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.

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.&#160;
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.&#160;
<ol>
	<li>Prepare a 0.1 mM aqueous solution of hydrochloric acid (HCl) to produce a pH of approximately 4.</li>
	<li>Add 32 g of the acid solution to 8 g of a compatible solvent with a higher boiling point and lower surface tension than water (such as dimethylformamide (DMF)). The addition of a co-solvent acts as a drying agent to induce a circulating flow within the ink droplet as the ink evaporates, leading to a uniform placement of nanoparticles over the surface of the droplet 21.</li>
	<li>Add 1.5 g of dispersing additive (45% active solution of propylene glycol and tetramethyl-5-decyne-4,7-diol in water).</li>
	<li>Add 10 g of ethylene glycol, as a humectant to prevent drying at the nozzles.</li>
	<li>Add 0.5 g of defoaming agent (20% active solution of acetylenic diol in methoxypolyethyleneglycol) to the ink to prevent air bubbles from developing.</li>
	<li>Perform a simple shake test by taking an aliquot of the ink into a closed container and shake by hand for 60 sec. If any foam is observed then add another 0.5 g of defoaming agent to the ink.</li>
	<li>Mix the solution for 8 hr using a magnetic stirring bar to ensure homogeneity at RT.</li>
	<li>Add 1.5 g of titanium dioxide (TiO2) nanoparticles with a primary particle size of 21 nm and surface area of 35 - 65 m2/g.</li>
	<li>Sonicate the mixture using an ultrasonic probe for 15 min&#160;at a frequency of 60 Hz.</li>
	<li>Measure the particle sizes, using an appropriate measurement technique such as dynamic light scattering (DLS) according to manufacturer&#39;s protocol, to ensure that they will flow easily through the nozzle openings. Make measurements under the same conditions (e.g., same solvent, pH, concentration of dispersant) to be used for the ink as each component may influence the formation of agglomerates within the ink. For successful jetting, the particles within the fluid should be 100 times smaller than the nozzle opening.</li>
	<li>Measure the viscosity of the ink, using an appropriate measurement technique such as a rotational viscometer according to manufacturer&#39;s protocol, to ensure reliable jetting from the printhead as inkjet printing requires low viscosity inks of between 2 and 20 centipoise (cP). Increase the viscosity through the addition of polymeric materials or cellulose-based materials; however these need to be removed after deposition to free up sites for the dye within the printed film22.</li>
	<li>Measure the surface tension of the ink, using an appropriate measurement technique such as a tensiometer according to manufacturer&#39;s protocol, to ensure reliable jetting. The jettable fluid formulation guidelines for inkjet printers suggest a surface tension between 28 and 33 mN/m to enable reliable printing.</li>
</ol>
2. Inkjet Printing
<ol>
	<li>Prior to printing, soak the glass substrates in a 2 wt% solution of cleaning detergent (a mixture of anionic and non-ionic surface active agents, stabilizing agents, alkalis, non-phosphate detergent builders and sequestering agents, in an aqueous base) in deionized water. Rinse the glass thoroughly with deionized water as soon as they are removed from the cleaning solution to remove traces of contamination and cleaning detergent.</li>
	<li>Measure the surface energy of the substrate, using an appropriate measurement technique such as a tensiometer according to manufacturer&#39;s protocol. For good adhesion, the surface energy of the substrate should not exceed the surface tension of the fluid by more than 10 - 15 mN/m. Modify the surface energy of the substrate using methods such as corona treatment23, plasma treatment24 and chemical etching25 if it is not suitable.</li>
	<li>Load the substrate into the printer according to manufacturer&#39;s protocol.</li>
	<li>Flush the print head with the ink through the port located on the side of the head to displace any air or cleaning solution within the reservoir and nozzles.</li>
	<li>Insert the printhead into the printer. Connect the printhead with the head personality board.</li>
	<li>Filter the ink through the correct size filter just before loading into the cartridge to remove large particle aggregates which can clog the nozzles. The printhead used in this work (e.g., Konica KM512) has nozzles with a diameter of 40 &#181;m; therefore inks should not contain particles with a diameter more than 400 nm. Pass the suspensions through a 5 &#181;m, followed by a 1.2 &#181;m polyvinylidene fluoride (PVDF) filter to remove any large particulates.</li>
	<li>Load the ink into the 150 ml syringe located above the print head, which supplies the ink to the print head. Attach the airtight cap on top of the syringe and turn on the vacuum pump.</li>
	<li>Purge the ink through the nozzles by pressing the &#39;purge&#39; button located on the vacuum pump.</li>
	<li>Through the geographic information system (GIS) print server, set-up the waveform and printing parameters. Note that the printer can print up to a speed of 1.5 meters per second, however for this ink a print speed of 0.3 meters per second has been found to provide optimal coating</li>
	<li>Open GIS user interface software and load the desired pattern.</li>
	<li>Print from the loaded cartridge according to manufacturer&#39;s protocol.</li>
	<li>Remove the substrate from the platen and heat the printed films at 150 &#186;C for 30 min, followed by 250 &#186;C for a further 30 min&#160;either on a hot plate or in an oven.</li>
</ol>
3. Analysis of the Printed Films
<ol>
	<li>Use an optical microscope or a scanning electron microscope (SEM) to look at the surface of the printed films at low magnification (100X) to analyze the surface morphology and at high magnification (35,000X) to analyze the porosity of the printed films. Check that the images show a uniform coverage with no cracks and good porosity. More detailed information on SEM operation can be found in the following references26,27.</li>
	<li>Measure the thickness of the printed layer, using an appropriate measurement technique such as a surface profiler according to manufacturer&#39;s protocol. The thickness and porosity of the TiO2 layer within DSSCs influence the amount of dye that can be absorbed onto the surface of the nanoparticles, which therefore influence the overall electrical conversion efficiency of the cell18. It is therefore an important parameter to evaluate. Use a surface profiler (precision of 1 nm) to measure the thickness of the printed films.</li>
	<li>Measure the transmittance of the film, using an appropriate measurement technique such as an ultraviolet-visible (UV-VIS) spectrophotometer to determine how much visible light will transmit through the printed film. Use manufacturer&#39;s protocol.</li>
</ol>
4. Making the Cell
<ol>
	<li>Make a dye solution by mixing 20 ml of ethanol and 2 mg of ruthenium dye in a glass beaker using a magnetic stirrer for 8 hr.</li>
	<li>Submerge the TiO2 coated glass in the solution at RT (20 to 25&#176;C) for 24 hr to allow the dye to absorb onto the surface of the TiO2 particles.</li>
	<li>Remove the TiO2 coated glass from the solution and place onto tissue paper to soak up any excess dye solution (with the TiO2 facing upwards to avoid contamination).</li>
	<li>Place the pre-cut 60 &#181;m thick thermoplastic sealing spacer on top of the conductive glass, around the TiO2 coating.</li>
	<li>Place the platinum coated counter electrode on top of the pre-cut 60 &#181;m thick thermoplastic sealing spacer so that the active sides of the anode and the cathode are facing each other. Allow enough overlap between the two pieces of glass so that an electrical contact can be made with the conductive glass. This should have a pre-drilled hole in the center to allow for electrolyte filling later.</li>
	<li>Heat on a hot plate to a temperature of 110 &#730;C and apply light pressure using tweezers over the area of the sealing spacer. After 30 sec&#160;the electrodes should be sealed together.</li>
	<li>Fill the gap between the two electrodes with an iodide/tri-iodide electrolyte in acetonitrile at a concentration of 50 mM, by injecting through the pre-drilled hole in the platinum coated glass using a syringe.</li>
</ol>

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J., Senthilarasu, S., Goodship, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Inkjet-Printed+TiO2+Nanoparticles+from+Aqueous+Solutions+for+Dye-Sensitized+Solar+Cells+(DSSCs).">Inkjet-Printed TiO2 Nanoparticles from Aqueous Solutions for Dye-Sensitized Solar Cells (DSSCs).</a> Energy Technology. 3, (2015).</li></ol>

The authors have nothing to disclose.

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.

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 &#181;m19. 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.

<table>
	<tbody>
		<tr>
			<td>Titanium dioxide</td>
			<td>Sigma Aldrich</td>
			<td>718467</td>
			<td></td>
		</tr>
		<tr>
			<td>Deionized water&#160;</td>
			<td></td>
			<td></td>
			<td>Supplied from a filter in the laboratory</td>
		</tr>
		<tr>
			<td>Hydrochloric acid, 2M(2N)&#160;</td>
			<td>Fisher Scientific</td>
			<td>J/4250/17</td>
			<td></td>
		</tr>
		<tr>
			<td>Dimethylformamide (DMF)</td>
			<td>Fisher Scientific</td>
			<td>D/3840/08</td>
			<td></td>
		</tr>
		<tr>
			<td>Ethanol</td>
			<td>VWR Chemicals</td>
			<td>20721.33</td>
			<td></td>
		</tr>
		<tr>
			<td>Dispersing additive&#160;</td>
			<td>Air Products</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Defoaming agent</td>
			<td>Air Products</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Ethylene glycol</td>
			<td>Fluka</td>
			<td>107-21-1</td>
			<td></td>
		</tr>
		<tr>
			<td>Polyvinylidene fluoride (PVDF) syringe filter</td>
			<td>VWR International</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Cleaning detergent&#160;</td>
			<td>Fisher Scientific</td>
			<td>10335650</td>
			<td></td>
		</tr>
		<tr>
			<td>Fluorine doped tin oxide (FTO) glass, 8 &#937;/sq</td>
			<td>Pilkington</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Ruthenizer dye</td>
			<td>Solaronix</td>
			<td>21613</td>
			<td></td>
		</tr>
		<tr>
			<td>Pre-cut 60 &#181;m thick thermoplastic sealing film&#160;</td>
			<td>Solaronix</td>
			<td>74301</td>
			<td></td>
		</tr>
		<tr>
			<td>50 mM iodide/tri-iodide electrolyte&#160; in acetonitrile</td>
			<td>Solaronix</td>
			<td>31111</td>
			<td></td>
		</tr>
		<tr>
			<td>Platinum coated FTO glass&#160;</td>
			<td>Solaronix</td>
			<td>74201</td>
			<td></td>
		</tr>
		<tr>
			<td>Vac&#39;n&#39;Fill Syringe</td>
			<td>Solaronix</td>
			<td>65209</td>
			<td></td>
		</tr>
		<tr>
			<td>Polyimide tape (6.35 mm)</td>
			<td>Onecall Farnell</td>
			<td>1676087</td>
			<td></td>
		</tr>
	</tbody>
</table>

digital printing of titanium dioxide for dye sensitized solar cells

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 &#181;m 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 &#176;C, compatible with flexible polyamide substrates which are stable up to temperatures of 350 &#730;C. The authors are continuing to develop this process route by investigating inkjet printing of other layers within dye sensitized solar cells.

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 (&#947;) from the sum of the contributions from dispersive interactions (&#947;d) and &#947;non-dispersive interactions (&#947;p). 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 &#181;m. 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 (&#951;). 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 &#730;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 &#181;m 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 &#181;m and 20 &#181;m 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.
<img alt="Figure 1" src="/files/ftp_upload/53963/53963fig1.jpg" /> 
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. <a href="https://www.jove.com/files/ftp_upload/53963/53963fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table>
	<tbody>
		<tr>
			<td rowspan="3"></td>
			<td>Short circuit current</td>
			<td>Open circuit voltage</td>
			<td rowspan="3">Fill factor</td>
			<td>Efficiency</td>
			<td>Thickness</td>
		</tr>
		<tr>
			<td>(mA/cm2)</td>
			<td>(mV)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td>(%)</td>
			<td>(&#181;m)</td>
		</tr>
		<tr>
			<td>Inkjet printed</td>
			<td>9.42</td>
			<td>760</td>
			<td>0.49</td>
			<td>3.5</td>
			<td>2.6</td>
		</tr>
		<tr>
			<td>Doctor bladed</td>
			<td>11</td>
			<td>756</td>
			<td>0.58</td>
			<td>4.8</td>
			<td>18</td>
		</tr>
	</tbody>
</table>
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 (&#951;) 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.

Watch this Scientific Journal Video about Digital Printing of Titanium Dioxide for Dye Sensitized Solar Cells at JoVE.com

Digital Printing of Titanium Dioxide for Dye Sensitized Solar Cells

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 ...

digital-printing-of-titanium-dioxide-for-dye-sensitized-solar-cells

Research

JoVE Journal

Engineering

12.7K Views.  University of Warwick. 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%.

Video: Digital Printing of Titanium Dioxide for Dye Sensitized Solar Cells

Micro 3D Printing Using a Digital Projector and its Application in the Study of Soft Materials Mechanics

Buckling is a classical topic in mechanics. While buckling has long been studied as one of the major structural failure modes1, it has recently drawn new attention as a unique mechanism for pattern transformation. Nature is full of such examples where a wealth of exotic patterns are formed through mechanical instability2-5. Inspired by this elegant mechanism, many studies have demonstrated creation and transformation of patterns using soft materials such as elastomers and hydrogels6-11. Swelling gels are of particular interest because they can spontaneously trigger mechanical instability to create various patterns without the need of external force6-10. Recently, we have reported demonstration of full control over buckling pattern of micro-scaled tubular gels using projection micro-stereolithography (P&mu;SL), a three-dimensional (3D) manufacturing technology capable of rapidly converting computer generated 3D models into physical objects at high resolution12,13. Here we present a simple method to build up a simplified P&mu;SL system using a commercially available digital data projector to study swelling-induced buckling instability for controlled pattern transformation.
A simple desktop 3D printer is built using an off-the-shelf digital data projector and simple optical components such as a convex lens and a mirror14. Cross-sectional images extracted from a 3D solid model is projected on the photosensitive resin surface in sequence, polymerizing liquid resin into a desired 3D solid structure in a layer-by-layer fashion. Even with this simple configuration and easy process, arbitrary 3D objects can be readily fabricated with sub-100 &mu;m resolution.
This desktop 3D printer holds potential in the study of soft material mechanics by offering a great opportunity to explore various 3D geometries. We use this system to fabricate tubular shaped hydrogel structure with different dimensions. Fixed on the bottom to the substrate, the tubular gel develops inhomogeneous stress during swelling, which gives rise to buckling instability. Various wavy patterns appear along the circumference of the tube when the gel structures undergo buckling. Experiment shows that circumferential buckling of desired mode can be created in a controlled manner. Pattern transformation of three-dimensionally structured tubular gels has significant implication not only in mechanics and material science, but also in many other emerging fields such as tunable matamaterials.

We demonstrate controlled pattern transformation of swelling gel tubes by elastic instability. A simple projection micro stereo-lithography setup is built using an off-the-shelf digital data projector to fabricate three-dimensional polymeric structures in a layer-by-layer fashion. Swelling hydrogel tubes under mechanical constraint display various circumferential buckling modes depending on dimension.

Buckling is a classical topic in mechanics. While buckling has long been studied as one of the major structural failure modes1, it has recently drawn new ...

Integrating a Triplet-triplet Annihilation Up-conversion System to Enhance Dye-sensitized Solar Cell Response to Sub-bandgap Light

The poor response of dye-sensitized solar cells (DSCs) to red and infrared light is a significant impediment to the realization of higher photocurrents and hence higher efficiencies. Photon up-conversion by way of triplet-triplet annihilation (TTA-UC) is an attractive technique for using these otherwise wasted low energy photons to produce photocurrent, while not interfering with the photoanodic performance in a deleterious manner. Further to this, TTA-UC has a number of features, distinct from other reported photon up-conversion technologies, which renders it particularly suitable for coupling with DSC technology. In this work, a proven high performance TTA-UC system, comprising a palladium porphyrin sensitizer and rubrene emitter, is combined with a high performance DSC (utilizing the organic dye D149) in an integrated device. The device shows an enhanced response to sub-bandgap light over the absorption range of the TTA-UC sub-unit resulting in the highest figure of merit for up-conversion assisted DSC performance to date.

An integrated device, incorporating a dye-sensitized solar cell and triplet-triplet annihilation up-conversion unit was produced, affording enhanced light harvesting, from a wider section of the solar spectrum. Under modest irradiation levels a significantly enhanced response to low energy photons was demonstrated, yielding a record figure of merit for dye-sensitized solar cells.

The poor response of dye-sensitized solar cells (DSCs) to red and infrared light is a significant impediment to the realization of higher photocurrents ...

Ambient Method for the Production of an Ionically Gated Carbon Nanotube Common Cathode in Tandem Organic Solar Cells

A method of fabricating organic photovoltaic (OPV) tandems that requires no vacuum processing is presented. These devices are comprised of two solution-processed polymeric cells connected in parallel by a transparent carbon nanotubes (CNT) interlayer. This structure includes improvements in fabrication techniques for tandem OPV devices. First the need for ambient-processed cathodes is considered. The CNT anode in the tandem device is tuned via ionic gating to become a common cathode. Ionic gating employs electric double layer charging to lower the work function of the CNT electrode. Secondly, the difficulty of sequentially stacking tandem layers by solution-processing is addressed. The devices are fabricated via solution and dry-lamination in ambient conditions with parallel processing steps. The method of fabricating the individual polymeric cells, the steps needed to laminate them together with a common CNT cathode, and then provide some representative results are described. These results demonstrate ionic gating of the CNT electrode to create a common cathode and addition of current and efficiency as a result of the lamination procedure.

A method of fabricating, in ambient conditions, organic photovoltaic tandem devices in a parallel configuration is presented. These devices feature an air-processed, semi-transparent, carbon nanotube common cathode.

A method of fabricating organic photovoltaic (OPV) tandems that requires no vacuum processing is presented. These devices are comprised of two ...

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly known as plasmonic effects, play an important role. Research in this field has, however, been impeded owing to the difficulty of fabricating devices containing the desired functional metal nanostructures. In order to provide a viable strategy to this issue, we herein show a transfer printing-based approach that allows the quick and low-cost integration of designed metal nanostructures with a variety of device architectures, including solar cells. Nanopillar poly(dimethylsiloxane) (PDMS) stamps were fabricated from a commercially available nanohole plastic film as a master mold. On this nanopatterned PDMS stamps, Ag films were deposited, which were then transfer-printed onto block copolymer (binding layer)-coated hydrogenated microcrystalline Si (&#181;c-Si:H) surface to afford ordered Ag nanodisk structures. It was confirmed that the resulting Ag nanodisk-incorporated &#181;c-Si:H solar cells show higher performances compared to a cell without the transfer-printed Ag nanodisks, thanks to plasmonic light trapping effect derived from the Ag nanodisks. Because of the simplicity and versatility, further device application would also be feasible thorough this approach.

A viable transfer printing-based methodology to introduce plasmonic metal nanostructures in solar cells is described. Using nanopillar poly(dimethylsiloxane) stamps, an Ag-based ordered nanodisk array was integrated with standard hydrogenated microcrystalline Si solar cells, which led to improved device performances due to plasmonic light trapping.

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly ...

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices have used devices prepared by spin coating, a process that is not amenable to large-scale fabrication. This mismatch in device fabrication makes it difficult to translate quantitative results obtained in the laboratory to the commercial level, making optimization difficult. Using a mini-slot die coater, this mismatch can be resolved by translating the commercial process to the laboratory and characterizing the structure formation in the active layer of the device in real time and in situ as films are coated onto a substrate. The evolution of the morphology was characterized under different conditions, allowing us to propose a mechanism by which the structures form and grow. This mini-slot die coater offers a simple, convenient, material efficient route by which the morphology in the active layer can be optimized under industrially relevant conditions. The goal of this protocol is to show experimental details of how a solar cell device is fabricated using a mini-slot die coater and technical details of running in situ structure characterization using the mini-slot die coater.

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Here, we present a protocol to fabricate organic thin film solar cells using a mini-slot die coater and related in-line structure characterizations using synchrotron scattering techniques.

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices ...

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical properties. The possibility of doping was investigated by adding monovalent cation halides with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+. A shift in the Fermi level and a remarkable decrease of sub-bandgap optical absorption, along with a lower energetic disorder in the perovskite, was achieved. An order-of-magnitude enhancement in the bulk hole mobility and a significant reduction of transport activation energy within an additive-based perovskite device was attained. The confluence of the aforementioned improved properties in the presence of these cations led to an enhancement in the photovoltaic parameters of the perovskite solar cell. An increase of 70 mV in open circuit voltage for AgI and a 2 mA/cm2 improvement in photocurrent density for NaI- and CuBr-based solar cells were achieved compared to the pristine device. Our work paves the way for further improvements in the optoelectronic quality of CH3NH3PbI3 perovskite and subsequent devices. It highlights a new avenue for investigations on the role of dopant impurities in crystallization and controls the electronic defect density in perovskite structures.

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we present a protocol to adjust the properties of solution-processed CH3NH3PbI3 through the incorporation of monovalent cation additives in order to achieve highly efficient perovskite solar cells.

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical ...

Electrospinning of Photocatalytic Electrodes for Dye-sensitized Solar Cells

This work demonstrates a protocol to fabricate a fiber-based photoanode for dye-sensitized solar cells, consisting of a light-scattering layer made of electrospun titanium dioxide nanofibers (TiO2-NFs) on top of a blocking layer made of commercially available titanium dioxide nanoparticles (TiO2-NPs). This is achieved by first electrospinning a solution of titanium (IV) butoxide, polyvinylpyrrolidone (PVP), and glacial acetic acid in ethanol to obtain composite PVP/TiO2 nanofibers. These are then calcined at 500 &#176;C to remove the PVP and to obtain pure anatase-phase titania nanofibers. This material is characterized using scanning electron microscopy (SEM) and powder X-ray diffraction (XRD). The photoanode is prepared by first creating a blocking layer through the deposition of a TiO2-NPs/terpineol slurry on a fluorine-doped tin oxide (FTO) glass slide using doctor blading techniques. A subsequent thermal treatment is performed at 500 &#176;C. Then, the light-scattering layer is formed by depositing a TiO2-NFs/terpineol slurry on the same slide, using the same technique, and calcinating again at 500 &#176;C. The performance of the photoanode is tested by fabricating a dye-sensitized solar cell and measuring its efficiency through J-V curves under a range of incident light densities, from 0.25-1 Sun.

The overall goal of this project was to use electrospinning to fabricate a photoanode with improved performance for dye-sensitized solar cells.

This work demonstrates a protocol to fabricate a fiber-based photoanode for dye-sensitized solar cells, consisting of a light-scattering layer made of ...

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for Cu(In,Ga)Se2 Solar Cells

The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction of degradation mechanisms and prevention of module field failure can consequently decrease investment risks as well as increase the electricity yield. An improved knowledge level can for these reasons significantly decrease the total costs of PV electricity. 
In order to better understand and minimize the degradation of PV modules, the occurring degradation mechanisms and conditions should be identified. This should preferably happen under combined stresses, since modules in the field are also simultaneously exposed to multiple stress factors. Therefore, two 'Combined Stress test with in situ measurement' setups have been designed and constructed. These setups allow the simultaneous use of humidity, temperature, illumination, and electrical biases as independently controlled stress factors on solar cells and minimodules. The setups also allow real-time monitoring of the electrical properties of these samples. This protocol presents these setups and describes the experimental possibilities. Moreover, results obtained with these setups are also presented: various examples about the influence of both deposition and degradation conditions on the stability of thin film Cu(In,Ga)Se2 (CIGS) as well as Cu2ZnSnSe4 (CZTS) solar cells are described. Results on the temperature dependency of CIGS solar cells are also presented.

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for CuIn,GaSe2 Solar Cells

Two &#39;Combined Stress test with in situ measurement&#39; setups, which allow real-time monitoring of accelerated degradation of solar cells and modules, were designed and constructed. These setups allow the simultaneous use of humidity, temperature, electrical biases, and illumination as independently controlled stress factors. The setups and various experiments executed are presented.

The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction ...

Indoor Experimental Assessment of the Efficiency and Irradiance Spot of the Achromatic Doublet on Glass (ADG) Fresnel Lens for Concentrating Photovoltaics

We present a method to characterize achromatic Fresnel lenses for photovoltaic applications. The achromatic doublet on glass (ADG) Fresnel lens is composed of two materials, a plastic and an elastomer, whose dispersion characteristics (refractive index variation with wavelength) are different. We first designed the lens geometry and then used ray-tracing simulation, based on the Monte Carlo method, to analyze its performance from the point of view of both optical efficiency and the maximum attainable concentration. Afterwards, ADG Fresnel lens prototypes were manufactured using a simple and reliable method. It consists of a prior injection of plastic parts and a consecutive lamination, together with the elastomer and a glass substrate to fabricate the parquet of ADG Fresnel lenses. The accuracy of the manufactured lens profile is examined using an optical microscope while its optical performance is evaluated using a solar simulator for concentrator photovoltaic systems. The simulator is composed of a xenon flash lamp whose emitted light is reflected by a parabolic mirror. The collimated light has a spectral distribution and an angular aperture similar to the real Sun. We were able to assess the optical performance of the ADG Fresnel lenses by taking photographs of the irradiance spot cast by the lens using a charge-coupled device (CCD) camera and measuring the photocurrent generated by several types of multi junction (MJ) solar cells, which have been previously characterized at a solar simulator for concentrator solar cells. These measurements have demonstrated the achromatic behavior of ADG Fresnel lenses and, as a consequence, the suitability of the modelling and manufacturing methods.

Indoor Experimental Assessment of the Efficiency and Irradiance Spot of the Achromatic Doublet on Glass ADG Fresnel Lens for Concentrating Photovoltaics

The achromatic doublet on glass (ADG) Fresnel lens makes use of two materials with differing dispersion to reduce chromatic aberration and increase attainable concentration. In this paper, a protocol for the complete characterization of the ADG Fresnel lens is presented.

We present a method to characterize achromatic Fresnel lenses for photovoltaic applications. The achromatic doublet on glass (ADG) Fresnel lens is ...

Developing High Performance GaP/Si Heterojunction Solar Cells

To improve the efficiency of Si-based solar cells beyond their Shockley-Queisser limit, the optimal path is to integrate them with III-V-based solar cells. In this work, we present high performance GaP/Si heterojunction solar cells with a high Si minority-carrier lifetime and high crystal quality of epitaxial GaP layers. It is shown that by applying phosphorus (P)-diffusion layers into the Si substrate and a SiNx layer, the Si minority-carrier lifetime can be well-maintained during the GaP growth in the molecular beam epitaxy (MBE). By controlling the growth conditions, the high crystal quality of GaP was grown on the P-rich Si surface. The film quality is characterized by atomic force microscopy and high-resolution x-ray diffraction. In addition, MoOx was implemented as a hole-selective contact that led to a significant increase in the short-circuit current density. The achieved high device performance of the GaP/Si heterojunction solar cells establishes a path for further enhancement of the performance of Si-based photovoltaic devices.

Here, we present a protocol to develop high-performance GaP/Si heterojunction solar cells with a high Si minority-carrier lifetime.

To improve the efficiency of Si-based solar cells beyond their Shockley-Queisser limit, the optimal path is to integrate them with III-V-based solar ...