Harnessing energy from the sun through solar panels is a popular form of renewable energy. They power a range of technologies, from calculators to hospitals and can even be found on spacecraft. Organic photovoltaics (OPVs) are a type of solar cell that are made from carbon-based semiconductors. They are typically thinner and more flexible than the silicon solar cells present in the large, dark solar panels we are used to seeing and are sometimes referred to as ’plastic solar cells’ as they share similar chemical compositions. OPVs boast a number of properties that enable their use across a wide range of applications, from construction to portable electronics and even agricultural polytunnels. Their materials are readily abundant, so the production costs are low, the colour and transparency can be easily controlled and they can be produced on a large scale by solution processing.
In the past, OPVs were based on fullerene materials, which are large cages of 60 or 70 carbon atoms, due to their good electron accepting abilities. Unfortunately, the structures of fullerene materials are difficult to modify, reducing the control over OPV performance, as film nanostructure or absorption of light cannot be optimised. Recent research has focussed on non-fullerene acceptor materials (NFAs) which enable more control over their composition, allowing researchers to alter optical, electrical and structural properties to make really efficient devices. However, despite the improved efficiencies NFA based compositions provide, issues remain surrounding the stability of devices - we need OPVs that will last a long time, while maintaining optimal efficiencies. This is where this recent work comes in.
In this study, published in Small, a group led by the University of Sheffield with researchers from Instituto Superior Técnico and University of Porto in Portugal, as well as the University of Manchester and ISIS, used Small-Angle Neutron Scattering (SANS) to study techniques for optimising the stability of NFA based OPVs.
“Organic solar cells have re-emerged as a really exciting photovoltaic technology due to a new class of highly efficient and tunable materials called non-fullerene acceptors. However, whilst the efficiency of these materials is impressive, they are often susceptible to structural instabilities over time.” -Dr Rachel Kilbride, Postdoctoral Research Associate in the Department of Chemistry, University of Sheffield and first author on the paper.
One of the ways to optimise OPVs that has been explored is by adjusting the chemical compositions with solvent additives. Solvent additives alter the way the OPV film dries as well as changing the solubilities of the donor/acceptor components, which can lead to a big effect on the nanostructure of the film. 1,8-diiodooctane (DIO) is a promising additive for this purpose and has been shown to alter the nanostructure in fullerene OPV systems, but little is known about its impact on non-fullerene acceptors (NFAs). In this experiment, the team used SANS on the Larmor instrument to investigate the impact of DIO on a typical polymer/NFA photovoltaic system, and a comparative polymer/fullerene system.
Organic photovoltaic thin film samples for SANS measurements on the Larmor diffractometer at ISIS. Credit: Rachel Kilbride
“Having access to ISIS and the technique of SANS over an extended period allowed us to track the changes in nanostructure as these materials age, like they would as solar cell devices. This is one of the priorities for this field: as efficiencies are now really good, what we need to understand is how to make more stable and long lasting nanostructured solar cells.” -Dr Andrew Parnell, Research Fellow in the Department of Physics and Astronomy, University of Sheffield.
Their findings demonstrated that solvent additive processing impacts the nanoscale structure and stability of fullerene and NFA based systems differently. This suggests that, with the increasing use of NFA blends, a different approach needs to be taken to optimise them, compared to fullerene systems.
“Mass production of solar cells that are made just from carbon and self-assemble into the right length scales are really important in the transition to a green future” says Dr Parnell. This research will be important to ensuring they are long-lasting. While these findings focus on solar cells, it is also noted by the authors that the conclusions can be applied to a range of optoelectronics that rely on solution processed blend films.
