Solar Cells Part 2

Solar cells from Blueberries:

This is essentially a dye sensitized solar cell.
To explain how this works in more detail we need to go back to Nature.

The problem with silicon solar cells is that achieving the optimum band gap is difficult. This was already explained in the last post. But wait, who am I kidding?! :P No one would have gone through that anyways. So let me explain again.

If the band gap is high, high energy photons can be captured. However, if the band gap is low, then more number of photons can be captured. There therefore must be an optimum band gap. Tandem solar cells, quantum dots are two of the ways used to capture all the wavelengths of light.

Also, in solid state devices. Silicon is used as the electron generator and an electric field is produced in it which causes charge transport. A simplistic explanation of this process is:- A photon is incident on n type Si. If it's energy is more than the band gap, it excites an electron which moves towards the bottom of the cell: p junction and then through an electrical wire to the load.
Now the problem is to prevent the recombination of the electron with a hole as it travels from the top negative junction to the bottom positive junction.

Dye sensitized cells solve the fist problem by mimicking photosynthesis. They consist of an electrolyte with two electrodes. Silicon basically acts as a charge transporter and does not generate the electron.

What happens is, a photon falls on the organic dye which excites the dye molecule to an excited state releasing an electron. This electron moves into the TiO2 and exits through the conductive layer into the electrical wires. This is how current flow occurs.

Then the electron returns to the cell via the bottom conductive layer. With the help of a catalyst (usually platinum), the electron enters the electrolyte and interacts with the tri iodide molecule. It reduces it to form an iodide ion. The iodide ion moves towards the activated dye molecule and transfers an electron to the dye. Thus the dye molecule reverts to its previous ground state and the iodide ion reverts back to tri iodide.

What happens in photosynthesis is, an electron is absorbed by a pigment. The energy of the electron is used to synthesize dihydro nicotinamide di hydrogen phosphate (NADPH), a molecule that will eventually produce carbohydrates. To get the pigment back to it's initial state, an electron is donated in the oxidation of water to produce oxygen.

The difference between the two processes is that the storage of energy in the form of starch occurs in leaves, while the storage of energy in a dye sensitized solar cell is still not possible.

The similarities of the two processes is evident. Both are regenerative cycles which involve redox reactions. They use a multi layer structure similar to a thylakoid membrane. (They are the site of the light dependent reactions in leaves)

In DYSC the organic dye replaces the pigment in Nature. The wide band gap semi conductor used replaces the oxidized NADPH and carbon dioxide as the electron acceptor. The electrolyte replaces the water and oxygen as the electron donor.

Now I will talk a little about the materials involved.

Typically a ruthenium metalorganic dye (Ru-centered) is used as a monolayer of light-absorbing material. The dye-sensitized solar cell depends on a mesoporous layer of nanoparticulate titanium dioxide to greatly amplify the surface area (200-300 m2/g TiO2, as compared to approximately 10 m2/g of flat single crystal). The photogenerated electrons from the light absorbing dye are passed on to the n-type TiO2, and the holes are passed to an electrolyte on the other side of the dye. The circuit is completed by a redox couple in the electrolyte, which can be liquid or solid. This type of cell allows a more flexible use of materials, and is typically manufactured by screen printing and/or use of Ultrasonic Nozzles, with the potential for lower processing costs than those used for bulk solar cells. However, the dyes in these cells also suffer from degradation under heat and UV light, and the cell casing is difficult to seal due to the solvents used in assembly.

The absorption of incident light depends upon the number of dye molecules attached per unit volume of the semi conductor. if the dye is on a flat surface less than 1% of light will be absorbed.
Light absorption is maximized by the use of sintered nano meter size anatase (tetragonal crystal structure) titanium dioxide. The surface are is increased by 200-300%. This surface has pores in the range of 20-500Angstrom. It is advantageous to use TiO2 because it is cheap, easily available, bio compatible and non toxic. The anatase phase is used because it has a wide band gap that is transparent to visible light and thus ensures that the light is absorbed only by the dye molecules.
The thickness of the TiO2 layer is a compromise between maximum surface area and minimum recombination losses.

They dyes used by commercial dye sensitized solar cells generally use ruthenium bi pyridyl based dyes. The wavelengths of light absorbed are 510-570nm and adequate conversion between 450-650nm. This absorption spectrum overlaps well with the diffuse sunlight spectrum. However such dyes are hard to synthesize.

A little about these dyes
Ruthenium bipyridyl complexes have been used as efficient sensitizers for photoelectrochemical cells based on nano-porous films of . It is found that cis-dithiocyanate-bis(2,2-bipyridyl-4,4-dicarboxylate) ruthenium (II) can be used as the sensitizer in a solid-state photovoltaic cell in which a monolayer of the sensitizer is sandwiched between nano-porous n- and p-CuI.

A significant photo current can be obtained using natural anthocyanin dyes which can be extracted from blueberries as natural water based substitutes. The absorption of cyanin on the surface of TiO2 is a rapid reaction and the OH counter ion is displaced from the ti(4) work site that combines with a proton donated by anthocyanin. This strong chemical affinity is the reason blue berries can be used :D.


An Experiment that was conducted: (i found this paper on the net)

A commercially bought titanium dioxide coated glass slide was stained with a berry dye (e.g. raspberries, blueberries, beetroot); the dye was made by crushing the fruit (or leaves) and purifying them in a solution of methanol, acetic acid and water . This slide was washed with water, and dried with propanol, and a transparent conducting glass slide (tin dioxide coated) was secured over it using metal clips.

The electrolyte solution (0.5M potassium iodide and 0.05M iodine in ethylene glycol) was drawn up into the porous titanium dioxide structure . Raspberry, blueberry, beetroot and orange leaf cells were constructed.

All cells produced a photocurrent when a voltage was applied , but its magnitude varied between the dyes; the region of negative current and positive voltage represents photocurrent activity. It was found that blueberries were most efficient –they produced a photocurrent of 0.2mA (~0.02% efficiency) - followed by raspberries, then beetroot. The orange leaf dyed cells produced virtually no current.

Initially the blueberries, raspberries and beetroot showed improvement, whereas there was none in the orange leaf dye. There was significant improvement over the first 9 hours, before degradation began. There was improved photocurrent as the concentration of electrolyte solution was increased for raspberries, blueberries and beetroot, but not for the orange leaf cell.

It is hard to take into account factors like electrolytic degradation and the oxidation of fruit etc.However people feel that the improvement is independent of the degradation process and when this degradation process becomes the dominant process then a decrease in efficiency is noted.

The improvement process can be explained by looking at the energy levels associated with the titanium dioxide, the dye and the electrolyte solution. In the blueberry cell: the ease with which an electron moves through the system is determined by the energy barriers, or band gaps that must be overcome.

The electron must have a significant energy to transfer from the electrolyte to the dye. As the cell is exposed to the light the cell heats up, causing evaporation of the electrolyte solution. This increases the concentration of KI and I2, and the energy level of the valence band is raised, making the transfer of electrons to the dye a far more favourable process. This explains why the photocurrent improved when placed underneath the light.

The conduction band of the blueberry dye lies above that of the titanium dioxide conduction band. Hence it is energetically favourable for the electrons excited from the valence band into the conduction band of the dye to be transferred to the conduction band of the titanium dioxide.

This is not the case in the orange leaves. The conduction band of the orange leaves lies below that of tiO2.
Thus in choosing materials it is important to look at the energy considerations.

A significant short coming of the DYSC model is the leaking of the electrolyte which reduces the life time of the cell. Although we can use sold electrolytes where the dye would eject an electron into the TiO2 and a hole into the solid electrolyte the efficiency is really low due to the poor penetration of the sold into the pores of TiO2.
Currently there is research going on, on the use of polymer gel to quasi solidify the electrolyte. The addition of Poly(viny1idene fluoride co-hexafluoropropylene) to the KI/I2 electrolyte has improved the fill factors and the energy conversion efficiency of the DYSC by about 17 %.

Thus dye sensitized solar cells seem to be a viable option for the future