Stability of Organic Solar Cells based on Non-Fullerene Acceptors

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.

This proposal is aligned with EPSRC's priority areas of Solar, Functional materials and Materials characterisation, and will contribute to the development of the UK as a more productive nation and a more resilient nation. The timeliness of this proposal is aligned with EPSRC's sustain investment on Solar Technology and growing investment on Materials for Energy Applications.The successful delivery of this project will lead to the design and manufacture of more stable fullerene-free organic solar cells (OSCs) and solar modules, thereby paving the way for their commercialisation and contributing to addressing the grand challenge of "clean growth" set by the UK's industrial strategy.
The research outcomes of the project will generate direct impact upon the industrial collaborators, Eight19 Ltd., NSG group and Armor Group. These companies have all identified stability as a major bottleneck for the commercialisation of their PV products, and are investing substantially in related R&D activities. These companies will contribute to this project by supplying industrial-level fullerene-free organic PV modules for stability evaluation and advanced characterisation (Eigh19 and NSG), development of fully-printed OSC products on flexible substrates (Eigh19), development of glass-based OSC products particularly for semi-transparent PV applications (NSG), hosting research visits to the project team members (Eight19 and NSG) and also by allocating the time of a number of their technical experts (all companies). The research outcomes of the project (e.g. new material and device designs, new processing routes) will be evaluated in terms of industrial impacts and will be implemented into the companies' manufacturing processes. Extensive research collaborations and knowledge exchange will be enabled through academic/industrial secondments and research visits of the project team members and company experts, and project progress will be updated on regular project collaborator meetings (held quarterly).
In addition to the companies above, other companies specialising in printed photovoltaics, including Heliatek, OPVIUS (former BELECTRIC OPV), CSEM and Oxford PV, are also potential beneficiaries of this project. The research outcomes of this project will not only help these companies to gain an in-depth understanding of the degradation mechanisms, but also to improve the design of their PV products for better stability.
This proposal will be strongly complementary to the established research activities in the UK and abroad in the development and applications of printed optoelectronic materials and devices (e.g. organic, dye-sensitised, perovskite and quantum dot solar cells, transistors, photodetectors and light-emitting diodes), thereby generating immediate impact on a range of research programs including material and device engineering, advanced characterisation, structure-function analysis, device physics and modelling. In addition, the research team members employed in this project will be equipped with skills and knowledge in printed optoelectronic materials and devices, thereby enhancing their future employability in related academic or industrial fields.