TECHNICAL FIELD

The present disclosure relates to a method for producing a poly (3, 4-ethylenedioxythiophene)

(PEDOT) film. The present disclosure further relates to a conductive PEDOT film. The present disclosure further relates to an electronic device comprising the conductive PEDOT film and the use of the conducting PEDOT film as an antistatic coating or an electrode in/of an electronic device.

BACKGROUND

Conducting PEDOT films are used in different fields like antistatic coatings, perovskite solar cells, organic solar cells, dye-sensitized solar cells, electrochemical transducers, electrochromic de vices, electroluminescent devices, thermoelectric de vices, smart windows, OLED's, optoelectronics and su percapacitors. Conducting PEDOT films may be manufac tured by e.g. electrochemical polymerization of PEDOT on conducting substrate, oxidative chemical vapor dep osition (oCVD) or vacuum vapor phase polymerization (WPP) technique. When forming a conductive PEDOT film for electronic devices, their conductivity, low sheet resistance, morphology (roughness average) and optical transparency is of importance.

SUMMARY

A method for producing a poly (3,4- ethylenedioxythiophene) (PEDOT) film on a substrate comprising a substrate and at least one PEDOT layer on at least one surface of the substrate is disclosed. The method may comprise: applying a solution compris- ing an oxidant and a base inhibitor on at least one surface of the substrate so as to form an oxidant coating on at least one surface of the substrate; sub jecting the oxidant-coated substrate formed to a polymerization step by exposing the surface (s) of the oxidant-coated substrate to 3, 4-ethylenedioxythiophene (EDOT) monomer vapour at a polymerization temperature to form a PEDOT layer on the surface (s) of the oxi dant-coated substrate; and wherein, during the polymerization step, the temperature of the oxidant- coated substrate is kept at a controlled substrate temperature and wherein the controlled substrate tem perature is 2 - 40 °C lower than the polymerization temperature .

Further, a conducting PEDOT film is dis closed. The conducting PEDOT film may comprise a non- conductive substrate; a PEDOT layer having anions from an oxidant/oxidants embedded in the PEDOT layer on the non-conductive substrate, wherein the conducting PEDOT film has conductivity of over 2100 S/cm and sheet re sistance of below 200 □/□.

Further, a conducting PEDOT film which is ob tainable by the method for producing a PEDOT film com prising a non-conductive substrate and at least one PEDOT layer on at least one surface of the non- conductive substrate as disclosed in the present dis closure is disclosed.

Further, an electronic device is disclosed. The electronic device may comprise the conducting PE- DOT film as disclosed in the present disclosure.

Further, uses of the conducting PEDOT film are disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the embodiments and constitute a part of this specification, illustrate various embodiments. In the drawings:

Fig. 1 illustrates the vapor phase polymerization cell for carrying out the vapour phase polymerization.

Fig. 2 illustrates the Raman Spectrum of 6 layer PEDOT film produced by the vapour phase polymerization method.

Fig. 3 illustrates AFM images of 1L) 1 layer of PEDOT, 2L) 2 layers of PEDOT, 3L) 3 layers of

PEDOT, 4L) 4 layers of PEDOT, 5L) 5 layers of PEDOT,

6L) 6 layers of PEDOT and SEM images of 1L, 3L and 6L

PEDOT .

Fig. 4 illustrates Cyclic voltammograms of 1L to 6L PEDOT (in 0.1 M TBABF ₄ in MeON with 100 mV/s scan rate) .

Fig. 5 illustrates Cyclic voltammograms of 1L PEDOT on glass and ITO coated glass in 6 mM Ferrocene solution with 20 mV/s scan rate.

DETAILED DESCRIPTION

The present application relates to a method for producing a poly (3, 4-ethylenedioxythiophene) (PE DOT) film comprising a substrate and at least one PE- DOT layer on at least one surface of the substrate, wherein the method comprises the steps of:

a) applying a solution comprising an oxidant and a base inhibitor on at least one surface of the substrate so as to form an oxidant coating on at least one surface of the substrate,

b) subjecting the oxidant-coated substrate formed in step a) to a polymerization step by exposing the surface (s) of the oxidant-coated substrate to 3,4- ethylenedioxythiophene (EDOT) monomer vapour at a polymerization temperature to form a PEDOT layer on the surface (s) of the oxidant-coated substrate, and wherein, during the polymerization step, the temperature of the oxidant-coated substrate is kept at a controlled substrate temperature and wherein the controlled substrate temperature is 2-40 °C lower than the polymerization temperature.

The expression that the PEDOT layer is "on" the surface of the substrate in the PEDOT film should be understood in this specification, unless otherwise stated, as meaning that the PEDOT layer is polymerized or formed to lie on or upon the substrate or is being at least partly embedded therein. The substrate may serve as a carrier or support structure for the PEDOT layer. The substrate can be changed and the material of the substrate can vary according to the application to which the PEDOT film is to be used.

The expression "film" should be understood in this specification, unless otherwise stated, as refer ring to a structure having its lateral dimensions sub stantially larger than its thickness. In that sense, a film may be considered as being a "thin" structure.

The polymerization temperature is the temperature of the monomer vapour during the polymerization step.

In one embodiment, the substrate is non conducting substrate. The expression that the sub strate is "non-conductive" should be understood in this specification, unless otherwise stated, as mean ing that the substrate has a sheet resistance of 10 Mohms/square or higher.

The non-conducting substrate may be a substrate such as a glass, a polymer, paper, cellulose, textile, fabric, wood, leather, cotton, ceramic, composites such as cellulose-wood composites, fibreglass, teflon, rubber, quartz, paint, carbon material and/or con-conducting mineral. The non conducting polymer substrate may be selected from the group consisting of plastic material selected from a group consisting of polyester, polyethylene terephthalate (PET) , polycarbonates (PC) , polyamide (PI), polyester sulfone (PES) , polystyrene (PS), and amorphous polyester (A-PET or PET-G) and mixtures thereof. The non-conducting substrate may be a flexible non-conducting substrate. Flexible substrates can be used for production of flexible PEDOT films that can be bent and folded.

The oxidant is a substance that induces polymerization of monomers, and can act as a dopant after polymerization of a conductive polymer. The embedded anions may remain within the PEDOT structure to act as dopant ions. The conductivity of PEDOT polymer may be due to the delocalization of electrons or holes upon oxidation (doping) . The oxidant may be selected from the group consisting of iron(III)p- toluenesulfonate, iron (III) chloride, p- toluenesulphonic acid, iodine, bromine, molybdophosphoric acid, ammonium persulfate, DL- tartaric acid, polyacrylic acid, copper chloride, ferric chloride, naphthalenesulfonic acid, camphorsulfonic acid, Iron ( I I I ) toluene sulfonate,

Iron (III) -perchlorate, Cu(C10 ₄₎₂ .6H ₂ 0, cerium (IV) ammonium nitrate, cerium (IV) sulfate and any mixtures thereof.

Base inhibitors decrease the activity of the oxidant and lower the rate of polymerization. This may alter the conductivity of deposited polymer. The base inhibitor may be selected from the group consisting of amine-based compounds and nitrogen atom-containing saturated or unsaturated heterocyclic compounds such as a pyridine-based, imidazole-based, or pyrrole-based compounds, water vapor, glycerol, and glycol derivatives, and mixtures thereof.

The oxidant solution of the present invention comprises the oxidant, the base inhibitor and a solvent. The solvent may be selected from the group consisting of organic solvents such as n-butanol methyl alcohol, 2-butyl alcohol, ethyl cellosolve, ethyl alcohol, cyclohexane, ethyl acetate, toluene, acetonitrile and methyl ethyl ketone and mixtures thereof .

In one embodiment, the PEDOT layer is doped.

In one embodiment, the method is carried out by vapour phase polymerization.

Vapour phase polymerization is a polymeriza tion technique where only the monomer is converted in to vapour phase. The VPP method may involve coating the substrate with an oxidant and base inhibitor solution followed by drying to remove traces of solvent.

In one embodiment, the vapour phase polymeri zation is carried out in atmospheric pressure. The va pour phase polymerization carried out in atmospheric pressure is easy to control. It does not need of pres sure control or sophisticated device or vacuum oven. The method may be used also for production of large area films.

The polymerization temperature influences the properties of resulting PEDOT film such as the rate of polymerization by controlling rate of volatilization of monomers and/or the mobility of a polymer chain. The decrease of the temperature increases the concentration of vapour molecules. The substrate temperature and the polymerization temperature control the rate of polymerization. The polymerization temperature and the substrate temperature influence the properties of the formed PEDOT film such as conductivity, transparency, a sheet resistance and morphology. The controlled substrate temperature is a factor in controlling the rate of polymerization which in turn controls the morphology of the PEDOT films. The controlled substrate temperature brings the uniform and homogeneous nature to the PEDOT films. In one embodiment, the solution comprising an oxidant and a base inhibitor is applied on one surface of the substrate and the oxidant coated surface of the substrate is exposed to EDOT monomer vapour at a polymerization temperature to form one PEDOT layer on the surface of the oxidant-coated substrate, thus pro ducing a one-layered PEDOT film.

The method for producing multilayer PEDOT film may be carried out by layer-by-layer vapour phase polymerization. The PEDOT films may be formed by depositing PEDOT layers with wash steps in between.

In one embodiment the method comprises steps of

c) applying the solution comprising the oxi- dant and the base inhibitor on at least one surface of the PEDOT layer formed in step b) so as to form an ox idant coating on the at least one surface of the PEDOT layer,

d) subjecting the oxidant-coated PEDOT layer formed in step c) to a polymerization step by exposing the surface (s) of the oxidant-coated PEDOT layer to 3, 4-ethylenedioxythiophene (EDOT) monomer vapour at a polymerization temperature to form a subsequent PEDOT layer on the surface (s) of the oxidant-coated PEDOT layer,

wherein, during the polymerization step, the temperature of the oxidant-coated PEDOT film is kept at a controlled substrate temperature and wherein the controlled substrate temperature is 2-40 °C lower than the polymerization temperature.

In one embodiment, the temperature of the oxidant-coated substrate is kept at a controlled substrate temperature during the polymerization step, and the controlled substrate temperature is 2 - 30 °C lower than the polymerization temperature.

In one embodiment, the temperature of the oxidant-coated substrate is kept at a controlled substrate temperature during the polymerization step, and the controlled substrate temperature is 3 - 20 °C lower than the polymerization temperature.

In one embodiment, the temperature of the oxidant-coated substrate is kept at a controlled substrate temperature during the polymerization step, and the controlled substrate temperature is 3 - 15 °C lower than the polymerization temperature.

In one embodiment, the temperature of the oxidant-coated substrate is kept at a controlled substrate temperature during the polymerization step, and the controlled substrate temperature is 5 - 15 °C lower than the polymerization temperature.

In one embodiment, the temperature of the oxidant-coated substrate is kept at a controlled substrate temperature during the polymerization step, and the controlled substrate temperature is 8 - 12 °C lower than the polymerization temperature.

In one embodiment, the solution comprising an oxidant and a base inhibitor is applied on the surface of the one PEDOT layer formed in step b) , the one layered PEDOT film, and the oxidant coated surface of the PEDOT layer is exposed to EDOT monomer vapour at a polymerization temperature to form a second PEDOT lay- er on the surface of the oxidant-coated PEDOT layer, thus producing a two-layered PEDOT film.

In one embodiment, the method comprises repeating step c) and step d) at least once for producing multilayer PEDOT film.

During polymerization step the substrate tem perature and the polymerization temperature are con trolled separately. In one embodiment, the polymeriza tion temperature is 55 - 95 °C and the controlled sub strate temperature is 40 - 70 °C. In one embodiment, the polymerization temperature is 60 - 85 °C and the controlled substrate temperature is 45 - 70 °C. In one embodiment, the polymerization temperature is 65 - 80 °C and the controlled substrate temperature is 55 - 70 °C, or the polymerization temperature is 67 - 77 °C and the controlled substrate temperature is 56 - 66 °C, or the polymerization temperature is 72 - 77 °C and the controlled substrate temperature is 61 - 66 °C. The vapour phase polymerization of the present in vention has no need of high temperature.

In one embodiment, the temperature of the oxidant-coated substrate and/or the oxidant-coated PEDOT film is kept at the controlled substrate temperature substantially during the entire polymerization step.

In one embodiment, the polymerization step is subdivided into sequential processing periods to tune the rate of polymerization.

In one embodiment, the polymerization step comprises two sequential processing periods, the temperature of the oxidant-coated substrate and/or the oxidant-coated PEDOT film is kept at the controlled substrate temperature throughout one of the processing periods .

In one embodiment, the polymerization step comprises three sequential processing periods, wherein the temperature of the oxidant-coated substrate and/or the oxidant-coated PEDOT film is kept at the controlled substrate temperature throughout the middle processing period.

During the processing period when the substrate is not kept at the controlled substrate temperature, the substrate temperature may change towards the ambient temperature, i.e. the polymerization temperature. The substrate temperature will cool towards the polymerization temperature, if the substrate temperature is higher than the polymerization temperature, or the substrate temperature will warm up towards the polymerization temperature, if it is lower than the polymerization temperature in the beginning of such a processing period .

In one embodiment, the method of the present application comprises the step of cleaning the substrate before step a) . In one embodiment, the substrate was cleaned by ultra-sonication with a solvent. The solvent may be selected from the group consisting of organic solvents such as acetone and EtOH, water and mixtures thereof. The cleaning step may be repeated with same or different solvents.

In one embodiment, the cleaned substrate is dipped in hot solution of H ₂ 0:NH ₄ 0H (25 %) :H ₂ 0 ₂ (30 %) as 5:1:1 volume ratio to remove any organic impurities left behind on the surface which was then followed by oxygen plasma treatment.

In one embodiment, the oxidant solution is spin-coated on at least one surface of the substrate in step a) . In one embodiment, the oxidant solution is spin-coated on at least one surface of the at least one surface of the PEDOT layer formed in step c) . In one embodiment, the oxidant solution is spin coated on the substrate at 800 - 3500 rpm. In one embodiment, the oxidant solution is spin coated on the substrate for 5 - 30 s.

In one embodiment, the method comprises cleaning, drying and/or heating the oxidant-coated non-conductive substrate from step a) before step b) . In one embodiment, the method comprises cleaning, drying and/or heating the PEDOT film from step c) before step d) . In one embodiment, the oxidant-coated non-conductive substrate is heated at 80 - 100 °C . In one embodiment, the PEDOT film from step c) is heated at 80 - 100 °C.

In one embodiment, the method comprises the step of annealing the PEDOT film at a temperature 50 - 100 °C after polymerization. The PEDOT film can be annealed to avoid stress fracture of the film during the washing step. The annealed PEDOT film may be washed to remove unreacted oxidant, monomer and any other impurities because the unconsumed oxidant and monomer traces may decrease the conductance. In one embodiment, the annealed PEDOT film is washed by dip rinsing thoroughly in ethanol followed by MeCN. In one embodiment, the annealed PEDOT film is dried. In one embodiment, the annealed PEDOT film is dried under dry nitrogen gas stream.

In one embodiment, the method comprises the step of cleaning the PEDOT polymer film received from step b) before the step c) . In one embodiment, the PEDOT polymer film is cleaned by annealing the PEDOT film at a temperature 50 - 100 °C after polymerization. In one embodiment, the annealed PEDOT film is washed by dip rinsing thoroughly in ethanol followed by MeCN. In one embodiment, the annealed PEDOT film is dried. In one embodiment, the annealed PEDOT film is dried under dry nitrogen gas stream.

The polymerization time may effect on the properies such as a sheet resistance and a roughness average of the formed PEDOT film.

In one embodiment, the duration of the polymerization step is 1 - 20 minutes. In one embodiment, the duration of the polymerization step is 1-10 minutes. In one embodiment, the duration of the polymerization step is 2 - 8 minutes. The polymerization step is fast.

In one embodiment, the duration of the processing period of the polymerization step, wherein the temperature of the oxidant-coated non-conductive substrate and/or the oxidant-coated PEDOT film is kept at a controlled substrate temperature is 20 - 80 % of the duration of the polymerization step.

In one embodiment, the duration of the polymerization step processing period, wherein the temperature of the oxidant-coated non-conductive substrate and/or the oxidant-coated PEDOT film is kept at a controlled substrate temperature is 30 - 60 % of the duration of the polymerization step. In one embodiment, the duration of the polymerization step processing period, wherein the temperature of the oxidant-coated non-conductive substrate and/or the oxidant-coated PEDOT film is kept at a controlled substrate temperature is 35 - 40 % of the duration of the polymerization step.

In one embodiment, the non-conductive substrate is glass. In one embodiment, the non- conductive substrate is polyethylene terephthalate (PET) .

In one embodiment, the oxidant is Iron (III) p-toluenesulfonate hexahydrate (FETOS)

In one embodiment, the base inhibitor is pyridine .

In one embodiment, the oxidant solution comprises or consists of FETOS and pyridine in n- butanol. In one embodiment, the oxidant solution comprises the oxidant at a proportion of 5 % - 50 % by weight relative to the volume of the oxidant solution.

The present application further relates to a conducting PEDOT film, comprising a non-conductive substrate, a PEDOT layer having anions from an oxidant/oxidants embedded in the PEDOT layer on the non-conductive substrate, wherein the conducting PEDOT film has conductivity of over 2100 S/cm and sheet resistance of below 200 □/□.

The present application further relates to a conducting PEDOT film obtainable by the method of the present application comprising a non-conductive substrate, a PEDOT layer having anions from an oxidant/oxidants embedded in the PEDOT layer on the non-conductive substrate, wherein the conducting PEDOT film has conductivity of over 2100 S/cm and sheet resistance of below 200 □/□. In one embodiment, the conducting PEDOT film has a conductivity of over 3200 S/cm and a sheet resistance below 21 □/□. In one embodiment, the conducting PEDOT film has a conductivity of over 3200 S/cm and a sheet resistance 12 □/□.

Van der Pauw method can be used to calculate sheet resistance (r _sheet ) for PEDOT films. The specific resistance (r) of the film is obtained by multiplying sheet resistance with film thickness (d) , equation ( 1 ) :

r = r _sheet d (1)

The conductivity (s) of the film is obtained according to equation (2)

s = 1 / r (2) CV and EIS techniques can be used for elec trochemical characterization. The charge (Q) is calcu lated by integration of the cyclic voltammogram in the potential range -0.25 to 0.75 V in the Origin software which is further processed by using following equa- tions to obtain capacitance values:

Areal capacitance, C _A = Q / (AV*A) (3)

Volumetric Capacitance, C _v = Q / (AV*V) (4)

Where, 'AV' is the potential window, Ά' is the area and 'V' is the volume of working electrode. In one embodiment, the conducting PEDOT film comprises 1 - 20 PEDOT layers on the non-conductive substrate. In one embodiment, the conducting PEDOT film comprises 1 - 15 PEDOT layers on the non- conductive substrate. In one embodiment, the conducting PEDOT film comprises 1 - 10 PEDOT layers on the non-conductive substrate. In one embodiment, the the roughness average of the conducting PEDOT film is below 3.5 nm.

Roughness average values of the PEDOT film are obtained from the AFM images by using WSXM software. The roughness average (R _a) is the mean of the difference, in absolute value, between the average height and the height of each single point of the sample. This number varies with the interval range. It shows how uniform or rough the PEDOT film is.

In one embodiment, the % transmittance of the conducting PEDOT film at 550 nm is over 30 % T. In one embodiment, the the % transmittance of the conducting PEDOT film is over 80 % T.

Agilent 8453 spectrometer can be used to record the UV-Vis spectra. The % Transmittance (%T) is calculated from the absorbance data by using following equation (5) ,

(%T) = (10 ^L (- Abs) ) *100 (5)

In one embodiment, the non-conductive substrate of the PEDOT film is glass. In one embodiment, the non-conductive substrate of the PEDOT film is polyethylene terephthalate (PET) .

In one embodiment, the PEDOT layer has anions from Iron (III) p-toluenesulfonate hexahydrate (FETOS) embedded in the PEDOT layer on the non-conductive substrate .

The thickness of the conducting PEDOT film may be designed in accordance with the properties of the conducting PEDOT film, especially the conductivity, resistance, roughness average or transmittance thereof or combinations of the properties. In one embodiment, the thickness of the PEDOT film is 10 - 500 nm, or 10 - 300 nm, or 20 - 200. The present application further relates to an electronic device comprising a conducting PEDOT film of the present application.

In one embodiment, the electronic device is a display, a flat panel display, an optoelectronic devise such as an organic light emitting diode OLED, an organic solar cell, a dye-sensitized solar cell, a perovskite solar cell, a smart window, a fuel cell, an organic electrochemical transistor, an electrochemical transducer, an electrochromic device, an electroluminescent device, an electroluminescent display, an organic capacitor, a supercapacitor, a sensor, a biosensor, an energy harvesting device, an antistatic material, a photovoltaic device, a storage device or a thermoelectric device. In one embodiment, the electronic device is an optoelectronic device. In one embodiment, the electronic device is a transparent electrode. In one embodiment, the electronic device is a supercapacitor.

The present application further relates to the use of the conducting PEDOT film of the present application as an antistatic coating or an electrode in/of an electronic device.

The method of the present application has the added utility of being simple, low cost and fast due to the atmospheric pressure controlled polymerization. Further, the method of the present application has the added utility of producing thin multilayered PEDOT films at nanometer scale. Further, the method can be used to a large surface area and is not limited to conducting substrates. The PEDOT films of the present application have the added utility of being very uniform and homogeneous as well as smooth and flexible. Further, the PEDOT films of the present application have the added utility of having high conductivity and transmittance. EXAMPLES

Reference will now be made in detail to various embodiments, an example of which is illustrated in the accompanying drawings.

The description below discloses some embodiments in such a detail that a person skilled in the art is able to utilize the embodiments based on the disclosure. Not all steps or features of the embodiments are discussed in detail, as many of the steps or features will be obvious for the person skilled in the art based on this specification.

For reasons of simplicity, item numbers will be maintained in the following exemplary embodiments in the case of repeating components.

Fig. 1 illustrates a vapor phase polymerization cell for carrying out vapour phase polymerization. The polymerization cell 1 comprises a stand 3 for the substrate 4 and for the metal block 5. Thermostatic baths 7 are attached to the polymerization cell 1 to keep the temperature of the monomer vapour 6 at the polymerization temperature. The thermostatic bath 8 is attached to the metal block 5 to keep the temperature of the substrate 4 at the controlled substrate temperature. The polymerization cell 1 comprises a lead 2 which is used to close the polymerization cell 1.

Example 1 - Preparation of the PEDOT film

Glass slides (substrates) (37.5 mm><25 mm) were cleaned by ultra-sonication with acetone, water and EtOH, respectively in each for 5 min. The cleaned substrates were dipped in hot solution (80 °C) of

H ₂ 0:NH ₄ 0H (25 %) :H ₂ 0 _{2 (} 30 %) as 5:1:1 volume ratio for 5 min to remove any organic impurities left behind on the surface which was then followed by oxygen plasma treatment for 5 min. 0.236 M FETOS solution containing 0.141 M pyridine was prepared in n-butanol (oxidant solution) . 60 mΐ of oxidant solution was spin coated on the substrate at 1450 rpm for 20 s. The oxidant coated substrate was dried on a hot plate at 90 °C for 90 s. The dried substrate was transferred to a preheated cell 1 containing EDOT monomer 6 at 75 °C in such a way that the coated surface faced down towards the vapors. Polymerization was carried out for 4 min in 3 different steps. The cell 1 was covered with a glass lid 2 during first 90 s. The cell 1 was thereafter uncovered and the substrate temperature was kept at 65 °C by a heated metal block 5 for the next 90 s. The metal block 5 was removed and the cell 1 was covered with a glass lid 2 for 60 s. Annealing of the film was done after polymerization on a hot plate at 90 °C for 90 s. After annealing, the film was let to cool to room temperature and dip rinsed thoroughly in ethanol followed by MeCN to remove unreacted oxidant, monomer and any other impurities. Washing was done to remove the unconsumed oxidant and monomer traces which may otherwise decrease the conductance. After washing, the film was dried under dry nitrogen gas stream. The procedure was repeated from the spin coating step to prepare 1 to 6 layers of PEDOT on the glass substrate 4 (1L to 6L PEDOT) . The temperature of the cell 1 and metal block 5 was controlled by thermostat baths 7, 5.

The sheet resistance values and surface morphology of the PEDOT films were monitored by changing the parameters for the VPP method. The remaining parameters of the method were kept constant while monitoring one particular parameter at a time. The polymerization was carried out by keeping the temperature in the cell 1, the polymerization temperature, at 55, 65, 75, 85°C for monitoring the VPP cell temperature. Polymerization time of 2, 4, 6, and 8 min were used to prepare the PEDOT films. The substrate temperature was not controlled while monitoring the VPP cell temperature and the polymerization time. PEDOT films were prepared at different substrate temperatures from 45 °C to 75 °C. The substrate temperature was changed by controlling the temperature of the metal block 5. The annealing of the PEDOT films was performed at 60, 70, 80, 90, 100, and 110 °C.

Characterization

Renishaw Qontor inVia Raman microscope was used to record Raman spectra (785 nm excitation) . Ag ilent 8453 (up to 1000 nm) and Cary 5E spectrophotome ter (VARIAN) (up to 2400 nm) were used to record UV- Vis-NIR measurements (background correction was done using an uncoated microscope glass slide) . Van der Pauw method was used to calculate sheet resistance ( r _sheet ) for 1L to 6L PEDOT. Resistance values were rec orded as the average of three measurements using the 4-point probe (square shape with a side a = 2.2 mm) and Keithley multimeter (Model 2000) . The specific re sistance (r) and conductivity (s) of the film was cal culated from equations (1) and (2) .

AFM measurements were carried out using Veeco diCaliber scanning probe microscope operated in a tap ping mode at room temperature. All AFM images were recorded using Bruker TESP-MT probe (resonant freq. 320 kHz, spring const. 42 N/m, length 125 ym, width 30 ym, Cantilever spec: 0.01-0.025 Qcm Antimony (n) doped Silicon, 4 ym thick, tip spec: 10-15 ym height, 8 nm radius) . WSXM software was used to determine mean roughness (roughness average (R _a ) ) values of PEDOT films . CV and EIS techniques were used for electro chemical characterization. CV measurements were car ried out in conventional 3 electrode configuration us ing 0.1M TBA-BF ₄ /MeCN . VPP prepared PEDOT films on glass slides covered with different number of PEDOT layers were used as working electrodes. The area of the working electrode was 1.13 cm ² . A Ag/AgCl wire and a platinum wire were used as reference and counter electrode, respectively. Ag/AgCl reference electrode was calibrated before and after every electrochemical measurement using ferrocene redox couple (E _I / ₂ (Fe/Fe ⁺ ) = 0.47 V) . Cyclic voltammograms were recorded using

Metrohm Autolab PGSTAT 101 potentiostat in a potential range from -0.25 V to 0.75 V at a scan rate of 100 mV/s for 1L to 6L PEDOT. The charge (Q) is calculated by integration of the cyclic voltammogram in the po tential range -0.25 to 0.75 V in the Origin software which is further processed by using equations (3) and (4) to obtain capacitance values. Results

The effect of the substrate temperature on the sheet resistance and the roughness average of PE DOT films are shown in table 1.

Table 1.

The effect of the polymerization temperature on the sheet resistance and the roughness average of PEDOT films are shown in table 2. Table 2.

The polymerization temperature is a factor influencing the properties of resulting PEDOT films such as the rate of polymerization by controlling rate of volatilization of monomers (reactant) , the mobility of a polymer chain, conductivity, and morphology. The roughness average (R _a) of the PEDOT film increased with an increase in the VPP cell 1 temperature (Table 1) . This increase in the roughness-average is due to the high concentration of vapor at elevated tempera tures forcing the condensation of vapor at the surface of a substrate. PEDOT films prepared were very uniform and homogeneous .

The roughness-average of the PEDOT films de- creased with an increase in the substrate temperature (Table 2) . This phenomenon is due to the fast conden sation of monomer vapor at a low temperature resulting in less homogeneous films. As the temperature is in creased, the condensation rate is decreased resulting in more uniform film formation. The same behavior also explains the decrease in the sheet resistance of the PEDOT film with the increase in substrate temperature up to 65 °C. The sheet resistance increased for films prepared at 75 °C which is due to less deposition of monomer vapor favored by the high substrate tempera ture .

The effect of polymerization time on the sheet resistance and roughness average of PEDOT films are shown in table 3.

Table 3.

PEDOT films were prepared at different polymerization times i.e. 2, 4, 6, and 8 min. The PE

DOT film prepared during 6 min showed the lowest roughness-average value and the PEDOT film prepared at 4 min polymerization time showed the lowest sheet re sistance (Table 3) . The sheet resistance decreased af- ter annealing. The film annealed at 60°, 70, 80, and

90 °C showed all similar sheet resistance. The de crease in sheet resistance in the range of 60 °C to 90 °C of annealing temperature ensures the curing and the completion of the polymerization process. Characterization of multilayer PEDOT films

Figure 2 illustrates the Raman Spectrum of 6 layer PEDOT film. The band at 577, 699, 989, 1095, 1255, 1367, 1415, and 1530 cm ^-1 are assigned to oxyeth- ylene ring deformation, symmetric C-S-C deformations, oxyethylene ring deformations, C-O-C deformations, O _a - C _c , _' inter-ring stretching, stretching, symmetric C _c< =C _p (-0) stretching, and O _a =O _b stretching, respective ly.

Figure 3 illustrates AFM images of 1L) llayer of PEDOT, 2L) 2 layers of PEDOT, 3L) 3 layers of PE DOT, 4L ) 4 layers of PEDOT, 5L) 5 layers of PEDOT, 6L) 6 layers of PEDOT and SEM images of 1L, 3L and 6L PE DOT. Figure 3 shows the AFM images of 1L to 6L PEDOT and SEM images of 1L, 3L and 6L with very uniform, ho mogeneous, sheet like nature of the films. Materials to be used in optoelectronics need to have high sur face smoothness. From the AFM images and roughness av erage values (Table 4), it can be observed that the roughness doesn't exceed 1 nm for 1L PEDOT and 4 nm for 6L. This indicates that there is a small increase in surface roughness but the uniform, homogeneous, sheet like nature was not affected with the increment of additional layers. Thickness measurements were ver ified with the average of multiple measurements across the large scan area of the PEDOT films. 1L PEDOT shows conductivity of 2178 S/cm and 6L PEDOT shows the maxi mum value of 3208 S/cm (Table 4) . Vapor phase polymer ized PEDOT films prepared using FETOS as oxidant form closely packed large conductive domains which increase the in-plane conductivity. Sheet resistance decreases from 194.56 □/□ for 1L PEDOT to 20.55 □/□ for 6L PEDOT (Table 4) . 2L PEDOT showed 57.7 % decrease in the sheet resistance from 1L PEDOT but almost the same conductivities due to the increase in thickness. 3L PEDOT shows 35.4 % decrease in sheet resistance from

2L PEDOT with a small increase in conductivity. De crease in the sheet resistance when going from 3L to 6L PEDOT is almost linear but 3L and 4L PEDOT show al most similar conductivity values. Decrease in sheet resistance and increase in conductance from 1L PEDOT to 6L PEDOT also indicate a fast charge transfer from layer to layer (Table 4) . This is facilitated by the efficient delocalization of the polaron and polaron pairs or bipolaron on PEDOT chains over different lay ers. This extended conjugation allows faster charge/electron transfer across the film.

Table 4 shows number of layers (L) , thickness (d) , sheet resistance (r _sheet ) , conductivity (s) , areal capacitance (C _A) , volumetric capacitance (C _v) , percent transmittance ( % T) , and roughness average (R _{a )} for VPP produced PEDOT 1-6 -layered films.

Table 4.

According to the polaron theory of conductiv ity, electron transfer occurs through the motion of polarons and bipolarons/polaron pairs along the poly mer chain with a reorganization of double and single bonds. Increased doping level results in an increase in conductivity. As can be seen from table 4, a single layer of PEDOT is highly transparent.

Figure 4 illustrates Cyclic voltammograms of 1L to 6L PEDOT (0.1 M TBABF ₄ in MeCN with 100 mV/s scan rate) .

Figure 5 illustrates Cyclic voltammograms of 1L PEDOT on glass and ITO coated glass in 6 mM Ferro cene solution with 20 mV/s scan rate.

In Figure 4, cyclic voltammograms of rectan gular shapes indicate reversible and efficient charg ing-discharging processes. Table 4 shows that there is an increase in capacitance values with every layer of PEDOT film. A film of 6 layers showed 10.67 mF/cm ² (areal capacitance) and 703.58 F/cm ³ (volumetric ca pacitance) capacitance values. In Figure 5, cyclic voltammogram of a single layer of PEDOT is compared to bare ITO coated glass in 6 mM ferrocene solution. From the difference in peak potentials PEDOT required 60 mV less than the ITO coated glass. Calculated charge val ues for 1L PEDOT and ITO coated glass are 42.93 mC and 36.48 mC, respectively. This can be explained by the higher surface area of PEDOT in comparison to ITO coated glass. Overall the ferrocene response of 1L PE DOT is superior over ITO coated glass.

The high transparency, surface area, conduc tivity, and a very low surface roughness of one- and multi-layer PEDOT film makes them usable in optoelec tronic devices and an alternative to ITO coated mate rials.

It is obvious to a person skilled in the art that with the advancement of technology, the basic idea may be implemented in various ways. The embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.

The embodiments described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment. A method, a conducting PEDOT film, an electronic device, or a use, disclosed herein, may comprise at least one of the embodiments described hereinbefore. It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. It will further be understood that reference to 'an' item refers to one or more of those items. The term "comprising" is used in this specification to mean including the feature (s) or act(s) followed thereafter, without excluding the presence of one or more additional features or acts.