Energy generation is not only important for meeting the requirements of consumers and industry, but is crucial for national security and economic competitiveness. Environmental sustainability is a global issue that needs to respond to existing damage from direct emissions as well as unplanned future events such as spills and leakages. Neutron techniques of analysis play some role in progressing all tech­nologies for renewable-energy generation, although this may be limited to structural materials for wind, marine and hydro energy generation, whilst neutron scattering in earth sciences plays a significant role in geothermal energy. Chapters in Part 1 concentrate on those aspects of energy generation that are mainstream for neutron — based methods, but are nevertheless relevant to the more general sustainable-energy technologies in energy generation.

Catalysis (Chap. 2) not only plays a central role in sustainable-energy generation where renewable feedstocks are used, but also plays a more general role in increasing efficiency, reducing energy consumption, and producing cleaner targeted products from fossil-based sources such as oil, natural gas, and coal. Active sites in catalysis are often present in only trace quantities, and characterizing these with neutrons is usually limited to model compounds. However, almost the whole range of neutron techniques of analysis have been used to help in the design, characterisation and optimisation of catalysts by measuring the structure of the catalysts themselves, and following the dynamics of the reactants and products. The hydrogen economy requires an efficient means of generating, storing, and using hydrogen, all of which involve some catalysis. However, because the most important role of catalysis is in energy generation, we gather all aspects of the topic in Chap. 2.

Although global CO2 emissions threaten today’s way of life, cost-effective methods to separate, capture, store, or use CO2 from fossil fuels represent a major challenge. As existing technologies use solvents that impose a heavy energy-pen­alty (about 30 % of the energy generated by the plant), a key scientific challenge is the development of materials that can interact with flue-gas streams to capture and concentrate CO2 with lower energy requirements. Porous materials such as

coordination polymers offer a new way to address this in that they not only possess the highest surface area of any known materials, but they can be engineered to be selective for CO2. Therefore, solid porous hosts represent one of the most prom­ising technologies for separating and storing gases of importance in the generation and use of energy. Studying the uptake of CO2 in such materials at the fundamental level is required to progress these towards commercialisation, with such studies allowing direct feedback into the synthesis of materials with enhanced CO2 uptake, selectivity, and chemical stability. Neutron scattering is essential in this research with in situ studies involving pressure and/or temperature being of particular importance. Both structural and dynamical information is important in this area in order to establish the gas-host interaction, and forms the basis of Chap. 3.

Structural materials are important to all forms of sustainable-energy production and neutron scattering is increasingly being used to characterize and understand fatigue and failure, and in this context we concentrate on materials for nuclear — energy applications in Chap. 4. Nuclear materials are particularly demanding because they must meet the mechanical demands not only under pressure, tem­perature, and chemical environment, but also under the effects of irradiation. Neutron-based characterisation of such materials mainly takes the form of neutron diffraction to understand how the microstructure and crystal structure characterize bulk material-properties. Superficially, this is a straightforward measurement, but in practice we need to understand how crystal structure, microstructure, chemical composition, and orientation are all coupled, and how these can be controlled to obtain (or avoid) particular properties. Major neutron-scattering centres now have at least one instrument that is conceived specially to do these types of experiment.

One of the greatest contributions neutron scattering has made in the study of sustainable energy-materials is in solar cells, which are divided into inorganic and organic in Chaps. 5 and 6, respectively. Photovoltaics (PV) is required for each of them, and is the direct conversion of light into electrical energy, the first PV device having been built by Edmond Becquerel who discovered [R. Williams, J. Chem. Phys. 32,1505 (1960)] thePV effect in 1839. The operating principles of this effect are based on a sequence of light-matter interactions that can be summarized as follows:

(i) Absorption of photons with a given energy matching the semiconducting properties of the device (band gap and intrinsic coefficients);

(ii) Free charge-carrier generation in inorganic semiconductors and bound exciton (pair of electron-hole) creation in the case of the organic analogous and exciton [E. A. Silinsh, V. Capek Organic Molecular Crystals — Interac­tion, Localization and Transport Phenomena (American Institute of Physics, New York, 1994)] dissociation;

(iii) Charge transport via relevant pathways;

(iv) Charge collection at dedicated electrodes and photocurrent generation.

PV solar cells convert solar energy into electricity via the PV effect in a variety of strategies that can be classified as (Fig. 1): Multijunction, single-junction GaAs, crystalline Si, thin-film, as well as organic and emerging (including hybrids).

Sustained development has been achieved for each of the PV strategies over the past decades (Fig. 1). It is important to notice that step changes in efficiency mainly arise with the discovery of a new strategy, but that this is not always upwards. This is because many factors contribute to the cost per Watt, and low efficiency may be counterbalanced by overall cost, which is composed of:

(i) Energy pay-back time;

(ii) Stability and lifetime;

(iii) Environmentally friendly materials and production;

(iv) Cost and supply of materials;

(v) Adaptability of shape/form;

(vi) Size/weight.

For PV materials there is a convenient separation of the materials in the broader classification of inorganic and organic. More attention has been devoted to inor­ganics, which have been known for well over a century and promise very high efficiency. In contrast, the first organic PV (OPV) was crystalline anthracene [H. Kallmann, M. Pope, J. Chem. Phys. 30, 585 (1959)], this having been first observed in 1959. OPVs have only been only attracting significant attention in the past decade (Fig. 2), at least in part as a result of robust concepts and principles for organic semiconductors set up around 1970 by Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa, who were awarded the Nobel Prize in 2000 for this contribution. This paved the way for the use of organic materials as PV devices, inducing and stimulating a tremendous interest in research on OPVs for both fundamental and technical purposes.

In inorganic (thin film, Chap. 5) and organic (Chap. 6) solar cells we focus our attention on case studies of model systems with the aim of showing how neutron techniques of analysis in combination with other techniques contribute to our understanding and resolution of specific challenges in PV materials. The failure of one of the earliest cells, Cu2S-CdS, due to Cu+ to Cu2+ conversion, illustrates the importance of understanding functional properties of PV materials at a number of levels. PV materials cover a vast range, from soft, almost liquid materials through to hard crystalline materials, with local and large-scale structure, interfaces, and dynamics over a wide range of timescales also being important. The challenge for diffraction is that the required semi-conductors are usually not only non-stoichi­ometric (due to doping), but also composed of elements with similar atomic numbers. The non-stoichiometry can lead to structural defects that affect the material and electronic properties so it is essential to have a method for identifying and characterising these differences. Chapter 5 shows how the neutron scattering cross-sections enable not only neighbouring elements to be distinguished, but also the study of defect non-stoichiometric structures. Chapter 6 concerns investigating the comparatively weak forces holding the organic and polymeric molecules together, these being both advantageous and disadvantageous for PV applications. Hence, whilst for inorganic PV materials the atoms can be regarded as localized, for organic PV materials molecular dynamics plays at least an equally important role as time-average atomic position. Dynamics on different timescales is responsible for recombination, charge-transfer, processing, and ultimately ageing, all of which are important to the PV’s function.

Chapter 6 shows not only the use of neutron diffraction, but also how different neutron spectroscopies can be used to unravel the dynamics that helps or hinders different aspects of the PV process.