Artem Sokol

05/17/2021

Engineering Proposal Final

Purpose

The purpose of this proposal is to address the need for a green and reusable source of energy and to discuss how an innovation of current solar panels can increase their potential to replace fossil fuel energy.

Summary

There is no doubt that modern society is heavily reliant on energy, we use it for practically everything and most people could not imagine a world without it. Unfortunately, our current sources of energy are hazardous and limited, and our ever-growing reliance is only catalyzing the problem. “Since 1970 carbon dioxide emissions have increased by 90% with emissions from fossil fuel combustion and industrial processes contributing about 78% of the total greenhouse emissions increase.” (epa.gov). Our increased usage and reliance on fossil fuel energy is directly correlated with the increase of hazardous emissions. It is essential that we find a solution to this problem before we damage our planet beyond repair and exhaust what limited supplies we have left.

The transition from fossil fuel energy to solar energy will provide the solution that is so desperately needed. Solar energy is both clean and reusable, and solar panel technology is reliable and constantly improving. However, many consumers are hesitant to transition to solar panels because of the upfront cost of purchasing a solar panel and the lower efficiency of energy production compared to fossil fuels. That is why I am proposing the implementation of a tandem solar cell comprised of a layer of silicon and a layer of perovskite. The mechanically stacked dual-terminal silicon/perovskite tandem solar cell, with the sub-cells independently fabricated and optimized, are attached by contacting the back electrode of the perovskite top cell with the metalized front contact of the silicon bottom cell. (Lamanna). The two-terminal cell can reach an efficiency of nearly 26%, a significant increase from the single layer silicon cell with an efficiency of only 12%. This innovation can also significantly decrease the cost of solar panels, as a tandem cell does not require high efficiency and therefore high-cost silicon or perovskite layers. Furthermore, perovskite and low efficiency silicon are easy to produce and abundant making them a much cheaper alternative.

Diagram 1 (https://ars.els-cdn.com/content/image/1-s2.0-S2542435120300453-fx1_lrg.jpg)

Introduction

Our supply of fossil fuels is slowly but surely running out, and our greenhouse emissions continue to increase. We are damaging our planet’s atmosphere and environment while simultaneously forcing ourselves into a potential energy deficit. We need a solution that can take the burden off our damaged planet and our depleted fossil fuel supply. Solar panels provide that solution as they are a clean and reusable source of energy. However, solar panels continue to be overlooked as a replacement for fossil fuels due to the higher cost and lower efficiency when compared with fossil fuel energy. To make solar panels more appealing to average consumers and ultimately major energy providers we must decrease their cost and increase their efficiency. I am proposing the implementation of a joint photo-voltaic cell comprised of a primary silicon layer and an auxiliary layer made from perovskite. By stacking a traditional silicon layer with a perovskite layer, scientists at Stanford University believe that they can increase the efficiency of a solar cell while also potentially decreasing the price. The Stanford researchers performed an experiment to test their hypothesis. During the experiment they used a perovskite solar cell with an efficiency of 12.7 percent and silicon solar cell with an efficiency of 11.4 percent. The results were promising as the tandem efficiency of cell jumped to 17 percent.  “By combining two cells with approximately the same efficiency, you can get a very large efficiency boost.” (Bailie, Stanford University). Improving the efficiency while also using a low cost and easy to produce material such as perovskite can dramatically decrease the cost of the solar panel. If each solar cell is more efficient at producing energy, the number of cells required to make a solar panel function will be decreased. Since incorporating a perovskite layer on an existing cell is much cheaper than manufacturing a whole new cell, the cost of the solar panel can be significantly decreased.

How it works

Diagram 2 (https://www.nature.com/articles/s42004-020-0283-4/figures/2)

Diagram 3 (https://pubs.rsc.org/en/Image/Get?imageInfo.ImageType=GA&imageInfo.ImageIdentifier.ManuscriptID=C9SE00948E&imageInfo.ImageIdentifier.Year=2020)

Tandem cells work by combining two independent silicon and perovskite sub-cells. Once combined these sub-cells function together to create a more efficient solar cell. There are various processes to convert two sub-cells into a tandem cell, in this proposal I will discuss the “dry two-step” conversion process. The dry two-step process is effective and much less complicated than alternative methods such as the evaporation incorporated process. “The processing conditions for organic material evaporation are challenging, and problems can exist in the large area process.” (Lee). Prior to beginning the process, the silicon and perovskite layers should be textured into a pyramidal shape as can be seen in diagram 2, “because high-efficiency silicon solar cells require surface texture to maximize light absorption.” (Lee). The dry two-step process is simple and as its name suggests only requires two steps. The first step is applying Indium Tin Oxide (ITO), Titanium Dioxide (TiO2), and Lead Oxide (PbO), in order to optimize the metalize contact at the surface of the silicon layer, and to form a precursor layer made up of Lead Oxide. The second step is applying a Methylammonium iodide (MAI) layer on top of the Lead Oxide precursor. The MAI layer will serve as an optimizer for the electrodes on the back of the perovskite cell. Once the perovskite layer is applied the metal contact of the silicon and the electrodes of the perovskite will attract causing the two layers to attach and creating a tandem cell. (Lee). The process is depicted in diagram 2. The process is identical for both low cost and high efficiency tandem cells the only difference is the quality of the silicon and perovskite sub-cells that are used. (Sofia). This is displayed in diagram 3.

Efficiency

Type of Device	Device	VOC (V)	JSC (mA cm−2)	FF (%)	PCE (%)	Area (cm2)
Stand-alone silicon solar cell	c-Si	0.61	36.80	75.4	16.8	1.20
Stand-alone silicon solar cell	Si HJT	0.69	37.57	79.8	20.7	1.43
Two-terminal tandem solar cell	perovskite/c-Si	1.68	16.72	75.6	21.2	1.20
Two-terminal tandem solar cell	perovskite/Si HJT	1.76	18.83	73.0	24.2	1.43
Two-terminal tandem solar cell	graphene-based perovskite/Si HJT	1.80	18.81	77.5	26.3	1.43

Table 1 (www.sciencedirect.com/science/article/pii/S2542435120300453.)

Table Key:

c-Si (standard single terminal silicon cell)

Si HJT (high efficiency single terminal silicon heterojunction cell)

perovskite/c-Si (standard perovskite/silicon tandem cell)

perovskite/Si HJT (standard perovskite and high efficiency silicon heterojunction tandem cell)

graphene-based perovskite/Si HJT (high efficiency graphene-based perovskite and high efficiency silicon heterojunction tandem cell)

PCE (power conversion efficiency)

To compete with and ultimately replace fossil fuel energy it is essential that solar energy and solar panels become more efficient at creating energy. The theoretical efficiency limit of a single terminal cell is 33.16 percent according to William Shockley and Hans Queisser, which is significantly lower than the highest observed efficiency of coal, natural gas, and oil which are 42 percent, 52 percent, and 45 percent, respectively. (Zeiss). Tandem cells, which operate with a two-terminal system, have the potential to overcome the efficiency limit of a single terminal cell, and compete with the high efficiency levels of fossil fuels. A silicon and perovskite tandem cell are the best option because both silicon and perovskite are abundant and low cost, they can also be merged with a variety of conversion processes including the simple dry two-step process. “To overcome the efficiency limit of a single-junction device, a perovskite/silicon tandem approach can be used because of several advantages of perovskite solar cells including tunable bandgap, easy fabrication, and high efficiency.” (Lee). As seen in Table 1 the tandem cells have a higher power conversion efficiency then the stand-alone silicon cells. A high efficiency silicon heterojunction cell has a lower efficiency then a standard silicon/perovskite tandem. A high efficiency graphene-based perovskite and high efficiency silicon heterojunction tandem cell have nearly a 10 percent higher efficiency then a standard stand-alone silicon cell. Tandem cells have the potential to overcome the efficiency limit of single terminal cells and have the capability to compete with the high efficiency rates of fossil fuel energy sources.

Cost

Diagram 4 (https://pubs.rsc.org/image/article/2020/se/c9se00948e/c9se00948e-f1_hi-res.gif)

Diagram 5 (https://ars.els-cdn.com/content/image/1-s2.0-S2542435118301910-gr1.jpg)

Diagram 5. Schematic Diagram of Modules

(A) Module A is composed of traditional silicon cells.

(B) Module B is composed of planar perovskite cells.

(C) Module C is composed of silicon/perovskite tandem cells.

(D) Module D is composed of perovskite/perovskite tandem cells.

Diagram 6 (https://ars.els-cdn.com/content/image/1-s2.0-S2542435118301910-gr2_lrg.jpg)

The goal of tandem cells is high efficiency and low cost to compete with fossil fuel productivity and increase consumer appeal. “Perovskite is a crystalline material that is inexpensive and easy to produce in the lab.” (Schwartz). Similarly, silicon is the second most abundant element on the Earth and is therefore easily accessible and low cost. Diagram 4 shows the models of low cost and high efficiency silicon cells and low cost and high efficiency perovskite/silicon tandem cells. In the diagram the low-cost cells are almost identical except for the addition of the perovskite cell, which is also true for the high efficiency cells. Therefore, the cost disparity between the stand-alone cells and the tandem cells would be very minimal as the cost of creating a perovskite layer is inexpensive. Furthermore, the higher efficiency of the tandem cells justifies the slight increase in price of a tandem cell compared to a stand-alone cell. With higher efficiency cells, solar panels and solar modules require less cells to function at an appropriate level, and ultimately the cost of energy is decreased. The cost of energy is measured by the Level Cost of Energy (LCOE) formula, which is: total lifetime cost of the module divided by the total lifetime energy production. (Sofia). In diagrams 5 and 6, the LCOE of four types of solar panel modules are displayed. Modules C and D which are comprised of perovskite/silicon tandem cells and perovskite/perovskite cells, respectively, have a lower LCOE then their respective stand-alone counterparts. Although, the initial cost of creating a tandem cell may be slightly more expensive, the ultimate cost of the solar panel and solar panel module will be significantly less.

Conclusion

Whether people want to admit it, or not fossil fuels are temporary. Our limited supplies continue to shrink and our usage is always increasing. We are not only in danger of sending ourselves into an energy crisis we are also damaging our planet and environment. Fracking and oil drilling continue to damage our environment and greenhouse emissions are deteriorating our atmosphere. We need a green and reusable alternative source of energy to replace the problematic use of fossil fuels. Solar energy provides the perfect solution as it is reliant only on the energy from the sun and is therefore completely reusable. Furthermore, solar panels generate energy through a clean process with no environmental hazards or harmful gas emissions. Unfortunately, the high cost low efficiency in comparison to fossil fuels makes solar energy and solar panels much less appealing to both average consumers and large energy distributors. My proposal of using silicon and perovskite tandem cells has the potential to increase the efficiency of solar cells while also decreasing the overall cost of a solar panel. The implementation of my proposal will allow for solar cells to compete with the efficiency and cost of fossil fuels. By providing a solution to the problems that current solar cell models face I hope that my proposal can catalyze the transition from fossil fuels to solar energy. We must find a permanent solution to our energy problem before we force ourselves into a global crisis and damage our planet beyond repair. Solar energy is the solution and use of tandem solar cells provides solar technology with the requirements it needs to compete with and ultimately replace fossil fuels.

References

1. Schwartz, Mark. “Inexpensive Material Could Be the Key to Cheaper, More Efficient Solar Cells.” Stanford School of Engineering, 13 Aug. 2017, engineering.stanford.edu/magazine/article/inexpensive-material-could-be-key-cheaper-more-efficient-solar-cells.
2. “Global Greenhouse Gas Emissions Data.” EPA, Environmental Protection Agency, 25 Mar. 2021, www.epa.gov/ghgemissions/global-greenhouse-gas-emissions-data.
3. Lamanna, Enrico, et al. “Mechanically Stacked, Two-Terminal Graphene-Based Perovskite/Silicon Tandem Solar Cell with Efficiency over 26%.” Joule, Cell Press, 17 Feb. 2020, www.sciencedirect.com/science/article/pii/S2542435120300453.
4. Lee, Sang-Won, et al. “Perovskites Fabricated on Textured Silicon Surfaces for Tandem Solar Cells.” Nature News, Nature Publishing Group, 25 Mar. 2020, www.nature.com/articles/s42004-020-0283-4#Fig2.
5. Sofia, Sarah E., et al. “Roadmap for Cost-Effective, Commercially-Viable Perovskite Silicon Tandems for the Current and Future PV Market.” Sustainable Energy & Fuels, The Royal Society of Chemistry, 11 Dec. 2019, pubs.rsc.org/en/content/articlelanding/2020/se/c9se00948e#!divAbstract.
6. Li, Zongqi, et al. “Cost Analysis of Perovskite Tandem Photovoltaics.” Joule, Cell Press, 24 May 2018, www.sciencedirect.com/science/article/pii/S2542435118301910.
7. Zeiss, Geoff. “Energy Efficiency of Fossil Fuel Power Generation.” Between the Poles, 31 Jan. 2010, geospatial.blogs.com/geospatial/2010/01/energy-efficiency-of-fossil-fuel-power-generation.html.