TECHNICAL FIELD

The present invention describes a method for the production of selective or laminar coatings in plastic electronics, in particular as part of non-volatile read only memory (ROM) devices and non-volatile erasable programmable ROM devices.

STATE OF THE ART

US6072716 to Jacobson et al. discloses an electrically erasable and rewritable memory structure based on a polymer compound with dispersed conductive particles or semiconductors, which includes carbon black or graphite. Particles are mixed into a polymer compound which can be deposited by low cost printing techniques. Then, the deposited compound has to be cured at low temperatures, allowing its use in association with flexible substrates.

The device presents negative differential resistance and can be electrically switched from low conducting OFF-state to a high conducting ON-state. While the OFF-state with low conductivity can be maintained for long periods, the ON-state is not stable under voltage pulses occurring in reading cycles and therefore will fade rather quickly, resulting in information loss with time.

In addition, like many metal-oxide memory switches, the disclosed devices exhibit "forming phenomena", meaning that they require a positive threshold voltage to switch the device for the first time which is greater than the voltage needed for subsequent switching events. Therefore the electronics to drive these memories must be more complex in order to take this effect into consideration.

Beside the "forming phenomena" the problem to be solved in the prior art devices is to reach the highest possible ON/OFF current ratio which makes an unambiguous detection of the correct state possible, even with common electronic devices.

DESCRIPTION OF THE INVENTION The object of the present invention is to create a flexible and transparent polymer compound with changeable electrical conductivity and resistivity in particular for use in organic electronics or plastic electronic applications, such as transparent displays or transparent circuitry, which shows an ON-state with high conductivity/low resistivity and an OFF-state with low conductivity/high resistivity resulting in a high ON/OFF current ratio.

Another objective of the present invention is to provide a polymer compound without the known forming phenomenon, when switching the conductivity for the first time, that is easier to manufacture and that can be more easily deposited from a liquid organic polymer mixture on substrates.

Another objective of the present invention is the use of the polymer compound in plastic electronics application to create a low cost programmable non-volatile erasable memory device with high ON/OFF ratio in particular to create a flexible and/or transparent non-volatile memory device. The present polymer compound achieves these objectives and is producible in a simple and fast way by using low-cost and commercially available organic raw materials, leading to a flexible and transparent polymer compound.

DESCRIPTION

Liquid organic polymer mixture to fabricate organic polymer compound The present application discloses a liquid organic polymer mixture consisting of at least one portion of polymer dissolved in a solvent and one portion of graphene, comprising sheets of sp2-bonded carbon atoms densely packed in a honeycomb crystal lattice forming one- atom-thick planes of carbon atoms. The graphene sheets are dispersed in the liquid organic polymer mixture. The liquid organic polymer mixture is printed or deposited from other low cost solution processes onto surfaces, where no further curing is necessary after the deposition.

A first fraction of liquid organic polymer mixture includes polymer (> 80 wt%) and graphene (< 20 wt%) forming between 50 wt% and 95 wt% of the liquid organic polymer mixture. A second fraction includes the volatile solvent forming between 5 wt% and 50 wt% of the liquid organic polymer mixture, depending on the used solvent.

The liquid organic polymer mixture forms a flexible and transparent organic polymer compound in the form of a matrix after the drying process, which is performed subsequent to partial or complete evaporation of solvent. In cases where the used solvent has a very high boiling point, a heating process or heating process plus vacuum may be used to increase the speed of film drying.

Due to the fact that the electrical resistivity of the dried organic polymer compound is switchable by applying short voltage pulses in the microseconds range, the conductivity of the organic polymer compound can be easily switched between an ON-state and an OFF- state. The switching and detection of the different conductivity states is possible by depositing the organic polymer mixture onto two pre- patterned electrodes. The organic polymer compound layer is an electrically switchable polymer matrix, which can change its conductivity between the initial ON-state and an OFF-state, if a voltage with a certain threshold is applied. By application of a short voltage pulse of below 5 V between electrodes, whose interspace is filled with the organic polymer compound, the low conductivity OFF- state can be reached. Typical reading voltages are in the range of IV, while switching voltages of about 2.5 V to 5 V are applied to switch from ON- state to OFF-state, although higher voltages can also be used to decrease the pulse duration. The high or low conductivity (ON or OFF state, respectively) of the compound between two electrodes defines one bit of information, which is non-volatile but can be erased and rewritten following simple steps.

Measurements showed very high ON/OFF current ratios of the polymer matrix of up to 10 ⁶ which makes an unambiguous detection of the present state possible. Measurements also showed that either the switched ON or the OFF-state was stable for an unlimited time, even if no external voltage is applied. It will also keep the information if a reading voltage (< = IV) is constantly applied. We measured a constant output signal for more than 72 hours with an applied bias of 1 Volt, with no sign of information loss or unintentional switch of ON/OFF-state. Thus, much longer retention times even under voltage stress are expected. We also measured more than 10 ⁶ reading cycles with short 1 V pulses, again with no sign of information loss. The same results were obtained in inert atmosphere (N ₂ ) or in normal ambient air. Therefore, the device works as a programmable read only memory. It is also possible to erase the information in the memory device, such that it works as an erasable-programmable read only memory (EPROM). In prior-art EPROM memories can be erased by illuminating the active area with ultraviolet light for several minutes. This requires the construction of EPROMS with expensive quartz windows, since normal glass or plastics would block the UV light. The EPROM disclosed here can be erased by a simple heating process at mild temperatures (typically between 100 and 200 °C for about 20- 30 minutes). This allows production of EPROMs without quartz glass and the possibility of erasing the whole memory at once (all bits) or only selective areas, for example by using a focused laser beam to heat selected bits.

Our experimental results showed that the present organic polymer compound does not exhibit the "forming phenomena" as known form prior art devices. The voltage threshold for switching the initial ON- state to the OFF-state for the first time equals the voltage needed for subsequent switching events, which results in electronics with lower specification.

Manufacturing process of the organic polymer mixture

The polymer is dissolved in the chosen solvent and then a specific amount of graphene is added. Suitable solvents are polymer-soluble solvents, like toluene, chlorobenzene, chloroform, acetone and tetrahydrofuran (THF), among others; as well as polymer-dispersible solvents, like water, alcohols and others.

Measurements have shown that raw graphene sheets or functionalized graphene sheets as described below can be used for the manufacturing of liquid organic polymer mixture with favourable properties. The solution is stirred and/or sonicated until good graphene sheet dispersion is obtained. The time for processing the graphene into solution depends on the polymer and solvent used, as well as on the functionalization of the graphene. Possible materials for the polymer are conducting or insulating organic polymers, thermoplastics, epoxys or other resins, which can form smooth thin films. Usable polymers can be either opaque or transparent. The organic polymer is said to be transparent when at least 50% of the ambient electromagnetic radiation in the range of the visible spectrum can pass the organic polymer. Preferred polymers are for example PEDOT-PSS, polythiophenes, polyaniline, poly- p(phenylene-vinylene), polysterene or polymethylmeacrilate.

As an example, when using polystyrene in benzyl alcohol solution, the ester functionalized graphene dispersion can be obtained within about 30 minutes under sonication at room temperature. The process can be speeded up at higher temperatures or by using homogenizers.

After the liquid organic polymer mixture is ready as described above, it can be stored for long times (we used stored solutions after more than a month). Before using stored solutions it is necessary to sonicate or stir for about 30 minutes the solutions to redisperse the graphene that precipitated.

To form a favorable organic polymer compound, the amount of graphene used should be below the percolation threshold in the specific matrix. A long-range connectivity between the graphene is to be avoided to prevent the formation of conductive and flexible films as known from the prior art. Only a small amount of graphene is necessary to allow forming of an electrically switchable polymer matrix forming conductive ON-states and non-conducting OFF-states. As an example, when using Polystyrene (PS) pellets, benzyl alcohol and ester functionalized graphene, the liquid organic polymer mixture should comprise between 0.5 weight percentage and 10 wt% of graphene and between 90 wt% and 99.5 wt% of the polymer and the solvent. In one experiment a liquid organic polymer mixture with such a graphene concentration was deposited between two at least approximately parallel electrodes in a distance of 10 μηη, which showed an intended switchability between ON/OFF-states. An organic polymer mixture including more than 85 wt% of the polymer dissolved in a solvent, mixed with up to 15 wt% of graphene resulted in a polymer matrix after deposition and drying which showed the intended electrically switchable behaviour. For a graphene concentration of 20 wt%, percolation does not allow the initial ON-state of the polymer matrix to be changed to a low conductivity OFF-state.

The liquid organic polymer mixture can be used to form selective or laminar coatings on surfaces of substrates. If the liquid organic polymer mixture only covers small areas of the substrate surface but still covers the interspace between electrodes, than a selective coating is achieved. If the organic polymer mixture is coated by for example an industrial roll on process, laminar coating of large surface areas are achievable, which is favorable for most electronic applications. Functionalizing of graphene sheets

The pristine graphene were refluxed in 9 M nitric acid for twenty four hours to carboxylate the graphene sheets and in order to remove catalytic metal particles also present on the raw graphene materials. Subsequently, 100 mg of nitric acid-treated graphene were sonicated using an ultrasonic probe (W-375, Heat-Systems Ultrasonic, Inc) for two hours in 450 ml of 8 : 1 (v:v) NH ₄ OH-ethanol solution. This treatment results in the partial esterification of the carboxylic acid groups. In the following, we will refer to this sample as esterified graphenes. The samples were then filtered through a 200 nm polycarbonate membrane and resuspended in ethanol. This step was repeated several times in order to completely remove the ammonia.

Application and use in plastic electronics

The special electrical properties of the liquid organic polymer mixture and the resulting organic polymer compound are applicable in plastic electronics applications. Due to the transparency of the resulting organic polymer compound, transparent electronic devices can be manufactured.

An emerging field in electronics is that of flexible and/or transparent electronics and/or optoelectronics. For flexible applications, organic semiconductors are expected to play an important role due to the easy deposition processes that can be made at low temperatures on flexible substrates and also to the possibility of producing 3-dimensional memories.

Read only memory (ROM) devices are a special case of memory which can be read but not changed under normal system operation. ROMs are non-volatile, meaning that the stored information is retained when the power is removed. This type of memory is required in most electronic applications such as bootstrapping of computer programs, game machines, electronic music instruments, routers, cordless phones among many others.

Commercial ROM devices are based mainly on Standard silicon technology. Mask programmable ROMs (MROMs) are the least expensive type of solid state memory. The programming is done during fabrication of the integrated circuit, where transistors are put in the circuit with either a normal threshold (on state) or a very high threshold (off state). Therefore MROMs are only attractive if the same memorized code will be used in large quantities. Programmable ROMs are typically more interesting since the same memory can be programmed for different uses by the costumers.

Commercial erasable programable read-only memory (EPROM) can be erased by exposure to strong ultraviolet light, typically for 10 minutes or longer and then rewritten with a process that requires application of relatively high voltages. The endurance of most EPROM chips exceeds 1000 cycles. For the erasing process these memories have to be irradiated with UV light and therefore are made with a quartz window to allow UV irradiation to reach the memory structure. The erasing by using UV light increases costs and complicates the technical process due to the necessity of the quartz window.

Low graphene content in the transparent organic polymer compound, together with the use of transparent electrodes (eg. Indium tin oxide (ITO)) allows the fabrication of transparent read-only memory devices using the present liquid organic polymer mixture. Since the resulting polymer matrix comprises very thin (hundreds of nanometers thick) organic polymer compound layers, the stacking of multiple (substrate/organic polymer compound) layers in a 3D geometry is possible to allow very high density memory storages. In general, usable materials for the substrate includes glass, silicon, transparent oxides (such as Indium tin oxide (ITO)), fabrics, plastic and metal foils.

The solution of organic liquid polymer mixture comprising raw or functionalized graphene as described above can be deposited onto substrates with pre-patterned electrodes building the memory device by different processes. The deposition processes include but are not restricted to: casting, spin-coating, doctor blading, ink-jet printing, spray coating, screen printing, pad printing, knife-over-edge coating, meniscus coating, slot die coating, gravure coating, curtain coating, multilayer slot coating, slide coating, flexographic printing, offset lithography, electrophotographic, magnetographic, electrographic coating or any combination of the above. The deposition time will depend on the used solvent and the deposition technique, but is typically in the order of few seconds up to minutes. After film deposition, the pre-patterned electrodes can be addressed by electronic components, e.g., as known in the art, in order to write or read information into the memory device formed by the substrate and the organic polymer compound. The number of bits that can be fabricated at one deposition step depends only on the number of pre- patterned electrodes onto the substrate, since most deposition methods from solution allow coating of large areas. The liquid organic polymer mixture can be coated on the substrate in a selective or laminar way, where we understand in a selective way the coating on small-areas.

If the substrate is flexible, the printing can be made in a roll-to-roll process allowing very high throughput and consequently reduced production costs. This is one advantage of using the present liquid organic polymer mixture consisting of graphene in memory devices, since the memory device remains mechanically stable after bending.

The possibility of using conducting polymers as the electrodes allows the whole memory device to be printed, for example in flexible substrates, decreasing production costs by the use of high throughput roll-to-roll manufacturing. Preferably, the substrate should be insulating. A conducting substrate (such as an aluminum foil) can also be used and the electrodes can be designed on the surface by different processes, such as etching. Another possibility is to deposit an insulating layer onto the conductive substrate before the electrodes are deposited onto the insulating layer.

As mentioned above, the flexibility allows using roll-to-roll for very fast production of the devices, which is similar to the production or printing of newspapers. This results in extremely low production costs. Also, it allows integration of memory devices in new flexible electronic devices which are either already commercialized or which are expected to enter the market any time soon. Examples of such flexible devices include but are not restricted to: electronic paper, electronic readers, displays, gift cards (e.g. that can play music), flexible music instruments, electronic product tags, among others. Transparency of the substrate and the organic polymer compound allows its use in visionary new products that are expected to enter the market in the future, such as transparent displays and solar cells, among others. As well as in applications where one does not want electronic devices to change the visual aspects of the product, such as in touch screens, electronic textiles and garments and/or in lighting elements.

Therefore, with the present invention, low cost programmable readonly memory devices with high ON-OFF ratio are producible. After the information has been written the device can be reset to the initial state by a mild temperature process, which allows rewriting with no need for the typical and expensive UV process used for conventional ROMs. The low temperature process is compatible with flexible substrates that, together with the good mechanical properties of the polymer matrix, allows for the development of flexible memory devices. In order to reset the memory device, the tempering process is to be executed for at least 30 minutes and the heating is done at temperatures above the glass transition temperature of the polymer compound, typically below 200°C. This heating process can also be done locally at selected areas of the ROM device, for instance, by using a focused laser beam.

The organic polymer compound/polymer matrix can be used in nonvolatile rewritable programmable read-only memory devices exhibiting high ON-OFF ratios, long retention times, low writing and reading voltages, low temperature induced memory resetting and low-cost deposition. Due to the flexibility of the polymer matrix the liquid organic polymer mixture can be deposited on flexible substrates and due to the small layer thickness ROMs with 3D architecture are manufacturable. The full memory can also be transparent for visible light allowing a range of new applications.