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Polymer - Carbon Nanotubes Active Systems for Photovoltaics

Final Report Summary - POCAONTAS (Polymer - Carbon Nanotubes Active Systems for Photovoltaics)

POCAONTAS – which stands for “POlymer-CArbOn NanoTube Active Systems for photovoltaics” – had the goal to develop highly efficient organic solar cells comprising carbon nanotube-polymer blends. Uniting leading groups in fundamental science and innovative companies in device technology and material design, control and understanding of the nanostructure of the photovoltaic blends is improved, which is key for optimum performance and stability.
Organic solar cells (OSCs) have many advantages compared to inorganic solar cells like, light weight, flexibility, colour tunablity, semi-transparency, and – depending on market volume – potentially lower costs. To increase the market volume of OSC, both efficiencies and operational lifetimes must be increased. Currently, the efficiencies of OSC are limited by recombination losses and low absorption yields, while the operational lifetimes are reduced by a series of degradation processes like, changes in the nanomorphology and others. Both problems can in principle be solved by the introduction of single-walled carbon nanotubes (SWNT) into the photoactive systems, because SWNT show high charge carrier mobilities and are chemically and thermally very stable. However, until now, photovoltaic efficiencies in systems containing SWNT stayed far below the best efficiencies of OSC without SWNT. This is due to a lack of control of the electronic and geometrical structure of the SWNT-polymer systems.
In POCAONTAS, we combine leading European research groups and companies in material science, advanced spectroscopy, device technology and quantum-chemical modeling, to achieve a full understanding of the interface between SWNT and the polymers that make up the photovoltaic blends. Based on this understanding, we are able to predict the geometrical and electronic structure of the “ideal” SWNT-polymer interface. Our long-standing experience in device technology and handling of multiphase optoelectronic materials allows us then to attain control over the nanostructure of the SWNT-polymer blends. We will therefore obtain OSC that can, for the first time, fully exploit the advantages of SWNT, because their nanostructure is – and stays – as needed for maximum efficiency.
In terms of scientific output, the fellows of the POCAONTAS network achieved a thorough understanding of the critical influence of nanomorphology on the function of the photovoltaic SWNT/polymer blends and organic solar cells comprising these blends. Thus, the percolation of charge in the SWNT networks inside blends was thoroughly studied, yielding relations between the maximum achievable extraction mobility and SWNT concentration. From this in turn a maximum “allowed” concentration of metallic tubes could be obtained. In samples containing a low concentration of SWNT, very far from the percolation threshold, it was demonstrated that the presence of SWNT significantly increases the bimolecular recombination coefficient. These findings together yielded a parameter range in which the presence of SWNT in photovoltaic blends can be advantageous. We highlight that the quantitative goal of 5% power conversion efficiency for SWNT-based organic solar cells could not be achieved. However, our fellows attained a full understanding for the reason of this limitation. This is beneficial for the research field, as shown by the invited talks and review articles of this network about this topic, but also for the fellows themselves, giving them in-depth insight into the relationship between nanostructure and performance.
In terms of training, we exploited the excellence and complementarity of our partner institutions to the maximum extent in both network-wide actions and in a tailored secondment program for each individual fellow. For maximum impact, these actions and secondments were adapted to the actual needs of the fellows in real time. As a consequence, the actually delivered training contents of the POCAONTAS network by far exceed the contractual deliverables, which already had maximum rating in the proposal phase. As planned, we delivered a highest level training program balanced between scientific (tutorials), technical (hands-on workshops) and complementary contents, always delivered by international experts in the respective fields, many of which are POCAONTAS members. But on top of that, we delivered extra events highlighting “hot” topics whose immediacy could not be foreseen in the proposal phase, such as perovskite solar cells and nanostructured materials for energy applications.
It is not a surprise that these efforts are paying off. First and above all, many of our fellows immediately found positions in academia and industry, working now in fields that fully reflect the training they received in POCAONTAS. For some of our fellows, the interdisciplinarity in the CVs was a decisive competitive advantage against other candidates on the job market. Furthermore, the partner institutes strongly intensified their long standing research tradition, such as IMDEA and BELECTRIC, or IIT and UW-PC, and new and promising research partnerships have been formed, such as UW-PC2 and IMPERIAL.