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Neutral-pH PEDOT:PSS as over-coating layer for stable silver nanowire flexible transparent conductive films
Organic Electronics 15 (2014) 3654–3659
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Neutral-pH PEDOT:PSS as over-coating layer for stable silver nanowire flexible transparent conductive films S. Chen a, L. Song b, Z. Tao b, X. Shao a, Y. Huang a, Q. Cui a, X. Guo a,⇑ a National Engineering Laboratory for TFT-LCD Material and Technologies, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai 200240, China b Shanghai OE Chemicals, 200232, China
a r t i c l e
i n f o
Article history: Received 18 August 2014 Received in revised form 13 September 2014 Accepted 29 September 2014 Available online 19 October 2014 Keywords: Transparent conductive film Silver nanowire PEDOT:PSS Stability
a b s t r a c t The silver nanowire (AgNW) mesh film with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the over-coating layer is a promising flexible transparent conductive film technology. In this work, experimental studies show that the hygroscopic and acid properties of the common PEDOT:PSS lead to poor stabilities of the composite films, due to the conductivity degradation of PEDOT:PSS by the water absorption and the acid corrosion of AgNWs by PEDOT:PSS. By using the modified PEDOT:PSS of neutral pH as the over-coating layer, the long term shelf-life time, thermal and current stressing stabilities are all significantly improved without sacrifice of transparency, electrical conductivity and mechanical flexibility. Under both cases of thermal aging test at 210 °C for 20 min and 12 h continuous current stressing at a current density of 30 mA/cm2, no obvious change of the conductivity is observed. The results clearly demonstrate that using the neutral-pH PEDOT:PSS as an over-coating layer can help to achieve flexible AgNW transparent conductive films with superior stability for flexible optoelectronic devices. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Rapidly growing interests in flexible optoelectronics, such as organic light emitting diodes (OLEDs), thin film solar cells and photonic sensors, demand transparent conductive films (TCFs) of excellent mechanical flexibility and low cost [1–6]. With problems of brittleness, and rising cost of indium, the commonly used indium tin oxide (ITO) is not an ideal choice [7]. The silver nanowire (AgNW) mesh is considered as a promising replacement, for its high dc conductivity and optical transmittance, excellent mechanical flexibility, capability with low cost, large area coating processes, and environmental and economic advantages for manufacturing [8-10]. However, several problems associated with AgNW mesh films need to be ⇑ Corresponding author. E-mail address: [email protected] (X. Guo). http://dx.doi.org/10.1016/j.orgel.2014.09.047 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
addressed for device applications, including rough surface, large wire-to-wire junction resistance and poor adhesion to the substrate [11,12]. The air stability is also of concern because the AgNWs are likely to be oxidized when being exposed to air [13,14]. Introducing an over-coating layer onto the AgNW mesh films appears to be an effective solution to overcome all or part of the above issues [15–26]. Among various over-coating materials, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is most commonly used because of its high transparency, good conductivity, high work function and ease for large area processing [27]. Although abundant work has been conducted on processing of flexible AgNW–PEDOT:PSS hybrid composite and device applications, there is lack of comprehensive study on the stabilities of the films, including shelf-life, thermal aging and current stressing stabilities, which are vital for device applications.
S. Chen et al. / Organic Electronics 15 (2014) 3654–3659
In this work, experimental studies show that the hygroscopic and acid properties of the normal PEDOT:PSS lead to poor stabilities of the composite films, due to the conductivity degradation of PEDOT:PSS by the water absorption and the acid corrosion of AgNWs by PEDOT:PSS [25]. By using a type of modified PEDOT:PSS with neutral pH as the over-coating layer, both the shelf-life and long term current stressing stabilities in air are significantly improved without sacrifice of transparency, electrical conductivity and mechanical flexibility. 2. Experimental The PEDOT:PSS (Clevios PH1000) used in this experiment was provided by Heraeus Clevios GmbH with a pH value of 1.5–2.5. The neutral-pH PEDOT:PSS solution with a pH value of 7.0–7.5 was obtained by adding guanidine aqueous solution into the acidic PEDOT:PSS. 5 wt% dimethyl sulfoxide (DMSO) solution was added to both PH1000 and the neutral-pH PEDOT:PSS dispersion to increase the conductivity. AgNWs supplied by Nanjing XFNANO were diluted in isopropyl alcohol (IPA) to form the AgNW dispersion with a concentration of 2 mg/ml for use. Glass or PET substrates were cleaned using detergent, deionized water, acetone, and IPA, and then treated with oxygen plasma to obtain hydrophilic surfaces for coating of AgNWs. The AgNW suspension was spin coated twice at 600 rpm for 40 s to form a continuous film on the substrate. PEDOT:PSS (PH1000 or the neutral-pH one) was then spin coated at 3000 rpm for 40 s onto the AgNW film, and then dried at 120 °C for 20 min to obtain the composite film. To investigate the reliability under current stressing, the films were patterned into strips with width of 5 mm and length of 2 cm by wiping, and then encapsulated by a polydimethylsiloxane (PDMS) layer to reduce the influence from the ambient. To form the PDMS layer, a 10:1 mixture of PDMS elastomer to cross-linker (Sylgard 184, Dow Corning) was prepared and stirred for 3 min, and coated onto the samples on glass substrate, followed by a curing process at 100 °C for 30 min. Electrical contacts were made using silver paste (SCP003, Electrolube). Specular transmission (T) spectra of the conductive films were obtained using a UV/VIS/NIR spectrophotometer (UV-3100PC, Mapada). The sheet resistance (Rs) was measured using a four point probe system (ST-2258A, Suzhou JG). The surface topography was analyzed using a scanning electron microscope (SEM) (TM3000, Hitachi). The current of the films were measured with a Keithley 4200 semiconductor characterization system. The stabilities of the films were characterized as the conductivity changes with the films being exposed in air ambient, under high temperature and current stressing in the air ambient. 3. Results and discussion The conventional PEDOT:PSS (PH1000) solution presents strong acidic property with pH value of 1.5–2.5, since the presence of SO3H groups in PSS release H+ in the solution. In this work, it is neutralized by using guanidine
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aqueous solution as illustrated in Fig. 1. The guanidine in water shows strong alkalinity because the NH2 and NH groups in the guanidine absorb H+ to form NH+3 and NH+2, leaving OH in the water. When the PEDOT:PSS solution mixed with guanidine aqueous solution, the OH- neutralizes the H+. With this approach, neutral-pH PEDOT:PSS aqueous dispersion with a pH value of 7.0–7.5 was obtained. The water absorption of the neutral-pH PEDOT:PSS film can also be well suppressed since the NH+2 group in the guanidine cation binds with SO3 to reduce the hygroscopic SO3H group. Fig. 2a compares the measured optical transmittance versus sheet resistance for the AgNW mesh film, and two kinds of PEDOT:PSS and AgNW–PEDOT:PSS composite films using PH1000 and the neutral-pH PEDOT:PSS, respectively. With a PEDOT:PSS over-coating layer, the sheet resistance of composite films decreases from 120 X/h to less than 30 X/h compared to that of the pure AgNW mesh film. Although the sheet resistance of the neutral-pH PEDOT:PSS is 600 X/h, which is lower than that of PH1000 with 250 X/h, the conductivities of the formed composite films using different PEDOT:PSS are similar (29 X/h and 28 X/h, respectively), indicating that the conductivity of the composite film is dominated by the wire-to-wire junction resistance, and the neutral-pH PEDOT:PSS over-coating layer can enhance the wire-to-wire current conduction as effectively as PH1000. The transmittance of both types of composite films is also similar (83.1% and 84.8%, respectively). Fig. 2b compares the mechanical flexibility of the AgNW mesh film and the composite films using different types of PEDOT:PSS. After 1200 bending cycles, there is less than 4% increase of sheet resistance for both types of composite films, while 18% for the AgNW mesh film. Obviously, both types of PEDOT:PSS as the overcoating layer can help to improve the mechanical flexibility, which is attributed to the enhanced wire-to-wire and wire-to-substrate adhesion. The results in Fig. 2 prove that the neutral-pH PEDOT:PSS coating layer can improve both the conductivity and mechanical flexibility of the AgNW film as same as the commercial PH1000. The long term stability in air for the different conductive films is shown in Fig. 3. The ambient temperature and humidity for the test are around 20 °C and 40%, respectively. As depicted in Fig. 3, the sheet resistance of the AgNW film increases by 19% after 108 h, which is caused by oxidation of AgNWs in air [14]. With an over-coating layer of PH1000 on top of AgNW film, the stability becomes even worse with 30% increase of the sheet resistance, which is thought to be caused by water absorption and oxygen-consuming corrosion of AgNWs with the hygroscopic and acid PH1000 PEDOT:PSS [25]. The neutral-pH PEDOT:PSS film is less hygroscopic and presents much less sheet resistance increase (<10%). With the neutral-pH PEDOT:PSS as the over-coating layer, the long term air stability of the composite film is significantly improved, with a sheet resistance increase of less than 9%, similar to that of the pure neutral-pH PEDOT:PSS film. The results in Fig. 3 proves that the neutral-pH PEDOT:PSS over-coating layer can function as a protection layer to prevent AgNWs from oxidation in air ambient.
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Fig. 1. Illustration of the formation of neutral-pH PEDOT:PSS.
Transmittance (%)
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AgNW PH1000 Neutral-pH PEDOT AgNW with PH1000 AgNW/neutral-pH PEDOT
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70
60
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100
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300
400
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Sheet Resistance (Ohm/sq)
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Bending Cycles
(b) Fig. 2. (a) The measured transmittance (at k = 550 nm) versus sheet resistance and (b) the measured sheet resistance over bending cycles (with a bending radius of 5 mm as illustrated in the inset) for the AgNW mesh film, and two kinds of PEDOT:PSS and AgNW–PEDOT:PSS composite films using PH1000 and the neutral-pH PEDOT:PSS, respectively.
Further aging tests were performed for all the conductive films at different annealing temperatures for 20 min in air. Fig. 4a compares the change of sheet resistance at different annealing temperatures from 90 °C to 210 °C for different films. The sheet resistance of AgNW film starts to decrease when the temperature increases to 150 °C, which is attributed to decomposition of the residues among
Fig. 3. The measured sheet resistance over time for all the conductive films being exposed in the air ambient.
AgNWs and fusing of AgNWs at higher temperature for enhancing wire-to-wire connections [28]. However, with the acid PH1000 as the over-coating layer, the sheet resistance of the formed composite film increases rapidly with the annealing temperature while the sheet resistance of pure PH1000 film doesn’t show obvious increase. Although PH1000 has similar stability under thermal annealing to the neutral-pH PEDOT:PSS, the composite film using the neutral-pH PEDOT:PSS as the over-coating layer presents significantly improved thermal stability compared to that using PH1000. It can thus be concluded that the temperature increase accelerates the oxygen-consuming corrosion of AgNWs in the acidic environment of PH1000, while with the neutral-pH PEDOT:PSS, the process can be effectively suppressed. Fig. 4b shows the top-view SEM images of the AgNW mesh film, and the composite films using different PEDOT:PSS after annealing at 210 °C for 20 min. Corrosion of AgNWs can be clearly seen in the composite film using the acid PH1000, while fine AgNW meshes appearing in the composite film using the neutral-pH PEDOT:PSS. The results indicate that, under high temperature, the acid PEDOT:PSS can cause deteriorated stability of the composite film due to corrosion of AgNWs, and with the neutralpH PEDOT:PSS, the thermal stability of the composite film can be greatly improved.
S. Chen et al. / Organic Electronics 15 (2014) 3654–3659
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Fig. 4. (a) The change of sheet resistance for different films at different annealing temperatures from 90 °C to 210 °C for 20 min in air ambient. (b) Top view SEM images of the AgNW, AgNW/PH1000 and AgNW/neutral-pH PEDOT:PSS films after annealing at 210 °C for 20 min.
Current stressing reliability of the conductive film is also critical to be investigated for device applications [22,29]. When current passes through the AgNW mesh electrode, the local temperature could increase due to Joule heating, particularly at the wire-to-wire junctions [30]. The samples of AgNW, AgNW-PH1000, AgNW/neutral-pH PEDOT:PSS strips with width of 5 mm and length of 2 cm were prepared, as shown in Fig. 5a. A PDMS encapsulation layer on top is to reduce the effects of oxygen and water in air. Due to the different conductivities of the films, different voltages were applied on these samples to obtain the same stressing current of 30 mA/cm2 for the test. Fig. 5b shows
the current density changes over time for the different films under fixed voltages, which are associated with the conductivity changes during current stressing. For the AgNW mesh film, the conductivity gradually increases with time, which can be attributed to enhanced wire-towire connections with decomposition of the residues among AgNWs and fusing of AgNWs by local Joule heating [29]. With the PH1000 over-coating layer, the conductivity of the composite film decreases dramatically during current stressing. According to the above discussions, this is mainly caused by corrosion of AgNWs by the acid PEDOT:PSS. For the composite film using neutral-pH PED-
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Patterned conductive film
Contacts with Ag paste PDMS encapsulation
(a)
superior thermal aging stability and current stressing reliability. Under both cases of thermal aging test at 210 °C for 20 min and 12 h continuous current stressing at a current density of 30 mA/cm2, no obvious change of the conductivity is observed with the AgNW/neutral-pH PEDOT:PSS composite film. All of these findings clearly demonstrate that using the neutral-pH PEDOT:PSS as an over-coating layer can help to achieve flexible AgNW transparent conductive films with superior stability in terms of shelf life, thermal aging and current stressing for flexible optoelectronic devices. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 61274083, 61334008), 863 Program of China (Grant No. 2014AA032702), Ph.D. Programs Foundation of Ministry of Education of China (20120073110093), and Scientific Research Foundation for Returned Scholars, Ministry of Education of China. References
(b) Fig. 5. (a) Photo image of the fabricated sample for current stressing test; (b) the measured current density changes over time for different conductive films.
OT:PSS, no degradation of conductivity is observed after 12 h continuous current stressing, showing the superior operation reliability. 4. Conclusion The AgNW mesh conductive film with a PEDOT:PSS over-coating layer is promising for many flexible optoelectronic device applications. However, the commonly used PEDOT:PSS is hygroscopic and presents conductivity degradation over time in air ambient due to water absorption, and when mixed with AgNWs, its acid property can cause corrosion of AgNWs, resulting in seriously deteriorated thermal stability and current stressing reliability. In this work, neutral-pH PEDOT:PSS was obtained by modifying the acid PEDOT:PSS using guanidine, and has significantly improved air stability compared to the commercial PH1000. When being applied to form the composite conductive film, the neutral-pH PEDOT:PSS over-coating layer is able to prevent AgNWs from oxidation in air ambient, and thus air stable composite conductive films can be achieved. The composite film using neutral-pH PEDOT:PSS presents similar conductivity, transmittance and mechanical flexibility to that using commercial PH1000. Since the neutral-pH PEDOT:PSS does not have corrosive effect on AgNWs, the formed composite conductive film presents
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Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Neutral-pH PEDOT:PSS as over-coating layer for stable silver nanowire flexible transparent conductive films S. Chen a, L. Song b, Z. Tao b, X. Shao a, Y. Huang a, Q. Cui a, X. Guo a,⇑ a National Engineering Laboratory for TFT-LCD Material and Technologies, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai 200240, China b Shanghai OE Chemicals, 200232, China
a r t i c l e
i n f o
Article history: Received 18 August 2014 Received in revised form 13 September 2014 Accepted 29 September 2014 Available online 19 October 2014 Keywords: Transparent conductive film Silver nanowire PEDOT:PSS Stability
a b s t r a c t The silver nanowire (AgNW) mesh film with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the over-coating layer is a promising flexible transparent conductive film technology. In this work, experimental studies show that the hygroscopic and acid properties of the common PEDOT:PSS lead to poor stabilities of the composite films, due to the conductivity degradation of PEDOT:PSS by the water absorption and the acid corrosion of AgNWs by PEDOT:PSS. By using the modified PEDOT:PSS of neutral pH as the over-coating layer, the long term shelf-life time, thermal and current stressing stabilities are all significantly improved without sacrifice of transparency, electrical conductivity and mechanical flexibility. Under both cases of thermal aging test at 210 °C for 20 min and 12 h continuous current stressing at a current density of 30 mA/cm2, no obvious change of the conductivity is observed. The results clearly demonstrate that using the neutral-pH PEDOT:PSS as an over-coating layer can help to achieve flexible AgNW transparent conductive films with superior stability for flexible optoelectronic devices. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Rapidly growing interests in flexible optoelectronics, such as organic light emitting diodes (OLEDs), thin film solar cells and photonic sensors, demand transparent conductive films (TCFs) of excellent mechanical flexibility and low cost [1–6]. With problems of brittleness, and rising cost of indium, the commonly used indium tin oxide (ITO) is not an ideal choice [7]. The silver nanowire (AgNW) mesh is considered as a promising replacement, for its high dc conductivity and optical transmittance, excellent mechanical flexibility, capability with low cost, large area coating processes, and environmental and economic advantages for manufacturing [8-10]. However, several problems associated with AgNW mesh films need to be ⇑ Corresponding author. E-mail address: [email protected] (X. Guo). http://dx.doi.org/10.1016/j.orgel.2014.09.047 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
addressed for device applications, including rough surface, large wire-to-wire junction resistance and poor adhesion to the substrate [11,12]. The air stability is also of concern because the AgNWs are likely to be oxidized when being exposed to air [13,14]. Introducing an over-coating layer onto the AgNW mesh films appears to be an effective solution to overcome all or part of the above issues [15–26]. Among various over-coating materials, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is most commonly used because of its high transparency, good conductivity, high work function and ease for large area processing [27]. Although abundant work has been conducted on processing of flexible AgNW–PEDOT:PSS hybrid composite and device applications, there is lack of comprehensive study on the stabilities of the films, including shelf-life, thermal aging and current stressing stabilities, which are vital for device applications.
S. Chen et al. / Organic Electronics 15 (2014) 3654–3659
In this work, experimental studies show that the hygroscopic and acid properties of the normal PEDOT:PSS lead to poor stabilities of the composite films, due to the conductivity degradation of PEDOT:PSS by the water absorption and the acid corrosion of AgNWs by PEDOT:PSS [25]. By using a type of modified PEDOT:PSS with neutral pH as the over-coating layer, both the shelf-life and long term current stressing stabilities in air are significantly improved without sacrifice of transparency, electrical conductivity and mechanical flexibility. 2. Experimental The PEDOT:PSS (Clevios PH1000) used in this experiment was provided by Heraeus Clevios GmbH with a pH value of 1.5–2.5. The neutral-pH PEDOT:PSS solution with a pH value of 7.0–7.5 was obtained by adding guanidine aqueous solution into the acidic PEDOT:PSS. 5 wt% dimethyl sulfoxide (DMSO) solution was added to both PH1000 and the neutral-pH PEDOT:PSS dispersion to increase the conductivity. AgNWs supplied by Nanjing XFNANO were diluted in isopropyl alcohol (IPA) to form the AgNW dispersion with a concentration of 2 mg/ml for use. Glass or PET substrates were cleaned using detergent, deionized water, acetone, and IPA, and then treated with oxygen plasma to obtain hydrophilic surfaces for coating of AgNWs. The AgNW suspension was spin coated twice at 600 rpm for 40 s to form a continuous film on the substrate. PEDOT:PSS (PH1000 or the neutral-pH one) was then spin coated at 3000 rpm for 40 s onto the AgNW film, and then dried at 120 °C for 20 min to obtain the composite film. To investigate the reliability under current stressing, the films were patterned into strips with width of 5 mm and length of 2 cm by wiping, and then encapsulated by a polydimethylsiloxane (PDMS) layer to reduce the influence from the ambient. To form the PDMS layer, a 10:1 mixture of PDMS elastomer to cross-linker (Sylgard 184, Dow Corning) was prepared and stirred for 3 min, and coated onto the samples on glass substrate, followed by a curing process at 100 °C for 30 min. Electrical contacts were made using silver paste (SCP003, Electrolube). Specular transmission (T) spectra of the conductive films were obtained using a UV/VIS/NIR spectrophotometer (UV-3100PC, Mapada). The sheet resistance (Rs) was measured using a four point probe system (ST-2258A, Suzhou JG). The surface topography was analyzed using a scanning electron microscope (SEM) (TM3000, Hitachi). The current of the films were measured with a Keithley 4200 semiconductor characterization system. The stabilities of the films were characterized as the conductivity changes with the films being exposed in air ambient, under high temperature and current stressing in the air ambient. 3. Results and discussion The conventional PEDOT:PSS (PH1000) solution presents strong acidic property with pH value of 1.5–2.5, since the presence of SO3H groups in PSS release H+ in the solution. In this work, it is neutralized by using guanidine
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aqueous solution as illustrated in Fig. 1. The guanidine in water shows strong alkalinity because the NH2 and NH groups in the guanidine absorb H+ to form NH+3 and NH+2, leaving OH in the water. When the PEDOT:PSS solution mixed with guanidine aqueous solution, the OH- neutralizes the H+. With this approach, neutral-pH PEDOT:PSS aqueous dispersion with a pH value of 7.0–7.5 was obtained. The water absorption of the neutral-pH PEDOT:PSS film can also be well suppressed since the NH+2 group in the guanidine cation binds with SO3 to reduce the hygroscopic SO3H group. Fig. 2a compares the measured optical transmittance versus sheet resistance for the AgNW mesh film, and two kinds of PEDOT:PSS and AgNW–PEDOT:PSS composite films using PH1000 and the neutral-pH PEDOT:PSS, respectively. With a PEDOT:PSS over-coating layer, the sheet resistance of composite films decreases from 120 X/h to less than 30 X/h compared to that of the pure AgNW mesh film. Although the sheet resistance of the neutral-pH PEDOT:PSS is 600 X/h, which is lower than that of PH1000 with 250 X/h, the conductivities of the formed composite films using different PEDOT:PSS are similar (29 X/h and 28 X/h, respectively), indicating that the conductivity of the composite film is dominated by the wire-to-wire junction resistance, and the neutral-pH PEDOT:PSS over-coating layer can enhance the wire-to-wire current conduction as effectively as PH1000. The transmittance of both types of composite films is also similar (83.1% and 84.8%, respectively). Fig. 2b compares the mechanical flexibility of the AgNW mesh film and the composite films using different types of PEDOT:PSS. After 1200 bending cycles, there is less than 4% increase of sheet resistance for both types of composite films, while 18% for the AgNW mesh film. Obviously, both types of PEDOT:PSS as the overcoating layer can help to improve the mechanical flexibility, which is attributed to the enhanced wire-to-wire and wire-to-substrate adhesion. The results in Fig. 2 prove that the neutral-pH PEDOT:PSS coating layer can improve both the conductivity and mechanical flexibility of the AgNW film as same as the commercial PH1000. The long term stability in air for the different conductive films is shown in Fig. 3. The ambient temperature and humidity for the test are around 20 °C and 40%, respectively. As depicted in Fig. 3, the sheet resistance of the AgNW film increases by 19% after 108 h, which is caused by oxidation of AgNWs in air [14]. With an over-coating layer of PH1000 on top of AgNW film, the stability becomes even worse with 30% increase of the sheet resistance, which is thought to be caused by water absorption and oxygen-consuming corrosion of AgNWs with the hygroscopic and acid PH1000 PEDOT:PSS [25]. The neutral-pH PEDOT:PSS film is less hygroscopic and presents much less sheet resistance increase (<10%). With the neutral-pH PEDOT:PSS as the over-coating layer, the long term air stability of the composite film is significantly improved, with a sheet resistance increase of less than 9%, similar to that of the pure neutral-pH PEDOT:PSS film. The results in Fig. 3 proves that the neutral-pH PEDOT:PSS over-coating layer can function as a protection layer to prevent AgNWs from oxidation in air ambient.
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Fig. 1. Illustration of the formation of neutral-pH PEDOT:PSS.
Transmittance (%)
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Bending Cycles
(b) Fig. 2. (a) The measured transmittance (at k = 550 nm) versus sheet resistance and (b) the measured sheet resistance over bending cycles (with a bending radius of 5 mm as illustrated in the inset) for the AgNW mesh film, and two kinds of PEDOT:PSS and AgNW–PEDOT:PSS composite films using PH1000 and the neutral-pH PEDOT:PSS, respectively.
Further aging tests were performed for all the conductive films at different annealing temperatures for 20 min in air. Fig. 4a compares the change of sheet resistance at different annealing temperatures from 90 °C to 210 °C for different films. The sheet resistance of AgNW film starts to decrease when the temperature increases to 150 °C, which is attributed to decomposition of the residues among
Fig. 3. The measured sheet resistance over time for all the conductive films being exposed in the air ambient.
AgNWs and fusing of AgNWs at higher temperature for enhancing wire-to-wire connections [28]. However, with the acid PH1000 as the over-coating layer, the sheet resistance of the formed composite film increases rapidly with the annealing temperature while the sheet resistance of pure PH1000 film doesn’t show obvious increase. Although PH1000 has similar stability under thermal annealing to the neutral-pH PEDOT:PSS, the composite film using the neutral-pH PEDOT:PSS as the over-coating layer presents significantly improved thermal stability compared to that using PH1000. It can thus be concluded that the temperature increase accelerates the oxygen-consuming corrosion of AgNWs in the acidic environment of PH1000, while with the neutral-pH PEDOT:PSS, the process can be effectively suppressed. Fig. 4b shows the top-view SEM images of the AgNW mesh film, and the composite films using different PEDOT:PSS after annealing at 210 °C for 20 min. Corrosion of AgNWs can be clearly seen in the composite film using the acid PH1000, while fine AgNW meshes appearing in the composite film using the neutral-pH PEDOT:PSS. The results indicate that, under high temperature, the acid PEDOT:PSS can cause deteriorated stability of the composite film due to corrosion of AgNWs, and with the neutralpH PEDOT:PSS, the thermal stability of the composite film can be greatly improved.
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Fig. 4. (a) The change of sheet resistance for different films at different annealing temperatures from 90 °C to 210 °C for 20 min in air ambient. (b) Top view SEM images of the AgNW, AgNW/PH1000 and AgNW/neutral-pH PEDOT:PSS films after annealing at 210 °C for 20 min.
Current stressing reliability of the conductive film is also critical to be investigated for device applications [22,29]. When current passes through the AgNW mesh electrode, the local temperature could increase due to Joule heating, particularly at the wire-to-wire junctions [30]. The samples of AgNW, AgNW-PH1000, AgNW/neutral-pH PEDOT:PSS strips with width of 5 mm and length of 2 cm were prepared, as shown in Fig. 5a. A PDMS encapsulation layer on top is to reduce the effects of oxygen and water in air. Due to the different conductivities of the films, different voltages were applied on these samples to obtain the same stressing current of 30 mA/cm2 for the test. Fig. 5b shows
the current density changes over time for the different films under fixed voltages, which are associated with the conductivity changes during current stressing. For the AgNW mesh film, the conductivity gradually increases with time, which can be attributed to enhanced wire-towire connections with decomposition of the residues among AgNWs and fusing of AgNWs by local Joule heating [29]. With the PH1000 over-coating layer, the conductivity of the composite film decreases dramatically during current stressing. According to the above discussions, this is mainly caused by corrosion of AgNWs by the acid PEDOT:PSS. For the composite film using neutral-pH PED-
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Patterned conductive film
Contacts with Ag paste PDMS encapsulation
(a)
superior thermal aging stability and current stressing reliability. Under both cases of thermal aging test at 210 °C for 20 min and 12 h continuous current stressing at a current density of 30 mA/cm2, no obvious change of the conductivity is observed with the AgNW/neutral-pH PEDOT:PSS composite film. All of these findings clearly demonstrate that using the neutral-pH PEDOT:PSS as an over-coating layer can help to achieve flexible AgNW transparent conductive films with superior stability in terms of shelf life, thermal aging and current stressing for flexible optoelectronic devices. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 61274083, 61334008), 863 Program of China (Grant No. 2014AA032702), Ph.D. Programs Foundation of Ministry of Education of China (20120073110093), and Scientific Research Foundation for Returned Scholars, Ministry of Education of China. References
(b) Fig. 5. (a) Photo image of the fabricated sample for current stressing test; (b) the measured current density changes over time for different conductive films.
OT:PSS, no degradation of conductivity is observed after 12 h continuous current stressing, showing the superior operation reliability. 4. Conclusion The AgNW mesh conductive film with a PEDOT:PSS over-coating layer is promising for many flexible optoelectronic device applications. However, the commonly used PEDOT:PSS is hygroscopic and presents conductivity degradation over time in air ambient due to water absorption, and when mixed with AgNWs, its acid property can cause corrosion of AgNWs, resulting in seriously deteriorated thermal stability and current stressing reliability. In this work, neutral-pH PEDOT:PSS was obtained by modifying the acid PEDOT:PSS using guanidine, and has significantly improved air stability compared to the commercial PH1000. When being applied to form the composite conductive film, the neutral-pH PEDOT:PSS over-coating layer is able to prevent AgNWs from oxidation in air ambient, and thus air stable composite conductive films can be achieved. The composite film using neutral-pH PEDOT:PSS presents similar conductivity, transmittance and mechanical flexibility to that using commercial PH1000. Since the neutral-pH PEDOT:PSS does not have corrosive effect on AgNWs, the formed composite conductive film presents
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