Method of improving electrical conductivity of PEDOT:PSS Technical Field

The present invention relates to a method of improving electrical conductivity of PEDOTPSS. The present invention also relates to a PEDOT:PSS film prepared according to the method.

Background

Optoelectronic devices such as liquid crystal displays (LCDs), light-emitting diodes (LEDs), solar cells, touch panel displays, and detectors, have been attracting considerable attention, due to their strong application in various areas. The market for optoelectronic devices is huge and is still rapidly expanding. At least one electrode of an optoelectronic device is required to be transparent in order to emit or harvest light. Traditionally, indium tin oxide (ITO) is the most popular material as the transparent electrode of optoelectronic devices. However, ITO has problems of scarce indium on earth, high mechanical brittleness that affects its application in flexible electronic devices, and poor adhesion to organic and polymeric materials. The indium price has been increasing. Therefore, new transparent conductive materials are needed to replace ITO as the transparent electrode.

Many materials, including conducting polymers, carbon nanotubes, and graphenes have been investigated as the transparent electrode of optoelectronic devices. Among them, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) can be dispersed in water and some organic solvents, and its films can be readily fabricated through conventional solution processing, such as spin coating. A PEDOTPSS film has high transparency in the visible range, high mechanical flexibility, and excellent thermal stability. However, PEDOTPSS suffers a problem of low conductivity. An as- prepared PEDOTPSS film fabricated from its aqueous solution usually has a conductivity below 1 S/cm, which is remarkably lower than that of ITO. Hence, it is important to significantly enhance the conductivity of PEDOTPSS films.

Methods have been reported to significantly improve the conductivity of PEDOTPSS, such as the treatment of PEDOTPSS with polar organic solvents such as ethylene glycol and dimethyl sulfoxide (DMSO) or the addition of an ionic liquid, an anionic surfactant, or dimethyl sulphate into the PEDOTPSS aqueous solution. Also proposed are methods comprising treating PEDOT:PSS films with a polar organic compound, a certain salt, a zwitterion, or cosolvent. However, the highest conductivity is about 700- 900 S/cm.

There is therefore a need for an improved process to obtain PEDOT:PSS with a higher electrical conductivity.

Summary of the invention

The present invention seeks to address at least one of the problems in the prior art, and provides a method for preparing poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOTPSS) with improved electrical conductivity.

According to a first aspect, there is a provided a method of improving the electrical conductivity of PEDOT:PSS comprising:

- providing a substrate with a PEDOTPSS film on a surface of the substrate; and

- heat treating the PEDOTPSS film with a fluorine-containing compound or an acid.

According to a particular aspect, the heat treating may comprise treating the PEDOTPSS film with a fluorine-containing compound or an acid at a suitable temperature. For example, the temperature may be about 120-180°C. In particular, the temperature is about 140-160°C.

Any suitable fluorine-containing compound may be used in the heat treating. For example, the fluorine-containing compound may be an organic fluorine-containing compound. In particular, the fluorine-containing compound may be an amphiphilic fluorine-containing compound or a geminal diol.

The amphiphilic fluorine-containing compound may be, but not limited to, hexafluoroacetone hydrate, hexafluoroisopropanol, 2,2,2-trifluoroethanol, heptafluorobutyric acid, trifluoroacetic acid, 2,2,3,3,3-pentafluoropropionic acid, 2,2,3,3,3-pentafluoropropanol, trifluoromethanesulfonic acid, or a combination thereof. The geminal diol may be, but not limited to, cyclohexanehexone octahydrate, hexafluoroacetone trihydrate, formaldehyde, acetaldehyde, acetone, perfluorobenzophenone, or a combination thereof.

Any suitable acid may be used in the heat treating. According to a particular aspect, the acid may be any acid which has a pK _a which is less than or equal to -2.8. For example, the acid may be, but not limited to, sulphurous acid, sulphuric acid, oxalic acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, phosphoric acid, nitric acid, methanesulfuric acid, fluorosulfuric acid, fluoroantimonic acid, hydrochlorous acid, chlorous acid, chloric acid, perchloric acid, fluoroboric acid, hexafluorophoric acid, chromic acid, boric acid, or a combination thereof.

The substrate may be any suitable substrate. For example, the substrate may be a transparent substrate and/or a flexible substrate. In particular, the substrate may be a plastic or glass substrate.

According to a particular aspect, the method may further comprise applying the PEDOTPSS film on the surface of a substrate prior to the providing the substrate. The substrate may be as described above. The applying may be by any suitable method. For example, the applying may be by, but not limited to, spin coating, spray coating, roller coating, chemical deposition, drop casting, ink-jet printing, rotogravure printing, doctor blading, wire bar coating, electrochemical deposition, or a combination thereof. In particular, the applying is by spin coating.

According to a particular aspect, the method may further comprise drying the heat treated PEDOT:PSS film. The method may further comprise repeating the heat treating the PEDOTPSS film for a second or subsequent time after the drying of the heat treated PEDOTPSS film.

According to a second aspect, the present invention provides a poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOTPSS) film obtained from the method according to the first aspect of this invention.

There is also provided an article of manufacture comprising the PEDOTPSS film obtained from the method according to the first aspect of the invention. The article of manufacture may be any suitable article. According to a particular aspect, the article of manufacture may be an optoelectronic device or a flexible electronic device. In particular, the article of manufacture may be, but not limited to, a light-emitting diode (LED), a flat panel display, a solar cell, a flexible display, a touch panel, a transparent electrode, an electrochromic display, a smart window, or a detector.

Brief Description of the Drawings

In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying illustrative drawings. In the drawings:

Figure 1 is a flow chart showing the general method of improving the electrical conductivity of PEDOTPSS according to the present invention;

Figure 2 shows the dependence of conductivity of PEDOT:PSS on the treatment temperature with H ₂ S0 solution;

Figure 3 shows the transmittance of a 1 10 nm thick PEDOTPSS film treated with hexafluoroacetone (HFA);

Figure 4 shows the conductivity of PEDOTPSS films after one treatment with H ₂ S0 ₄ solutions of different concentrations at 160°C;

Figure 5 shows the conductivity of PEDOTPSS films after one treatment with 1 M H ₂ S0 solution at different temperatures;

Figure 6 shows the temperature dependence of the normalized resistance of untreated and H ₂ S0 ₄ treated PEDOTPSS films;

Figure 7 shows the analysis of resistance-temperature relationship of the H ₂ S0 treated PEDOTPSS film with the VRH model;

Figure 8 shows the spectra of PEDOTPSS films - (a) transmittance spectra of PEDOTPSS films. The thickness was 24, 66, 109 nm for the H ₂ S0 ₄ treated PEDOTPSS film formed by spin coating the PEDOTPSS aqueous solution once, thrice and five times, respectively, (b) XPS spectra of untreated and H ₂ S0 ₄ treated PEDOTPSS films, (c) FTIR spectra of untreated and H ₂ S0 ₄ treated PEDOTPSS film and H ₂ S0 ₄ . The films were treated with 1 M H ₂ S0 ₄ for three times in (a), (b) and (c);

Figure 9 shows the AFM height images of (a) untreated PEDOTPSS film and (b) PEDOTPSS film treated with 1 M H ₂ S0 ₄ for three times. The unit for the AFM images is μηι;

Figure 10 shows (a) the device architecture of a polymer solar cell with a H ₂ S0 ₄ treated PEDOTPSS film as the transparent anode, (b) chemical structure of P3HT and PCBM, and (c) J-V characteristics of polymer PVs with ITO and H ₂ S0 ₄ treated PEDOTPSS film as the anode under AM 1 .5G illumination (100 mW/cm ² ); Figure 1 1 shows the transmittance spectra of PEDOTPSS films treated with HFA, HFP and 12.2 M formaldehyde solution;

Figure 12 shows (a) the temperature dependence of the normalized resistance of untreated and treated PEDOTPSS films and (b) analysis of resistance-temperature relationship of the untreated and treated PEDOT:PSS film with the VRH model. The resistances are normalized to that of the corresponding PEDOTPSS films at 1 10 K;

Figure 13 shows the UV absorption spectra of PEDOTPSS films before and after treatments with different solutions;

Figure 14 shows the S _2p XPS of untreated and treated PEDOTPSS films;

Figure 15 shows the AFM images of (a) untreated, (b) HFA treated, (c) HFP treated, (d) 36.5% formaldehyde treated, and (e) 2,2,2-trifluoroethanol treated PEDOTPSS films. The unit for the AFM images is μιτι;

Figure 16 shows the conductivities of PEDOTPSS films after treatment with HFA solutions of various concentrations at 140°C;

Figure 17 shows the conductivities of PEDOTPSS films after treatment with HFA.3H ₂ 0 at various temperatures;

Figure 18 shows the transmittance spectra of PEDOTPSS film;

Figure 19 shows (a) the temperature dependence of the normalized resistance of untreated and HFA treated PEDOTPSS films and (b) analysis of resistance- temperature relationship of the untreated and HFA treated PEDOTPSS film with the VRH model. The resistances are normalized to that of the corresponding PEDOTPSS films at 1 10 K;

Figure 20 shows the UV absorption spectra of PEDOTPSS films before and after treatment with HFA;

Figure 21 shows the S _2p XPS of untreated and HFA treated PEDOTPSS films;

Figure 22 shows the AFM images of (a) untreated and (b) HFA treated PEDOTPSS films. The unit is μηι;

Figure 23 shows cyclic voltammograms of PEDOTPSS films untreated, treated with HFA.3H ₂ 0 and an aqueous solution of 15 wt% HFA.3H ₂ 0 in 0.1 M NaCI solution; and

Figure 24 shows J-V characteristics of polymer PVs with ITO and HFA treated PEDOTPSS electrodes under 100 mW/cm ² AM 1 .5G illumination.

Detailed Description of the invention

The exemplary embodiments aim to provide a simple method of improving the electrical conductivity of PEDOTPSS. The method of the present invention significantly enhances the electrical conductivity of PEDOTPSS. The electrical conductivity of PEDOTPSS achieved by using the method of the present invention is comparable to the electrical conductivity of ITO on polyethylene terephthalate (PET) and close to the electrical conductivity of ITO on glass. Accordingly, the PEDOTPSS with enhanced electrical conductivity obtained from the method of the present invention may be used as a replacement for ITO as transparent electrodes in optoelectronic devices. Further, the method of the present invention is a cheap and scalable method, ensuring that the cost of providing PEDOT:PSS with enhanced electrical conductivity remains affordable.

According to a first aspect, there is a provided a method of improving the electrical conductivity of PEDOTPSS comprising:

- providing a substrate with a PEDOT:PSS film on a surface of the substrate; and

- heat treating the PEDOTPSS film with a fluorine-containing compound or an acid.

The method 100 for improving the electrical conductivity of PEDOTPSS may generally comprise the steps as shown in Figure 1. Each of these steps will now be described in more detail.

Step 102 comprises applying poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOTPSS) solution on a surface of a substrate to form a PEDOTPSS film 1 18 on the surface of the substrate. PEDOTPSS generally has the structure as shown in Scheme 1 below.

Scheme 1 : Chemical structure of PEDOT:PSS

PEDOTPSS can be dispersed in water and some organic solvents. PEDOTPSS can processing techniques. Such methods may include coating and printing. PEDOTPSS films have high transparency in the visible range, high mechanical flexibility and good thermal stability. It has been understood that PEDOTPSS has a core/shell structure in water and in PEDOTPSS films. The shell is rich of insulating PSS, while the core is rich of conductive PEDOT.

The substrate onto which the PEDOTPSS solution is applied may be any suitable substrate for the purposes of the present invention. The substrate may be a transparent substrate and/or a flexible substrate. In particular, the substrate may be a plastic or glass substrate. For example, the plastic substrate may be a substrate containing polypropylene (PP), polycarbonate (PC), polyimide (PI), polyethersulfone (PES), polyethylene terephthalate (PET), polymethylmethacrylate (PMMA), polyethylene (PE), polyethylenenaphatalene (PEN), polyvinylalcohol (PVA), polyvinylchloride (PVC), cyclic olefin copolymer (COC), styrene polymer, or a mixture or copolymer thereof. However, it would be understood by a person skilled in the art that the substrates suitable for the present invention are not specifically limited to the substrates mentioned above. Even more in particular, the substrate may be a glass substrate or a PET substrate.

The step 102 of applying may be carried out under conditions suitable for the purposes of the present invention and may comprise any suitable method of applying the PEDOTPSS solution on the surface of the substrate. For example, the step 102 of applying may be by any suitable deposition method. The step 102 of applying may comprise chemical deposition or physical deposition of the PEDOTPSS on the substrate surface. In particular, the step 102 of applying may comprise wet chemistry, spin coating, spray coating, roller coating, chemical solution deposition, drop casting, electrohydrodynamic deposition, ink-jet printing, rotogravure printing, doctor blading, wire bar coating, electrochemical deposition, spin on glass (SOG) or a combination thereof, of the PEDOTPSS solution on the substrate surface. Even more in particular, the step 102 of applying comprises spin coating the PEDOTPSS on the surface of the substrate. The step 102 of applying may also comprise chemical vapour deposition, plasma-enhanced chemical vapour deposition, thermal evaporator, electron beam evaporator, sputtering, pulsed laser deposition, cathodic arc deposition, physical vapour deposition, and molecular beam epitaxy. The substrate may be pre-cleaned prior to the step 102 of applying the PEDOT:PSS solution on the substrate surface. Any suitable pre-cleaning method may be used prior to the step 102.

The PEDOT:PSS film 1 18 formed on the substrate surface may have any thickness suitable for the purposes of the present invention. For example, the PEDOTPSS film 1 18 may be a thin PEDOTPSS film. For example, the PEDOTPSS film 1 18 may have a thickness between 3 nm to 1 mm. In particular, the PEDOTPSS film 1 18 may have a thickness of less than 500 nm, 400 nm, 300 nm, 200 nm, 100 nm, 50 nm, 25 nm, 20 nm, 15 nm, 10 nm, or 5 nm. The PEDOTPSS film 118 may be a single layer or multiple layers, and wherein each layer of the PEDOTPSS film 1 18 may be of the same or different thickness from the other layer.

Step 104 comprises drying the PEDOTPSS film 1 18 obtained from step 102. The step 104 of drying may be carried out under any suitable conditions. For example, the step 104 of drying may be carried out at a suitable temperature for a suitable period of time. In particular, the step 104 of drying may be carried out at about 00-150°C. The step 104 of drying may be carried out for about 1 -20 minutes.

The PEDOTPSS film 1 18 after the step 104 of drying may then be subjected to a step 106 of heat treating the PEDOTPSS film 1 18. It should be noted that if at the step 106 a substrate with a pre-applied PEDOTPSS film on its surface is used, the steps 102 and 104 may not be required for the method 100.

The step 106 of heat treating the PEDOTPSS film 1 18 may comprise treating the PEDOTPSS film 1 18 with a fluorine-containing compound or an acid at a suitable temperature. Following the step 106, a heat treated PEDOTPSS film 120 with improved electrical conductivity compared to the PEDOTPSS film 18 is obtained. For example, the temperature may be about 120-180°C. In particular, the temperature may be about 125-175°C, 130-165°C, 140-160°C, 145-150°C. Even more in particular, the temperature may be about 140-160°C.

The fluorine-containing compound may be any suitable compound which comprises at least one fluorine atom for the purposes of the present invention. For example, the fluorine-containing compound may be an organic fluorine-containing compound. In compound or a geminal diol. Even more in particular, the fluorine-containing compound may be a compound which hydrolyzes into a geminal diol with water.

For the purposes of the present invention, an amphiphilic fluorine-containing compound is defined as a chemical compound which comprises at least one fluorine atom and which possess both hydrophilic and hydrophobic properties. The fluorine-containing compound may be, but not limited to, hexafluoroacetone hydrate (HFA), hexafluoroisopropanol, 2,2,2-trifluoroethanol, heptafluorobutyric acid, trifluoroacetic acid, 2,2,3,3, 3-pentafluoropropionic acid, 2,2,3,3,3-pentafluoropropanol, trifluoromethanesulfonic acid, or a combination thereof.

For the purposes of the present invention, a geminal diol is defined as a chemical compound which has at least two hydroxyl groups which are bonded to the same atom. Geminal diols are usually generated through hydrolysis of ketones or aldehydes. The equilibrium constant for the conversion of a ketone or aldehyde into a geminal diol depends on the molecular structure. Any suitable geminal diol may be used for the purposes of the present invention. For example, the geminal diol may be, but not limited to, cyclohexanehexone octahydrate, hexafluoroacetone trihydrate, formaldehyde, acetaldehyde, acetone, perfluorobenzophenone, or a combination thereof.

In particular, the fluorine-containing compound may be hexafluoroacetone (HFA). HFA hydrolyzes into 1 ,1 ,1 ,3,3,3-hexafluoropropane-2,2-diol (HFP20H) with water. This is shown in Scheme 2 below.

Scheme 2: Hydrolysis of HFA

HFP20H is a geminal diol with two -OH groups connected to the middle carbon atom and is highly amphiphilic arising from the hydrophobic -CF ₃ and hydrophilic -OH groups. The formation of the geminal diol is due to the two electron-withdrawing -CF ₃ groups. The equilibrium constant of the hydrolysis is 10 ⁶ at room temperature. Thus, HFA.3H ₂ 0. Treatment of PEDOTPSS with a geminal diol enhances the conductivity of PEDOTPSS even more significantly than normal polyols with only one -OH group connected to a carbon atom. The amphiphilic groups can preferentially interact with the hydrophobic PEDOT and hydrophilic PSS chains of PEDOTPSS, and these interactions can give rise to conductivity enhancement of PEDOTPSS.

As mentioned above, the PEDOTPSS film has a core/shell structure in which the shell is rich of insulating PSS, while the core is rich of conductive PEDOT. When the film is treated with HFA, the HFA-induced PSSH reduction reduces the thickness of the less conductive shell and benefits the charge transport across the PEDOT chains. Thus, the energy barrier for the interchain and inter-domain charge hopping is lowered, giving rise to significant conductivity enhancement. While the above explanation specifically refers to HFA, it would be understood by a person skilled in the art that the same applies to any fluorine-containing compound used for the step 106 of heat treating the PEDOTPSS film 1 18.

Any suitable acid may be used in the step 106 of heat treating the PEDOTPSS film 1 18. According to a particular aspect, the acid may be an acid which has a pK _a which is less than or equal to -2.8. For example, the acid may be, but not limited to, sulphurous acid, sulphuric acid, oxalic acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, phosphoric acid, nitric acid, methanesulfuric acid, fluorosulfuric acid, fluoroantimonic acid, hydrochlorous acid, chlorous acid, chloric acid, perchloric acid, fluoroboric acid, hexafluorophoric acid, chromic acid, boric acid, or a combination thereof. In particular, the acid may be sulphuric acid (H ₂ S0 ₄ ).

For example, when the acid used in the step 106 of heat treating is sulphuric acid, removal of PSSH is induced, and the presence of the sulphate ions cause the conformational change of the PEDOT chains in the PEDOTPSS film 1 18. In particular, H ₂ S0 ₄ dissociates into H ⁺ and HS0 ₄ ^" ions. This dissociation has a pK _a of -6.4 in water. The pK _a value of H ₂ S0 ₄ is higher than the pK _a value of PSSH (PSSH -» PSS ^" + H ⁺ ). When the PEDOTPSS film 1 18 is heat treated with H ₂ S0 ₄ , H ⁺ from H ₂ S0 ₄ will associate with PSS ^" that exists as the counter anions of positively charged PEDOT in the untreated PEDOTPSS film 1 18 by following the scheme H ⁺ + PSS ^" -> PSSH. The total reaction can be written as: This results in the replacement of some PSS ^" with HS0 ₄ ^" as the counter anions of PEDOT. The PSSH chains are neutral and do not have any Coulombic interaction with PEDOT, so that phase separation occurs between the hydrophilic PSSH and hydrophobic PEDOT chains. In addition, the replacement of PSS ^' with HS0 ₄ ^" results in the conformational change of conjugated PEDOT chains. PEDOT has a coil conformation as the result of the coiled PSS ^" conformation and the Coulombic interactions between PSS ^" and positively charged PEDOT in the untreated PEDOTPSS film 1 18. The coil conformation causes the localization of the positive charges on the PEDOT chains. The disappearance of the Coulombic attractions between PEDOT and PSSH can thus induce conformational change of PEDOT from the coil to extended-coil or linear structure. The removal of PSSH leads to the reduction of the energy barrier width for the interchain and inter-domain charge hopping, whereas the conformational change of PEDOT makes the positive charges on PEDOT more delocalized. Both effects contribute to the conductivity enhancement of PEDOTPSS. While the above explanation specifically refers to H ₂ S0 ₄ , it would be understood by a person skilled in the art that the same applies to any other acid used for the step 06 of heat treating the PEDOTPSS film 1 18.

The method 100 may further comprise a step 108 of drying the heat treated PEDOTPSS film 120. The step 108 of drying may or may not be carried out under similar conditions to the step 104 of drying the PEDOTPSS film 1 18. For example, the step 108 of drying the heat treated PEDOTPSS film 120 may be carried out at about 100-150°C. The step 108 of drying may be carried out for about 1 -20 minutes. In particular, the step 108 may be carried out at about 160°C for about 5 minutes.

The heat treated PEDOTPSS film 120 is then subjected to a step 1 10 of cooling the heat treated PEDOTPSS film 120. The step 10 of cooling may be carried out under any suitable conditions. For example, the step 1 10 of cooling may be carried out until the heat treated PEDOTPSS film 120 reaches a suitable temperature. In particular, the step 1 10 of cooling may be carried out until the heat treated PEDOTPSS film cools down to room temperature or about 25°C.

The heat treated PEDOTPSS film 120 may then be rinsed according to step 1 12. The step 1 12 of rinsing may be with any suitable solvent. For example, the heat treated PEDOTPSS film 120 may be rinsed with deionised water. The step 1 12 of rinsing may be repeated more than one time.

Following the step 1 12 of rinsing, the heat treated PEDOT:PSS film 120 is then dried according to step 1 14. The step 1 14 of drying may be similar to the steps 104 and 108, as described above.

Optionally, the step 106 of heat treating the PEDOT:PSS film 120 may be repeated at least once. This may result in a PEDOT:PSS film 122 which has an electrical conductivity which is even higher than the electrical conductivity of the PEDOT:PSS film 120. Steps 108 to 1 14 may also be repeated following the repetition of the step 106.

Optionally, the steps 102 to 106 may be repeated to obtain a thicker PEDOT:PSS film 122 which has an electrical conductivity which is even higher than the electrical conductivity of the PEDOT:PSS film 120. Steps 108 to 1 14 may also be repeated following the repetition of the steps 102 to 106.

The prepared PEDOT:PSS films 120 and 122 have desirable properties. As mentioned above, the electrical conductivity of the PEDOT:PSS films 120 and 122 are enhanced compared to the untreated PEDOTPSS film 1 18. In fact, the electrical conductivity of the PEDOTPSS films 120 and 122 are enhanced by an order of more than 10 ³ . Further, despite the heat treatment, the PEDOT:PSS films 120 and 122 are able to maintain the transparency of the PEDOT:PSS and have transmittance which is higher than 90% in the wavelength below 550 nm. The PEDOT:PSS films 120 and 122 are also thermally stable. In this way, the PEDOTPSS films 120 and 122 may be used in optoelectronic devices in view of the high conductivity and transparency.

In addition to obtaining the PEDOTPSS films 120 and 122 which have an improved electrical conductivity compared to that the PEDOTPSS film 1 18, the method 100 is a simple and cost-effective method. Further, the method 100 is a scalable method, thus allowing the method 100 to be scaled up to an industrial scale.

Even more in particular, the method 100 may be suitable for fabricating thin films of PEDOTPSS with high conductivity, high transparency and high stability on flexible substrates. The method 100 results in the penetration of the fluorine-containing compound or acid into PEDOTPSS, thereby resulting in the improvement of the electrical conductivity of PEDOT:PSS. In particular, the penetration of the fluorine-containing compound or acid causes the PEDOT chains to separate from the PSS chains, which gives rise to a reduction in the Coulombic attraction between PEDOT and PSS. A phase segregation occurs between the hydrophobic PEDOT and hydrophilic PSS chains. This is in contrast to the method described in US 2011 /0165704, in which a layer of anions are attached on the surface of PEDOTPSS structure, thereby affecting the work function of the PEDOTPSS. It would be evident to a person skilled in the art that such an attachment of a layer of anions on the surface of PEDOTPSS would not improve the conductivity of the PEDOTPSS.

According to another aspect of the present invention, there is provided a poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOTPSS) film obtained from or obtainable by the method described above. The PEDOTPSS film may have desirable properties such as an improved electrical conductivity suitable. For example, the electrical conductivity may be comparable to that of indium tin oxide (ITO) films. In particular, the PEDOTPSS film may be as described in relation to the heat treated PEDOTPSS films 120 and 122.

The present invention further provides an article of manufacture comprising the PEDOTPSS film 120 or 122. The article of manufacture may be any suitable article of manufacture which requires a PEDOTPSS film with high electrical conductivity. In particular, the article of manufacture may be any suitable article of manufacture in which ITO films are currently used. For example, the article of manufacture may comprise optoelectronic devices or a flexible electronic device. The flexible electronic device may comprise flexible optoelectronic devices. In particular, the article of manufacture may be, but not limited to, a light-emitting diode (LED), a flat panel display, a solar cell, a flexible display, a touch panel, a transparent electrode, an electrochromic display, a smart window, or a detector.

Having now generally described the invention, the same will be more readily understood through reference to the following examples which are provided by way of illustration, and are not intended to be limiting. EXAMPLES Example 1

Materials and method

PEDOTPSS aqueous solution (Clevios™ PH1000) was obtained from H.C. Starck. The concentration of PEDOTPSS was 1 .3% by weight, and the weight ratio of PSS to PEDOT was 2.5 in solution. PEDOTPSS films were prepared by spin coating the PEDOTPSS aqueous solution on eleven 1 .3 x 1.3 cm ² glass substrates, which were pre-cleaned successively with detergent (Sigma-Aldrich), de-ionized (Dl) water, acetone (Sigma-Aldrich) and isopropanol (IPA) (Sigma-Aldrich). The PEDOTPSS films were dried at 1 10°C on a hot plate for 30 min. The treatment was performed by dropping 100 μΐ_ solvent or solution as indicated in Table 1 on each of the eleven PEDOTPSS films on a hot plate at 140°C. The films were then dried for about 5 minutes.

After being dried and cooled down to room temperature, the conductivity of each of the PEDOTPSS films were measured by the van der Pauw four-point probe technique with a source meter (Keithley instruments, Series 2400). The electrical contacts were made by pressing indium on the four corners of each PEDOTPSS film on the glass substrate.

The UV-Vis-NIR absorption spectra of the polymer films were taken with a Varian Cary 5000 UV-Vis-NIR spectrometer. The thicknesses of the polymer films were determined with an Alpha 500 step profiler.

Results and discussion

As seen from Table 1 , the conductivity of an as-prepared PEDOTPSS film from the PH1000 aqueous solution was measured. This PEDOTPSS film was not subjected to any heat treatment. It was found that the conductivity of this untreated PEDOTPSS film was about 0.3 S/cm. The conductivity was significantly enhanced after treatment with different solvents and solutions, specifically fluorine-containing compounds and acids.

The conductivity increased to 1 160 S/cm and 1500 S/cm for the PEDOTPSS films treated with hexafluoroacetone at 140°C and with 0.5 M sulohuric acid at 140°C. respectively. These conductivities were significantly higher than that of the PEDOTPSS films treated with ethylene glycol or DMSO. When a PEDOT:PSS film was treated with ethylene glycol at 160°C, the conductivity was only 700 S/cm, and it was 600 S/cm by adding 5% DMSO into the PEDOTPSS aqueous solution. Table 1 also lists the conductivities of PEDOTPSS films treated with other fluorine-containing compounds and acids. It can be seen that all the conductivities were over 1 ,000 S/cm, which is significantly higher than that by the treatment with ethylene glycol or DMSO. It can be seen that if it is desired to avoid using strong acids such as sulphuric acid and trifluoromethanesulfonic acid, the other acids, such as sulphurous acid and oxalic acid, or fluorine-containing compounds may be used for improving the electrical conductivity of the PEDOTPSS films.

Table 1 : Conductivities of PEDOTPSS films after treatment with different solvents at different temperatures The improvement in the conductivity enhancement was dependent on the temperature during the treatment. Figure 2 shows the conductivities of PEDOT:PSS films treated with H ₂ S0 ₄ at different temperatures. The conductivity was more . than 1 ,200 S/cm when the temperature during the treatment with the H ₂ S0 ₄ was more than 120°C. Figure 2 shows that the optimal temperature or treatment with H ₂ S0 ₄ was about 140°C.

The conductivity of the PEDOT:PSS films was further improved by multiple heat treatments with the solvent/solution. For example, when a PEDOT:PSS film treated with hexafluoroacetone was subjected to a repeat of the heat treatment with hexafluoroacetone again, the conductivity of the PEDOT:PSS film was 1650 S/cm.

The treatment with the different solvents and solutions did not deteriorate the PEDOT'.PSS films. The PEDOT:PSS films were smooth and homogeneous after the heat treatment. The heat treatment also did not affect the transparency of the PEDOT:PSS film. Figure 3 shows the transmittance spectrum of a 1 10 nm thick PEDOT:PSS film treated with hexafluoroacetone in the visible range. As seen from the figure, the transmittance is about 90.6% at 550 nm. The transmittance is higher than 90% at a wavelength below 550 nm, and decreases slightly but still higher than 85% at a wavelength over 550 nm.

Example 2

Treatment of PEDOT PSS films with H ₂ S0 ₄

PEDOT:PSS aqueous solution (Clevios PH1000) from H. C. Starck was used. The concentration of PEDOTPSS was 1 .3% by weight, and the weight ratio of PSS to PEDOT was 2.5 in solution. All other chemicals were obtained from Sigma-Aldrich. All the materials were used without further purification.

PEDOTPSS films were prepared by spin coating the PEDOT:PSS aqueous solution on 1 .3 x 1 .3 cm ² glass or PET substrates, which were pre-cleaned successively with detergent, de-ionized (Dl) water, acetone and isopropanol (IPA). The PEDOTPSS films were dried at 120°C on a hot plate for 15 minutes. The H ₂ S0 ₄ treatment was performed by dropping 100 μΙ_ H ₂ S0 ₄ solution on a PEDOTPSS film on a hot plate at 160°C. The films were dried for about 5 minutes. They were cooled down to room tpmnprati irp anrl thpn \Λ/ΡΓΡ rinspri with Hpinni7Prt watpr thrpp timoQ Finallv thp trpatprl PEDOTPSS films were dried at 160°C for about 5 minutes again. Some PEDOTPSS films were treated with a H ₂ S0 solution multiple times. Thick PEDOT.PSS films were prepared by spin coating the PEDOTPSS aqueous solution multiple times. The PEDOTPSS films were dried after each spin coating and then treated with H ₂ S0 ₄ .

Characterisation of PEDOT:PSS films

The conductivities of the PEDOT.PSS films were measured by the van der Pauw four- point probe technique with a Keithley 2400 source/meter. The electrical contacts were made by pressing indium on the four corners of each PEDOT SS film on glass substrate. The temperature dependences of the resistivity of the untreated and H ₂ S0 ₄ treated PEDOTPSS films were tested using a Janis Research VPF-475 dewar with liquid nitrogen as coolant and a Conductus LTC- 1 temperature controller. The UV-Vis- NIR absorption spectra of the PEDOTPSS films were taken with a Varian Cary 5000 UV-Vis-NIR spectrometer, and the AFM images of the PEDOTPSS films were obtained using a Veeco NanoScope IV Multi-Mode AFM with the tapping mode. The X- ray photoelectron spectroscopy (XPS) spectra were collected with an Axis Ultra DLD X- ray photoelectron spectrometer equipped with an Al K _a X-ray source (1486.6 eV). The thicknesses of the PEDOTPSS films were determined with an Alpha 500 step profiler. The ac impedance spectra were obtained using an SI 1260 impedance analyzer.

Device fabrication and characterisation

The highly conductive PEDOTPSS films on glass were used as the anode of polymer solar cells. It was coated with a 50 nm thick PEDOTPSS film (Clevios Al 4083 by H. C. Starck). The active layer was formed by spin coating a 1 ,2-dichlorobenzene solution consisting of 20 mg/mL P3HT (poly(3-hexylthiophene-2,5-diyl)) and 20 mg/mL PCBM ([6, 6]-phenyl-C ₆ i -butyric acid methyl ester) at 800 rpm for 1 minute in a glove box filled with nitrogen. It was dried at room temperature for 3 hours and had a thickness of about 220 nm. The devices were completed by thermally depositing a 20 nm-thick Ca layer and subsequently a 200 nm-thick Al layer in a vacuum of 10 ^~6 mbar. Each polymer solar cell had an active area of 0.09 cm ² . The polymer solar cells were encapsulated with a UV-curable epoxy and glass sheets in the glove box. They were then taken out for electrical testing. The photovoltaic performance of the devices was measured with a computer-programmed Keithley 2400 source/meter. The light source was a Newport's Oriel class A solar simulator, which simulated the AM1 .5G sunlight (100 mW/cm ² ) and was certified to the JIS C 8912 standard.

Results and discussion

As prepared PEDOT:PSS films from the Clevios PH1000 aqueous solution had a conductivity of 0.3 S/cm. In contrast, the conductivity of the H ₂ S0 ₄ treated PEDOTPSS films was significantly enhanced. Figure 4 presents the conductivities of PEDOTPSS films after being treated with H ₂ S0 solutions of different concentrations.

The conductivity increased to more than 1000 and 2000 S/cm after treatment with 0.05 M and 1 M H ₂ S0 ₄ , respectively. The maximum conductivity reached 2400 S/cm when the H ₂ S0 ₄ concentration was 1 .5 M.

The conductivity enhancement depended on the temperature during the treatment, as shown in Figure 5. The optimal treating temperature was 160°C. The effect of the temperature during the H ₂ S0 ₄ treatment on the conductivity of the PEDOTPSS film may be considered to be related to the thermal properties of the PEDOT: PSS films.

The conductivity of the PEDOTPSS films was further enhanced by multiple treatments with H ₂ SO ₄ as presented in table 2 below. A conductivity of 3065 S/cm was observed after the PEDOTPSS film was treated with 1 M H ₂ S0 ₄ at 160°C for three times. This conductivity is comparable to that of ITO, and is significantly higher than that demonstrated by other methods in the prior art.

Key - "a": PEDOTPSS films were treated for one time; "b": PEDOTPSS films were treated for 3 times; and "c": DMSO was added into the PEDOTPSS aqueous solution

Table 2: Conductivities and thicknesses of PEDOT:PSS films after treatment with a solvent/solution at 160°C The H ₂ S0 ₄ treated PEDOT:PSS films also had good stability in the conductivity. No appreciable change was observed in the conductivity after two months. The conductivity also did not change much after several months.

The resistance of the PEDOTPSS films before and after the H ₂ S0 ₄ treatment was investigated from 313 K down to 1 10 K. The results obtained are shown in Figure 6. When the temperature was below 230 K, the resistance of the H ₂ S0 treated PEDOTPSS film decreased with the elevating temperature. It became almost constant at temperature above 230 K. It can therefore be concluded that the H ₂ S0 ₄ treated PEDOTPSS film behaves almost like a metal or semimetal.

Figure 7 shows the analysis of the temperature dependence of the resistance with the one-dimensional variable range hopping (VRH) model, where Γ ₀ = 16/£ _fi N(£ _F )L _// L _i ² is the energy barrier between localized states, N(E _F ) is the density of the states at the Fermi level, and (L _x ) is the localization length in the parallel (perpendicular) direction. The log R vs T ^{"1 2} has a linear relationship at the low temperature range below 230 K, and the T ₀ value estimated from this linear relationship is 29 K. It deviates from the linear relationship at T>230 K. The temperature dependence of the resistance of the H ₂ S0 ₄ treated PEDOTPSS film is saliently different from that of the untreated PEDOTPSS film. The latter has a linear log R vs T ^"1/2 relationship in the whole temperature range from 1 10 to 300 K, and the estimated T ₀ value is 1901 K. Thus, the H ₂ S0 ₄ treatment can significantly reduce the energy barrier for the interchain and inter-domain charge hopping.

The thickness of PEDOTPSS films is reduced after the treatment with H ₂ S0 ₄ as shown in Table 2. The sheet resistance of the PEDOTPSS film is 136 Ω sq ^" after the treatment with 1 M H ₂ S0 ₄ for three times. Thick PEDOTPSS films with a low sheet resistance can be obtained by spin coating the PH1000 solution for multiple times. Treatment with 1 M H ₂ S0 ₄ was carried out on each PEDOTPSS layer. The conductivity of the H ₂ S0 ₄ treated multiple-layer PEDOTPSS film was slightly lower still higher than 2500 S/cm. A sheet resistance of 67 Ω sq ^"1 on H ₂ S0 ₄ treated three- layer PEDOTPSS films was observed which had a thickness of 66 nm and a sheet resistance of 39 Ω sq ^"1 on H ₂ S0 ₄ treated five-layer PEDOT:PSS films which had a thickness of 109 nm. These sheet resistances are comparable or lower than that of ITO coated on polyethylene terephthalate (PET), which has a typical sheet resistance of 50 Ω sq ^"1 . The H ₂ S0 ₄ treatment did not affect the transparency. Figure 8(a) presents the transmittance spectra in the visible range of three H ₂ S0 ₄ treated PEDOTPSS films with different thicknesses. The transmittance of the 66 nm-thick PEDOTPSS film, whic has a sheet resistance of 67 Ω sq ^"1 , is 87% at 550 nm. The transmittance is higher than 90% in the wavelength below 500 nm, and decreases a little but still higher than 80% at the wavelength above 550 nm. The transmittance of the 109 nm-thick PEDOTPSS film, which has a sheet resistance of 39 Ω sq ^"1 , is more than 80% at 550 nm. The thickness of the ITO layer on ITO/PET is about 100 nm, quite close of the thickness of the thickest PEDOTPSS film in Figure 8(a). Thus, the optical absorption coefficient of the H ₂ S0 ₄ treated PEDOTPSS films is slightly higher than ITO.

The PEDOTPSS films before and after the H ₂ S0 ₄ treatment were further characterized by X-ray photoelectron spectroscopy (XPS), UV absorption spectroscopy, FTIR spectroscopy and atomic force microscopy (AFM). As shown in Figure 8(b), the two XPS bands between 166 and 172 eV are the S _2p band of the sulfur atoms in PSS, whereas the two XPS bands between 162 and 166 eV are the S _2p band of the sulfur atoms in PEDOT. The S _2p XPS intensity ratio of PEDOT to PSS saliently increased after the H ₂ S0 treatment. This indicated the removal of some PSSH chains from the PEDOTPSS film. In addition, the XPS spectra indicate that the C/S molar ratio changed from 6.4 to 4.2 after the H ₂ S0 ₄ treatment. The change in the C/S molar ratio is consistent with the removal of PSSH from PEDOTPSS. This conclusion is confirmed by the UV absorption spectra. The two absorption bands in the UV range originate from the aromatic ring of PSS. Their intensities remarkably drop after the H ₂ S0 ₄ treatment.

The PEDOTPSS films were also characterized by the FTIR spectroscopy (Figure 8(c)). The FTIR bands near 600 cm ^"1 are the S-0 stretching modes of the sulfate ions. These bands are absent in the FTIR spectrum of the untreated PEDOTPSS film, while they appear for the H ₂ S0 treated PEDOTPSS film. Therefore, it can be concluded that some sulfate ions remain in the PEDOTPSS film after the H ₂ S0 ₄ treatment even after The surface morphology of the PEDOT:PSS films changed after the H ₂ S0 ₄ treatment, as seen in Figure 9. The untreated PEDOT:PSS film was quite smooth with a RMS roughness of 1 .63 nm. The RMS roughness increased to 2.95 nm after the H ₂ S0 treatment. Moreover, entangled wires with a diameter of tens of nanometers could be observed on the H ₂ S0 ₄ treated PEDOT:PSS films. The AFM images suggest the conformational change of the polymer chains during the H ₂ S0 ₄ treatment, which may be related to the change in the energy barrier of charge hopping as observed by the temperature dependence of the resistances.

The conduction mechanisms of untreated and H ₂ S0 ₄ treated PEDOT:PSS films can be similar to that for other conducting polymers. Ionic conduction makes no or negligible contribution to the high conductivity. This is also evidenced by the ac impedance spectra, in which the modulus of the ac impedance was almost flat in the whole frequency range for the H ₂ S0 ₄ treated PEDOTPSS film. But the untreated PEDOTPSS film exhibited a relaxation process in the impedance at the frequency above 10 ⁴ Hz. This change is similar to the observation by our and other labs, and it is attributed to the segregation of some PSSH chains from the PEDOTPSS films.

The above results indicate that the H ₂ S0 ₄ treatment induces the removal of PSSH, the presence of sulfate ions and the conformational change of the PEDOT chains in PEDOTPSS films.

The H ₂ S0 ₄ treated PEDOTPSS films were used to replace ITO as the transparent electrode of polymer solar cells. The device architecture and the chemical structure of P3HT and PCBM are shown in Figure 10(a) and Figure 10(b), respectively. The H ₂ S0 ₄ treated PEDOTPSS film had a thickness of 70 nm. The current density (J )-voltage (V) curves of these solar are presented in Figure 10(c). The device with the H ₂ S0 ₄ treated PEDOTPSS film anode exhibited high photovoltaic performance: J _sc of 9.29 mA/cm ² , V _oc of 0.59 V, FF of 0.65, and PCE of 3.56%. The photovoltaic performance was comparable to that of the control devices using ITO anode, as can be seen from the results in Table 3.

When untreated PEDOTPSS films were used as the anode of polymer PVs, the devices exhibited very poor photovoltaic performance, as seen in Table 3. This can be attributed to the low conductivity of the untreated PEDOTPSS films.

Example 3

Treatment of PEDOT:PSS films with geminal diols or fluorine-containing compounds

PEDOTPSS films were prepared in the same manner as described in Example 2. Once the PEDOTPSS films were obtained, treatment was performed by dropping 100μΙ_ geminal diol or fluorine-containing compound or an aqueous solution of the same on a PEDOTPSS film on a hot plate at 140°C. The films were dried for about 5 minutes. They were cooled down to room temperature, and then were rinsed with deionized water. Finally, the treated PEDOTPSS films were dried at 140°C for about 5 minutes again.

Characterisation of PEDOT:PSS films

The conductivities, temperature dependences of the resistivities, UV-Vis-NIR absorption spectra, AFM images, XPS spectra and thicknesses of the PEDOTPSS films were measured in the same manner as described in Example 2 above.

Results and discussion

The conductivity of the untreated PEDOTPSS film was found to be 0.3 S/cm. Tables 4 and 5 provide the conductivities of the PEDOTPSS films after the treatment with the different compounds. Treatment with Melting point Boiling point Conductivity

(°C) (°C) (S/cm) ^a

Cyclohexanehexone 99 345 349 (0.1 )

octahydrate

HFA trihydrate -129 -28 1164

Formaldehyde -92 -19 862 (12.2 M)

Acetaldehyde -124 20 83 (18.2 M)

Acetone -95 56 1

Perfluorobenzophenone 90-95 359 -0.3 (1 M)

Key - "a": PEDOTPSS films were treated with a neat compound or a maximum concentration solution. The numbers in the parentheses indicate the concentrations for the aqueous solutions used for the treatment

Table 4: Conductivities of PEDOT:PSS films treated with geminal diols

Key - "a": PEDOTPSS films were treated with a neat compound or a maximum concentration solution. The numbers in the parentheses indicate the concentrations for the aqueous solutions used for the treatment

Table 5: Conductivities of PEDOT:PSS films treated with amphiphilic fluorine-containing compounds

As can be seen from the results obtained, a treatment of the PEDOT:PSS films with 12.2 M formaldehyde aqueous solution, which corresponds to the concentration of 36.7% by weight, enhanced the conductivity from 0.3 to 862 S/cm, while a treatment with acetone hardly improved the conductivity. The conductivity of PEDOTPSS films treated with acetaldehyde was 83 S/cm, which was higher than that treated with acetone but lower than that treated with formaldehyde.

The conductivity of PEDOT:PSS film treated with cyclohexanehexone was also investigated. Only its geminal dioi structure, dodecahydroxycyclohexane, exists in water. Cyclohexanehexone is a solid at room temperature, and it has limited solubility in water. The conductivity of PEDOTPSS film enhanced to 349 S/cm after a treatment with 0.1 M cyclohexanehexone aqueous solution. This conductivity was even higher than the nnnriimtivitv M 77 R/ ml nf PFDOTPSS films treated with 0 1 M H A Perfluorobenzophenone, which has a carbonyl group connected with two fluorobenzene rings, is a solid at room temperature, and its aqueous solution gives rise to no change in the conductivity of PEDOT:PSS. It may be difficult for this molecule to convert into its geminal diol structure because the attachment of two OH groups to the C atom of the carbonyl group can destroy the conjugation of the carbonyl group with the two fluorobenzene rings.

All the fluorine-containing compounds listed in Table 5 are liquid at room temperature. Conductivity as high as 1022 S/cm was observed on EPDOTPSS films treated with hexafluoroisopropanol (HFP). This conductivity is quite close to the conductivity of the PEDOT:PSS film treated with HFA. The conductivity enhancement cannot be attributed to the effect by geminal diol, because HFP does not convert into a geminal diol in water. However, HFP has the amphiphilicity similar to HFP20H owing to the presence of two -CF ₃ groups and the -OH group.

The conductivity enhancement became less significant when 2,2,2-trifluoroethanol was used for the treatment. The conductivity was only 1 1 S/cm after a treatment with 2,2,2- trifluoroethanol. Therefore, it can be concluded that the presence of two hydrophobic groups or two hydrophilic groups in the molecule was important for the significant conductivity enhancement.

This was further confirmed by investigating the treatments of PEDOT.'PSS films with trifluoroacetic acid and heptafluorobutyric acid. The carboxylic group can be considered as two polar groups, a carbonyl group and a hydroxyl group. Though trifluoroacetic acid is different from 2,2,2-trifluoroethanol only by the carbonyl group, the former had a conductivity of 473 S/cm, much more significant than the latter. The conductivity enhancement became even more significant for heptafluorobutyric acid, which has more fluoro atoms on the molecule.

Optical and electrical properties of treated PEDOT:PSS films

As seen above, although a treatment with a geminal diol or an amphiphilic fluorine- containing compound can significantly enhance the conductivity of PEDOT:PSS films, the treatment did not affect the transparency in the visible range. Figure 1 1 shows the transmittance spectra of PEDOT:PSS films treated with HFA, HFP and formaldehyde. more than 90% in the visible range from 400 to 700 nm. The HFA-treated PEDOTPSS films, as well as the HFP and formaldehyde treated PEDOTPSS films had comparable transmittance and surface resistance to ITO on plastic.

The conduction mechanism of PEDOTPSS films treated with HFA, HFP and formaldehyde were studied by measuring their resistances from room temperature down to 1 10 K (Figure 12a). At temperatures below 200 K, the resistances of the PEDOTPSS films increased with the lowering temperature, which was similar to that of untreated PEDOTPSS films. However, the temperature dependences of the resistances of the treated PEDOTPSS films were different from that of untreated PEDOTPSS films when the temperature was higher than 200 K. The resistances were almost constant, especially for the PEDOTPSS films treated with HFA and HFP.

Figure 12b shows the analysis of the temperature dependences of the resistances with the one-dimensional variable range hopping (VRH) model, which has been described in Example 2 above. The untreated PEDOTPSS film has a linear logR vs T ^{"1 2} in the whole temperature range from 1 10 to 310 K, and the estimated T ₀ value from this linear relationship is 1901 K. The logR vs T ^{"1 2} relations of the HFA and HFP treated PEDOTPSS films had a linear relationship only at the low temperature range below 200 K, and the T ₀ value estimated from the linear relationships are 99 and 127 K, respectively. The T ₀ value of PEDOTPSS film treated with formaldehyde at T < 200 K is 194 K. The T ₀ values in the low temperature range were consistent with the conductivities of the PEDOTPSS films at room temperature, which were 0.3, 1 64, 1022, and 862 S/cm for the untreated, HFA, HFP, and formaldehyde treated PEDOTPSS films, respectively. T ₀ decreased with the increasing conductivity. Thus, the treatment with a geminal diol or an amphiphilic fluorine-containing compound lowers the energy barrier for the interchain and inter-domain charge hopping.

The logR vs T ^"1/2 curves of the PEDOTPSS films treated with HFA, HFP and formaldehyde show that the polymer films experience a phase transition at around 230 K. The T ₀ value of PEDOTPSS films treated with formaldehyde decreased to 54 K at the temperature range from 230 to 310 K. The resistances of the PEDOTPSS films treated with HFA and HFP were almost insensitive to temperature at the temperature range of more than 230 K. Thus, the phase transition at around 230 K for the PEDOTPSS films treated with HFA and HFP was an insulator-to-metal Dhase transition. However, X-ray diffraction band for untreated PEDOTPSS films and PEDOTPSS films treated with HFA, HFP and formaldehyde was not observed.

Mechanism for the conductivity enhancements

The mechanism for the conductivity enhancements of PEDOT:PSS by treatment with geminal diols and amphiphilic fluorine-containing compounds was studied by various chemical and physical characterizations. The UV absorption spectra of PEDOTPSS films are shown in Figure 13. The intensities of the two absorption bands, which originate from the aromatic rings of PSS, dropped after treatment with HFA or HFP. This change indicates the decrease of the PSSH amount in PEDOTPSS after the treatment. On the other hand, the intensities of the two absorption bands hardly changed after a treatment with 2,2,2-trifluoroethanol. This indicates that the treatment with 2,2,2-trifluoroethanol did not give rise to remarkable change in the composition of PEDOTPSS. The changes in the UV absorption spectra of PEDOTPSS films after the treatment with different compounds are consistent with the effects of these compounds on the conductivity of the PEDOTPSS films.

The reduction of PSSH from the PEDOTPSS films after treatment was confirmed by the XPS spectra of the PEDOTPSS films (Figure 14). The two XPS bands with binding energies between 166 and 172 eV are the S _2p bands of the sulfur atoms in PSS, whereas the two XPS bands with binding energies between 162 and 166 eV are the S _2p bands of the sulfur atoms in PEDOT. The S _2p XPS intensity ratio of PEDOT to PSS increased after the HFA and HFP treatment, while it only slightly changed after the treatment with 2,2,2-trifluoroethanol.

Moreover, no fluorine XPS signal was detected on PEDOTPSS films treated with HFA and the other fluorine-containing compounds. Hence, these compounds did not remain in the PEDOTPSS films after the treatment. They completely vaporized -during the treatment and/or were rinsed away by water.

The results indicate that the mechanism for the conductivity enhancement of PEDOTPSS by the geminal diols and amphiphilic fluorine-containing compounds is similar to that by HFA and polyol. The polyol treatment of PEDOTPSS gives rise to the phase segregation of PSSH chains from PEDOTPSS and the conformational change

("if t o ΡΡΠ Τ hainc Tho amnhin ili f li iririno-rv-intaininn mmnnnnHc o lon ni a rica tr\ changes in the composition and structure of PEDOT:PSS. The amphiphilic fluorine- containing compounds induced phase segregation of the PSSH chains which can be ascribed to the shielding of the Coulombic attraction between PEDOT and PSS by the amphiphilic fluorine-containing compounds. Some of the PSSH chains were removed from the polymer films during the rinsing step with water. The phase segregation reduces the amount of insulator PSSH in the polymer film and also the conformation of the PEDOT chains.

The surface morphology of the PEDOT:PSS films also changed after the treatment with geminal diols and fluorine-containing compounds as seen from the AFM results shown in Figure 15. The rms roughness was 1 .63 nm, 1 .68 nm, 1 .44 nm, and 2.27 nm for the untreated, HFA, HFP, and formaldehyde treated PEDOTPSS films. The apparent rms roughness increased to 4.88 nm for the PEDOTPSS film treated with 2,2,2- trifluoroethanol. This was mainly due to the appearance of pores in the 2,2,2- trifluoroethanol-treated PEDOTPSS film. A close-up look on the AFM image showed changes in the morphology of the PEDOTPSS films after the treatment. Fiber structures were observed on the PEDOTPSS films treated with HFA, HFP and formaldehyde, while they were absent for the untreated and 2,2,2-trifluoroethanol- treated PEDOTPSS films. The change in the AFM images indicates phase segregation of PSSH chains from PEDOTPSS films after the treatment. It also indicates the conformational change of the PEDOT chains during the treatment.

Example 4

Treatment of PEDOT:PSS films with hexafluoroacetone (HFA)

PEDOTPSS films were prepared in the same manner as described in Example 2. Once the PEDOTPSS films were obtained, treatment was performed by dropping 100μΙ_ pure HFA.3H ₂ 0 (Sigma-Aldrich) or an aqueous solution of the same on a PEDOTPSS film on a hot plate at 140°C. The films were dried for about 5 minutes. They were cooled down to room temperature, and then were rinsed with deionized water. Finally, the treated PEDOTPSS films were dried at 140°C for about 5 minutes again. Some PEDOTPSS files were treated multiple times. Thick PEDOTPSS films were prepared by spin coating the PEDOTPSS aqueous solution multiple times. The PEDOTPSS films were dried after each spin coating, and an HFA treatment was carried out after each spin coating.

Characterisation of PEDOT:PSS films

The conductivities, temperature dependences of the resistivities, UV-Vis-NIR absorption spectra, AFM images, XPS spectra and thicknesses of the PEDOTPSS films were measured in the same manner as described in Example 2 above. Cyclic voltammetric (CV) measurements were carried out with a ECO CHEMIE AUTOLAB PGSTAT 302N + FRA2 system in 0.1 M NaCI solution with a Au disc coated with a PEDOTPSS film as the working electrode. The PEDOTPSS films for the CVs were prepared by dropping PEDOTPSS aqueous solution on a Au disc with a diameter of 2 mm and subsequently drying at 120°C. A Pt wire and Ag/AgCI (3 M NaCI) were used as the counter and reference electrodes, respectively. The scan rate was 50 mV/s. The X-ray diffractions (XRD) of the thin films were analyzed using a Bruker D8 Advance X- ray diffractometer equipped with Cu Kai radiation (40 kV, 40 mA).

Device fabrication and characterisation

The highly conductive PEDOTPSS films on glass were used as the anode of polymer solar cells (PSCs). 50 nm thick PEDOTPSS film (Clevios Al 4083 by H. C. Starck) was spin coated on a HFA-treated PEDOTPSS film as a buffer layer. The active layer was formed by spin coating a 1 ,2-dichlorobenzene solution consisting of 18 mg/mL P3HT and 18 mg/mL PCBM at 500 rpm for 1 minute in a glove box filled with nitrogen. It was dried at room temperature for 3 hours and had a thickness of about 200 nm. The devices were completed by thermally depositing a 40 nm-thick Ca layer and subsequently a 200 nm-thick Al layer in a vacuum of 10~ ⁶ mbar. Each PSC had an active area of 0.1 1 cm ² . The P3HT and PCBM were used as the donor and acceptor in the active layer.

The polymer solar cells were encapsulated with a UV-curable epoxy and glass sheets in the glove box. They were then taken out for electrical testing. The photovoltaic performance of the devices was measured with a computer-programmed Keithley 2400 source/meter. The light source was a Newport's Oriel class A solar simulator, which simulated the AM1 .5G sunlight (100 mW/cm ² ) and was certified to the JIS C 8912 Results and discussion

Treatment of PEDOT:PSS films with HFA significantly enhanced the conductivity of the films. Figure 16 presents the conductivities of PEDOTPSS films after treatment with aqueous solutions of different HFA concentrations. The as-prepared PEDOTPSS films had a conductivity of 0.3 S/cm. The conductivity was significantly enhanced after the treatment, and the conductivity increased with the increasing HFA concentration. The highest conductivity of 1 164 S/cm was observed for the treatment ith pure HFA.3H ₂ 0. This conductivity is saliently higher than that of PEDOTPSS treated with ethylene glycol or DMSO once. The highest conductivity is only 735 S/cm for the PEDOTPSS films treated with ethylene glycol, and it is 680± 50 S/cm by adding 5% DMSO into the PH1000 aqueous solution as reported in the prior art.

The conductivity of PEDOTPSS films was further enhanced through the treatment with HFA multiple times. It reached 1325 S/cm after treatment with HFA.3H20 four times. This conductivity is comparable to that of the PEDOTPSS films reported by YH Kim et al. by adding EG into the Clevious PH 000 PEDOTPSS aqueous solution and subsequent treatment of the polymer films in an EG bath. The two-step treatment of PEDOTPSS with EG as reported by YH Kim et al. was reported and it was observed that the conductivity was only 882 S/cm. The lower conductivity than that of YH Kim et al. is probably related to the different Clevious PH1000 PEDOTPSS batches used in the different experiments.

The conductivity enhancement also depended on the temperature during the treatment (Figure 17). The conductivity increased with the elevating temperature from 80 to 140°C, and then dropped slightly when the temperature was further increased to 200°C, but the conductivity was still more than 100 S/cm even when the treatment temperature was 200°C. The HFA treated PEDOTPSS films were more thermally stable than those treated with other compounds, such as salts, zwitterions, acids and cosolvents as in the methods of the prior art.

As prepared PEDOTPSS films had a thickness of 60 nm. The thickness decreased to 50 nm after treatment with HFA once, and HFA treated PEDOTPSS films had a sheet resistance of 172 Ω sq ^"1 . In order to lower the sheet resistance, thick PEDOTPSS films were prepared by spin coating the Clevios PH1000 aqueous solution multiple times. An HFA treatment was carried out after spin coating each PEDOTPSS layer. Four-layer PEDOTPSS films had a thickness of 164 nm and a conductivity of 1319 S/cm after the HFA treatment. The corresponding sheet resistance was 46 Ω sq ^"1 . It is comparable or even lower than that of ITO coated on PET, which is about 50 Ω sq ^"1 .

Optical and electrical properties of PEDOT:PSS films before and after the HFA treatment

The PEDOTPSS films were smooth and homogeneous after the HFA treatment. Their transmittance in the visible range was not affected by the HFA treatment. Figure 18 shows the transmittance spectra of HFA treated PEDOTPSS films with different thicknesses in the visible range. The transmittance of the 50 nm-thick PEDOTPSS film, which has a sheet resistance of 172 Ω sq ^"1 , is 94% at 550 nm. The transmittance is above 90% in the visible wavelength range from 400 to 700 nm. The transmittance of the 164 nm-thick PEDOTPSS film, which has a sheet resistance of 46 Ω sq ^"1 , is more than 80% at 550 nm. The high conductivity and transparency indicate that the HFA treated PEDOTPSS films are suitable as the transparent conductive electrode of optoelectronic devices.

The conductivity of PEDOTPSS films was studied from room temperature down to 1 10 K (Figure 19a). When the temperature was below 200 K, the resistance of the HFA treated PEDOTPSS film decreased with the temperature increase like untreated PEDOT: PSS. It became almost constant at temperatures above 200 K. This indicates that the HFA treated PEDOTPSS film behaves almost like a metal or semimetal. It is very rare to observe metallic behavior for conductive PEDOT films, particularly when they are formed by solution processing. This is consistent with the high conductivity of the HFA-treated PEDOTPSS films.

Figure 19b shows the analysis of the temperature dependences of the resistances of untreated and HFA treated PEDOTPSS films with the one-dimensional variable range hopping (VRH) model, as described in Example 2. The untreated PEDOTPSS film has a linear logR vs T ^"1/2 in the whole temperature range from 1 10 to 310 K, and the estimated T ₀ value from this linear relationship is 1901 K. The logR vs T ^{"1 2} relations of the HFA-treated PEDOTPSS film had a linear relationship only at the low temperature range below 200 K, and the T ₀ value estimated from the linear relationship is 99 K. It the conductivities of the PEDOTPSS films at room temperature, which were 0.3 and 1164 S/cm, for the untreated and HFA treated PEDOTPSS films, respectively. T ₀ decreased with the increasing conductivity. Thus, the treatment with HFA lowers the energy barrier for the interchain and inter-domain charge hopping.

Mechanism for the conductivity enhancement by the HFA treatment

The mechanism for the conductivity enhancement through the HFA treatment was studied by various chemical and physical characterizations. The UV absorption spectra of untreated and HFA treated PEDOTPSS films are shown in Figure 20. The two absorption bands in the UV range originate from the aromatic rings of PSS. Their intensity drops after the HFA treatment. This change indicates the decrease of the PSSH amount in PEDOTPSS.

The reduction of PSSH from the PEDOTPSS films after the HFA treatment is confirmed by XPS spectra of the PEDOTPSS films (Figure 21 ). The two XPS bands with binding energies between 166 and 172 eV are the S _2p band of the sulfur atoms in PSS, whereas the two XPS bands with binding energies between 162 and 166 eV are the S _2p band of the sulfur atoms in PEDOT. The S _2p XPS intensity ratio of PEDOT to PSS increases after the HFA treatment.

No fluorine XPS signal was detected on HFA treated PEDOT: PSS films. Thus, no HFA remained in the PEDOTPSS films after the treatment. HFA completely vaporized during annealing of the PEDOTPSS films, because the boiling points are 106°C and - 28°C for HFA.3H ₂ 0 and unhydrated HFA, respectively.

The surface morphology of the PEDOTPSS films changed after the HFA treatment, as revealed by the AFM images (Figure 22). Both the untreated and HFA treated PEDOTPSS films have quite a smooth surface. The rms roughness was 1 .63 and 1 .68 nm for the untreated and HFA treated PEDOTPSS films, respectively, but a remarkable difference in the AFM images can be observed. The untreated PEDOTPSS film consisted of mainly polymer particles with a size of about 50 nm. These polymer particles turned into entangled wires with a diameter of tens of nanometers after the HFA treatment. The AFM results are consistent with the energy barriers for the charge hopping obtained by the temperature dependences of the conductivities of the untreated and HFA treated PEDOTPSS films. The conductive nanowires can promote the charge hopping in comparison with the particles.

The AFM images suggest the conformational change of the polymer chains after the HFA treatment. It is further evidenced by the electrochemical activity of untreated and HFA treated PEDOTPSS films, which is quite sensitive to the chain conformation. CVs of PEDOTPSS films untreated, treated with HFA.3H ₂ 0 and an aqueous solution of 15 wt% HFA.3H ₂ 0 are presented in Figure 23. The untreated PEDOTPSS film exhibited electrochemical activity only at potentials higher than -0.2 V vs. Ag/AgCI, whereas additional electrochemical activity appears at potentials down to -0.8 V vs. Ag/AgCI for the HFA treated PEDOTPSS films. The electrochemical activities of the untreated and treated PEDOTPSS films are consistent with the conductivities of the PEDOTPSS films. The higher conductivity of the PEDOTPSS films corresponds to more redox behavior in a more negative potential range. The PEDOTPSS film treated with an aqueous solution of 15 wt% HFA.3H ₂ 0 had a conductivity of 532 S/cm, the peak potentials of the redox pair in the lowest potential range was -0.57 V and -0.54 V vs. Ag/AgCI, while this redox pair was not observed for the untreated PEDOTPSS film. The peak potentials of this redox pair shifted to -0.68 V and -0.64 V vs. Ag/AgCI for the PEDOTPSS film treated with HFA.3H20. The additional electrochemical activity for the HFA treated PEDOTPSS film was due to the decrease in the PSSH amount and the conformational change of the PEDOT chains. The PEDOT chains with a linear or extended-coil conformation were electrochemically reduced and subsequently oxidized, while the reduction and oxidation were difficult for the PEDOT chains with a coil conformation surrounded by a relatively thick shell rich of PSSH.

These experimental results indicate that HFA induced the reduction of PSSH and the conformational change of the PEDOT chains in PEDOTPSS films. The geminal diol, HFP20H corresponding to HFA, is an amphiphilic compound with the hydrophobic - CF ₃ and hydrophilic -OH groups. The hydrophobic -CF ₃ groups of HFP20H preferentially interact with the hydrophobic PEDOT chains, whereas the hydrophilic - OH groups of HFP20H preferentially interact with the hydrophilic PSS chains of PEDOTPSS as shown in Scheme 3 below.

PEDOT PSS PEDOT PSS

Scheme 3: Schematic representation of PEDOT:PSS before and after HFA treatment.

As a result of these interactions, the PEDOT chain is separated from the PSS chain by HFA. This gives rise to the reduction in the Coulombic attraction between PEDOT and PSS. Thus, phase separation occurs between the hydrophobic PEDOT and hydrophilic PSS chains. The Coulombic attraction even disappears when the PSS chains become PSSH chains by taking protons from other PSS chains. These PSSH chains can segregate from PEDOT:PSS and are removed during the post-treatment rinse with water.

HFA induces the conformational change of the PEDOT chains in the PEDOTPSS films. The hydrophobic PEDOT chains are stabilized by the hydrophilic PSS chains in water, so that the PEDOT chains follow the coil conformation of PSS chains due to the Coulombic attraction between them. The coil conformation is conserved in the as- prepared PEDOTPSS films. The PEDOT chains turn to an extended-coil or linear conformation as a result of the Coulombic screening between PSS and PEDOT by HFA. The linear PEDOT chains have stronger inter-chain interactions, which facilitates the inter-chain charge transport, thus leading to an improved conductivity of the PEDOTPSS film.

Application of highly conductive PEDOT:PSS in polymer PVs

The HFA treatment significantly enhances the conductivity while not affecting the transparency in the visible range of the PEDOTPSS films. As observed by ultraviolet photoelectron spectroscopy (UPS), the work function of PEDOTPSS films almost does not change after the HFA treatment. The HFA treated PEDOTPSS films are thus suitable for use as the transparent electrode of PSCs to replace ITO. The current density (J)-voltage (V) curves of the PSCs are provided in Figure 24. The device with short-circuit current density (J _sc ) of 9.44 mA/cm ² , open-circuit voltage (V _oc ) of 0.59 V, fill factor (FF) of 0.64, and power conversion efficiency (PCE) of 3.57%. The photovoltaic performance is comparable to that of the control device using an. ITO anode. The photovoltaic performance of the control device was: J _sc of 10.00 mA/cm ² , V _oc of 0.59 V, FF of 0.70, and PCE of 4.13%.

Whilst the foregoing description has described exemplary embodiments, it will be understood by those skilled in the technology concerned that many variations in details of design, construction and/or operation may be made without departing from the present invention. References

1 . YH Kim et al, Adv. Funct. Mater., 201 1 , 21 :1076