NeutronOPV · New neutron techniques to probe bulk heterojunction solar cells with graded morphologies – understanding the link between processing, nanostructure and device performance

New neutron techniques to probe bulk heterojunction solar cells with graded morphologies – understanding the link between processing, nanostructure and device performance

By using sufficiently high efficiency solar cell devices, all global energy requirements could theoretically be met by harvesting energy from the sun. Current silicon-based solar cells are limited in their applications because they are heavy, inflexible, fragile and expensive to produce at a large scale. Conversely, polymer solar cells (PSCs) open up new perspectives in the solar cell market due to their potential for large-area, lightweight and flexible devices and ease of manufacture at relatively low cost making them a very attractive alternative to their inorganic counterparts. The low production costs associated with PSCs might be the key for opening solar energy to new markets such as those developing countries where people wouldn’t normally think about investing and generating their own electricity. The typical active layer in a PSC, known as bulk-heterojunction (BHJ), is composed of a blend of an electron donating polymer and an electron accepting fullerene such as PC71BM. In the last decade, PSCs have developed steadily attaining in very recent years power conversion efficiencies (PCEs) > 10% for single layer devices in systems such as PffBT4T-2OD/PC71BM. The processing conditions used in the preparation of polymer:fullerene BHJs play a crucial role in the corresponding nanoscale morphology and this also plays a critical role on the corresponding device efficiencies. Therefore, being able to probe the nanoscale morphology of BHJs is of the uttermost importance. This project’s primary aim was to develop the potential of Small Angle Neutron Scattering (SANS) for studying the morphology of thin BHJ films as used in real PSCs. The secondary aim was to contribute to a better understanding of the relationship between processing conditions, active layer morphology and device performance in PSCs, providing the understanding needed to guide the search for practical processing routes. As conclusions, in this project we have demonstrated that SANS is a very good and highly sensitive technique for studying these BHJ morphologies being able for example to spot morphological differences that would go unnoticed under the most frequently currently used techniques, particularly atomic force microscopy (AFM). The secondary aim was also achieved as we have unravelled, in our opinion with an unprecedented level of detail, the mechanism or one of the mechanisms of action of additives in the improvement of the efficiency of BHJs.

Data: CORDIS, © European Union