Published: May 4th, 2016
This paper investigates the suitability of inkjet printing for the manufacturing of dye-sensitized solar cells. A binder-free TiO2 nanoparticle ink was formulated and printed onto a FTO glass substrate. The printed layer was fabricated into a cell with an active area of 0.25 cm2 and an efficiency of 3.5%.
Silicon solar cell manufacturing is an expensive and high energy consuming process. In contrast, dye sensitized solar cell production is less environmentally damaging with lower processing temperatures presenting a viable and low cost alternative to conventional production. This paper further enhances these environmental credentials by evaluating the digital printing and therefore additive production route for these cells. This is achieved here by investigating the formation and performance of a metal oxide photoelectrode using nanoparticle sized titanium dioxide. An ink-jettable material was formulated, characterized and printed with a piezoelectric inkjet head to produce a 2.6 µm thick layer. The resultant printed layer was fabricated into a functioning cell with an active area of 0.25 cm2 and a power conversion efficiency of 3.5%. The binder-free formulation resulted in a reduced processing temperature of 250 °C, compatible with flexible polyamide substrates which are stable up to temperatures of 350 ˚C. The authors are continuing to develop this process route by investigating inkjet printing of other layers within dye sensitized solar cells.
Conventional silicon solar cells are made from highly pure materials that require expensive and high-energy consuming specialist equipment. These conventional silicon cells incorporate a p-n junction that requires highly pure materials at the interface to generate electron-hole pairs. Dye-sensitized solar cells (DSSCs) have a fundamentally different working principle, where charge generation takes place at the materials interface. This means that processing under vacuum, ultrahigh temperatures or the use of clean room facilities are not required1. Therefore they are seen as a potentially low cost alternative; however up-scaling from small laboratory test cells into large prototypes for industrial manufacturing involves overcoming several issues including the rapid patterning of substrates.
Electronics manufacturing generally requires a degree of patterning, which is either achieved by masking or selective removal of the material after deposition. These steps can be removed through the use of "additive" digital printing techniques such as inkjet printing or spray coating. Digital printing is a promising method for direct deposition of functional materials for electronic devices. The technique can be described as printing from a digital-based pattern directly to a variety of substrates2. They are non-contact methods, which will not damage or contaminate the substrate surface and deposit material only where it is required, resulting in little or no wastage3. These techniques have been highlighted as being ideally suited to being scaled up to high-volume production3. Since digital printing methods use liquid forms of materials dispersed in a solvent, it is critical to understand the deposition of ink to determine the applications of the technique.
DSSCs have three main components: a porous layer of wide bandgap metal oxide material, a dye that covers the particles, and a "charge transporter" that infiltrates the pores within the porous layer of semiconductor. These are sandwiched in between a transparent conductive electrode and a counter electrode4. The counter electrode is coated with a catalytic material for electron transfer, which in most cases is platinum. Under illumination, the dye molecules will absorb energy in the form of photons. The dye molecules then become excited and charge separation occurs at the interface of the titanium dioxide and the dye. Electrons are ejected into the adjacent metal oxide particles and 'holes' are left behind on the dye molecule. The injected electrons travel through the metal oxide particles and reach the transparent conductive electrode. When a load is connected, the electrons move to the counter electrode through the external circuit and are finally reunited with their counter charges through the redox couple present in the electrolyte1. The nano-structured metal oxide layer within DSSCs plays a critical role in the overall performance of the cell, with material choice, processing methods and nature of the structure all having influencing factors5-10. One of the most important requirements for the photoanode is that it needs to have an extremely large surface area. This is achieved through the deposition of nanoparticle materials, commonly TiO21,11. This has been fabricated by countless different processes, however wet coating techniques such as screen-printing and doctor-blading, are still the most popular approach9,12,13.
Inkjet technology is a potential manufacturing route for dye-sensitized solar cells. It uses the movement of a piezoelectric crystal to expel a fixed quantity of liquid through a nozzle onto the desired substrate. This deposition method allows material to be jetted very accurately but also at high frequency with a potentially high print speed or deposition rate. Inkjet technology is sensitive to the viscosity of the ink used and this was previously a barrier to the development of functional inks. Recent work in the development of solvents suitable for ink formulation has helped to alleviate this problem, and printing of electronic components using 2D layered materials such as graphene has been demonstrated14. The viscosity of nanoparticle suspensions such as these has been found to depend on the nanoparticle size and concentration15. High concentrations of nanoparticles result in higher viscosities, therefore particle loadings are usually around 10 wt% to avoid nozzle blockages16, however higher concentrations have been achieved17.
The key advantages of inkjet technology include it being non-contact, additive patterning and maskless18. The latter two attributes are due to the ability to position many nozzles together on one or more printheads, with each nozzle separately addressable by the control software. This allows highly complex, multi-layered patterns to be created very rapidly as the printheads move across the substrate. No masking between materials or layers is required as the position of each ink drop is accurately controlled, in some systems to an accuracy of ~1.5 µm19. One of the key benefits is that inkjet technology is mature, with significant development carried out in the latter half of the twentieth century. The result is that the inkjet is a very scalable technology, with roll-to-roll systems capable of printing accurately onto flexible substrates at rates of many meters per second. Traditionally this was used for high volume production, e.g., newspapers. However, developments in technology have allowed the inkjet to be used in roll-to-roll production of electronic circuits using nanoparticulate silver inks20. The inkjet is therefore an attractive process for the potential production of dye-sensitized solar cells by digital printing.
1. Ink Formulation
Note: Ink formulations are often kept a highly guarded secret by manufacturers. Successful formulations balance jetting, drop formation, wetting and drying behavior alongside functional performance. Usually a functional material is dispersed in a solvent and at least one other component to make them jettable. This section details the development of a TiO2 ink for use within inkjet printing. A small batch of ink was prepared by the following method.
Caution: Ink preparation should be performed in a suitably vented area, e.g., under a fume hood, whilst wearing eye protective goggles and latex gloves.
2. Inkjet Printing
3. Analysis of the Printed Films
4. Making the Cell
A TiO2 ink was formulated according to the procedure outlined. The size of particles suspended within the ink was measured using dynamic light scattering (DLS) and an average particle size of 80 nanometers (nm) was observed. The viscosity of the ink in this work was found to be 3 cP, measured using a rotational viscometer with a small sample adapter and an 18 mm spindle diameter. The surface tension was measured using a tensiometer and was calculated to be an average of 26 mN/m.
The surface energy of the FTO glass was calculated according to the European Standard EN 828 for determining the wettability of a solid surface by measuring the contact angle and surface free energy. Ten drops of three different liquids (water, ethylene glycol and diiodomethane) were dispensed onto a plane test piece surface. For each drop, the left and right contact angle were measured. From the averaged contact angles of each liquid combined with its surface tension, the surface free energy of the test piece is calculated. The Fowkes method calculates the total surface energy (γ) from the sum of the contributions from dispersive interactions (γd) and γnon-dispersive interactions (γp). This method resulted in a surface free energy of 26.45 mN/m for the FTO coated glass.
Printing was carried out according to the procedure above to produce 5 mm squares. The thickness of the printed layer on the glass was measured using a surface profiler. The maximum thickness at the center of the printed layer was measured to be 2.6 µm. The transmittance of the coated glass was measured using a UV-VIS spectrometer. At a wavelength of 700 nm, a 60% transmittance was measured for the TiO2 printed film compared with 78% for the FTO glass.
Photovoltaic devices were produced according to the procedure outline above and characterized directly after fabrication to minimize the effect of degradation caused by water and oxygen in the air. There are five electrical performance parameters that are used to characterize and compare solar cells28. The values of short circuit current (Isc) and open circuit voltage (Voc) can be derived from the current-voltage (I-V) curve. These can then be used to determine the fill factor (FF) and power conversion efficiency (η). The FF gives a ratio of the cells actual maximum power output to the product of the open circuit voltage and short circuit current29. This is a key parameter in evaluating the performance of solar cells. A high FF means low electrochemical losses, whereas a low FF indicates there is room for improvement. Several factors are known to influence the FF including the quality and interface of layers within the cell. DSSCs incorporating an iodide/triiodide redox couple with record efficiencies of 11.9% report fill factors of 0.7130. All of these parameters need to be determined under standard test conditions where the device temperature is 25 ˚C, spectral irradiance distribution of the light has an air mass of 1.5, total irradiance measured (Em) at the solar cell is 100 mW/cm2 . Theoretical maximum for the conversion efficiency for a single p-n junction cell has been widely reported as 37.7%31, however for DSSCs it has been reported that the maximum efficiency is closer to 15.1% with an absorption onset at 920 nm32.
The output current and voltages were measured using a source meter whilst the cells were illuminated with a 100 mW/cm2 light source fitted with a filter to match the spectral irradiance distribution with an air mass of 1.5. The results were compared to a cell produced using a doctor-bladed TiO2 layer using a commercially available paste which has a blend of anatase particles 20 nm and 450 nm. The printed layer had an area of 0.25 cm2 and an average thickness of 18 µm which was measured using a surface profiler. A comparison of the photoelectric performance between the two devices is shown in Figure 1 and Table 1.
Several studies have investigated the relationship between the thickness of the TiO2 layer and the conversion efficiency within DSSCs. The results vary significantly, with optimum film thickness reported from anywhere between 9.5 µm and 20 µm 33-39. Table 1 outlines the thicknesses of the TiO2 printed layers and the efficiencies. The thickness of the inkjet printed TiO2 is significantly less than the doctor bladed TiO2, resulting in a lower efficiency. Future work will investigate the use of organic binders within the ink formulation to increase the thickness of the inkjet printed layer.
Figure 1. Performance Curves of DSSCs with Inkjet Printed and Doctor Bladed TiO2 Layers. Current-density/voltage curves for DSSCs incorporating an inkjet printed TiO2 layer and a doctor-bladed TiO2 layer. The short circuit current density in the device with the inkjet printed TiO2 layer is significantly lower than the device with the doctor bladed TiO2 layer resulting in a lower overall conversion efficiency. Please click here to view a larger version of this figure.
|Short circuit current
|Open circuit voltage
Table 1. Key Performance Characteristics of the Cells in Figure 1. This table compares the key parameters of the solar cell including open circuit voltage (Voc), short circuit current (Isc) which determine the efficiency (η) under the specified light condition are presented. The parameters of a cell produced using a doctor-bladed TiO2 layer have also been included for comparison. The fill factors (FF) of both devices are quite low which is generally attributed to a high internal resistance within the cell.
A particular challenge when formulating inks is the natural tendency for nanoparticles to cluster together. These are known as either aggregates or agglomerates, depending on the nature and strength of the bonds between the particles. The energy of simply stirring particles into water or binder is not great enough to overcome the particle attractive forces preventing the breakup of agglomerates. Ball milling, high shear mixing or ultrasonication are commonly used to break up agglomerated nanoparticles. Various anionic, nonionic, and cationic surfactants and polymers can also be used to provide long-term stabilization. By minimizing the number of these agglomerates, a good quality suspension can be achieved. The fluids should be filtered through the correct size filter just before loading into the cartridge to remove large particle aggregates which can clog the nozzles.
The particle size within the TiO2 layer also has been shown to influence the overall efficiency of DSSCs. The photocatalytic activity of titanium dioxide increases as particle sizes decrease due to an increase in the specific surface area40. A study comparing the efficiency of DSSCs incorporating TiO2 nanoparticles with 5 different sizes ranging from 400 nm to 14 nm and found that those with smaller particle sizes resulted in better electrical conversion efficiencies33.
Inkjet printing is a non-contact deposition technique capable of multi-pass printing. This presents the unique opportunity to rapidly fabricate multilayer devices in one operation on a wide range of substrates with minimal material waste. It also potentially provides a way to integrate other components (such as batteries) into the system through the printing of functional materials41. Although the representative results shown for the inkjet printed devices do not perform as well as the doctor-bladed devices, it demonstrates the potential for the deposition technique. With further ink optimization, it could perform on a comparable level to currently used methods and may provide further scope for cost-effective, environmentally friendly integration of photovoltaic cells onto a wide range of substrates. We hope to improve the efficiency of the inkjet printed devices by increasing the thickness of the printed layer closer to that of the doctor-bladed TiO2 and will continue to look at the printing of other materials and layers within DSSCs.
The authors have nothing to disclose.
This research is gratefully undertaken with support from the Engineering and Physical Sciences Research Council (EPSRC) funded through a doctoral training grant. Open access article processing charges (APCs) were funded by Research Councils UK (RCUK). All of the data is provided in full in the results section of the paper. Representative results have previously been published by the authors42.
We would like to thank Dr. Senthilarasu Sundaram from the University of Exeter for his help in characterising the electrical performance of the cells.
|Supplied from a filter in the laboratory
|Hydrochloric acid, 2M(2N)
|Polyvinylidene fluoride (PVDF) syringe filter
|Fluorine doped tin oxide (FTO) glass, 8 Ω/sq
|Pre-cut 60 µm thick thermoplastic sealing film
|50 mM iodide/tri-iodide electrolyte in acetonitrile
|Platinum coated FTO glass
|Polyimide tape (6.35 mm)
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