Inkjet-printed graphene electrodes for dye-sensitized solar cells
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).
References (90)
- et al.
Room-temperature fabrication of graphene films on variable substrates and its use as counter electrodes for dye-sensitized solar cells
Solid State Sci.
(2011) - et al.
Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide
Carbon
(2007) - et al.
Anthocyanins and betalains as light-harvesting pigments for dye-sensitized solar cells
Sol. Energy
(2012) - et al.
Natural dyes as photosensitizers for dye-sensitized solar cell
Sol. Energy
(2006) - et al.
Calculation of the photocurrent-potential characteristic for regenerative, sensitized semiconductor electrodes
Sol. Energy Mater. Sol. Cells
(1996) - et al.
Efficient photosensitization of nanocrystalline TiO2 films by tannins and related phenolic substances
J. Photochem. Photobiol. A Chem.
(1996) - et al.
Bio-photovoltaic conversion device using chlorine-e6 derived from chlorophyll from Spirulina adsorbed on a nanocrystalline TiO2 film electrode
Biosens. Bioelectron.
(2004) - et al.
Blue sensitizers for solar cells: natural dyes from calafate and jaboticaba
Sol. Energy Mater. Sol. Cells
(2006) - et al.
Fruit extracts and ruthenium polypyridinic dyes for sensitization of TiO2 in photoelectrochemical solar cells
J. Photochem. Photobiol. A Chem.
(2003) - et al.
Shiso leaf pigments for dye-sensitized solid-state solar cell
Sol. Energy Mater. Sol. Cells
(2006)
Counter-electrode function in nanocrystalline photoelectrochemical cell configurations
Coord. Chem. Rev.
Testing of dye sensitized TiO2 solar cells I: Experimental photocurrent output and conversion efficiencies
Sol. Energy Mater. Sol. Cells
Effect of the thickness of the Pt film coated on a counter electrode on the performance of a dye-sensitized solar cell
J. Electroanal. Chem.
Dissolution of platinum in methoxy propionitrile containing LiI/I2
Sol. Energy Mater. Sol. Cells
A printable graphene enhanced composite counter electrode for flexible dye-sensitized solar cells
Nano Energy
Low cost photovoltaic modules based on dye sensitized nanocrystalline titanium dioxide and carbon powder
Sol. Energy Mater. Sol. Cells
Developments in conducting polymer based counter electrodes for dye-sensitized solar cells – an overview
Eur. Polym. J.
Counter electrodes for DSC: application of functional materials as catalysts
Inorg. Chim. Acta
Dye-sensitized solar cell using natural dyes extracted from rosella and blue pea flowers
Sol. Energy Mater. Sol. Cells
Stability of n-type doped conducting polymers and consequences for polymeric microelectronic devices
Synth. Met.
Reversible water-solubilization of single-walled carbon nanotubes by polymer wrapping
Chem. Phys. Lett.
Fabrication and characterization of silver core and porous silica shell nanocomposite particles
Microporous Mesoporous Mater
Enhanced electrochemical performance of the counterelectrode of dye sensitized solar cells by sandblasting
Electrochim. Acta
A carbon gel catalyst layer for the roll-to-roll production of dye solar cells
Carbon
Education and solar conversion: demonstrating electron transfer
Sol. Energy Mater. Sol. Cells
Transparent carbon films as electrodes in organic solar cells
Angew. Chem.
Roll-to-roll production of 30-inch graphene films for transparent electrodes
Nat. Nanotechnol.
Two-dimensional graphene bridges enhanced photoinduced charge transport in dye-sensitized solar cells
ACS Nano
The edge- and basal-plane-specific electrochemistry of a single-layer graphene sheet
Sci. Rep.
Electric field effect in atomically thin carbon films
Science
Large-area synthesis of high-quality and uniform graphene films on copper foils
Science
Liquid-phase exfoliation of nanotubes and graphene
Adv. Funct. Mater
Liquid exfoliation of defect-free graphene
Acc. Chem. Res.
Solution-phase exfoliation of graphite for ultrafast photonics
Phys. Status Solidi
High-yield production of graphene by liquid-phase exfoliation of graphite
Nat. Nanotechnol.
A review of chemical vapour deposition of graphene on copper
J. Mater. Chem.
Optically transparent cathode for dye-sensitized solar cells based on graphene nanoplatelets
ACS Nano
Inkjet-printed graphene electronics
ACS Nano
Heterostructures produced from nanosheet-based inks
Nano Lett.
Functional inks of graphene, metal dichalcogenides and black phosphorus for photonics and (opto)electronics
Proc. SPIE
Inkjet printing of high conductivity, flexible graphene patterns
J. Phys. Chem. Lett.
Inkjet Technology for Digital Fabrication
Porphyrin-Sensitized Solar Cells with Cobalt (II/III)-Based Redox Electrolyte Exceed 12 Percent Efficiency
Science
Vegetable-based dye-sensitized solar cells
Chem. Soc. Rev.
A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films
Nature
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