end, and the central controller.

Figure 8: Task scheduler

Image pre-processing
In the image pre-processing part, the original image first needs to be adjusted to fit the physical resolution of the display. The image resizing algorithm performs average down sampling by assigning the value of the pixels in the new image per the average value of it neighboring pixels in the original pixels. Then, before the color space mapping algorithm is applied to convert the RGB value of the pixels to the potential level of the pixels, the image is enhanced to improve the visual quality of the outcome. Afterwards, the processed image will be divided into small pieces with the exact size of the modules and stored in the database.

Figure 9: Image Pre-processing Steps
Player (UI) design
The player is designed as a website so that it can be used across different platforms and support remote control from multiple-users. The data of the player is stored in the database, and the web-based interface reflects the real time data stored in the database. There are five main sections of the player: the homepage, the status page, the control center page, the system test page and the user management page. Only the administrator has the authorization to log into the user manage and manage the user database, while other verified users are allowed to look up the status of the server and perform operations like image upload, delete and queue sequence editing.

Figure 10: User Interface Example

Central controller
The server is powered by Apache and Lighttpd and hosted on the Raspberry Pi (Rpi) which is also the central controller. Thus, the server is able to respond to the users’ request instantly without any network delay, and the communication between the central controller and the server is avoided. Another advantage of using the Rpi is that the system will be very easy to expand since the Rpi is based on Linux operation platform, such as the Wifi module and Bluetooth module. The system has built-in driver for such devices. The task scheduler will read the parameters from the database and pass the status information from server to the central controller. Once the central controller received the information, it will send commands or data to the module controllers. However, to ensure the commands and data are delivered by the module controller smoothly, a communication protocol is specially designed.

Assembling and testing of the final display unit
In the final stage of the project the electrochromic matrices were assembled with the backboard for testing and validation. The electrochromic sheets used in the final display (Figure 11) were composed of 12x12 pixels that correspond to 12x12 cm (with each pixel having 1x1 cm). The employed electrochromic sheets make use of an active matrix structure to assure an independent control of each pixel.

Figure 11: Example of an electrochromic sheet used to build the display

The purpose was the assembling of the display sheets on a traditional billboard using the defined methods to provide continuation of the addressing bus between each sheet and to enable connection to the EC display driver. The display within the task was constituted by a base unit, the PCB back board, in which 4 electrochromic sheets are laminated to form a 24X24 pixels unit with approximately 25x25 cm. that can then be combined to form larger displays, with dimensions that can be adjusted to client’s requirements and application. The software, developed in WP4, that is used to control the displayed image is a web based application through which the displayed images can be controlled.
The electrochromic display is assembled in several stages. The first is to prepare the PCB backboard for the electrochromic sheets by applying a conductive epoxy resin on the copper pads present in the front side and an adhesive layer. The conductive epoxy is dispensed in each of the pads to assure a good electrical connection to the pixels, while the adhesive layer is used to fixate the ECD after lamination. After the preparation of the backboard the electrochromic sheets previously prepared are then laminated onto the backboard, establishing electrical connection with the backboard pads through the conductive epoxy. With the assembling of the 4 matrices on to a backboard several experiments were performed to validate the system that consisted in an on and off pattern applied to the display. With these tests, it was possible to verify the correct addressing of the pixels using the backboard, the possibility of individual addressing the pixels (that was one of the main reasons for the new approach), and establish the assembling method and materials to be used. Even though the good results that were obtained some issues arose related to the need of improving the contrast of the display, with the increase of deposited electrochromic material, and eliminate the induced damages to the pixels by improving the addressing and electronic control for the individual pixels.

Characterization under controlled laboratory conditions
Due to experimental constrains, the generality of the characterization tests were performed with single electrochromic devices (ECDs) or small 3x3 matrices. In both cases, the structure and composition of the ECDs were similar to the ones that composed the final prototype. Figure 14 shows typical samples used to perform the different characterization tests.

Figure 12: Examples of ECDs developed to perform characterization tests. A – Red individual ECDs, B – Green individual ECDs and C - Blue individual ECDs
The mentioned matrices were characterized in terms of reflectance, optical contrast, coloration efficiency, color coordinates (using CIE standards), switching rate, stability (under repeated switching), power consumption, optical memory, and durability under accelerated environmental conditions (temperature and UV exposure).

Reflectance analysis and optical contrast

The reflectance spectra of Red-, Green- and Blue-ECDs were recorded for the oxidized and neutral state, with Figure 13 depicting the obtained spectra.

Figure 13: Reflectance spectra for Red (A), Green (B) and Blue (C) ECDs in the oxidized neutral state

From the analysis of the reflectance spectra was determined the wavelength at which the reflectance variation is maximal, λmax. In terms of optical contrast, the different ECDs present similar values of reflectance variation (between 20.5 and 25.4) and the coloration efficiency varies between 92 and 104 mC-1.

Table 1: Study of the optical contrast. Values of the reflectance variation, ΔR, at λmax for the Red-, Green- and Blue- ECDs.
ECD λmax / nm ΔR / %
Red-ECP 520 25.4
Green-ECP 700 21.3
Blue-ECP 585 20.5

CIELAB coordinates

The displayed color of the electrochromic polymers: Red-, Green and Blue-ECDs were studied determining the CIELAB coordinates from the reflectance spectra recorded using a spectrometer with an integration sphere. For the CIELAB coordinates determination, spectra were recorded between 400 - 700 nm, with Δλ = 10 nm, without correction for de the bandpass (2 nm), using the weighting factors from table 6.19 of ASTM E 308 – 01, using the Illuminant D65 and the CIE 1964 supplementary standard observer 10 °.

Table 2: CIELAB coordinates for electrochromic devices
ECD L* a* b*
Red-ECP neutral (colored) 65.1 20.7 14.0
oxidized (uncolored) 74.7 -1.9 7.9
Variation 9.6 -22.5 -6.0
Green-ECP neutral (colored) 74.3 -7.4 12.5
oxidized (uncolored) 76.6 -2.3 12.4
Variation 2.3 5.2 -0.1
Blue-ECP neutral (colored) 77.1 1.8 -1.7
oxidized (uncolored) 84.4 -1.7 5.9
Variation 7.3 -3.5 7.6

The different ECDs are cathodically colored, i.e. upon oxidation they switch from a colored state to an uncolored state. Therefore, as expected for all the different ECDs, the L* coordinate (which represents the lightness of color) increases upon oxidation. The ECD with the higher L* variation is the red one. This ECD also have a better performance in terms of color variation, its coordinate a* presents the higher variation upon oxidation. The ECD with poorer color performance is the green one, which coordinate L* only varies 2.3 units between the neutral (colored) state and the oxidized (uncolored) state. It is worthwhile to note that color adjustments of the ECDs cannot be easily implemented as their colors arise from the electronic spectra of the different electrochromic polymers which in turn intrinsically depend on the polymer chemical structure.

Switching rate

The switching rate was evaluated by measuring the switching time for coloration (coloration time) and for discoloration (discoloration time), specifically, the time required by each electrochromic device to reach 95 or 5 % of the reflectance variation when reduced or oxidized respectively. Figure 14 shows how the switching times for the coloration and discoloration of the electrochromic devices were determined.

Figure 14: Determination of the switching times for coloration and discoloration of a Red (A), a Green (B) and a Blue (C) ECD.
The switching times for both processes (coloration and discoloration) of the different ECDs are summarized in the following table.

Table 3: Switching times for coloration and discoloration process of Red, Blue and Green ECDs
Switching time/s
ECD Coloration process Discoloration process
Red-ECP 8.0 5.1
Green-ECP 16.0 3.9
Blue-ECP 7.3 1.4

For the different ECDs, the coloration process is faster than the discoloration one. All switching times are less than 10 s, as exception of the coloration of the Green ECD which is 16.0 s. It is worthwhile to note that the switching time decreased with the increase in applied potential. However, higher potentials could jeopardize the stability of the ECDs as they can trigger undesired side reactions. Therefore, the applied potentials must be low as possible.

Stability (under repeated switching)

The stability of the electrochromic devices was evaluated by repeatedly switch the coloration state applying a square wave electrical signal with step potentials of 2.5 and -2.5 V, of 5 s duration each.

Figure 15: Study of the optical stability under repeated switching: A – Red ECD; B – Blue ECD and C – Green ECD. The potential was stepped between 2.5 and -2.5 V, each step has a duration of 5 s.

The ECD with worst performance was the blue one, which with only 100 cycles has lost more than 50 % of optical contrast. Regarding to the other ECDs, the green and the red one, results were more promissory. Although they also show a decrease of optical contrast upon repeated switch, it was less pronounced. In all cases, the problem appears to be in the return to the uncolored state. One possible explanation could be the electrochromic film passivation, i.e. loss of the electroactivity, caused by overoxidation. Another possible explanation for the results is the presence of undesired side reactions involving contaminants within the samples. In this last case, the results could be largely improved by reduction of potential contamination sources such as atmospheric water vapor and atmospheric oxygen.

Power consumption

The electric power consumption (P) required to color and discolor each electrochromic pixel was determined by applying an electric pulse of the -2.5 V and other of 2.5 V respectively, with 20 s of duration and being the electrical current recorded over that time (Figure 16).

Figure 16: Study of the ECDs power consumption. The electrical current vs time for pixels of the different EC polymers (Red, Green and Blue) with the application of electrical pulses 2.5 V and -2.5 V

The electrical power consumption was calculated integrating the electrical current, with the obtained results summarized in the following table:

Table 4: Power consumption for coloration and discoloration processes
Power Consumption (W)
ECD Coloration process Discoloration process
Red-ECP 0.0213 0.0161
Green-ECP 0.0092 0.0110
Blue-ECP 0.0083 0.0051

Regarding to the power consumption of a display (which contains approximately 10 000 ECDs per 1 m2, with an average power consumption of 0.0118 W), the maximum power consumption will be 118 W/m2. This value corresponds to a situation of all ECDs changing at each 20 s, however, the actual power consumption value is expected to be significantly lower. Due to the optical memory, after the color switching to render the intended image, the whole display will only require a small amount of energy to maintain the displayed image.

Coloration efficiency

The coloration efficiency (η) is defined as the change in optical density (OD) per unit of the inserted charge, Q. Considering OD = log10(1/T), the color efficiency can be defined as: η = log10(Tox/Tneut)/Q.
A high coloration efficiency gives a large optical modulation at small insertion/extraction of charge. Its value could positive or negative. When absorption increases with ion intercalation, the coloration efficiency is positive and the phenomena is called cathodic electrochromism, when the absorption decrease, the coloration efficiency is negative and the phenomena is called anodic electrochromism.

Table 5: Study of the coloration efficiency
ECD λmax (nm) Coloration efficiency/mC-1
Blue-ECP 585 104
Red-ECP 520 101
Green-ECP 720 92

Optical memory
The open circuit memory (optical memory) can be defined as the time at which the device maintains its color under open circuit conditions. To evaluate the optical memory experiments, 1 s pulse of +2.5 V or -2.5 V was applied to the different electrochromic devices, and the transmittance was recorded at λmax immediately after during 500 s, being the devices kept under open-circuit conditions (Figure 17).

Figure 17: Optical memory study. Absorbance at λmax recorded over time for red (A), green (B) and blue (C) electrochromic films immediately after application of an electric pulse of 2.5 V (oxidized state) and -2.5 V (reduced state) during 1 s

Durability under accelerated environmental conditions
In order to evaluate the durability of developed systems under accelerated environmental conditions, different samples were submitted to an accelerated ageing in an artificial chamber. To evaluate the samples degradation resulting from the accelerated ageing tests, before and after submitting the samples to the accelerated ageing, the contrast and the switching time (for coloration and discoloration) were measured. The results can be found at Table 6 and Table 7, respectively. Figure 18 depicts how the switching time was determined for the different ECDs.
In general, for all ECDs, after being submitted to accelerated ageing, the optical contrast decreases, indicating a worst performance, likely due to undesired reactions triggered by UV radiation. On the other hand, in all analysed samples, after the accelerated ageing, the switching time is shorter. That is somehow expected due to the decrease of the optical contrast.

Figure 18: Determination of the switching times for coloration and discoloration of a Red (A), a Green (B) and a Blue (C) ECD after accelerated ageing

Table 6: Study of the optical contrast. Values of the reflectance variation, ΔR, at λmax for the Red-, Green- and Blue-ECDs, before and after accelerated ageing
ΔR / %
ECD λmax Before accelerated ageing After accelerated ageing
Red-ECP 520 25.4 23.0
Green-ECP 700 21.3 18.8
Blue-ECP 585 20.5 19.7

Table 7: Switching times for coloration and discoloration process of Red, Blue and Green ECDs, before and after accelerated ageing
Switching time/s
Before accelerated ageing After accelerated ageing
ECD Coloration process Discoloration process Coloration process Discoloration process
Red-ECP 8.0 5.1 2.4 3.7
Green-ECP 16.0 3.9 14.0 4.4
Blue-ECP 7.3 1.4 1.2 0.9

These results demonstrate the need to protect the developed systems with UV protective transparent coatings.
In conclusion ECOPIX designed addressable 1x1cm red, blue and green ECDs based on the electrochromic conductive polymers, a gel polymer electrolyte, a counter-electrode, and transparent conductive electrode materials, sandwiched between two flexible plastic sheets of polyethylene terephthalate (PET). Stepwise oxidation of each ECD produces at least 20 different tones of the same color, in addition to the clear/transparent state. The partners developed ECD design to provide ease of production using standard printing and coating equipment, and will fully define the manufacturing steps to take place using that equipment. The use of flexible plastic sheet substrates enables the display to be thin, light and conformable, and to employ cost effective high volume roll to roll (R2R) processing methods for manufacture.

Potential Impact:
The commercial dissemination of ECOPIX was led by the RTDs via their networks of clients throughout Europe, and via industry trade fairs and events which they regularly attend. For these and other events, the partners will develop marketing material such as brochures and posters. A white paper was produced for mailing to target audiences. A demonstration video was produced and published on project website. The RTDs seek to publish non-confidential aspects of their work, subject to approval by the Exploitation Board.
All partners were experienced either in commercial or scientific dissemination and all contributed to the dissemination of foreground. The consortium was aware that part of the ECOPIX exploitation strategy is based on trade secrets and first mover advantage. Therefore, some dissemination activities (e.g. until a patent application has been submitted) were postponed, to protect the project's intellectual property.
The SMEs will exploit the foreground directly (via the manufacture and sale of ECOPIX) and indirectly, by licensing the ECOPIX technology to other SMEs. Via direct exploitation, ECOPIX SMEs will supply components to other companies. The ECOPIX technology is designed to enable display manufacturers / integrators to purchase the modules and electronics and assemble them into complete displays. This approach enables ECOPIX SME partners to maximize their technological offerings and reduce the end price of the product. Alternatively, the consortium SMEs may also manufacture complete display units for sale to outdoor media companies and site owners as a turn-key solution. Moreover, the unique characteristics of ECOPIX, will allow the SMEs to present competitive bids for tenders.
Each consortium SME will have the right to use; and license (non-exclusively); ECOPIX in its own country of establishment. There is no direct competition between consortium SMEs because they complementary different business activities in the supply chain. If any SME wishes to license ECOPIX to companies in other countries, it must inform the other partner SMEs in advance of licensing plans. Licenses of ECOPIX to third parties will include strict confidentiality conditions to protect the trade secrets, and the know-how and (possibly) patent rights necessary to manufacture and commercialize the ECOPIX hardware and software components.
The main aim of the consortium partners is the commercialization of the ECOPIX product. ECOPIX does not suffer the paper waste problem or manual labor costs issues of traditional billboards. ECOPIX does not suffer the high initial and energy costs of LED and LCD digital billboards, and generates less e-waste at end of life. The consortium SME will use the new networks they develop in the project to identify business angels and venture capital firms likely to be interested in the portfolio of results from the ECOPIX project.
We note that ECOPIX could also find application in indoor advertising, but would require some additional investment and development to meet the higher display resolution requirements of that market. As such, the impact analysis considers only the target OOH target, which is already substantial.
During the project duration:
Number of events at which the ECOPIX technology was presented – 10
Number of industry visitors at trade shows and exhibitions – 80,000
Number of face to face meetings - 0
Number of policy makers reached - 0
Number of press releases and articles published in the press - 1
Number of hits on the project website - 5331
Followers on social media networks – 0

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Nathaniel Van Parijs