Subscriber access provided by UNIVERSITY OF ADELAIDE LIBRARIES
Article
Low temperature solution processed random silver nanowire as promising replacement for Indium Tin Oxide Arastoo Teymouri, Supriya Pillai, Zi Ouyang, Xiaojing Hao, Fangyang Liu, Chang Yan, and Martin A. Green ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13085 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 13, 2017
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 Materials & Interfaces 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 13
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 Materials & Interfaces
Low temperature solution processed random silver nanowire as promising replacement for Indium Tin Oxide Arastoo Teymouri*, Supriya Pillai, Zi Ouyang, Xiaojing Hao, Fangyang Liu, Chang Yan, Martin A. Green School of Photovoltaic and Renewable Energy (SPREE), University of New South Wales (UNSW), Sydney 2052, Australia KEYWORDS: Silver nanowire, Transparent conductive layer, Low temperature process, Conductive Atomic Force Microscopy, Solar cell
ABSTRACT: A low temperature solution-based process for depositing silver nanowire (AgNW) networks for use
as transparent conductive top electrode is demonstrated. These AgNWs when applied to Cu2ZnSnS4 solar cells outperformed indium tin oxide as the top electrode. Thinner nanowires allow the use of lower temperatures during processing while longer wires allow lowered sheet resistance for the same surface coverage of NWs, enhancing the transmittance/conductance trade-off. Conductive atomic force microscopy and percolation theory were used to study the quality of the NW network at the microscale. Our optimised network yielded a sheet resistance of 18 Ω/□ and ~95% transmission across the entire wavelength range of interest for a deposition temperature as low as of 60°C. Our results show that AgNWs can be used for low temperature cell fabrication using cheap solutionbased processes that could also be promising for other solar cells constrained to low processing temperatures such as organic and perovskite solar cells.
Introduction Silver nanowire (AgNW) networks as transparent conductive (TC) film have shown comparable optoelectronic 1,2 features with Indium Tin Oxide (ITO) in terms of sheet resistance and optical transmitance . Many studies have 3-5 been conducted in the last decade to develop possible applications of AgNWs as TC layers , but there are still some significant issues in the applicability of AgNWs. These include high surface roughness and more importantly the fact that the conductivity of AgNW network is enabled only after a high temperature annealing heat5-8 treatment . To overcome high resistance at the wire-to-wire junction, which prevents electrons from easily trav9 ersing from one wire to another, annealing at high temperatures (≥150 °C) has been widely applied as a stand10 ard treatment . At such temperatures, good contact between adjacent nanowires can be made, achieving sheet 11 resistance as low as 10 Ω/□ . High temperature annealing could damage heat-sensitive devices in cases where AgNWs act as top electrodes 12 13 14 of absorber layer. Emerging organic , perovskite and Cu2ZnSnS4 (CZTS) solar cells are temperaturesensitive devices; the highest desirable processing temperature is preferably 200 °C and even below 100 °C for some highly-sensitive devices so that the structure of those solar cells remains unscathed. Studies have placed more efforts on replacing rather than optimising the heat treatment, which makes the fabrication process more 15 16-18 19-22 complicated . For instance, physical pressing , coupling of AgNWs with other conductive materials and 23-25 chemical treatment of AgNWs have been tried, but have not yet resulted in advantageous outcomes com26 pared to ITO . Firstly, these methods can rarely reach as low a sheet resistance as 10 Ω/□ as obtained by annealing heat-treatment. Secondly, all added materials and treatments lead to higher cost of the final product. Thirdly, the complexity of production restricts them to laboratory scale implementation. Though many drawbacks 27-30 of the AgNWs can be rectified by using lithography processes , which provides a controllable deposition technique to relieve the high junction resistivity. The high associated cost of production is the main barrier for these approaches to be nominated as an ITO replacement. In this work, we aim to show that low temperature treatment is capable of providing low sheet resistance and high transmittance in random AgNWs networks for applications where processing temperatures are constrained. Optical nanowelding using hotspots created by excitation of localised surface plasmon resonances at the junction of the silver nanowires has previously been successfully demonstrated to provide large interconnected networks 31 . This light induced welding technique can be used for low thermal budget substrates such as targeted in this ACS Paragon Plus Environment
ACS Applied Materials & Interfaces
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 13
study. However, the associated junction temperatures in past work have not been measured and it is expected that these may exceed 150°C. The subsequent impact of hotspots on the underlying material was also not clear and may cause degradation of the absorber layer for heat sensitive devices. A detailed investigation hence needs to be conducted to isolate any negative effects on the solar cell due to such localised heating before the positive effects of a good conductive network can be understood. More recently partial welding for Ag NWs under the ex2 32 posure of natural sunlight with power density of 1W/m was achieved . Other room temperature processes for 33 deposition of AgNWs using polymer dispersed liquid crystals has also been reported. However, these techniques are not suitable to be adopted into device/solar cell processing.
In this work, we propose a simple, more controlled and monitored environment using a solution-based technique for deposition and low temperature treatment to achieve good conductivity and a high transmittance, large area AgNW networks with properties comparable to those obtained from high temperature treatment. In this case the AgNW is used as top electrode for heat sensitive solar cells ie. the AgNW is deposited after the absorber layer and hence constrained by temperature and extra chemical processing that can degrade the performance of the cell. Hence, the low temperature process is the focus of this study and best results are achieved at the temperature of 60°C with thinner and longer high aspect ratio NWs (here aspect ratio refers to the ratio of the length to the diameter for individual nanowires). Our AgNWs network quality is validated using conductive atomic force microscopy (C-AFM) and numerical fit parameters using a percolation equation and we demonstrate feasibility of applying these structures on CZTS cells with superior performance to those with conventional ITO layers.
METHODOLOGY AgNWs with three different aspect ratios (length to diameter) of 200, ~700 and ~3000 were tested. The AgNW diameters were 150nm, 90nm and 70nm, with lengths 30µm, 60µm and 200 µm respectively. To obtain a consistent TC layer, three solution-based methods for thin-film deposition were examined, specifically: spin-coating, drop-casting and dip-coating. Eventually, drop-casting was selected owing to its controllability in relation to the time of solvent evaporation (wetness and dryness of the solution), pattern and surface roughness of AgNWs film as explained later. The AgNWs were sourced from ACS Materials Group with a concentration of 20mg/mol in Isopropyl alcohol (IPA). The dispersion was then diluted with ethanol to 0.04 mg/mol and the solution was gently stirred to prevent breakage of nanowires. The AgNWs were then coated on to the substrate (0.02 ml in each deposition) by dropcasting. Different substrates, such as quartz and quartz coated with ZnO buffer layer were used to investigate the compatibility of the interface between the AgNWs and the substrate for potential applications as TC layer in thinfilm solar cells. The deposition technique of drop-casting was optimized to coat homogenously on 2.5cm × 2.5cm quartz substrates. However, we are also optimizing other methods like spray-coating to have more controlled deposition on large area solar cells for future work. Three different techniques of heat treatment were examined for each AgNW diameter and deposition approach. In the first technique, described as the annealing treatment, AgNWs were heated in the oven at a set temperature after deposition for 20 min at a wide range of temperatures from 30-200°C with increment of 10 °C and from 200-400°C with increment of 25°C each to investigate the effect on sheet resistance. The second technique, post-treatment, was conducted on a hot plate after AgNWs deposition. After completely drying out the solvent at room temperature, each sample was gradually heated up at a rate of 30°C/min and the sheet resistance of samples was measured at sequential temperatures using the four-point probe technique. In the third technique called in-situ treatment, the process was the same as the post-treatment process except that the samples were heated up on the hot plate immediately after deposition without drying. In order to achieve a uniform coating immediately after deposition, samples were shaken to decrease the surface roughness and reach a uniform pattern. Four-point probe measurement was conducted with Jandel Model RM3 for each sample at 10 different points to attain the average value of sheet resistance. Transmittance was examined by CARY 500scan UV-Vis-NIR spectrophotometer. Final samples were imaged by Focused Ion Beam Scanning Electron Microscopy (FIB-SEM). The FIB-SEM images were taken at an angle of 54° where the contact points of nanowires can be distinctly scrutinized by virtue of the side-view. Conductivity of the AgNW network in microscopic scale was verified by conductive atomic force microscopy (C-AFM). A variety of samples including different substrates and concentrations of AgNWs were imaged by JEOL 3500 and PeakForce Tuna C-AFM, respectively. Gwyddion software was used to analyse roughness using AFM images of the AgNWs network deposited on quartz. Surface coverage is defined as the area covered by the NWs / total area and was calculated using the image processing software ImageJ. Surface coverage value was taken as average of 10 different regions on a sample using SEM images of similar
ACS Paragon Plus Environment
Page 3 of 13
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 Materials & Interfaces
magnification. The external quantum efficiency (EQE) of AgNWs replacing the ITO on CZTS solar cells was measured by spectral response/QE/IPCE measurement system.
RESULTS AND DISCUSSION Traditional annealing treatment in the oven revealed that high temperature annealing > 200°C provides enough 6-7, 34 energy to fuse adjacent AgNWs at the contact points, which is consistent with the literature ; although the impact is slightly different for AgNWs with the different aspect ratios used in this study. Figure 1 shows the effect of temperature on sheet resistance for various aspect ratio NWs. With increased temperature, the sheet resistance first decreased attributed to better joint formation and reduced contact resistance and then increased 35-37 . This separation occurs at the NWdue to snapping of the NWs after passing the optimum temperature point 38 NW junction due to Rayleigh instability (see SEM images in Figure 1). It shows that the higher aspect ratio and thinner nanowires started to fuse at lower temperatures below 150°C. This can be explained using the sizedependent cohesive theory where the melting temperature for nanosolids like nanoparticles, NWs or nanofilms 39 can decrease with size . In some samples, it was interestingly observed that gently drying out solvent in the range of 50-70°C immediately after deposition gave a conductive AgNWs network. Previously, a slight reduction in sheet resistance was also observed in the samples with annealing treatment at temperature around 60°C (Figure 1). Hence, the two alternative methods of post-treatment and in-situ treatment mentioned above were employed to further investigate at low temperature (
