High-Performance Transparent and Flexible Electrodes Made by

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High-performance transparent and flexible electrodes made by flash-light sintering of gold nanoparticles Renyun Zhang, Magnus Engholm, Magnus Hummelgård, Henrik Andersson, Jonas Örtegren, and Håkan Olin ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01649 • Publication Date (Web): 30 Nov 2018 Downloaded from http://pubs.acs.org on December 10, 2018

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ACS Applied Energy Materials

High-performance transparent and flexible electrodes made by flash-light sintering of gold nanoparticles

Renyun Zhang,†,* Magnus Engholm, ‡ Magnus Hummelgård, † Henrik Andersson,‡ Jonas Örtegren, † Håkan Olin†

†

Department of Natural Sciences, Mid Sweden University, Holmgatan 10, SE-85170 Sundsvall,

Sweden ‡

Department of Electronics Design, Mid Sweden University, Holmgatan 10, SE-85170

Sundsvall, Sweden *Email: [email protected] KEYWORDS: transparent electrode, flexible electrode, gold nanoparticles, flash-light sintering, high performance

ABSTRACT

ACS Paragon Plus Environment

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Metallic nanowire-based transparent electrodes (TEs) are potential alternatives to indium tin oxide (ITO). To achieve a high performance [sheet resistance (Rs) 90%], the nanowires must have a high length-to-diameter (L/D) ratio to minimize the number of wire-to-wire junctions. Attempts to produce TEs with gold nanowires have been made, and the results reveal difficulties in achieving the requirements. A successful strategy involves creating templated gold nanonetworks through multiple procedures. Here, we present a simple and efficient method that uses flash-light sintering of a gold nanonetwork film into gold TEs (Rs: 82.9 Ω/sq, T%: 91.79%) on a thin polycarbonate film (25 µm). The produced gold TEs have excellent mechanical, electrical, optical and chemical stabilities. Mechanisms of the formation of gold nanonetworks and the effect of flash-light have been analyzed. Our findings provide a scalable process for producing transparent and flexible gold electrodes with a total processing time of less than 8 min without the use of heating, vacuum processing, organic chemicals and without any material loss. This is possible because all the gold nanoparticles have been aggregated and filtrated on the filter membranes. The area density of gold is 0.094 g/m2 leading low material costs, which is very competitive with the price of commercial TEs.

Transparent electrodes (TEs) are one of the essential components used for various optoelectronic devices, such as displays and solar cells1. New kinds of devices like electronic skins2 and nanogenerators3–5 are also an emerging application area TEs. Transparent conductive oxides such as ITO have been used in these devices for a very long time. However, the brittleness6,7 of the ITO film limits its application in modern electronics where flexibility is required. Promising alternatives to ITO are porous metallic structures such as nanowire networks8 and nanomeshes9.


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Silver10–12 and copper13–15 are the most studied metals due to the easy synthesis of ultra-long nanowires with a high L/D ratio16–18, which can greatly reduce the number of wire-to-wire junctions. However, some issues, such as the instability of the metal in air and harsh environments6, limit their applications. In contrast to silver and copper, gold does not suffer from these issues. However, TEs made of gold nanowires1,19–21 are less frequently reported due to either the difficulties in synthesizing nanowires with high L/D ratios or the coalescence of the thin nanowires, which limits the electrical conductivity19. Instead of making gold nanowire networks, evaporating gold on fibre or polymer templates to form gold nanotroughs7,22 or nanomeshes23,24 is a very successful method for producing gold TEs. Recently, a direct printing method was also developed to print metallic grids that have a low sheet resistance at high transmission25. Here, we report a new, efficient method to produce gold TEs with excellent mechanical stability by flash-light processing gold nanoparticles films into nanoetworks on a polycarbonate membrane. The total processing time from chemical solution to the final product is less than 8 min, where the key procedure for creating the TE takes several milliseconds, much faster than other methods. This method requires no heating or processing in vacuum. In addition, there is no loss of material because all gold nanoparticles have been aggregated and filtered on the filter membrane

Results Fabrication and characterization of the gold TEs The process for producing gold nanonetwork TEs (Figure 1a) is based on the light-induced sintering of aggregated gold nanoparticles. Gold nanoparticles were first synthesized by reducing a solution of Au3+ with NaBH4. The reaction was performed for 3, 5, 10 and 30 s before the addition of NaOH to induce aggregation of the gold nanoparticles. The aggregated nanoparticles were


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deposited on a polycarbonate membrane (0.1 µm pore size) by filtration followed by flash-light sintering13,26 (FLS). The FLS process sintered the gold nanoparticles into nanowires and at the same time condensed the polycarbonate membrane into a transparent film. The filtered gold nanoparticles formed a network on the polycarbonate film (Figure 1b, f) and were converted to a gold nanonetwork after flash-light processing (Fig 1c, d, e, g). The SEM images show that the gold nanonetwork was not evenly distributed on the membrane, which is due to the uneven distribution of the holes on the membrane. An SEM image of the polycarbonate film has been given in the supporting information (Figure S1). The energy from the flash-light tube was absorbed by the gold nanoparticles, resulting in fast sintering of adjacent nanoparticles, which usually takes a much longer time27–29. The short pulse duration of the flash prevented the formation of bigger gold nanoparticles as indicated by the microscopic images. The flash-light process also condensed the porous polycarbonate membrane (the pores on the membrane are highlighted by arrows in Figure 1b) into a solid transparent film, as indicated by the disappearance of the pores on the front side (Figure 1c, d). However, if one looks at the back side of the polycarbonate membrane, there are some holes that had not been fully filled (Figure S2 in supporting information). Anyway, the polycarbonate membrane became transparent (Figure 1h, i) after the FLS. The mechanisms of the condensing process are discussed below.


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Figure 1. Fabrication of gold TEs. (a) Processes for fabricating gold TEs, (b) An SEM image of filtered gold nanoparticle aggregates on a polycarbonate membrane. The red arrows indicate the holes on the filter membrane. Scale bar: 2 µm. (c) An SEM image of a gold TE after flash-light processing. Scale bar: 2 µm. (d) and (e) Higher magnification SEM images of gold TEs after flashlight processing captured in SE and BSE imaging mode. Scale bar: 1 µm. (f) and (g) TEM images of gold nanoparticle aggregates before and after flash-light processing. (h) A photograph showing the gold nanoparticle aggregates deposited on the polycarbonate membrane without and with flashlight processing. (i) A photograph of a fully processed gold TE.


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Figure 2 Performance of the gold TEs. (a) Plots of the transmission at 550 nm and sheet resistance versus volume of the filtered gold suspension. (b) Transmission of gold TEs produced with different volumes of the filtered gold suspension. (c) Plot of the transmission at 550 nm versus sheet resistance of the gold TEs. The red dashed line is a fit of the experimental data using equation (1). (d) Comparison of the performance of our gold TE with that of other reported gold TEs.

Electrical and optical properties of the gold TEs The optical and electrical properties of the gold TEs could be tuned by adjusting the volume of gold nanoparticle suspension filtered through the polycarbonate membrane. We found that both the transmittance and the sheet resistance of the gold TEs was non-linearly related to the volume of gold nanoparticle suspension filtered (Figure 2a). This result may be related to the size of the


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gold nanoparticle aggregates. A larger volume of suspension requires a longer time for filtration, providing a longer time for the aggregates to grow larger. Figure 2b shows the optical transmittance of gold TEs obtained from the filtration of 20, 25, 30 and 35 ml of the gold nanoparticle suspension. As observed, the transmittance decreases as the volume of the suspension increases. The results indicate that the transmission was quite constant in the measured wavelength range of 400~900 nm. Figure 2c presents the transmittance at 550 nm versus the sheet resistance of the gold TEs. The percolative figure of merit () was determined to be 87 by fitting the experimental data with equation

[

𝑇= 1+

()

1 𝑍0 Π 𝑅𝑠

1 𝑛+1

]

―2

(1)

where T is the trans at 550 nm, Z0 is the impedance of free space (377 Ω), Rs is the sheet resistance, and n is the percolation exponent. The value of  is higher than the median value of 31.7 for metallic nanowire networks6, while it is lower than the value of 365 for an 80 nm gold nanotrough7. The percolation exponent was 0.001, which is lower than other reported values6,7, indicating a very narrow distribution of the inter-junction resistance30. Haze is an important parameter for metal nanowire-based TEs since the nanowires scatter light. In our case, the values of the haze of the gold TEs were very low due to the small size of the gold nanowires coalesced from gold nanoparticles. The measured haze values of the gold TEs including the substrates at 550 nm were 1%, 0.8%, 1.7% and 2.2%, as presented in Figure 2c. Such values are very low compared to those of other gold TEs.


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We also studied the flexibility of the gold TEs. The results indicate no significant change in the conductivity after 400 bending cycles (Figure 3a) at a radius of 1.5 mm, showing an excellent flexibility. We further folded the gold TE and firmly pressed the crease with a finger. However, no significant change in conductivity was observed (Figure 3b) after folding the TE 4 times and pressing the crease at the same place.

Figure 3. Mechanical stability of the gold TEs. (a) Resistance change of a TE during 400 bending cycles at a radius of 1.5 mm. (b) Resistance change of a TE before and after 3 folding cycles. (c) Resistance change of a TE before and after scratching the surface 10 times with a spent ball-point pen. (d) Resistance change of a TE before and after scratching the surface 3 times with a stylus for tablets. (e) Resistance change of a TE before and after 40 cycles of peeling scotch tape from the surface. (f) I/V curves of a TE before and after 3 crushing cycles.


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The adhesion of the gold nanonetwork to the polycarbonate membrane was also evaluated by performing two different tests. The first one is to scratch the gold TEs with a spent ball-point pen and a stylus for tablets. The results are presented in Figure 3c, where the resistance of the gold nanonetwork between the electrodes was monitored in real time during scratching with a spent ball-point pen. A resistance change of less than 5% was observed after scratching 10 times with a force similar to that used for regular writing. However, when a larger force was used, the resistance increased by over 10%. The change in resistance recovered to approximately 5% after 3 min of relaxation, although repeated scratching with a large force led to a permanent reduction in conductivity. Following each scratch with a stylus for tablets, the resistance increased by approximately 2% (Figure 3d) due to the high amount of friction created. The second test is to peel scotch tape off the gold TEs, and the results indicate that there was no significant change in the resistance after 40 peeling cycles (Figure 3e). We also tested the mechanical stability of the gold TEs under extreme conditions like crushing. Figure 3f shows the source/drain (I/V) measurement results of a TE before and after 3 cycles of crushing and unfolding. The results indicate that the resistance increased from 81 Ω to 147 Ω after 3 cycles of crushing and unfolding. This performance is better than that of the gold nanotroughs7, where the resistance increased from 73 Ω to 137 Ω after 1 crushing and unfolding cycle.

Stabilities of the gold TEs. Mechanical stability. We tested the mechanical stabilities of the gold TEs by measuring the electrical properties after bending, folding, scratching, peeling and crushing. The results of all these tests indicated that the gold TEs had excellent mechanical stabilities. Such excellent stabilities are based on strong adhesion between the gold nanonetwork and the polycarbonate


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membrane as well as the thinness of the polycarbonate membrane. Strong adhesion was created by flash-light processing, where the light heated the gold nanoparticles and increased the temperature to over 170 °C, leading to melting of the polycarbonate in contact with the gold nanoparticles. This process created very good contact between these two materials, resulting in strong adhesion.

Figure 4. Stability of a gold TE. (a) Electrical stability of a gold TE. The data show the sheet resistance of a gold TE right after it was produced and after 30 days. The average data were calculated from 10 values measured at 10 random places on the gold TE. (b) Chemical stability of a TE. The TE was treated with H2O, NaOH (10 M), H2SO4 (9.2 M) and ethanol (99.7%) for 3 min each. The liquids were dropped on a gold TE belt covering the edges, and the resistance changes were measured using a multimeter.

Electrical stability. One of the problems associated with thin gold nanowire (< 5 nm)-based TEs is that the electrical properties are unstable due to the coalescence of gold. However, such a problem did not occur with our gold TEs. The reason is that the strong adhesion between the gold and polycarbonate eliminated coalescence of the gold. Figure 4a shows the measured sheet


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resistance of a gold TE at day 0 and day 30, indicating no loss of conductivity. Such electrical stability is very important for applying gold TEs in real electronics. The as-deposited gold nanoparticle aggregate film with the same conditions but before flash-light sintering had a sheet resistance of 1411.9 Ω/sq; however, the conductivity was lost 5 s after the bias was applied. Such behavior is due to the loss of contact between the gold nanoparticles. Chemical stability. Chemical stability is another concern for metal-based TEs since most of the metals are not stable upon exposure to humidity and acids. Gold is the best choice because it has the lowest chemical activity. We exposed the gold TEs to water, NaOH (10 M), H2SO4 (9.2 M) and ethanol (99.7%) for 3 min each and measured the resistance change between two electrodes. The results (Figure 4b) indicate that the electrical properties of the gold TE were stable under these conditions.

Discussion Mechanisms. The most important mechanism of our method is that the gold nanoparticles forms long chains after the addition of NaOH. The addition of NaOH increases the ionic strength in the gold nanoparticle solution, causing the decrease of Debye length (-1, Figure 5a) and thus the decrease of the electrostatic repulsion potential (Velec)31. According to DLVO (Derjaguin–Landau– Verwey–Overbeek) theory, the interaction potential energy is the sum of Velec and Waals attraction potential (VvdW). When the Velec is lower than the VvdW, the gold nanoparticles start to aggregate. The Velec and the VvdW at gold nanoparticles center-to-center separation of R can be calculated with equations31: 𝑉𝑒𝑙𝑒𝑐(𝑑) = 2𝜋𝜀𝑠𝜀0𝑟Ψ20ln {1 + 𝑒𝑥𝑝 [ ―𝑟𝜅(𝑅 ― 2)]}

(2)


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𝑅2 ― 4 𝑉𝑣𝑑𝑊(𝑑) = ― [ 2 + + ln ] 6 𝑅 ― 4 𝑅2 𝑅2 𝐴𝐻

2

2

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(3)

Herein, r is the radius of the gold nanoparticles, R=d/r, s is the dielectric constant of the solvent that is water in this case, 0 is dielectric constant of vacuum,  is the inverse Debye length, 0 is the surface potential of the nanoparticles, AH is the Hamaker constant of the gold nanoparticles (about 4 x 10-19 J). Figure 5b shows the plot of the VvdW, the Velec and the interaction potential (Vtotal) versus R. The plotting results (Figure 5b) indicate that at this experimental condition the gold nanoparticles intend to aggregate. When two gold nanoparticles get coupled, the repulsion potential changed, leading to higher potential at the end (Vend) than on the sides (Vside) that can be predicted by equations31: 𝑉𝑒𝑛𝑑 = 𝜌𝐴ln 2𝑛𝑟

(4)

𝑉𝑠𝑖𝑑𝑒 = 2𝜌𝐴ln 𝑛𝑟

(5)

where  is the charge density, A is the surface area, r is the radius of the gold nanoparticles, n is the number of the nanoparticles. The result of the potential change is that other gold nanoparticles will attach to the ends of the dimer formed by the first two nanoparticles, forming linear trimers. The more nanoparticles attached to the end, the bigger difference between the Vend and the Vside (Figure 5c), resulting in a gold nanoparticle chain. Figure 5d shows the plot of the Vside/Vend versus the number of the 4 nm gold nanoparticles that used in our experiments, indicating the increasing potential difference.

The theoretical model mentioned above predict the growth of gold nanoparticles chains. However, in the real case, the gold nanoparticles will grow into network structures (Figure 1f) instead of the


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single chains. The reason is that the gold nanoparticles do not have perfect spherical shapes, which will lead to different Vend and Vside values that changes the position of the next adding nanoparticle.

Figure 5. The growth of gold nanonetworks. (a) Change of Debye length of gold nanoparticles due to the addition of NaOH, (b) Plots of the DLVO interaction potential energy of gold nanoparticles. (c) the ratio between the Vside and the Vend versus number of gold nanoparticles. (d) schematic drawing of the growth of gold nanoparticles chain when Vside > Vend.

Fabrication process. One of the key process steps in the fabrication of the gold TEs is the NaOH-induced aggregation of gold nanoparticles. The as-synthesized gold nanoparticles possessed an absorption band at approximately 500 nm (Figure 6a). However, the aggregated gold nanoparticles exhibited a wide absorption band (Figure 6a) that covers almost the entire visible spectral range. This wide spectral range resulted in efficient light absorption, which heated the


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gold nanoparticles in milliseconds and led to their sintering. The size of the gold nanoparticles was approximately 4 nm, while the addition of NaOH created aggregates with sizes ranging from 100 to 600 nm (Figure 6b). The addition of NaOH also changed the surface potential of the gold nanoparticles from a narrow range of approximately -10 mV to a broad range with a peak at approximately -20 mV (Figure 6c). The change in the surface potential led to the growth of the gold nanoparticle networks, which was beneficial for the coalescence process induced by flashlight irradiation. To produce high-performance gold TEs, it is very important to add the NaOH at the right time. The method for synthesizing gold nanoparticles was very simple, where 40 µM (final concentration) NaBH4 was added into a 17.8 µM HAuCl4 aqueous solution while stirring at 600 rpm. An NaOH solution was then added 3 s, 5 s, 10 s, or 30 s after the addition of NaBH4. Interestingly, the time delay significantly influenced the performance of the gold TEs. Figure 7a shows a plot of the transmittance at 550 nm as a function of the sheet resistance for all produced gold TEs. By fitting the data using equation (1), we found that the percolative figure of merit decreased as the waiting time increased. At waiting times of 3 s and 5 s, gold TEs with sheet resistances lower than 100 Ω/sq at 90% transmission were produced, in which values of 82.9 Ω/sq and 91.79% were obtained for the 3-s delay time and of 89.4 Ω/sq and 90.4% for the 5-s delay time.


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Figure 6. Characterization of the synthesized gold nanoparticles. (a) UV-Vis absorption of the gold nanoparticles before and after the addition of NaOH. The inset shows photographs of the suspension before and after the addition of NaOH. (b) The size distribution of the gold nanoparticles and the gold nanoparticle aggregates after the addition of NaOH. (c) Surface Zeta potential of the gold nanoparticles and the gold nanoparticle aggregates after the addition of NaOH.

Figure 7. Gold TEs produced with different waiting times. (a) T% at 550 nm versus sheet resistance for TEs produced with different waiting times. The  values were obtained by fitting the experimental data with equation (1). (b) The absorbances of the gold nanoparticle suspension at different reaction times.


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Since the reactions happens very fast, it is not easy to characterize the nanoparticles. We designed a monitoring system to observe the absorption of the gold nanoparticles during the reactions. The results show that the reaction happens very fast, and no obvious shift in the absorption peaks was found. However, the absorption bands became slightly sharper as the reaction time increased (Figure 7b), indicating that the gold nanoparticles became larger32. The nanoparticle size measured after 30 s of reaction using DLS methods was approximately 4 nm (Figure 6b); thus, the nanoparticle sizes at 3 s and 5 s were smaller than that. Transmittance of the TEs. Analysis of an SEM image (87 µm2) indicated that the area density of the gold nanonetwork was 37% for a TE having 90% transmission. Therefore, the none-gold area contributed 63% transmission, and the gold nanonetwork itself had 27% transmission. The total transmission (T%) of a gold TE can be expressed by 𝑇% = 𝑇𝐸% + 𝑇𝐴𝑢% = 100% ∙ 𝐴𝐸 + 𝑇𝑡% ∙ 𝐴𝐴𝑢

(6)

where TE is the transmission of the empty area, TAu is the transmission of the gold nanonetwork, AE is the area percentage of empty space, Tt is the transmission of the gold film with a thickness of t, and AAu is the area percentage of the gold nanonetwork. The average size of the gold nanowires in the nanonetwork is approximately 8 nm as measured with TEM. Such nanowires have approximately 75% transparency33 to visible light. Thus, the nanonetwork could contribute 27.75% transmission to the gold TEs. By plus the 63% transmission of the empty area, the total transmission is 90.75% that is very close to the measured value (90%). Temperature rise on the gold nanoparticles. The flash-light irradiation was absorbed by the gold nanoparticles, resulting in coalescence and the formation of a gold nanonetwork. The formation of a gold nanonetwork could be realized in less than 1 millisecond. However, at least


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three consecutive flashes were required to produce gold TEs with good conductivity. The heated gold nanonetwork further heated the polycarbonate membrane and led to condensation of the porous membrane, resulting in a transparent film. One can also create a transparent polycarbonate film by placing the membrane in an oven at 170 °C for 10 min, which is a much longer time than that required for flash-light processing. Thus, the temperature induced on the gold nanonetwork should be much higher than 170 °C to cause such changes in a short time.

It is known that gold nanoparticles can be heated by light through plasmonic heating34,35, and the temperature of the gold nanoparticles is decided by their size, shape and interparticle distance36. In most cases, the temperature changes induced by gold nanoparticles are simulated on single nanoparticles irradiated by lasers. Neumann and co-workers simulated the temperature change (∆T) of water surrounding gold nanoparticles using

𝑉𝑁𝑃𝑃𝑎𝑏𝑠

(7)

∆𝑇 = 4𝜋𝑘0𝑅𝑁𝑃

where VNP is the volume of the nanoparticle, Pabs is the local light-induced heating of the nanoparticle, k0 is the thermal conductivity of water, and RNP is the radius of the gold nanoparticle. We calculated ∆T in our system (Pabs is 8.1  1015 W/m3), in which the gold was surrounded by air. The value we obtained was 0.44 °C, which is far below the real value. Such an error may be caused by neglecting the influence of the interparticle distance. Mezeme and Brosseau36 showed that the temperature change of gold is exponentially related to the distance between gold nanoparticles. In our case, the gold nanoparticles are in direct contact with each other; thus, equation (3) is not suitable for our case.


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Harris and co-workers37 proposed an equation to predict ∆T based on the heat transfer coefficient, h. 𝑃

∆𝑇 = ℎ4𝜋𝑟2

𝑝

(8)

In this equation, P is the heat transfer rate, h is the heat transfer coefficient, and rp is the radius of the gold nanoparticles. P can be calculated by38 𝑃 = 𝐼𝐶𝑎𝑏𝑠

(9)

where I is the light intensity and Cabs is the particle absorption cross-section. The uncertainty in this model is the value of h since there is no known data that apply on the nanoscale. A value of 10 000 (W/m2)/K is suggested37 for a short heating time, which would apply to our case. Taking such a value and the calculated P value of 1.91  10-10 W, we obtained a ∆T value of 396 K. This number gives a nanoparticle temperature of 421 °C, which can lead to condensation of the porous polycarbonate membrane. However, due to the uncertainty in h, the calculated data might not represent the real value. Flash-light. The flash-light process is the key factor in this method. The flash head we used here is a Profoto D1 1000, which has a circle-shaped flash tube. Such a flash tube created an uneven distribution of light power, where the power was higher in the centre. For this reason, it was better to adjust the power to as low a value as possible and perform multiple flashes to avoid the ablation of gold at the centre of the polycarbonate film. A solution to this problem is to use light sources that can distribute light evenly. Besides, the selection of the light source could be optimized in the future. Kim’s group has demonstrated that the combination of Xenon light and UV light can produce silver nanowire39, or silver nanowire/graphene40 electrode with better performances. Moreover, one can also tune the wavelength41 of the flash light to achieve better performance.


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In summary, we report here a simple and efficient method for fabricating gold TEs from gold nanoparticles. With the help of flash-light processing, the gold nanoparticle film can be turned in gold TEs in milliseconds. Such a method produces TEs with excellent mechanical and electrical stabilities. The simplicity of the method has great potential to be scaled up for industrial production.

Materials and Methods Preparation of gold nanoparticles. All chemicals were purchased from Sigma-Aldrich and used without further purification. Gold nanoparticles42 were synthesized through the reaction of HAuCl4 and NaBH4. Briefly, 15 µl of HAuCl4 (0.025 M) was added into 20 ml of Milli-Q water while stirring, followed by the addition of 4 µl of freshly prepared NaBH4 (0.2 M) in an ice bath. To aggregate the gold nanoparticles, 0.2 ml of NaOH (10 M) was added after 3, 5, 10 and 30 s. The suspension was stirred for 30 s before filtration.

Flash-light processing. Before flash-light processing, different amounts of gold nanoparticle suspensions were filtered through polycarbonate membranes (VCTP04700, MilliPore). After filtration, the membranes were fixed on a metal frame to reduce shrinkage of the membrane during flash-light processing. Flash-light processing was done by flashing the membrane with a D1 Profoto Studio head. All membranes were flashed two consecutive times at an energy of 33.7 J with a duration (t0.5) of 1/1300 s followed by another flash at 66.4 J with a duration (t0.5) of 1/1500 s. The pulse energy of each setting on the flash tube was measured with a pyroelectric energy sensor (ES245C) from Thorlabs.


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Characterization of the TEs. SEM was done on a Maia3 (TESCAN) microscope. TEM was done on a JEOL 2000FX (JEOL) microscope. Electrical measurements were done using a PXI4132 source measure unit card (NATIONAL INSTRUMENT). The sheet resistances were measured on a KEITHLEY 2602B source measurement unit. The transmissions were measured using a CCD spectrometer (EO Edmund). Optical measurements (transmittance and absorbance) were performed with an experimental setup consisting of a fibre-coupled Xenon light source (HPX-2000) and a UV/Vis spectrometer (USB 2000+) from Ocean Optics. A transmission spectrum was acquired every 250 ms during the synthesis of the Au nanoparticles and aggregation to Au networks in solution under magnetic stirring. The haze of the TEs was measured by using an integrating sphere (Avantes AvaSphere-50) in the transmission setup. A small piece (5 x 5 mm) of each TE was placed at the entrance of the integrating sphere, enabling measurement of the total transmittance, Tt, and sample diffusion, Ds. The haze, H, was calculated according to H = (Ds – Di)/Tt, where Di is the instrument diffusion.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions R.Z. conceived the idea and performed the experiments, R.Z. and M.E. Performed the optical measurements, M.H. and H.A. supported with the electrical measurements, R.Z., M.E., M.H., H.A., J.Ö., and H. O. discussed the results, and R.Z. and M.E. wrote the manuscript.


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Funding Sources This work is supported by the Knowledge Foundation, Tillväxtverket, Landstinget Västernorrland, J. Gust Richert stiftelse, and the European regional development foundation.

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SYNOPSIS


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High-Performance Transparent and Flexible Electrodes Made by

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