Session A5 Paper #6192

Disclaimer — This paper partially fulfills a writing requirement for first year (freshman) engineering students at the University of Pittsburgh Swanson School of Engineering. This paper is a student, not a professional, paper. This paper is based on publicly available information and may not be provide complete analyses of all relevant data. If this paper is used for any purpose other than these authors’ partial fulfillment of a writing requirement for first year (freshman) engineering

students at the University of Pittsburgh Swanson School of Engineering, the user does so at his or her own risk.

METAL OXIDES: PAVING THE WAY TO PEROVSKITE CELLS

Desmond Zheng, dez11@pitt.edu, Sanchez 10:00, Patrick Flaherty, pjf28@pitt.edu, Vidic 2:00

Abstract —With the imminent decline of fossil fuels, a new clean form of energy is emerging to take its place at the apex of the energy industry: solar power. Enormous strides in the research of solar panel production have resulted in a product with higher levels of efficiency and lower costs than ever before: perovskite solar cells. While exploring the background of this rapidly developing solar cell, we analyzed a main obstacle to the mass production of perovskite solar cells in great detail: excessive exposure to moisture in the environment. Despite being a revolutionary new form of solar cells, perovskite solar cells must overcome this roadblock to be mass produced.

Perovskite solar cells can be a pivotal part of the future once the issue of degradation is overcome. The main solution currently being researched is the use of a metal oxide coating, which can combat the problem of the cells experiencing rapid degradation by preventing any contact with moisture in the air. When the metal oxide coating is perfected, perovskite solar cells will be as sustainable as they are efficient. Once this technology is fine-tuned, companies can capitalize on producing more cost effective solar panels with greater efficiencies. This paper will explore the use of a metal oxide coating on perovskite solar cells to surmount their weakness to moisture in the environment and enable them to reshape the landscape of sustainable energy generation across the globe.

Key Words—Protective Coating, Metal Oxide, Moisture Degradation, Perovskite, Solar Cell, Solar Efficiency

A FUTURE PREEMPTIVELY PEROVSKITE

As the usage of fossil fuels decline around the globe, renewable energies have emerged to take their place. A premier choice in today’s renewable energy market is solar power. Recently, a new form of solar technology has materialized: perovskite solar cells. These cells, composed from perovskite, an organic-metal halide compound [1], possess staggering levels of efficiency and economical pricing; however, one fatal flaw in the perovskite

compound has caused companies to be hesitant about mass production. In the open environment, water vapor or moisture from the air will cause perovskite in the cells to rapidly degrade and become essentially useless [2]. This dilemma does not currently allow perovskite to be a sustainable marketable solar cell. Research of possible solutions has been conducted for the unique compound of perovskite in solar cells.

Prior to its use in solar cells, perovskite had no major significance as a component of modern technology. As a material, perovskite possesses a structure of ABX3, which Muhammad Ahmed, a researcher at the National University of Sciences and Technology in Islamabad, Pakistan, describes in the scientific journal, International Journal of Photoenergy, by writing that “ABX3 describes the crystal structure of perovskite class of materials, where A and B are cations and X is an anion of different dimensions with A being larger than X [3].” This is the first time that this crystal structure, in the form CH3NH3PbI, has been introduced into the world of solar power, and it is being met with remarkable success.

Shown in Figure 1 [4], perovskite’s crystalline structure is what sets it apart from other solar cell materials. Materials that have this particular molecular structure are typically used for semiconductor, superconductor, and thermoelectric-based research, as they contain key conducting properties. ABX3 (perovskite) molecules are synthesized by solid-state mixing of commonly found elements, or by drying the solution of precursor salts [3]. These methods of synthetization are not difficult to utilize on a massive scale. A main component of perovskite is lead, which is another key element as to why corporations could manufacture perovskite with ease [4]. The use of lead to synthesize validates that perovskite is a sustainable compound to accumulate or synthesize, as lead exists in large amounts around the world. The easier it is for a material to be synthesized or accumulated, the less the material will cost for the

University of Pittsburgh Swanson School of Engineering2016/02/12

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companies who try to manufacture it. This feature is vital for perovskite cells to succeed.

FIGURE 1 [4]

Cubic perovskite structure, where cation A is CH3NH3, cation B is Pb, and the anion X is a

halogen (I).Prior to any mass production, all problems of

perovskite solar cells must be solved. The primary problem of these cells, degradation from moisture, has caused setbacks in this technology since its birth. Multiple solutions exist currently, but the most prominent solution available is to coat metal oxide, such as titanium oxide (TiO2), onto the perovskite to seal the cell from outside exposure [3]. This solution has been met with relative success, but is nowhere near the caliber of quality a metal oxide coating would need to be to adequately protect perovskite and ensure its long-term sustainability. Another method that is currently being tested is a vapor-applied metal oxide coating that would protect the solar cell. This method has been met with similar success to the TiO2 coating, but nonetheless, is a step in the right direction for the future of perovskite solar cell technology [5]. These coatings will be discussed in greater detail later, but their success as an easily applicable solution is crucial for the long term viability of perovskite solar cell technology.

Perovskite solar cells are intended to change the energy industry and how consumers use energy to power their devices. Researchers from the Johannes Kepler University in Austria, have already implemented these cells into next-generation technology such as autonomous drones, exoskeletons, and robotics [1]. Perovskite cells may not be ready for the present-day market, but they provide a glimpse into the bright future. To understand the breakthrough and setbacks of perovskite, it is preeminent to examine the difficulties in laboratory and large-scale production of perovskite cells and the main solutions to absolve these major issues.

PERILS OF PEROVSKITE

Perovskite solar cells hold tremendous potential in the industries of energy production and

manufacturing. Despite their increasing capacity, perovskite cells face a major dilemma when used outside a laboratory setting. Without being enclosed in a vacuum, perovskite rapidly degrades from water vapor in the air. Currently, this is the major obstacle preventing perovskite cells from being mass produced and sold as the premier option in solar power. Due to this important reason, moisture presents itself as a perilous opponent to perovskite solar cells [6].

The battle between perovskite and water vapor can be explained by a sequence of chemical reactions undertaken in the presence of ultraviolet radiation, oxygen, and water. Shown in Figure 2, perovskite, or CH3NH3PbI3, is separated into separate ions under ultraviolet radiation and then amongst contact with the water vapor, or H2O, is broken down completely into hydrogen iodide, or HI [3]. This molecule cannot be used in a photochemical reaction to draw electrons from sunlight. Further research on perovskite cells shows that a humidity level of fifty-five percent is sufficient to deteriorate perovskite [3]. As areas with higher levels of sunlight generally tend to contain higher quantities of humidity, this problem cannot be ignored or the use of perovskite cells will be restricted around the world.

FIGURE 2 [3]

The chemical reactions responsible for degradation amongst contact with moisture

Another problem combatting the progress of perovskite is the presence of lead (Pb) in the compound. For perovskite to be a sustainable compound, it must meet the ecological factor of sustainability: “meeting human needs without compromising health of ecosystems” [7]. The main form of perovskite used currently is CH3NH3PbI3, which contains lead, a toxic metal during all processes of manufacturing and deployment. Concerns raised over the environmental impact of the lead in perovskite can be neutralized [3]. Though the perovskite uses lead as a main component of the compound, the main component can be any material that follows the formula ABX3, where A and B are cations and X is an anion [3]. A solution to this environmental issue is the use of

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other elements in its place as the B cation [3]. An element gaining traction to replace lead is tin (Sb), which lays in the same group on the periodic table and thus, shares similar properties with lead. Tests have shown that tin forms a similar compound with a more ideal bandgap, which is key to electrical conductivity [4]. Further experimenting with various other types of elements could prove a better and safer replacement to lead.

Perovskite must overcome environmental and chemical issues before becoming a mainstay solar cell. To do so, research must be conducted and possible solutions must be tested. Possible elements to replace lead and neutralize any harmful effects on the environment have been produced and can only effectively work when perovskite cells can survive contact with the outside world. Subsequently, the main solution currently being designed for moisture degradation is a metal oxide coating, which be discussed in detail in the next section [2].

MEDDLING WITH METALS

Perovskite solar cells can become major factors in the solar industry, but are plagued by the issues of degradation and nonoperation when exposed to the smallest amounts of moisture. This flaw of perovskite is undermining the research and advancement of this breakthrough solar cell, but can potentially be solved with the use of a metal oxide coating. The goal of the metal oxide is to provide a transparent, waterproof barrier that will protect the cell from water vapor as much as possible while affecting the total efficiency of the perovskite cell as little as possible [2]. Creating a viable, sustainable metal oxide coating would justify producing perovskite cells. This is a very promising solution to the major problem of perovskite cells, and now the specific types of coatings will be discussed.

Titanium Oxide

A promising front runner being developed to cover these solar cells is the compound, titanium oxide (TiO2). This particular coating requires a spin-coating of TiO2 that goes over the perovskite solar cell [3]. TiO2 has shown success in laboratory experiments, but has not yet reached the caliber needed for commercial use. These tests have shown that TiO2 has a very minute effect on the photoelectric excitement of perovskite, and could prove to be a very effective sealant to keep the moisture in the air away from the sensitive

perovskite material. In addition, TiO2 is not a very difficult material to be applied to the surface of perovskite solar cells [6]. The spin-coating method to apply TiO2 could be done on a large scale to cover these solar cells [6]. While TiO2 has proven an effective coating, other metal oxides are currently being researched as possible alternatives.

Zinc Oxide

Another potential metal oxide that could be used for coating perovskite solar cells is zinc oxide, or ZnO. The scientific journal, Nature Nanotechnology, says that “This ZnO nanoparticle film exhibits a continuous and smooth surface with a roughness of <2 nm and a particle size of <10 nm” [8]. This statement effectively means that titanium oxide provides a smooth coating over a perovskite cell while the particles are miniscule, which allows more sunlight to come into contact with the perovskite. The ability to prevent degradation while leaving the high levels of efficiency intact is vital for the long-term sustainability and use of perovskite solar cells in industries and homes. While no research has been done to support one versus the other, ZnO could be more efficient at allowing sunlight to contact the perovskite, leading it to be a more economical solution as directly it allows more electricity to be produced [8].

Titanium and zinc oxide are only two types of metal oxides currently being examined and tested on as possible coatings for perovskite solar cells. Countless other metal oxides exist and could be better solutions for the issue of degradation [3]. Any assortment or combination of these metal oxides may allow perovskite to become a sustainable compound to use in solar cells. However, before searching for a suitable coating, researchers must prove the worthiness of a metal oxide coating as the best solution for moisture dilapidation. To do so, scholars and scientists must implement these preliminary oxide compounds to produce effective results in perovskite cells.

HOW WILL THIS COATING BE IMPLEMENTED?

Implementing a metal oxide coating requires careful research of the compound being used, the chemistry amongst bonding, and the aftereffects against the environment. Universities, such as the University of California at Los Angeles [8] and Japan Institute for Chemical Research and Kyoto University [5], have begun implementing metal oxides onto current perovskite solar cell models. These preliminary tests could be the first stepping stone into the future of energy production.

At the Japan Institute for Chemical Research with collaboration with Kyoto University, graduate students alongside their professors have fabricated

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a metal oxide coating of TiO2 using a chemical bath deposition method (CBD). Another method of coating a metal oxide layer is using a spin-coating of TiO2, which deposits the oxide with an acidic solution of titanium isopropoxide in ethanol. This is then heated to 500ºC, which requires costly thermal equipment, preventing these types of metal oxide coated perovskite cells to be cost-effective [5]. The researchers at the Japanese institute are developing the CBD method as a low-cost alternative.

FIGURE 3 [5]

A schematic of the perovskite cell during the CBD process

Compared to the 500ºC of a spin-coating, a chemical bath deposition of metal oxides only requires 150ºC [5]. The lower temperature would lead to an easier and inexpensive manufacturing process. To begin the CBD process, the perovskite was laid on top of the TiO2 (or TiOx in Figure 3), which had been treated in an aqueous solution of titanium oxysulfate and hydrogen peroxide. This compound was placed on an indium tin oxide to further sustain the titanium oxide. Gold placed on top of the compound was deposited in the hole transport layer, spiro-OMeTAD, which allows electrodes to transfer from light to the perovskite cell [5]. The Japanese university collaboration succeeded in fabricating the perovskite solar cell with a TiO2 layer using the chemical bath deposition method. Data from this experiment estimated that power conversion efficiency of this cell was almost the same as perovskite solar cells without an oxide coating [5]. With further research or communication with other researchers, such as the University of California at Los Angeles, the efficiency levels of perovskite cells can be sustained at an equivalent or superior level [8].

At the University of California at Los Angeles, researchers have created a different type of metal oxide coating, using nickel oxide (NiOx) and zinc oxide (ZnO) nanoparticles as an outside layer [8]. These nanoparticles act as a hole and electron transport layer for electrodes from the sunlight.

Their prototype cell possesses a ‘p-i-n structure’ containing indium tin oxide, NiOx, perovskite, ZnO, and aluminum [8]. The nanoparticles act as a shielding around the perovskite and aluminum, protecting the perovskite from water vapor. Research conducted on the prototype cell has returned great results: ninety percent power conversion efficiency after sixty days exposed to outside air at room temperature [8]. Typical perovskite cells without a metal oxide coating can only last five days before complete degradation from moisture [8]. This prototype solar cell, coupled with the Japanese titanium oxide cell, shows great promise in overcoming the current limitations of perovskite cells.

LIMITATIONS OF PEROVSKITE CELLS

Perovskite cells seem like the perfect solution for the future of energy. They are an economical, sustainable solution to the ensuing problem of depleting fossil fuels within the next one hundred years. Despite these positive attributes, a difficult problem of degradation surrounds perovskite solar cell technology that will prevent them from being mass-produced until a resolution is produced. The material of perovskite cannot have any contact with water vapor. The resulting dilapidation does not just occur when water condenses onto the solar cell material. It also occurs when even the slightest amount of moisture interacts with the delicate surface of the perovskite [2]. In a current laboratory setting, perovskite solar cells must be tested inside of a vacuum so that absolutely no water vapor comes in contact with the material. That is a condition that makes perovskite relatively difficult to experiment on, compared to other types of solar cells.

However daunting, researchers have been exploring innovative solutions to this potentially crippling problem of moisture-induced cell degradation. The most popular solution is the application of a metal oxide coating to the outside of the solar cell. This coating would serve as a waterproofing agent for the cell, as well as inhibiting the cell's electrical efficiency as little as possible [8]. Once this problem is solved permanently, perovskite will be able to be mass-produced and will be able to enter the marketplace with an unstoppable force.

Perovskite solar cells have shown extremely high promise, despite not being able to be exposed to water. The tests inside of the vacuum chamber demonstrate that perovskite has extremely high potential to be effective as a solar material, but this technology will be absolutely impractical until it can overcome its moisture degradation

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problem. Perovskite cells are typically hard to make and test currently due to this problem.

Manufacturing Issues

Due to the dilapidation that occurs, the manufacturing of perovskite cells is a very expensive and difficult process. Assuming that a long-term solution to this problem is arrived at, steps would have to be made for the process of perovskite solar cell production to ensure the cell is not harmed before the metal oxide coating can be applied [3]. This is a very concerning scenario, because air holds a small amount of water vapor, and it is possible that the perovskite solar cell could be ruined before it exits the assembly line if it is exposed to this moisture during the process. Preventing exposure during the manufacturing process is potentially the biggest problem that would arise when this technology is mass produced. Researchers and engineers will have to devise a plan for manufacturing perovskite solar cells that does not involve the cells being exposed to air during the process. These manufacturing issues will surface after it is confirmed that perovskite solar cell metal oxide coatings have the durability to actively protect the solar cells from moisture for very long periods of time [10].

Lifespan and Sustainability

Currently, the lifetime of a perovskite solar cell is very dependent on the durability of the metal oxide protective coating that surrounds it. As soon as this metal oxide begins to wear and allow water to seep through, the cell will deteriorate to a state beyond repair, rendering the solar cell useless. Current metal oxide coating tests have shown that with a proper coating, a perovskite solar cell can remain functional for a full four days of non-stop electricity generation when exposed to air [2]. This length of time before deterioration makes perovskite solar cells currently useless in terms of practicality. Four days is a very short length of time for a solar cell to remain operational. This length of time would have to increase to years before perovskite solar cells are considered useful and economical is the business world. This length of time is expected to see a great increase as metal oxide coatings are further researched and the construction methods of perovskite solar cells improve.

These small developments of the perovskite technology may lead the average consumer to wrongly conclude that perovskite does not have the capacity to be sustainable. Sustainability can be

viewed from multiple perspectives, primarily agricultural, ecological, and economic [7]. For example, as stated previously, the ecological perspective is defined as “meeting human needs without compromising health of ecosystems” [7]. This can conversely be compared to the economic definition which is very similar except its priority is to save money, not the environment while furthering the effectiveness of a product. Although this issue may be viewed from multiple perspectives, one component remains the same through all of them: one thing is conserved while another thing is advanced. Perovskite solar cells definitely hold potential to be sustainable, and should be viewed and judged with an open mind. Once perfected, perovskite cells will be able to flawlessly operate for decades on end. Their efficiency and cost are phenomenal for being still so early in development. This technology, with so much potential, will change the way the world thinks about solar power.

Perovskite is very efficacious in terms of the various definitions of sustainability. For example, perovskite solar cells are economically sustainable because they are less costly and more efficient than the silicon solar cells that they will be competing with in the future on the marketplace. Perovskite will be a very enticing investment option for the business world due to its economic sustainability. In terms of ecological sustainability, perovskite solar cells are phenomenal due to their ability to convert solar radiation into useable power. In addition, perovskite generates a neutral effect on the environment compared to most other major forms of power generation, such as hydroelectric, which may alter rivers and lakes, and fossil fuels, which contribute heavily to the global warming effect. With the help of perovskite solar cells, power generation as a whole can be brought to a higher standard of ecological sustainability due to the eco-friendly nature of perovskite solar cell technology. Perovskite solar cell technology offers a powerful and sustainable source of energy that surmounts the limits of current solar cells.

APPROACHING THE APEX

With proper research and development, perovskite solar cells could approach the apex of solar cell technology relatively soon. Scientists from universities all over the globe, ranging from California to Japan, are collaborating on improving this technology. The prominent solution being developed is the metal oxide coating we have discussed throughout this paper. Once the issue of degradation has been overcome, real-world tests can be carried out in the

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open environment. In addition, research can be further conducted to improve the power-per-weight and levels of efficiency. However, compared to current solar cells, perovskite has already begun to take its place on the solar throne.

Success in Solar

Solar cells first came into public awareness in the 1970’s. Since their introduction, solar cells have developed into their own industry. The most familiar solar cell is comprised of silicon and is the most cost-effective solar cell currently. In 2014, the cost of a silicon solar cells was 72 cents per watt, while researchers believe that perovskite cells could cost as low as 10 to 20 cents per watt [2]. This price could make solar cells viable for any family, regardless of income status. However, due to companies driving the development of solar technology, the solar industry can only grow if profits grow. According to Figure 4, silicon cells have shown no improvement on their twenty-five percent efficiency level in the past twenty years. Costs have been driven down, yet these same cells have not converted higher rates of electricity [9]. Instead, leaving silicon as the current mainstay of solar, researchers have developed other new types of solar cells, such as perovskite-based cells.

FIGURE 4 [9]

Graph displaying the levels of efficiencies by year (data collected by the NREL)

Perovskite solar cells first emerged in 2009 in a laboratory at Toin University in Yokohama, Japan, where the researchers created perovskite nanometer crystals by developing the compound out of a chemical solution [2]. Adding solar cell metal contacts on the perovskite crystals resulted in a transfer of electrons, resulting in the first perovskite cell. This cell would be the first step in becoming a promising alternative to silicon cells. They reached

twenty percent efficiency in three years, while it took silicon almost a decade to achieve. Despite this, perovskite cells come with a condition: these cells are non-stable [2]. As we discussed at length throughout this paper, research has shown glimpses of a solution that would not lower the superb efficiency while protecting the cell from the elements [9]. Since these debacles must be solved before manufacturing, perovskite cells do not have a large presence in the marketplace, but prototypes are still available to examine these remarkable cells.

What’s Available Right Now?

Despite their high power-per-weight and levels of efficiency, perovskite solar cells are currently not available to the majority of the public. However, working prototypes have been produced in research laboratories. In Changzhou University in Changzhou, China, researchers have developed functioning perovskite cells. These cells have been used in different settings, such as different levels of humidity, to examine the efficiency in these controlled environments [6]. In addition, as discussed earlier, the graduate students and professors at the Japan Institute for Chemical Research and Kyoto University fabricated perovskite solar cell prototypes with a titanium oxide coating. This promising prototype has shown only a slight decrease in efficiency and the ability to be used outside of a laboratory environment [5]. Furthermore, other prototypes created with various metal oxides have been researched, but will only be improved on once the problem of moisture degradation has a definite solution.

A European collaboration between Austrian universities recently manufactured flexible perovskite solar cells using an acrylic elastomer. This will be discussed in the later section on the future possibilities of perovskite solar cells, but the ability to be used in any setting certainly grants perovskite technology countless possibilities [1]. These prototypes may not be available to the vast majority of the world, but the potential has companies ready for mass production once perovskite cells have been deemed serviceable in the outside environment.

PEROVSKITE’S POTENTIAL

Perovskite solar cell technology is something that has enormous potential to forever change the energy market worldwide. Solar cells made of perovskite have made the same advancements in two years compared to the twenty years it took contemporary silicon cells [6]. From transparent cells to vastly increased efficiencies, perovskite offers a versatile and renewable alternative to current forms

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of fossil fuel generated energy and its solar brethren. An example of this is the idea of replacing the windows of a skyscraper with fully transparent perovskite solar cells. Not only would it retain the grandeur of the skyscraper, but would also make the skyscraper completely self-sufficient in terms of power generation. Being transparent, these cells would absorb the electrons from the sunlight, but would not prevent the light from illuminating the rooms inside. By replacing the windows with clear perovskite cells, a skyscraper could generate electricity for itself and surrounding buildings [1]. These are the types of innovations with perovskite that will help revolutionize the energy industry as we know it, and help future generations foster renewable sources of energy.

Solar energy is a clean, efficient, and economical source of energy that has the potential to reshape the way that we approach electricity generation, especially in urban environments. As shown in Figure 5, perovskite cells possess a record power-per-weight of twenty-three watts per gram. Compared with earlier models of solar cells, this level of energy efficiency per weight is unprecedented and will allow perovskite cells to take solar power and the energy industry to brighter future. Along with a record power-per-weight, perovskite has reached a confirmed efficiency level of 17.9% in 2014 [1]. This level is significant due to the exponential rise compared to the main-stay silicon solar cell. Perovskite cells could reach the apex of the solar industry once they have proven able to function in the outside environment. If properly integrated into skyscrapers in the form of windows, perovskite could drastically lower the amount of power used in urban centers [1]. This would enable perovskite cells to aid cities in sustainable self-production of their electricity, which has never been achieved in the course of human history. Coupled with this fact, perovskite cells would allow buildings to be carbon-neutral, whereas the buildings would otherwise add onto the daily carbon emissions humans cause and contribute to the global warming effect. In addition, perovskite cells could be used to power the ensuing drone revolution. As drones are better implemented into society, a viable power source to keep them afloat for longer will be required. As a possible solution, perovskite cells also have a minuscule carbon footprint over their lifetime, but before fulfilling its potential and being used throughout society, any impact, positive or negative, of perovskite on humanity must be considered [2].

Figure 5 [1]

A graph of solar cells and their power-per-weight

IMPACT ON THE FUTURE AND SOCIETY

Due to their vast potential, perovskite solar cells are poised to take a major part of modern society and the future. Compared to current silicon solar cells, perovskites hold a major advantage. Perovskite cells possess a record power-per-weight and efficiency levels on par with silicon [4]. In approximately three years, perovskite has almost matched silicon in its efficiency and surpassed it in power-per-weight. Furthermore, perovskite could become the cheapest solar cell available because it can be made at lower temperatures, requiring less energy and therefore less money. These same cells can be converted to flexible solar cells, allowing them to be placed and utilized in any location or surface [2]. However, before reaching the forefront of the solar industry, perovskite solar technology must prove its application and cost to society.

With their dramatic rise amongst their solar brethren, perovskite solar cells do have some opposition other than the issue of moisture degradation. The primary perovskite used is a lead-based compound, which may pose a toxic threat to the global environment. However, preliminary research has shown that the lead in perovskite may be able to be replaced by tin, countering any lead-based issues [4]. In addition, current silicon cells possess similar amounts of lead or cadmium, another toxic element, leading to phase-outs by governments, where perovskite presents a positive alternative [3]. Despite these possible challenges, perovskite cells display a prime opportunity to improve current technology and society as a whole.

Along with their high power-per-weight and efficiency, perovskite solar cells are ultra-lightweight and flexible solar cells, an unheard of combination until now [11]. The perovskite solar cells are only two micrometers thick with a one

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micrometer layer of metal oxide and a polyurethane commercial coating. Merged with an acrylic elastomer, perovskite cells become a flexible solar cell, able to be used on any emerging technologies, including flexible OLED screens and autonomous drones. The ability to be placed on any surface will allow perovskite solar cell technology to be used in any environment. As shown in Figure 6, perovskite cells can undergo a considerable amount of strain, which is significant to be utilized in real-world situations. Tests undertaken on flexible perovskite cells show only a five percent energy loss over a thousand cycles [1]. The ability to conform itself to any surface and sustain its high performance over repeated use will allow perovskite cells to have countless incorporations in technology and society.

FIGURE 6 [1]

Three-dimensional rendering of a perovskite cell undergoing approximately 40% compressive strain

Perovskite solar cells have many applications in the future due to their blend of critical properties. Once commercialized, these cells have the ability to end the global energy crisis, surpassing silicon and other solar cells [3]. Furthermore, perovskite solar cells have the flexibility to be used to power instruments such as weather balloons, satellites, and unmanned aerial vehicles [1]. The high power-per-weight ratio coupled with their lightweight properties enable perovskite cells to be used in future robotics, such as exoskeletons. Currently, these cells have been successfully used to power micro-aerial robotics, which demonstrates a future in travel, emergency and rescue, and security applications [1]. These exciting prospects and properties of perovskite solar cells lead us to have a great outlook on the future of solar technology.

THE OUTLOOK

Perovskite solar cell technology is a very promising form of energy that should be gaining traction within the next ten years. Research has provided tremendous evidence of the raw potential that these new solar cells have, especially when compared to their silicon counterparts. From skyscrapers to solar farms, this new form of energy is just as versatile as it is powerful [1]. In their short existence, perovskite solar cells have almost topped the efficiencies achieved by traditional silicon solar cells, and perovskite is not stopping there [11]. Extensive research is being done around the world in laboratories dedicated making this technology more efficient and applicable. This technology seems like the perfect solution to the problem of finding a sustainable source of renewable energy, but perovskite solar cells have a long way to go before they can be ready for consumers.

As noted previously, the main problem with perovskite solar cell technology is that the solar cell deteriorates and loses functionality when it is exposed to the slightest amounts of water vapor in the air [2]. The solar cell cannot be put on the market as long as this problem persists. The moisture issue is the only thing standing in the way of their elevated market potential, as a solution has not yet been perfected. This problem is in the process of being solved by researchers, who are applying a metal oxide coating to the surface of perovskite solar cells in an attempt to seal the water out while retaining the efficiency of the solar cell [8]. These metal oxides currently can keep moisture out for the short period of time of a few days, but are not where they need to be yet to be effective in the market [10].

Despite this dilemma, perovskite solar cells have massive potential to be a main source of energy for the world. These new solar cells are cheaper, much more efficient than their silicon counterparts, and can be integrated into the marketplace easily due to their overall versatility and flexibility as a material. As research continues on this promising technology, it will be noteworthy to see how a material such as perovskite can reshape solar power’s effect on the world.

REFERENCES

[1] M. Kaltenbrunner, G. Adam, E. Glowacki, et al. (2015). “Flexible high power-per-weight perovskite solar cells with chromium oxide-metal contacts for improved stability in air.” Nature Materials. (online article).[2] V. Sivaram, S. Stranks, H. Snaith. (2015). “Out-Shining Silicon.” Scientific American. (online article).

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http://search.ebscohost.com/login.aspx?direct=true&db=aph&AN=103166956&site=ehost-live.http://search.ebscohost.com/login.aspx?direct=true&db=aph&AN=109580502&site=ehost-live. p. 1032-1089[3] M. Ahmed, A. Habib, S. Javaid. (2015). “Perovskite Solar Cells: Potentials, Challenges, and Opportunities.” International Journal of Photoenergy. (online article). http://www.hindawi.com/journals/ijp/2015/592308/[4] M. A. Green, A. Ho-Baillie, H. J. Snaith. (2014). “The Emergence of Perovskite Solar Cells.” Nature Photonics. (online article). http://rdcu.be/f4Nt.[5] K. Yamamoto, Y. Zhou, T. Kuwabara, et al. (2014). “Low Temperature TiOx Compact Layer by Chemcial Bath Deposition Method for Vapor Desposited Perovskite Solar cells.” Photovoltaic Specialist Conference. (online article). http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6925218&tag=1. p. 1573-1576.[6] M. Lv, X. Dong, X. Fang, B. Lin, S. Zhang, et al. (2015). “Improved photovoltaic performance in perovskite solar cells based on CH3NH3PbI3films fabricated under controlled relative humidity.” RSC Advances. (online article). https://www.engineeringvillage.com/share/document.url?mid=cpx_M288a5a5f1511695320eM608710178163171&database=cpx.[7] J. Morelli (2011). “Environmental Sustainability: A Definition for Environmental Professionals”. Journal of Environmental Sustainability. (online article).[8] J. You, L. Meng, T. Song, et al. (2015). “Improved air stability of perovskite solar cells via solution-processed metal oxide transport layers.” Nature Nanotechnology. (online article). http://www.nature.com/nnano/journal/v11/n1/full/nnano.2015.230.html.[9] M. Gunther. (2015). “Meteoric rise of perovskite solar cells under scrutiny over efficiencies..” Royal Society of Chemistry. (online article). http://www.rsc.org/chemistryworld/2015/02/meteoritic-rise-perovskite-solar-cells-under-scrutiny-over-efficiencies.[10] T. Minemoto, M. Murata (2014). “Device modeling of perovskite solar cells based on structural similarity with thin film inorganic semiconductor solar cells”. Journal of Applied Physics. (online article). http://web.a.ebscohost.com/ehost/pdfviewer/pdfviewer?sid=b450ca22-125d-4a0f-bb19-127939446a7c%40sessionmgr4003&vid=16&hid=4212[11] (2015). "High-efficiency, high-reliability perovskite solar cells realized by a low-temperature solution process." National Institute for Materials Science. (online article). www.sciencedaily.com/releases/2015/09/150911141223.htm.

ADDITIONAL SOURCES

“A Current, Significant Engineering Topic.” University of Pittsburgh Swanson School of Engineering Freshman Program. (2014). (Video). http://www.library.pitt.edu/other/files/il/fresheng/index.html

(2015). “New Technique Stabilizes Perovskite Solar Cells.” Advanced Materials & Processes. (online article). http://search.ebscohost.com/login.aspx?direct=true&db=aph&AN=111081644&site=ehost-live. p. 14.

(2016). “Sustainability Mission.” Alcoa Inc. (online article). http://www.alcoa.com/sustainability/en/home.asp.

ACKNOWLEDGEMENTS

We would like to acknowledge our family and friends who aided us. We would also like to thank our writing instructor Mrs. Nancy Koerbel, our co-chair Alyssa Srock, our chair Mr. Richard Velan, our conference director Dr. Dan Budny, and our sources of inspiration, Gatsby and Lexie.

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