Manufacturing dye-­‐sensitized solar cells By Kasra Amini (year 3 summer student, UCL) Introduction: Semiconductor materials are used greatly in modern electronics, such as in: radios, telephones, computers, light-­‐emitting diodes, solar cells and many other devices. Titanium dioxide, TiO2, is a white semiconductor which is not sensitive to visible light. Hence, another material must be used to sensitize the TiO2 (titania) particles; a layer of dye molecules coat the titania particles in order to absorb visible light. The first dye-­‐sensitized solar cell was produced by O’Regan and Gratzel in 1991 [3,4]. In the dye-­‐sensitized solar cell there exist two electrodes, an anode and cathode, which are made of a specific type of glass that has a Transparent Conductive Oxide (TCO) coating on one side. Figure showing schematics of DSSC [1]. On the negative terminal of the cell, the anode, there is a porous network of titania particles which are coated with a sensitizing dye. Its porosity is a key feature as it has roughly a thousand times greater a surface area than the equivalent flat area, where light can easily pass through the transparent glass and can be caught [1]. Subsequently, the conductive surface collects charges. The positive terminal of the cell, the cathode, is coated with a catalytic material such as platinum or carbon in order to catalyse the electron transfer process and for the circuit to be complete. An electrolyte solution, such as iodine solution, separates the electrodes and ensures charge transportation through a redox couple. Iodide/tri-­‐iodide would be the redox couple if iodine solution. The photovoltaic effect is at the heart of how sunlight can be turned into electricity. Upon absorption of light, an electron is promoted to the conduction band of the semiconductor (TiO2), where there is a “hole” left behind. Hence, upon continued exposure to sunlight there is a separation of electrons and “holes”, a charge separation in the solar cell. Naturally, these opposite charges would like to combine together again, in a process called radiative recombination [2]. They do so after the electrons are transported from the anode to the cathode via an external circuit. As stated previously, TiO2 must be covered with a sensitizing dye in order to absorb visible light. Upon absorption of light, dye molecules can give up an electron and inject it into the titanium dioxide particle to which it is attached to. In this case, the electron is injected into the titania and the “hole” is left behind on the dye molecule. However, this oxidised dye molecule does not exist for long as the electron will recombine with the oxidised dye molecule. The injected electrons migrate through the titania particles and reach the TCO side of the glass at the anode and travel through an external circuit to the cathode -­‐ the movement of electrons produces an electric current. The electrons can then travel from the cathode’s surface, which is coated with an electron transfer catalyst such as Platinum, to the oxidized dye molecule through the electrolyte solution that has a redox couple. This cyclic efficacy will be repeated as the electron and “hole” has recombined and ready to be separated again. It is clear that kinetics is of paramount importance, where fast kinetics wins; the fastest route for the electron to return back to the dye molecule is via the external circuit and through the electrolyte solution [1]. Apparatus list: -­‐
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Transparent Conductive Oxide (TCO) glass Iodine solution (50mM) from Sigma-­‐Aldrich. Product #: 318981. Batch #: 31096PP. Sensitizing dye (Ruthenizer 535-­‐bisTBA) from Solaronix. Product #: 21622. Batch #: 404c/110811FM. Platinum or carbon. TiO2 (P25 titania) from Sigma-­‐Aldrich. Product #: 7184. Batch #: 4160031198. Multimeter; PeakTech Digital Multimeter 4000. S/N: 08020091572. Bulldog clips. Superglue. Method: Preparation of TCO glass squares: Squares of TCO glass were prepared with dimensions of 40x40mm. A cutting line was scribed onto the TCO glass so as to achieve the desired dimension. The glass was subsequently broken into two pieces by gently bending the plate from both sides of the line. These figures are from reference [1]. The glass plates were cleaned before deposition; a soft sponge was used in order to avoid scratching the conductive side. This figure is from reference [1]. Preparation of anode: A hot plate was set to a temperature of 300oC and was left for 10 min. The side of the glass plate which has the TCO layer was identified by gently and carefully sliding your thumb across the surface, where the surface provided some friction; the uncoated side had no such friction. The glass plate was placed on the hot plate and left for 10 min. A thermocouple identified he glass plate’s temperature to be ca. 185.9oC. The titania solution was prepared by adding a certain volume of titania slurry (at a specific weight %) and a certain volume of D.I. water. The artist spray brush was ready to use. One had to spray the titania on with a constant action, 4 inches above the glass plate. Around 4-­‐5 coats of spray onto the glass plate were required to produce a nice, thick enough coating. The pressure of the compressed air was around 30-­‐40 psi. Lower pressures, around 10-­‐20psi, could also be used to spray coat lighter materials such as thin stainless steel sheets. The sprayed titania should deposit instantly upon contact with the glass plate. The temperature of the hot plate was slowly reduced in order to prevent cracking of the glass plate. Once cooled the coated glass plate was sintered in the oven for 15-­‐20 min at 450oC. Glass plate was left to cool down after sintering for around 1 hour. Staining the Titania: As we saw earlier, it is essential to stain the titania coating with a sensitizing dye. There are two different methods that could be used in doing so depending on the sophistication of the dye solar cell. The first is to use a high performance synthetic dye, Ruthenizer 535-­‐bisTBA. A typical concentration of 3 x 10-­‐4 M in methanol was prepared; around 10 mg of dry 535-­‐dye powder was dissolved in 25 mL of methanol in a 100mL beaker, and stirred using a magnetic stirrer for around 1 hour to ensure that no traces of solid were present. A deep purple coloured solution was produced. These figures are from reference [1]. The titania coated glass plate was slowly immersed into the staining bath so as to avoid damage to the glass plate; the titania coated side of the glass was facing upwards. This figure is from reference [1]. The beaker was sealed and placed into the dye solution for 4 – 8 hours at room temperature and contact with light was avoided. After the allotted time, the glass plate was taken out of the staining solution and placed on a piece of tissue to dry. These figures are from reference [1]. The second method uses frozen berries which are a good source of natural dyes that absorb visible light. Raspberries or blackberries were crushed in a container so as to extract the dark red juice. These figures are from reference [1]. Once there was enough juice, the anode was place into the container and stained for 4 – 8 hours at room temperature. These figures are from reference [1]. Again, contact with light was avoided and the container sealed. After the allotted the anode was placed onto tissue paper to dry. Completing the DSSC: Another TCO glass plate, used as the cathode, must have a catalyst coated onto the conductive side of the glass; platinum was used as the catalyst, where a platinum spotting machine was used to coat the conductive side. Alternatively, a pencil could be used to coat carbon onto the conductive side of the cathode, which is another catalyst that could also be used. These figures are from reference [1]. A few drops of 50 mM Iodine solution, electrolyte solution, was placed onto the dye-­‐coated side of the anode, where the cathode was placed on top of the anode in an offset manor so as to allow electrical contact with each electrode without short-­‐circuiting the solar cell. The platinum coated side of the cathode was on top of the dye coated side of the anode, where iodine was separating both electrodes from one another. These figures are from reference [1]. Bulldog clips were used to keep the solar cell in place and to complete the opened vessel solar cell. These figures are from reference [1]. Alternatively, you could make a closed-­‐vessel solar cell by placing a thin line of superglue on the counter electrode (on the side which has platinum coated), making sure it’s a square gasket. Let it dry and then place iodine solution inside of the gasket. Carefully place a thinner line of superglue on top of the dried superglue placed previously and then place the anode (titania coated electrode) on top of the thin line of glue and let that dry. Again make sure the electrodes are placed on top of one another in such a way that they seem to be offset. A multimeter was attached to the solar cell in such a way that the negative end was attached to the anode (titania electrode) and the positive end to the cathode (counter-­‐electrode). A voltage of around 0.6 V should be typically recorded under full sun illumination. Results: The titania slurries were made as follows: (1) 2549 -­‐ spherical: -­‐ 9.603 mL of slurry (24.76 wt% of titania in slurry), equivalent to 3.0g of titania nanoparticles, and 30.397 mL of D.I. water were placed into a 50mL beaker and stirred for 10 min. This is a 7.5 wt% solution. Coating was very good. -­‐ Coating was ok. When this coating was used in the DSSC, the voltage output produced was the highest seen, with 0.68 V produced. (2) 2551 -­‐ needles: -­‐ A 7.5 wt% sample solution was produced where 3.0g of NPs was dissolved in 27.884 mL of D.I. water. Initially, poor coatings were produced. Decision was made to produce concentrated sample solutions. Note that the needle slurry solid actually had 31.24wt% of titania. -­‐ A 30 wt% solution was produced where 12.3159g of NPs was dissolved in 27.884 mL of D.I. water. Much better coatings were produced. (3) Citric acid coated titania: -­‐ 25.3378 mL of slurry (11.84 wt% of titania in slurry) was dissolved in 14.662 mL of D.I. water -­‐ Coating was ok. When this coating was used in DSSC, the voltage output was moderate, with a value of 0.5 V. This sample of titania was nowhere near as good as the 2549 spherical titania in splitting water and producing hydrogen, and hence, this citric acid coated titania coating was not as good a photocatalyst as in the 2549 coating. Hence, the reason why a lower voltage was obtained with this coating sample. References: 1) “Dye Solar Cells for Real – The assembly guide for Making Your Own Cells”, Solaronix 2) S. Ito et al., “High-­‐efficiency (7.2%) flexible dye-­‐sensitized solar cells with Ti-­‐metal substrate for nanocrystalline-­‐TiO2 photoanode”, Chem. Commun., 2006, 4004-­‐4006. 3) B. O’Regan and M. Gratzel, Nature, 1991, 353, 737. 4) M. Gratzel, Nature, 2001, 414, 338.