Elsevier

Carbon

Volume 105, August 2016, Pages 33-41
Carbon

Inkjet-printed graphene electrodes for dye-sensitized solar cells

https://doi.org/10.1016/j.carbon.2016.04.012Get rights and content

Abstract

We present a stable inkjet printable graphene ink, formulated in isopropyl alcohol via liquid phase exfoliation of chemically pristine graphite with a polymer stabilizer. The rheology and low deposition temperature of the ink allow uniform printing. We use the graphene ink to fabricate counter electrodes (CE) for natural and ruthenium-based dye-sensitized solar cells (DSSCs). The repeatability of the printing process for the CEs is demonstrated through an array of inkjet-printed graphene electrodes, with ∼5% standard deviation in the sheet resistance. As photosensitizers, we investigate natural tropical dye extracts from Pennisetum glaucum, Hibiscus sabdariffa and Caesalpinia pulcherrima. Among the three natural dyes, we find extracts from C. pulcherrima exhibit the best performance, with ∼0.9% conversion efficiency using a printed graphene CE and a comparable ∼1.1% efficiency using a platinum (Pt) CE. When used with N719 dye, the inkjet-printed graphene CE shows a ∼3.0% conversion efficiency, compared to ∼4.4% obtained using Pt CEs. Our results show that inkjet printable graphene inks, without any chemical functionalization, offers a flexible and scalable fabrication route, with a material cost of only ∼2.7% of the equivalent solution processed Pt-based electrodes.

Introduction

Graphene has attracted significant interest in a variety of electronic and optoelectronic applications, including in photovoltaics. This is because it can serve multiple purposes: as a transparent conducting electrode [1], [2], as a channel for charge transport [3] and as a counter electrode material [4]. It has been reported that the edge plane sites of graphene exhibit faster electron-transfer kinetics than the basal plane sites [5]. Thus, exfoliated graphene flakes/platelets may offer good electrocatalytic properties. Coupled with this, electrical conductivity and chemical stability makes graphene an attractive counter electrode material.

Graphene can be prepared via various top-down and bottom-up approaches; the most widely exploited ones being mechanical cleavage [6], chemical vapour deposition (CVD) [7], and solution based methods such as chemical exfoliation [8] and ultrasonic-assisted liquid phase exfoliation (UALPE) [9], [10], [11], [12]. Though micromechanically cleaved graphene is ideal for fundamental studies due to the high quality of the exfoliated material, the low yield of the process renders it unsuitable for large-scale applications [6]. In recent years, CVD has been scaled up to produce large-area, high quality graphene [2], [7] accompanied by a significantly improved understanding of the growth mechanisms and graphene–catalyst interaction [13]. However, the high temperature production process and subsequent transfer to target substrate is not always ideal for photovoltaic devices. In addition, CVD grown mono- or few-layer graphene has limited exposed edges, a key requirement for enhanced electrocatalytic properties [5], [14]. Solution processing, meanwhile, allows scalable production of dispersions consisting of single- and few-layer graphene flakes under ambient conditions [9], [10], [11], [12]. These can be exploited as inks using existing functional printing and coating techniques, enabling graphene to be deposited onto substrates such as silicon and glass as well as flexible materials [15], [16], [17], [18]. Inkjet printing is of particular interest, allowing additive patterning, direct writing without the use of masks or stencils and low cost [15], [17], [19].

Amongst the different photovoltaic devices, dye-sensitized solar cells (DSSCs) offer certain advantages, including low cost materials [20], [21], economic fabrication, good low-light conversion efficiencies (>12%) and many colour design possibilities [22], [23]. The DSSC structure consists of a ∼10 μm thick mesoporous network of nanocrystalline semiconductor oxide (e.g. titanium dioxide, TiO2) deposited onto a conducting electrode or transparent conducting electrode (e.g. FTO, ITO etc), and an electrolyte. The porous network of nanocrystals (10–30 nm) provides the large surface area necessary for adsorption of a thin layer of dye molecules, typically a ruthenium(II) bipyridyl dye [24] or an organic dye [21], [25], [26], to allow for optimum light harvesting. Absorption of light by a dye molecule creates an excited molecular electronic state. The dye rapidly returns to its original oxidation state via electron transfer from iodide ions in the I/I3 redox electrolyte. The I3 ions formed by oxidation of I diffuse a short distance (<50 μm) through the electrolyte to the cathode, which is coated with a thin layer of platinum catalyst, where the regenerative cycle is completed by electron transfer to reduce I3 to I [27].

Two of the most important factors which influence the performance of a DSSC are the dye used as a sensitiser, and the counter-electrode material. The key properties of the dye are the absorption across the solar spectrum and the adsorption/adhesion of the dye molecules to the surface of the semiconductor oxide [28]. The most widely used sensitizers are those based on heavy transition metal co-ordination compounds (e.g. ruthenium (Ru) polypyridyl complexes) [23], [29] due to their efficient metal-to ligand charge transfer, intense charge-transfer absorption across the visible range and long excited lifetime [23]. However, Ru-polypyridyl based complexes are expensive (∼1200 $/g) [30], and contain a heavy rare earth metal, which is environmentally undesirable [31]. Thus, investigation into environmentally-friendly, economic and readily available dyes, including those extracted from plants, remains a strong interest [21], [25], [26], [32], [33].

Dyes produced from plant extracts are advantageous due to their wide availability, simple extraction process, usability without further purification, environmental sustainability, and low cost [21], [25], [26], [32], [33]. Anthocyanin molecules (water-soluble vacuolar pigments [34]) are found in tissues of higher plants (i.e. land plants that have lignified tissues or xylems for transporting water and minerals throughout the plant), and are responsible for their red–blue range of pigments, depending on pH [21], [25], [26], [32], [33], [34], [35], [36]. Anthocyanin molecules contain carbonyl and hydroxyl groups, which can adsorb on to the surface of porous TiO2 films, leading to photoelectron transfer from the anthocyanin molecule to the conduction band of TiO2 [21].

The counter-electrode, meanwhile, should be catalytically active and electrically conducting. It should also exhibit a low over-potential for rapid reduction of the redox couple to carry the generated photocurrent across the width of the solar cell. Platinum (Pt) is the most commonly used counter electrode material in DSSCs [37]. However, while the required platinum loading for optimum performance of the solar cell is small (∼3.2 g/m2) [38], [39], the dissolution of the platinum film in the corrosive I3/I electrolyte and high-temperature heat treatment (∼300–400 °C) [37], [40] required for good Pt-substrate adhesion limits their use on flexible substrates and in low-cost applications. This has necessitated the search for chemically stable and cost-effective counter-electrode materials for DSSCs such as various carbon nanomaterials [14], [41], [42], [43], [44], [45], [46] conductive organic polymers [47] and inorganic semiconductors [48]. In this context, graphene has emerged as a promising low-cost electrode material candidate, which provides both the conductive pathways and catalytic properties [41], [42], [43], [49], [50], [51], [52], [53], [54].

The performance of DSSCs with a graphene counter electrode is dependent on the structure of graphene. Although CVD graphene produces continuous layers with high electrical conductivity and comparable charge transfer resistance (Rct) to platinum [54], it has a very limited number of active sites for I/I3 electrocatalysis [55]. On the other hand, single and few-layer graphene nano-flakes obtained via solution processing have high density of active edge sites, offering high catalytic activity, and modest electrical conductivity [15], [17], [18] at a fraction of the cost. In addition, solution processed graphene can be used in combination with a variety of deposition technologies (such as inkjet [15], [16], [17], [18], [56], screen printing [42], spray coating [17], [44] or flexographic printing [43]) forming a versatile platform not only for DSSCs but also for any devices requiring such electrode materials for large scale fabrication. We note that there have been reports in the literature of graphene inks being used for DSSCs [14], [41], [42], [43]. However, these typically use functionalised forms of graphene such as graphene oxide [41], [42], [43]. While the presence of functional groups on graphene sheets can aid their exfoliation and dispersion into solvents, the electrical properties of the material are compromised, and cannot be fully restored to those of pristine graphene [57].

Here, we present formulation of an inkjet printable pristine graphene ink for the fabrication of graphene-based CEs as a Pt alternative. Through the use of a polymer stabiliser, we tune the rheology of the ink, allowing stable and repeatable CE printing. We also investigate three natural tropical dye extracts as photosensitizers as a low-cost alternative to standard Ru-based dyes. We demonstrate that with only ∼2.7% of the materials cost of platinum our printed graphene counter electrode, without any structural or chemical optimization, shows comparable performance to Pt CEs with both natural and Ru-based dyes as sensitizers.

Section snippets

Extraction of natural dyes

Clean, fresh specimens of Hibiscus sabdariffa, Caesalpinia pulcherrima and Pennisetum glaucum are oven dried at 60 °C and crushed into fractionlets. To extract the natural dyes, 5 g of each of these samples are put in 60 mL of ethanol for 5 days at room temperature without exposure to light. The dye solutions are then filtered to remove the solid residues. The concentrations of the dyes in the final solutions are ∼0.4 g/L.

Characterisation of natural dyes

Anthocyanins and their derivatives, which belong to a group of natural

Graphene ink preparation

The graphene ink is formulated from graphene prepared via ultrasonic-assisted liquid phase exfoliation (UALPE). In this process, bulk graphite flakes are mixed into a solvent, and the mixture is ultrasonicated. The ultrasound causes high-frequency pressure variations in the solvent, forming localised microcavities. These cavities are inherently unstable and rapidly collapse, producing high shear forces sufficient to overcome the weak interlayer van der Waals forces and exfoliate graphene flakes

DSSC fabrication

Fabrication of a DSSC involves several steps. First, two FTO (fluorine-doped SnO2) conductive glass substrates (∼2 cm × 2 cm, 8 Ω/□, Solaronix) are cleaned in a detergent solution using an ultrasonic bath for 10 min, rinsed with deionised water and ethanol, followed by oven drying at 90 °C. One of the FTO substrates is then immersed in a 40 mM aqueous solution of TiCl4 (Sigma Aldrich) at ∼70 °C for 30 min (for good mechanical adhesion of the TiO2) and washed with deionised water and ethanol

Conclusions

We have demonstrated low-cost, environmentally friendly alternatives for two key elements of DSSCs. Plant-extract dyes have been used as photosensitisers, in place of ruthenium-based dyes, where their simple extraction procedure, wide availability, and environmentally friendly nature make them promising alternative sources of sensitizers for DSSCs. Meanwhile, inkjet-printed graphene has been used as a counter-electrode in the place of platinum. A polymer-stabilised isopropanol-based graphene

Acknowledgements

Authors acknowledge support from CAPREX, Cambridge Africa Alborada Fund, Carnegie-University of Ghana Next Generation of Africa Academics programme and the Royal Academy of Engineering (RAEng) through a research fellowship (Graphlex).

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