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| Fig. 1: Solar Panels. (Source: Wikimedia Commons) |
Solar energy is the fastest growing electricity source in the world. [1] Harnessing just 0.01% of the 173,000 terawatts of solar energy that continuously hits the Earth per day would satisfy the whole world's energy usage. [2] Although there are many advantages for adopting solar energy, current traditional photovoltaic solar cells are costly, have low efficiency rates, and are weather and time-dependent. [3]
Recently, motivated by current shortcomings of conventional solar cells, researchers at the University of British Columbia led by Dr. Vikramaditya G. Yadav developed a biogenic solar cell fabricated from bacteria. This successfully converted light to solar energy more efficiently than biogenic solar cells previously fabricated. Nonetheless biogenic solar cells are still far from reaching industry-standards for production-grade solar cells. [4]
Commercial solar panels contain solar cells made with inorganic, crystalline silicon materials that can be excited by light stimulus to release electrons (See Fig. 1). [4] However, in the case of biogenic solar cells, the material used is biological, organic material. Previously, research in synthesizing biogenic or biohybrid solar cells were focused on extracting photosynthetic natural dyes from bacteria. These processes were costly, complicated, and involved toxic solvents, which degraded the dyes and led to decreased efficiency in harnessing solar energy. [5]
Without utilizing these detrimental extraction processes and leaving the natural dyes in the bacteria, Yadav and his team used genetically engineered Escheria Coli to synthesize a biogenic photovoltaic material. [5] The E. Coli were genetically engineered to produce large amounts of lycopene, which is a natural dye found in plants that efficiently captures light. The bacteria were covered with titanium dioxide, which acted as a transparent conductor, and then added to a glass surface. Through testing, the researchers discovered that the E. Coli-covered glass generated 0.686 mA/cm2, which is the highest recorded short-circuit current density for a biogenic photovoltaic cell, significantly more than the previously recorded 0.362 mA/cm2. Another key number recorded was the open-circuit potential of 0.289V. They claimed that under conditions that closely simulated outdoor sunlight, the difference between the recordings under normal and low light was insignificant, indicating possible suitability for usage under low light conditions.
However, there are areas highlighted that are in need of improvement. For instance, the study described that the total external efficiency for the conversion of incident sunlight usable electricity of the dye-sensitized solar cells (DSSC) incorporating cells@TiO2 was 0.057%, which was significantly lower than the 13% external efficiency achieved by conventional DSSCs. To sanity-check this number, assuming that the power of the sunlight on a solar cell at mid-day is about 1000 watts/m2 or 0.1 Watts/cm2, this biogenic solar cell power would be
which amounts to an efficiency of only 0.02%. This is significantly lower than the 13% of a cheap industry-level solar cell and the number that Yadav's study described. Therefore, Yadav likely did not expose the solar cell to mid-day level sunlight to generate the previously described short-circuit current density and open-circuit potential. Furthermore, it is far from reaching the level necessary to be successfully used in production-grade solar cells. [5]
Yadav and his team's proof-of-concept model nearly doubled the current density output and reduced production costs for biogenic photovoltaic cells, which is a promising sign for future developments in the field. Moving forward, Yadav mentioned that the "holy grail" would be to discover a process that would not kill the bacteria, allowing them to produce dye continuously. For future testing, gains in efficiency could be made by potentially incorporating platinum as the counter electrode, using more light-sensitive dyes, and other techniques. However, it is unclear whether biogenic solar cells will actually provide cost-savings in the current situation because solar technologies have become much more affordable in the recent years. For reference, LEC for traditional energy sources are between 7.04 to 11.90 US cents/kWh and the LEC for solar PV technologies are between 9.78 to 19.33 US cents/kWh. [6]
Overall, Yadav's study is a promising improvement in the development of biogenic solar cells. However, there are many more advancements that need to be made before biogenic solar cells can compete with current industry-standard inorganic solar cells.
© Crystal Zheng. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] S. Sengupta, "Solar Power Is Burning Bright. But It's Hardly Twilight for Fossil Fuels,"New York Times, 5 Apr 18.
[2] R. C. Anderson, Business Lessons from a Radical Industrialist (McClelland and Stewart, 2011).
[3] J. Mohtasham, "Review Article - Renewable Energies," Energy Procedia 74, 1289 (2015).
[4] R. W. Miles, G. Zoppi, and I. Forbes, "Inorganic Photovoltaic Cells," Materials Today 10, No. 11, 20 (2007).
[5] S. K. Srivastava et al., "A Biogenic Photovoltaic Material," Small 14, 1800729 (2018).
[6] M. Cai et al., "Cost-Performance Analysis of Perovskite Solar Modules," Adv. Sci. 4, 1600269 (2016).
