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Solution-Processable transparent conducting electrodes via the self-assembly of silver nanowires for organic photovoltaic devices
Accepted Manuscript Solution-Processable Transparent Conducting Electrodes via the Self-Assembly of Silver Nanowires for Organic Photovoltaic Devices B. Tugba Camic, Hee Jeong Shin, M. Hasan Aslan, Fevzihan Basarir, Hyosung Choi PII: DOI: Reference:
S0021-9797(17)31150-5 https://doi.org/10.1016/j.jcis.2017.09.112 YJCIS 22864
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
8 August 2017 27 September 2017 29 September 2017
Please cite this article as: B. Tugba Camic, H. Jeong Shin, M. Hasan Aslan, F. Basarir, H. Choi, Solution-Processable Transparent Conducting Electrodes via the Self-Assembly of Silver Nanowires for Organic Photovoltaic Devices, Journal of Colloid and Interface Science (2017), doi: https://doi.org/10.1016/j.jcis.2017.09.112
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Solution-Processable Transparent Conducting Electrodes via the SelfAssembly of Silver Nanowires for Organic Photovoltaic Devices
B. Tugba Camic1,2, Hee Jeong Shin3, M. Hasan Aslan1, Fevzihan Basarir4 and Hyosung Choi*3
1
2
3
Department of Physics, Gebze Technical University, 41400 Gebze, Kocaeli, Turkey
Sabanci University Nanotechnology Research and Application Center (SUNUM), 34956 Tuzla, İstanbul, Turkey
Department of Chemistry and Research Institute for Natural Sciences, Hanyang University, 04763 Seoul, South Korea 4
Next Chemicals & Plastics, 34490 Ikitelli, Istanbul, Turkey
*Corresponding Author Prof. Hyosung Choi Address: 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, South Korea E-mail address: [email protected] Tel: +82-2-2220-2619 Fax: +82-2-2298-0319
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Abstract Solution-processed transparent conducting electrodes (TCEs) were fabricated via the selfassembly deposition of silver nanowires (Ag NWs). Glass substrates modified with (3aminopropyl)triethoxysilane (APTES) and (3-mercaptopropyl)trimethoxysilane (MPTES) were coated with Ag NWs for various deposition times, leading to three different Ag NWs samples (APTES-Ag NWs (PVP), MPTES-Ag NWs (PVP), and APTES-Ag NWs (COOH)). Controlling the deposition time produced Ag NWs monolayer thin films with different optical transmittance and sheet resistance. Post-annealing treatment improved their electrical conductivity. The Ag NWs films were sucessfully characterized using UV–Vis spectroscopy, field emission scanning electron microscopy, optical microscopy and four-point probe. Three Ag NWs films exhibited low sheet resistance of 4-19 Ω/sq and high optical transmittance of 65-81% (at 550 nm), which are comparable to those of commercial ITO electrode. We fabricated an organic photovoltaic device by using Ag NWs as the anode instead of ITO electrode, and optimized device with Ag NWs exhibited power conversion efficiency of 1.72%.
Keywords: Silver nanowires, transparent conducting electrode, self-assembly deposition, organic photovoltaics
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1. Introduction Transparent
conducting electrodes (TCEs) are critical components in various
optoelectronic devices such as touch screens [1], liquid crystal displays [2], organic light emitting diodes [3], and solar cells [4]. Although indium tin oxide (ITO) has been widely used for TCEs because of its low sheet resistance and high transmittance at visible wavelengths, ITO has many disadvantages. ITO requires a sputtering technique for deposition, as it is a brittle material, and reserves of indium are limited. Therefore, the development of solutionprocessable indium-free TCEs are essential for the application of printing techniques and the fabrication of low-cost and flexible optoelectronic devices [5]. Recent research has suggested various alternatives to conventional ITO, such as silver nanowires (Ag NWs) [6], carbon nanotubes (CNTs) [7], graphene [8], and conducting polymers [9]. Among these materials, Ag NWs are regarded as the most promising electrode material due to their exceptional solution compatibility, high transparency, and low sheet resistance [10] [11]. However, the surface roughness of Ag NWs is a significant drawback, leading to short circuits and high sheet resistance. Another drawback of Ag NWs is their poor adhesion with the substrate. Although these problems limit the application of Ag NWs to optoelectronic devices, devices based on Ag NW electrodes exhibit a comparable performance to ITO-based devices. On the other hand, devices based on CNTs and graphene electrodes both exhibit lower device efficiencies than ITO-based devices. [12]. There are a variety of methods to produce Ag NWs ,including spin coating [13], spray coating [14], brush painting [15], Meyer-rod-coating [16], drop-casting [17], and dry transferring [18]. These solution processes are all cost competitive with conventional vacuum deposition for ITO production [19] and produce an electrode with low sheet resistance and high transmittance. However, it is difficult to obtain a uniform network of Ag NWs over a large area using drop-casting, Meyer-rod-coating, spray-coating or dry transferring methods.
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This creates a challenge for large-area device fabrication [20]. Although those coating methods can produce large-scale and uniform nanowire networks, they generate large amounts of waste material after coating, which is a major problem in terms of cost effectiveness. Self-assembled coatings are convenient for controlling the degree of nanostructure coverage. This method is easily tuned to film thickness, porosity, or electrical and optical properties. In addition, self-assembled molecules provide excellent adhesion between the nanostructure and substrate. In this work, Ag NW-based TCEs were successfully developed via a self-assembling coating method. The Ag NWs were functionalized with 3-mercaptopropionic acid (MPA) to obtain the negatively-charged Ag NWs (COOH). Then, Ag NWs were coated onto the glass substrates
modified
with
3-aminopropyltriethoxysilane
(APTES)
and
(3-
Mercaptopropyl)trimethoxysilane (MPTES) via self-assembly. The distribution and density of Ag NWs were studied as a function of immersion time of the modified substrate in the Ag NW solution. We characterized the Ag NW samples using UV–Vis spectrometry, field emission scanning electron microscopy (FE-SEM), optical microscopy (OM) and the fourpoint probe method.
2. MATERIALS AND METHODS 2.1. Materials (3-Aminoproply)triethoxysilane (APTES), (3-Mercaptopropyl)trimethoxysilane (MPTES), 3-Mercaptopropionic acid (MPA) (99%), N,N-dimethylformamide (DMF, 99.8%), methanol and isopropyl alcohol (IPA) were obtained from Sigma–Aldrich Co. LLC. St. Louis, USA. Silver nanowire (Ag NW) dispersed in IPA (20 mg/ml) with an average diameter of 50 nm and length of 5-10 µm was obtained from ACS Materials. Glass slides (Menzel-Glaser, 15 × 15 mm2) were sonicated in acetone and ethanol for 15 minutes and treated with piranha 4
solution (7:3 H2SO4/H2O2) at 90 °C for 1 h. Then, they were sonicated in deionized water (DI water) for 5 min and dried under N2 flow.
2.2. Modification of AgNWs Carboxylic acid-functionalized Ag NWs (Ag NWs (COOH)) were synthesized according to a procedure in the literature [21], except using MPA instead of cysteamine hydrochloride (CA). First, the Ag NWs (10 mg) were centrifuged to remove IPA and dispersed in 50 ml DMF. Then, MPA (30 mg) was added to the Ag NW solution and stirred for 24 h at room temperature. The Ag NWs-COOH were then separated by centrifugation and the final product was dispersed in DI water.
2.3. Preparation of transparent conducting films Self-assembly of Ag NWs was performed for the preparation of TCEs. First, the glass substrates were functionalized by immersion in either 3% APTES or 3% MPTES solution for 3 h, followed by rinsing with excess methanol and drying under N2 flow. Then, the substrates were annealed at 100 °C for 1 h. The APTES-functionalized substrates were dipped into Ag NWs (polyvinylpyrrolidone (PVP)) (0.5 mg/ml, in IPA) or Ag NWs (COOH) solution (0.5 mg/ml, in DI water), while MPTES-functionalized substrates were dipped into Ag NWs (PVP) solution (0.5 mg/ml, in IPA) for an immersion time between 1.5 h and 24 h. Self-assembled films were sequentially rinsed with IPA and DI water, and then dried under N2 flow and at 50 °C for 30 min. Finally, the films were annealed in a tube furnace at 230 °C for 15 min under atmospheric condition.
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2.4. Device fabrication and characterization The Ag NW TCE was used as the anode of the OPV device. A vanadium triisopropoxide oxide (V2O5) precursor solution was diluted in IPA with (V2O5 : IPA = 1 : 60 [vol.%]) and spin-casted onto the Ag NW TCE substrate as the hole transport layer (HTL) under ambient conditions. A P3HT:PCBM solution (P3HT:PCBM=1:0.8 [wt%] dissolved in a mixed solvent of dichlorobenzene:diphenylether (97:3 [vol.%])) was then spin-casted onto the HTL as the active layer in an N2-filled glove box with thermal annealing at 100 °C for 10 min. A 100 nmthick aluminum electrode was deposited onto the active layer using a thermal evaporator under a vacuum of 10-6 torr. Device performance was measured under 1 sun illumination (100 mW cm-2) in a N2 filled glove box without encapsulation.
2.5. Characterizations The surface composition of Ag NWs (COOH) was detected using X-ray photoelectron spectroscopy (XPS) (K-Alpha, Thermo). Surface charges of Ag NWs (PVP) and Ag NWs (COOH) were determined by zeta potential analysis (Nano ZS, Malvern Instruments). The surface morphologies of the TCEs were analyzed by optical microscopy (ECLIPSE L150, Nikon) and field emission scanning electron microscopy (FE-SEM) (JEOL 63335F JSM). Sheet resistances of TCEs were measured using the four-point probe technique (RM3000, Jandel) while the optical properties were examined using UV−vis spectrometry (Lambda 750, Perkin-Elmer). The root mean square rouhgness of TCEs were measured with atomic force microscopy (AFM) (XE-100, PSIA).
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3. RESULTS AND DISCUSSION 3.1. Characterizations of Ag NWs (PVP) and Ag NWs (COOH) Ag NWs were modified with MPA to obtain negatively charged Ag NWs. The thiol groups (-SH) of MPA interact with the Ag atoms, forming carboxylic acid-functionalized silver nanowires (Ag NWs (COOH)). The XPS analysis was used to evaluate the composition of the Ag NWs after surface modification. As illustrated in Figure S1, the XPS spectra of Ag NWs (COOH) exhibited peaks at 162.2, 163.2 and 164.6 eV, which can be attributed to the S–Ag bond between MPA and Ag NWs, the S–H bond of unreacted MPA, and the S–C bond of MPA, respectively. These are in good agreement with the results in the literature [22,23], which shows the XPS spectra of Ag NWs-NH2. These results confirm the successful surface modification of Ag NWs. The surface charges and stability of Ag NWs (PVP) and Ag NWs (COOH) dispersions were investigated via zeta potential analysis. The surface charges of Ag NWs (PVP) and Ag NWs (COOH) were found to be negative values of -30.2 eV and -31.3 eV in IPA (and DI water), respectively (Figure 1). Particles with highly negative or positive zeta potentials (more than 30 mV or less than -30 mV) are considered to provide sufficient mutual repulsion to ensure a stable dispersion [19]. This implies that Ag NWs (PVP) and Ag NWs (COOH) have sufficient surface charge to maintain a stable dispersion.
Figure 1. Zeta potential of (a) Ag NWs (PVP) (b) Ag NWs (COOH).
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3.2. Electrical and optical characterization of TCEs Silanization of surfaces is a common method for immobilizing metallic nanoparticles, which involves covering the silanol groups on the surface of the substrate with alkoxysilane compounds [24]. APTES and MPTES were used for the silanization of the glass substrate (Figure 2). APTES and MPTES promoted adhesion between the glass substrate and Ag NWs due to the strong interaction between the amine/ thiol group and metal surface. The Silanol (Si-OH) groups of APTES interact with the surface of the glass substrate, while the amine groups (-NH2) of the the APTES interact with Ag NWs. Similarly, the silanol groups of MPTES interact with surface of the glass substrate, while the thiol groups (-SH) of MPTES interact with Ag NWs. Self-assembly of the nanowires onto the glass substrate was achieved by utilizing the electrostatic interaction between the Ag NWs and the APTES-(or MPTES-) functionalized glass substrate. In addition, thermal curing of the silanized substrate was carried out to limit heterogeneous multilayer formation by inducing polymerization between any free silanol groups [25].
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Figure 2. Schematic illustration for (a) APTES- and (b) MPTES-functionalization of the glass substrates.
Figure 3a-3c show the optical microscope images of APTES-Ag NWs (PVP), MPTES-Ag NWs (PVP), APTES-Ag NWs (COOH) monolayer films. The density of the Ag NWs on the surface was found to increase with the increased deposition time, which demonstrates the success of self-assembly deposition. Moreover, it is clear that increasing the deposition time caused a decrease in the size of the voids.
Figure 3. Optical microscope images of the (a) APTES-Ag NWs (PVP), (b) MPTES-Ag NWs (PVP), (c) APTES-Ag NWs (COOH) monolayer films as functions of the immersion time. The scale bar is 30 μm.
Macro images of the self-assembled Ag NW films are shown in Figure S2. We found that for longer exposure times, the coverage of the Ag NWs on the surface increased, which again demonstrates the success of self-assembly deposition. Analysis of the macro and optical images was confirmed by optical transmittance analysis. After deposition for 24 h, we obtained Ag NWs (PVP), Ag NWs (COOH) films on the APTES-modified surface and Ag NWs (PVP) film on the MPTES-modified surface, with optical transmission at 550 nm of 65%, 66% and 81%, respectively (Figure 4). Denser Ag NWs (PVP) and Ag NWs (COOH) networks were obtained on the APTES-modified surface
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than for Ag NWs (PVP) on the MPTES-modified surface. While Ag NWs (PVP) and Ag NWs (COOH) ionic bonds with the amine groups of APTES, Ag NWs (PVP) forms covalent bonds to the thiol groups of MPTES.
Figure 4. Optical transmittance of (a) APTES-Ag NWs (PVP) (b) MPTES-Ag NWs (PVP) (c) APTES-Ag NWs (COOH) monolayer films.
The sheet resistances were investigated as a function of deposition time. As deposition time increased, the sheet resistance decreased as low as 9 Ω/sq, 80 Ω/sq, 261 kΩ/sq for APTES-AgNWs (PVP), MPTES-AgNWs (PVP), APTES-Ag NWs (COOH), respectively. This could be explained by the increasingly dense packing of Ag NWs, which leads to smaller void spaces and thus a more electrically conductive pathway, resulting in low sheet resistance. Annealing of the Ag NW films resulted in a further decrease in sheet resistance (Table 1). This is attributed to the removal of the PVP and MPA during the annealing process. The APTES-Ag NWs (PVP), APTES-Ag NWs (COOH) films exhibited lower sheet resistance than MPTES-Ag NWs (PVP), due to the dense coverage created by ionic bonds. As shown in Figure 5, sheet resistances of the APTES-Ag NWs (PVP), APTES-Ag NWs-COOH films decreased by ~15 Ω/sq after 6 h of deposition time, while the sheet resistance of the MPTESAg NWs (PVP) film decreased by ~126 Ω/sq. The optimum deposition time was found to be 18 h for both APTES-Ag NWs (PVP), MPTES-Ag NWs (PVP), and APTES-Ag NWs (COOH) films, producing films with 4 Ω/sq, 19 Ω/sq, and 3.9 Ω/sq sheet resistance, respectively.
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Table 1. Sheet resistance, optical transmittance and FOM values of self-assembled Ag NW.
Figure 5. Sheet resistance as a function of deposition time for self-assembled Ag NW films.
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The effect of annealing on the surface morphology of the Ag NW films was investigated using field emission scanning microscope (FE-SEM) analysis. As shown Figure S3, the fusion of nanowires occured after 15 min annealing (230 °C). However, Ag NWs started to break after more than 15 min of annealing, which created a high sheet resistance. Annealing at 200 °C for 30 min demonstrated almost the same values of sheet resistance. The optimum annealing conditions were found to be 230 oC for 15 min or 200 oC for 30 min. The root mean square (rms) roughness of electrode is one of important factors to affect the device performance since surface roughness determines effective interfacial area between layers, trap-assisted recombination by interfacial traps and surface energy.[26-28] As reported previously, ITO had smooth surface roughness of 1.1 nm [29], whereas APTES-Ag NWs (PVP)-6h exhibited extremely high surface roughness of 49.0 nm (Figure S4). In spite of low sheet resistance and high transmittance of Ag NWs, it is possible that this high surface roughness has negative effect on device efficiency of OSCs. The figure of merit (FOM) equation, which is the electrical to optical conductivity ratio (σDC/σOP), was utilized to evaluate the performance of the Ag NW films as the electrode. The FOM equation is defined as FOM = Z0/2RS(T-1/2-1) where, Z0 is the impedance of free space [377 Ω], RS is the sheet resistance of the film [Ω/sq], and T is the transmittance of the film. A higher FOM value of electrode material indicates that TCE has a better performance in terms of transmittance and conductivity. The APTES-Ag NWs (PVP), MPTES-Ag NWs (PVP), APTES-Ag NWs (COOH) films exhibited FOM values of 204.1, 99.6, and 221.6 at immersion time of 18 h, respectively. We obtained highest FOM value for APTES-Ag NWs (COOH), which is higher than that of ITO (T= 85%, RS= 15 Ω/sq, FOM= 148.5) [30].
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Figure 6. (a) Structure and (b) J-V curves of the devices with ITO or Ag NWs as the anode under illuminated AM 1.5G (solid square), dark current (open square).
To evaluate the potential of Ag NWs as an electrode in solar cells, we fabricated OPVs using Ag NWs as the anode. APTES-Ag NWs (PVP)-6 h electrode was chosen as anode due to its high optical transmittance (86.4%) and adequately high FOM value (165.7). In addition, there is no significant difference of density of the Ag NWs on the substrate before and after the tape test which demonstrate the adhesion between glass and Ag NW was superior quality. We
employed
a
simple
and
conventional
device
structure
of
ITO
(or
Ag
NWs)/V2O5/P3HT:PCBM/Al (Figure 6a). Although Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS) is widely used as hole transport layer (HTL) due to excellent hole selectivity, its wettability with Ag NWs electrode is too poor to form uniform HTL. Therefore, we chose vanadium oxide (V2O5) as the HTL on Ag NWs electrode in this work. The P3HT and PCBM were used as electron donor and acceptor, respectively, for the active layer. The P3HT is one of the cheap and common polymers in OPVs. Figure 6b exhibits the current density-voltage (J-V) curves of the devices based on ITO and Ag NWs. We optimized device efficiency by controlling the thickness of all layers. The optimum device with Ag NWs exhibited a short circuit current density (JSC) of 7.23 mA cm-2, open
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circuit voltage (VOC) of 0.54 V, and fill factor (FF) of 0.44, corresponding to a power conversion efficiency (PCE) of 1.72%. Reference device with conventional ITO electrode exhibited JSC of 9.27 mA cm-2, VOC of 0.60 V, FF of 0.48, and PCE of 2.70%. Although efficiency of the device with Ag NWs is still lower than that of the device with ITO, this implies that Ag NWs film can be promising candidate of TCEs as an alternative to ITO electrode.
CONCLUSION In this work, we successfully developed Ag NWs TCEs via self-assembly deposition method. The negatively charged Ag NWs were successfully coated on APTES or MPTESmodified glass substrates by electrostatic interaction. We controlled sheet resistance and optical transmittance by varying the deposition time of substrates in the Ag NWs solution. We obtained optimized Ag NWs films at deposition time of 18 h and they exhibited sheet resistance of 4-19 Ω/sq and optical transmittance of 66-82% (at 550 nm). Among three different Ag NWs samples (FOM value of MPTES-Ag NWs (PVP) = 99.6), APTES-Ag NWs (PVP) and APTES-Ag NWs (COOH) had high FOM values of 204.1 and 221.6, which are higher than that of conventional ITO electrode (148.5). The P3HT:PCBM OSCs based on Ag NWs showed high device efficiency of 1.72%, indicating that Ag NWs prepared by selfassembly deposition can be a promising candidate for TCEs as an alternative to ITO electrode in optoelectronic devices.
Acknowledgements B. T. Camic and H. J. Shin equally contributed to this work. This research was supported by the Technology Development Program to Solve Climate Changes of the National Research
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Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2016M1A2A2940914) and TUBITAK (Grant No: 113M772).
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Supplementary Information
Solution-Processable Transparent Conducting Electrodes via the SelfAssembly of Silver Nanowires for Organic Photovoltaic Devices
B. Tugba Camic1,2, Hee Jeong Shin3, Fevzihan Basarir*4,5, M. Hasan Aslan1, Hyosung. Choi*3
1
2
Department of Physics, Gebze Technical University, 41400 Gebze, Kocaeli, Turkey
Sabanci University Nanotechnology Research and Application Center (SUNUM), 34956 Tuzla, İstanbul, Turkey
3
Department of Chemistry and Research Institute for Natural Sciences, Hanyang University, 04763 Seoul, South Korea
4
Materials Institute, TUBITAK Marmara Research Center (MRC), 41470 Gebze, Kocaeli, Turkey 5
Next Chemicals & Plastics, 34490 Ikitelli, Istanbul, Turkey
19
Figure S1. XPS S2p spectra of Ag NWs (COOH).
Figure S2. Macro images of (a) APTES-Ag NWs (PVP) (b) MPTES-Ag NWs (PVP) (c) APTES-Ag NWs (COOH) monolayer films for 1.5, 3, 6, 9, 12, 18, 24 h.
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Figure S3. Tilted SEM images of Ag NW film annealed at 230 °C for 15 min.
Figure S4. Topological AFM images of Ag NWs on glass.
21
Figure S5. The optical microscope images of (a) APTES-Ag NWs (PVP) and (b) APTES-Ag NWs (PVP) after 3-M tape test. The size of scale bar is 60 ㎛.
22
Graphical abstract
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S0021-9797(17)31150-5 https://doi.org/10.1016/j.jcis.2017.09.112 YJCIS 22864
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
8 August 2017 27 September 2017 29 September 2017
Please cite this article as: B. Tugba Camic, H. Jeong Shin, M. Hasan Aslan, F. Basarir, H. Choi, Solution-Processable Transparent Conducting Electrodes via the Self-Assembly of Silver Nanowires for Organic Photovoltaic Devices, Journal of Colloid and Interface Science (2017), doi: https://doi.org/10.1016/j.jcis.2017.09.112
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Solution-Processable Transparent Conducting Electrodes via the SelfAssembly of Silver Nanowires for Organic Photovoltaic Devices
B. Tugba Camic1,2, Hee Jeong Shin3, M. Hasan Aslan1, Fevzihan Basarir4 and Hyosung Choi*3
1
2
3
Department of Physics, Gebze Technical University, 41400 Gebze, Kocaeli, Turkey
Sabanci University Nanotechnology Research and Application Center (SUNUM), 34956 Tuzla, İstanbul, Turkey
Department of Chemistry and Research Institute for Natural Sciences, Hanyang University, 04763 Seoul, South Korea 4
Next Chemicals & Plastics, 34490 Ikitelli, Istanbul, Turkey
*Corresponding Author Prof. Hyosung Choi Address: 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, South Korea E-mail address: [email protected] Tel: +82-2-2220-2619 Fax: +82-2-2298-0319
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Abstract Solution-processed transparent conducting electrodes (TCEs) were fabricated via the selfassembly deposition of silver nanowires (Ag NWs). Glass substrates modified with (3aminopropyl)triethoxysilane (APTES) and (3-mercaptopropyl)trimethoxysilane (MPTES) were coated with Ag NWs for various deposition times, leading to three different Ag NWs samples (APTES-Ag NWs (PVP), MPTES-Ag NWs (PVP), and APTES-Ag NWs (COOH)). Controlling the deposition time produced Ag NWs monolayer thin films with different optical transmittance and sheet resistance. Post-annealing treatment improved their electrical conductivity. The Ag NWs films were sucessfully characterized using UV–Vis spectroscopy, field emission scanning electron microscopy, optical microscopy and four-point probe. Three Ag NWs films exhibited low sheet resistance of 4-19 Ω/sq and high optical transmittance of 65-81% (at 550 nm), which are comparable to those of commercial ITO electrode. We fabricated an organic photovoltaic device by using Ag NWs as the anode instead of ITO electrode, and optimized device with Ag NWs exhibited power conversion efficiency of 1.72%.
Keywords: Silver nanowires, transparent conducting electrode, self-assembly deposition, organic photovoltaics
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1. Introduction Transparent
conducting electrodes (TCEs) are critical components in various
optoelectronic devices such as touch screens [1], liquid crystal displays [2], organic light emitting diodes [3], and solar cells [4]. Although indium tin oxide (ITO) has been widely used for TCEs because of its low sheet resistance and high transmittance at visible wavelengths, ITO has many disadvantages. ITO requires a sputtering technique for deposition, as it is a brittle material, and reserves of indium are limited. Therefore, the development of solutionprocessable indium-free TCEs are essential for the application of printing techniques and the fabrication of low-cost and flexible optoelectronic devices [5]. Recent research has suggested various alternatives to conventional ITO, such as silver nanowires (Ag NWs) [6], carbon nanotubes (CNTs) [7], graphene [8], and conducting polymers [9]. Among these materials, Ag NWs are regarded as the most promising electrode material due to their exceptional solution compatibility, high transparency, and low sheet resistance [10] [11]. However, the surface roughness of Ag NWs is a significant drawback, leading to short circuits and high sheet resistance. Another drawback of Ag NWs is their poor adhesion with the substrate. Although these problems limit the application of Ag NWs to optoelectronic devices, devices based on Ag NW electrodes exhibit a comparable performance to ITO-based devices. On the other hand, devices based on CNTs and graphene electrodes both exhibit lower device efficiencies than ITO-based devices. [12]. There are a variety of methods to produce Ag NWs ,including spin coating [13], spray coating [14], brush painting [15], Meyer-rod-coating [16], drop-casting [17], and dry transferring [18]. These solution processes are all cost competitive with conventional vacuum deposition for ITO production [19] and produce an electrode with low sheet resistance and high transmittance. However, it is difficult to obtain a uniform network of Ag NWs over a large area using drop-casting, Meyer-rod-coating, spray-coating or dry transferring methods.
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This creates a challenge for large-area device fabrication [20]. Although those coating methods can produce large-scale and uniform nanowire networks, they generate large amounts of waste material after coating, which is a major problem in terms of cost effectiveness. Self-assembled coatings are convenient for controlling the degree of nanostructure coverage. This method is easily tuned to film thickness, porosity, or electrical and optical properties. In addition, self-assembled molecules provide excellent adhesion between the nanostructure and substrate. In this work, Ag NW-based TCEs were successfully developed via a self-assembling coating method. The Ag NWs were functionalized with 3-mercaptopropionic acid (MPA) to obtain the negatively-charged Ag NWs (COOH). Then, Ag NWs were coated onto the glass substrates
modified
with
3-aminopropyltriethoxysilane
(APTES)
and
(3-
Mercaptopropyl)trimethoxysilane (MPTES) via self-assembly. The distribution and density of Ag NWs were studied as a function of immersion time of the modified substrate in the Ag NW solution. We characterized the Ag NW samples using UV–Vis spectrometry, field emission scanning electron microscopy (FE-SEM), optical microscopy (OM) and the fourpoint probe method.
2. MATERIALS AND METHODS 2.1. Materials (3-Aminoproply)triethoxysilane (APTES), (3-Mercaptopropyl)trimethoxysilane (MPTES), 3-Mercaptopropionic acid (MPA) (99%), N,N-dimethylformamide (DMF, 99.8%), methanol and isopropyl alcohol (IPA) were obtained from Sigma–Aldrich Co. LLC. St. Louis, USA. Silver nanowire (Ag NW) dispersed in IPA (20 mg/ml) with an average diameter of 50 nm and length of 5-10 µm was obtained from ACS Materials. Glass slides (Menzel-Glaser, 15 × 15 mm2) were sonicated in acetone and ethanol for 15 minutes and treated with piranha 4
solution (7:3 H2SO4/H2O2) at 90 °C for 1 h. Then, they were sonicated in deionized water (DI water) for 5 min and dried under N2 flow.
2.2. Modification of AgNWs Carboxylic acid-functionalized Ag NWs (Ag NWs (COOH)) were synthesized according to a procedure in the literature [21], except using MPA instead of cysteamine hydrochloride (CA). First, the Ag NWs (10 mg) were centrifuged to remove IPA and dispersed in 50 ml DMF. Then, MPA (30 mg) was added to the Ag NW solution and stirred for 24 h at room temperature. The Ag NWs-COOH were then separated by centrifugation and the final product was dispersed in DI water.
2.3. Preparation of transparent conducting films Self-assembly of Ag NWs was performed for the preparation of TCEs. First, the glass substrates were functionalized by immersion in either 3% APTES or 3% MPTES solution for 3 h, followed by rinsing with excess methanol and drying under N2 flow. Then, the substrates were annealed at 100 °C for 1 h. The APTES-functionalized substrates were dipped into Ag NWs (polyvinylpyrrolidone (PVP)) (0.5 mg/ml, in IPA) or Ag NWs (COOH) solution (0.5 mg/ml, in DI water), while MPTES-functionalized substrates were dipped into Ag NWs (PVP) solution (0.5 mg/ml, in IPA) for an immersion time between 1.5 h and 24 h. Self-assembled films were sequentially rinsed with IPA and DI water, and then dried under N2 flow and at 50 °C for 30 min. Finally, the films were annealed in a tube furnace at 230 °C for 15 min under atmospheric condition.
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2.4. Device fabrication and characterization The Ag NW TCE was used as the anode of the OPV device. A vanadium triisopropoxide oxide (V2O5) precursor solution was diluted in IPA with (V2O5 : IPA = 1 : 60 [vol.%]) and spin-casted onto the Ag NW TCE substrate as the hole transport layer (HTL) under ambient conditions. A P3HT:PCBM solution (P3HT:PCBM=1:0.8 [wt%] dissolved in a mixed solvent of dichlorobenzene:diphenylether (97:3 [vol.%])) was then spin-casted onto the HTL as the active layer in an N2-filled glove box with thermal annealing at 100 °C for 10 min. A 100 nmthick aluminum electrode was deposited onto the active layer using a thermal evaporator under a vacuum of 10-6 torr. Device performance was measured under 1 sun illumination (100 mW cm-2) in a N2 filled glove box without encapsulation.
2.5. Characterizations The surface composition of Ag NWs (COOH) was detected using X-ray photoelectron spectroscopy (XPS) (K-Alpha, Thermo). Surface charges of Ag NWs (PVP) and Ag NWs (COOH) were determined by zeta potential analysis (Nano ZS, Malvern Instruments). The surface morphologies of the TCEs were analyzed by optical microscopy (ECLIPSE L150, Nikon) and field emission scanning electron microscopy (FE-SEM) (JEOL 63335F JSM). Sheet resistances of TCEs were measured using the four-point probe technique (RM3000, Jandel) while the optical properties were examined using UV−vis spectrometry (Lambda 750, Perkin-Elmer). The root mean square rouhgness of TCEs were measured with atomic force microscopy (AFM) (XE-100, PSIA).
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3. RESULTS AND DISCUSSION 3.1. Characterizations of Ag NWs (PVP) and Ag NWs (COOH) Ag NWs were modified with MPA to obtain negatively charged Ag NWs. The thiol groups (-SH) of MPA interact with the Ag atoms, forming carboxylic acid-functionalized silver nanowires (Ag NWs (COOH)). The XPS analysis was used to evaluate the composition of the Ag NWs after surface modification. As illustrated in Figure S1, the XPS spectra of Ag NWs (COOH) exhibited peaks at 162.2, 163.2 and 164.6 eV, which can be attributed to the S–Ag bond between MPA and Ag NWs, the S–H bond of unreacted MPA, and the S–C bond of MPA, respectively. These are in good agreement with the results in the literature [22,23], which shows the XPS spectra of Ag NWs-NH2. These results confirm the successful surface modification of Ag NWs. The surface charges and stability of Ag NWs (PVP) and Ag NWs (COOH) dispersions were investigated via zeta potential analysis. The surface charges of Ag NWs (PVP) and Ag NWs (COOH) were found to be negative values of -30.2 eV and -31.3 eV in IPA (and DI water), respectively (Figure 1). Particles with highly negative or positive zeta potentials (more than 30 mV or less than -30 mV) are considered to provide sufficient mutual repulsion to ensure a stable dispersion [19]. This implies that Ag NWs (PVP) and Ag NWs (COOH) have sufficient surface charge to maintain a stable dispersion.
Figure 1. Zeta potential of (a) Ag NWs (PVP) (b) Ag NWs (COOH).
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3.2. Electrical and optical characterization of TCEs Silanization of surfaces is a common method for immobilizing metallic nanoparticles, which involves covering the silanol groups on the surface of the substrate with alkoxysilane compounds [24]. APTES and MPTES were used for the silanization of the glass substrate (Figure 2). APTES and MPTES promoted adhesion between the glass substrate and Ag NWs due to the strong interaction between the amine/ thiol group and metal surface. The Silanol (Si-OH) groups of APTES interact with the surface of the glass substrate, while the amine groups (-NH2) of the the APTES interact with Ag NWs. Similarly, the silanol groups of MPTES interact with surface of the glass substrate, while the thiol groups (-SH) of MPTES interact with Ag NWs. Self-assembly of the nanowires onto the glass substrate was achieved by utilizing the electrostatic interaction between the Ag NWs and the APTES-(or MPTES-) functionalized glass substrate. In addition, thermal curing of the silanized substrate was carried out to limit heterogeneous multilayer formation by inducing polymerization between any free silanol groups [25].
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Figure 2. Schematic illustration for (a) APTES- and (b) MPTES-functionalization of the glass substrates.
Figure 3a-3c show the optical microscope images of APTES-Ag NWs (PVP), MPTES-Ag NWs (PVP), APTES-Ag NWs (COOH) monolayer films. The density of the Ag NWs on the surface was found to increase with the increased deposition time, which demonstrates the success of self-assembly deposition. Moreover, it is clear that increasing the deposition time caused a decrease in the size of the voids.
Figure 3. Optical microscope images of the (a) APTES-Ag NWs (PVP), (b) MPTES-Ag NWs (PVP), (c) APTES-Ag NWs (COOH) monolayer films as functions of the immersion time. The scale bar is 30 μm.
Macro images of the self-assembled Ag NW films are shown in Figure S2. We found that for longer exposure times, the coverage of the Ag NWs on the surface increased, which again demonstrates the success of self-assembly deposition. Analysis of the macro and optical images was confirmed by optical transmittance analysis. After deposition for 24 h, we obtained Ag NWs (PVP), Ag NWs (COOH) films on the APTES-modified surface and Ag NWs (PVP) film on the MPTES-modified surface, with optical transmission at 550 nm of 65%, 66% and 81%, respectively (Figure 4). Denser Ag NWs (PVP) and Ag NWs (COOH) networks were obtained on the APTES-modified surface
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than for Ag NWs (PVP) on the MPTES-modified surface. While Ag NWs (PVP) and Ag NWs (COOH) ionic bonds with the amine groups of APTES, Ag NWs (PVP) forms covalent bonds to the thiol groups of MPTES.
Figure 4. Optical transmittance of (a) APTES-Ag NWs (PVP) (b) MPTES-Ag NWs (PVP) (c) APTES-Ag NWs (COOH) monolayer films.
The sheet resistances were investigated as a function of deposition time. As deposition time increased, the sheet resistance decreased as low as 9 Ω/sq, 80 Ω/sq, 261 kΩ/sq for APTES-AgNWs (PVP), MPTES-AgNWs (PVP), APTES-Ag NWs (COOH), respectively. This could be explained by the increasingly dense packing of Ag NWs, which leads to smaller void spaces and thus a more electrically conductive pathway, resulting in low sheet resistance. Annealing of the Ag NW films resulted in a further decrease in sheet resistance (Table 1). This is attributed to the removal of the PVP and MPA during the annealing process. The APTES-Ag NWs (PVP), APTES-Ag NWs (COOH) films exhibited lower sheet resistance than MPTES-Ag NWs (PVP), due to the dense coverage created by ionic bonds. As shown in Figure 5, sheet resistances of the APTES-Ag NWs (PVP), APTES-Ag NWs-COOH films decreased by ~15 Ω/sq after 6 h of deposition time, while the sheet resistance of the MPTESAg NWs (PVP) film decreased by ~126 Ω/sq. The optimum deposition time was found to be 18 h for both APTES-Ag NWs (PVP), MPTES-Ag NWs (PVP), and APTES-Ag NWs (COOH) films, producing films with 4 Ω/sq, 19 Ω/sq, and 3.9 Ω/sq sheet resistance, respectively.
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Table 1. Sheet resistance, optical transmittance and FOM values of self-assembled Ag NW.
Figure 5. Sheet resistance as a function of deposition time for self-assembled Ag NW films.
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The effect of annealing on the surface morphology of the Ag NW films was investigated using field emission scanning microscope (FE-SEM) analysis. As shown Figure S3, the fusion of nanowires occured after 15 min annealing (230 °C). However, Ag NWs started to break after more than 15 min of annealing, which created a high sheet resistance. Annealing at 200 °C for 30 min demonstrated almost the same values of sheet resistance. The optimum annealing conditions were found to be 230 oC for 15 min or 200 oC for 30 min. The root mean square (rms) roughness of electrode is one of important factors to affect the device performance since surface roughness determines effective interfacial area between layers, trap-assisted recombination by interfacial traps and surface energy.[26-28] As reported previously, ITO had smooth surface roughness of 1.1 nm [29], whereas APTES-Ag NWs (PVP)-6h exhibited extremely high surface roughness of 49.0 nm (Figure S4). In spite of low sheet resistance and high transmittance of Ag NWs, it is possible that this high surface roughness has negative effect on device efficiency of OSCs. The figure of merit (FOM) equation, which is the electrical to optical conductivity ratio (σDC/σOP), was utilized to evaluate the performance of the Ag NW films as the electrode. The FOM equation is defined as FOM = Z0/2RS(T-1/2-1) where, Z0 is the impedance of free space [377 Ω], RS is the sheet resistance of the film [Ω/sq], and T is the transmittance of the film. A higher FOM value of electrode material indicates that TCE has a better performance in terms of transmittance and conductivity. The APTES-Ag NWs (PVP), MPTES-Ag NWs (PVP), APTES-Ag NWs (COOH) films exhibited FOM values of 204.1, 99.6, and 221.6 at immersion time of 18 h, respectively. We obtained highest FOM value for APTES-Ag NWs (COOH), which is higher than that of ITO (T= 85%, RS= 15 Ω/sq, FOM= 148.5) [30].
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Figure 6. (a) Structure and (b) J-V curves of the devices with ITO or Ag NWs as the anode under illuminated AM 1.5G (solid square), dark current (open square).
To evaluate the potential of Ag NWs as an electrode in solar cells, we fabricated OPVs using Ag NWs as the anode. APTES-Ag NWs (PVP)-6 h electrode was chosen as anode due to its high optical transmittance (86.4%) and adequately high FOM value (165.7). In addition, there is no significant difference of density of the Ag NWs on the substrate before and after the tape test which demonstrate the adhesion between glass and Ag NW was superior quality. We
employed
a
simple
and
conventional
device
structure
of
ITO
(or
Ag
NWs)/V2O5/P3HT:PCBM/Al (Figure 6a). Although Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS) is widely used as hole transport layer (HTL) due to excellent hole selectivity, its wettability with Ag NWs electrode is too poor to form uniform HTL. Therefore, we chose vanadium oxide (V2O5) as the HTL on Ag NWs electrode in this work. The P3HT and PCBM were used as electron donor and acceptor, respectively, for the active layer. The P3HT is one of the cheap and common polymers in OPVs. Figure 6b exhibits the current density-voltage (J-V) curves of the devices based on ITO and Ag NWs. We optimized device efficiency by controlling the thickness of all layers. The optimum device with Ag NWs exhibited a short circuit current density (JSC) of 7.23 mA cm-2, open
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circuit voltage (VOC) of 0.54 V, and fill factor (FF) of 0.44, corresponding to a power conversion efficiency (PCE) of 1.72%. Reference device with conventional ITO electrode exhibited JSC of 9.27 mA cm-2, VOC of 0.60 V, FF of 0.48, and PCE of 2.70%. Although efficiency of the device with Ag NWs is still lower than that of the device with ITO, this implies that Ag NWs film can be promising candidate of TCEs as an alternative to ITO electrode.
CONCLUSION In this work, we successfully developed Ag NWs TCEs via self-assembly deposition method. The negatively charged Ag NWs were successfully coated on APTES or MPTESmodified glass substrates by electrostatic interaction. We controlled sheet resistance and optical transmittance by varying the deposition time of substrates in the Ag NWs solution. We obtained optimized Ag NWs films at deposition time of 18 h and they exhibited sheet resistance of 4-19 Ω/sq and optical transmittance of 66-82% (at 550 nm). Among three different Ag NWs samples (FOM value of MPTES-Ag NWs (PVP) = 99.6), APTES-Ag NWs (PVP) and APTES-Ag NWs (COOH) had high FOM values of 204.1 and 221.6, which are higher than that of conventional ITO electrode (148.5). The P3HT:PCBM OSCs based on Ag NWs showed high device efficiency of 1.72%, indicating that Ag NWs prepared by selfassembly deposition can be a promising candidate for TCEs as an alternative to ITO electrode in optoelectronic devices.
Acknowledgements B. T. Camic and H. J. Shin equally contributed to this work. This research was supported by the Technology Development Program to Solve Climate Changes of the National Research
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Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2016M1A2A2940914) and TUBITAK (Grant No: 113M772).
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Supplementary Information
Solution-Processable Transparent Conducting Electrodes via the SelfAssembly of Silver Nanowires for Organic Photovoltaic Devices
B. Tugba Camic1,2, Hee Jeong Shin3, Fevzihan Basarir*4,5, M. Hasan Aslan1, Hyosung. Choi*3
1
2
Department of Physics, Gebze Technical University, 41400 Gebze, Kocaeli, Turkey
Sabanci University Nanotechnology Research and Application Center (SUNUM), 34956 Tuzla, İstanbul, Turkey
3
Department of Chemistry and Research Institute for Natural Sciences, Hanyang University, 04763 Seoul, South Korea
4
Materials Institute, TUBITAK Marmara Research Center (MRC), 41470 Gebze, Kocaeli, Turkey 5
Next Chemicals & Plastics, 34490 Ikitelli, Istanbul, Turkey
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Figure S1. XPS S2p spectra of Ag NWs (COOH).
Figure S2. Macro images of (a) APTES-Ag NWs (PVP) (b) MPTES-Ag NWs (PVP) (c) APTES-Ag NWs (COOH) monolayer films for 1.5, 3, 6, 9, 12, 18, 24 h.
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Figure S3. Tilted SEM images of Ag NW film annealed at 230 °C for 15 min.
Figure S4. Topological AFM images of Ag NWs on glass.
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Figure S5. The optical microscope images of (a) APTES-Ag NWs (PVP) and (b) APTES-Ag NWs (PVP) after 3-M tape test. The size of scale bar is 60 ㎛.
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Graphical abstract
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