Introduction: Solar Energy Conversion

This thematic issue contains reviews of various aspects of Solar Energy Conversion. The sun provides the largest energy source known to man, with more energy from sunlight striking the earth in 1 h than all of the energy consumed on the planet in an entire year. Solar panels provide a known, scalable technology to capture and convert sunlight into electricity. Moreover, the costs of Si-based photovoltaic panels have declined continuously in the past decade, to the point where solar electricity is now cost-competitive in certain regions and niche markets. Nevertheless, solar energy conversion continues to attract fervent efforts devoted to the discovery and development of new materials, concepts, devices, and systems that can provide new and/or dramatically improved functionality and scalability.

Almost a decade ago, the potential and promise of research efforts in solar energy conversion was documented in an extensive report that was the output of a workshop sponsored by the Department of Energy, entitled “Basic Research Needs for Solar Energy Utilization”. Although much progress has been made in the subsequent period, several key opportunities, as well as knowledge and capability gaps, remain to be developed. The articles contained in this thematic issue cover an important, representative cross section of these outstanding issues in the field.

At present, the cost of Si-based solar panels comprises less than 1/3 of the cost of a fully installed solar electricity system. Improvements in efficiency of the active region of the panel directly reduce the area required, and thus the area for the so-called balance of systems, of such installations. Accordingly, reductions in the cost of manufacturing existing Si solar panels will have less impact on the cost of installed solar electricity systems than increases in the efficiency of the panels themselves. Research directed toward materials and structures that can provide efficiencies in excess of the Shockley–Quiesser limit, i.e. above 32% solar-to-electricity conversion efficiency under unconcentrated sunlight, could therefore provide a disruptive technological approach for photovoltaic systems. Materials that can exploit new physical processes, such as avoiding thermalization losses in quantum dots, are consequently being intensely explored. Another approach involves the development of efficient thin-film materials that can allow for flexible, processable panels, which would facilitate installation and avoid the expense associated with shipping and handling bulky and heavy glass-coated panels. Yet another approach is to develop materials that would allow for tandem structures that can mate seamlessly with Si photovoltaic cells, while being compatible with existing, cost-effective, Si panel manufacturing processes. Yang and Lu both review different aspects of research on flexible, processable, organic solar cells, whereas Sargent reviews the development of colloidal quantum dot systems both for thin-film flexible materials and also as materials that could form devices having efficiencies in excess of the Shockley-Quiesser limit. Chabal et al. review methods for modification of Si surfaces, to facilitate integration of other absorbers and device structures into existing Si photovoltaic manufacturing processes while obtaining large improvement in efficiency through use of tandem devices and improved surface passivation methods.

As reviewed by Chen et al., a second, conceptually distinct opportunity involves the use of concentrated sunlight to produce either electricity or chemical fuels. New thermoelectric materials that could operate satisfactorily at the high temperatures produced by concentrated sunlight could provide a system with no moving parts that would provide alternatives to current technology that involves use of concentrated sunlight to heat a thermal fluid and then, in turn, produce hot steam that is converted into electricity by use of an engine running either a Rankine or Brayton cycle. Additionally, thermochemical cycles to directly split water into renewable hydrogen have been proposed, and research opportunities are associated with lowering the temperature of the needed process steps as well as with managing the optical, thermal, mechanical, and chemical fluxes of the inputs and outputs in a seamless, integrated fashion to allow for construction of an operating, manufacturable solar-driven reactor.

A third opportunity involves the conversion and storage of the energy in sunlight through the direct or indirect production of chemical fuels. Other than the nucleus of an atom, chemical fuels provide the most energy-dense means of storing energy known to date. The energy density of a typical Li-ion battery is 200 kW/kg, whereas the energy density of gasoline is 12,000 W-h/kg. Hence, the production of fuels from sunlight would provide an approach to capture, convert, and store sunlight in a form that is fully compatible with existing energy systems and infrastructure. Such a system must efficiently capture and convert sunlight as well as incorporate catalysts that act on abundant, sustainable feedstocks such as N₂, H₂O, and/or CO₂ and transform them chemically into value-added fuels such as NH₃, alcohols or hydrocarbons, or H₂. Natural photosynthesis, of course, performs this function, albeit with a low sunlight-to-fuels efficiency and with fragile catalytic sites utilized to effect the requisite chemical transformations. Choi et al. review the electrochemical synthesis of materials for solar-driven water splitting. By closer analogy to natural photosynthesis, which involves fixation of CO₂ to energy-rich carbon-containing sugars, Bocarsly et al. review the use of catalysts on semiconductor electrode surfaces to effect the solar-driven photoelectrochemical conversion of H₂O and CO₂ to value added chemicals and fuels. Fujita et al. review the hydrogenation of CO₂ to formate and methanol. In contrast, Brudvig et al. review molecular catalysts for water oxidation, a key step in the sustainable production of fuels at global scale, and Meyer et al. review the use of molecular systems to perform all of the needed functions, including light absorption and catalysis.

Collectively, the articles in this Thematic Issue provide a comprehensive assessment of the state-of-the-art in these selected, key areas in solar energy conversion. The progress in lowering the cost of solar panels, batteries, and other carbon-free or carbon-neutral energy conversion and storage technologies attests to the power of investment in R&D and innovation, which will ultimately provide options and better choices for construction of a clean energy system than those available to date. Continued progress in this area is of strategic importance for mitigation of climate change effects as well as for energy security, to ultimately allow decarbonization of both the electricity and transportation sectors of a global energy system. In addition, research in the field of solar energy conversion is intellectually stimulating and offers opportunities to explore new physics, chemistry, materials science, and engineering. Hence the present issue is timely and will hopefully continue to stimulate progress toward development of a truly cost-effective, functional, scalable, and deployable system that can capture, convert, and store the energy of sunlight beneficially for use by generations to come.