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

Periodic Reporting for period 1 - NeutronOPV (New neutron techniques to probe bulk heterojunction solar cells with graded morphologies – understanding the link between processing, nanostructure and device performance)

Reporting period: 2015-07-01 to 2017-06-30

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.
In this project we have prepared bulk-heterojunction (BHJ) thin films with graded variations in morphology and we have characterized them using neutron scattering techniques, such as Small Angle Neutron Scattering (SANS) and Neutron Reflectivity (NR), as well as a broad range of standard laboratory based techniques.
As main results, we have proved the effectiveness of Small Angle Neutron Scattering (SANS) and the thin film stacking methodology for studying the morphology of thin films as used in real BHJ polymer solar cells. This understanding was achieved in test cases of the full relationship between processing route, nanoscale morphology and device performance, as in the study of the effect of the additive 1,8-diiodooctane (DIO) and of thermal annealing on the morphology and efficiency of PffBT4T-2OD/PC71BM devices.
There is currently a wide interest in organic and hybrid optoelectronic devices such as polymer photovoltaics due to their potential for large-area, lightweight and flexible devices at relatively low cost.
Despite some great improvements in device efficiencies achieved over the last decade, these systems still need a better understanding of how different processing routes lead to different nanoscale morphologies which perform differently. For this reason, being able to probe the nanoscale morphology of these BHJ systems is of the uttermost importance. However, one of the major difficulties in determining precise phase compositions using standard electron- and x-ray-based techniques lies in the limited contrast between the amorphous phases.
In this project we have demonstrated a successful methodology for probing the morphology of BHJs, namely SANS in stacks of thin BHJ films, that other researchers in academia and industry can use to help bringing these organic photovoltaic technologies closer to production.
Some potential outcomes are a much more frequent and generalized use of SANS in the probing of the morphology of these systems, which will help bringing to the market this more sustainable source of energy. This will create new business opportunities and stimulate economic growth.
Left: Schematic of an OPV device; Right: Example SANS data on the system PffBT4T-2OD/PC71BM