One of the most pressing challenges of the 21st century is meeting the ever-increasing demand for energy consumption whilst reducing the environmental impact of energy production and storage. Solar energy conversion devices have the potential of providing a source of renewable energy and offsetting carbon emissions. Tremendous strides have been made in achieving ever increasing power conversion efficiencies for all types of solar cells balanced by the need for cost reduction, limited environmental impact, improved stability and longer lifetimes. New characterization, fabrication and testing techniques have continuously demonstrated higher power conversion efficiency and improved designs.
PLOS ONE recently launched a call for papers on the design and testing of photovoltaic solar cell materials to explore how the long stability and degradation of these devices is affected and what we can do to improve them. Here, four of our Guest Editors, David P. Fenning, Maria Antonietta Loi, Hongxia Wang and Graeme Blake, share their experiences and thoughts on the opportunities and challenges of photovoltaic materials research.
What excites you most about working in this field?
Graeme Blake – It is exciting to join the worldwide effort to reduce the world’s reliance on energy from fossil fuels, and to play a small part in the ongoing energy revolution. Solar energy is abundant and relatively reliable in most populated areas of the planet, which is not always the case for other forms of renewable energy. Solar energy is already contributing more to the energy mix every year, and if we are able to further lower the costs and improve the energy conversion efficiency in the coming years, research in this area can make a significant contribution to halting global warming and to realizing a reliable, cheap energy source that is available to all.
David Fenning – Photovoltaics (PVs) appear to be at a tipping point on the path to becoming truly ubiquitous, meaning that innovations now and in the coming years hold promise to have exponential impacts at scale. In recent years there has also been an explosion of activity and creativity in materials and device design leading to remarkable improvements in performance almost uniformly across materials systems. In many ways, this suggests to me that the field may be really just scratching the surface of its full potential.
Maria Antonietta Loi – It is a very important subject, in the next years we will need to make a transition to renewable energy, and we would need better systems to convert solar light into electricity. We still do not have perfect semiconductors and perfect device system, so it is important to work hard to improve them.
Hongxia Wang – The most exciting thing of working in the field of next generation solar cells, such as dye-sensitized solar cells, CZTS solar cells, and in particular the most recent perovskite solar cells, is the unprecedented progress of energy conversion efficiency with these new types of solar cells devices. You never know when a new world-record device efficiency will be announced.
Are there any special challenges associated with carrying out research in the design, fabrication and testing of Photovoltaic Solar Cell Materials?
Hongxia Wang – There are some general rules when designing a material for PVs such as the optical properties (optimal bandgap, large light absorption coefficient) and electrical properties (charge carrier density, defects, energy band position and alignment) that enable effective light absorption, charge carrier separation and collection. However, some materials that may theoretically be ideal for PVs, may be challenging to make and their potential hard to realise due to the complexity of the device architecture required. Solar cell devices integrate several types of materials which often involve not only the light absorber, but also charge extraction layers, a window layer and electrical contact(s). Both the energy and chemical match between the layers are critical to make a good device. Some promising PV materials such as organic semiconductors or organic-inorganic perovskites are sensitized to environmental factors such as moisture, oxygen, UV light, or even electrical fields. These restrictions can create challenges for fabrication and testing.
Graeme Blake – My research is definitely at the fundamental end of the spectrum, searching for new materials that might be cheaper, easier to process and do a better job in terms of energy conversion efficiency. This type of research carries the risk that significant time and money is spent trying to develop materials that in the end do not make the grade, either because their performance is not good enough, they are not stable enough under long-term exposure to ambient conditions, or their use is not cost-effective. Such risks carry with them a degree of difficulty in obtaining sufficient funding.
Maria Antonietta Loi – These devices are complex, many layered, with different materials and it is often very difficult to pin-point where problems are.
David Fenning – One of the most interesting aspects of working in solar cells is the inherently interdisciplinary nature of the work. Solid state physics, optical, electronic, and materials engineering are all required, as are insights into synthesis/fabrication and even systems level and economic considerations. It sits at the intersection of a variety of traditional disciplines, which makes it fun but also challenging.
What kinds of research are you most excited about in this area?
David Fenning – I think the recent strong convergence and cross-fertilization of ideas from previously disparate corners of photovoltaic devices is exciting and something we can learn from when faced with future challenges. For example, the design of highly-conductive but selective contact layers is emerging from historical developments in OPV and silicon — two communities that normally have not overlapped strongly. Literal convergence in tandem cell formats is also an exciting direction ripe for innovation in device architecture and integration. It is also almost a given that I find the emergence of semiconducting halide perovskites and the distinct physics they bring to be particularly motivating. Especially in light of our increasing capability to design and screen materials computationally, it is exciting to think how much broader and more powerfully we can explore.
Maria Antonietta Loi – Difficult to say, there are so many new ideas, new materials, new device structures. Everything is exciting.
Hongxia Wang – In the past 17 years, my research has been primarily focused on 3rd generation solar cells, including dye-sensitized solar cells, perovskite solar cells and CZTS based thin film solar cells. My particular interest in this area is to understand the fundamental operation mechanism of the devices through comprehensive characterisation and theoretical modelling. Meanwhile, my research group has also done extensive research in developing new materials or methods for making light absorbing materials for PVs to enhance the energy conversion efficiency and stability of the solar cells. I believe that a deep understanding of the function of each material in the device is critical to develop effective strategies that solve the issues associated with these solar cells through material and device engineering.
Graeme Blake – I am involved in the area of hybrid perovskite solar cell materials. These materials are exciting because their energy conversion efficiency has been improved from close to zero to percentages equivalent to single-crystal silicon in less than a single decade. The rate of progress in these materials has been astonishing, and there is now a big worldwide effort to develop these materials further. Hybrid perovskites can be processed at low temperatures (an important factor in lowering production costs and making fabrication more sustainable) and even printed to easily produce large-area solar cell devices. They are very flexible materials in that many elements of the periodic table, as well as a wide range of small organic molecules can be incorporated, which allows parameters such as their electronic band gap, light absorption spectrum and chemical reactivity to be tuned. For this reason only a small area of parameter space has been explored until now, and in my opinion, there is great scope for further optimization of their properties.
This Call for Papers brings together interdisciplinary perspectives from materials synthesis, device optimization, computational studies and many more. What do you see as the importance of interdisciplinarity in this field?
Maria Antonietta Loi – The devices are complex, the material challenges are complex. If we want to progress, we need the help of people with very different expertise.
Hongxia Wang – Photovoltaic devices are a complex system that involves chemistry, physics, materials science and engineering. Fabrication of high-performance solar cells requires deep multidisciplinary knowledge in these areas. It has become more important than ever to have a comprehensive understanding of the critical materials/processes that control device performance and how to improve them to make high performance solar cells at low cost. Therefore this special issue that brings interdisciplinary perspectives on solar cells is timely and important in my opinion.
David Fenning – Interdisciplinarity is in its very nature. I think having diverse perspectives and expertise attacking challenges in photovoltaic is the best way we can move forward.
Graeme Blake – In order to design better photovoltaic materials, we need to obtain a deep understanding of the chemistry and physics of both the materials themselves, and also of the way that they function in-situ as part of the overall light harvesting device operating under ambient conditions. This naturally requires interdisciplinarity and close communication between physicists, chemists and engineers. Although much of the work in developing new materials and device designs is necessarily experimental in nature, computational models and simulations have developed to the point where they have become essential as a guide to experimentalists in order to explain the observed electronic and photonic properties of a particular material, and to predict new materials that might have improved properties. The new insight gained via computational studies can then be fed back into the development loop where the chemists have target materials to synthesize and the physicists can study their properties in detail and build improved devices.
What do you think the main barriers are for researchers?
David Fenning – A significant challenge is that at the end of the day our goal is to produce a commodity, electricity, where there are existing large incumbents, a natural but deep conservatism since we all want the lights to turn on when we flip a switch, and a race to the bottom in price competition. This makes it hard to advance new technologies from the lab to market and creates less of an R&D pull than might exist from other industries where marginal value is greater.
Also, because of its interdisciplinarity, researchers can sometimes be silo-ed within departments or traditional fields of the physical sciences or engineering, while working on particular aspects of photovoltaics. I think there is a real and continuing need for centre-level collaborations and specialists conferences to bring researchers together to spark creativity and innovation.
Maria Antonietta Loi – Generally the limits are time, money and mass. Money and mass can shorten the time, but there are limits to the acceleration of research.
Graeme Blake – The huge number of researchers and institutes now working in this field can be a hindrance as well as a help for progress. It is difficult to keep up to date with the volume of scientific literature constantly appearing, and also to judge where the genuine breakthroughs are, and which work is possibly misleading. The strong competition in the academic environment between rival research groups and institutes can also lead to the hasty publication of less than reliable work. Otherwise, as is the case for most academic research, the main barrier is obtaining sufficient funding to continue high quality research in the face of continuous budget cuts for science in many countries.
Hongxia Wang – The barrier(s) for individual researchers varies and depends on many factors. Some researchers are limited by access to key resources such as specialized facilities, whereas others may be limited by the availability of research funding and PhD students.
One of the main focuses of this call for papers is how to design stable photovoltaic materials for applications in various environments. Why do you think this is important and what extra value does it provide?
Maria Antonietta Loi – Stability is fundamental to bring new materials from the labs to production. Without the certainty that the material can be stabilised no companies would invest in further research and implementation into products.
David Fenning – Without question, stability is critical. It is one of a trifecta for technical relevance: efficiency, cost and stability. A short-lived solar cell may provide value to niche applications, but to have societal-level impact on energy production it is hard to imagine sacrificing much if any of the 25+ year operational lifetimes demonstrated in fielded silicon modules. I think stability is undervalued in R&D.
Hongxia Wang – The research community of PVs has witnessed the unprecedented progress of perovskite solar cells (PSCs) in terms of energy conversion efficiency in the past ten years. The current world record of PSCs is above 25%, which is comparable to the record efficiency of monocrystalline silicon solar cells. Compared to silicon-based PV technology, the high performance of PSCs is achieved by cost-effective solution processing and can be fabricated by simple methods such as roll-to-roll, printing, inkjet etc. Clearly PSCs are a very promising PV technology, which has the potential to deliver cost-effective solar electricity if commercialized. All the major players in the PV market have paid attention to this technology. Besides energy conversion efficiency, device stability and operational lifetime are equally important for the commercialisation of PV technology. Currently device stability is the major concern for PSCs due to the sensitivity of the perovskite light absorber to moisture, electrical fields, UV light, and phase transition of the perovskite materials at relatively low temperatures as well as the ionic migration of materials. This Call will provide a platform showing the most recent research progress addressing the critical device stability of PSCs.
Graeme Blake – Often photovoltaic materials show a high degree of promise when investigated under lab conditions, but when transferred to various “real” environments they do not perform so well, or they degrade with time making them economically unfeasible for use. Although such materials can still be interesting to investigate from a fundamental point of view, it is of course important to develop new materials that contain abundant, non-toxic elements, that can still be processed easily on a large scale, but are stable on multi-year timescales in the open air and can thus eventually take the big step towards commercial production. The development of more stable materials with commercial potential will also help to secure more funding for ongoing research.
What do you think the future of renewable energy/technology is?
Hongxia Wang – The global issues of climate change partially if not solely as a consequence of CO2 emission due to the combustion of fossil fuels, and the limited reserve of fossil fuels in the earth’s crust have forced all parties to work together to develop technologies that can generate electricity by harnessing renewable energy such as sunlight, wind, hydraulic power, geothermal etc. According to the International Energy Agency the share of renewables in the global energy market will increase by 20% in the next five years . In 2023, renewables will provide almost 30% of power demand in the market. Clearly the demand for renewable energy/technology will increase continuously in the future.
David Fenning – Renewable energy is simply a necessity to mitigate carbon pollution and enable the development of a more sustainable society. Fortunately, as levelized costs continue to fall, new large markets will open up with the potential to create a virtuous cycle where expanded markets drive more technology development, which opens new applications and so on. In the face of climate change and its severe weather impacts, integrated renewable energy production and storage technology also offers unique advantages in creating robust and modular energy grids. These aspects of renewable energy systems also facilitate electrification efforts globally, in a quite literal sense helping to power human development.
Graeme Blake – In my opinion we must use a broad mixture of renewable energy sources. Solar, wind, hydroelectric, geothermal and tidal power will all have their place in the coming decades depending on the local environment. A big challenge is still the storage of energy on a large scale, either in “batteries” or by chemical means, such that power is still available on cloudy or wind-free days. Energy storage on the household level must become affordable. The ultimate renewable energy source may be nuclear fusion, but that is still likely half a century away from reality. In the meantime we will probably need to keep nuclear fission reactors running to make up the inevitable shortfall and inherent unreliability in most existing renewable sources.
Maria Antonietta Loi – The future is brilliant and will be full of many different options. I doubt we will have a good solution for every application.
What challenges do you think the field will need to overcome in the next 20 years?
Graeme Blake – I think that photovoltaic technology has a very bright future. It can be used at various scales from the household level to large commercial solar power plants and is usable to a significant degree at most locations in the world. It is not intrusive, does not harm local ecosystems, and is relatively maintenance-free compared to other sources such as wind and tidal power. The main challenge is reducing costs per kilowatt hour to lower than those of fossil fuel sources, while using earth-abundant, non-toxic and stable materials that do not cost a lot of energy to produce in the first place. Once these goals are achieved then economics will take over and governments/power providers will be persuaded to take up photovoltaic technology on an increasingly large scale.
Maria Antonietta Loi – We will need more efficient devices to be able to reduce the surface necessary to produce the electricity for a household. To do that we will need new concepts and new materials.
Hongxia Wang – With the popularity of mobile electronic devices, one of the challenges in the field of PVs will be the development of reliable, light weight, portable or flexible/packable PVs that can provide electricity whenever there is sunlight. Another area will be the integration of PVs with energy storage devices to provide continuous and reliable energy. Practically, by looking at the cost of a solar cell module, the majority comes from installation and transportation rather than the materials. How to reduce costs associated with installation and transportation is critical to deliver low cost solar electricity. These challenges deserve more attention in the research community.
David Fenning – The field itself needs to focus on maintaining a “sustainable” path for its own development. Recently there has been rapid worldwide growth at both ends of the spectrum: at the early stages of laboratory research and in commercial deployment. Growth to unprecedented scales cannot sacrifice quality, and I would argue must be met in fact with ever-improving quality since expectations rise with increased visibility. However, ensuring for example that energy yields can be maintained over a decade or two with a new technology can be difficult to do in a predictive manner, often requiring extensive and long-duration testing. To sustain or accelerate technical development, advances in predictive modelling of material and device evolution over the time span of years is needed.
The submission deadline for the Photovoltaic Solar Cell Materials – Design, Fabrication and Testing Call for Papers is the 6th of December 2019. For full details of the scope and the full editorial team see https://collections.plos.org/s/solar-cells
About our Guest Editors
Guest Editor, PLOS ONE
David is an Assistant Professor in the Department of NanoEngineering at UC San Diego, where he directs the Solar Energy Innovation Laboratory. His research focuses on defect engineering to improve performance and reliability in silicon and hybrid perovskite solar cells and on CO2 electrocatalysis for energy storage and green fuels. He specializes in the use of synchrotron-based X-ray microscopies to understand the relationships between local chemistry, structure, and performance in energy conversion materials.
Maria Antonietta Loi
Guest Editor, PLOS ONE
Maria Antonietta studied physics at the University of Cagliari in Italy where she received a PhD in 2001. In the same year, she joined the Linz Institute for Organic Solar cells, of the University of Linz, Austria as a postdoctoral fellow. Later she worked as a researcher at the Institute for Nanostructured Materials of the Italian National Research Council in Bologna, Italy. In 2006, she became an assistant professor and Rosalind Franklin Fellow at the Zernike Institute for Advanced Materials of the University of Groningen, The Netherlands, where she is now full professor and chair of the Photophysics and OptoElectronics group. In 2018 she received the Physicaprijs from the Dutch physics association for her outstanding work on organic-inorganic hybrid materials
Guest Editor, PLOS ONE
Hongxia has a PhD degree in Condensed Matter Physics from the Institute of Physics, Chinese Academy of Science, Master’s degree and Bachelor’s degree in Chemistry from the Central South University, China. She is currently a full Professor at Queensland University of Technology (QUT), Australia. Her research group is dedicated to the development of new routes to enhance the performance and stability of next generation solar cells, in particular perovskite solar cells and energy storage devices such as supercapacitors, through innovative material and device engineering. She was the recipient of several prestigious fellowships including the “Australian Research Council (ARC) Future Fellowship” and the “Australian Postdoctoral Fellowship (Industry)”.
Guest Editor, PLOS ONE
Graeme is an Assistant Professor at the Zernike Institute for Advanced Materials, University of Groningen, Netherlands. He received his PhD in inorganic chemistry at the University of Oxford, then worked as a postdoc split between Argonne National Laboratory and the ISIS neutron scattering facility, UK, before joining the faculty at the University of Groningen. His research interests include the chemical synthesis and characterisation of hybrid perovskite-related materials, with a special focus on their crystallography. He is also interested in magnetic materials, especially multiferroic order, skyrmion phases, and magnetism arising from p-electrons in oxygen, and in addition, investigates the chemistry and physics of thermoelectric materials such as chalcogenides.
 ‘Renewables 2018 – Market analysis and forecast from 2018 to 2023’, https://www.iea.org/renewables2018/