Solar energy conversion

The rapidly increasing world-wide energy consumption, in combination with the climate and environmental consequences of today's energy production, calls for a significant increase of renewable energy in our energy mix. Solar energy is abundant, but so far a very small part of the energy we use. If we can find ways to convert solar energy into forms of energy that we know how to handle and use, a large part of the “energy problem” is solved. Methods and materials that we develop have to fulfil a number of requirements: lead to devices producing electricity from fuel at a competitive price; use environmentally benign materials and processes; only use abundant elements; use processes and materials suitable for large scale production; the materials and devices utilized should have a long lifetime.

For conversion of solar energy into other forms of energy, that we already know how to use, there are three choices, conversion to electricity for immediate use, conversion to a fuel that can be stored for later use, or conversion to heat. In this themed issue the two first approaches are discussed. For solar electricity production silicon solar cells already exist for commercial applications and have quite decent power conversion efficiency. Advanced multi-junction semiconductor cells for research or very specialized applications provide much higher power conversion efficiencies. For the reasons given above and further discussed in several of the articles in this issue, other types of solar cell materials are needed, like plastic or hybrid materials. Several of the papers in this issue describe work being carried out to develop such solar cells.

The sun is not always shining and some applications, like the powering of motor vehicles, cannot be conveniently done directly with solar cells. Therefore there is a need to store the solar energy in a fuel that can be used when it is needed. Of course, solar generated electricity can be used to produce a fuel via electrolysis, but it would be better (probably more efficient) if this could be done directly—solar energy to fuel.

The natural process for solar fuel generation, photosynthesis, has a low overall light-to-biomass conversion efficiency of approximately 1%. However, the quantum efficiency for converting and storing the light energy as energy rich intermediates can approach 100% and power conversion efficiencies for these processes are tens of percent. Many different approaches for solar fuel production are explored. Using Nature's solution as a blue print, supramolecular, multichromophoric systems are designed with photosensitizers and functions for water splitting and fuel production. Molecular hydrogen is the most commonly considered fuel, but large scale solar energy driven production of carbon based fuels (using CO₂ as starting material) is an appealing idea. Such a completely integrated system, with light driven oxygen and hydrogen production has yet to be demonstrated. Improving and modifying biological organisms for higher biomass production or hydrogen production is an interesting way to capitalize on Nature's evolution of highly specialized processes towards our goal. Several variants of biology inspired or biology based solar fuel production are discussed in this issue.

Fully inorganic systems based on metal oxide and semiconductor materials, as well as bio-inorganic conjugates, are additional promising options for solar fuel production considered here.

Development of a new material with the complex function of photocatalysis involving multielectron transfer processes requires interactions between many scientific fields. Organic or inorganic synthesis to produce a material, often with a very specific nanostructure, has to be combined with characterization of the structure, its optical and electrical properties involving both experimental and theoretical work. When the targeted system involves biological organisms or parts thereof, biochemistry and molecular biology methods of course play an important role. The papers of this issue give several nice examples of such multidisciplinary work. I wish you much pleasure reading this themed issue!

Villy Sundström

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