The major challenge of our society in the twenty-first century is the worldwide development of clean and sustainable sources of energy. Solutions for renewable energy generation on a global scale are crucial for countering the permanent threat of global warming, which requires us to act faster than the actual decline of fossil fuel reserves itself would necessitate. This message was clearly expressed and received by representatives of more than 190 nations at the recent UNO climate conference in Copenhagen, where politicians managed to keep alive the historic chance for a new, worldwide binding climate change agreement. As soon as we prioritize our worldwide responsibility over national interests, mankind will face a hitherto unprecedented level of requirement for novel materials and technologies to enable the ambitious goals for reducing CO2 emission.
In view of this process, it has to be emphasized that one of the most important and immediate tasks at hand is to invigorate and focus research into the basic and translational science needed for new energy-conversion and energy-generation technologies. Timing is of utmost importance in that process: history proves the importance of starting innovative research early enough. With rising levels of complexity as exemplified by energy materials, research needs to adequately advance applications. In fact, the implementation of applications is typically triggered by material innovations, and not vice versa. This is especially true in materials science. In contrast to typical engineering topics such as device technology or process development, innovation in materials science is nearly impossible to predict. As a result, a stable and sustainable research environment with an excellent platform for scientific interaction and intellectual exchange is vital for success.
A whole range of innovations is required in energy conversion and storage technologies in order to address future needs. Among them are mainstream materials and technologies for solar cells, high energy density batteries and solid state lighting, but also for fuel cells as well as hydrogen generation and storage. Furthermore, the relevance of today's niche technologies like thermoelectric energy conversion or artificial photosynthesis to cutting carbon emissions may well skyrocket within a short time. The properties of novel composite materials will determine the success of these future energy systems, with innovative materials science as the enabling technology. This is illustrated convincingly by the amazing progress made to date in organic solar cells or in thermoelectric materials.
How to direct this research requires urgent decisions and action that will have far-reaching ramifications. The most important question concerns the funding principle: can we put such a crucial need as energy research into the hands of high-tech entrepreneurs, which will help finance and drive clean energy technologies under the powerful motivation of generating (and maximizing) private profit, or is a government-directed, entirely taxpayer-funded lighthouse project the correct environment since it can focus on the research mission and attract the best brains worldwide to a concerted effort?
As with most complex questions, there is no clear answer, no binary truth of one or zero. The discussion has already been around for quite some time,1 and is ongoing. What certainly has emerged is that the direction of research through targeted funding is of particular value to fundamental research far from commercialization and high-tech entrepreneurs' interest, as the latters' strength undoubtedly lies in taking the most promising of a choice of candidate technologies through to commercialization rather than in the initial, exploratory research. The German government's introduction of a renewable energy feed-in tariff for the German national power grid already some years ago has proven that governments are capable of setting technological trends and incentives to the benefit of all, society and industry, and it is to hope that the 2009 UN Copenhagen conference will trigger more decisions in kind.
The challenges post-Copenhagen, namely to match the increasing energy desire to overall worldwide CO2 emission reduction while stabilizing energy consumption in the developing world, are immense. Given the urgency for new technologies and solutions, governments worldwide may well need to synchronize their funding efforts to ensure that progress in energy technology is limited only by the creativity of scientists rather than lack of funding, collaboration or information.
Such concerted efforts require two things: suitable means for knowledge dissemination, and bringing the most important results and successes to widespread attention. Together with the publisher and the editorial office of Advanced Materials, we have therefore taken on the challenge to highlight and further emphasize outstanding results in energy-related materials research. As a result, Advanced Materials will henceforth feature this recurring focus section entitled Advanced Energy Materials.
Innovation in renewable energy is at the center of focus, complemented by research towards reduction of energy consumption and advancement of devices towards mass application. Readers will find materials, devices, and processing for photovoltaics, primary and secondary batteries, hydrogen storage and generation, efficient energy conversion such as solid state lighting, thermoelectrics, and biomimetic materials for artificial photosynthesis as well as coverage of the materials aspects of solar thermal energy and other topics. At this stage we should clarify that traditional energy generation by means of fossil and nuclear fuels or the design and use of structural materials for wind or hydroelectric power generation are not covered by the special section. These engineering disciplines, though clearly energy-related, find coverage and different readerships elsewhere.
We are delighted to herewith present the inaugural issue of Advanced Energy Materials and its representative topic mix from various fields of energy research where material innovation plays the leading role: René Janssen, Guillermo Bazan, and Yang Yang report new findings on organic solar cells (energy generation), Mario Leclerc reviews materials for polymer solar cells and efficient lighting, complemented by Paolo Samorì summarizing progress in solid state lighting as well as other energy saving applications. Hui Cheng reviews the current state-of-the-art in battery material research (energy storage), and Gerhard Wegner reports novel results on proton conductors for high temperature fuel cells (energy conversion).
Suitably designed tandem cell technology is regarded as the essential step to achieve efficiencies beyond 10% for organic solar cells. René Janssen demonstrates that combined analysis of optical absorption and electrical characteristics of the individual single junction subcells is the key to identify optimum material choice as well as device layout of the corresponding tandem cell. Based on combined experimental and simulated data, his contribution demonstrates that the short-circuit current of a polymer tandem cell can exceed that of the current-limiting subcell, opening up new alternative methods to further optimize performance. Yang Yang demonstrates in his manuscript that tandem structures with a 3-terminal (3T) configuration, in which two subcells are connected in parallel through a transparent conducting interlayer, open up easy opportunities to independently either match the current or the voltage of the two subcells. Guillermo Bazan points out the importance of precisely controlling the morphology of bulk heterojunction composites at the time of thin film deposition by processing with solvent additives. In this contribution, he shows a substantial improvement in the morphology of a polymer BHJ solar cell by introducing an additive that is a good solvent for both components, leading to better mixing of the individual phases.
Mario Leclerc reviews the recent progress made on the synthesis and characterization of conjugated polymers based on bridged phenylenes, namely poly(2,7-fluorene)s, poly(2,7-carbazole)s, and poly(2,7-dibenzosilole)s, for applications in solar cells and white light emitting diodes. In his review he presents and explains the main strategies and the remaining challenges to develop reliable and low-cost renewable sources of energy and energy-saving lighting based on these material classes. Paolo Samorì and numerous collaborators including Klaus Müllen, David Beljonne, Richard Friend and Alan Rowan highlight recent results obtained on a new class of macromolecules that possess a very rigid backbone and semiconducting side chains that point away from this backbone. These perylene-substituted polyisocyanides are investigated in a combined experimental and theoretical approach. The (opto)electronic properties of the new functional architectures outline their potential for field effect transistor, solid state lighting and solar cell applications.
The state-of-the-art in material research for energy storage systems, such as thermal energy storage, mechanical energy storage, electromagnetic energy storage, hydrogen energy storage, and electrochemical energy storage is reviewed by Hui Cheng. The strategies for designing and fabricating these advanced energy storage materials are discussed in detail for high-performance hydrogen storage materials and for electrochemical energy storage materials for Li-ion batteries and supercapacitors.
Gerhard Wegner reports that poly(alkylene biguanides) turn out to be excellent ion conductors at temperatures well above 100 °C and up to 200 °C when conjugated with acids like HCl. This and fascinating structure-property relationships in the poly(alkylene biguanides) make further research in this class of materials well worthwhile.
Finally, we would like to acknowledge our Editorial Advisory Board colleagues for their willingness to participate and lend their expertise to Advanced Energy Materials. We thank all the authors for their efforts in submitting a rich variety of high-quality peer-reviewed contributions. We also wish to thank the editorial and production staff at Wiley VCH for their outstanding support and assistance. To our readers, we express our hope that Advanced Energy Materials will provide a valuable forum for top quality publications and scientific discourse for the energy research community, and that it will quickly establish itself as a helpful resource in this important field.