Protection-free Ag Nanowires as a Transparent Conductive Electrode

Subscriber access provided by UNIV OF DURHAM

Article

Protection-free Ag Nanowires as a Transparent Conductive Electrode for Improved CIGS-based Photovoltaic Performances Jingling Liu, Longfei Guo, Xinsheng Liu, Shuang Li, Xiaolan Liu, Xingfen Shen, Songfeng Chang, Ke Cheng, and Zu-liang Du ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00252 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on February 3, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Energy Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Energy Materials

Protection-free Ag Nanowires as a Transparent Conductive Electrode for Improved CIGS-based Photovoltaic Performances Jingling Liu†, Longfei Guo†, Xinsheng Liu, Shuang Li, Xiaolan Liu, Xingfen Shen, Songfeng Chang, Ke Cheng, Zu-liang Du* Key Laboratory for Special Functional Materials of Ministry of Education, Collaborative Innovation Center of Nano Functional Materials and Applications, Henan University, Kaifeng 475004, Henan Province, People’s Republic of China † Authors J.L., and L.G. contributed equally * Corresponding author: [email protected] KEYWORDS: Ag NWs; Steric effect; Chemical characteristics; CIGS; Solar cell

ABSTRACT: A rather low efficiency of CIGS-based solar cells (mostly less than 1%), is generally reported, where Silver nanowires (Ag NWs) are employed as a top transparent conductive electrode (TCE). The weak adhesion and small contact area between Ag NWs and ntype buffer layer, remain an acknowledged issue to be addressed. Here, a modified polyol reduction process was elaborately proposed, based on the regulation of PVP molecules steric effect and cationic chemical characteristics. Ag NWs with controllable lengths and diameters

ACS Paragon Plus Environment

1

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 23

were successfully synthesized, to meet the optimization of the photovoltaic performance. The mixed PVP consisting of short and long chains works very effectively, in increasing the length and shortening the diameter. We attribute this to the large steric hindrance-induced protective shielding on {100} planes by insertion of the short chains into the long ones. Cationic chemical characteristics, on the effect of Ag morphological evolution, were also referred and carefully conducted. Importantly, a champion efficiency of 4.97% on pristine Ag NWs without protection and post-treatment as a TCE was achieved, contributed to the enhanced adhesion and increased contact area between Ag NWs and the top of buffer layer.

1. INTRODUCTION With the popularization of optoelectronic devices, transparent conducting electrodes (TCEs) are of great success in the applications of touch-panel displays, smart windows, and solar cells.1-3 Currently, indium tin oxide (ITO) is awarded as the most universal material employed as TCEs due to its excellent transparency and conductivity. However, ITO is often having hard knocks as a TCE due to: (a) the use of rare metals of indium; (b) a high-cost vacuum deposition based process; (c) the poor mechanical flexibility. Thus, ITO is now being gradually replaced with other transparent electrode materials, such as silver nanowires (Ag NWs), conjugated polymers, carbon nanotubes (CNTs), and graphene.4,5 Ag NWs, as an alternative, have received much concern because of their excellent conductivity, high transparency, along with a low-cost green solution based fabrication process.6-9 Ag NWs as a TCE employed in CIGS-based solar cell have been sketchily studied in previous reports. Whereas, based on the pure Ag NWs electrode, a rather low value of efficiency, less than 1 % mostly, was reported as summarized in table S1.10-15 This is mainly caused by the weak


2

Page 3 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


adhesion and small contact area between Ag NWs and n-type buffer layer. Thus, enormous attempts have been tried to improve photovoltaic performance by loading a protecting layer or post-treatment on Ag NWs. However, few efforts were made to embark on the intrinsic Ag NWs control. As certified in reports, the optical and electrical properties of the TCEs are highly dependent on the length, diameter and distribution of the Ag NWs, which is also strongly associated with the photovoltaic performance: (a) at a certain transmittance, the smaller diameter possesses the higher density, and the better connection between the wires; (b) at a given sheet resistance, increasing the length, equivalent to a reduce in the number of wires, leads to an increase in TCE transmittance; (c) the distribution possibly increases the effective contact area. Therefore, it is highly possibly to improve the photovoltaic performance with Ag NWs as a top TCE, by the control of the diameter, length and distribution of the nanowires. A series of strategies have been developed to synthesis Ag NWs.16-18 The most promising one is undoubtedly awarded to the polyol process, once taking the cost, yield, and simplicity as the criteria. It is a thermal induced reduction process of Ag+ by the polyol, in the presence of poly(vinylpyrrolidone) (denoted as PVP) as a capping agent.19-21 Here, two factors are deemed to play a significant role in the synthesis of Ag NWs: (a) the structure of the crystal seed, which dominates the growth direction of Ag NWs; (b) the growth rate of Ag atoms, which highly correlates to the diffusion rate of Ag+/Ag0 species, along with the deposition rate. In polyol-mediated synthetic approach, PVP was firstly conducted by controlling the growth direction and rate, due to its selective adsorption capacity on {100} planes, which allows the unprotected {111} planes to grow securely.22-24 The size of steric hindrance induced by the PVP molecular chains represents the strength of the protection. On the other hand, chloride additives,


3


Page 4 of 23

such as NaCl, KCl and so on, are another matter of vital importance to the formation of seed crystals and morphological evolution.7,25-28 Unfortunately, the systematic study of the cation categories of chlorides additives on the effect of Ag morphologies along with diameter and length, still remains obscure. Thus, a design based on a modified polyol reduction process was proposed to control the diameter and length of Ag NWs, by adjusting PVP molecules steric effect and cation chemical properties, to realize the improvement of CIGS-based photovoltaic performance. The effect of PVP molecules steric effect and cation chemical properties on the Ag morphologies was systematically conducted. Compared with the previous reports on Ag NWs employed TCE without protecting layer and post-treatment for CIGS solar cell devices, the highest efficiency of 4.97% was achieved. It is possibly attributed to the improved adhesion and increased contact area between ultra-long pliable nanowires and the top of buffer layer. Our design guides a new direction to improve the solar cell efficiency by simply adjusting the characteristics of Ag NWs itself. Furthermore, our findings also provide a promising way to finely control the synthesis of other metal nanowires. 2. EXPERIMENTAL SECTION Synthesis of Ag Nanostructures. Ag NWs were synthesized by a renowned salt-mediated polyol process with elaborate modifications. Briefly, 3 mmol of AgNO3 was firstly dissolved in ethylene glycol (EG, 99%). Subsequently, 0.06 mmol of chlorides with different cations, and l.4 g of polyvinylpyrrolidone (PVP) with different long chains dissolved in EG were rapidly added into AgNO3/EG solution under vigorous stirring. Afterwards, the mixture was transferred into a Teflon-lined stainless steel autoclave (100-ml capacity), maintained at 160 °C for 6 h to ensure the complete reaction. After naturally cooling to room temperature, the suspension solution was


4

Page 5 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


centrifuged, and washed for several times with ethanol. Finally, the collection was redispersed and stored in ethanol for further utilization. We varied the PVP chain length (Mw=58k, 360k and 1300k) and chloride categories with different cations (Li+, Na+, K+, NH4+, Zn2+, Ca2+, Cu2+, and Sn2+), while PVP with Mw=1300k and trace salt of NaCl were used in most of case. To be more specific, herein, the detailed experimental conditions, along with finial products are listed in Table S2.The samples was also denoted as Px and Cx as in the list, mainly according to the two research objects of PVP molecules and trace chlorides, while x does not stand for any specific meanings. Solar Cell Device Fabrication. For solar cell fabrication, radio-frequency sputtered Cu(In,Ga)Se2 (CIGS) film was employed as the absorber layer being available in our previous work.29,30 Briefly, a preformed layer of CIGS/In was firstly deposited by sputtering quaternary CIGS target and indium target continuously onto the Mo coated soda lime glass (SLG) substrate. Subsequently, the as-deposited CIGS/In precursor film was placed in a graphite box with 100 mg Se powder, and annealed by a rapid thermal process (RTP) with a two-step annealing system of 250 and 500 °C. Finally, it was converted to be Cu-poor CIGS films applied in our device. Later on, the CIGS absorber was covered with a 60 nm thick CdS buffer layer deposited by chemical bath deposition as buffer layer, and a standard window layer of intrinsic-ZnO (i-ZnO) with an 80 nm thickness by RF sputtering. Finally, 2%wt of Ag NWs ink was spin coated onto the top as a TCE, with a masked Ag grid to make an individual cell with an area of 0.21 cm-2. In the end, an architecture of SLG/Mo/CIGS/CdS/i-ZnO/Ag NWs device was constructed, and further evaluated. Characterization. The morphologies were observed via field emission scanning electron microscopy (FE-SEM, JEOL JSM – 7401F) and field emission high-resolution transmission


5


Page 6 of 23

electron microscopy (FE-TEM, JEM-2010). The structures were characterized by X-ray diffraction (XRD) (MRD-Philips) with Cu kα radiation. UV-Vis absorbance and transmittance spectra were recorded by UV−visible (UV−vis) spectrometer (Lambda 950, PerkinElmer). The current density−voltage (J-V) curve of the solar cell device was measured by using a Keithley 2400 source meter under the standard AM 1.5 global illumination with a light intensity of 100 mW/cm2, which was calibrated by a Si reference cell. 3. RESULTS AND DISCUSSION

Figure 1. SEM images of Ag morphologies induced by different chain length of PVP molecules: (a) P1 (Mw=58k); (b) P2 (Mw=360k); (c) P3 (Mw=1300k), and (d) Diameter distribution of P1, P2 and P3 samples. (Scale bar: 500 nm)


6

Page 7 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PVP molecules are highly appraised as an excellent capping agent on governing anisotropic growth of Ag NWs, especially in polyol reduction approach. Thus, an in-depth study has been made on the amount of PVP and the ratio of PVP to silver nitrate on impact of Ag NWs formation. Taking advantage of the literature, a suitable amount of PVP has been used to rule out the effect of PVP on morphological evolution, and clarify the role of PVP molecules with different chain length. For this purpose, three types of PVP molecules with short (Mw=58k), intermediate (360k) and long (1300k) chains, along with their mixtures, are applied in our system. Firstly, Ag NWs promoted by single types of PVP molecules are presented in Table S2 and Figure 1. Apparently, Ag NWs appear significantly different in term of micro-morphologies. On the whole, as the chain length increases, the nanowires trend to be more uniform in size and smaller in diameter, along with the increased yield. In P1 sample with PVP molecule weight of 58k, the morphology is particularly intricate which is consisted of nanowires with an average diameter of ~55 nm and regular polyhedral nanoparticles in Figure 1a. In case of P2 sample with the intermediate chain length of 360k, the number of nanoparticles gets less, but with larger nanowires of ~125 nm in diameters in Fig 1b. When the chain length further increases, ultra-long hyperpure Ag NWs with a uniform diameter of ~38 nm, formed, as in Figure 1c. For short chains of PVP, the smaller size with less steric hindrance, leaves the protection of {100} planes loose, causing the inefficient block of reduced Ag species accessing onto the surfaces, which induces large size of nanowires and symbiotic nanoparticles. When the length of chains is long to be a certain degree, because of its large size of molecule tails, forming a hard protecting layer on {100} plane, the large steric hindrance may tightly block the diffusion of Ag species into the


7


Page 8 of 23

surfaces. When the protection layer is incomplete like P2, it still can cause the large size of nanowires and symbiotic nanoparticles.

Figure 2. SEM images of Ag morphologies induced by mixed chain length of PVP molecules: (a) P12 (1:1 wt% Mw=58k and 360k); (b) P13 (1:1 wt% Mw=58k and 1300k); (c) P23 (1:1 wt% Mw=360k and 1300k), (d) Diameter distribution of P12, P13 and P23 samples, and (e) schematic drawing of different chain length of PVP molecules incorporated with Ag NWs. (Scale bar: 500 nm) In case of mixed PVP promoted Ag NWs, the morphologies are distinctly getting improved, in the evaluation mode of lengths, diameters, and yields of nanowires. In sample of P13 with the mixture of long and short chains in Figure 2, the nanowires become more uniform, along with reduced diameters to be ~32 nm and increased length up to ~58 µm, compared with those of single type PVP molecules. In case of the intermediated participated mixtures of P12 and P23, the proportion of nanoparticles reduced rapidly, in contrast with that of single intermediate PVP


8

Page 9 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


promoted Ag NWs as displayed in Figure 2a and 2c. The possible explanations can be as sketched in Figure 2e: (1) the coverage of PVP on {100} planes could be improved, by insertion of the shorter chain molecules into the long chain molecules; (2) the gaps between adjacent PVP molecules could be reduced, the diffusion of Ag to the surface being efficiently blocked; (3) the steric effect is also reinforced by the insertion of the shorter chain molecules into the long chain molecules. The corresponding TEM of P13 sample was presented in Figure 3a, showing the uniform diameter of ~32 nm and obvious decahedral tips, indicating of the good protection on Ag seeds for selective growth of Ag NWs. So as to investigate the influence of single and mix PVP molecules promoted Ag NWs on the phase structure, XRD analysis for typical P3 and P13 samples were carried out as presented in Figure 3b. Both P3 and P13 samples displayed five distinct diffraction peaks at 38.1, 44.2, 64.4, 77.4 and 81.5°, which are indexed as (111), (200), (220), (311), and (222) planes of the cubic face-centered silver (JCPDS file No. 04-0783),24,31 testifying there is no obvious difference of the phase structure between single and mix PVP molecules promoted Ag NWs, except the size.

Figure 3. (a) TEM image of P13 sample, and (b) XRD patterns of P3 and P13 samples. Inset of (a) is the enlarged tip part of single P13 nanowire. (Scale bar: 100 nm)


9


Page 10 of 23

The additives of chlorides such as NaCl or KCl, are commonly used to promote Ag NWs growth, identified to be favorable of the selective growth of AgCl seeds, and the oxidative etching evoked by reductive cations, like Cu+, Fe2+.25,27 Trace salts assisted Ag NWs growth has drawn wide concerns, however, systematic study of the categories of chlorides additives, especially in irreducible salts, on the effect of Ag morphologies still has a large number of vacancies. Thus, we propose to systematically research the categories of chlorides additives on the impact of Ag morphological evolution. To expel other factors, we fixed the concentrations of trace salts along with other experimental conditions, besides the type of cations. To better understand the role of the cations, a number of controlled experiments were conducted. On the account of the diversity of chemical properties, such as ionic radii, surface charges, and ionic strength, a series of chlorides additives, keeping the molar concentration of Cl- the same, were employed in our system, which are LiCl, NaCl, KCl, NH4Cl, ZnCl2, CaCl2, CuCl2 and SnCl2, respectively as listed in Table S2.


10

Page 11 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a-c) SEM images of Ag NWs induced by different trace salts: (a) C1 sample (trace salt: LiCl); (b) C3 sample (trace salt: KCl); (c) C4 sample (trace salt: NH4Cl). (d-f) UV absorption spectra of Ag morphologies promoted by different trace salts: (d) C1, C2, C3, C4 samples after 0.5 h reaction; (e, f) C1 and C4 samples as a function of reaction time, respectively. (Scale bar: 500 nm) As seen in Figure 4(a-c), the as-synthesized Ag NWs promoted by LiCl, NaCl, KCl are similarly thin in diameter, long in length, while, in case of that by NH4Cl, the diameter becomes thick, and the length shortens, along with a certain amount of particles. This is possibly attributed to the differences in the chemical properties, like ionic radii, ionic potential, and ionic strength. Initial AgCl crystals as the seeds for nanowires growth, which are stabilized by PVP molecules, are electropositive due to the extra adsorption of Ag+ cations. Therefore, the stability of AgCl crystals is of vital importance. Any factor, which causes the fluctuation of AgCl seeds or their surface properties, may induce the morphological change. With the addition of chlorides, tiny heterogeneous cations can be adsorbed in company with Ag+ adsorption, to some extent affecting the surface properties of AgCl seeds. In case of the samples with Li+, Na+, K+, having very similar chemical properties due to the same main group, and strong ionic character but different ion radii, the finial morphologies did not appear to be much different. The relative thicker diameter with Li+ could be attributed to the stronger polarization evoked by higher ionic potential (φ) defined as Z/r (Z: the electric charge, r: ionic radius), but it is still very weak, resulted the similar phenomenon with that of Na+, K+. A big difference in NH4Cl induced Ag NWs emerges, consisted of nanowires with larger diameter and a certain amount of nanoparticles. We ascribe this to the unstable effect of the Ag seeds etched by NH4+cations. At a high temperature of 160 °C, the solubility of AgCl is generally enhanced, which etches the AgCl


11


Page 12 of 23

seeds, and forms Ag(NH3)2+ complexing due to its high stability constant (1.12 × 107). On the other side, the released Cl- ions can form fresh AgCl seeds to accelerate nanoparticles or nanowires formation. To confirm our claim, we performed the UV–visible absorption of trace salts assisted Ag NWs as presented in Figure 4(d-f) and Figure S2. After 0.5 h, a prominent peak at 405 nm, appeared in all C1, C2, and C3 samples, indicating the AgCl seeds already formed. After 1 hour, a blue shift of 405 to 372 nm, evidenced the formation of Ag NWs, and a new peak at 350 nm, which has been commonly associated with Ag nanowires, also belongs to the optical signature of silver nanowires.24-25,32 Those peaks have been stabilized within 1 hours, stating the fast growth process of Ag NWs. In case of that by NH4Cl, quite a different situation arises in the initial stage. A sharp peak at 372 nm appeared in advance, compared with that of C1, C2, and C3 three samples, demonstrating that it is compatible with our propose of NH4+ cation etched acceleration process. After 1 h, it also stabilized, showing the similar but broader peaks than those of C1, C2, and C3 samples, which indicates the larger distribution of nanowires.


12

Page 13 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. (a-c) SEM images of Ag NWs induced by different trace salts: (a) C5 sample (trace salt: ZnCl2); (b) C6 sample (trace salt: CaCl2); (c) C7 sample (trace salt: CuCl2). (d-f) UV absorption spectra of Ag morphologies promoted by different trace salts: (d) C5, C6, C7, C8 samples after 0.5 h reaction; (e, f) C5 and C7 samples as a function of reaction time, respectively. (Scale bar: 500 nm) Very different phenomenon was observed in regard to the finial morphologies impelled by ZnCl2, CaCl2, CuCl2 and SnCl2 as shown in Figure 5(a-c), and Figure S3. More effect of bivalent metallic cations on the morphologies was observed. Ag NWs also formed in the samples with Zn2+ and Ca2+, which is firstly discovered in our work. While regular polyhedrons formed in that with Cu2+, irregular particles in that with Sn2+. The difference between two kinds of salts can be seen in the UV–visible absorption in Figure 5(d-f), Figure S4 and S5. In initial 30 minus, four samples already shows different trend with that of alkali salts, indicating the different growth process. A broad peak at 405 nm emerges in all samples, attributed to the sign of Ag colloidal seeds. A weak peak at 320 nm can be seen in all sample, is possibly ascribed to coordination of M2+ with PVP. After 1 hour, the sign peaks of Ag NWs appeared in C5 and C6 samples, while, a broad peak at 420 nm arises for C7 sample, which is ascribed to the SPR of regular polydedrons, and along with the time going, the peaks disappeared in C8 sample, stating the irregular bulk formed in the end. This can be predicated as the strong interaction of bivalent metallic cations (defined as Mn+) with the AgCl seeds as illustrated in Figure 6. Under high temperature and pressure, AgCl keeps the kinetic equilibrium: AgCl⇔Ag+ + Cl-. Under the condition of M2+, on one side, AgCl seeds prefer to absorbing the M2+ rather than Ag+ due to more surface charges, blocking the Ag+/Ag0 diffusion, on the other side, the addition of M2+ may influence the solubility balance of AgCl. For


13


Page 14 of 23

Zn2+ and Ca2+, to some extent, the blocking reacts, but the effect on the solubility balance of AgCl could be neglected due to their insensitive properties. Thus, the similar nanowires in samples with Zn2+ and Ca2+, was synthesized. For Cu2+ case, due to its complicated complex reaction with Cl-, leading to the seed anisotropic etched because of the re-dissolving of AgCl. From another point of view, some Cu+ cations reduced by EG can anisotropically react with the adsorbed atomic oxygen, and prompt the formation of the well-regular polyhydrons. For Sn2+, which is a strong oxidant, the following reaction may occur: 2 → As a result, Sn2+ accelerated the reduction rate of Ag+. The fast reduction of Ag+, on one side, destroys the anisotropy due to the adsorption of Ag on the AgCl seed, and on the other side, promotes the aggregation of Ag. Thus, the finial irregular nanoparticles were formed.

Figure 6. The illustration of Ag NWs evoked by trace salts.


14

Page 15 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


From the above analysis, some commonplaces in the formation of Ag NWs promoted between alkali metal, and divalent metal with electrochemical inactive chlorides, are dissolution and redissolution, and complexation and recomplexation processes. Withal, other important parameters affecting the Ag NWs growth are surface charges, ionic polarization and redox ability. Those are distinctly important for the study of metal nanowires growth. Ag NWs have been reputed as TCEs substituting for ITO owing to their promising optical and electrical performance.28,33 To excavate their potential as TCEs, we employed our pristine Ag NWs without any protection layer and post-treatment as a TCE applied in CIGS-based photovoltaic devices. The P3 and P13 samples, due to their smaller diameter and ultra-long length, was employed in the PV device with a architecture of SLG/Mo/CIGS/CdS/i-ZnO/Ag NWs by a spin coating technique as shown in of Figure 7.


15


Page 16 of 23

Figure 7. (a) The SEM image of the spin coated Ag NWs as a TCE on CIGS-based PV device; (b) I-V curve of P3 and P13 prepared solar cell devices; (c) The illustrated structure of CIGSbased PV device; (d) Graphical explanation for unpliable and pliable Ag NWs as a TCE on buffer layer surface. Table 1. Solar cell parameters based on P3 and P13 samples (short-circuit current (Jsc), open circuit voltage (Voc), fill factor (FF), photon to current conversion efficiency (η), series resistance (RS), shunt resistance (Rsh)).

Samples

Jsc (mA/cm2)

Voc (V)

FF (%)

η (%)

Rs (Ω·cm2 )

Rsh (Ω·cm2 )

Transmittance (%) (at 550 nm)

P3

20.03

0.379

54.5

4.13

21.82

821.4

81.6

P13

23.09

0.380

56.6

4.97

16.8

812.7

82.2

The conducting network of Ag NWs can be clearly seen on the top surface as shown in Figure 7a, with the sheet resistances of 28.4, and 25.2 Ω/□ under transparency more than 80% as shown in Figure S6, for P3 and P13, respectively, fully meeting the requirements of TCEs. The influence of Ag NWs-based TCEs on solar cell performance is presented in Figure 7b and Table 1. Obviously, both samples possess an excellent efficiency over 4 %, which is much higher than those of relative reports as summarized in Table S1. A highest efficiency of 4.97% with a Jsc of 23.09 mA/cm2, a Voc of 0.380 V and a FF of 56.6% was achieved for the P13 sample. Although P3 has lower efficiency of 4.13% than that of P13, it also gains a good parameter value with a Jsc of 20.03 mA/cm2, a Voc of 0.379 V and a FF of 54.5%. The exciting results in our work could be understood as the followings illustrated in Figure 7d: (1) the improved adhesion between pliable Ag NWs and the top of buffer layer surface, facilitating the electron flow from the buffer layer to the top electrode; (2) the increased contact area for more electron going through; (3) the


16

Page 17 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


emergent electron flow along the buffer layer surface due to the good adhesion. For the difference between the two samples, we attribute this to the following reasons: (a) thinner diameter induced higher transmittance; (b) better adhesion and junction, resulting in the smaller contact resistance, more beneficial to the charge transfer. Although it was verified that the pristine Ag NWs provide a good potential for TCEs utilized in solar cell applications, the efficiency is still low compared with that of ITO-based device, due to the leak of photo-excited electrons from the voids space between Ag NWs. Thus, more efforts are underway in our group to further improve the solar cell efficiency. 4. CONCLUSIONS To address the issue of low efficiency of PV devices on pristine Ag NWs as a TCE resulted from weak adhesion and small contact area, we have designed a facile polyol-modified reduction approach to synthesize a wide range of Ag NWs with controllable lengths and diameters. The length of PVP molecules chains, along with the cationic categories of chloride, on the effect of Ag morphological evolution, was systematically studied. The mixed long and short chains of PVP molecules were found to dramatically increase the length and shrink the diameter, with the aid of the efficient passivation of PVP molecules on {100} planes of Ag seeds by insertion of the short chains in between long chains. Interestingly, cations have been found to be very sensitive to the Ag NWs formation, due to their delicate nature differences, for instance, ionic radii, surface charges, ionic strength, as well as redox abilities. As expected, our pristine Ag NWs without protection layer and post-treatment as a TCE applied in photovoltaic device with an architecture of SLG/Mo/CIGS/CdS/i-ZnO/Ag NWs, have achieved an impressive efficiencies of 4.13, and 4.97 %, respectively.


17


Page 18 of 23

ASSOCIATED CONTENT Supporting Information Additional figures and images are provided, along with explanations in the main text. The Supporting Information is available free of charge. AUTHOR INFORMATION Corresponding Author * E-mail address: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Program for Changjiang Scholars and Innovative Research Team in University (No. PCS IRT_15R18), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (B20140004), and the National Natural Science Foundation of China (Nos. 61376061, 11274093, 51572070). REFERENCES 1. Ye, S.; Rathmell, A. R.; Chen, Z.; Stewart, I. E.; Wiley, B. J. Metal Nanowire Networks: The Next Generation of Transparent Conductors. Adv. Mater. 2014, 26, 6670-6687. 2. Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. OneDimensional Nanostructures: Synthesis, Characterization, and Applications. Adv. Mater.


18

Page 19 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2003, 15, 353-389. 3. Yun, H. J.; Kim, S. J.; Hwang, J. H.; Shim, Y. S.; Jung, S.-G.; Park, Y. W.; Ju, B.-K. Silver Nanowire-IZO-Conducting Polymer Hybrids for Flexible and Transparent Conductive Electrodes for Organic Light-Emitting Diodes. Sci. Rep. 2016, 6, 34150. 4. De, S.; Lyons, P. E.; Sorel, S.; Doherty, E. M.; King, P. J.; Blau, W. J.; Nirmalraj, P. N.; Boland, J. J.; Scardaci, V.; Joimel, J.; Coleman, J. N. Transparent, Flexible, and Highly Conductive Thin Films Based on Polymer−Nanotube Composites. ACS Nano 2009, 3, 714720. 5. Hsu, P.-C.; Kong, D.; Wang, S.; Wang, H.; Welch, A. J.; Wu, H.; Cui, Y. Electrolessly Deposited Electrospun Metal Nanowire Transparent Electrodes. J. Am. Chem. Soc. 2014, 136, 10593-10596. 6. Dong, H.; Wu, Z.; Jiang, Y.; Liu, W.; Li, X.; Jiao, B.; Abbas, W.; Hou, X. A Flexible and Thin Graphene/Silver Nanowires/Polymer Hybrid Transparent Electrode for Optoelectronic Devices. ACS Appl. Mater. Interfaces. 2016, 8, 31212-31221. 7. Liang, X.; Zhao, T.; Hu, Y.; Zhu, P.; Sun, R.; Lu, D. q.; Wong, C. CuCl2 and Stainless Steel Synergistically Assisted Synthesis of High-Purity Silver Nanowires on a Large Scale. RSC Adv. 2014, 4, 47536-47539. 8. Kim, T.; Canlier, A.; Kim, G. H.; Choi, J.; Park, M.; Han, S. M. Electrostatic Spray Deposition of Highly Transparent Silver Nanowire Electrode on Flexible Substrate. ACS Appl. Mater. Interfaces. 2013, 5, 788-794. 9. Jiu, J.; Tokuno, T.; Nogi, M.; Suganuma, K. Synthesis and Application of Ag Nanowires via a Trace Salt Assisted Hydrothermal Process. J. Nanopart. Res. 2012, 14, 975. 10. Kim, A.; Won, Y.; Woo, K.; Jeong, S.; Moon, J. All-Solution-Processed Indium-Free


19


Page 20 of 23

Transparent Composite Electrodes based on Ag Nanowire and Metal Oxide for Thin-Film Solar Cells. Adv. Funct. Mater. 2014, 24, 2462-2471. 11. Kim, A.; Won, Y.; Woo, K.; Kim, C.-H.; Moon, J. Highly Transparent Low Resistance ZnO/Ag Nanowire/ZnO Composite Electrode for Thin Film Solar Cells. ACS Nano 2013, 7, 1081-1091. 12. Chung, C.-H.; Song, T.-B.; Bob, B.; Zhu, R.; Duan, H.-S.; Yang, Y. Silver Nanowire Composite Window Layers for Fully Solution-Deposited Thin-Film Photovoltaic Devices. Adv. Mater. 2012, 24, 5499-5504. 13. Singh, M.; Rana, T. R.; Kim, S.; Kim, K.; Yun, J. H.; Kim, J. Silver Nanowires Binding with Sputtered ZnO to Fabricate Highly Conductive and Thermally Stable Transparent Electrode for Solar Cell Applications. ACS Appl. Mater. Interfaces. 2016, 8, 12764-12771. 14. Shin, D.; Kim, T.; Ahn, B. T.; Han, S. M. Solution-Processed Ag Nanowires + PEDOT:PSS Hybrid Electrode for Cu(In,Ga)Se2 Thin-Film Solar Cells. ACS Appl. Mater. Interfaces. 2015, 7, 13557-13563. 15. Singh, M.; Jiu, J.; Sugahara, T.; Suganuma, K. Thin-Film Copper Indium Gallium Selenide Solar Cell Based on Low-Temperature All-Printing Process. ACS Appl. Mater. Interfaces. 2014, 6, 16297-16303. 16. Skrabalak, S. E.; Wiley, B. J.; Kim, M.; Formo, E. V.; Xia, Y. On the Polyol Synthesis of Silver Nanostructures: Glycolaldehyde as a Reducing Agent. Nano Lett. 2008, 8, 2077-2081. 17. Xu, J.; Hu, J.; Peng, C.; Liu, H.; Hu, Y. A Simple Approach to the Synthesis of Silver Nanowires by Hydrothermal Process in the Presence of Gemini Surfactant. J. Colloid Interface Sci. 2006, 298, 689-693. 18. Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. DNA-Templated Assembly and Electrode


20

Page 21 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Attachment of a Conducting Silver Wire. Nature 1998, 391, 775-778. 19. Chen, D.; Qiao, X.; Qiu, X.; Chen, J.; Jiang, R. Large-Scale Synthesis of Silver Nanowires via a Solvothermal Method. Journal of Materials Science: Mater. El. 2010, 22, 6-13. 20. Schuette, W. M.; Buhro, W. E. Polyol Synthesis of Silver Nanowires by Heterogeneous Nucleation; Mechanistic Aspects Influencing Nanowire Diameter and Length. Chem. Mater. 2014, 26, 6410-6417. 21. Coskun, S.; Aksoy, B.; Unalan, H. E. Polyol Synthesis of Silver Nanowires: An Extensive Parametric Study. Cryst. Growth Des. 2011, 11, 4963-4969. 22. Visaveliya, N.; Kohler, J. M. A Self-Seeding Synthesis of Ag Microrods of Tuned Aspect Ratio: Ascorbic Acid Plays a Key Role. Nanotechnology 2013, 24, 345604. 23. Whitcomb, D. R.; Clapp, A. R.; Bühlmann, P.; Blinn, J. C.; Zhang, J. New Perspectives on Silver Nanowire Formation from Dynamic Silver Ion Concentration Monitoring and Nitric Oxide Production in the Polyol Process. Cryst. Growth Des. 2016, 16, 1861-1868. 24. Yang, W.; Li, J.; Zhong, Y.; Qian, H.; Li, Z.; Hu, Y. Facile Cl--Mediated Hydrothermal Synthesis of Large-Scale Ag Nanowires from AgCl Hydrosol. CrystEngComm 2013, 15, 2598-2600. 25. Yang, Z.; Qian, H.; Chen, H.; Anker, J. N. One-pot Hydrothermal Synthesis of Silver Nanowires via Citrate Reduction. J. Colloid Interface Sci. 2010, 352, 285-291. 26. Ran, Y.; He, W.; Wang, K.; Ji, S.; Ye, C. A One-Step Route to Ag Nanowires with a Diameter Below 40 nm and an Aspect Ratio Above 1000. Chem. Commun. 2014, 50, 1487714880. 27. Jiu, J.; Sugahara, T.; Nogi, M.; Suganuma, K. Ag Nanowires: Large-Scale Synthesis via a Trace-Salt-Assisted Solvothermal Process and Application in Transparent Electrodes. J.


21


Page 22 of 23

Nanopart. Res. 2013, 15, 1588. 28. Wang, S.; Tian, Y.; Ding, S.; Wang, C. The Role of Chloride Ions in Rapid Synthesis of Ultra-Long Silver Nanowires for Flexible Electrodes. Mater. Res. Express 2016, 3, 075007. 29. Hu, J.; Cheng, K.; Han, K.; Kuang, Z.; Liu, J.; Hu, B.; Tian, Q.; Wu, S.; Du, Z. 10.3% Efficient Cu(In, Ga)Se2 Solar Cells Fabricated by Selenization of Cu-In-Ga Precursors. Mater. Lett. 2016, 174, 114-117. 30. Cheng, K.; Huang, Y.; Liu, J.; Xue, M.; Kuang, Z.; Lu, Z.; Wu, S.; Du, Z. Chalcogenide Solar Cells Fabricated by Co-Sputtering of Quaternary CuIn0.75Ga0.25Se2 and In Targets: Another Promising Sputtering Route for Mass Production. J. Alloys Compd. 2016, 684, 237244. 31. Liu, S.; Sun, B.; Li, J.-g.; Chen, J. Silver Nanowires with Rounded Ends: Ammonium Carbonate-Mediated Polyol Synthesis, Shape Evolution and Growth Mechanism. CrystEngComm 2014, 16, 244-251. 32. Yang, C.; Tang, Y.; Su, Z.; Zhang, Z.; Fang, C. Preparation of Silver Nanowires via a Rapid, Scalable and Green Pathway. Journal of Materials Science & Technology 2015, 31, 16-22. 33. Sannicolo, T.; Lagrange, M.; Cabos, A.; Celle, C.; Simonato, J. P.; Bellet, D. Metallic Nanowire-Based Transparent Electrodes for Next Generation Flexible Devices: a Review. Small 2016, 12, 6052-6075.


22

Page 23 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC GRAPHICS

The pristine Ag NWs as a top TCE applied in CIGS-based photovoltaic device has achieved an impressive efficiency of 4.97 %.


23

Protection-free Ag Nanowires as a Transparent Conductive Electrode

Recommend Documents