Featured paper: Additivated perovskites for more stable solar cells.

Posted on Wednesday 30 de September de 2020

[Paper: Effect of the incorporation of poly(ethylene oxide) copolymer on the stability of perovskite solar cells. Jeann Carlos da Silva, Francineide Lopes de Araújo, Rodrigo Szostak, Paulo Ernesto Marchezi, Raphael Fernando Moral, Jilian Nei de Freitas and Ana Flávia Nogueira. J. Mater. Chem. C, 2020,8, 9697-9706].

"Sandwich" of materials that form the perovskite solar cell developed by the Brazilian team. — “Sandwich” of materials that form the perovskite solar cell developed by the Brazilian team.

Thanks to the contributions of research groups from different countries, perovskite-based solar cells have quickly become competitive in terms of energy conversion efficiency – the percentage of solar energy that is converted into electrical energy – reaching values above 25%. Unfortunately, the good efficiency achieved for these solar cells does not remain throughout their use, mainly because of the instability of their active layer. Composed of materials from the perovskite family, this layer of the sandwich-like solar cell is responsible for absorbing light. Due to moisture, as well as light itself, perovskite degrades and threatens the life cycle of a solar cell.

The problem has been the focus of many researchers in the area, among them, those from the Laboratory of Nanotechnology and Solar Energy (LNES) at Unicamp (Brazil), led by Professor Ana Flávia Nogueira. In recently reported research in the Journal of Materials Chemistry C (impact factor 7.059), LNES members were able to produce more stable perovskite films which allowed manufacturing solar cells with lower efficiency losses over time.

The addition of copolymer P(EO/EP) improved the stability of MAPbI3 perovskite.

The strategy adopted was to add to the perovskite a compound that gives it stability without affecting its crystalline structure, from which important properties emerge for solar cell performance. The chosen additive, a copolymer (polymer formed by two different monomers), was added in different concentrations to a solution of lead iodide and methylammonium iodide, which, when crystallized, formed a modified and more stable perovskite film.

The researchers used the spin coating technique to prepare filmes of pure perovskite and “additivated” perovskite. In a material degradation test, the authors exposed the samples to ambient light and humidity for nine days and observed their degradation, which was visible to the naked eye by the yellowing of the films, whose original color is almost black. In the samples with additive, the degradation was delayed by a few days when compared to the pure perovskite samples.

Another test carried out by the team showed the films’ ability to regenerate after an initial degradation caused by exposure to a humidifier. The samples with the additive not only degraded less, but also spontaneously regenerated, almost entirely, thirty seconds after removing the moisture source – a phenomenon known as healing – as can be seen in this video.

“This work demonstrated that incorporating a copolymer based on poly(ethylene oxide) to the perovskite layer can delay and, in some cases, even reverse the degradation process of the film with relation to moisture and lighting,” summarizes Jeann Carlos da Silva, co-author of the article.

box_en To study in detail the structure and composition of the films, the authors used a series of characterization techniques, including an X-ray diffraction technique (in situ GWAXS), available at the Brazilian National Synchrotron Light Laboratory (LNLS), which allowed to monitor the manufacturing process of the films. Based on the set of characterization results, the authors were able to explain the mechanism that generates the protective effect in perovskite films with additives. According to them, the effect occurs mainly due to the interaction performed by the copolymer, through hydrogen bonds, with the methylammonium cation of the perovskite. In films without the additive, light and moisture cause part of the methylammonium to shift into the gas state and then leave the perovskite structure, generating the degradation, which is partially irreversible. In the films with the additive, the copolymer retains the methylammonium, which generates films that are more stable and have greater regenerative capacity.

“This study also allowed to investigate the crystallization dynamics of the perovskite containing the copolymer, as well as to understand the formation mechanisms of perovskite/copolymer in humidity and lighting conditions,” highlights Francineide Lopes de Araújo, co-author of the article. “In addition, through characterization techniques such as in situ X-ray diffraction, the study explores an important area in order to understand the material, offering an important contribution to the scientific community and opening new investigation perspectives for the application of polymers in the process of forming and manufacturing perovskite solar cells,” she adds.

Finally, the scientific team manufactured solar cells using perovskite films with and without additives as active layer, and compared their energy conversion efficiency. Initially, the presence of the copolymer decreased the efficiency of the devices, since, as it is an insulating material, it impairs the transfer of electrical charges. However, in the stability tests, when the devices were exposed to humidity and light for twenty days, the perovskite cells with additives performed better.

In numbers: while pure perovskite solar cells started at 17% efficiency and maintained 47% of that value at the end of the test, perovskite devices containing 1.5 mg mL-1% copolymer had an initial efficiency of around 15 %, but retained 68% of efficiency after the 20 days of testing.

“Unfortunately, the problem of stability of perovskite solar cells could not be definitively solved through this research, however, an important way to protect the material was explored, mainly against aggressive exposure to moisture and light, which in the future can be combined with other protection mechanisms,” summarizes Jeann Carlos da Silva. “The research also reinforces the feasibility of incorporating extrinsic compounds to perovskite as protective agents,” he adds.

This study began at LNES in 2016, in the master’s research of Jeann Carlos da Silva, shortly after the development, in that same laboratory, of the first perovskite solar cell prototype in Brazil. The research was completed with the collaboration of the postdoctoral fellow Francineide Lopes de Araújo and other members and former members of the group, always under the guidance of Professor Ana Flávia.

The study was funded by Brazilian agencies FAPESP, CNPq and CAPES, and is the subject of the project “Perovskite Solar Cells for Artificial Photosynthesis” of the Center for Innovation on New Energies (CINE) with support from Shell and Fapesp.

Authors of the paper. From the left: Jeann Carlos da Silva, Francineide Lopes de Araújo, Rodrigo Szostak, Paulo Ernesto Marchezi, Raphael Fernando Moral, Jilian Nei de Freitas and Ana Flávia Nogueira.

Featured scientist: interview with Carlos Graeff.

Posted on Friday 30 de March de 2018

Fascinated by science since he was a child, with a representative at his home (his father, a renowned neuroscientist), Carlos Frederico Oliveira Graeff, born at Ribeirao Preto (state of São Paulo), chose the area of Physics as his university studies. He obtained his bachelor’s (1989), master (1991) and doctor (1994) degrees in Physics from the University of Campinas (Unicamp). During his master’s and doctorate program, supervised by professor Ivan Chambouleyron, he took his first steps as a researcher in the Materials area with studies on materials based on germanium and silicon. During his doctorate he participated in a research internship at the Max Plank Institut für Festkörperforschung in Germany.

He returned to Germany in 1994 until 1996 for a postdoctoral period to work on electronic magnetic resonance, semiconductors and electronic devices at the Walter Schottky Institute of the Technische Universität München (TUM), with a grant from the German foundation Alexander Von Humboldt.

Upon returning to Brazil, he became a professor at the Department of Physics and Mathematics of the University of São Paulo (USP), where he remained for 10 years. In 2006, he joined the Faculty of Sciences of Bauru at the State University of São Paulo (UNESP) as a full professor, where he is still teaching and researching. Throughout his academic career, Graeff has been visiting professor or researcher at several institutions in France, China and Switzerland.

From 2007 to 2009, Graeff was coordinator of the Post-Graduate Program in Materials Science and Technology (POSMAT) at UNESP – Bauru campus. Between 2009 and 2014, he was the coordinator of the newly created Materials Area of CAPES, responsible for the evaluation of Brazilian post-graduate programs in Materials, among other functions. From 2011 to 2013, Graeff was president of the Humboldt Club of Brazil and in 2012 and 2013 he was scientific director of B-MRS. The scientist also fulfilled or performs management or advisory functions at Brazilian agencies FAPESP and CAPES, and at IUPAC (International Union of Pure and Applied Chemistry).

In 2017, after having participated in the editorial board of several international journals, he was appointed associate editor in the photovoltaic area of the journal Solar Energy (impact factor 4,018), of Elsevier publishing house. Also in 2017, he became Dean of Research at UNESP, a post he holds until now.

With an h index of 28, Graeff is the author of about 200 indexed papers that have more than 2,500 citations, according to Google Scholar. In three decades of scientific work, together with his team at the Laboratory of New Materials and Devices at UNESP and his numerous national and international collaborators, Graeff has contributed to the field of materials research with multiple subjects. Among his most cited articles there are studies on synthetic diamond, silicon and germanium heterostructures, conjugated polymers, latex and melanin (biological material with semiconductor properties that are promising for the development of bioelectronic devices).

The researcher has also worked in the area of photovoltaic energy (direct conversion of solar radiation into electricity), with numerous contributions to the development of solar cells based on different materials (dyes, perovskites and organic semiconductors). On this subject of photovoltaic energy, Carlos Graeff will offer a plenary lecture at the XVII B-MRS Meeting, to be held in Natal (RN) from September 16 to 20.

The following is an interview with this outstanding researcher of our community.

B-MRS Newsletter: How or why did you become a scientist? Did you always want to be a scientist? Also, briefly tell us what led you to work in the field of materials.

Carlos Graeff: My father, Frederico Graeff, is a well-known researcher and perhaps one of the most important influences in my decision. My aunts were also teachers and researchers, so from an early age I had access to the world of science from my home, which has always fascinated me. The decision to study physics was largely due to the various books I read and from the television Cosmos series presented by Carl Sagan. The decision to work in the Materials area came later on during my baccalaureate in physics after the first courses in condensed matter physics and semiconductors. From the beginning of the graduate studies I worked in materials, and soon I was attracted by the interfaces of physics with chemistry and biology in very different subjects of materials science and engineering.

B-MRS Newsletter: What do you believe are your main contributions to the Materials area? Please consider all aspects of scientific activity.

Carlos Graeff: It is always difficult to choose key contributions. In my case in particular it is easy to see, reading my CV, a very eclectic trajectory in terms of studied materials and applications. Using originality as a preference, I will dwell on three themes; the first is the production of CoS (cobalt sulfide) the basis of ecological paints for the production of electrodes for solar cells. We have achieved a simple, industrial and ecological method to replace platinum in dye-based solar cells. In the second theme, we have proposed several alternative methods for the synthesis of melanin, the material involved in tanning, and with this we have been able to produce biocompatible materials with very special characteristics with regard to, for example, solubility. We are identifying a very important defect for this material using, as a main tool, computational simulations combined with spectroscopic techniques. We are sure this material will be important in the emerging area of bioelectronics. In the third theme, we describe in detail the whole degradation process of organic semiconductors, identifying routes for the production of high sensitivity dosimeters for applications in hospitals and clinics that use, for example, gamma rays for cancer treatments and diagnosis. We also have had very unique contributions in the physics of electrically detected magnetic resonance, increasing the sensitivity and the general understanding of the physical phenomena involved. In addition to these fundamental contributions, I was responsible, proudly and with satisfaction, for the implementation of the materials area at CAPES. Another source of satisfaction regards the good students I was fortunate enough to mentor, many of them brilliant scientists. I helped and coordinated the assemblage of several laboratories both here in Brazil and abroad, most recently I helped set up a magnetic resonance laboratory in China.

B-MRS Newsletter: Now we invite you to leave a message for our readers who are starting their scientific careers.

Carlos Graeff: I started my master’s degree in 1989, a time that was perhaps as troubled as the current one, do not get discouraged! With focus and a bit of luck it is always possible to create new ideas, build a solid career and contribute to our beautiful country. We are going through a great revolution, with the emergence of new technologies that will profoundly transform society. Intelligence will increasingly play a decisive role in the direction of our society, be prepared to work in this new world of great opportunities. Always seek out dialogue with specialists from the most different areas of knowledge and from various countries. Quite possibly, in the coming years we will unravel the mysteries of how the brain works, we will master limitless forms of energy and ecological production, generate artificial intelligence. Open up to what is new, be bold, Brazil needs your citizen and entrepreneurial spirit.

B-MRS Newsletter: You will deliver a plenary lecture at the XVII B-MRS Meeting. Leave an invitation to our community.

Carlos Graeff: Photovoltaic energy is reaching its commercial maturity, we are living an unprecedented energy revolution. In the lecture I will show some updated data on the perspectives of using photovoltaic cells in Brazil and in the world; its principles of operation; the challenges for scientists and material engineers in this relentless race for increasingly efficient, durable and environmentally friendly materials, processes and devices. I will present our group’s latest results on this topic.

Interview with Prof. Kirk Schanze (UTSA, USA), editor-in-chief of ACS Applied Materials & Interfaces.

Posted on Wednesday 30 de August de 2017

In the research group of Professor Kirk Schanze, conjugated polyelectrolytes (CPEs) have been the subject of both fundamental studies and applications. The group has already explore CPEs as fluorescent sensors, in solar cells and as biocidal materials.

On September 13, in Gramado, Kirk Schanze, who is a Professor at the University of Texas at San Antonio (UTSA) and editor-in-chief of ACS Applied Materials & Interfaces, will take some time out of his busy schedule to deliver a plenary lecture on CPEs in the XVI B-MRS Meeting.

Schanze graduated in Chemistry from Florida State University in 1979. Four years later, he earned his Ph.D., also in Chemistry, from the University of North Carolina at Chapel Hill. Soon after, he was appointed a Miller Postdoctoral Fellow at the University of California, Berkeley. In 1986, he joined the University of Florida (UF) as a professor of the Department of Chemistry. There, he chaired the Division of Organic Chemistry, held the Prominski Chair of Chemistry, and founded the Schanze Group, which today continues its research activities at UTSA. In 2016, Schanze left UF to hold the Robert A. Welch Distinguished University Chair in Chemistry at UTSA.

Between 2000 and 2008, Schanze served as senior editor of the prestigious journal Langmuir. Shortly thereafter, he became the first editor-in-chief of ACS Applied Materials & Interfaces, which had just been released.

Prof. Schanze has authored about 300 papers and 20 patents. According to Google Scholar, his scientific production has more than 16,000 citations and his h index is 71. He is fellow of the American Chemical Society (ACS). He was a visiting professor at the Harbin Institute of Technology (China) and the Tokyo Metropolitan University (Japan) in 2011, at the Ecole Normale Supérieure Cachan (France) in 2008 and at the Chemical Research Promotion Center (Taiwan) in 2007. He has received distinctions from the American Chemical Society, National Science Foundation, University of Florida, Japan Society for Promotion of Science, and Japanese Photochemical Association, among other entities.

Here follows an interview with the scientist.

B-MRS newsletter: – In your opinion, what are your main scientific and/ or technological contributions to the field of conjugated polyelectrolytes? Describe them briefly and feel free to share a few references of your papers, patents or books.

Kirk Schanze: – We were among the first groups to study conjugated polyelectrolytes, which are water soluble conjugated polymers. Following are some of the key contributions from our group to this field:

a) Our lab was the first to report the synthesis of a water soluble, fluorescent poly(phenylene ethynylene) sulfonate (PPE-SO3) and describe the application to fluorescence sensing of ions in water at ultralow concentration.[1]

b) We were the first to report the use of a fluorescent conjugated polyelectrolyte as a sensor for enzyme activity, which is an important biosensing application.[2]

c) Our lab has developed the applications of cationic conjugated polyelectrolytes to sensing phosphatase enzyme activity. These enzymes are important in a number of biologically significant processes. [3,4]

d) Working in collaboration with Prof. David Whitten of the University of New Mexico, we have developed cationic conjugated polyelectrolytes as a novel class of antibacterial agents.[5,6]

References:

[1] C. Tan, M. R. Pinto and K. S. Schanze, “Photophysics, Aggregation and Amplified Quenching of a Water-Soluble poly(Phenylene ethynylene)”, Chem. Commun. 2002, 446-447, 10.1039/B109630C.

[2] M. R. Pinto and K. S. Schanze, “Amplified Fluorescence Sensing of Protease Activity with Conjugated Polyelectrolytes”, Proc. Nat. Acad. Sci. USA, 2004, 101, 7505, 10.1073/pnas.0402280101.

[3] Zhao, X.; Liu, Y.; Schanze, K. S., “A Conjugated Polyelectrolyte Based Fluorescence Sensor for Pyrophosphate”, Chem. Commun. 2007, 2914-2916, 10.1039/b706629e.

[4] Zhao, X. Y.; Schanze, K. S., “Fluorescent Ratiometric Sensing of Pyrophosphate via Induced Aggregation of a Conjugated Polyelectrolyte”, Chem. Commun. 2010, 46, 6075-6077, 10.1039/c0cc01332c.

[5] Ji, E.; Corbitt, T. S.; Parthasarathy, A.; Schanze, K. S.; Whitten, D. G., “Light and Dark-Activated Biocidal Activity of Conjugated Polyelectrolytes”, ACS Appl. Mater. Interfaces 2011, 3, 2820-2829, 10.1021/am200644g.

[6] 299. Huang, Y.; Pappas, H. C.; Zhang, L.; Wang, S.; Cai, R.; Tan, W.; Wang, S.; Whitten, D. G.; Schanze, K. S., “Selective Imaging and Inactivation of Bacteria over Mammalian Cells by Imidazolium Substituted Polythiophene”, Chem. Mater. 2017, 2017, 29, 6389–6395, 10.1021/acs.chemmater.7b01796.

B-MRS Newsletter: – You have been the Editor-in-Chief of ACS Applied Materials & Interfaces since its release, haven´t you? In less than 10 years, the journal hit an impact factor of 7,504. To what factors do you attribute this good result?

Kirk Schanze: – ACS Applied Materials & Interfaces (AMI) publishes papers that come from a currently very active area of materials research, specifically applied materials/interfaces. There is a large community of scientists and engineers around the globe who are working in this field. AMI has a global community of editors and editorial board members who represent their regions. Indeed, the newest editor who has joined our editorial board is Prof. Osvaldo Oliveira Jr. of the University of Sao Paulo!

B-MRS Newsletter: – We often see papers from the Brazilian Materials Community at ACS Applied Materials & Interfaces. Could you share with our readers some numbers about the participation of Brazilian authors in the journal?

Kirk Schanze: – ACS Applied Materials & Interfaces has published more than 100 papers with authors or co-authors from Brazil. Many of these papers have been highly cited in the field of materials science. Examples of highly cited papers are:

K. Poznyak†, J. Tedim†, L. M. Rodrigues†‡, A. N. Salak†, M. L. Zheludkevich*†, L. F. P. Dick‡ and M. G. S. Ferreira†§ Novel Inorganic Host Layered Double Hydroxides Intercalated with Guest Organic Inhibitors for Anticorrosion Applications, ACS Appl. Mater. Interfaces, 2009, 1 (10), pp 2353–2362, DOI: 10.1021/am900495r (co-author from Rio Grande do Sul Federal University in Porto Alegre)
Heberton Wender*†, Adriano F. Feil†, Leonardo B. Diaz†, Camila S. Ribeiro‡, Guilherme J. Machado†, Pedro Migowski§, Daniel E. Weibel‡, Jairton Dupont§, and Sérgio R. Teixeira*† Self-Organized TiO2 Nanotube Arrays: Synthesis by Anodization in an Ionic Liquid and Assessment of Photocatalytic Properties, ACS Appl. Mater. Interfaces, 2011, 3 (4), pp 1359–1365, DOI: 10.1021/am200156d

B-MRS Newsletter: – Please, leave an invitation to your plenary talk.

Kirk Schanze: – Everyone is invited to attend my talk which will highlight our work of conjugated polyelectrolyte as applied in the field of energy- and bio- materials chemistry.

More information

On XVI B-MRS Meeting website, click on the photo of Kirk Schanze and see his mini CV and the abstract of his plenary lecture: http://sbpmat.org.br/16controter/home/

Interviews with plenary speakers of the XV Brazil-MRS Meeting: Anders Hagfeldt (EPFL, Switzerland).

Posted on Monday 19 de September de 2016

In the late 1950s, solar cells where used for the first time in artificial satellites. Today, these devices that produce electricity from sunlight thanks to the property of some materials to release electrons when absorbing photons, are part of the energy matrix of many countries, besides being used in all sort of spacecraft. Several technologies based on different materials have been developed to make this sustainable production of electricity. However, research in the area is still very intense. While silicon solar cells dominate the current market, other technologies can compete with silicon in economic and environmental terms.

In a plenary lecture at the XV Brazil-MRS Meeting, Anders Hagfeldt, Professor at École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, will talk about recent advances on some solar cells technologies that are alternative to the silicon one, in particular those based on perovskite materials and those based on dye sensitized thin films (known as dye-sensitized solar cells, DSSCs). Hagfeldt has been performing research on both types of solar cells, and succeeded in improving their efficiency using different methods and new materials.

Hagfeldt obtained his diploma of Master of Science in Physics and Chemistry from Uppsala University (Sweden) in 1989 and started his doctoral studies in the same university. In 1993, he concluded his PhD with a thesis on microporous and polycrystalline semiconductor electrodes. Then he went to EPFL, in Switzerland, where he was a postdoctoral fellow with Prof. Michael Grätzel, the inventor of the DSSCs.

In 1994 he went back to his alma mater (Uppsala), first as a junior researcher and then as a Professor in Chemical Physics and Physical Chemistry. He was a Visiting Professor at the Royal Institute of Technology (Sweden) from 2005 to 2010, and at the Institute of Materials Research and Engineering (Singapore) from 2008 to 2010. In 2009, he co-founded Dyenamo, a company dedicated to materials and equipment for solar energy applications.

Since 2014, Hagfeldt is a Full Professor of Physical Chemistry at EPFL, where he heads the Laboratory of Photomolecular Science. Besides, he is a Visiting Professor at Uppsala University and Nanyang Technological University (Singapore). He is a member of the European Academy of Sciences, Royal Swedish Academy of Sciences, Royal Society of Sciences in Uppsala, and the Royal Swedish Academy of Engineering Sciences.

Anders Hagfeldt´s was ranked in various international lists of highly cited researchers, such as the Thomson Reuter’s list of the top 1% most cited in Chemistry (2014-2016) and the top-100 material scientists of the past decade by Times Higher Education (2011). In fact, Hagfeldt authored above 400 papers with more than 47.000 citations and has an h-index of 103, according to Google Scholar. Moreover, he is the author of 9 patent applications.

Here follows a short interview with Professor Anders Hagfeldt.

SBPMat newsletter: – Could you state, very briefly, which are the main advantages and disadvantages of the different solar cells technologies, in terms of efficiency, cost and other relevant criteria?

Anders Hagfeldt: – When it comes to different technologies, the very dominant technology has been for long time based on silicon material, the “silicon solar cells”. In terms of market share, I think that around 90% of all solar cells stored in the world are silicon solar cells. They are mainly produced in China. Silicon has always had the advantage of being efficient and very stable, robust, durable. So, you can get, for example, guarantees of lifetime of over 20 years. If you go few years back, it was always said that silicon solar cells were too expensive and that they wouldn´t never be competitive with other energy technologies, such as fossil fuels and so on. However, five years ago, the production volume has increased a lot and the prize has became unexpectedly lower. Actually, silicon solar cells can be considered quite cheap today. Therefore, silicon solar cells are very good. They have good performance at low cost and match the price in kilowatt-hour of other energy technologies such as burning liquid hydrocarbons, for example.

The next class of technology is called “thin-film solar cells”, and it comprises two different materials. One is known as CIGS, that comes from copper indium gallium selenide, which is the material that absorbs sunlight. The other material is cadmium telluride (CdTe). The latter solar cells are manufactured in a big company in the United States called First Solar. Both of these technologies have the promise to be cheaper than silicon technology. They are almost as efficient as silicon: 10% to 15% less efficient. Both materials are disposed in very thin films, so the materials cost is low and, probably, they can be produced cheaper than silicon solar cells. Therefore, the key thing for thin films to be able to compete with silicon is to be a little bit higher in efficiency and scaling up the production.

These are the three main commercially available technologies today. If we go to research activities, the most “hype” at the moment is the perovskite solar cell. It had its breakthrough only four years ago, and during these few four years, it has reached similar efficiencies as thin-film solar cells. That has been the fastest development in solar cells. Perovskite solar cells will probably be cheaper than thin-film solar cells, but they are still in a research stage. The main question mark of perovskite solar cells has been on stability. In that point, we have made some breakthroughs. Two papers of us have been accepted by the Science journal reporting very promising stability data for perovskite solar cells. That is something I will report on the Brazil-MRS Meeting as a key or latest result of perovskite technology. It is very exciting, perovskite solar cells are not fundamentally unstable, they show promising stability. However, there is a lot of work to be done in terms of scaling up and further stability testing and development.

Dye-sensitized solar cells (DSC). — Dye-sensitized solar cells (DSSC).

The other technology I will talk about is dye-sensitized solar cells (DSSC), which is also in research or demonstration level. They are lower in efficiency, so, what we look for in that technology today is niche applications. These cells are based on dyes, which means that you can make them in different colors, and use them in windows, buildings and so on. At the moment, DSSC cannot compete with silicon for large scale, but there is interest for buildings, consumer electronics, rechargeable batteries, keyboards and so on.

SBPMat newsletter: – In your opinion, which are the main next challenges in the field of solar cell research and development?

Anders Hagfeldt: – I can divide this answer in two parts.

Firstly, there is still room and it is still important to make solar cells more efficient to lower the cost of the kilowatt-hour produced. There is a kind of dream target I see. This is not really in my expertise, but I listen to more industrial people talking about U.S. dollar cents per kilowatt-hour (how much costs for solar cell to produce a kilowatt-hour). And it seems that now this cost can go down to 4 cents per kilowatt-hour, which is very good, but it could be cheaper. People say that it can go down to 2 cents per kilowatt-hour. The best thing you can do today to lower the cost per kilowatt-hour of solar cells is to try to increase their efficiency. That is where you see the potential of the perovskite solar cells, because they have had such a fast development and they are already at the same level of established technologies. It seems promising that these perovskite solar cells can show even higher efficiencies than the silicon ones. That is a big challenge but I think it is not impossible.

The second thing is that solar cells is intermittent, that means that they only produce electricity when there is sunshine. During nights and evenings, this is a problem. I come from Sweden that is a country where more electricity is needed when there is less sunshine. That means that you also have to find the storage for electricity.

These key challenges are not for solar cell itself, but for the whole picture of solar energy.

More and more solar cells are being used to produce electricity, but that creates problems to the utility grid, because there can be too much electricity when it is sunny day and too little when it is not sunshine. Therefore, we need to work on storage. I think that for small scale, batteries are interesting, but for larger scale, we need to find how to produce fuel from sunlight or from the electricity produced. And that can be hydrogen, that is something people look into a lot, but also for example methanol.

SBPMat newsletter: – Leave an invitation for our readers to attend your plenary lecture “The Versatility of Mesoscopic Solar Cells”.

Anders Hagfeldt: – I am very happy to go to Brazil. I have been around before, some years back, but I am very excited to meet new people to discuss our research at the meeting. Everyone will be very welcome and I will be very happy to discuss all kind of ideas and questions.

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Link to the abstract of Anders Hagfeldt´s plenary lecture at the XV Brazil-MRS Meeting: http://sbpmat.org.br/15encontro/speakers/abstracts/9.pdf

Featured paper: Analytical contribution to sustainable energy.

Posted on Friday 29 de January de 2016

[Paper: Influence of charge carriers mobility and lifetime on the performance of bulk heterojunction organic solar cells. D.J. Coutinho, G.C. Faria, D.T. Balogh, R.M. Faria. Solar Energy Materials and Solar Cells, Volume 143, Pages 503-509 (December 2015). DOI:10.1016/j.solmat.2015.07.047]

Analytical contribution to sustainable energy

A scientific study entirely conducted at the São Carlos Institute of Physics from the University of São Paulo (IFSC-USP) has made significant contribution to the assessment of performance of organic solar cells, devices that are able to produce electricity from sunlight – a renewable, clean, safe and practically inexhaustible source of energy. The results of this piece of research were recently published on the journal Solar Energy Materials & Solar Cells, which has an impact factor of 5.337.

BHJ-OSC — Composition of the bulk-heterojunction organic solar cell used in the experiments reported in the paper. In the active layer, the configuration of the electron acceptor (Blue) and donor (Red) materials.

With a structure comparable to a sandwich, the organic solar cell is comprised of layers of nanometric thickness, made of several materials that execute specific functions in the device.

The so-called “active layer”, the one responsible for the main stages of transforming light (flow of photons) into electric current (flow of electrically charged particles), is made of semiconducting organic materials (whose molecules have carbon atoms). In the electronic band structure of traditional semiconductors, electrons located in the so-called “valence band” jump from their state when they absorb photons, leaving vacant spaces, or holes, and occupying new places in the so-called “conduction band”. In organic semiconductors, the mechanism that produces the electron-hole pairs is similar, with the difference that, instead of a direct transition from one band to the other, there is a molecular exciton (a system containing one negative charge, and one positive charge), which is easily dissociated, producing free charges (electrons and holes).

For the next stage in the conversion of light into electricity to occur, the active layer of the organic solar cells must have many interface regions between two types of materials: the donor and the acceptor of electrons (usually an electronic polymer and a fullerene derivative, respectively). If the exciton, in its few picoseconds of existence, manages to reach an interface region, the forces keeping the electron and the hole together are broken, so the donation of the electron from the polymer to the fullerene happens. At this moment, if no traps are on the way to prevent their movement, electrons and holes flow in opposite directions, attracted and collected by electrode elements, producing an electric current that can be used in an external circuit.

In this succession of stages, efficiency losses in the conversion of solar energy into electrical energy may happen due to several factors. One example is the recombination of electrons and holes after the dissociation of the exciton, which prevents these charge carriers to flow freely. Other examples include defects or impurities in active layer materials, which act as traps for the charge carriers, decreasing their mobility.

The paper published on Solar Energy Materials and Solar Cells reports the results of a series of experiments conducted for the purpose of studying, in detail, the mobility and lifetime of charge carriers (electrons and holes), as a function of temperature, in a bulk-heterojunction organic solar cell produced at IFSC. In this kind of device, the electron donor and acceptor materials coexist in a particular configuration (a nanometric film with a dual-phase structure) that increases the interface area between the two, compared to other possible configurations.

The authors also presented in the paper the results of electric current measurements, based on external applied voltage (J-V) under lighting – one of the most relevant experiments in the characterization of solar cells. In fact, this experiment is necessary for assessing the efficiency of a solar cell.

BHJ-OSC_caracterização — Organic solar cell during electrical characterization under artificial lighting equivalent to sunlight. In the prototype pictured above, on a 5 x 5 cm plate, five devices are connected in series, producing a total of approximately 2V. The individual efficiency of each device studied is around 4%.

In order to adjust and analyze the experimental results, the authors developed a model, based on a set of equations. This model filled a gap in the scientific literature as, up to its release, these analyses were made from approximations, being inaccurate, or using numerical methods, which require hard time-consuming work.

“To this day, there is no formal description of the J-V curve”, says Roberto Mendonça Faria, full professor at IFSC-USP and corresponding author of the paper. “Our research had the merit of developing a J-V analytical expression, which successfully reproduces the characteristics of an organic solar cell in the event the positive and negative carriers have equal mobility”, he points out. With this expression, he adds, it is possible to carry out a more precise assessment of the cells performance, even for cases in which the electrons and holes do not have the exact same mobility.

faria coutinho — Left side: Roberto Mendonça Faria corresponding author of the paper). Right side: Douglas José Coutinho (first author).

The paper also features all the analyses the team managed to do from the experimental results and the model, mainly in regard to some factors leading to efficiency loss in the conversion of light into electricity.

This way, the authors of the paper made a contribution to the challenge of producing sustainable energy. “Energy production is crucial for humanity to keep its economic and social development, but it cannot go on with its terrible side effects, polluting the planet and contributing to global warming”, says Faria.

The results reported in the paper comprise the Master’s and Doctoral studies of Douglas José Coutinho, advised by Professor Faria and financed by Brazilian research funding agencies, FAPESP (São Paulo Research Foundation) and CNPq (National Council for Scientific and Technological Development), including through the CNPq National Institute of Science and Technology for Organic Electronics (INEO).