Research Article pubs.acs.org/journal/ascecg
Recycling Indium Tin Oxide (ITO) Electrodes Used in Thin-Film Devices with Adjacent Hole-Transport Layers of Metal Oxides Minh Trung Dang,*,† Josianne Lefebvre,‡ and James D. Wuest*,† †
Département de chimie, Université de Montréal, Montréal, Québec H3C 3J7, Canada Département de génie physique, École Polytechnique de Montréal, Montréal, Québec H3T 1J4, Canada
‡
S Supporting Information *
ABSTRACT: Many thin-film optoelectronic devices use electrodes made of tin-doped indium oxide (ITO), which is acceptably conductive, as well as virtually transparent and colorless. Regrettably, indium is an uncommon element and its price continues to rise, so it is increasingly important to recover ITO electrodes from devices that are no longer needed. Previous work has shown that simple sonication in neutral water can separate intact ITO electrodes from other components in typical devices, in which the active components and ITO are separated by an ionic buffer layer of poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Sonication in water appears to be effective because it favors selective penetration and dissolution of PEDOT:PSS, thereby freeing the underlying ITO electrode. However, PEDOT:PSS is being replaced in emerging devices by the use of various metal oxides as hole-transport materials. We have now found that ITO electrodes in these new devices can be recycled by sonication in dilute aqueous base. The layers of ITO undergo only minor changes in composition and morphology, and the recovered electrodes can be reused many times to fabricate new devices without loss of performance. KEYWORDS: Recycling, Tin-doped indium oxide (ITO), Thin-film devices, Metal oxides
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include nanotubes,3,9 and films of graphene offer flexibility and 90% transparency, with an electrical resistance lower than that of conventional layers of ITO.10−12 Conductive polymers, including poly(aniline)13,14 and poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS),15,16 may also be suitable alternatives to ITO. Despite a typically lower conductivity, these polymers are more flexible, less expensive, and easier to process and manufacture. Forms of ZnO doped with In, Ga, or Al also have the potential to replace ITO.1,17 However, despite the ongoing search for substitutes, ITO electrodes are still found in most thinfilm molecule-based optoelectronic devices. The best of these devices are beginning to rival classical inorganic analogues in performance. For example, the efficiencies of the best OPVs incorporating ITO electrodes now exceed 10%.18−23 This success has triggered an international surge of research exploring new active molecular materials, their nanomorphology, and their underlying photophysical behavior, thereby increasing the need for glass-coated ITO electrodes in research and development.18,24,25 Finding ways to recover and reuse them has therefore become a priority. Such work is an important step toward the eventual goal of achieving sustainable production of green optoelectronic devices in which all
INTRODUCTION Tin-doped indium oxide (ITO), which is a solid solution of SnO2 in In2O3, is widely used to make electrodes in thin-film optoelectronic devices. ITO offers acceptable electrical conductivity, is virtually colorless and transparent in thin layers, and can be deposited by well-established methods.1−4 The rate of consumption of ITO is increasing rapidly because it is used in commercial technologies such as flat-panel displays. Regrettably, indium is an uncommon element normally obtained as a byproduct of the extraction of base metals.2,5 The low abundance of indium, combined with the high cost of extraction and rapidly growing demand, has led to steep increases in price. For example, the annual average price in 2014 was 22% higher than in 2013.6 Moreover, preparing thin layers of ITO requires advanced techniques for depositing and doping,2 and even when hightemperature methods of deposition under vacuum are replaced by low-temperature sol−gel methods, the costs of production remain high.7 It is also important to note that typical thin-film devices, such as organic light-emitting diodes (OLEDs), fieldeffect transistors (OFETs), and photovoltaic devices (OPVs), use ITO electrodes fabricated in geometrically well-defined patterns by lithography or wet etching. This inescapable processing further increases the cost of devices. Life-cycle assessments have shown that ITO accounts for 87% of the energy expended in making typical OPV devices.8 For these reasons and others, such as the limited mechanical flexibility of layers of ITO, effective substitutes are actively being sought. For example, prospective carbon-based replacements © 2015 American Chemical Society
Received: September 14, 2015 Revised: October 28, 2015 Published: October 30, 2015 3373
DOI: 10.1021/acssuschemeng.5b01080 ACS Sustainable Chem. Eng. 2015, 3, 3373−3381
Research Article
ACS Sustainable Chemistry & Engineering
have been widely employed in thin-film photovoltaic devices.18,33 P3HT was purchased from Rieke Inc., and PC60BM was generously provided by Solaris Chem Inc. A mixture of P3HT and PC60BM (1:0.8 by weight) was stirred at 40 °C overnight in chlorobenzene to give a solution containing P3HT at a concentration of 10 mg/mL. The solution was deposited on the underlayer of VOx by spin coating at 500 rpm for 60 s, and the films were then left in a covered glass Petri dish to allow slow evaporation and solvent annealing to occur at 25 °C, thereby favoring the self-organization of P3HT and PC60BM to form effective bulk heterojunctions.18 The average thickness of the resulting photoactive layer was measured by profilometry to be 100 ± 10 nm. Finally, a top electrode of Al with an average thickness of 100 nm was deposited thermally on the P3HT:PC60BM layer under vacuum (3 × 10−7 mbar). This yielded thin-layer devices with a glass/ITO/VOx/P3HT:PC60BM/ Al sequence of layers (Figure 1). The devices were subsequently
components are derived cheaply from renewable sources or can be recovered after use by simple methods. In 2014, Elshobaki and co-workers reported a method for recycling ITO electrodes used in standard molecule-based thinfilm optoelectronic devices.26 In such devices, ITO is typically separated from the active layer by a buffer of PEDOT:PSS, which is introduced to facilitate the movement of charges across the interface. In the method of Elshobaki et al., devices are first washed with chloroform to dissolve the active layer and then with water to remove the buffer layer of PEDOT:PSS, thereby liberating the ITO electrodes. Independently, we reported a greener way to recover ITO electrodes and reuse them without loss of performance.27 Our method involves simple sonication of spent devices in water at 40 °C, which leads to dissolution of PEDOT:PSS, separates upper layers containing the active layer and top electrode, and yields recovered ITO substrates that can be reused many times to fabricate new devices without loss of efficiency. The success of both methods of recycling hinges on the presence of PEDOT:PSS, which is selectively soluble in water, thereby allowing clean separation of ITO-coated glass substrates from the other components of standard thin-film devices. Despite this requirement, the reported methods of recycling are valuable because the use of PEDOT:PSS as a hole-transport layer remains widespread. However, the search for better substitutes continues at a rapid pace. In particular, devices that use layers of oxides of transition metals such as vanadium, molybdenum, tungsten, and nickel show greater stability,28,29 in part because the characteristic acidity and hygroscopic nature of PEDOT:PSS promote degradation of the underlayer of ITO.28 As a result, many new thin-layer devices such as OPVs and OLEDs now employ transition metal oxides as buffer layers,30 making it important to find ways to ensure that ITO electrodes in these devices can be recycled. We now report that washing such devices with dilute aqueous base (0.02 N NaOH) is an effective way to recover ITO electrodes that can be reused without a decrease in performance. In our study, we used OPVs fabricated with layers of vanadium oxide as representative devices, but we believe that similar methods can be used to recover intact ITO electrodes from devices incorporating layers of other transition metal oxides.
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Figure 1. Schematic representation of thin-film OPVs that were subjected to recycling. subjected to annealing in a glovebox at 165 °C for 30 min, which is known to improve the organization of P3HT and PC60BM in the photoactive layer and improve performance.18 Our devices had a cross bar configuration, and photovoltaic performances were measured using a shadow mask with an aperture that determined the active area of the devices (12.6 mm2). Assessment of the Performance of Thin-Film OPVs. As described previously,27 J−V curves were obtained using a Keithley 2400 Source Measure Unit, photocurrent was measured under standard illumination using a 150 W Oriel solar simulator (AM1.5G, 100 mW/ cm2), the intensity of the light was determined using a monosilicon detector calibrated by the National Renewable Energy Laboratory, and the reported measures of photovoltaic performance were taken as the average of values determined for all devices. Recycling by Recovering Glass/ITO Substrates and Reusing Them To Fabricate New Thin-Film OPVs. As set out in the cycle (C) of production and recovery described in our previous study,27 thin-film devices fabricated with pristine ITO substrates are considered to be reference cells and are labeled C0, whereas devices created from recycled ITO electrodes are labeled as Cn, where n is the number of previous uses of the substrate. After measurements of performance had been completed, reference cells were immersed in aqueous NaOH (0.02 N) and sonicated at 40 °C using a Branson 3510 ultrasonic cleaner. After 15 min, ITO substrates were totally freed of supernatant layers of Al, P3HT:PC60BM, and VOx. In contrast, clean intact ITO substrates could not be recovered by treating devices with other common agents that were tested, including neutral water, acetone, ethanol (95%), 2propanol, and glacial acetic acid. ITO substrates recovered after immersion in dilute aqueous base were then used to fabricate new thin-film devices as described above, starting with the same ultrasonic cleaning with a series of solvents, exposure to UV/ozone, and deposition of additional layers, including the buffer layer of VOx, the photoactive layer of P3HT:PC60BM, and the top electrode of Al. The resulting cells, now containing ITO recycled once, were labeled C1. The same cycles were repeated to demonstrate the ability to create new devices incorporating ITO substrates recycled multiple times. In each cycle, we simultaneously prepared devices using pristine ITO substrates, which were used as benchmarks to allow their
EXPERIMENTAL SECTION
Fabrication of Thin-Film OPVs. OPVs were fabricated by a standard protocol described previously.27 Patterned bottom electrodes of ITO-coated glass (12 mm × 18 mm, with a sheet resistance of ∼20 Ω/ square) were provided by either Colorado Concept Coatings LLC or the Kintec Company (Hong Kong). Before use, commercial electrodes were sequentially cleaned in an ultrasonic bath using deionized water, acetone, 95% aqueous ethanol, and 2-propanol. To destroy any residual contaminants, the cleaned substrates were then exposed to UV/ozone for 15 min.31 We prepared representative buffer layers of metal oxide by dissolving vanadium(V) oxytriisopropoxide in anhydrous 2-propanol (1:120 volume:volume),32 stirring the solution at 25 °C for 1 h, depositing it on ITO substrates by spin coating in air at a speed of 1000 rpm for 30 s, and then storing the layers at 25 °C in contact with air for 1 h to allow hydrolysis and formation of solid oxide, represented by the formula VOx. The average thickness of the resulting layers of oxide was 27 nm, as measured by spectroscopic ellipsometry. Once the ITO substrates had been coated with VOx, all subsequent steps in the production and characterization of devices were conducted in a glovebox under N2, with residual concentrations of O2 and H2O of
