Team:UiOslo Norway/Design

UiOslo

This is my design...

Background

Different types of solar cells are available in the market at the moment, such as silicon-based solar cells and dye-sensitized solar cells (DSSC). The most abundant of the solar cells is the silicon-based solar cells, which perform well in the sunny days but are not as efficient in low light conditions. DSSC, on the other hand, does not have high efficiency. An answer to this problem could be to find an additional product that can help increase the overall efficiency of solar cells. One option is a biogenic solar cell that uses living bacteria with pigments to absorb light and convert it into high energy electron for electricity production.

Our biogenic solar cell will utilize high-intensity wavelength of light that is otherwise poorly absorbed by silicon-based solar cells. The reason these wavelengths are not absorbed that well is because of the band gap of silicon, which is the minimum energy required to excite the outermost electron. For electrons in silicon to be excited, the amount of energy found at 830 nm wavelength (1.1eV) is needed. This means that silicon can absorb all the energy found at this wavelength.

On the other side of the visible light spectra, there are shorter wavelengths with higher energy. For example, the blue light (2.5eV) has more energy than silicon can absorb. As only one electron can be excited by one photon, and considering the fact that silicon is incapable of utilizing all the energy found at these high-energy wavelengths, the extra energy (1.4eV) will be lost in the form of heat [1]. Because of the loss of energy, the efficiency of the solar cell will not be as high as it could be. Biogenic solar cells with pigments that absorb high energy photons can help reduce energy loss. The power output of biogenic solar cells does not compare well to the traditional solar cells but can become the value for investment in areas with limited daylight where others cannot provide high efficiency. Our choice to use lycopene as the pigment in our solar cell makes it is capable of utilizing high-intensity wavelength.

Pigment

We decided to use lycopene because of the well-studied nature of the pigment and the success in producing it using bacterial systems[2]. It is also an intermediate in the synthesis of different carotenoids, which will make it easier to introduce other carotenoid pigments later on. Furthermore, lycopene is a transmembrane protein with many conjugated pairs of electrons, which can be excited and result in effective light absorption in the range of 450-510 nm[3]. Because lycopene is a transmembrane protein, it opens up for transport of the excited electrons out of the pigment and into a circuit without help from an adaptor protein, which would have otherwise been necessary if the pigment was localized in the cytoplasm. This property makes lycopene an option for the solar cell.

E. coli does not produce lycopene naturally, but it produces both isopentenyl diphosphate and farnesyl diphosphate, which are substrates needed at the start of the lycopene biosynthesis. Adding crtE, the gene encoding the enzyme geranylgeranyl diphosphate synthase allows for the synthesis of geranylgeranyl diphosphate. The gene crtB, phytoene synthase, can further be added to move the biosynthesis forward by synthesizing phytoene. The crtI gene assures that phytoene desaturase is produced, which results in the production of neurosporene. Due to the nature of the enzyme transcribed from crtI, neurosporene will be processed further to produce lycopene.

Cloning

Gibson cloning was used to insert required genes into a plasmid one gene at a time. First, crtE was inserted into a pBAD vector, creating the recombinant plasmid pBAD-crtE. Once confirmed that the insert was correct, the cloning proceeded with the insertion of crtB into the recombinant plasmid, resulting in pBAD-crtEB. Finally, crtI was added to make the final plasmid containing all three genes, pBAD-crtEBI. The gene set-up was preceded by a promoter and a ribosomal binding site (RBS).

After the individual cloning we had to verify that the insert was correct by gel electrophoresis and sequencing. To test the protein expression, we used a SDS-page.

Our biogenic solar cell

The finished biogenic solar cell is built like a dye-sensitized solar cell. On the top we have a conductive glass plate covered with semi-conductive bacteria. We have two conductive glass plates, where the top one is covered by semi-conductive bacteria producing lycopene, working as the anode, and the bottom one made with drop-casted carbon, being the cathode. The cathode and anode are coupled together by electrical wires and electrolyte solution between them, resulting in a closed circuit.

Making the semi-conductive bacteria

In the solar panel the bacteria cells need to be semi conductive in order to give and receive electrons to create a current. One way of making them semi-conductive is to coat the cells in titanium dioxide nanoparticles, TiO₂. TiO₂ lowers the amount of energy needed to make the excited electrons in the pigments leave the cells and go into a circuit creating electrical energy. The nanoparticles are linked to the cells by tryptophan forming a non-covalent link (a tryptophan-mediated supramolecular interphase) between the TiO₂ and the bacteria, between the cells and the nanoparticles. The bacteria covered with TiO₂ create a paste that are fixed to a conductive glass. This part of the solar cell function like as an anode.

Function

Light travels through the conductive glass plate coated with semi conductive cells, where it excites an electron in the pigment, which in turn is transferred through the titanium dioxide to the conductive glass plate and into the circuit. The excited electron travels through the circuit, to the other conductive glass plate where it is further transferred to an electrolyte solution. The electrolyte solution transfer electrons to the anode where it is released and can go back into the circuit. We chose to use an electrolyte solution containing Iodide red-ox pair due to its use in
previous experiments[2].

Keeping the cells alive

As the bacteria continuously use nutrients and build up waste products, it is essential to keep the media fresh to ensure the survival of the bacteria. So, at a predetermined time, calculated to provide nutrients to maintain cells in near stasis condition, the valves closing the system will open and will flush out the old media, while providing fresh media. Since the electrolyte solution will be flushed out together with the waste products, the media will contain an electrolyte solution. The transfer of the chemicals happens through a semipermeable membrane which helps prevent physical stress to the cells due to the flow of media.

Maintenance of the solar cell

Keeping the pigment-producing bacteria alive in the solar cell, offers a great advantage in terms of maintenance. Since the cells are continuously producing the red lycopene, any portions that have dead cells beyond a certain threshold will not be able to produce lycopene, resulting in a visible color change. After detection of the color change, the damaged portion can be swapped for a new one.

Models to increase the efficiency

Lycopene is an intermediate in the synthesis of different carotenoids. Because of this, new carotenoids can be introduced into the system by inserting additional genes in their biosynthesis pathway into the bacteria. These carotenoids, along with other pigments like phycocyanin and phycoerythrin, can be added to the solar cell model for it to cover the visible spectrum, and ensure increased efficiency of the whole solar panel.

Solar cells can also be made up of multiple plates with different bacteria or different pigments where each layer will absorb their respective, specific light wavelength, increasing the overall efficiency of the solar panel. The use of layers can also be an advantage if we want to use different types of bacteria in the same setup that otherwise would not easily grow in close proximity to each other.

References

Kittel, C.(1986). Introduction to Solid State Physics, 6th Ed., New York:John Wiley, p. 185.
Srivastava, S.K., Piwek, P., Ayakar, S.R., Bonakdarpour, A., Wilkinson, D.P., Yadav, V.G., 2018. A Biogenic Photovoltaic Material. Small 14, 1800729. doi:10.1002/smll.201800729
Lavecchia, Roberto & Zuorro, Antonio. (2019). Use of cell-wall degrading enzymes to improve lycopene extraction from tomato processing waste.