Comparison of Dye-Sensitized Solar Cells: Titanium Oxide vs. Zinc Oxide

This lab explored the efficiency and viability of dye-sensitized solar cell under certain conditions. The modification from week 1 to week 2 was using zinc oxide instead of titanium oxide as the wide-band gap semiconductors. The maximum current conducted from the TiO2 cell was 30.1 μA, under direct sunlight, while the Zinc Oxide cell’s maximum current was 5.1 μA under direct sunlight. According to the results, titanium oxide solar cell was better at measuring the current and voltage under different lights. This does not support the predicted outcome as the conduction band of Zinc is greater than that of Titanium, thus its current and voltage should have been higher as well.

The test under the lamp at different angles was also not effective at producing the predicted results and this could have been due to the exhaustion of the cell from being under the light for a certain period of time and the classroom lights still being on. The greatest source of error might have been due to zinc oxide being washed off after being bathed in the blackberry solution in week two’s experiment.

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Since the oxide solution is the semi-conductor band this would have caused great error in the reading of the current and potential.

Introduction

Traditionally, solar cells use silicon doped with phosphorous (n-type, meaning it contains extra electrons) or boron (p-type, meaning it is deficient in electrons) to create a semiconductor. A semiconductor, essentially, is a material with a gap between the conduction and the valence bands. In silicon-based solar cells, when the sun or photons hit the cell, electrons present in the n-type semiconductor absorb the energy and transfer to “holes” which is the missing electrons in the p-type semiconductor.

This flow of electrons is what creates a current which can be conducted when conductive contacts are situated on the top and bottom of the p-n type semiconductors (1). This current can then be drawn off to use as electricity. The current, which is generated by light getting absorbed by electrons from the n-type semiconductor and moving to the p-type semiconductor, is hosted between the valence and the conduction band. Because silicon cells are costly, yet very effective an alternative, cost-effective solar cells exist known as dye-sensitized cells, which are examined in this lab. The cells that were constructed contained wide band-gap semiconductors in nanoparticle form (Titanium Oxide) and chromophores, molecules that absorb light. When light hits the cell, the dye molecules, chromophore, absorb it immediately and an electron is promoted to an excited state. This causes the electron to move from the highest occupied level, HOMO, to the lowest unoccupied molecular orbital, LOMO, leaving the chromosphere in an oxidized state (positively charged). The excited electron is then transferred to Titanium Oxide nanoparticle, TiO2, where it is collected by the conductive electrode which generates the current that is visible on the multimeter. The electrolyte solution, Iodine, is used to reduce the chromosphere to the ground state or a neutral form, so that it can absorb another light photon. In this lab, the current and voltage of the dye-sensitized cells were observed to determine how well electricity is conducted.

In week one, the dye-sensitized cell was made with titanium oxide solution as the wide band-gap semiconductor, while in week two, zinc oxide was used instead. The different semiconductors were the independent variables and the resulting current was used to determine which dye-sensitized cell was more efficient at conducting current. Because voltage and current were used, Ohm’s Law became relevant as it states that current and potential are directly proportional. While, voltage recorded is dependent upon the difference in energy levels of the TiO2 or ZnO conduction band and the electrolyte, the current is dependent upon the amount of electrons that are able to relax from the LIMO of the blackberry dye to the conduction band of the TiO2. The energy difference between the valence band (the LUMO of the electrolyte solution) and fermi level is what determines the potential difference, which is also the voltage measured in the lab (2). Fermi level is essentially the energy of the conductive band (HOMO-unoccupied orbital at 0K). Additionally, as mentioned before, the efficiency of dye-sensitized cells is less than that of the silicon-based cells, whose current results from electron flow following the movement of electrons excited by photons between the n-type semiconductors and p-type semiconductors. Nonetheless, dye-sensitized cells are cheaper to construct than the silicon-based cells and are also easier to build. Therefore, more research is required to determine how to maximize the efficiency of dye-sensitized cells, but they are still a promising step towards green energy.

Experimental

First, connect the black alligator clip to the middle socket and the red clip to the right socket of a digital multimeter. Next obtain and identify the conductive side of two different glass pieces using the mutimeter and the alligator clips. Keeping track of which side is conductive, rinse one piece of glass with a small amount of acetone and place 4 pieces of tapes on the conductive side of the glass so that three sides have tape on 1-2mm in from the edge and the fourth side has tape 4-5mm from the edge. Meanwhile, in a mortar, thoroughly grind 1.5 g of titanium oxide and 8.5 g

of deionized water and while grinding add 0.015 g of trimeric acid. Grind for approximately 15-20 minutes. Next, place 3-4 drops of titanium oxide on the untapped portion of the conductive glass. Using a glass rod, make an even, thin coating of the suspension on the surface. When the solution is set, remove the tape and bake the slide in the oven for 1 hour. While the slide is baking, obtain 3-4 blackberries in a petri dish and add 10mL of dH2O and crush the berries using hands until the mixture is uniform in consistency. Set this mixture aside. Rinse the second piece of conductive glass with ethanol and hold it over a flame with thongs and form a carbon coating on the surface.

Once the slide is done baking, let it cool to room temperature then place the coated side down in the berries mixture and soak for approximately 10 minutes. After 10 minutes, rinse the glass with dH2O and ethanol and gently dry with a Kimwipe. Next, cut a piece of Parafilm large enough to cover the size of the glass and lightly place the piece of Parafilm over the glass with the TiO2 coating and cut out a window for the coated area. Stick the Parafilm to the glass. Carefully, add 3-4 drops of iodide electrolyte solution to the well created in the plate and place the carbon-coated glass on top of the TiO2-coated glass such that the two pieces are offset. Finally clip the two glass pieces together with binder clips and remove any air bubbles and dry ay excess Iodide solution with a Kimwipe.

Connect the black alligator clip to the conductive side of the titanium oxide plate and the red alligator clip to the carbon-coated plate. Find the current and

voltage in various locations like in and out of direct sunlight, overhead lamp, laboratory florescent lights and window. Then place the cell under an overhead lamp and until a complete circle is achieved, rotate the cell in 45 degree increments, jotting down the current at every position. Repeat the same process in week two except with Zinc Oxide solution instead of the Titanium Oxide.

Results

Week 1

Table 1: A summary of the current (μA) the voltage (V) observed in various locations with the Titanium Oxide solar cell.

Table 2: A summary of the current (μA) observed at various angles under a lamp with the Titanium Oxide solar cell.

Week 2

Table 3: A summary of the current (μA) the voltage (V) observed in various locations with the Zinc Oxide solar cell.

Table 4: A summary of the current (μA) observed at various angles under a lamp with the Zinc Oxide solar cell.

Discussion

The main objective of this lab was to examine how much electricity is produced by different dye-sensitized solar cells. This was measure using voltage (V), which is the difference in charge between two points and current (μA), which is the rate at which electric charge is flowing. In the first week, the solar cell was constructed using titanium oxide as the wide-band semiconductor. The recorded current of the solar cell under direct sunlight was the greatest at 0.8 μA before exposure and 30.1 μA after exposure. Although the current found under direct sunlight and the window had similar differences before and after exposure, the recorded potential values and the current had significant differences. This is due to the absorbance spectrum of the blackberry dye. The blackberry dye absorbs light from the UV range to the IR range of wavelengths so when the solar cell is exposed to a greater range of wavelength, that is used to excite a wide arrange of electrons in the dye. The window was able to filter out the light in the UV spectrum which caused there to be a lower current to be recorded because the dye was absorbing less wavelength of the current. This concept can be applied to the lamp as well since that also emits a light of certain wavelengths with less range that the direct sunlight. This can also account for the lower current values as well. Ohm’s law was upheld as well because as the current increased at a certain location, the voltage also increased. For example, under the lamp after exposure, the current was 6.6 μA and the voltage was 0.2 V, whereas next to the window the current was 9.5 μA and the voltage was 0.208 V. This shows how Ohm’s law was upheld in the first week’s experiment.

In week two of the experiment, Zinc Oxide was used as the wide-band gap semiconductor instead of the Titanium Oxide. This is because Zinc has a higher conduction band than that of Titanium Oxide, thus the fermi level of Zinc Oxide is also higher in energy. Since the fermi level is higher, the voltage should also be higher because the difference between the valence band (LUMO of the electrolyte solution) and the fermi level is what governs the potential difference. Thus, the reasoning behind this modification was valid; zinc had a higher conductive band because of its higher fermi level than titanium thus why zinc should have conducted more current.

Even though it was expected that the zinc oxide solar cell would conduct more energy, the results proved otherwise. The current conducted by the solar cell from week 2 under various locations was overall lower than that of the solar cell from week 1. For instance, under direct sunlight the current recorded for the zinc oxide solar cell was just 5.1 μA, whereas the titanium oxide solar cell’s current was 30.1 μA. Potential source of error could have been misuse of the multimeter and the alligator clips which were old and quite inefficient and inconsecutive with its readings. Multiple times, the alligator clips would give different readings based on their placement on the solar cell. Additionally, as seen in figure 3, during week two, the zinc oxide layer from the glass washed off after being bathed in the berry solution and since the oxide solution is the semiconductor of the cell, this caused there to be a large error in the conduction of the current. More error could have been caused by the Parafilm not being sealed on the cell properly causing the iodide solution, the electrolyte, to escape from the cell, resulting in inaccurate conduction of current and potential. This was especially true for week one as it was the first time working with Parafiilms. Nonetheless, Ohm’s law was still upheld by week two’s results as the recorded potential was proportional to the recorded current. The titanium oxide solar cell produced more current in comparison to the zinc oxide cell, but it was still not significant amount of current to power phone, calculator or watches which all require current measured in amps and not microamps and the current generated by the cells was all merely in microamps (1 amp = 1x106 microamps). Nonetheless, this is a feasible result because the cell constructed in both weeks was very small and also it was constructed using organic dye rather than inorganic dyes which are inherently better at conducting electricity and are used by large industries.

Another measurement that was taken to test the effectiveness of the solar cell was its current under a lamp at different angles. Essentially, the cell was turned 45 degrees under the lamp in order to determine how this would impact the measured current. In the titanium oxide cell, the highest recorded current was 7.6 μA at 360 degrees. Meanwhile, in the zinc oxide cell, the highest current was 0.7 μA at 45 degrees. The implication of this test was to see whether the solar cell’s efficiency enhanced at certain angles. Real life implication of this are that throughout the day, the sun’s movement results in different angles of sunlight, thus the angle for the solar cell is expected to remain constant relative to the sunlight. This is the reason why solar panels are installed on roofs, where they are maintained at a constant angle relative to the sunlight. Therefore, this test was done to determine at what angle the solar cells should be installed in order to generate maximum current. This test did not give a specific angle at which the solar cells should be installed, rather it was expected that for both weeks the highest current would be read at 180 degrees which is directly beneath the lamp. The error observed may have been due to the solar cell being exhausted by being under the lamp for a long period of time that it produces less current at each angle. Another source of error could have been due to the classroom lights still being on when measuring the current under the overhead lamp.

The ruggedness and functionality of the solar cells constructed both weeks is poor and it would be greatly difficult to power any device with them because they cannot generate enough current. 1 microamp is 1x106 amps and the results from both weeks produced mere microamps, and powering actual devices like calculators, watches and flashlights require current measured in amps. This is due to the use of organic dye instead of inorganic dye which is more efficient. Since inorganic dyes are more opaque than organic dyes, they absorb more light, thus they have more photons to excite electrons in the chromophore from the HOMO to the LUMO, which would generate greater and more efficient electricity. Another problem with solar cells is their dependency on sunlight. At night, the electricity that is produced in the day is consumed in order to produce further electricity. This issue also extends to cloudy or foggy weather which all can inhibit the efficiency of the electricity produced. A possible solution that is proposed for this issue is the combination of solar cells with hydrogen fuel cells (3). In the day, the generated electricity can be used to split O2 and H2 gas and at night, this gas can be used to generate electricity by the fuel cells. This produce water as a waste product which can just be recycled in the atmosphere.

An interesting topic to further explore would be using vegetable-based dye instead of berries-based dye. Generally, vegetables are cheaper than berries because of their importation costs and growing practices. Additionally, vegetable such as beets, red cabbage and kale can provide dye that is much more rich and opaque than the dye of berries. Therefore, vegetable-based dyes would be cost effective and still provide rich dyes to generate current from solar cells. Additionally, some berries have seeds that stick to the cell when transferring the dye and in an attempt to remove the seeds the oxide solution may also come off, as evident in figure 3, thus why vegetables that don’t have seeds could be an alternative to using berries as the dye.

Conclusion

This lab explored the efficiency and viability of dye-sensitized solar cell under certain conditions. The modification from week 1 to week 2 was using zinc oxide in place of titanium oxide. The maximum current conducted from the Titanium Oxide cell was 30.1 μA, under direct sunlight, while the Zinc Oxide cell’s maximum current was 5.1 μA under direct sunlight. According to the results, titanium oxide solar cell was better at measuring the current and voltage under different lights. This does not support the predicted outcome as the conduction band of Zinc is greater than that of Titanium, thus its current and voltage should have been higher as well. The test under the lamp at different angles was also not effective at producing the predicted results and this could have been due to the exhaustion of the cell from being under the light for a certain period of time and the classroom lights still being. Possible sources of error might have been due to zinc oxide being washed off after being bathed in the blackberry solution in week two’s experiment. Since the oxide solution is the semi-conductor band this would have caused great error in the reading of the current and potential. Additionally, the alligator clips were old and non-consistent with their readings as the readings varied when moved on the cell. More error could have been due to Parafilm not sealed on glass properly, causing the electrolyte solution to escape from the cell. Overall, this lab proves that dye sensitized solar cells require further refinements in order to be more effective at conducting large scale electricity.