Solar Energy Capture Materials
The importance of harnessing the power from the sun to satisfy our growing energy demands without harming the planet is now widely accepted. ‘Solar energy conversion’ includes many categories of technology, including solar thermal, solar fuels and solar-to-electricity (photovoltaic). This book focuses on photovoltaic technology, which now contributes a sizeable percentage of power generation globally, with new markets appearing each year and with over 100 GW of solar-to-electricity capacity installed annually. The large majority of this commercial technology is based on crystalline silicon, which captures the energy from sunlight and converts it to electricity using the pholtovoltaic effect, a phenomenon first reported by Edmond Becquerel in 1839. Evolution of the technology throughout the 1950s led to applications of solar cells for space applications, and strong developments in the 1980s lead to establishment of the commercial technology that is familiar today. Research since then has focused intensively on bringing down the cost and energy required to manufacture the modules. While the efficiency of solar to electrical power conversion is the established figure of merit for solar cells, processability is becoming increasingly important as industry looks towards integrating solar power into new applications, such as sensors and portable electronic devices. New categories of photovoltaic devices have also emerged, such as thin-film solar cells and devices based on nanotechnology, which provide routes to high-throughput manufacturing methods and new applications, such as built-in photovoltaics and wearable technology.
At the heart of these devices are semiconductors, which can be made from extended structures of both inorganic and organic materials. Rapid development in recent years has focused on the discovery and engineering of these materials towards capturing light and converting it to electricity more efficiently. This volume describes the synthesis and properties of the inorganic materials which underpin the current state-of-the-art and possible opportunities for future research. Synthetic chemistry remains fundamental to progress in energy capture materials and targets include increasing the absorptance of materials, replacing rare elements with earth-abundant materials, avoiding the toxic components and manipulating grain boundaries or interfaces to promote charge-transport and minimise charge-recombination processes. The evolution of photovoltaic technology is discussed, including solution-processed materials for continuous manufacturing, reaching high conversion efficiency through stacked or tandem architectures, and device assembly on flexible substrates for easier systems integration. In addition to the materials, advances in characterisation and modelling of charge transfer and transport in the devices continues to enable scientists to understand the fundamental processes occurring in the devices. The knowledge generated is increasingly important as the materials and device configurations become more complex.
The five chapters of this volume cover different approaches to capturing light with inorganic materials. The first chapter introduces silicon devices, which make up most of the PV industry. These are still researched intensively to try and bring down the energy used to make the silicon wafers and to capture as much light as possible. The chapter on compound semiconductor solar cells introduces the concept of heterojunction-based devices and describes synthesis routes to the chalcogenide materials, which dominate thin-film technology. This chapter highlights how both the composition and crystalline properties of the material determine light absorption. The chapter on dye-sensitized solar cells introduces the concept of hybrid devices, which combine organic and inorganic materials. These devices differ from established technology in two ways, first by containing a semiconductor–liquid junction and second because light absorption and charge transport are performed by separate components (a dye, a transparent metal oxide and a redox electrolyte). These devices perform particularly well under diffuse light conditions and are potentially useful for indoor applications. Research in this area focuses on tandem configurations to increase the efficiency and replacing the liquid with a solid-state charge-transport layer. The chapter on solution-processable materials builds on this concept and explores the cutting-edge research into perovskite solar cells. Research in this area has exploded over the last few years as conversion efficiencies have risen steeply to match those of established PV technology. The unique properties of these materials, in terms of charge-separation and transport, are described. Finally, the application of computational methods to aid the discovery of new materials is described, using the example of copper-based absorber materials. Density functional theory is applied to investigate the structural, electronic, and optical properties of emerging copper-based chalcogenides, to identify compositions that could be applied in ultrathin light-absorbing films in very high-efficiency devices in the future.
I am delighted that very active researchers who are working at the forefront of the solar energy field agreed to prepare the chapters in this volume. I thank them for their excellent contributions across the breadth of this important research area. I hope that this book will provide a useful and up-to-date introduction to the area and the role that materials chemistry can play in improving the way we use the abundant and freely available source of energy for the welfare and security of everyone on the planet. There is still much chemical space to explore: the semiconductor industry is still dominated by only a few chemical elements!
Elizabeth A. Gibson