Lifetime and Durability Study of Perovskite Solar Cells

Transcription

1 Lifetime and Durability Study of Perovskite Solar Cells A senior capstone proposal Sam Sprawls Abstract This proposal requests permission and guidance in completing a senior capstone project to test the durability and lifetime of perovskite solar cells. Perovskite solar cells are a new and promising technology with the fastest growth in efficiency demonstrated for a solar cell. However, little is known about how perovskites withstand over time when subjected to real world conditions. I plan to test this using the help of the MORE center to fabricate perovskite cells and the SDLE center to run durability and lifetime tests. Special methodologies for testing the perovskite cell will be designed to ensure that the lead used to create the solar cell does not contaminate the lab equipment or environment. Equipment from both facilities will be used to ensure that a standardized cell is being tested, and to diagnose where and why failure occurred in the lifetime and durability studies. By completing this project, my goal is to contribute knowledge about lifetime and durability of perovskite solar cells so that future researchers can move perovskite solar cells towards the energy market. In this proposal I have also included my qualifications to lead this project, the university scientists whose knowledge and facilities I will rely on for this project, a project timeline and a project budget. i

2 Table of Contents Abstract... i Project Description... 2 Literature Review... 2 Introduction to Solar Cells... 2 Measuring Solar Cells... 3 Introduction to Perovskites... 4 Qualifications of the Researcher... 5 Anticipated Audience Involvement... 5 Research Plan and Schedule... 6 Budget... 7 Bibliography

3 Project Descriptions The goal of my project is to use lifetime and durability science studies to assess the working lifetime of perovskite solar cells and reasons for failure in real world conditions. Solar cell technology has grown rapidly in the 21 st century. It is a clean, renewable energy that has the potential to replace current standard energy sources. With a growing population and an even faster growing global middle class, there is a large need for increased energy production. Many countries have created goals to increase the percentage of their power coming from clean energy; solar cells have become a prime alternative. Perovskite solar cells could take over the solar cell industry with their high efficiency and low cost. However, more tests must be done to optimize the technology. The lifetime and durability of perovskite cells is not fully understood. Because of this gap of knowledge, I am requesting to study perovskite solar cells in the Solar Durability and Lifetime Extension (SDLE) center for my senior research project. My plan is to work with Ken Singer and Ina Martin in the Materials for Opto/Electronics Research and Education (MORE) center over the summer to develop a standardized methodology to fabricate perovskite solar cells. Then, using the analysis tools in both centers, I hope to understand what causes failure in perovskite solar cells and hypothesize what could be done to improve lifetime and durability of perovskite cells. Solar cells rightfully have many research teams around the globe searching for breakthroughs in the technology. Examining perovskite solar cells to better understand their faults will allow the scientific community to determine the direction of future research. Running lifetime and durability tests on perovskites in this early stage will allow researches to better focus their research to move perovskites towards the energy market faster. Following is a literature review discussing the validity of this argument. I will introduce the field of solar energy, its importance and the tools to measure solar cells. I will then introduce perovskites and what work needs to be done with that technology. Following this will be my qualifications for working on this project, who I will be presenting this proposal to and what help I will need from them, the timeline of the proposed project and the budget of the project. Literature Review Introduction to Solar Cells Climate change is occurring and calls for the development of energy sources that do not produce green house gases. Increased flooding and varying weather patterns have already affected many people s lives [1]. Continuing on this path poses great risks to humanity s current way of life. The United States of America and the United Nations have recognized these risks and have proposed measures to limit climate 2

4 change. The National Academy of Sciences offers many suggestions to reducing U.S. emissions including pricing of emissions by means of a tax or cap- and- trade system. [1] The most recent report by the Intergovernmental Panel on Climate Change states, International cooperation is necessary to significantly mitigate climate change impacts (robust evidence, high agreement). This is principally due to the fact that greenhouse gases mix globally in the atmosphere. [2] If policy makers follow these suggestions, clean energy sources such as solar cells must be developed to replace current harmful energy sources. Measuring Solar Cells For solar cells to make a successful large scale move into the energy marketplace, three criteria must be successfully met: efficiency, lifetime and cost. While policy making can bolster solar cell technology sales, the ideal is for solar cells to be competitive on the market without subsidies [3]. This goal is defined as grid parity and is measured using a levelized cost of electricity (LCOE). A LCOE takes into account all expenses in creating the energy and finds the cost per unit of energy produced (usually in units of US dollars per kilowatt hour). Important factors in this model are production cost, infrastructure cost and lifetime. While the majority of the production costs have typically been in the cell itself, the solar panel case and electrical wiring are also factors. The cost of new organic and perovskite solar cells is much lower than silicon. Organic solar cells have recently run into the difficulty of not having high enough efficiency to offset the cost of the standard solar cell panel materials (cases, wiring, sun tracking stands, etc.). Infrastructure costs have similar challenges as production costs. In the United States, an extremely large energy grid is already set up. Therefore, storage is not as necessary as it would be in an undeveloped country, where there is not a large demand for energy or an existing electrical grid. However, in the United States the cost of land for large scale solar plants must be considered. With increased efficiency, less land, installation and panel materials are needed to provide the same amount of energy [3]. Higher efficiency leads to lower cost. The cost of a solar array is important to determine if a solar cell has the potential to be introduced into the market. Durability and lifetime determine if a solar cell will stay in the market. If a product does not last as long as expected, the product is considered a disappointment. The expected lifetime of a solar cell influences the LCOE. The LCOE is the total cost to produce divided by the total power produced. Since maintenance of solar cells is very cheap compared to production and installation costs, if a cell lasts twice as long to an otherwise identical cell, it has almost half the LCOE. One of the advantages of silicon solar cells is their long lifespan and durability. Silicon cells typically loose 1% of their relative output annually and continue to operate for 35 years. Also, they are extremely durable, having withstood the harsh climate of space. [3] While perovskite cells cost less and are on track to rival the efficiency of silicon cells, the lifetime and durability of perovskite cells will determine if they can compete with silicon. 3

5 Introduction to Perovskites Miyasaka s paper on the use of perovskites as a photovoltaic material capable of use in a solar cell was published in At that time, the cells had poor stability and an efficiency of 3.8% [4], compared to silicon cells with a record of 25.6% [5]. Soon it was found that a lead- based perovskite functioned better, leading to discoveries of improvements in both efficiency and stability [4]. Now, perovskites have reached an efficiency of 20.1%. Yet two main concerns arise with perovskite solar cells: their poor stability in humid air and the risk that these devices may release lead. [6] If either concern is legitimate, perovskite solar cells must be redesigned. Lifetime and durability science (L&DS) tests must be run on perovskite solar cells to understand how much work has to be done to make perovskite cells ready for the market and give understanding to where that work must be done (what part of the solar cell causes failure). In 2011, NREL PV came to the conclusion that while initial performance and safety qualifications are imperative to the field of solar energy, the value proposition of solar energy relies on metrics for lifetime performance. [7] NREL s interest in L&DS spurred many other researchers to follow. For new solar cell technologies, an array of tests is recommended. Outdoor testing is a must for new technologies, but it is not enough because the time frames are so long. [8] Wohlgemuth argues that no current lab test can replace testing a solar cell in the real world where unforeseen complications can arise. However, to increase the pace of degradation, accelerated environmental tests must also be done. Stress tests such as thermal cycling or humidity and wind The figure above shows the layers comprising a perovskite solar cell. It is unknown which layer or boundary between layers causes problems as the cell functions over an extended lifetime. loading should be used at various levels and in various combinations to understand the cause of the failure so that it may be corrected [8]. As perovskite solar cells continue to see growth in efficiency, L&DS tests should be run to understand when failures occur and how to correct those failures. Solar cell technology has shown capabilities in replacing green house gas producing energy sources. To make a shift towards solar cells in the energy market, a high efficiency, low cost, long lasting solar cell is necessary. Perovskite solar cells have met the first two requirements but the durability to harsh climates and average lifetime has not yet been scrutinized. Real world and accelerated stress studies of perovskite solar cells must be done. 4

6 Qualifications of the Researcher I have a worked in multiple engineering research labs that specialize in renewable energy and solar cells over the past six years. In high school I volunteered for three years at the CONN center at the University of Louisville s Speed Engineering School. The CONN center is a chemical engineering lab specializing in advanced materials for renewable energy. Here I gained experience in designing scientific projects by creating a new project each of the last two years I worked in the lab. I also gained experience with advanced imaging technique by using a scanning electron microscope. Also, using a solar simulator, I ran efficiency measurements on a photovoltaic (PV) device as part of my project on water splitting. In college, I worked in Case Western s MORE center during a summer research project and during the school year building organic photovoltaic cells. Here I learned the advanced science behind solar cell technology. This included learning different methods for characterizing the solar cell and the layers of a solar cell, including ellipsometry and contact angle measurements. I began to understand some of the difficulties behind fabricating solar cells. While working at the MORE center, I successfully fabricated a functioning solar cell. During the summer of 2014, I worked at the University of Utah fabricating perovskite solar cells. I learned the difference in the methodology of creating perovskite cells and the new challenges that perovskite solar cells create. Here I sharpened my skills of using also a vapor and electron beam deposition chamber and learned spectroscopy, another methods for characterizing solar cells. My experience working in three renewable energy laboratories over the last six years has including working with perovskite solar cells as well as developing experience with various lab equipment and techniques that are utilized in solar cell research. Because of these experiences, I am qualified to test the durability and lifetime of perovskite solar cells. Anticipated Audience Involvement For this project I plan to use the MORE center to fabricate the solar cells and the SDLE center to test the durability and lifetime of the solar cells. I am asking Dr. Ken Singer and Dr. Ina Martin for their guidance and approval to work in the MORE center. I will need guidance and training to create an appropriate methodology for fabricating perovskites using the lab equipment in the MORE center. I will also talk with Dr. Burda about fabricating the perovskite active layer. He has a PhD student who is currently working with perovskite layers and I will consult with them to better understand how to create a perovskite layer. If his lab has capabilities that would be suited for creating perovskites, then I will request permission to use his facilities. 5

7 For the second part of my project, I will be requesting the help of Dr. French, the SDLE center Director. I will need the use of his facilities to test the lifetime of the perovskite solar cells. Because of the unique situation that perovskites contain lead, I will need Dr. French s expertise and guidance in how to create a new methodology for testing the cells without contaminating either the environment in outdoor tests or the lab equipment in accelerated durability testing chambers. Research Plan and Schedule As this is my senior project I will mainly be conducting the research over the two semesters in which I am taking my senior capstone class. Since I am cooping this fall I will begin my project in the 2016 spring semester and finish the project in the 2016 fall semester. Perovskite Research 3 Weeks I will research various methodologies for fabricating perovskite solar cells and determine a plan that can be accomplished and replicated using the facilities at CWRU. Training 2 Weeks I will need to be trained on the MORE center equipment that is necessary for fabrication of perovskite solar cells. Making and testing Cells Durability Test Design 8 Weeks I will work to fabricate most layers independant of each other to test their properties and ensure I am fabricating the correct materials. Then I need to build a full cell and test it to determine if further improvements are necessary. 4 Weeks Work with SDLE center to create a comprehensive methodolgy for testing perovskite cells durability and lifetime. I hope to work both inside and outside but will need to ensure that no contamination occurs. Test Cell Durability 10 Weeks Working both at the solar farm in real world conditions and with lab equipment that speeds up degredation, I will run stress tests on the perovskite solar cells. I will then analyze the solar cells to determine where and why degredation is occuring. 6

8 Budget I anticipate that this project will require approximately $3000 in materials. This would include purchasing the transparent conductive oxide layer, and the chemicals required to fabricate the other layers. I would then expect there to be a cost of about $1000 for using the fabrication deposition chamber in the MORE center. I would anticipate approximately another $1000 on imaging and other techniques to test that the perovskite layers and cell are being created in a reproducible manner. The material used to encase the cells and to create safety measures to insure that no contamination of lead occurs will cost about $2500. Lastly, I expect another $1500 would be needed for using lab machines to test the degradation of the perovskite cells. The total requested is $9,000. Bibliography [1] Committee on America s Climate Choices, America s Climate Choices, The National Academies Press, [2] R. Stavins and Z. Ji, Climate Change 2014 Mitigation of Climate Change, Cambridge University Press, Chapter 13, [3] A. Luque and S. Hegedus, Handbook of Photovoltaic Science and Engineering, John Wiley and Sons, Ltd, 2 nd Edition, [4] M. Grätzel, The light and shade of perovskite solar cells, Nature Materials, vol. 13 pp , September [5] Best Research Cell Efficiencies. National Renewable Energy Laboratory, March < [6] Perovskite Fever, Nature Materials, vol. 13, pp837. September [7] M. Murray and R. French Solar radiation durability of materials, components and systems for photovoltaics, Photovoltaic Specialists Conference (PVSC), th IEEE. [8] J. Wohlgemuth et.al. Long Term Reliability of Photovoltaic Modules, Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4 th World Conference, vol. 2, pp ,