Stability of Organic Solar Cells based on Non-Fullerene Acceptors

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Organic and other types of solution-processed solar cells are a highly promising alternative to conventional silicon-based photovoltaics (PV) as a lightweight, flexible, disposable and truly building-integrated PV technology with extremely quick energy payback. However, their limited stability has now been widely recognised as a common bottleneck for their commercialisation, with exposure to various environmental factors (e.g. light, heat, oxygen, humidity) leading to rapid losses of their performance, the origin of which often remains widely unclear.Fullerenes have been ubiquitously used as an electron acceptor and transport material in organic solar cells (OSCs) in the past two decades. Only until the last 3-4 years, non-fullerene acceptor materials have been brought to the forefront of the development of OSCs as a more efficient, lower-cost and more versatile alternative to fullerenes, with the performance of fullerene-free OSCs already significantly exceeding that of fullerenes-based OSCs. Nevertheless, the majority of research efforts to date have only been dedicated to further optimising their efficiency, leaving a clear gap in the understanding of their stability and degradation mechanisms, another key consideration for their commercialisation.This proposal is designed to address three very important yet largely unanswered questions in the development of stable fullerene-free OSCs: 1-What are the mechanisms causing the degradation of fullerene-free OSCs; 2-Can we understand these degradation mechanisms both comprehensively and quantitatively; and 3-What controls these degradation mechanisms and how to address them? To answer these questions, this proposal will develop a new research methodology to study OSC degradation, which has not been established previously. By performing time-resolved and inter-correlated optical, structural and functional analysis of PV films and devices degraded in a locally-controlled environment, this methodology is capable of capturing the real-time information of the fundamental processes leading to device performance losses during the degradation process, thereby establishing a quantitative relationship between the degradation mechanisms and the resulting OSC degradation behaviour. Specifically, the evolution (i.e. time-resolved) of several advanced, performance-determining device parameters, as well as that of chemical and structural changes during the same degradation process (i.e. inter-correlated), will be recorded and further analysed in order to reconstruct the OSC degradation behaviour. Only fullerene-free OSCs will be studied in this project, but the new methodology can be universally applied to study other types of solar cells, such as polymer:fullerene, quantum dots, dye-sensitised and perovskite solar cells. A core focus of this project is the quantitative analysis of the impacts of major degradation mechanisms of fullerene-free OSCs as a function of their material and device design. The PI has already led the research efforts in quantitatively investigating the degradation of fullerenes and their impacts upon OSC stability, which laid the foundations for the development of the new research methodology proposed here. Based on the quantitative knowledge acquired, this proposal also aims to develop comprehensive material and device design rules capable of guiding the systematic optimisation of the stability of fullerene-free OSCs. This proposal will build upon the established research expertise and facilities in energy materials and devices at Cardiff University, in close collaboration with Swansea University and Imperial College London. The project will be carried out in partnership with 1) Eight19 Ltd., a UK-based SME specialising in the commercialisation of OSC products; 2) NSG group, a UK-based, world-leading company in glass and glazing products (e.g. glass-based PV products) 3) Armor group, a France-based company specialising in printing and coating technologies.